Automation With PLC

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Automation with PLC

Dr. Mohammad Ababneh Department of Mechatronics Engineering

The Hashemite University

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Table of Contents

Table of Contents ....................................................................................................................... ii

Preface ............................................................................................................................................ 1

Chapter 1: Introduction .......................................................................................................... 2 1.1 Introduction.......................................................................................................................... 2 1.2 Why PLC? ............................................................................................................................. 3 1.3 PLC Hardware .................................................................................................................... 4 1.5 “PLC Mixer Process Control” Example ......................................................................... 7

Chapter 2: Digital Systems Review .................................................................................... 9 2.1 Introduction.......................................................................................................................... 9 2.2 Logical Gates..................................................................................................................... 11 2.2.1 The Logical AND function ........................................................................................... 11 2.2.2 The Logical OR function.............................................................................................. 12 2.2.3 The Logical NOT function ........................................................................................... 12 2.2.4 The Logical NAND function ........................................................................................ 13 2.2.5 The Logical NOR function........................................................................................... 13 2.2.6 The Logical XOR function ........................................................................................... 14

Chapter 3: Hard Wired versus Programmed Logic..................................................... 15 2.1 Hardwired Stop/Start Motor Control Circuit ............................................................ 15 2.2 Examples of Hard Wired versus Programmed Logic ............................................. 16

Chapter 4: Bit Logic Instructions ..................................................................................... 21 1.1 Normally Open Contact .................................................................................................. 21 1.2 Normally Closed Contact ............................................................................................... 22 1.3 Output Coil ......................................................................................................................... 23 1.4 Midline Output .................................................................................................................. 23 1.5 Output Invert Power Flow.............................................................................................. 23 1.6 Save RLO into BR Memory ............................................................................................ 24 1.7 XOR function ..................................................................................................................... 24 1.8 Set Coil ................................................................................................................................ 25

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1.9 Reset Coil ........................................................................................................................... 25 1.10 Set-Reset Flip Flop ........................................................................................................ 26 1.11 Reset-Set Flip Flop ........................................................................................................ 26

Chapter 5 Timers ..................................................................................................................... 28 5.1 Introduction........................................................................................................................ 28 5.2 Time Base .......................................................................................................................... 28 5.3 Bit Configuration in the Time Cell ............................................................................... 29 5.4 Choosing the right Timer................................................................................................ 29 5.5 Timers ................................................................................................................................. 30 5.5.1: Pulse S5 Timer (S_PULSE) ........................................................................................ 31 5.5.2: Extended Pulse S5 Timer ( S_PEXT) ...................................................................... 33 5.5.3: On-Delay S5 Timer (S_ S_ODT) ............................................................................... 34 5.5.4: Retentive On Delay S5 Timer (S_ODTS)................................................................ 36 5.5.5: Retentive On-Delay S5 Timer (S_ODTS)................................................................ 38

Chapter 6: Counters................................................................................................................ 40 6.1: Up-Down Counter ........................................................................................................... 41 6.2: Up Counter........................................................................................................................ 42 6.3: Down Counter .................................................................................................................. 44

Chapter 7: Data Types ........................................................................................................... 46 7.1 Overview of the Data Types.......................................................................................... 46 7.2 Elementary Data Types ................................................................................................. 47

Chapter 8: More PLC Instructions.................................................................................... 50 8.1 Overview of All Ladder Instructions ........................................................................... 50 8.2 Add double integer instruction (ADD_DI) .................................................................. 53 8.3 Compare integer instruction.......................................................................................... 54 8.4 Shift right integer instruction ........................................................................................ 55 8.5 SQRT Square Root Instruction...................................................................................... 56 8.6 Subtract Real (SUB_R) .................................................................................................... 56 8.7 WAND_W (Word) AND Word ........................................................................................ 57

Chapter 9: Computer Numerical Control (CNC).......................................................... 59 9.1 Introduction........................................................................................................................ 59 9.2 Fundamentals of CNC .................................................................................................... 61

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9.3 Motion control - the heart of CNC ................................................................................ 62 9.4 Understanding absolute versus incremental motion ............................................. 64 9.5 Assigning program zero ................................................................................................. 64 9.6 Telling the machine what to do - the CNC program ............................................... 65 9.7 Decimal point programming .......................................................................................... 67 9.7 Know your machine ........................................................................................................ 67 9.8 What is G-Code Programming? (CNC Machine)....................................................... 69 9.9 Directions of motion (axes) ............................................................................................ 72

References ................................................................................................................................... 77

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Preface

In these days Programmable Logic Controller (PLC) plays a main in industrial

control. Except for power systems, Logic Relay Control is something from the past, and PLC is used in most of the industrial automation. In fact, it is said that if you use more than six relays in machine, then it is more cost effective to replace them with a PLC.

We believe that Siemens PLC has the loin share in the PLC market worldwide.

Therefore, it is very beneficial to provide our students with this expertise. In addition, learning PLC is like learning computer programming, in that if you master one PLC system, it is easy to understand other PLC systems.

The book is divided into nine chapters; Chapters one through three

introduces the PLC subject, digital logic and logic relay. These three chapters are based on reference [1].Chapters four through present programming material about bit logic, timers, counters, data types and rest of Siemens PLC instructions. These five chapters are based on references [2] and [3]. Chapter nine present the CNC material and based on references [4], [5], and [6].

This textbook is designed for undergraduate course in PLC automation using

Siemens PLC. The lack of textbooks in this field was huge motivation to produce this book.

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Chapter 1: Introduction

This chapter introduces the subject of the Programmable Logic Controller

(PLC). The PLC is simply a microprocessor-based device that is used mainly to control industrial processes. Unlike the personal computer, the PLC is designed for multiple inputs and multiple outputs, and is designed for the harsh industrial environment. It reads process variables through sensors (like pressure, temperature, weight, etc), these inputs are used based on a digital-controller (saved in the PLC) to output certain outputs (like actuator, motor relay, horn, … etc) to control the industrial processes. The figure below shows a PLC cabinet.

Figure 1: PLC inside a control cabinet

1.1 Introduction

PLC is widespread in the industry and is the corner stone for almost all

industrial machines and equipment. It used in many applications, such packaging, extruding, slitting, winding, defect-detection …etc. And it hard to find a plant without PLCs in it.

PLC are available from many manufacturers such Seimens, Allen Bradley, Omron, Mitsubishi and Telemechanique. Seimens PLCs are most spread worldwide. And Allen Bradley PLCs are more popular in the North America market. Omron and Mitsubishi some portion of the international market. And telemechanique the least popular among the above brands. This book will cover the material that are common to PLCs and will use examples that applicable to Seimens PLCs.

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1.2 Why PLC? A programmable logic controller (PLC) is a specialized computer used to

control machines and process. It uses a programmable memory to store instructions and execute specific functions that include On/Off control, timing, counting, sequencing, arithmetic, and data handling.

A big advantage of a PLC Control System is it eliminates much of the hard wiring that was associated with conventional relay control circuits. Where the program takes the place of much of the external wiring that would be required for control of a process.

Also increased reliability is another important advantage of PLC. Once a

program has been written and tested it can be downloaded to other PLCs. Since all the logic is contained in the PLC’s memory, there is no chance of making a logic wiring error.

Figure 2: An industrial process In addition PLCs are more flexible than traditional controllers, where original

equipment manufacturers (OEMs) can provide system updates for a process by simply sending out a new program. And it is easier to create and change a program in a PLC than to wire and rewire a circuit. End-users can modify the program in the field.

Furthermore lower cost is a huge benefit. Originally PLCs were designed to

replace relay control logic. The cost savings using PLCs have been so significant that relay control is becoming obsolete, except for power applications. Generally, if an application requires more than about 6 control relays, it will usually be less expensive to install a PLC.

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Figure 3: A control relay

A PLC can communicate with other controllers or computer equipment. They can be networked to perform such functions as: supervisory control, data gathering, monitoring devices and process parameters, and downloading and uploading of programs. PLCs have faster response time and they operate in real-time which means that an event taking place in the field will result in an operation or output taking place. Machines that process thousands of items per second and objects that spend only a fraction of a second in front of a sensor require the PLC’s quick response capability. PLCs are easier to troubleshoot and have resident diagnostic and override functions that allow users to easily trace and correct software and hardware problems. 1.3 PLC Hardware

In general a PLC has four main components, these are power supply,

processor, input cards, output cards. Furthermore, a programming device is used to enter and troubleshoot programs and problems as shown in the figure below.

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Figure 4: PLC System

The CPU is the “brain” of the PLC. It consists of a microprocessor for

implementing the logic, and controlling the communications among the modules. Designed so the desired circuit can be entered in relay ladder logic form. The processor accepts input data from various sensing devices, executes the stored user program, and sends appropriate output commands to control devices.

The DC supply provides DC power to other

modules that plug into the PLC rack. In large PLC systems, this power supply does not normally supply power to the field devices. In small and micro PLC systems, the power supply is also used to power field devices.

I/O section consists of input modules and output modules. Input card forms the interface by which input field devices are connected to the controller. The terms “field” and “real world” are used to distinguish actual external devices that exist and must be physically wired into the system.

Figure 5: Input interface

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Output modules forms the interface by which output field devices are connected to the controller. PLCs employ an optical isolator which uses light to electrically isolate the internal components from the input and output terminals.

A personal computer (PC) is the most commonly used programming device. The personal computer communicates with the PLC processor via a serial or parallel data communications link.

The computer monitor is used to display the logic on the screen.

Figure 6: Output interface The software allows users to create, edit, document, store and troubleshoot

programs. If the programming unit is not in use, it may be unplugged and removed. Removing the programming unit will not affect the operation of the user program.

PLC Operating Cycle

During each operating cycle,

the controller examines the status of input devices, executes the user program, and changes outputs accordingly. The completion of one cycle of this sequence is called a scan. The scan time, the time required for one full cycle, provides a measure of the speed of response of the PLC.

Figure 7: PLC operating cycle

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1.5 “PLC Mixer Process Control” Example

Mixer motor to automatically stir the liquid in the tank when the temperature and pressure reach preset values. Alternate manual pushbutton control of the motor to be provided.

The temperature and pressure

sensor switches close their respective contacts when conditions reach their preset values.

Figure 8: Mixer process Control

Process Control Relay Ladder Diagram

Figure 9: Relay ladder diagram

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Motor starter coil is energized when

both the pressure and temperature switches are closed or when the manual pushbutton is pressed.

PLC Input Module Connections

The same input field devices are used. These devices are wired to the input module according to the manufacturer’s labeling scheme as shown in the figure on the side.

PLC Output Module Connections

Same output field device is used and wired to the output module as shown in the figure on the side PLC program

where ---| |--- (Normally Open Contact) --| / |--- (Normally Closed Contact) ---( ) (Output Coil).

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Chapter 2: Digital Systems Review

Digital systems have been in used since the mid of last century. And since the

1980’s the digital revolution have been started and affected all the industrial aspects, one important aspect is the introduction of PLC which nothing but a special digital computer.

This chapter covers the basics for digital systems, this is necessary to

understand subject of PLCs.

2.1 Introduction The radix or base of a number system determines the total number of

different symbols or digits used by the system. The decimal system has a base of 10. In the decimal system, 10 unique numbers or digits ( 0 through 9) are used: the total number of symbols is the same as the base, and the symbol with the largest value is 1 less than the base.

The decimal system can be summarized as follows:

Ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 Base: 10 Weights: 1, 10, 100, 1000, …(powers of base 10)

The binary system has a base of 2. The only allowable digits are 0 and 1. Digital Signal Waveform: with digital circuits it is easy to distinguish between two voltage levels - +5 V and 0 V, which can be related to the binary digits 1 and 0.

