Servomechanisms

Servomechanisms are classified on the basis of whether they depend upon information sampled at the output of the system for comparison with the input instructions. The simplest servomechanisms are called open-loop servomechanisms and do not feed back the results of their output. Open-loop servomechanisms do not verify that input instructions have been satisfied and they do not automatically correct errors.

An example of an open-loop servomechanism is a simple motor used to rotate a television-antenna. The motor used to rotate the antenna in an open-loop configuration is energized for a measured time in the expectation that antenna will be repositioned correctly. There is no automatic check to verify that the desired action has been accomplished. An open-loop servomechanism design is very unsatisfactory as a basis for an antenna rotator, just as it is usually not the best choice for other applications.

When error feedback is included in the design the result is called a closed-loop servomechanism. The servo's output result is sampled continuously and this information is continuously compared with the input instructions. Any important difference between the feedback and the input signal is interpreted as an error that must corrected automatically. Closed-loop servo systems automatically null, or cancel, disagreements between input instructions and output results.

The key to understanding a closed-loop servomechanism is to recognize that it is designed to minimize disagreements between the input instructions and the output results by forcing an action that reduces the error.

A more sophisticated antenna rotator system, compared to the open-loop version described earlier, will use the principles of the closed-loop servomechanism. When it is decided that the antenna is to be turned to a new direction the operator will introduce input information that creates a deliberate error in the servomech anism's feedback loop. The servo's electronic controller senses this purposely-introduced change and energizes the rotator's motor. The antenna rotates in the direction that tends to null the error. When the error has been effectively canceled, the motor is turned off automatically leaving the antenna pointing in the desired direction. If a strong wind causes the antenna turn more slowly than usual the motor will continue to be energized until the error is canceled. If a strong wind repositions the antenna improperly the resulting error will cause the motor to be energized once again, bringing the antenna back into alignment.

Another example of a simple closed-loop servomechanism is a thermostatically-controlled gas furnace. A sensor called a thermostat determines that heat is required, closing a switch that actuates an electric circuit that turns on the furnace. When the building's temperature reaches the set point the electric circuit is de-energized, turning off the fuel that supplies the flame. The feedback loop is completed when warmed air of the desired temperature is sensed by the thermostat.