1. CONCEPT OF PLC SYSTEMS:

Programmable Logic Controllers or PLC’s evolved as industries sought economical ways to automate their production lines. The PLC’s took the place of relay based control systems which were comparatively slower, less reliable, and which required formidable wiring and maintenance requirements.PLC’s operate by monitoring input signals from the INPUT DEVICES. When changes are detected in the signals, the PROGRAMMABLE LOGIC CONTROLLER (PLC) SYSTEM reacts, through user programmed internal logic, to produce output signals. These signals actuate the OUTPUT DEVICES.

INPUT DEVICES: A device connected to the PLC and sends a signal to the PLC is called an Input device. The signal it sends is called an Input Signal. Signal enters the PLC via interfacing relays through terminals or through pins on a connector on a unit. The place where the signal enters the PLC is called an input point. This input point is allocated a location in memory that reflects its status i.e., either ON or OFF. This memory location is called an input bit. The CPU, in its normal processing cycle, monitors the status of all input points and turns ON or OFF corresponding to input bits accordingly. PLC’S can receive inputs from either automated or manual devices. Data can be received from user via a push button switch, keyboard, or similar device. Automated inputs can come from variety of devices: limit switches, timers, photoelectric, proximity switches etc.

OUTPUT DEVICES:

Devices connected to the PLC and takes the output signals from PLC are called output devices. There are also output bits in memory that are allocated output points on unit through which output signals are sent to output devices via interfacing relays, i.e., an output bit is turned ON to send a signal to the output device through an output point. The CPU periodically turns the output points ON or OFF according to the status of the output bits. A PLC can output to wide variety of devices in automated control. Some of the most common devices are motors, solenoid valves, relays, indicator lights, buzzers, alarms, servomotors, stepper motors etc. Microprocessor based CONTROLLER SYSTEM eliminates much of the wiring and rewiring that was necessary with the conventional relay based system. Instead the

programmed logic provides the “wiring network” which can be changed as required by simply reprogramming the PLC. PLC’s historically originated from relay based control systems. Although the integrated circuits (IC’s) and internal logic of the PLC have taken the place of the discrete relays, timers, counters etc. actual PLC operation proceeds as if those discrete devices were still in place. PLC control, however, also provides computer capabilities and accuracy to achieve a great deal more flexibility and reliability than is possible with relays. The symbols and other control concepts used to describe PLC operation also come from relay based control and form the basis of the ladder diagram programming method.

RELAY Vs. PLC TERMINOLOGY:

2. DIFFERENT GENERATION OF PLC SYSTEMS:

There are different makes of PLC systems namely OMRON, ALLEN BRADELY, HIMA, GE FANUC,TATA HONEYWELL,SIEMENS etc available in the market. We in IFFCO, have all the above makes of PLC’s in use. However we widely use OMRON make PLC’s of different generations. The same are being discussed below in detailed.

2.1 SYSMAC C-20 :

These are the first generation PLC’s. These are low cost, versatile industrial control system that can be quickly installed and easily operated by technicians with little or no experience with programmable controllers. All functions for a small-scale control system are integrated in a single housing suitable for applications, which require fewer I/O control points. The C 20 comes in two versions, a basic unit and an expandable unit.The basic unit has the capacity of 1194 program statements, 28 I/O points and 136 internal auxiliary relays. The expandable unit is functionally identical to the base unit except that it can be expanded using expansion I/O units to include up to 140 I/O points. Due to its compact design the C20 incorporates the detachable I/O terminals and microprocessor functions in a single housing called the CPU. The detachable programming console functions as the programming device. It is upwardly compatible with the full line of SYSMAC C series programmable controllers. The same shall be discussed in detail during the PLC programming.

2.2 SYSMAC C- 200, 500, 2000:

These can be called second generation PLC’s suitable for applications requiring higher I/O count above 150. These are back plane versions with rack mounted PLC, I/O, POWER SUPPLY CARDS. The system can be expanded and expansion I/O racks can be connected through I/O connecting cables. I/O units are available with 5, 8, 12, 16 or 32 points. The basic configuration consists of a CPU RACK with Power supply and I/O cards. C2000 PLC’s have both simplex and duplex mode of operations.

2.3 SIMPLEX PLC SYSTEM:

Simplex PLC system consists of single CPU unit that controls the operations. Failure of the CPU unit will hamper the control operations in the plant, reducing the reliability of the system. A simple schematic of such systems can be shown as below:

SCHEMATIC DIAGRAM OF SIMPLEX PLC SYSTEMS

2.4 DUPLEX PLC SYSTEM:

To increase the reliability of the PLC system, duplex PLC systems have been developed with double CPU units. They can be serviced easily without shutting down operation in the rare event that one of these units should fail.Duplex CPU units function on hot standby method. It is a method where the two CPU units are simultaneously performing calculating functions. One CPU unit performs the control functions (active CPU) and the other is placed on standby while it is performing the calculation functions. If the CPU unit performing control functions detects an error and stops, then control functions switch immediately over to the CPU unit that is on standby. The two CPU units are synchronized to ensure a smooth transition between them.

