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                Buffering, feedback, feedforward: mechanisms of control

   While the perturbations resisted in a [2]control relation can originate either
   inside (e.g. functioning errors or quantum fluctuations) or outside of the system
   (e.g. attack by a predator or changes in the weather), functionally we can treat
   them as if they all come from the same, external source. To achieve its goal in
   spite of such perturbations, the system must have a way to block their effect on
   its essential variables. There are three fundamental methods to achieve such
   regulation: buffering, feedback and feedforward (see Fig. 1).

Buffering: absorbing perturbations

   Buffering is the passive absorption or damping of perturbations. For example, the
   wall of a thermostatically controlled room is a buffer: the thicker or the better
   insulated it is, the less effect fluctuations in outside temperature will have on
   the inside temperature. Other examples are the shock-absorbers in a car, and a
   reservoir, which provides a regular water supply in spite of variations in rain
   fall. The mechanism of buffering is similar to that of a stable equilibrium:
   dissipating perturbations without active intervention. The disadvantage is that
   it can only dampen the effects of uncoordinated fluctuations; it cannot
   systematically drive the system to a non-equilibrium state, or even keep it
   there. For example, however well-insulated, a wall alone cannot maintain the room
   at a temperature higher than the average outside temperature.

   [ControlMechanisms.gif]

   Fig. 1: basic mechanisms of regulation, from left to right: buffering,
   feedforward and feedback. In each case, the effect of disturbances D on the
   essential variables E is reduced, either by a passive buffer B, or by an active
   regulator R.

Feedforward: anticipating perturbations

   [3]Feedback and feedforward both require action on the part of the system, to
   suppress or compensate the effect of the fluctuation. For example, a thermostat
   will counteract a drop in temperature by switching on the heating. Feedforward
   control will suppress the disturbance before it has had the chance to affect the
   system's essential variables. This requires the capacity to anticipate the effect
   of perturbations on the system's goal. Otherwise the system would not know which
   external fluctuations to consider as perturbations, or how to effectively
   compensate their influence before it affects the system. This requires that the
   control system be able to gather early information about these fluctuations.

   For example, feedforward control might be applied to the thermostatically
   controlled room by installing a temperature sensor outside of the room, which
   would warn the thermostat about a drop in the outside temperature, so that it
   could start heating before this would affect the inside temperature. In many
   cases, such advance warning is difficult to implement, or simply unreliable. For
   example, the thermostat might start heating the room, anticipating the effect of
   outside cooling, without being aware that at the same time someone in the room
   switched on the oven, producing more than enough heat to offset the drop in
   outside temperature. No sensor or anticipation can ever provide complete
   information about the future effects of an infinite [4]variety of possible
   perturbations, and therefore feedforward control is bound to make mistakes. With
   a good control system, the resulting errors may be few, but the problem is that
   they will accumulate in the long run, eventually destroying the system.

Feedback: correcting perturbations after the fact

   The only way to avoid this accumulation is to use [5]feedback, that is,
   compensate an error or deviation from the goal after it has happened. Thus
   feedback control is also called error-controlled regulation, since the error is
   used to determine the control action, as with the thermostat which samples the
   temperature inside the room, switching on the heating whenever that temperature
   reading drops lower than a certain reference point from the goal temperature. The
   disadvantage of feedback control is that it first must allow a deviation or error
   to appear before it can take action, since otherwise it would not know which
   action to take. Therefore, feedback control is by definition imperfect, whereas
   feedforward could in principle, but not in practice, be made error-free.

   The reason feedback control can still be very effective is continuity: deviations
   from the goal usually do not appear at once, they tend to increase slowly, giving
   the controller the chance to intervene at an early stage when the deviation is
   still small. For example, a sensitive thermostat may start heating as soon as the
   temperature has dropped one tenth of a degree below the goal temperature. As soon
   as the temperature has again reached the goal, the thermostat switches off the
   heating, thus keeping the temperature within a very limited range. This very
   precise adaptation explains why thermostats in general do not need outside
   sensors, and can work purely in feedback mode. Feedforward is still necessary in
   those cases where perturbations are either discontinuous, or develop so quickly
   that any feedback reaction would come too late. For example, if you see someone
   pointing a gun in your direction, you would better move out of the line of fire
   immediately, instead of waiting until you feel the bullet making contact with
   your skin.

   Reference:
   Heylighen F. & Joslyn C. (2001): "[6]Cybernetics and Second Order Cybernetics",
   in: R.A. Meyers (ed.), Encyclopedia of Physical Science & Technology , Vol. 4
   (3rd ed.), (Academic Press, New York), p. 155-170
     ____________________________________________________________________________

   [7]Copyright© 2001 Principia Cybernetica - [8]Referencing this page

   Author
   F. [9]Heylighen, & C. [10]Joslyn,

   Date
   Aug 31, 2001

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  16. http://pespmc1.vub.ac.be/PROPCTRL.html
  17. http://pespmc1.vub.ac.be/MAKANNOT.html
  18. http://pespmc1.vub.ac.be/hypercard.acgi$annotform?

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