Feedback

Essential to all automatic-control mechanisms is the feedback principle, which enables a designer to endow a machine with the capacity for self-correction. A feedback loop is a mechanical, pneumatic, or electronic device that senses or measures a physical quantity such as position, temperature, size, or speed, compares it with a pre-established and takes whatever pre-programmed action is necessary to maintain the measured quantity within the limits of the acceptable standard.

The feedback principle has been used for centuries. An outstanding early example is the fly ball governor, invented in 1788 by the Scottish engineer James Watt to control the speed of the steam engine. In this device a pair of weighted balls is suspended from arms attached to a spindle, which is connected by gears to the output shaft of the engine. At the top of the spindle the arms are linked by a lever with a valve that regulates the steam input. As the engine speeds up beyond the desired rate, causing the spindle to rotate faster, the fly balls are driven upwards by the centrifugal effect. The action of the fly balls partly closes the input valve, reducing the amount of steam delivered to the engine. The common household thermostat is another example of a feedback device.
In manufacturing and production, feedback loops require that acceptable limits or tolerances be established for the process to be performed; that these physical characteristics be measured and compared with the set of limits; and, finally, that the feedback system be capable of correcting the process so that the measured items comply with the standard. Through feedback devices, machines can start, stop, speed up, slow down, count, inspect, test, compare, and measure. These operations are commonly applied to a wide variety of production operations that can include milling, boring, bottling, and refining.