PLCs (programmable logic controllers) are found everywhere in industry today. Proper PLC system design can provide many years of service for a machine or process and greatly improve efficiency and profitability for the owner.
Why do we use a PLC? PLCs can be “relay replacers,” providing logic, timing, and counting capabilities that would be difficult to do with discrete components (see Figure 1). PLCs can perform math functions, making calculations based on external signals and/or HMI (human-machine interface) input. A good example of PLC use is with VFDs (variable frequency drives). PLCs can now be networked to many VFDs via Ethernet/IP or other fieldbus type communications protocols. By using the communications protocol, a great deal of wiring can be eliminated.
Before we apply a PLC to a machine or process, let’s look at some design and maintenance practices that are critical to a successful implementation.