7.1 Objectives

As a result of studying this chapter, and after having completed the relevant exercises, the student should be able to:

• Select the correct PID-control algorithm for field interaction and for computer-optimized calculations

• Clearly distinguish between process noise and control loop instability, which are often similar in appearance

• List the correct sequence of steps to handle the different problems of noise and instability.

7.2 Comparative Descriptions of Real and Ideal Controllers

The ideal PID-controller is not suitable for direct field interaction, therefore it is called the non-interactive PID-controller. It is highly responsive to electrical noise on the PV input if the derivative function is enabled.

The real PID-controller is especially designed for direct field interaction and is therefore called the interactive PID-controller. Due to internal filtering in the derivative block the effects of electrical noise on the PV input is greatly reduced.

7.3 Description of the Ideal or the Non-Interactive PID Controller

The non-interactive form of controller is the classical teaching model of PID algorithms. It gives a student a clear understanding of P, I and D control, since:

• P-control, I-control and D-control can be seen independently of each other. Then, PID is effectively a combination of independent P, I and D-control actions. This can be seen in Figure 7.1.

• Since P, I and D algorithms are calculated independently in an ideal PID-controller, this form of controller is recommended if an ideal process variable exists.

Figure 7.1: Ideal PID-controller