Controller Algorithms and Tuning

In the previous articles of this category were described the purpose of control, defined individual elements within control loops, and demonstrated the symbology used to represent those elements in an engineering drawing. The examples of control loops used thus far have been very basic. In practice, control loops can be fairly complex. The strategies used to hold a process at setpoint are not always simple, and the interaction of numerous setpoints in an overall process control plan can be subtle and complex. In this article, you will be introduced to some of the strategies and methods used in complex process control loops. The goal of this article is to:

❑ Differentiate between discrete, multistep, and continuous controllers.
❑ Describe the general goal of controller tuning.
❑ Describe the basic mechanism, advantages and disadvantages of the following mode of controller action:
• Proportional action;
• Intergral action;
• Derivative action;
❑ Give examples of typical applications or situations in which each mode of controller action would be used.
❑ Identify the basic implementation of P, PI and PID control in the following types of loops:
• Pressure loop;
• Flow loop;
• Level loop;
• Temperature loop.

Controller Algorithms

The actions of controllers can be divided into groups based upon the functions of their control mechanism. Each type of controller has advantages and disadvantages and will meet the needs of different applications. Grouped by control mechanism function, the three types of controllers are:

❑ Discrete controllers
❑ Multistep controllers
❑ Continuous controllers

Discrete controllers

Discrete controllers are controllers that have only two modes or positions: on and off. A common example of a discrete controller is a home hot water heater. When the temperature of the water in the tank falls below setpoint, the burner turns on. When the water in the tank reaches setpoint, the burner turns off. Because the water starts cooling again when the burner turns off, it is only a matter of time before the cycle begins again. This type of control doesn’t actually hold the variable at setpoint, but keeps the variable within proximity of setpoint in what is known as a dead zone (Picture 1).

Picture 1: Discrete Control

Multistep controllers

Multistep controllers are controllers that have at least one other possible position in addition to on and off. Multistep controllers operate similarly to discrete controllers, but as setpoint is approached, the multistep controller takes intermediate steps. Therefore, the oscillation around setpoint can be less dramatic when multistep controllers are employed than when discrete controllers are used (Picture 2).

Picture 2: Multistep Control Profile

Continuous controllers

Controllers automatically compare the value of the PV to the SP to determine if an error exists. If there is an error, the controller adjusts its output according to the parameters that have been set in the controller.

Picture 3: Automatic Feedback Control


The tuning parameters essentially determine (Picture 3):

>> How much correction should be made? The magnitude of the correction (change in controller output) is determined by the proportional mode of the controller.
>> How long should the correction be applied? The duration of the adjustment to the controller output is determined by the integral mode of the controller.
>> How fast should the correction be applied? The speed at which a correction is made is determined by the derivative mode of the controller.