Figure 1: Binary waveform

The binary system can be summarized as follows:

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Two digits: 0, 1 Base: 2 Weights: 1, 2, 4, 8, 16, 32, …(powers of base 2)

The size of the programmable

controller relates to the amount of user program that can be stored. The 1 K word memory size shown can store 1,024 words, or 16,380 (1,024 x 16) bits of information using 16-bit words or 32,768 (1,024 x 32) using 32 bit words.

Figure 2: PLC Processor Memory Size

Utilizing binary concept, many things can be thought of as existing in one of

two states. These two states can be defined as “high” or “low”, “on” or “off”, “yes” or “no”, and “1” or “0”.

Figure 3: Binary concept

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2.2 Logical Gates

In this section we will presents the most common logical gates such as AND, OR, NOT, NAND, NOR and XOR. Examples will be provided to demonstrate some applications of these logical gates. 2.2.1 The Logical AND function

The outcome or output is called Y and the input signals are called A, B, C,

etc. Binary 1 represents the presence of a signal or the occurrence of some event, while binary 0 represents the absence of the signal or nonoccurrence of the event.

AND Gate Example

The AND gate operates like a series circuit. The light will be “on” only when both switch A and switch B are closed.

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2.2.2 The Logical OR function

An OR gate can have any number of inputs but only one output. The OR gate output is 1 if one or more inputs are 1. OR Gate Example

The OR gate operates like a parallel circuit. The light will be “on” if switch A or switch B is closed. 2.2.3 The Logical NOT function

The NOT function has only one input and one output.

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The NOT output is 1 if the input is 0. The NOT output is 0 if the input is 1. Since the output is always the reverse of the input it is called an inverter.

NOT Gate Example Acts like a normally closed pushbutton in series with the output. The light will be “on” if the pushbutton is not pressed. The light will be “off” if the pushbutton is n pressed. 2.2.4 The Logical NAND function

The NAND gate functions like an AND gate with an inverter connected to its output. The only time the NAND gate output is 0 is when all inputs are binary 1. 2.2.5 The Logical NOR function

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The NOR gate functions like an OR gate with an inverter connected to its output. The only time the NAND gate output is 1 is when all inputs are binary 0. 2.2.6 The Logical XOR function

The XOR function has two inputs and one output. The output of this gate is HIGH only when one input or the other is HIGH, but not both. It is commonly used for comparison of two binary numbers. Example: Write down the final equation for the following digital circuit

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Chapter 3: Hard Wired versus Programmed Logic

The term hardwired logic refers to logic control functions that are determined

by the way devices are interconnected. Hardwired logic can be implemented using relays and relay ladder schematics. Hardwired logic is fixed and it is changeable only by altering the way devices are connected. 2.1 Hardwired Stop/Start Motor Control Circuit

Stop/start motor control circuit is one of the most used circuit in the industry. The figure below shows a picture of the cabinet and relay ladder schematic

Figure 1: Stop/start motor control circuit and its relay ladder schematics

The input and output field devices remain the same as those required for the

hardwired circuit. A rung is the contact symbolism required to control an output. Each rung is a combination of input conditions connected from left to right with the symbol that represents the output at the far right. The instructions used are the relay equivalent of normally open (NO) and normally closed (NC) contacts and coils

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Figure 2: Ladder logic for stop/start motor control circuit

2.2 Examples of Hard Wired versus Programmed Logic

Example 1:

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Example 2:

Example 3:

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Example 4:

Example 5:

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Example 6:

Example 7:

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Example 8 (Interlock System):

Example 9:

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Chapter 4: Bit Logic Instructions

Bit logic instructions work with two digits, 1 and 0. These two digits form the

base of a number system called the binary system. The two digits 1 and 0 are called binary digits or bits. In the world of contacts and coils, a 1 indicates activated or energized, and a 0 indicates not activated or not energized.

The bit logic instructions interpret signal states of 1 and 0 and combine them

according to Boolean logic. These combinations produce a result of 1 or 0 that is called the “result of logic operation” (RLO).

The logic operations that are triggered by the bit logic instructions perform a

variety of functions. There are bit logic instructions to perform the following functions:

1. ---| |--- Normally Open Contact (Address) 2. ---| / |--- Normally Closed Contact (Address) 3. ---( ) Output Coil 4. ---( # )--- Midline Output 5. ---|NOT|--- Invert Power Flow 6. ---(SAVE) Save RLO into BR Memory 7. XOR Bit Exclusive OR

The following instructions react to an RLO of 1:

1. ---( S ) Set Coil 2. ---( R ) Reset Coil 3. SR Set-Reset Flip Flop 4. RS Reset-Set Flip Flop

Other instructions react to a positive or negative edge transition to perform

the following functions:

1. ---(N)--- Negative RLO Edge Detection 2. ---(P)--- Positive RLO Edge Detection 3. NEG Address Negative Edge Detection 4. POS Address Positive Edge Detection

These instructions are discussed in details in the following sections. 1.1 Normally Open Contact

---| |--- (Normally Open Contact) is closed when the bit value stored at the

specified <address> is equal to "1". When the contact is closed, ladder rail power flows across the contact and the result of logic operation (RLO) = "1".

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Otherwise, if the signal state at the specified <address> is "0", the contact is

open. When the contact is open, power does not flow across the contact and the result of logic operation (RLO) = "0".

When used in series, ---| |--- is linked to the RLO bit by AND logic. When

used in parallel, it is linked to the RLO by OR logic. Example

Power flows if one of the following conditions exists:

The signal state is "1" at inputs I0.0 and I0.1 Or the signal state is "1" at input I0.2 Industrial control

1.2 Normally Closed Contact

--| / |--- (Normally Closed Contact) is closed when the bit value stored at the specified <address> is equal to "0". When the contact is closed, ladder rail power flows across the contact and the result of logic operation (RLO) = "1".

Otherwise, if the signal state at the specified <address> is "1", the contact is

opened. When the contact is opened, power does not flow across the contact and the result of logic operation (RLO) = "0".

When used in series, ---| / |--- is linked to the RLO bit by AND logic. When

used in parallel, it is linked to the RLO by OR logic. Example

Power flows if one of the following conditions exists:

The signal state is "1" at inputs I0.0 and I0.1 Or the signal state is "1" at input I0.2

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1.3 Output Coil ---( ) (Output Coil) works like a coil in a relay logic diagram. If there is power flow to the coil (RLO = 1), the bit at location <address> is set to "1". If there is no power flow to the coil (RLO = 0), the bit at location <address> is set to "0". An output coil can only be placed at the right end of a ladder rung. Multiple output elements (max. 16) are possible (see example). A negated output can be created by using the ---|NOT|--- (invert power flow) element.

Example

The signal state of output Q4.0 is "1" if one of the following conditions exists:

The signal state is "1" at inputs I0.0 and I0.1 Or the signal state is "0" at input I0.2.

The signal state of output Q4.1 is "1" if one of the following conditions exists: The signal state is "1" at inputs I0.0 and I0.1 Or the signal state is "0" at input I0.2 and "1" at input I0.3

1.4 Midline Output ---( # )--- (Midline Output) is an intermediate assigning element which saves the RLO bit (power flow status) to a specified <address>. The midline output element saves the logical result of the preceding branch elements. In series with other contacts, ---( # )--- is inserted like a contact. A ---( # )--- element may never be connected to the power rail or directly after a branch connection or at the end of a branch. A negated ---( # )--- can be created by using the ---|NOT|--- (invert power flow) element. Example

1.5 Output Invert Power Flow

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--|NOT|--- (Invert Power Flow) negates the RLO bit. Example


The signal state is "1" at input I0.0 Or the signal state is "1" at inputs I0.1 and I0.2.

1.6 Save RLO into BR Memory ---(SAVE) (Save RLO into BR Memory) saves the RLO to the BR bit of the status word. The first check bit /FC is not reset. Example

The status of the rung (=RLO) is saved to the BR bit. 1.7 XOR function For the XOR function, a network of normally open and normally closed contacts must be created as shown in the example below. XOR (Bit Exclusive OR) creates an RLO of "1" if the signal state of the two specified bits is different. Example

The output Q4.0 is "1" if (I0.0 = "0" AND I0.1 = "1") OR (I0.0 = "1" AND I0.1 = "0").

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1.8 Set Coil ---( S ) (Set Coil) is executed only if the RLO of the preceding instructions is "1" (power flows to the coil). If the RLO is "1" the specified <address> of the element is set to "1". An RLO = 0 has no effect and the current state of the element's specified address remains unchanged. Example



If the RLO is "0", the signal state of output Q4.0 remains unchanged. 1.9 Reset Coil ---( R ) (Reset Coil) is executed only if the RLO of the preceding instructions is "1" (power flows to the coil). If power flows to the coil (RLO is "1"), the specified <address> of the element is reset to "0". A RLO of "0" (no power flow to the coil) has no effect and the state of the element's specified address remains unchanged. The <address> may also be a timer (T no.) whose timer value is reset to "0" or a counter (C no.) whose counter value is reset to "0". Example

The signal state of output Q4.0 is reset to "0" if one of the following conditions exists:


If the RLO is "0", the signal state of output Q4.0 remains unchanged. The signal state of timer T1 is only reset if:

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the signal state is "1" at input I0.3. The signal state of counter C1 is only reset if:

the signal state is "1" at input I0.4. 1.10 Set-Reset Flip Flop SR (Set-Reset Flip Flop) is set if the signal state is "1" at the S input, and "0" at the R input. Otherwise, if the signal state is "0" at the S input and "1" at the R input, the flip flop is reset. If the RLO is "1" at both inputs, the order is of primary importance. The SR flip flop executes first the set instruction then the reset instruction at the specified <address>, so that this address remains reset for the remainder of program scanning. The S (Set) and R (Reset) instructions are executed only when the RLO is "1". RLO "0" has no effect on these instructions and the address specified in the instruction remains unchanged.

Parameter Data Type Memory Area Description <address> BOOL I, Q, M, L, D Set or reset bit S BOOL I, Q, M, L, D Enable set instruction R BOOL I, Q, M, L, D Enable reset instruction Q BOOL I, Q, M, L, D Signal state of <address> Example

If the signal state is "1" at input I0.0 and "0" at I0.1, memory bit M0.0 is set and output Q4.0 is "1". Otherwise, if the signal state at input I0.0 is "0" and at I0.1 is "1", memory bit M0.0 is reset and output Q4.0 is "0". If both signal states are "0", nothing is changed. If both signal states are "1", the reset instruction dominates because of the order; M0.0 is reset and Q4.0 is "0". 1.11 Reset-Set Flip Flop RS (Reset-Set Flip Flop) is reset if the signal state is "1" at the R input, and "0" at the S input. Otherwise, if the signal state is "0" at the R input and "1" at the S input, the flip flop is set. If the RLO is "1" at both inputs, the order is of primary

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importance. The RS flip flop executes first the reset instruction then the set instruction at the specified <address>, so that this address remains set for the remainder of program scanning. The S (Set) and R (Reset) instructions are executed only when the RLO is "1". RLO "0" has no effect on these instructions and the address specified in the instruction remains unchanged.

Example

If the signal state is "1" at input I0.0 and "0" at I0.1, memory bit M0.0 is set and output Q4.0 is "0". Otherwise, if the signal state at input I0.0 is "0" and at I0.1 is "1", memory bit M0.0 is reset and output Q4.0 is "1". If both signal states are "0", nothing is changed. If both signal states are "1", the set instruction dominates because of the order; M0.0 is set and Q4.0 is "1".

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Chapter 5 Timers

This chapter introduces the subject of Timers to the reader. Timers

have an area reserved for them in the memory of your CPU. This memory area reserves one 16-bit word for each timer address. The ladder logic instruction set supports 256 timers.

5.1 Introduction

Bits 0 through 9 of the timer word contain the time value in binary code. The time value specifies a number of units. Time updating decrements the time value by one unit at an interval designated by the time base. Decrementing continues until the time value is equal to zero.

