PLC Basics for beginner

1.1 Function of a PLC

A PLC is a microprocessor-based controller with multiple inputs and outputs. It uses a programmable 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:

  • Low cost
  • Reliability
  • Reprogramability

1.2 Inputs 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 to protect the internal PLC circuitry from damage.

The PLC outputs are connected directly or indirectly (e.g. through a relay) to actuator controls. Examples include solenoids on directional control valves, motors, motor contactors, alarms and warning lights.

There are three main types of output module:

Relay (volt-free): The signal from the PLC operates a relay within 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 for low power direct current applications.

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

1.3 PLC Architecture and Wiring Diagrams

PLC Connections

PLC wiring diagram

Shows a pictorial view of the PLC with its connections. In practice, we work with a simplified scheme as shown in the figure above.

1.4 Network Protocols

The wiring diagram in Fig above 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 single analog signal can be represented on the circuit. The 4-20 mA standard is slowly being replaced by network or fieldbus communications. Fieldbus is a multi-drop digital two-way communication link between intelligent devices. Fieldbus allows the connection 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 application. 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.

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