Question

What is the function of a PID controller? List the applications of PID modes of control.

Answer 1

List the applications of PID modes of control.

Furnace Temperature Control

Furnaces typically involve heating and holding large amounts of raw material at high temperature. It’s commonplace for the material involved to have a large mass. As a result it possesses a high degree of inertia – the material’s temperature doesn’t change quickly even when high heat it applied. This characteristic results in a relatively steady PV signal, and it allows the Derivative term to effectively correct for Error without excessive changes to either the Controller Output or the Final Control Element .

Neutralization pH Control

pH is widely viewed in industry as a challenge to control. For one: pH is highly non-linear – its behavior changes from one operating range to another. For another: The buffering effects of some material can curb what would otherwise be volatile dynamics until the buffer is saturated. While the dynamics of pH are challenging from a control perspective, they are well suited for the PID form of the controller. Specifically, the dynamics of pH tend to be slow as the amount of caustic or acid that’s typically added to a process is relatively small when compared to the volume of existing liquid. The slower dynamics allow Derivative to improve control without overworking the Final Control Element .

Batch Temperature Control

In contrast with the furnace example mentioned above batch temperature control is basically operated as a closed system. While bubbling and other process noise will certainly be evident in the data, noise on the whole is less of an issue in a closed system. Another aspect of batch temperature control relates to temperature itself. While heat can be applied to either maintain or increase temperature, many batch temperature control processes do not include a cooling loop with which to counter the effects of heat. Said another way: Heat can be added, but it can’t be subtracted. The net effect are dynamics which are both slow and nonlinear, and noise within the data is limited. These characteristics make for an ideal application of PID Control.