Synonyms: control circuit, control loop, closed-loop control, feedback interaction
Information processing structures that comprise a control process with inclosed degenerative feedback (thus forming a loop) are referred to as feedback control systems. Feedback control permits that vitally important parameters of body function like osmolality, pH and Oxygen-tension as well as plasma level of hormones and body temperature remain constant or in a compliant range (Homeostasis).
Example of a 0th order linear feedback control system with load:
e(t) = x(t) - yR(t)
yS(t) = G1 e(t) = G1 [x(t) - yR(t)]
y(t) = yS(t) + z(t) = G1 [x(t) - yR(t)] + z(t)
yR(t) = G2 y(t)
y(t) = G1 x(t) - G1 G2 y(t) + z(t)
x: set point, e: error, y: controlled variable, yS: manipulated variable, yR: measured variable, z: load, disturbance variable, G1: amplification factor of direct branch, G2: amplification factor of feedback path.
See the legend for an explanation of symbols.
Selected types of feedback control cover:
- Unity feedback systems
- Linear feedback control
- Nonlinear feedback control
Feedback control systems play essential roles in the organism. Examples are:
- corticotropic feedback controls
- gonadotropic feedback controls
- thyrotropic feedback control
- homeostasis of blood glucose level
- control of blood pressure
- control of respiration and plasma pH
- thermoregulation in homeotherms
- synchronisation of root and crown growth in plants
Mathematical description and modellingEdit
Common methods for describing the relation among structure and behaviour of feedback control systems are:
- Time domain analysis (as outlined in the example above)
- Frequency domain analysis (covering transfer functions)
- s-plane analysis including zero-pole investigation
- State space description.