You can pre-load a time value using either of the following formats: W#16#wxyz Where w = the time base (that is, the time interval or resolution) Where xyz = the time value in binary coded decimal format S5T#aH_bM_cS_dMS Where H = hours, M = minutes, S = seconds, and MS = milliseconds;

a, b, c, d are defined by the user. The time base is selected automatically, and the value is rounded to

the next lower number with that time base. The maximum time value that you can enter is 9,990 seconds, or 2H_46M_30S.

S5TIME#4S = 4 seconds s5t#2h_15m = 2 hours and 15 minutes S5T#1H_12M_18S = 1 hour, 12 minutes, and 18 seconds

5.2 Time Base

Bits 12 and 13 of the timer word contain the time base in binary code.

The time base defines the interval at which the time value is decremented by one unit. The smallest time base is 10 ms; the largest is 10 s.

Time Base Binary Code for the Time Base 10 ms 00 100 ms 01 1 s 10 10 s 11

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Values that exceed 2h46m30s are not accepted. A value whose

resolution is too high for the range limits (for example, 2h10ms) is truncated down to a valid resolution. The general format for S5TIME has limits to range and resolution as shown below:

Resolution Range 0.01 second 10MS to 9S_990MS 0.1 second 100MS to 1M_39S_900MS 1 second 1S to 16M_39S 10 seconds 10S to 2H_46M_30S

5.3 Bit Configuration in the Time Cell When a timer is started, the contents of the timer cell are used as the

time value. Bits 0 through 11 of the timer cell hold the time value in binary coded decimal format (BCD format: each set of four bits contains the binary code for one decimal value). Bits 12 and 13 hold the time base in binary code.

The following figure shows the contents of the timer cell loaded with

timer value 127 and a time base of 1 second:

Each timer box provides two outputs, BI and BCD, for which you can

indicate a word location. The BI output provides the time value in binary format. The BCD output provides the time base and the time value in binary coded decimal (BCD) format. 5.4 Choosing the right Timer

This overview is intended to help you choose the right timer for your

timing job.

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S_PULSE (Pulse timer): The maximum time that the output signal remains at 1 is the same as the programmed time value t. The output signal stays at 1 for a shorter period if the input signal changes to 0. S_PEXT (Extended pulse timer): The output signal remains at 1 for the programmed length of time, regardless of how long the input signal stays at 1. S_ODT (On-delay timer): The output signal changes to 1 only when the programmed time has elapsed and the input signal is still 1. S_ODTS (Retentive on-delay timer): The output signal changes from 0 to 1 only when the programmed time has elapsed, regardless of how long the input signal stays at 1. S_OFFDT (Off-delay timer): The output signal changes to 1 when the input signal changes to 1 or while the timer is running. The time is started when the input signal changes from 1 to 0. 5.5 Timers

In the subsection will discuss the five timers in details.

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5.5.1: Pulse S5 Timer (S_PULSE) S_PULSE (Pulse S5 Timer) as shown in the figure below starts the

specified timer if there is a positive edge at the start (S) input. A signal change is always necessary in order to enable a timer. The timer runs as long as the signal state at input S is "1", the longest period, however, is the time value specified by input TV. The signal state at output Q is "1" as long as the timer is running. If there is a change from "1" to "0" at the S input before the time interval has elapsed the timer will be stopped. In this case the signal state at output Q is "0".

The timer is reset when the timer reset (R) input changes from "0" to

"1" while the timer is running. The current time and the time base are also set to zero. Logic "1" at the timer's R input has no effect if the timer is not running.

The current time value can be scanned at the outputs BI and BCD. The

time value at BI is binary coded, at BCD it is BCD coded. The current time value is the initial TV value minus the time elapsed since the timer was started.

Figure 5.1: Pulse Timer

Parameter Data

Type Memory Area

Description

T no. Timer T Timer identification number; range depends on CPU

S BOOL I, Q, M, L, D Start input TV S5TIME I, Q, M, L, D Preset time value R BOOL I, Q, M, L, D Reset input BI WORD I, Q, M, L, D Remaining time value, integer format BCD WORD I, Q, M, L, D Remaining time value, BCD format Q BOOL I, Q, M, L, D Status of the timer Table: Pulse Timer

Example

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If the signal state of input I0.0 changes from "0" to "1" (positive edge in

RLO), the timer T5 will be started. The timer will continue to run for the specified time of two seconds (2 s) as long as I0.0 is "1". If the signal state of I0.0 changes from "1" to "0" before the timer has expired the timer will be stopped. If the signal state of input I0.1 changes from "0" to "1" while the timer is running, the time is reset.

The output Q4.0 is logic "1" as long as the timer is running and "0" if

the time has elapsed or was reset.

Pulse Timer Coil ( SP ) In addition timer can be use by using Pulse Timer Coil. ( SP ) (Pulse Timer Coil) starts the specified timer with the <time value> when there is a positive edge on the RLO state. The timer continues to run for the specified time interval as long as the RLO remains positive ("1"). The signal state of the counter is ”1” as long as the timer is running. If there is a change from "1" to "0" in the RLO before the time value has elapsed, the timer will stop. In this case, a scan for "1" always produces the result "0". Example:

If the signal state of input I0.0 changes from "0" to "1" (positive edge in RLO), the timer T5 is started. The timer continues to run with the specified time of two seconds as long as the signal state of input I0.0 is "1". If the signal state of input I0.0 changes from "1" to "0" before the specified time has elapsed, the timer stops.

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The signal state of output Q4.0 is "1" as long as the timer is running. A signal state change from "0" to "1" at input I0.1 will reset timer T5 which stops the timer and clears the remaining portion of the time value to "0". 5.5.2: Extended Pulse S5 Timer ( S_PEXT) S_PEXT (Extended Pulse S5 Timer) starts the specified timer if there is a positive edge at the start (S) input. A signal change is always necessary in order to enable a timer. The timer runs for the preset time interval specified at input TV even if the signal state at the S input changes to "0" before the time interval has elapsed. The signal state at output Q is "1" as long as the timer is running. The timer will be restarted ("re-triggered") with the preset time value if the signal state at input S changes from "0" to "1" while the timer is running. The timer is reset if the reset (R) input changes from "0" to "1" while the timer is running. The current time and the time base are set to zero. The current time value can be scanned at the outputs BI and BCD. The time value at BI is binary coded, at BCD is BCD coded. The current time value is the initial TV value minus the time elapsed since the timer was started.

Parameter Data

Type Memory Area

Description

T no. Timer T Timer identification number; range depends on CPU

S BOOL I, Q, M, L, D Start input TV S5TIME I, Q, M, L, D Preset time value R BOOL I, Q, M, L, D Reset input BI WORD I, Q, M, L, D Remaining time value, integer format BCD WORD I, Q, M, L, D Remaining time value, BCD format Q BOOL I, Q, M, L, D Status of the timer Example

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If the signal state of input I0.0 changes from "0" to "1" (positive edge in RLO), the timer T5 will be started. The timer will continue to run for the specified time of two seconds (2 s) without being affected by a negative edge at input S. If the signal state of I0.0 changes from "0" to "1" before the timer has expired the timer will be re-triggered. The output Q4.0 is logic "1" as long as the timer is running.

Extended Pulse Timer Coil ( SE ) ( SE ) (Extended Pulse Timer Coil) starts the specified timer with the specified <time value> when there is a positive edge on the RLO state. The timer continues to run for the specified time interval even if the RLO changes to "0" before the timer has expired. The signal state of the counter is ”1” as long as the timer is running. The timer will be restarted (re-triggered) with the specified time value if the RLO changes from "0" to "1" while the timer is running. Example

If the signal state of input I0.0 changes from "0" to "1" (positive edge in RLO), the timer T5 is started. The timer continues to run without regard to a negative edge of the RLO. If the signal state of I0.0 changes from "0" to "1" before the timer has expired, the timer is re-triggered. The signal state of output Q4.0 is "1" as long as the timer is running. A signal state change from "0" to "1" at input I0.1 will reset timer T5 which stops the timer and clears the remaining portion of the time value to "0".

5.5.3: On-Delay S5 Timer (S_ S_ODT) S_ODT (On-Delay S5 Timer) starts the specified timer if there is a positive edge at the start (S) input. A signal change is always necessary in order to enable a timer. The timer runs for the time interval specified at input TV as long as the signal state at input S is positive. The signal state at output Q is "1" when the timer has elapsed without error and the signal state at the S input is still "1". When the signal state at input S changes from "1" to "0" while the timer is running, the timer is stopped. In this case the signal state of output Q is "0".

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The timer is reset if the reset (R) input changes from "0" to "1" while the timer is running. The current time and the time base are set to zero. The signal state at output Q is then "0". The timer is also reset if there is a logic "1" at the R input while the timer is not running and the RLO at input S is "1". The current time value can be scanned at the outputs BI and BCD. The time value at BI is binary coded, at BCD is BCD coded. The current time value is the initial TV value minus the time elapsed since the timer was started. Example

If the signal state of I0.0 changes from "0" to "1" (positive edge in RLO), the timer T5 will be started. If the time of two seconds elapses and the signal state at input I0.0 is still "1", the output Q4.0 will be "1". If the signal state of I0.0 changes from "1" to "0", the timer is stopped and Q4.0 will be "0" (if the signal state of I0.1 changes from "0" to "1", the time is reset regardless of whether the timer is running or not). On Delay Timer Coil (SD) On Delay Timer Coil (SD) starts the specified timer with the <time value> if there is a positive edge on the RLO state. The signal state of the timer is "1" when the <time value> has elapsed without error and the RLO is still "1". When the RLO changes from "1" to "0" while the timer is running, the timer is reset. In this case, a scan for "1" always produces the result "0". Example

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If the signal state of input I0.0 changes from "0" to "1" (positive edge in RLO), the timer T5 is started. If the time elapses and the signal state of input I0.0 is still "1", the signal state of output Q4.0 will be "1". If the signal state of input I0.0 changes from "1" to "0", the timer remains idle and the signal state of output Q4.0 will be "0". A signal state change from "0" to "1" at input I0.1 will reset timer T5 which stops the timer and clears the remaining portion of the time value to "0". 5.5.4: Retentive On Delay S5 Timer (S_ODTS) S_ODTS (Retentive On-Delay S5 Timer) starts the specified timer if there is a positive edge at the start (S) input. A signal change is always necessary in order to enable a timer. The timer runs for the time interval specified at input TV even if the signal state at input S changes to "0" before the time interval has elapsed. The signal state at output Q is "1" when the timer has elapsed without regard to the signal state at input S. The timer will be restarted (re-triggered) with the specified time if the signal state at input S changes from "0" to "1" while the timer is running.

The timer is reset if the reset (R) input changes from "0" to "1" without regard to the RLO at the S input. The signal state at output Q is then "0".

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The current time value can be scanned at the outputs BI and BCD. The time value at BI is binary coded, at BCD it is BCD coded. The current time value is the initial TV value minus the time elapsed since the timer was started. Example

If the signal state of I0.0 changes from "0" to "1" (positive edge in RLO), the timer T5 will be started. The timer runs without regard to a signal change at I0.0 from "1" to "0". If the signal state at I0.0 changes from "0" to "1" before the timer has expired, the timer will be re-triggered. The output Q4.0 will be "1" if the timer elapsed. (If the signal state of input I0.1 changes from "0" to "1", the time will be reset irrespective of the RLO at S.) Retentive On-Delay Timer Coil (SS) Retentive On-Delay Timer Coil (SS) starts the specified timer if there is a positive edge on the RLO state. The signal state of the timer is "1" if the time value has elapsed. A restart of the timer is only possible if it is reset explicitly. Only a reset causes the signal state of the timer to be set to "0". The timer restarts with the specified time value if the RLO changes from "0" to "1" while the timer is running. Example

If the signal state of input I0.0 changes from "0" to "1" (positive edge in RLO), the timer T5 is started. If the signal state of input I0.0 changes from "0" to "1" before the timer has expired, the timer is re-triggered. The output Q4.0 will be "1" if the

38

timer elapsed. A signal state "1" at input I0.1 will reset timer T5, which stops the timer and clears the remaining portion of the time value to "0". 5.5.5: Retentive On-Delay S5 Timer (S_ODTS) S_OFFDT (Off-Delay S5 Timer) starts the specified timer if there is a negative edge at the start (S) input. A signal change is always necessary in order to enable a timer. The signal state at output Q is "1" if the signal state at the S input is "1" or while the timer is running. The timer is reset when the signal state at input S goes from "0" to "1" while the timer is running. The timer is not restarted until the signal state at input S changes again from "1" to "0".

