A programmable logic controller (PLC) Basics and introductions

 

Introduction to PLC

 

A programmable logic controller (PLC) is a special microprocessor-based controller that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and processes and are designed to be operated by engineers with perhaps a limited knowledge of computers and computing languages. The designers of the PLC have pre-programmed it so that the control program can be entered using a simple, rather intuitive, form of language. The term logic is used because programming is primarily concerned with implementing logic and switching operations, e.g. if A or B occurs switch on C, if A and B occurs switch on D. Input devices, e.g. sensors such as switches, and output devices in the system being controlled, e.g. motors, valves, etc., are connected to the PLC. The operator then enters a sequence of instructions, i.e. a program, into the memory of the PLC. The controller then monitors the inputs and outputs according to this program and carries out the control rules for which it has been programmed.

Definition:

PLC can be defined as a digitally operating electronic system, designed for use in an industrial environment, which uses memory for the internal storage of user-oriented instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic to control, through digital or analog inputs and outputs, various types of machines or processes.

 

 

PLCs have the great advantage that the same basic controller can be used with a wide range of control systems. To modify a control system and the rules that are to be used, all that is necessary is for an operator to key in a different set of instructions. There is no need to rewire. The result is a flexible, cost effective, system which can be used with control systems which vary quite widely in their nature and complexity.

PLCs are similar to computers but whereas computers are optimised for calculation and display tasks, PLCs are optimised for control tasks and the industrial environment. PLCs are:

1 Rugged and designed to withstand vibrations, temperature, humidity and noise.

2 Have interfacing for inputs and outputs already inside the controller.

3 Are easily programmed and have an easily understood programming language which is primarily concerned with logic and switching operations.

Typically a PLC system has the basic functional components of processor unit, memory, power supply unit, input/output interface section, communications interface and the programming device. Figure below shows the basic arrangement.

 

 

1.       The processor unit or central processing unit (CPU) is containing the microprocessor and this interprets the input signals and carries out the control actions, according to the program stored in its memory, 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 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 and data stored from the input for processing and for the output for outputting.

5.        The input and output sections are where the processor receives information from external devices and communicates information to external devices. Devices giving discrete or digital signals are ones where the signals are either off or on. Thus a switch is a device giving a discrete signal, either no voltage or a voltage. Analogue devices give signals whose size is proportional to the size of the variable being monitored. For example, a temperature sensor may give a voltage proportional to the temperature.

6.       The communications interface is used to receive and transmit data on communication networks from or to other remote PLCs. It is concerned with such actions as device verification, data acquisition, synchronisation between user applications and connection management.

Advantages of using PLCs:

 

• ·         Highly reliable

·         Highly versatile (universal applicability)
·         Simple troubleshooting
·         Simple installation
·         Quick modification of the program (highly flexible)
·         Capable of tasks not possible with relays
·         Low space requirement
·         Low power consumption
·         High processing speed
·         No moving parts, hence no wearing parts
 

Disadvantages of using PLCs:

• ·         High initial cost ( for a simple process )

·         Sensitive to dust, high temperature and high humidity
·         Repair must be made by a quali_ed personnel
·         Not very widespread
·         No uniform programming language

 

Selecting a PLC:

 

1. ·         Number of I/Os
2. ·         Kind of signals : digital or analog
3. ·         Fieldbus system or standalone
4. ·         Modular or compact
5. ·         Operating voltage
6. ·         Positive or negative triggerred input
7. ·         Relay or transistor output
8. ·         Operating systems

 

PLC manufacturers:

ABB Allen-Bradley ALSTOM/Cegelec Beck Electronic/Festo Beckhoff Bosch Fisher & Paykel Fuji Electric GE-Fanuc Hitachi Honeywell, LG Industrial Systems Matsushita (Europe)/Aromat Mitsubishi Omron Rockwell Automation Schneider Electric Siemens/Moore Products Toshiba Yamatake Yokogawa