History

Early machines were controlled by means of mechanical devices like cams, gears, levers etc. As the complexity grew, it was needed to have more sophisticated control systems. These systems were developed to operate with electrical power and contained wired relays and switches/contacts as control elements. In the 1960s and 1970s electromechanical relays, timers, counters, and sequencers were the industrial standard. Many control panels contained hundreds of electromechanical devices and, in some cases, a mile or more wires.

These control elements were wired as required to provide control logic necessary for the particular type of machine operation. Hardwired relays and switch logic was cumbersome and time consuming to modify. Wiring had to be removed and replaced to implement a new control scheme. One more negative aspect of this electromechanical control was low reliability in contrast to extremely high maintenance cost to keep this machine operative.

A new more versatile means of controlling this equipment and modifying the control circuitry was needed. Instead of achieving the desired control or automation through physical wiring of control devices, in a new controller it was achieved through a program or software and it was referred to as a programmable controller, abbreviated as PC. To differentiate it from a personal computer, also abbreviated as PC, the new controller is now referred to as programmable logic controller – PLC.

The first commercially successful PLC was developed in 1969 by Bedford Associates (Bedford, MA) MODICON for General Motors hydramatic division. Through the late 1970s, improvements were made in PLC programs to make them somewhat user friendly. In 1972, with the introduction of the microprocessor chip, the computer power for all kinds of automation systems increased and lowered the computing cost. Robotics, automation devices and computers of all types, including the PLC consequently underwent many improvements. PLC programs written in high level language become more understandable to more people and PLCs become more affordable.

Programmable Logic Controller

Definition

A programmable logic controller is currently defined by the NEMA (National Electrical Manufacturers Association) as a “Digital electronic device that uses the programmable memory to store instructions and to implement specific functions such as logic, sequence, timing, counting and arithmetic operations to control machines and processes.”

Older PLCs were capable of handling only discrete inputs and outputs (i.e. ON-OFF type signals), while today’s system can accept and generate analog voltages and currents as well as a wide range of voltage levels and pulsed signals. Its purpose is to monitor crucial process parameters and adjust process operations accordingly.

The PLC is basically a programmed interface between the field input devices like push-buttons, limit switches, sensors etc. and final control elements like actuators - motors, solenoids, valve dampers, drivers - or status indicating output devices like lamps, LEDs, hooters etc.

➤ CPU (Central Processing Units)

• The central processing unit, the brain of the system, is the control portion of the PLC. It has three sub-parts:

1. Power Supply:-

• The power supply provides power to the memory system, processor and I/O Modules.

• It converts the higher level AC line Voltage to various operational DC values for electronic circuitry.

• It filters and regulates the DC voltages to ensure proper computer operations.

2. Processor:-

• The processor, the heart of the CPU is the computerized part of the CPU in the form of Microprocessor / Microcontroller chip.

• It supervises all operations in the system and performs all tasks necessary to fulfill the PLC function.

• It reads the information i.e status of externally connected input devices with input module.

• It stores this information in memory for later use.

• It carries out mathematical and logic operations as specified in the application program.

• After solving the user's program, it writes the result values in the memory.

• It sends data out to external devices like output modules, so as to actuate field hardware.

• It performs peripheral and external device communication.

• It performs self-diagnostics.

3. Memory:-

• The memory is the area of the CPU in which data and information is stored and retrieved. The total memory area can be subdivided into the following three Sections.

a. System Memory

• It is used to store an executive program or system software. An operating system of the PLC is a special program that controls the action of the CPU and consequently the execution of the user's program.

b. DATA Memory:-

• It is used to store numerical data required in math calculation, bar code data etc.

• The input image memory consists of memory locations used to hold the ON or OFF states of each input field device, in the input status file.

• The output status file consists of memory locations that stores the ON or OFF states of hardware output devices in the field. Data is stored in the output status file as a result of solving the user program and is waiting to be transferred to the output module's switching device.

c. Program Memory

• It contains a user's application program.

➤ Programmer/Monitor

• The Programmer/Monitor (PM) is a device used to communicate with the circuits of the PLC. The programming unit allows the engineer/technicians to enter the edit the program to be executed.

Scan Cycle

• A PLC program is generally executed repeatedly as long as the controlled system is running.

• The status of physical input points is copied to an "I/O Image Table". (This memory area accessible to Processor so it can read Input status of the Inputs)

• The program is then run from its first instruction rung down to the last rung.

• It takes some time for the processor of the PLC to evaluate all the rungs and update the I/O image table with the status of outputs.

• "This scan time may be a few milliseconds (3ms to 10ms) for a small program or on a fast processor, but older PLCs running very large programs could take much longer (say, up to 100ms) to execute the program."

Communication Protocol

• "It is a set of rules for data transmission when PLC is connected to Network. Cable is used to connect different external device”.

• Types of the different communication Protocols are as under:-

• RS 232, RS 422, RH 485 (Serial Communication)

• CAN BUS, MOD BUS, DEVICE NET, PROFIBUS-PA, PROFIBUS-DP

• ETHERNET, ETHERCAT, INTER BUS-S, SERIPLEX

• Every protocol has a different "Baud rate".

  • • • Baud Rate:- "Rate of data transmission on network."

      • Unit:- Bits/Second.

      • Range:- 120 bits/sec To 100 Mbps.

Programming Language

• Ladder Diagram. (Note: - Mostly use everywhere.)

• Function Block Diagram.(FBD)

• Sequential Function Chart.(SFC)

• Structured Text Language.(ST)

PLC Selection Criteria

• Inputs and Outputs Requirement.

• CPU Memory.

• Compatible Protocol.

• Integration of High-Level Application.

• Reliability & Flexibility.

• Spare Parts and Maintenance.

• Cost of Hardware.

• Local Support Availability.