The timer is reset when the reset (R) input changes from "0" to "1" while the timer is running. The current time value can be scanned at the outputs BI and BCD. The time value at BI is binary coded, at BCD it is BCD coded. The current time value is the initial TV value minus the time elapsed since the timer was started. Example

If the signal state of I0.0 changes from "1" to "0", the timer is started. Q4.0 is "1" when I0.0 is "1" or the timer is running. (if the signal state at I0.1 changes from "0" to "1" while the time is running, the timer is reset). Off-Delay Timer Coil (SF) Off-Delay Timer Coil (SF) starts the specified timer if there is a negative edge on the RLO state. The timer is "1" when the RLO is "1" or as long as the timer is running during the <time value> interval. The timer is reset when the RLO goes

39

from "0" to "1" while the timer is running. The timer is always restarted when the RLO changes from "1" to "0". Example

If the signal state of input I0.0 changes from "1" to "0" the timer is started. The signal state of output Q4.0 is "1" when input I0.0 is "1" or the timer is running. A signal state change from "0" to "1" at input I0.1 will reset timer T5 which stops the timer and clears the remaining portion of the time value to "0". Example 5.1: Mixing tank is used prepare a medicine of two ingredients as follows: 1. The first pump will be turn from start/stop station to sump the first

ingredient and will stay on until the medium level switch is reached. 2. Immediately a second pump is turned on until high level is reached. 3. A mixer is turned on for 32 seconds. 4. Then the liquid is dumped until a low level switch. Write a PLC code to represent this process.

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Chapter 6: Counters

Counters have an area reserved for them in the memory of your CPU. This memory area reserves one 16-bit word for each counter address. The ladder logic instruction set supports 256 counters. The counter instructions are the only functions that have access to the counter memory area. Bits 0 through 9 of the counter word contain the count value in binary code. The count value is moved to the counter word when a counter is set. The range of the count value is 0 to 999. Bit Configuration in the Counter is shown in the figure below. You provide a counter with a preset value by entering a number from 0 to 999, for example 127, in the following format: C#127. The C# stands for binary coded decimal format (BCD format: each set of four bits contains the binary code for one decimal value). Bits 0 through 11 of the counter contain the count value in binary coded decimal format. The following figure shows the contents of the counter after you have loaded the count value 127, and the contents of the counter cell after the counter has been set.

There are three types of counters as follows

1. Up-Down Counter (S_CUD) 2. Down Counter (S_CD) 3. Up Counter (S_CU)

These types are discussed in details in the next three sections.

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6.1: Up-Down Counter

A positive edge (i.e. a change in signal state from 0 to 1) at input S of the Up-Down Counter instruction sets the counter with the value at the Preset Value (PV) input.

A signal state of 1 at input R resets the counter. Resetting the counter places

the value of the count at 0. The counter is incremented by 1 if the signal state at input CU changes from 0 to 1 (that is, there is a positive edge) and the value of the counter is less than 999.

The counter is decremented by 1 if the signal state at input CD changes from

0 to 1 (that is, there is a positive edge) and the value of the counter is more than 0. If there is a positive edge at both count inputs, both operations are executed and the count remains the same.

A signal state check for 1 at output Q produces a result of 1 when the count

is greater than 0; the check produces a result of 0 when the count is equal to 0. The range of the count value is 0 to 999. You can vary the count value within this range by using the Up-Down Counter, Up Counter, and Down Counter instructions.

You provide a counter with a preset value by entering a number from 0 to

999, for example 127, in the following format:C#127.

Example:

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A change in signal state from 0 to 1 at input I 0.2 sets counter C 10 with the value 55 in binary coded decimal format. If the signal state of input I 0.0 changes from 0 to 1, the value of counter C 10 is increased by 1, except when the value of counter C 10 is equal to 999. If input I 0.1 changes from 0 to 1, counter C 10 is decreased by 1, except when the value of counter C 10 is equal to 0. If I 0.3 changes from 0 to 1, the value of C 10 is set to 0. Q 4.0 is 1, when C 10 is not equal to “0”.

Set Counter Value (SC) Set Counter Value (SC) executes only if there is a positive edge in RLO. At that time, the preset value transferred into the specified counter. Example

The counter C5 is preset with the value of 100 if there is a positive edge at input I0.0 (change from "0" to "1"). If there is no positive edge, the value of counter C5 remains unchanged.6.2:

6.2: Up Counter The up-counter parameters are described in the table below.

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Example:

A change in signal state from 0 to 1 at input I 0.2 sets counter C 10 with the value 901 in binary coded decimal format. If the signal state of I 0.0 changes from 0 to 1, the value of counter C 10 is increased by 1, unless the value of C 10 is equal to 999. If I 0.3 changes from 0 to 1, the value of C 10 is set to 0. The signal state of output Q 4.0 is 1 if C 10 is not equal to 0.

Up Counter Coil (CU) Up Counter Coil (CU) increments the value of the specified counter by one if there is a positive edge in the RLO and the value of the counter is less than "999". If there is no positive edge in the RLO or the counter already has the value "999", the value of the counter will be unchanged. Example

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If the signal state of input I0.0 changes from "0" to "1" (positive edge in RLO), the preset value of 100 is loaded to counter C10. If the signal state of input I0.1 changes from "0" to "1" (positive edge in RLO), counter C10 count value will be incremented by one unless the value of C10 is equal to "999". If there is no positive edge in RLO, the value of C10 will be unchanged. If the signal state of I0.2 is "1", the counter C10 is reset to "0". 6.3: Down Counter The down-counter parameters are described in the table below.

Example:

A change in signal state from 0 to 1 at input I 0.2 sets counter C 10 with the value 901 in binary coded decimal format. If the signal state of I 0.0 changes from 0 to 1, the value of counter C 10 is increased by 1, unless the value of C 10 is equal to 999. If I 0.3 changes from 0 to 1, the value of C 10 is set to 0. The signal state of output Q 4.0 is 1 if C 10 is not equal to 0.

Off-Delay Timer Coil (SF) -( CD ) (Down Counter Coil) decrements the value of the specified counter by one, if there is a positive edge in the RLO state and the value of the counter is more

45

than "0". If there is no positive edge in the RLO or the counter has already the value "0", the value of the counter will be unchanged. Example

If the signal state of input I0.0 changes from "0" to "1" (positive edge in RLO), the preset value of 100 is loaded to counter C10. If the signal state of input I0.1 changes from "0" to "1" (positive edge in RLO), counter C10 count value will be decremented by one unless the value of C10 is equal to "0". If there is no positive edge in RLO, the value of C10 will be unchanged. If the count value = 0, then Q4.0 is turned on. If the signal state of input I0.2 is "1", the counter C10 is reset to "0". Example: Count starting from zero when I1.0 start is pressed and I1.2 is energized.

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Chapter 7: Data Types

7.1 Overview of the Data Types A data type is the combination of value ranges and operations in a single unit. The data types decide:

• the type and interpretation of the data elements, • the permitted ranges for the data elements, • the permitted operations that can be executed on an address of a data type • the notation of the constants of the data type.

Elementary Data Types

Elementary data types define the structure of data elements that cannot be subdivided into smaller units. They correspond to the definition in the DIN EN 1131-3 standard. An elementary data type describes a memory area with a fixed length and stands for bit, integer, real, time period, time-of-day and character values. The following data types are predefined in S7. Group Data Types Explanation Bit Data Types BOOL

BYTE WORD DWORD

Date elements of this type occupy either 1 bit, 8 bits, 16 bits or 32 bits

Character Types CHAR Data elements of this type occupy exactly 1 character in the ASCII character set

Numeric Types INT DINT REAL

Data elements of this type are available for processing numeric values.

Time Types TIME DATE TIME_OF_DAY S5TIME

Data elements of this type represent the various time and date values in STEP 7.

Complex Data Types S7 supports the following complex data types:

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Data Type Explanation DATE_AND_TIME DT

Defines an area of 64 bits (8 bytes). This data type stores date and time (as a binary coded decimal) and is a predefined data type in S7.

STRING Defines an area for a character string of up to 254 characters (data type CHAR).

ARRAY Defines an array consisting of elements of one data type (either elementary or complex).

STRUCT Defines a group of data types in any combination of types. It can be an array of structures or a structure consisting of structures and arrays.

User-Defined Data Types You can create your own user-defined data types in the data type declaration. Each one is assigned a unique name and can be used any number of times. Once it has been defined, a user-defined data type can be used to generate a number of data blocks with the same structure. Parameter Types Parameter types are special data types for timers, counters and blocks that can be used as formal parameters. Data Type Explanation TIMER This is used to declare timer functions as parameters. COUNTER This is used to declare counter functions as parameters. BLOCK_xx This is used to declare FCs, FBs, DBs and SDBs as

parameters. ANY This is used to allow an address of any data type as a

parameter. POINTER This is used to allow a memory area as a parameter. ANY Data Type In S7, you can use variables of the ANY data type as formal parameters of a block. You can also create temporary variables of this type and use them in value assignments. 7.2 Elementary Data Types Bit Data Types Data of this type are bit combinations occupying either 1 bit (data type BOOL), 8 bits, 16 bits or 32 bits. A numeric range of values cannot be specified for the data types: byte, word, and double word. These are bit combinations that can be used only to formulate Boolean expressions.

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Type Keyword Bit Width Alignment Value Range Bit BOOL 1 bit Begins at the least

significant bit in the byte 0, 1 or FALSE, TRUE

Byte BYTE 8 8 bits Begins at the least significant byte in the word.

-

Word WORD 16 bits Begins at a WORD boundary.

-

Double word

DWORD 32 bits Begins at a WORD boundary.

-

Character Types Data elements of this type occupy exactly one character of the ASCII character set. Type Keyword Bit Width Value Range

Single character CHAR 8 Extended ASCII character set Numeric Data Types These types are available for processing numeric values (for example for calculating arithmetic expressions). Type Keyword Bit Width Alignment Value Range Integer INT 16 Begins at a WORD

boundary.

-32_768 to 32_767

Double integer

DINT 32 Begins at a WORD boundary.

-2_147_483_648 to 2_147_483_647

Floating-point number (IEEE floating-

REAL 32 Begins at a WORD boundary.

-3.402822E+38 to -1.175495E-38 +/- 0 1.175495E-38 to 3.402822E+38

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point number) Time Types Data of this type represent the various time and date values within STEP 7 (for for setting the date or for entering the time value for a time). Type Keyword Bit

Width Alignment Value Range

S5 time S5TIME

S5T 16 Begins at a

WORD boundary.

T#0H_0M_0S_10MS to T#2H_46M_30S_0MS

Time period: IEC time in steps of 1 ms.

TIME T

32 Begins at a WORD boundary.

-T#24D_20H_31M_23S_647MS to T#24D_20H_31M_23S_647MS

Date IEC data in steps of 1 day

DATE D

16 Begins at a WORD boundary.

D#1990-01-01 to D#2168-12-31

Time of day time in steps of 1 ms.