A simple schematic of such systems can be shown as below:

SCHEMATIC DIAGRAM OF DUPLEX OLC SYSTEM

2.5 SYSMAC CVM1D DUPLEX :

These are considered the latest generation of PLC’s in the market. They incorporate double systems for higher reliability. These PLC’S are available as simplex systems with double power supply units or as a duplex systems with double CPU and double POWER SUPPLY units.Power supply units are connected in parallel. Even if one of the power supply units breaks down, the PLC can continue to operate normally and the faulty unit can be replaced without interruption. With double CPU units and Power supply units, a CPU unit or a Power supply unit can be serviced easily without shutting down operation in the rare event that one of these units fail. I/O units on CPU, CPU exp. and I/O exp. racks can be replaced from the programming console while the system is running. It has high speed processing of 0.125 micro sec and I/O point capacity of 2048.

A typical schematic of such PLC’s is as shown below:

SCHEMATIC DIAGRAM OF SYSMAC CVM1D DUPLEX PLC SYSTEM

2.6 SYSMAC CPM2A, CPM2C:

These are the latest version of PLC’s in the micro PLC series. It is a compact PLC having advanced functions having high speed processing. The processing speed is 0.64 micro sec. I/O capacity is max 60 points with basic unit and 120 points with the expansion unit Program capacity is 4096 words. Supports programmable terminals or display screens via built in RS232 port for visual confirmation of operating conditions and debugging. They have provision for both DIN rail and surface mounting.

The diagram above explains the different generation of PLC systems that are used in industry. However the detailed hardware configurations and software programming techniques varies from manufacturer to manufacturer and also from one generation of PLC’s to the other generation. With the advancement

SYSMAC CPM2A, CPM2C PLC SYSTEM

in technology, the new generation PLC’s can cater to analog control applications in addition to basic concept of sequential logic control.3. DIFFERENT COMPONENTS OF PLC SYSTEMS:

A simple schematic scheme of a PLC is shown below:

A typical PLC has three basic components. They are detailed as below.

3.1 INPUT/OUTPUT SECTION

This section consists of wiring and interfacing relays that connect the PLC to the equipment being controlled (Input / Output devices). The detailed circuit configuration is as below:

A TYPICAL PLC BLOCK

3.2 CENRAL PROCESSING UNIT ( CPU )

The CPU contains the control circuitry as well as the memory that stores the control program that guides equipment operation. It is the heart of the PLC and organizes all controller activity by scanning the control plan along with the status of the inputs and executes specified commands to specified outputs.

3.2.1 PROGRAMMING DEVICE

This device is used to enter the control program into the CPU memory. Operation keys in the instructions used to sequentially control the application process. Relay ladder diagram programming technique is generally used. It can be a dedicated terminal compatible to a particular brand of PLC or a PC terminal loaded with the proprietary programming software communicating with the PLC.There are three different types of memory chips are used in PLC’s for storing theprogram. These are EPROM, EEPROM and RAM. The size varies with the no ofprogramming lines. In case of RAM, battery back up is provided in order to retain the program when the power is off. Units with battery back up can retain the data for approximately five years at room temperatures ( 25 0 c ).

Power supply shall be either 24 V DC or 110 / 220 AC used with the PLC.

3.2.2 SCAN TIME:When a PLC operating, it executes the program to control the external system. In the process a series of operations are performed inside the PLC. These are common processes such as resetting the watchdog timer and checking the user program memory, Data input and output, Instruction execution, Peripheral device command servicing. The total time required for a PLC to perform all of these internal operations is called scan time. It primarily varies from the no of I/O units, no of program instructions and any peripheral devices connected to the system. It is typically in the order of milli seconds.

3.3.3 RESPONSE TIME:

Response time is the time from when the PLC receives an input signal until it outputs the control signal. It is also in the order of milli seconds.4. INTRODUCTION TO PROGRAMMING:

To understand the basic programming techniques, we select C20 OMRON PLC systems as the learning kit and programs are developed and run on the same. As already mentioned above, relay ladder diagram method is followed in programming the PLC. However there are eight basic programming steps to follow in creating the control program for the PLC.Determine what the controlled system must do and in what order. Controlled system is basically a part of the process, which is controlled by the PLC system. For example:Let us consider the case of lube oil system where the lube oil supply header is maintained at a desired set pressure by pumping through the main oil pump. In case the main oil pump trips, oil pressure drops. When it reaches the low set point, the stand by oil pump should take start to maintain the set pressure and sustain the process. But Pump to stop only on manual. Also there shall be a provision to start the pump manually in case of emergency. Display lamps shall indicate the low-pressure alarm. There shall be a logic bypass arrangement for on line maintenance of lube oil pressure switch. Lamp test to check the display lamp. This is the logic to be implemented in the PLC system. Based on above we have to assess the Number of I / O’s required. The same is as below:

Assign identification nos. to input and output devices, that is, designate the external devices that will send signals to and receive signals from the PLC via interfacing relays.PLC ’s use the concept of I / O channels to identify the I / O terminals or points. Each channel consists of 16 points.

