Process Control with PLCs
In its simplest form, a control process has three main parts: the sensor, the controller, and the actuator. The sensor sends information or a reference signal from the process to the controller that implements a feedback or closed-loop control system and uses actuators to drive the process. Sensors can measure any type of parameter from a process. Examples of actuators in a mechanical system are motors, drives, and pumps, but also include pinions, pulleys, and chains.
This directly maps on the processing model of a PLC which runs as a continuous loop where all inputs are read, processing takes place and outputs are being generated. The frequency of the loop is identical to the (highest) sampling frequency. Lower sampling frequencies can be supported by splitting the loop into parts where some parts are executed every loop and other parts e.g. only every alternative loop. If the total processing takes (too) long this might enforce a lower overall loop frequency or splitting the loop as mentioned before. In a simplistic approach there is no concept of a system heartbeat and the loop just runs as fast as it can. In general power optimizations are not relevant for PLC applications hence sleep and wake-up situations are not present.
This continuous loop paradigm is a direct inheritance from the way a PLC was conceived. PLC is an acronym for ‘programmable logic controller’. At the beginning of the electronic revolution (1960s-1970s), controlling devices were designed with discrete electronic components in a fixed topology. Changing project specifications meant redesigning and reengineering the whole control logic, with physical components shuffled and moved around. With PLCs, the design efforts for the control algorithm are confined almost entirely into the software.
PLCs have specific advantages that make them attractive for manufacturing applications. They are modular, simple and robust and suppliers ensure availability of replacement parts for years. Very often production lines have to run for many years and uptime is a very important criterion. However the most important feature of a PLC is that it is hard real-time. A real-time system guarantees that all inputs, outputs, and computations process within a specific time constraint, often referred to as a deadline. A PLC considers missing a deadline to be a total failure. For specific operations hard real-time guarantees are essential.