Biomedical Cybernetics

Feedback control

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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:

0th order feedback control

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)

y = G1x + z / (1 + G1 G2)

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:

Feedback systems are also classified according to their behaviour with regard to a set point. Regulators hold the controlled variable steady, while tracking or servo systems track a reference signal.


Feedback control systems play essential roles in the organism. Examples are:

  • Osmoregulation
  • 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:

Additionally, the behaviour of control circuits may be studied with simulative methods (in silico modelling).


Physiological systems theory, a subsection of medical cybernetics deals with mathematical description and analysis of feedback control systems and other information processing structures.

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