Controller control theory: Encyclopedia II - Controller control theory - Types of Control

Controller control theory - Types of Control

In control theory there are two basic types of control. These are feedforward and feedback. The input to a feedback controller is the same as what it is trying to control - the controlled variable is "fed back" into the controller. The thermostat of a house is an example of a feedback controller. This controller relies on measuring the controlled variable, in this case the temperature of the house, and then adjusting the output, whether or not the heater is on. However, feedback control usually results in intermediate periods where the controlled variable is not at the desired setpoint. With the thermostat example, if the door of the house were opened on a cold day, the house would cool down. After it fell below the desired temperature (setpoint), the heater would kick on, but there would be a period where the house were colder than desired.

Feedforward control can avoid the slowness of feedback control. With feedforward control, the disturbances are measured and accounted for before they have time to affect the system. In the house example, a feedforward system may measure the fact that the door is opened and automatically turn on the heater before the house can get too cold. The difficulty with feedforward control is that the effect of the disturbances on the system must be perfectly predicted, and there must not be any surprise disturbances. For instance, if a window were opened that was not being measured, the feedforward controlled thermostat may let the house cool down.

To achieve the benefits of feedback control - controlling unknown disturbances and not having to know exactly how a system will respond to disturbances, and the benefits of feedforward control - responding to disturbances before they can affect the system, there are combinations of feedback and feedforward that can be used.

Some examples of where feedback and feedforward control can be used together are dead-time compensation, and inverse response compensation. Dead time compensation is used to control devices that take a long time to show any change to a change in input, for example, change in composition of flow through a long pipe. A dead time compensation control uses an element (also called a Smith predictor) to predict how changes made now by the controller will affect the controlled variable in the future. The controlled variable is also measured and used in feedback control. Inverse response compensation involves controlling systems where a change at first affects the measured variable one way but later effects it in the opposite way. An example would be eating candy. At first it will give you lots of energy, but later you will be very tired. As can be imagined, it is difficult to control this system with feedback alone, therefore a predictive feedforward element is necessary to predict the reverse affect that a change will have in the future.

Adapted from the Wikipedia article "Types of Control", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki