Basic Control System unit5

E3145/5/1 INTRODUCTION TO PLC

INTRODUCTION TO PLC

OBJECTIVES

General Objective : To apply and integrate the concept of PLC

Specific Objectives : At the end of the unit you will be able to :

Identify the PLC terminology

Explain the PLC historical background

Describe the functions of PLC

Explain the advantages of PLC

Draw the PLC block diagram including processor unit, memory, the power supply unit, input and output interface and the programming device

Explain and draw the logic function LD, OR, AND, AND NOT, LD NOT, OUT, TIM, CNT

UNIT5


Design and develop ladder program

5.0 EXPLANATION OF PLC

In today’s fast-moving, highly competitive industrial world, a company must be flexible, cost-effective and efficient if it wishes to survive. In the process and manufacturing industries, this has resulted in a great demand for industrial control systems in order to streamline operations in terms of speed, reliability and product output.

Control systems such as hard-wire relay, logic and computer systems, can and do provide effective control of industrial processes and plant. However, each of this systems has limitations or disadvantages that may often be overcome through the use of a Programmable Logic Controller (PLC).

What is a Programmable Logic Controller?A PLC is a digital electronics system which uses a programmable

memory for implementing specific functions such as logic, sequencing,

INPUTINPUT

Do you have a motorcycle? If the answer is yes, explain to your students of the PLC system which is used in the traffic light.


timing, counting and arithmetic to control through analog or digital input/output modules, various types of machines or processes.

5.1 PLC TERMINOLOGY

In general, a control systems is a collection of electronic devices and equipment which are in place to ensure the stability, accuracy and smooth transition of a process or a manufacturing activity. It takes any form and varies in scale of implementation, from a power plant to a semiconductor machine. As a result of rapid advancement of technology, complicated control tasks accomplished with a highly automated control system, which may be in the form of Programmable Controller (PLC) and possibly a host computer , etc.

Besides signal interfacing to the field device (such as operator panel, motors, sensors, switches, solenoid valves and etc.), capabilities in network communication enable a big scale implementation and process coordination besides providing greater flexibility in realizing distributed control system. Every single component in a control system plays an important role regardless of size. For instance, as shown in Figure 5.1 the PLC would not know the happenings around it without any sensing devices. It is also unable to activate any moving mechanism if there is no motor installed. And if necessary, an area host computer has to be in place to coordinate the activities in a specific area at the shop floor.


Figure 5.1 : It could also be an application as small as single PLC controlling a single or some output service

5.2 PLC HISTORICAL BACKGROUND

PLCs were used mainly in the motor industries in the early 1970s where they replaced large relay panels. Not only do PLCs take up much lesser space than the relay system, they are also more reliable in operation over longer periods. One important point to take note is that PLCs are very flexible in terms of modifying or changing the control sequences. It is now possible to change or modify the control system without having to connect or disconnect a single wire. It is simply done by changing the program (software), using a program console or graphic programming panel attached to the programmer logic controller.

PLCs highly desireable in a wide variety of industrial-plant and process-control application.

(Source : Manual OMRON (1999), Beginner Guide To PLC)


Industry demands on PLCs are increasing rapidly and this has encouraged manufacturers to develop whole families of microprocessors-based systems having various level of performance. Available PLCs now range from small self-contained units to sophisticated modular systems with a range of add-on function modules for tasks such as analog input/output and communications. This modular system allows the expansion or upgrading of a control system with minimum costs and interruption.

In 1968, a group of engineers from General Motors developed the concept of PLC with an initial specification. The PLC must be :

1) Easy to program and reprogram2) Easy to maintain and repair3) More reliable in an industrial environment4) Smaller in size than its relay equivalent5) Cost-competitive

5.2.1 Developments of PLC

YEARS DESCRIPTION

1968 The concept of PLC was developed

1969 A hardware CPU controller was used with logic instructions. It comes with 1K of memory and 128 I/O points

1974 Using several processors within a PLC with add-on timers and counters functions. Arithmetic operations are also included. This PLC has 12K of memory and 1024 I/O points

1976 Remote I/O systems were introduced.

1977 Microprocessor-based PLC was introduced

1980 Intelligent I/O modules were developed

1983 Small low-cost PLCs were introduced

1985 onwards With networking capabilities


Table 5.1 PLC historical background(Source : Manual SMC International Training (1999), Introduction to PLC Controller)

Activity 5A

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

PART 1Questions 1 to 5 have four options: A, B, C or D. Choose the correct answer from the options given.

5.1 The term PLC stands for :A Personal Logic ComputerB Programmable Local ComputerC Personal Logic ControllerD Programmable Logic Controller


5.2 Which of the following is most likely to be the voltage level used internally in a PLC, excluding the voltage levels that might occur during conditioning in output/input channels:

A 5VB 24VC 110VD 240V

5.3 The cycle time of a PLC is the time it takes to:A Read an input signalB Read all the input signalsC Check all the input signals against the programD Read all the inputs, run the program and update all outputs

PART 2

5.4 What is a Programmable Logic Controller (PLC)


Feedback To Activity 5A

PART 1

5.1 D5.2 A5.3 D

PART 2


5.4 A PLC is a digital electronics system which uses a programmable memory for implementing specific functions such as logic, sequencing, timing, counting and arithmetic to control through analog or digital input/output modules, various types of machines or processes.

5.3 FUNCTIONS OF PLC

INPUTINPUT

PLCs are made to operate for long periods of time in adverse industrial environment. For example, temperature, humidity etc.


PLCs are used in many “real world” applications. If there are industries present, chance are good that there is a PLC present. If you are involved in machining, packaging, material handling, automated assembly or countless other industries you are probably already using them. If you are not, you are wasting money and time. Almost any application that needs some type of electrical control has a need for a PLC.

For example, let’s assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for. We can do this with a simple external timer. But what if the process included 10 switches and solenoids? We would need 10 external timers. What if the process also needed to count how many times the switches individually turned on? Thus we need a lot of external counters.

As you can see the more bigger process you need, we have a PLC. We can simply program the PLC to count its inputs and turn the solenoids for the specified time.

There are so many applications where PLCs are used in the various industries. Below is the list of applications :

i. conveyor systemii. swimming pooliii. traffic light systemiv. lift control systemv. food processingvi. pick and place robot controlvii. packaging machineviii. security control systemix. car manufacturing plantx. printing industriesxi. air condition control


xii. cement manufacturing

5.4 ADVANTAGES OF PLC

The following are the major advantages that can be distinguishably realized :

i. The wiring of the system usually reduces by 80% compared to conventional relay control system.

ii. The power consumption is greatly reduced as PLC consumes much less power

iii. The PLC self diagnostic functions enable easy and fast trouble shooting of the system

iv. Modification of control sequence or application can easily be done by programming through the console or computer

software without changing of I/O wiring, if no additional Input or Output devices are required.

v. In PLC system spare parts for relay and hardware timers are greatly reduced as compared to conventional control panel.

vi. The machine cycle time is improved tremendously due to the speed of PLC operation is a matter of milliseconds. Thus, productivity increases.

vii. It cost much less compared to conventional system in situations when the number of I/Os is very large and control functions are complex.

viii. The reliability of the PLC is higher than the mechanical relays and timers.

ix. An immediate print out of the PLC program can be done in minutes. Therefore, hardcopy of documentation can be easily maintained.


Activity 5B


5.5 State five (5) applications of PLC.

5.6 State the five (5) advantages of PLC.


Feedback To Activity 5B

5.5 a) Conveyor system b) Swimming pool

c) Traffic light systemd) Lift control systeme) Food processing

5.6

(i) The wiring of the system usually reduces by 80% compared to conventional relay control system.


(ii) The power consumption is greatly reduced as PLC consumes much less power

(iii) The PLC self diagnostic functions enable easy and fast trouble shooting of the system

(iv) Modification of control sequence or application can easily be done by programming through the console or computer software without changing of I/O wiring, if no additional Input or Output devices are required.

(v) In PLC system spare parts for relay and hardware timers are greatly reduced as compared to conventional control panel.