TIME_OF_DAY TOD 32 Begins at a

WORD boundary.

TOD#0:0:0.0 to TOD#23:59:59.999

If the set value is higher than the upper limit of the range, the upper limit value is used. With variables of the data type S5TIME, the resolution is limited, in other words, only the time bases 0.01 s, 0.1 s, 1 s, 10 s are available. The compiler rounds the values accordingly. If the set value is higher than the upper limit of the range, the upper limit value is used.

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Chapter 8: More PLC Instructions

8.1 Overview of All Ladder Instructions

All Ladder Instructions are sorted according to Siemens International Mnemonics in the table below. Quite many instructions were discussed in earlier chapters. However, in the following sections we shall discuss samples of the rest of instruction that were not addressed earlier. English Mnemonics

Program Elements Catalog

Description

---| |--- Bit logic Instruction Normally Open Contact (Address) ---|/|--- Bit logic Instruction Normally Closed Contact (Address) ---( ) Bit logic Instruction Output Coil ---(#) Bit logic Instruction Midline Output ==0 ---| |--- Status bits Result Bit Equal 0 >0 ---| |--- Status bits Result Bit Greater Than 0 >=0 ---| |--- Status bits Result Bit Greater Equal 0 <=0 ---| |--- Status bits Result Bit Less Equal 0 <0 ---| |--- Status bits Result Bit Less Than 0 <>0 ---| |--- Status bits Result Bit Not Equal 0 ABS Floating point ABS Instruction Establish the Absolute

Value of a Floating-Point Number ACOS Floating point Instruction Establish the Arc Cosine

Value ADD_DI Integer Math

Instruction Add Double Integer

ADD_I Integer Math Instruction

Add Integer

ADD_R Floating point Instruction

Add Real

ASIN Floating point Instruction

Establish the Arc Sine Value

ATAN Floating point Instruction

Establish the Arc Tangent Value

BCD_DI Convert BCD to Double Integer BCD_I Convert BCD to Integer BR ---| |--- Status bits Exception Bit Binary Result ----(CALL) Program control Call FC SFC from Coil (without

Parameters) CALL_FB Program control Call FB from Box CALL_FC Program control Call FC from Box

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English Mnemonics


Description

CALL_SFB Program control Call System FB from Box CALL_SFC Program control Call System FC from Box ----(CD) Counters Down Counter Coil CEIL Convert Ceiling CMP >=D Compare Compare Double Integer

(==, <>, >, <, >=, <=) CMP >=I Compare Compare Integer (==, <>, >, <, >=, <=) CMP >=R Compare Compare Real (==, <>, >, <, >=, <=) COS Floating point Instruction Establish the Cosine Value ----(CU) Counters Up Counter Coil DI_BCD Convert Double Integer to BCD DI_R Convert Double Integer to Floating-Point DIV_DI Integer Math Instruction Divide Double Integer DIV_I Integer Math Instruction Divide Integer DIV_R Floating point Instruction Divide Real EXP Floating point

Instruction Establish the Exponential Value

FLOOR Convert Floor I_BCD Convert Integer to BCD I_DI Convert Integer to Double Integer INV_I Convert Ones Complement Integer INV_DI Convert Ones Complement Double Integer ---(JMP) Jumps Unconditional Jump ---(JMP) Jumps Conditional Jump ---(JMPN) Jumps Jump-If-Not LABEL Jumps Label LN Floating point Instruction Establish the Natural

Logarithm ---(MCR>) Program control Master Control Relay Off ---(MCR<) Program control Master Control Relay On ---(MCRA) Prgram control Master Control Relay Activate ---(MCRD) Program control Master Control Relay Deactivate MOD_DI Integer Math

Instruction Return Fraction Double Integer

MOVE Move Assign a Value MUL_DI Integer Math

Instruction Multiply Double Integer

MUL_I Integer Math Instruction

Multiply Integer

MUL_R Floating point Instruction

Multiply Real

---( N )--- Bit logic Instruction Negative RLO Edge Detection NEG Bit logic Instruction Address Negative Edge Detection NEG_DI Convert Twos Complement Double Integer NEG_I Convert Twos Complement Integer NEG_R Convert Negate Floating-Point Number

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English Mnemonics


Description

---| NOT |--- Bit logic Instruction Invert Power Flow ---( OPN ) DB call Open Data Block: DB or DI OS ---| |--- Status bits Exception Bit Overflow Stored OV ---| |--- Status bits Exception Bit Overflow ---( P )--- Bit logic Instruction Positive RLO Edge Detection POS Bit logic Instruction Address Positive Edge Detection ---( R ) Bit logic Instruction Program control Return ROL_DW Shift/Rotate Rotate Left Double Word ROR_DW Shift/Rotate Rotate Right Double Word ROUND Convert Round to Double Integer RS Bit logic Instruction Reset-Set Flip Flop ---( S ) Bit logic Instruction Set Coil ---( SAVE ) Bit logic Instruction Save RLO into BR Memory ---( SC ) Counters Set Counter Value S_CD Counters Down Counter S_CU Counters Up Counter S_CUD Counters Up-Down Counter ---( SD ) Timers On-Delay Timer Coil ---( SE ) Timers Extended Pulse Timer Coil ---( SF ) Timers Off-Delay Timer Coil SHL_DW Shift/Rotate Shift Left Double Word SHL_W Shift/Rotate Shift Left Word SHR_DI Shift/Rotate Shift Right Double Integer SHR_DW Shift/Rotate Shift Right Double Word SHR_I Shift/Rotate Shift Right Integer SHR_W Shift/Rotate Shift Right Word SIN Floating point

Instruction Establish the Sine Value

S_ODT Timers On-Delay S5 Timer S_ODTS Timers Retentive On-Delay S5 Timer S_OFFDT Timers Off-Delay S5 Timer ---( SP ) Timers Pulse Timer Coil S_PEXT Timers Extended Pulse S5 Timer S_PULSE Timers Pulse S5 Timer SQR Floating point

Instruction Establish the Square

SQRT Floating point Instruction

Establish the Square Root

SR Bit logic Instruction Set-Reset Flip Flop ---( SS ) Timers Retentive On-Delay Timer Coil SUB_DI Integer Math

Instruction Subtract Double Integer

SUB_I Integer Math Instruction

Subtract Integer

SUB_R Floating point Instruction

Subtract Real

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English Mnemonics


Description

TAN Floating point Instruction

Establish the Tangent Value

TRUNC Convert Truncate Double Integer Part UO ---| |--- Status bits Exception Bit Unordered WAND_DW Word logic

Instruction AND Double Word

WAND_W Word logic Instruction

AND Word

WOR_DW Word logic Instruction

OR Double Word

WOR_W WOR_W OR Word WXOR_DW Word logic

Instruction Exclusive OR Double Word

WXOR_W Word logic Instruction

Exclusive OR Word

8.2 Add double integer instruction (ADD_DI) The block diagram for the ADD_DI is shown in the figure below

The inputs and outputs are described in the table below. Parameter Data Type Memory Area Description EN BOOL I, Q, M, L, D Enable input ENO BOOL I, Q, M, L, D Enable output IN1 DINT I, Q, M, L, D

or constant First value for addition

IN2 DINT I, Q, M, L, D or constant

Second value for addition

OUT DINT I, Q, M, L, D Result of addition

ADD_DI (Add Double Integer) is activated by a logic "1" at the Enable (EN) Input. IN1 and IN2 are added and the result can be scanned at OUT. If the result is outside the permissible range for a double integer (32-bit), the OV bit and OS bit will be "1" and ENO is logic "0", so that other functions after this math box which are connected by the ENO (cascade arrangement) are not executed.

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Example:

The ADD_DI box is activated if I0.0 = "1". The result of the addition MD0 + MD4 is output to MD10. If the result was outside the permissible range for a double integer, the output Q4.0 is set. 8.3 Compare integer instruction The block diagrams for different Compare Integer instructions are shown in the figure below

The inputs and outputs are described in the table below. IN1 INT I, Q, M, L, D or constant IN2 INT I, Q, M, L, D or constant Parameter Data Type Memory Area Description box input BOOL I, Q, M, L, D Result of the previous logic operation box output BOOL I, Q, M, L, D Result of the comparison, is only

processed further if the RLO at the box input = 1

IN1 INT I, Q, M, L, D or constant

First value to compare

IN2 INT I, Q, M, L, D or constant

Second value to compare

IN1 and IN2 are compared according to the type of comparison you choose: == IN1 is equal to IN2 <> IN1 is not equal to IN2 > IN1 is greater than IN2 < IN1 is less than IN2

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>= IN1 is greater than or equal to IN2 <= IN1 is less than or equal to IN2 If the comparison is true, the RLO of the function is "1". It is linked to the RLO of a rung network by AND if the compare element is used in series, or by OR if the box is used in parallel. Example:

Output Q4.0 is set if the following conditions exist:

There is a signal state of "1" at inputs I0.0 and at I0.1 AND MW0 >= MW2

8.4 Shift right integer instruction The block diagram for the Shift Right Double Integer instruction is shown in the figure below

The inputs and outputs are described in the table below. Parameter Data Type Memory Area Description EN BOOL I, Q, M, L, D Enable input ENO BOOL I, Q, M, L, D Enable output IN DINT I, Q, M, L, D Value to shift N WORD I, Q, M, L, D Number of bit positions to shift OUT DINT I, Q, M, L, D Result of shift instruction

SHR_DI (Shift Right Double Integer) is activated by a logic "1" at the Enable

(EN) Input. The SHR_DI instruction is used to shift bits 0 to 31 of input IN bit by bit to the right. The input N specifies the number of bits by which to shift. If N is larger than 32, the command acts as if N were equal to 32. The bit positions shifted in from the left to fill vacated bit positions are assigned the logic state of bit 31 (sign bit for the double integer). This means these bit positions are assigned "0" if the integer is positive and "1" if the integer is negative. The result of the shift instruction can be scanned at output OUT. The CC 0 bit and the OV bit are set to "0" by SHR_DI if N is not equal to 0. ENO has the same signal state as EN. Example:

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The SHR_DI box is activated by logic "1" at I0.0. MD0 is loaded and shifted right by the number of bits specified with MW4. The result is written to MD10. Q4.0 is set. 8.5 Subtract real instruction 8.5 SQRT Square Root Instruction The block diagram for the SQRT instruction is shown in the figure below

The inputs and outputs are described in the table below. OUT REAL I, Q, M, L, D Description Parameter Data Type Memory Area Description EN BOOL I, Q, M, L, D Enable input ENO BOOL I, Q, M, L, D Enable output IN REAL I, Q, M, L, D

or constant Input value: floating-point

OUT REAL I, Q, M, L, D Output value: square root of floating-point number

SQRT establishes the square root of a floating-point number. This instruction

issues a positive result when the address is greater than "0". Sole exception: the square root of -0 is -0. 8.6 Subtract Real (SUB_R) The block diagram for the Subtract Real is shown in the figure below

The inputs and outputs are described in the table below.

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Parameter Data Type Memory Area Description EN BOOL I, Q, M, L, D Enable input ENO BOOL I, Q, M, L, D Enable output IN1 REAL I, Q, M, L, D

or constant First value for subtraction

IN2 REAL I, Q, M, L, D or constant

Value to subtract

OUT REAL I, Q, M, L, D Result of subtraction

SUB_R (Subtract Real) is activated by a logic "1" at the Enable (EN) Input. IN2 is subtracted from IN1 and the result can be scanned at OUT. If the result is outside the permissible range for a floating-point number (overflow or underflow), the OV bit and OS bit will be "1" and ENO is logic "0", so that other functions after this math box which are connected by the ENO (cascade arrangement) are not executed. Example:

The SUB_R box is activated by logic "1" at I0.0. The result of the subtraction MD0 - MD4 is output to MD10. If the result was outside the permissible range for a floating-point number or if the program statement was not processed, the output Q4.0 is set. 8.7 WAND_W (Word) AND Word

The block diagram for the WAND_W (Word) AND Word is shown in the figure below.

