Given the wide variety of PLCs available, industrial engineers must go through the process of evaluating different devices in order to select the one that is best suited to their task. To do this, they must review the requirements of their system and the applications that will use the PLC. Similarly, information and requirements must be gathered about the I/O capacity of the PLC, and the type of output being targeted must be specified. For example, the electrical requirements of the I/O modules must be documented, including input device voltages, output device voltages, and currents. It is also important to determine whether the PLC should support special operations and advanced features beyond simple discrete (on/off) logic.
Another important aspect of PLC selection involves the memory and CPU (central processing unit) of the PLC device. To determine CPU requirements, it is important to know the speed of the industrial process or machine to be controlled. This involves determining the fastest action required, as well as time urgency and response times for various operations. Communication requirements also need to be considered during the PLC selection process. Specifically, data sharing requirements must be identified, as well as the devices that must communicate with the PLC (e.g., computers, touch screens in workstations). In this way, any required communication devices (e.g., modems, cables) can also be identified.
The PLC selection process must also consider the need to interact with the operator through certain interfaces, such as push buttons or LED (light emitting diode) digital displays. Specifically, the selected PLC must support the display of the correct messages to the operator, as well as any required alarms and notifications. Similarly, the operator should be provided with the means to enter data as required by the application.
The PLC selection process must also consider the physical environment in which the automation equipment will be deployed. This is important for the use of properly hardened equipment, i.e., equipment that is rugged and able to withstand the shocks imposed by the target environment at hand.
Finally, there are non-technical criteria that determine the selection decision, including the cost of the equipment and the quality of complementary services offered, such as training and after-sales support. The various criteria must be weighed against their relevant importance to the application at hand. This will drive the selection of a product with the appropriate modules, value-added programmable features, operator interface and cost.
Overall, PLCs are one of the most commonly used devices in industrial control applications. They are also expected to remain at the center of industrial control in the era of the fourth industrial revolution (Industry 4.0)[3]. However, emerging Industry 4.0 applications will provide ways to drive PLC operations in an intelligent and data-driven manner. Specifically, PLC operations in Industry 4.0 will be driven not only by sensors and cyber-physical devices, but also by data analytics on the cloud. This promises to improve the accuracy and intelligence of next-generation industrial automation systems. In this context, it makes sense for young engineers to learn more about PLCs and their operation.