Plc Principle

Design Characteristics (P1.1)

1. a) Explain the three types of Programmable logic controllers. Give an example for application of these types of programmable logic controllers

The three types of programmable logic controllers

i. UNITARY

The Unitary PLC contains every feature of a basic system in one box. They are most likely located on or close to the machine being controlled.

ii. MODULAR

These use a range of modules that slot together to build up a system. The basic modules are the power supply, the main module containing the CPU, the input module and the output module. Other modules such as A/D converters may be added, the main advantage is that the number of input and output terminals can be expanded to cope with changes to the hardware system.

Modular PLCs may be designed to be fixed direct to a back panel. Usually they are arranged on a rack or rail and mounted inside a large cabinet for protection and security.

iii. RACK MOUNTING

This is a similar concept to the modular design but the modules. They will be arranged in rows on racks inside cabinets. They do not plug into each other but plug into a standard communication slot in the rack. All the slots are connected by standard network or vus so that they all communicate with each other and with the external system s. this cuts down on the quantity of cable work needed. The system is flexible and expandable.

Modular PLCs in an automated system

Modular PLCs consist of optional components required for a more complex control application, as selected and assembled by the user.

The Modular PLCs include the following components, as demonstrated in Figure

1- The CPU module, containing the CPU and its memory.

2- Input and output modules (I/O modules), to allow the PLC to read sensors and control actuators.

3- A power supply module, to provide power to the CPU and often to provide power to drive sensors and low-power actuators connected to I/O modules.

4- A rack or bus, so that the CPU module can...