Controllers

The actions of controllers can be divided into groups based upon the
functions of their control mechanism. Each type of contoller 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 (Figure 7.15).

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
(Figure 7.16).















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. The tuning parameters essentially determine:
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.