There are examples of feedback controllers everywhere. There are 3 essential elements of a feedback control system.
- Actuator that manipulates and causes an action
- Sensor that measures the response
- Controller that adjusts the actuator in response to the measurement
The controller receives the sensor measurement and compares it to the target (set point) to determine what adjustment is needed to the actuator. Some actuators are binary such as ON/OFF while others are continuous such as percent open (0-100%). Below are a couple examples of common feedback controllers.
Example 1: Building Temperature Control
One well-known example of a controller is the temperature control of a building in winter includes a heater (actuator), sensor (temperature thermistor), and controller (thermostat). The controller switches the heater on when the temperature drops below a certain level and switches the heater off when the temperature rises above a certain level. A disturbance variable may be changes in the outside air temperature or a change in indoor temperature due to a door opening.
|Actuator||Valve or switch in heater, Fuel to the furnace|
|Controller Set Point||Desired temperature indoors|
|Sensor||Temperature sensor such as a thermocouple or thermistor|
|Disturbance||Doors opening, wind, temperature outside|
Example 2: Automobile Speed Control
Consider an automobile with an automatic cruise control. The driver may adjust the velocity set point for the controller. The controller adjusts the gas pedal position (actuator) in response to changes in the speedometer reading (sensor). A disturbance variable may be an approaching hill or wind that would cause a deviation of the speed from the desired set point.
|Actuator||Gas Pedal Position, Fuel to the Engine|
|Controller Set Point||Desired Speed (mph or km/sec)|
|Sensor||Speedometer, measured velocity|
|Disturbance||Hills, wind, other cars|
There is different terminology when talking about common controllers such as Proportional Integral Derivative (PID) or advanced controllers such as Model Predictive Control (MPC). Below is a table of some of the terminology and associated abbreviations.
|Element||Common Control (PID)||Advanced Control (MPC)|
|Actuator||Controller Output (CO) or Output (OP)||Manipulated Variable (MV)|
|Controller||Set Point (SP)||Set Point (SP) or Range (SPHI/SPLO)|
|Sensor||Process Variable (PV)||Controlled Variable (CV)|
|Disturbance||Disturbance Variable (DV)||Disturbance Variable (DV)|