Servomechanisms

A gas-furnace controller example above illustrates a potential problem with servomechanisms that must be solved when they are designed. If not properly engineered, closed-loop servomechanisms tend to be unstable. They must not overcontrol. The controller must be intelligent enough to shut down the actuator just before satisfaction is accomplished. Just as a car driver must slow down gradually before stopping at an intersection, a servomechanism must anticipate the effects of inertial mass. The inertia may be mechanical or it may be thermal, as in the case of the gas furnace. If the furnace flame were to continue to burn until the air temperature reaches the exact set point on the temperature selector, the residual heat in the furnace firebox would continue to heat the house, raising the temperature excessively. The room temperature will overshoot the desired value, perhaps uncomfortably. Most space-heating furnace control thermostats include a heat-anticipation provision designed to minimize thermal overshoot. A properly-adjusted anticipation control turns off the furnace's flame before the room temperature reaches the desired set point, allowing the temperature to coast up to the desired value as the furnace cools.