PID controllers
PI, PII variants

In some applications, two PI controllers cascaded (output of first controller is connected to input of the second), so the resulting system performs double integration. The controller with double integration is usually called a PII or PI2.

As shown above, the actual software implementation of PID controller is not difficult. Of course, in real life application, much more complicated structures of PID control are used. Many PLC offer dedicated instructions of functional blocks for PID control.

The main challenge is to adjust controller parameters (coefficients), to obtain control performance as good as possible (stable, fast, but without unwanted overshoot, with minimal energy consumption and cautious about controlled process and actuators).

The control design is determined mainly by properties of controlled plant and its identification is a basic requirement to follow design methodology, often described in literature. However, this is often difficult to do. The other theoretical requirements of linear control design are that the plant should be a linear and time invariant system. Due to limited scope of this text, we will only show some illustrative examples.