Computer Control Devices in Automation

This chapter deals with the control devices used in automation such as Programmable Logic Devices (PLD), Programmable Logic Controller (PLC), Programmable Automation Controller (PAC), Personal Computer (PC), etc. The goal of the control devices in automation is to achieve an efficient, robust and reliable system control. Basically, system control devices include input devices (for raw data input), processing devices (for processing raw data into information), output devices (to disseminate the processed data and information), and storage devices (for the retention of processed data and information). The sensors feed the main controller with the input data acquired from the environment. Following the processing of the data, the decision is made by the main controller on the control action to take and this decision is communicated to the actuator for execution. The actuator in turn drives the final control device to implement the control action. The programming language is crucial in achieving optimum efficiency. While the PLC follows a scan-based program execution, PC software is usually event-driven. In terms of cost efficiency, indicators such as performance, expandability, and ruggedness are important considerations. The initial cost of a PC may be higher than that of a PLC as a PC is more suitable for processing of complex network loads. PLC may be initially inexpensive but as the demand for processing power increases, the PC-based system becomes more cost-effective. In terms of expandability, PLC usually offers support to standard industrial equipment but when an external control is needed, a PC is more suited. PLC does not require additional protection equipment compared to PC.