Relationship of program control and feedback control in an automated system.

Machines that cannot function without (feedback) control

Many basic devices must be manufactured in such a way that their behaviour can be modified by means of some external control. Generally, the same effect cannot be brought about (in practice and sometimes even in theory) by any intrinsic modification of the characteristics of the device. For example, transistor amplifiers introduce intolerable distortion in sound systems when used alone, but properly modified by a feedback control system they can achieve any desired degree of fidelity. Another example involves powered flight. Early pioneers failed, not because of their ignorance of the laws of aerodynamics but because they did not realize the need for control and were unaware of the basic principles of stabilizing an inherently unstable device by means of control. Jet aircraft cannot be operated without automatic control to aid the pilot, and control is equally critical for helicopters. The accuracy of inertial navigation equipment cannot be improved indefinitely because of basic mechanical limitations, but these limitations can be reduced by several orders of magnitude by computer-directed statistical filtering, which is a variant of feedback control.

Control of machines

In many cases, the operation of a machine to perform a task can be directed by a human (manual control), but it may be much more convenient to connect the machine directly to the measuring instrument (automatic control); e.g., a thermostat may be used to turn on or off a refrigerator, oven, air-conditioning unit, or heating system. The dimming of automobile headlights, the setting of the diaphragm of a camera, and the correct exposure for colour prints may be accomplished automatically by connecting a photocell directly to the machine in question. Related examples are the remote control of position (servomechanisms) and speed control of motors (governors). It is emphasized that in such cases a machine could function by itself, but a more useful system is obtained by letting the measuring device communicate with the machine in either a feedforward or feedback fashion.