PLC Basic

1. Fungction of a PLC

A PLC is a microprocessor-bassed controller with multiple inputs and outputs. It uses aprogrammable memory to store instructions and carry out functions to control machines and processes.

The PLC performs the logic functions of relays, timers, counters and sequencers. It has the following advantages:

2. Input and Outputs

The PLC inputs give it information about the machine or process that it is controlling. These are typically switches and sensors. The switches are connected to an input module that provides the interface between the switches or sensors and the PLC.

Input module circuits have opto-isolators ti protect the internal PLC circuity from damage.

The PLC output are connected directly or indirectly (e.g. though arelay) to actuator controls. Example include solenoids on directional control valves, motors, motor contactors, alarms and warning lights,

There are tree main types of output module:

Relay (volt-free): The signal from the PLC operates a relay wothin the output module connecting the control voltage to the output port and hence to the actuator.

Transistor: A transistor is used to switch on the output. This is faster than a relay output but is only suitable dor low power direct curent applications.

Triac: This soid state device is used for switchinf alternating current devices. It requires some form of over current protection.

3. PLC Architecture and Wiring Diagrams

4. Network Protocols

The wirinf diagram in fig.5 shows the inputs and outputs connected directly (hard wired) to the PLC. The devices shown are on/off or digital in nature but the signal to the PLC is analog. Many commonly used devices conform to a 4-20 mA standard whereby signals of 4mA and 20mA form respectively the minimum and maximum values of an analog signal.

With analog devices, a separate cable needs to be run between the end device and the control system because only a signal analog signal can be represented on the circuit. The 4-20mA standard is slowly being replaced by netwoek or fieldbus communications. Fieldbus allows the connection of a number of a number of sensors all located in the same area to the same cable. Fieldbus comes in many varieties depending on the manufacturer and appliation. Examples include ASibus, Profibus, Devicenet and Modbus.

A more recent trend is the development of Industrial Ethernet which has the capacity to transport large quantities of data not only for process control but also to integrate the process with management information systems.