The inputs and outputs are described in the table below. Parameter Data Type Memory Area Description EN BOOL I, Q, M, L, D Enable input ENO BOOL I, Q, M, L, D Enable output IN1 WORD I, Q, M, L, D First value for logic operation IN2 WORD I, Q, M, L, D Second value for logic operation OUT WORD I, Q, M, L, D Result word of logic operation

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WAND_W (AND Words) is activated by signal state "1" at the enable (EN) input

and ANDs the two word values present at IN1 and IN2 bit by bit. The values are interpreted as pure bit patterns. The result can be scanned at the output OUT. ENO has the same logic state as EN. Example:

The instruction is executed if I0.0 is "1". Only bits 0 to 3 of MW0 are relevant, the rest of MW0 is masked by the IN2 word bit pattern: MW0 = 01010101 01010101 IN2 = 00000000 00001111 MW0 AND IN2 = MW2 = 00000000 00000101 Q4.0 is "1" if the instruction is executed.

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Chapter 9: Computer Numerical Control (CNC)

The abbreviation CNC stands for computer numerical control, and refers

specifically to a computer "controller" that reads G-code instructions and drives a machine tool, a powered mechanical device typically used to fabricate components by the selective removal of material. CNC does numerically directed interpolation of a cutting tool in the work envelope of a machine. The operating parameters of the CNC can be altered via software load program. A picture of basic CNC machine is shown below

Figure 1: A miniature mill showing the basic parts of a mill.

9.1 Introduction

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While most people never heard of this term, CNC has touched almost every form of manufacturing process in one way or another. If you are working in manufacturing, it's likely that you are dealing with CNC on a regular basis.

CNC machines typically replace some existing manufacturing processes. Take

one of the simplest manufacturing processes, drilling holes, for example. A drill press can of course be used to machine holes. (It's likely that almost everyone has seen some form of drill press, even if you don't work in manufacturing.) A person can place a drill in the drill chuck that is secured in the spindle of the drill press. They can then (manually) select the desired speed for rotation (commonly by switching belt pulleys), and activate the spindle. Then they manually pull on the quill lever to drive the drill into the workpiece being machined.

As you can easily see, there is a lot of manual intervention required to use a

drill press to drill holes. A person is required to do something almost every step along the way. While this manual intervention may be acceptable for manufacturing companies if but a small number of holes or workpieces must be machined, as quantities grow, so does the likelihood for fatigue due to the tediousness of the operation. And do note that we've used one of the simplest machining operations (drilling) for our example. There are more complicated machining operations that would require a much higher skill level (and increase the potential for mistakes resulting in scrap workpieces) of the person running the conventional machine tool. (We commonly refer to the style of machine that CNC is replacing as the conventional machine). The figure shows a picture of a typical CNC machine.

Figure 2: Typical CNC machine. By comparison, the CNC equivalent for a drill press (possibly a CNC

machining center or CNC drilling & tapping center) can be programmed to perform this operation in a much more automatic fashion. Everything that the drill press operator was doing manually will now be done by the CNC machine,

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including: placing the drill in the spindle, activating the spindle, positioning the workpiece under the drill, machining the hole, and turning off the spindle.

Everyone involved in the manufacturing environment should be well aware of

what is possible with these sophisticated machine tools. The design engineer, for example, must possess enough knowledge of CNC to perfect dimensioning and tolerancing techniques for workpieces to be machined on CNC machines. The tool engineer must understand CNC in order to design fixtures and cutting tools for use with CNC machines. Quality control people should understand the CNC machine tools used within their company in order to plan quality control and statistical process control accordingly. Production control personnel should be abreast of their company's CNC technology in order to make realistic production schedules. Managers, foremen, and team leaders should understand CNC well enough to communicate intelligently with fellow workers. And, it goes without saying that CNC programmers, setup people, operators, and others working directly with the CNC equipment must have an extremely good understanding of CNC.

In this chapter, we will explore the basics of CNC, showing you much of what

is involved with using these sophisticated machine tools. At the completion of this chapter, you should have a good understanding of how and why CNC functions as it does and know those things you must learn more about in order to work with any style of CNC machine tool. Furthermore, you should be quite comfortable with the fundamentals of CNC and be able to communicate intelligently with others about your CNC machine tools. 9.2 Fundamentals of CNC

While the specific intention and application for CNC machines vary from one

machine type to another, all forms of CNC have common benefits. It helps to understand why these sophisticated machines have become so popular. Here are but a few of the more important benefits offered by CNC equipment.

The first benefit offered by all forms of CNC machine tools is improved

automation. The operator intervention related to producing workpieces can be reduced or eliminated. Many CNC machines can run unattended during their entire machining cycle, freeing the operator to do other tasks. This gives the CNC user several side benefits including reduced operator fatigue, fewer mistakes caused by human error, and consistent and predictable machining time for each workpiece. Since the machine will be running under program control, the skill level required of the CNC operator (related to basic machining practice) is also reduced as compared to a machinist producing workpieces with conventional machine tools.

The second major benefit of CNC technology is consistent and accurate

workpieces. Today's CNC machines show almost unbelievable accuracy and repeatability specifications. This means that once a program is verified, two, ten, or one thousand identical workpieces can be easily produced with precision and consistency.

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A third benefit offered by most forms of CNC machine tools is flexibility. Since

these machines are run from programs, running a different workpiece is almost as easy as loading a different program. Once a program has been verified and executed for one production run, it can be easily recalled the next time the workpiece is to be run. This leads to yet another benefit, fast change-overs. Since these machines are very easy to setup and run, and since programs can be easily loaded, they allow very short setup time.

9.3 Motion control - the heart of CNC

The most basic function of any CNC machine is automatic, precise, and consistent motion control. Rather than applying completely mechanical devices to cause motion as is required on most conventional machine tools, CNC machines allow motion control in a revolutionary manner. All forms of CNC equipment have two or more directions of motion, called axes. These axes can be precisely and automatically positioned along their lengths of travel. The two most common axis types are linear (driven along a straight path) and rotary (driven along a circular path).

Instead of causing motion by turning cranks and handwheels as is required

on conventional machine tools, CNC machines allow motions to be commanded through programmed commands. Generally speaking, the motion type (rapid, linear, and circular), the axes to move, the amount of motion and the motion rate (feedrate) are programmable with almost all CNC machine tools.

Accurate positioning is accomplished by the operator counting the number of

revolutions made on the handwheel plus the graduations on the dial. The drive motor is rotated a corresponding amount, which in turn drives the ball screw, causing linear motion of the axis. A feedback device confirms that the proper amount of ball screw revolutions have occurred.

A CNC command executed within the control (commonly through a program)

tells the drive motor to rotate a precise number of times. The rotation of the drive motor in turn rotates the ball screw. And the ball screw causes drives the linear axis. A feedback device at the opposite end of the ball screw allows the control to confirm that the commanded number of rotations has taken place.

How axis motion is commanded - understanding coordinate systems It would be infeasible for the CNC user to cause axis motion by trying to tell each axis drive motor how many times to rotate in order to command a given linear motion amount. (This would be like having to figure out how many turns of the handle on a table vise will cause the movable jaw to move exactly one inch!) Instead, all CNC controls allow axis motion to be commanded in a much simpler and more logical way by utilizing some form of coordinate system. The two most popular coordinate systems used with CNC machines are the rectangular coordinate system and the polar coordinate system. By far, the most popular of these two is the rectangular coordinate system.

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One very common application for the rectangular coordinate system is graphing. Almost everyone has had to make or interpret a graph. Since the need to utilize graphs is so commonplace, and since it closely resembles what is required to cause axis motion on a CNC machine, let's review the basics of graphing.

As with any two dimensional graph, this graph has two base lines. Each base

line is used to represent something. What the base line represents is broken into increments. Also, each base line has limits. In our productivity example, the horizontal base line is being used to represent time. For this base line, the time increment is in months. Remember this base line has limits - it starts at January and end with December. The vertical base line is representing productivity. Productivity is broken into ten percent increments and starts at zero percent productivity and ends with one hundred percent productivity.

Let's take what we now know about graphs and relate it to CNC axis motion.

Instead of plotting theoretical points to represent conceptual ideas, the CNC programmer is going to be plotting physical end points for axis motions. Each linear axis of the machine tool can be thought of as like a base line of the graph. Like graph base lines, axes are broken into increments. But instead of being broken into increments of conceptual ideas like time and productivity, each linear axis of a CNC machine's rectangular coordinate system is broken into increments of measurement. In the inch mode, the smallest increment is usually 0.0001 inch. In the metric mode, the smallest increment is 0.001 millimeter. (By the way, for rotary axes the increment is 0.001 degrees.)

Just like the graph, each axis within the CNC machine's coordinate system

must start somewhere. With the graph, the horizontal baseline started at January and the vertical base line started at zero percent productivity. This place where the vertical and horizontal base lines come together is called the origin point of the graph. For CNC purposes, this origin point is commonly called the program zero point (also called work zero, part zero, and program origin).

For this example, the two axes we happen to be showing are labeled as X and

Y but keep in mine that program zero can be applied to any axis. Though the names of each axes will change from one CNC machine type to another (other common names include Z, A, B, C, U, V, and W), this example should work nicely to show you how axis motion can be commanded.

The program zero point establishes the point of reference for motion

commands in a CNC program. This allows the programmer to specify movements from a common location. If program zero is chosen wisely, usually coordinates needed for the program can be taken directly from the print.

With this technique, if the programmer wishes the tool to be sent to a position

one inch to the right of the program zero point, X1.0 is commanded. If the programmer wishes the tool to move to a position one inch above the program zero point, Y1.0 is commanded. The control will automatically determine how many times to rotate each axis drive motor and ball screw to make the axis reach

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the commanded destination point. This lets the programmer command axis motion in a very logical manner.

With the examples given so far, all points happened to be up and to the right

of the program zero point. This area up and to the right of the program zero point is called a quadrant (in this case, quadrant number one). It is not uncommon on CNC machines that end points needed within the program fall in other quadrants. When this happens, at least one of the coordinates must be specified as minus.

9.4 Understanding absolute versus incremental motion

All discussions to this point assume that the absolute mode of programming is used. The most common CNC word used to designate the absolute mode is G90. In the absolute mode, the end points for all motions will be specified from the program zero point. For beginners, this is usually the best and easiest method of specifying end points for motion commands. However, there is another way of specifying end points for axis motion.

In the incremental mode (commonly specified by G91), end points for motions

are specified from the tool's current position, not from program zero. With this method of commanding motion, the programmer must always be asking "How far should I move the tool?" While there are times when the incremental mode can be very helpful, generally speaking, this is the more cumbersome and difficult method of specifying motion and beginners should concentrate on using the absolute mode.

Be careful when making motion commands. Beginners have the tendency to

think incrementally. If working in the absolute mode (as beginners should), the programmer should always be asking "To what position should the tool be moved?" This position is relative to program zero, NOT from the tools current position.

Aside from making it very easy to determine the current position for any

command, another benefit of working in the absolute mode has to do with mistakes made during motion commands. In the absolute mode, if a motion mistake is made in one command of the program, only one movement will be incorrect. On the other hand, if a mistake is made during incremental movements, all motions from the point of the mistake will also be incorrect.

9.5 Assigning program zero

Keep in mind that the CNC control must be told the location of the program zero point by one means or another. How this is done varies dramatically from one CNC machine and control to another. One (older) method is to assign program zero in the program. With this method, the programmer tells the control how far it is from the program zero point to the starting position of the machine.

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This is commonly done with a G92 (or G50) command at least at the beginning of the program and possibly at the beginning of each tool.