(vi) The machine cycle time is improved tremendously due to the speed of PLC operation is a matter of milliseconds. Thus, productivity increases.

(vii) It cost much less compared to conventional system in situations when the number of I/Os is very large and control functions are complex.

(viii) The reliability of the PLC is higher than the mechanical relays and timers.

5.5 PLC BLOCK DIAGRAM

Typically a PLC system has five basic components. These are the processor unit, memory, the power supply unit, input/output interface section and the programming device. Figure 5.2 shows the basic arrangement.

INPUTINPUT


Figure 5.2 Block Diagram of PLC(Source : Manual OMRON (1999), Beginner Guide To PLC)

1. The processor unit or Central Processing Unit (CPU) is theunit containing the microprocessor. This unit interprets the input signals and carries out the control actions, according to the program stored in its memory and communicating the decisions as action signals to the outputs.

2. The power supply unit is needed to convert the mains a.c voltage to the low d.c voltage (5V) necessary for the processor and the circuits in the input and output interface modules.

3. The programming device is used to enter the required program into the memory of the processor. The program is developed in the device and then transferred to the memory unit of the PLC.

4. The memory unit is where the program is stored, that is to be used for the control actions to be exercised by the microprocessor. Ladder program, Timer and Counter Values are stored in the user memory. Depending on user’s need,


various types of memory are available for choice : Read-Only Memory (ROM), Random Access Memory (RAM), Erasable Programmable Read Only Memory (EPROM) and Electrically Erasable Programmable Read-Only Memory (EEPROM).

5. The input and output sections are where the processor receives information from external devices and communicates information to external devices

5.6 PLC BASIC INSTRUCTIONS

A very commonly used method of programming PLCs is based on the use of the ladder diagrams. Writing a program is then equivalent to drawing a

The most important from any PLC programming language is that it is easily understood and used in a control system.

In addition, PLCs consists of three functional areas : processing, memory and input/output.


switching circuit. The ladder diagram consists of two vertical lines representing the power rails. Circuits are connected as horizontal lines, i.e the rungs of the ladder, between these two verticals.

In drawing a ladder diagram, certain conventions are adopted:

1. The vertical lines of the diagram represent the power rails between which circuits are connected.

2. Each rung on the ladder defines one operation in the control process.3. A ladder diagram is read from left to right and from top to bottom, Figure

5.3 shows the scanning motion employed by the PLC. The top rung is read from left to right. Then the second rung down is read from left to right and so on. When the PLC is in its run mode, it goes through the entire ladder program to the end, the end rung of program being clearly denoted, and then promptly resumes at the start. This procedure of going through all the rungs of the program is termed a cycle.

Figure 5.3 The rung ladder(Source : Manual SMC International Training (1999), Introduction to PLC Controller)


4. Each rung must start with an input or inputs and must end with at least one output. The term input is used for a control action, such as closing the contacts of a switch, used as an input to the PLC. The term output is used for a device connected to the output of a PLC, e.g. a motor.

5. Electrical devices are shown in their normal condition. Thus a switch which is normally open until some object closes it, is shown as open on the ladder diagram. A switch that is normally closed is shown closed.

6. A particular device can appear in more than one rung of a ladder. For example, we might have a relay which switches on one or more devices. The same letters and/or numbers are used to label the device in each situation.

7. The inputs and outputs are all identified by their addresses, the notation used depending on the PLC manufacturer. This is the address of the input or output in the memory of the PLC. The OMRON series of PLCs precedes input elements by an 00 and output elements by an 01 and uses the following numbers :

Inputs 00000 - 00011 (12 possible inputs)Outputs 01000 - 01007 ( 8 possible outputs)

Meanwhile the Mitsubishi F series of PLCs precedes input elements by an X and output elements by a Y and uses the following numbers :

Inputs X400 - 407 , 410 - 413 , 500 - 507, 510 - 513 (24 possible inputs)

Outputs Y430 - 437, 530 - 537 (16 possible outputs)

Ladder Diagrams’ Explanation

Ladder diagram uses standard symbols to represent the circuit components and functions found in a control system.

