(Control Theory : Classical Control : Feedback control)

In feedback systems, the variable being controlled--such as speed or temperature--is measured by a sensor and the information is fed back to the process to influence the controlled variable.

Elementary feedback control can be represented in a block diagram:

                                             Disturbance Input
                                                    |
                                                    |
         +-----------+        +----------+       +-------+
   Input | Reference |  +     |          |       |       |    Output ->
   ------|  Sensor   |---C----| Actuator |-------| Plant |----o-----
         +-----------+  -|    |          |       |       |    |
                         |    +----------+       +-------+    |
                         |                                    |
                         |            +--------+              |
                         |            | Output |              |
                         +------------| Sensor |--------------+
                                      |        |
                                      +--------+

Where the components are defined as follows:

• Plant: The system output to be controlled
• Disturbance: Noise affecting the operation of the plant
• Actuator: Device capable of influencing the controlled variable of the process
• Reference sensor: Device capable of measuring the input to the plant
• Output sensor: Device capable of measuring the output of the plant
• Comparator: Device (C in diagram) that subtracts the feedback signal from the reference signal (for negative feedback)

The best way to visualise the dynamics of a feedback control system is, of course, via example. Suppose the temperature in your home is controlled by a thermostat (a stretch, I know, but bear with me here) and that we can represent the system with the following block diagram:

                                                      Heat
                                                      Loss
    Desired                                             |                    Room
   Temperature                +-------+   +---------+   |   +-------+     Temperature
             +------------+   |  Gas  |   |         |  -|   |       |              
   ----------| Thermostat |---| Valve |---| Furnace |---C---| House |-------o------
             +------------+   |       |   |         |  +    |       |       |
                    |         +-------+   +---------+       +-------+       |
                    |                                                       |
                    |                                                       |
                    +-------------------------------------------------------+ 

When the room temperature is significantly below the desired temperature, the thermostat will be on and the control logic will transmit power to the gas valve, which will open. This will cause the furnace to fire, and assuming that it is properly designed, the heat supplied to the house will be much larger than the heat lost and the temperature in the room will rise until it exceeds the thermostat setting by a small amount. At this point, the furnace will turn off, the temperature will gradually drop again, eventually triggering the thermostat once more, and the cycle will continue to maintain the desired temperature!

From amoeba to space stations, this fundamental idea is literally everywhere.