Another, newer and better way to assign program zero is through some form

of offset. Commonly machining center control manufacturers call offsets used to assign program zero fixture offsets. Turning center manufacturers commonly call offsets used to assign program zero for each tool geometry offsets. More on how program zero can be assigned will be presented during key concept number four.

To this point, our primary concern has been to show you how to determine

the end point of each motion command. As you have seen, doing this requires an understanding of the rectangular coordinate system. However, there are other concerns about how a motion will take place. Fore example, the type of motion (rapid, straight line, circular, etc.), and motion rate (feedrate), will also be of concern to the programmer. We'll discuss these other considerations during key concept number three.

9.6 Telling the machine what to do - the CNC program

Almost all current CNC controls use a word address format for programming. (The only exceptions to this are certain conversational controls.) By word address format, we mean that the CNC program is made up of sentence-like commands. Each command is made up of CNC words. Each CNC word has a letter address and a numerical value. The letter address (X, Y, Z, etc.) tells the control the kind of word and the numerical value tells the control the value of the word. Used like words and sentences in the English language, words in a CNC command tell the CNC machine what it is we wish to do at the present time.

One very good analogy to what happens in a CNC program is found in any set

of step by step instructions. Say for example, you have some visitors coming in from out of town to visit your company. You need to write down instructions to get from the local airport to your company. To do so, you must first be able to visualize the path from the airport to your company. You will then, in sequential order, write down one instruction at a time. The person following your instructions will perform the first step and then go on to the next until he or she reaches your facility.

In similar manner, a manual CNC programmer must be able to visualize the

machining operations that are to be performed during the execution of the program. Then, in step by step order, the programmer will give a set of commands that makes the machine behave accordingly.

Though slightly off the subject at hand, we wish to make a strong point about

visualization. Just as the person developing travel directions MUST be able to visualize the path taken, so MUST the CNC programmer be able to visualize the movements the CNC machine will be making BEFORE a program can be successfully developed. Without this visualization ability, the programmer will not be able to develop the movements in the program correctly. This is one

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reason why machinists make the best CNC users. An experienced machinist should be able to easily visualize any machining operation taking place.

Just as each concise travel instruction will be made up of one sentence, so

will each instruction given within a CNC program be made up of one command. Just as the travel instruction sentence is made up of words (in English), so is the CNC command made up of CNC words (in CNC language).

The person following your set of travel instructions will execute them

explicitly. If you make a mistake with your set of instructions, the person will get lost on the way to your company. In similar fashion, the CNC machine will execute a CNC program explicitly. If there is a mistake in the program, the CNC machine will not behave correctly.

O - Program number (Used for program identification) X- absolute X position Y- absolute Y position Z- absolute Z position A- position (rotary around X) B- position (rotary around Y) C- position (rotary around Z) U- Relative axis parallel to X V- Relative axis parallel to Y W- Relative axis parallel to Z M- code (another "action" register or Machine code(*)) (otherwise referred to as a "Miscellaneous" function") F- feed rate S- spindle speed N- line number R- Arc radius or optional word passed to a subprogram/canned cycle P- Dwell time or optional word passed to a subprogram/canned cycle T- Tool selection I- Arc data X axis J- Arc data Y axis. K- Arc data Z axis, or optional word passed to a subprogram/canned cycle D- Cutter diameter/radius offset H- Tool length offset

As you can see, many of the letter addresses are chosen in a rather logical

manner (T for tool, S for spindle, F for feedrate, etc.). A few require memorizing. There are two letter addresses (G and M) which allow special functions to be designated. The preparatory function (G) specifies is commonly used to set modes. We already introduced absolute mode, specified by G90 and incremental mode, specified by G91. These are but two of the preparatory functions used. You must reference your control manufacturer's manual to find the list of preparatory functions for your particular machine.

Like preparatory functions, miscellaneous functions (M words) allow a variety

of special functions. Miscellaneous functions are typically used as programmable switches (like spindle on/off, coolant on/off, and so on). They are also used to

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allow programming of many other programmable functions of the CNC machine tool.

To a beginner, all of this may seem like CNC programming requires a great

deal of memorization. But rest assured that there are only about 30-40 different words used with CNC programming. If you can think of learning CNC manual programming as like learning a foreign language that has only 40 words, it shouldn't seem too difficult.

9.7 Decimal point programming

Certain letter addresses (CNC words) allow the specification of real numbers

(numbers that require portions of a whole number). Examples include X axis designator (X), Y axis designator (Y), and radius designator (R). Almost all current model CNC controls allow a decimal point to be used within the specification of each letter address requiring real numbers. For example, X3.0625 can be used to specify a position along the X axis.

On the other hand, some letter addresses are used to specify integer

numbers. Examples include the spindle speed designator (S), the tool station designator (T), sequence numbers (N), preparatory functions (G), and miscellaneous functions (M). For these word types, most controls do NOT allow a decimal point to be used. The beginning programmer must reference the CNC control manufacturer's programming manual to find out which words allow the use of a decimal point.

Here we discuss other programmable functions. All but the very simplest CNC

machines have programmable functions other than just axis motion. With today's full blown CNC equipment, almost everything about the machine is programmable. CNC machining centers, for example, allow the spindle speed and direction, coolant, tool changing, and many other functions of the machine to be programmed. In similar fashion, CNC turning centers allow spindle speed and direction, coolant, turret index, and tailstock to be programmed. And all forms of CNC equipment will have their own set of programmable functions. Additionally, certain accessories like probing systems, tool length measuring systems, pallet changers, and adaptive control systems may also be available that require programming considerations.

The list of programmable functions will vary dramatically from one machine

to the next, and the user must learn these programmable functions for each CNC machine to be used. In key concept number two, we will take a closer look at what is typically programmable on different forms of CNC machine tools.

9.7 Know your machine

A CNC user MUST understand the makeup of the CNC machine tool being

utilized. While this may sound like a basic statement, a CNC user must be able to view the machine from two distinctly different perspectives. Here in key concept number two, we will be viewing the machine from a programmer's

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perspective. Much later, in key concept number seven, we will look at the machine from an operator's viewpoint.

Many forms of CNC machines are designed to enhance or replace what is

currently being done with more conventional machines. The first goal of any CNC beginner should be to understand the basic machining practice that goes into using the CNC machine tool. The more the beginning CNC user knows about basic machining practice, the easier it will be to adapt to CNC.

Think of it this way. If you already know basic machining practice as it relates

to the CNC machine you will be working with, you already know what it is you want the machine to do. It will be a relatively simple matter of learning how to tell the CNC machine what it is you want it to do (learning to program). This is why machinists make the best CNC programmers, operators, and setup personnel. Machinists already know what it is the machine will be doing. It will be a relatively simple matter of adapting what they already know to the CNC machine.

For example, a beginner to CNC turning centers should understand the basic

machining practice related to turning operations like rough and finish turning, rough and finish boring, grooving, threading, and necking. Since this form of CNC machine can perform multiple operations in a single program (as many CNC machines can), the beginner should also know the basics of how to process workpieces machined by turning so a sequence of machining operations can be developed for workpieces to be machined.

This point cannot be overstressed. Trying to learn about a particular CNC

machine without understanding the basic machining practice related to the machine would be like trying to learn how to fly an airplane without understanding the basics of aerodynamics and flight. Just as a beginning pilot will be in for a great number of problems without understanding aerodynamics, so is the beginning CNC user have difficulty learning how to utilize CNC equipment without an understanding of basic machining practice.

From a programmer's standpoint, as you begin to learn about any new CNC

machine, you should concentrate on four basic areas. First, you should understand the machine's most basic components. Second, you should become comfortable with your machine's directions of motion (axes). Third, you should become familiar with any accessories equipped with the machine. And fourth, you should find out what programmable functions are included with the machine and learn how they are programmed.

While you do not have to be a machine designer to work with CNC equipment,

it is important to know how your CNC machine is constructed. Understanding your machine's construction will help you to gauge the limits of what is possible with your machine. Just as the race car driver should understand the basics of suspension systems, breaking systems, and the workings of internal combustion engines (among other things) in order to get the most out of a given car, so must the CNC programmer understand the basic workings of the CNC machine in order to get the most from the CNC machine tool.

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For a universal style slant bed turning center, for example, the programmer

should know the most basic machine components, including bed, way system, headstock & spindle, turret construction, tailstock, and work holding device. Information regarding the machine's construction including assembly drawings is usually published right in the machine tool builder's manual. As you read the machine tool builder's manual, here are some of the machine capacity and construction questions to which you should find answers.

What is the machine's maximum RPM? How many spindle ranges does the machine have (and what are the cut-off

points for each range? What is the spindle and axis drive motor horsepower? What is the maximum travel distance in each axis? How many tools can the machine hold? What way construction does the machine incorporate (usually square

ways, dovetail, and/or linear bearing ways)? What is the machine's rapid rate (fastest traverse rate)? What is the machine's fastest cutting feedrate?

These are but a few of the questions you should be asking yourself as you

begin working with any new CNC machine. Truly, the more you know about your machine's capacity and construction, the easier it will be to get comfortable with the machine. 9.8 What is G-Code Programming? (CNC Machine)

G-code is a common name for the programming language that drives NC and CNC machine tools. It was developed by EIA in the early 1960s, a final revision was approved in February 1980 as RS274D.

Due to the lack of further development, the sheer variety of machine tool

configurations, and little demand for interoperability, few machine tool controllers (CNCs) adhere to this standard. Extensions and variations have been added to it independently by manufacturers, meaning that operators have to know the dialects and quirks of the particular machines they use, and CAM systems have had to limit themselves to the lowest common denominator of all the tools that they support.

Many manufacturers tried to overcome this difficulty of remaining compatible

by following the lead of a machine tool controller built by Fanuc. Unfortunately, Fanuc does not remain consistent with RS-274 or its own previous standard, and has been slow at adding new features and exploiting the increase in computing power. For example, they changed g70/g71 to g20/21; they used parentheses for comments which caused difficulty when they introduced mathematical calculations; they started to use nanometers just recently (requires 64 bit); they introduced the nurbs to overcome slow fetching of blocks from memory (instead of caching).

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G-code is also the name of any word in a CNC program that begins with the letter G, and generally is a code telling the machine tool what type of action to perform, such as:

Rapid move controlled feed move in a straight line or arc series of controlled

feed moves that would result in a hole being bored, a workpiece cut (routed) to a specific dimension, or a decorative profile shape added to the edge of a workpiece.

Common Fanuc G Codes G00 Fast positioning G01 Linear interpolation G02 CW circular interpolation G03 CCW circular interpolation G10/G11 Data writing/Data write cancel G17 X-Y plane selection G18 X-Z plane selection G19 Y-Z plane selection G20 Programming in inches G21 Programming in mm G28 Return to home position G31 Skip function (used for probes and tool length measurement systems) G33 Constant pitch threading G34 Variable pitch threading G40 Tool radius compensation off G41 Tool radius compensation left G42 Tool radius compensation right G90 Absolute programming G91 Incremental programming G94/G95 Inch per minute/Inch per revolution feed G96/G97 Constant cutting speed (Constant surface speed)/Constant rotation speed (constant RPM)

A standardized version of G-code known as BCL is used, but only on very few

machines. G-code is understood by Gerber photoplotters, machine tool controls, and

CNC machinists. CNC is written by hand for volume production jobs. In this environment, the inherent inefficiency of CAM-generated g-code is unacceptable.

G-code files may be generated by CAM software such as Alphacam, Artcam,

Edgecam, Featurecam, GibbsCAM, Mastercam, OneCNC, Plasma cam, Router-CIM, SmartCAM, Surfcam, etc. Those applications typically use translators called post-processors to output code optimized for a particular machine type or family. Post-processors are often user-editable to enable further customization, if necessary. G-code is also output by specialized CAD systems used to design printed circuit boards. Such software must be customized for each type of machine tool that it will be used to program.