Ladder symbols


Input, normally-open contact

Input, normally-closed contact

Inputs in series connections

Inputs in parallel connections

Outputs device

Figure 5.4 : Ladder Symbols(Source : Manual SMC International Training (1999), Introduction to PLC Controller)

Logic Instructions (Mnemonic)

The instruction set consists of logic instructions also known as mnemonics that represent the actions to be performed within a given PLC.


Each program instruction consists of two parts : a mnemonic operation component also known as opcode and an address or data component that identifies a particular element (e.g output) within the PLC. Table 5.2 below is an example of a mnemonic code.

Address Instructions Data

00000 LD 00000

00001 OUT 01000

00002 END(01)

Table 5.2 Mnemonic Code(Source : Manual SMC International Training (1999), Introduction to PLC Controller)

Here the instruction refers to output (01) number 0.

Input/Output (I/O) Assignment

These instructions are used to program logic circuits in ladder diagram form, by assigning all physical inputs and outputs with an operand (address) suitable to the PLC being used. The I/O assignment used differ between manufacturers but certain common terms exist.

Logic Instructions and Graphic Programming

Logic instructions are used as the basic programming language for PLCs. Although logic instructions are easy to earn and use, it can be very time – consuming to check and relate a large coded program to the actual circuit function. Furthermore, logic instruction tends to vary between different types of PLC. A factory or plant may use a range of different PLCs, in which confusion can result over differences in the instruction sets.


Name/Mnemonic Symbol Description

LOAD(LD)

Creates a normally open condition as the first condition off the bus bar. All instruction lines begin with either LOAD or LOAD NOT.

LOAD NOT(LD NOT)

Creates a normally closed condition as the first condition off the bus bar. All instruction lines begin with either LOAD or LOAD NOT.

AND(AND)

Combines a normally open condition in series with a previous condition.

AND NOT(AND NOT)

Combines a normally closed condition in series with a previous condition.

OR(OR)

Combines a normally open condition in parallel with a previous condition.

OR NOT(OR NOT)

Combines a normally closed condition in parallel with a previous condition.

AND LOAD(AND LOAD)

Combines two group of conditions in series. These groups are called blocks.

OR LOAD(OR LOAD)

Combines two parallel groups of conditions. These group are called block.


OUTPUT(OUT)

Specifies an output bit that is to be turned ON for an ON execution condition and OFF for an OFF condition.

OUTPUT NOT(OUT NOT)

Specifies an output bit that is to be turned OFF for an ON execution and ON for an OFF condition.

TIMER(TIM)

Creates a 0-1 s decrementing timer that starts from the set value (SV) when the execution condition turns ON.

COUNTER(CNT)

Counts down the number of times the input conditions turns ON. Each time the input condition turns ON, the present value (PV) is reduced by 1 and when the count reaches 0, the Completion Flag (accessed through the counter number) turns ON.

NO OPERATION(NOP) None

Does nothing. Can be inserted into a program when modification are made that would otherwise change program address.

Table 5.3 Basic Instructions(Source : Manual SMC International Training (1999), Introduction to PLC Controller)

TIM

CP CNTR


5.7 DESIGN AND WRITE PROGRAM LADDER

Examples of Logic Function (Source : Manual SMC International Training (1999), Introduction to PLC Controller)

