Description

A brief description of industrial automation

In factory automation, field devices such as switchgear, variable speed drives (VSDs) and instrumentation communicate to a programmable logic controller (PLC) which reads all inputs from the plant or machine, processes the information and then transfers the result to the outputs. The field devices communicate to the PLC via either hardwired signals or a network, typically RS485-based Profibus or the more advanced ProfiNet, which is based on Ethernet.

The network distributes input/output (I/O) channels, making distributed I/O necessary to prevent all the hardwired field devices from having to be connected to a single cabinet. This would mean many long wires and a lot of extra work

The engineer writes the software to collect these signals from the field devices and to collate them in the PLC, which uses the signals to control the plant.

For example, when the level-reading instruments in a tank register a high level, the PLC reacts by stopping the pump filling the tank and by starting the motor for the tank mixer. Software programming languages used by PLCs include LADDER, Function Block Diagram (FBD), Statement List (STL) and Sequence Flow Charts.

Human-machine interface (HMI) panels are used onsite to link to the PLC and to show graphically whether the machine or process plant is operating optimally. In the field, these panels are normally located on the PLC cabinet. They provide a partial view of the plant operation as opposed to an entire plant automation visualisation overview.

Where the factory or plant has multiple PLC areas, a plant overview visualisation system (SCADA) would link to all PLCs, gather data and provide the operator with a plant overview.