A Real-Time Approach to Process Control, 3rd Edition

4

Basic Control Modes

The previous chapter discussed basic feedback control concepts including the vital role of the controller. Again, the purpose of the controller in regulatory control is to maintain the controlled variable at a predetermined set point. This is achieved by a change in the manipulated variable using a pre-programmed controller algorithm. This chapter will describe the basic control modes or algorithms used in controllers in feedback control loops.

4.1 On–Off Control

The most rudimentary form of regulatory control is on–off control. This type of control is primarily intended for use with final control elements (FCEs) that are non-throttling in nature, that is, some type of switch as opposed to a valve. An excellent example of on–off control is a home heating system. Whenever the temperature goes above the set point, the heating plant shuts off, and whenever the temperature drops below the set point, the heating plant turns on. This behaviour is shown by Equation 4.1:

(4.1)

The controller output, mv, is equal to 0% or off whenever PV exceeds the set point, SP. Whenever the process variable is below the set point, the controller output is equal to 100% or on.

The most useful type of process where on–off control can be successfully applied is a large capacitance process where tight level control is not important, that is, for the case of flow smoothing. A good example ...