1. AND function

ADDRESS INSTRUCTIONS DATA

00000 LD 00000

00001 AND 00001

00002 AND 00002

00003 OUT 01000

2. OR function


00000 LD 00001

00001 OR 00002

00002 OR 00003

00003 OUT 01001

3. LOAD and LOAD NOT function

01000

00000 00001 00002

00001

01001

00002

00003



00000 LD 00000

00001 OUT 01000

00002 LD NOT 00001

00003 OUT 01001

4. AND and AND NOT function


00000 LD 00000

00001 AND NOT 00001

00002 AND 00002

00003 OUT 01002

5. OR and OR NOT function


00000 LD NOT 00000

00001 OR NOT 00001

00002 OR 00002

00003 OUT 01003

01000

01001

00000

00001

0100200000 00001 00002

01003

00000

00001

00002


6. AND and OR function


00000 LD 00000

00001 AND 00001

00002 OR 00002

00003 AND 00003

00004 OUT 01004

7. END function


00000 LD 00000

00001 AND NOT 00001

00002 OUT 01005

00003 END (01)

Activity 5C


PART 1

5.7 Decide whether each of these statements is True (T) or False (F).Figure shows a ladder diagram rung for which:

(i) The input contacts are normally open

00000 00001 00003

00002

01004

01005

END

00000 00001


(ii) There is an output when there is an input to the contacts

A (i) T (ii) TB (i) T (ii) FC (i) F (ii) TD (i) F (ii) T

PART 2

5.8 Draw a block diagram showing in very general terms the main units in a PLC.

5.9 State five (5) advantages of PLC.

5.10 What is the address of the input or output in the memory of the PLC manufacturer below :

(i) OMRON series(ii) MITSUBISHI F series

5.11 Draw the ladder symbols to represent the circuit components and function in a control system such as :

(i) Input, normally -open contact(ii) Input, normally – closed contact(iii) Inputs in series connections(iv) Inputs in parallel connections(v) Output devices


Feedback To Activity 5C

PART 1

5.7 A

PART 2


5.8 Block diagram of PLC

5.9 i. The wiring of the system usually reduces by 80% compared to conventional relay control system.

ii. The power consumption is greatly reduced as PLC consume much less power

iii. The PLC self diagnostic functions enable easy and fast trouble shooting of the system

iv. Modification of control sequence or application can easily be done by programming through the console or computer software without changing of I/O wiring, if no additional Input or Output devices are required.

v. In PLC system spare parts for relay and hardware timers are greatly reduced as compared to conventional control panel.


5.10 (i) OMRON series

Inputs 00000 - 00011 (12 possible inputs)Outputs 01000 - 01007 ( 8 possible outputs)

(ii) MITSUBISHI F series

Inputs X400 - 407 , 410 - 413 , 500 - 507, 510 - 513 (24 possible inputs)

Outputs Y430 - 437, 530 - 537 (16 possible outputs)

5.11 Ladder symbols

a) Input, normally-open contact

b) Input, normally-closed contact

c) Inputs in series connections


d) Inputs in parallel connections

e) Outputs device

KEY FACTS

1. PLC stands for PROGRAMMABLE LOGIC CONTROL.

2. PLC system has five basic components : processor unit, memory, the power supply unit, input/output interface section and the programming device.


SELF-ASSESSMENT

You are approaching success. Try all the questions in this self-assessment section and check your answers with those given in the Feedback on Self-Assessment given on the next page. If you face any problems, discuss it with your lecturer. Good luck.

Q5-1 What is the characteristic of a PLC ?

Q5-2 Why do we need a PLC ?

Q5-3(a) Draw the logic instructions below:

(i) AND NOT(ii) TIM(iii) OUTPUT(iv) OR(v) OR NOT(vi) LOAD NOT

(b) Draw the ladder rungs to represent: Two switches which are normally open and both have to be closed for a motor to operate.

(c) Convert the ladder diagrams given below to its equivalent mnemonic codes.

01000

00000 00001 00002 00003

00004

00005


Feedback To Self-Assessment

Have you tried the questions????? If “YES”, check your answers now.

Q5-1 The PLC must be :a) Easy to program and reprogramb) Easy to maintain and repairc) More reliable in an industrial environmentd) Smaller in size than its relay equivalent

e) Cost-competitive

Q5-2 PLC are used in bigger processes because we can simply program the PLC to count its inputs and turn the solenoids for the specified time.

Q5-3(a)

i.

ii.

iii.

TIM


iv.

v.

vi.

vii.

(b)


(c)

Address Instruction Data

00000 LD 01000

00001 AND NOT 01001

00002 LD 00002

00003 AND 00003

00004 OR 00004

00005 OR 00005

00006 AND LD ---

00007 OUT 01000

00008 END(01)

Basic Control System unit5

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