A four digit no is used to identify an I/O point. The left two digits indicate the channel no. and the right two digits indicate the point within the channel.

For eg:“0000” identifies the first point of the first channel and “0105” identifies the sixth point of second channel. It may be noted that both channel and point designations start from zero.Part of the output points is not used to send the output signals directly to the output devices but function as internal relays. These relays do not control external devices directly but they are used to control other relays, timers and counters. They are also assigned specific nos. These called internal auxiliary relays. Timers and counters are also assigned specific nos. But the same number cannot be assigned for both timer and a counter.I / O assignment is proprietary detail of the PLC manufacturer with ref to the nos.assigned for I/P and O/P channels, I/P and O/P points, internal relays, counters and timers.The actual number of channels and the I/O points available depend upon the hardware configuration of the system.

Draw a diagram, using relay ladder symbols. This represents in the correct sequence, all of the required functions and their relationship.

A ladder diagram consists of two vertical lines running down the sides with linesbranching in between them. The vertical lines are called bus bars. The branching lines are the instruction lines called rungs. Along the instruction line are placed the contacts/input conditions that lead to the output to the right bus bar.

Code the ladder symbols into a form that can be input to the CPU by programming console.When finished writing the ladder diagram, the next step is to encode the diagram into a language the PLC can understand. This language consists of addresses, instructions and data. The addresses are locations in the memory where instructions and data are stored. The instructions are used to tell the PLC what to do using the data that follows each instruction.For example let us consider the ladder diagram developed above.To code this, we first need to specify the memory address from which this program starts. In this example we will use “0000” as the starting address. To this address, an instruction that indicates the beginning of the program

must be written. This is LD instruction or LOAD. The first contact in the circuit must be stored as data of the LD instruction. Like wise rest of the program follows as below:

When finished we always have to tell the PLC when a program is over. So we assign the END statement to address 0012.The number of instructions vary from a typical 27 to 275 which will be listed out in the users manual of the PLC with symbols.Transfer these written instructions to the CPU via the programming console.

Before we go any further, it is required to know how a programming console looks like and the key sequence followed to enter the written instructions to the CPU.As already explained before a programming console can be a dedicated terminal compatible to a particular brand of PLC or a PC terminal loaded with the proprietary programming software communicating with the PLC.Here we limit ourselves to the dedicated terminals used for programming the PLC.The keyboard consists of numeric keys 0 to 9:These keys are used to input the numeric values used for program data. For example these keys would be used to input the input / output numbers and timer / counter numbers and values.CLR key used to clear the display.

A typical programming console looks as below:

INSTRUCTION KEYS:

These keys are used to place the instructions in the program.For example:AND instruction is used for ANDing two contacts.LD instruction used for loading a specified input.OUT instruction used for outputting to a specified output point.The detailed functions of the individual keys along with symbols and mnemonic codes shall be obtained from list of instructions in the PLC users manual.

OPERATION KEYS:

These sets of keys are used to edit the programs. Particularly the following three keyswhich are in variably used.

Key is used to scroll through the program one step at a time backward until you reach the beginning of the program. The displayed memory address of the program will decrement once for each press.

Key is used to scroll through the program one step at a time forward till you reach the end of the program. The displayed memory address of the program will increment once for each press.

WRITE Key: During the programming when the instruction and its data are written, this key used to register the instruction in the PLC memory at the address desired.

MODE SWITCH:

It is a 3-position switch used to select the operating mode of the PLC.\

• Run mode: the run mode is used to begin the PLC operation. When turned ON, the PLC begins controlling the equipment using the program written into the PLC memory.• Monitor mode: the monitor mode allows to visually monitors the operation in progress. For instance if you want to check whether a particular relay is in thecorrect state or not, we can move to the address (or step) that refers to the relay.• Program mode: PLC is kept in program mode while programming.CONSOLE DISPLAY:It is an easy to read LCD display through which we interact with the PLC.Now with the basic understanding of the programming console we can now start entering the program developed in the previous step:

5. ERRORS:

There are basically two levels of errors- fatal and non-fatal.

A fatal error, such as a memory error, prevents the PC from operating.A non fatal error, such as a battery failure, allows the operation to proceed but still must be corrected. In case both types of errors have occurred, the error messages for fatal errors take precedence over the other and must be corrected first.FORCED SET / RESET: During the execution of the program, this operation is used to force set or reset (for one scan time) the operating status of each i/o relay, internal auxiliary relay, holding relay, timer or counter. The most common use of this function is during the trial run of the controlled system.Programs are saved in EPROM’S, CASSETE TAPES, FLOPPY DISKS ETC.