Some CNC machines use "Conversational" programming, which is a wizard-like programming mode that either hides G-code or completely bypasses the use

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of G-code. Some popular examples are Mazak's Mazatrol, Hurco's Ultimax and Mori Seiki's CAPS conversational software.

Example

This is a generic program that demonstrates the use of G-Code to turn a 1" diameter X 1" long part. Assume that a bar of material is in the machine and that the bar is slightly oversized in length and diameter and that the bar protrudes by more than 1" from the face of the chuck. (Caution: This is generic, it might not work on any real machine! Pay particular attention to point 5 below.)

Tool Path for program

Line Code Description

N01 M216 Turn on load monitor

N02 G00 X20 Z20 Rapid move away from the part, to ensure the starting position of the tool

N03 G50 S2000 Set Maximum spindle speed

N04 M01 Optional stop

N05 T0303 M6 Select tool #3 from the carousel, use tool offset values located in line 3 of the program table, index the turret to select new tool

N06 G96 S854 M42 M03 M08

Variable speed cutting, 854 ft/min, High spindle gear, Start spindle CW rotation, Turn the flood coolant on

N07 G00 X1.1 Z1.1

Rapid feed to a point 0.1" from the end of the bar and 0.05" from the side

N08 G01 Z1.0 F.05

Feed in horizontally until the tool is standing 1" from the datum

N09 X0.0 Feed down until the tool is on center - Face the end of the bar

N10 G00 Z1.1 Rapid feed 0.1" away from the end of the bar

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N11 X1.0 Rapid feed up until the tool is standing at the finished OD

N12 G01 Z0.0 Feed in horizontally cutting the bar to 1" diameter all the way to the datum

N13 M05 M09 Stop the spindle, Turn off the coolant

N14 G28 G91 X0 Home X axis in the machine coordinate system, then home all other axes

N15 M215 Turn the load monitor off

N16 M30 Program stop, pallet change if applicable, rewind to beginning of the program

9.9 Directions of motion (axes) The CNC programmer MUST know the programmable motion directions (axes)

available for the CNC machine tool. The axes names will vary from one machine tool type to the next. They are always referred to with a letter address. Common axis names are X, Y, Z, U, V, and W for linear axes and A, C, and C for rotary axes. However, the beginning programmer should confirm these axis designations and directions (plus and minus) in the machine tool builder's manual since not all machine tool builders conform to the axis names we show.

As discussed in key concept number one, whenever a programmer wishes to

command movement in one or more axes, the letter address corresponding to the moving axes as well as the destination in each axis are specified. X3.5, for example tells the machine to move the X axis to a position of 3.5 inches from the program zero point in X (assuming the absolute mode of programming is used.

Most CNC machines utilize a very accurate position along each axis as a

starting point or reference point for the axis. Some control manufacturers call this position the zero return position. Others call it the grid zero position. Yet others call it the home position. Regardless of what it is called, the reference position is required by many controls to give the control an accurate point of reference. CNC controls that utilize a reference point for each axis require that the machine be manually sent to its reference point in each axis as part of the power up procedure. Once this is completed, the control will be in sync with the machine's position.

The third area a beginning CNC user should address is related to other

possible additions to the basic machine tool itself. Many CNC machine tools are equipped with accessories designed to enhance what the basic machine tool can do. Some of these accessories may be made and supported by the machine tool builder. These accessories should be well documented in the machine tool builder's manual. Other accessories may be made by an after-market manufacturer, in which case a separate manual may be involved.

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Examples of CNC accessories include probing systems, tool length measuring devices, post process gauging systems, automatic pallet changers, adaptive control systems, bar feeders for turning centers, live tooling and C axis for turning centers, and automation systems. Truly, the list of potential accessory devices goes on and on.

The programmer must also know what functions of the CNC machine are

programmable (as well as the commands related to programmable functions). With low cost CNC equipment, often times many machine functions must be manually activated. With some CNC milling machines, for example, about the only programmable function is axis motion. Just about everything else may have to be activated by the operator. With this type of machine, the spindle speed and direction, coolant and tool changes may have to be activated manually by the operator.

With full blown CNC equipment, on the other hand, almost everything is

programmable and the operator may only be required to load and remove workpieces. Once the cycle is activated, the operator may be freed to do other company functions.

Reference the machine tool builder's manual to find out what functions of

your machine are programmable. To give you some examples of how many programmable functions are handled, here is a list a few of the most common programmable functions along with their related programming words.

An "S" word is used to specify the spindle speed (in RPM for machining

centers). An M03 is used to turn the spindle on in a clockwise (forward) manner. M04 turns the spindle on in a counter clockwise manner. M05 turns the spindle off. Note that turning centers also have a feature called constant surface speed which allows spindle speed to also be specified in surface feet per minute (or meters per minute)

A "T" word is used to tell the machine which tool station is to be placed in the

spindle. On most machines, an M06 tells the machine to actually make the tool change. Tool change (on turning centers) A four digit "T" word is used to command tool changes on most turning centers. The first two digits of the T word specify the turret station number and the second two digits specify the offset number to be used with the tool. T0101, for example specifies tool station number one with offset number one.

M08 is used to turn on flood coolant. If available M07 is used to turn on mist

coolant. M09 turns off the coolant. Automatic pallet changer. An M60 command is commonly used to make pallet changes. An M60 command is commonly used to make pallet changes.

As stated, programmable functions will vary dramatically from one machine

to the next. The actual programming commands needed will also vary from builder to builder. Be sure to check the M codes list (miscellaneous functions) given in the machine tool builder's manual to find out more about what other functions may be programmable on your particular machine. M codes are

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commonly used by the machine tool builder to give the user programmable ON/OFF switches for machine functions. In any case, you must know what you have available for activating within your CNC programs.

For turning centers, for example, you may find that the tailstock and tailstock

quill is programmable. The chuck jaw open and close may be programmable. If the machine has more than one spindle range, commonly the spindle range selection is programmable. And if the machine has a bar feeder, it will be programmable. You may even find that your machine's chip conveyor can be turned on and off through programmed commands. All of this, of course, is important information to the CNC programmer.

You Must Understand The Motion Types Available On Your CNC Machine.

During key concept number one, we discussed how end points for axis motion are commanded utilizing the rectangular coordinate system. We are concerned with describing how the CNC machine determines the END POINT position for each motion. To effectively command motion on most CNC machines requires more than just specifying end points for positioning movements.

CNC control manufacturers try to make it as easy as possible to make

movement commands within the program. For those styles of motion that are commonly needed, they give the CNC user interpolation types.

To understanding interpolation, say for example, you wish to move only one

linear axis in a command. Say you wish to move the X axis to a position one inch to the right of program zero. In this case, the command X1. would be given (assuming the absolute mode is instated). The machine would move along a perfectly straight line during this movement (since only one axis is moving). Now let's say you wish to include a Y axis movement to a position one inch above program zero in Y (with the X movement). We'll say you are trying to machine a tapered or chamfered surface of your workpiece in this command. For the control to move along a perfectly straight line to get to the programmed end point, it must perfectly synchronize the X and Y axis movements. Also, if machining is to occur during the motion, a motion rate (feedrate) must also be specified. This requires linear interpolation.

During linear interpolation commands, the control will precisely and

automatically calculate a series of very tiny single axis departures, keeping the tool as close to the programmed linear path as possible. With today's CNC machine tools, it will appear that the machine is forming a perfectly straight line motion. However, Figure 3.1 shows what the CNC control is actually doing during linear interpolation. Figure 3.1 - Actual motion generated with linear interpolation. Notice the series of very tiny single axis movements. The step size is equal to the machine's resolution, usually 0.0001 in or 0.001 mm.

In similar fashion, many applications for CNC machine tools require that the

machine be able to form circular motions. Applications for circular motions include forming radii on turned workpieces between faces and turns and milling radii on contours of machining center workpieces. This kind of motion requires

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circular interpolation. As with linear interpolation, the control will do its best to generate as close to a circular path as possible.

While your particular CNC machine may have more motion types (depending

on your application), let's concentrate on becoming familiar with the three most common types of motion. These three motion types are available on almost all forms of CNC equipment. After briefly introducing each type of motion, we'll show an example program that stresses the use of all three.

These motion types share two things in common. First, they are all modal.

This means they remain in effect until changed. If for example, several motions of the same kind are to be given consecutively, the corresponding G code need only be specified in the first command. Second, the END POINT of the motion is specified in each motion command. The current position of the machine will be taken as the starting point.

Rapid motion (also called positioning). This motion type (as the name implies)

is used to command motion at the machine's fastest possible rate. It is used to minimize non-productive time during the machining cycle. Common uses for rapid motion include positioning the tool to and from cutting positions, moving to clear clamps and other obstructions, and in general, any non-cutting motion during the program.

You must check in the machine tool builder's manual to determine a

machine's rapid rate. Usually this rate is extremely fast (some machines boast rapid rates of well over 1000 IPM!), meaning the operator must be cautious when verifying programs during rapid motion commands. Fortunately, there is a way for the operator to override the rapid rate during program verification.

The command almost all CNC machines use to command rapid motion is

G00. Within the G00 Command, the end point for the motion is given. Control manufacturers vary with regard to what actually happens if more than one axis is included in the rapid motion command. With most controls, the machine will move as fast as possible in all axes commanded. In this case, one axis will probably reach its destination point before the other/s. With this kind of rapid command, straight line movement will NOT occur during rapid and the programmer must be very careful if there are obstructions to avoid. With other controls, straight line motion will occur, even during rapid motion commands.

Straight line motion (also called linear interpolation). This motion type allows

the programmer to command perfectly straight line movements as discussed earlier during our discussion of linear interpolation. This motion type also allows the programmer to specify the motion rate (feedrate) to be used during the movement. Straight line motion can be used any time a straight cutting movement is required, including when drilling, turning a straight diameter, face or taper, and when milling straight surfaces. The method by which feedrate is programmed varies from one machine type to the next. Generally speaking, machining centers only allow the feedrate to be specific in per minute format (inches or millimeters per minute). Turning centers also allow feedrate to be specified in per revolution format (inches or millimeters per revolution).

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A G01 word is commonly used to specify straight line motion. Within the G01,

the programmer will include the desired end point in each axis. Circular motion causes the machine to make movements in the form of a circular path. This motion type is used to generate radii during machining. All feedrate related points made during our discussion of straight line motion still apply.

Two G codes are used with circular motion. G02 is commonly used to specify

clockwise motion while G03 is used to specify counter clockwise motion. To evaluate which to use, you simply view the movement from the same perspective the machine will view the motion. For example, if making a circular motion in XY on a machining center, simply view the motion from the spindle's vantage point. If making a circular motion in XZ on a turning center, simply view the motion from above the spindle. In most cases, this is as simple as viewing the print from above.

Additionally, circular motion requires that, by one means or another, the

programmer specifies the radius of the arc to be generated. With newer CNC controls this is handled by a simple "R" word. The R word within the circular command simply tells the control the radius of the arc being commanded. With older controls, directional vectors (specified by I, J, and K) tell the control the location of the arc's center point. Since controls vary with regard to how directional vectors are programmed, and since the R word is becoming more and more popular for radius designation, our examples will show the use of the R word. If you wish to learn more about directional vectors, you must reference your control manufacturer's manual.

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References

[1] Petruzella, Frank, “Programming Logic Controller”, Third Edition, 2005. [2] Siemens, “Ladder Logic (LAD) for S7-300 and S7-400 Programming”, Edition 03/2006. [3] Siemens, “SIMATIC Manager Software © STEP 7 S7/M7/C7”, Version V5.3 +SP3. [4] www.cncbookstore.com. [5] www.cncci.com. [6] www.midwestwirechicago.com.

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