On/off control: A simple control system in which the device being controlled is either completely on if there is a deviation from set point, or completely off if you are at the set point or within the hysteresis.
Proportional (P) control: Control in which the value of the control output is proportional to the deviation from the set point. When the process variable enters the proportional band, the control output decreases as your process variable approaches the set point.
Usually, the control output will be 50% when you reach the set point. Integral (I) control: Control that corrects for a ÒdroopÓ or offset condition that can occur when using only proportional control. The deviation from set point is integrated over a selected time interval and added to the proportional signal in order to move or ÒresetÓ the proportional band. The selected time interval is sometimes called the number of resets per minute. Usually used with proportional control for proportional-integral (PI) control.
Derivative (D) control: Control in which the value of the control output is dependent on the rate of increase (or decrease) of the process variable from the set point. The derivative of the deviation from set point is taken and added to the proportional signal. Always used with at least proportional control for proportional-derivative (PD) control. Helps prevent overshoot during process disturbances.
PID control: Proportional-integral-derivative control. Control in which the value of the control output is a linear combination of the error signal, its integral, and its derivative. Provides precise control and is used for systems that have frequent disturbances.
Fuzzy logic PID: Uses a set point modifier to create a temporary set point that allows the controller to recover more quickly and smoothly. The temporary set point changes continuously as you get closer to the actual set point, so that overshoot at start-up and during process disturbances is virtually eliminated. Optimizes system performance.
SPST relay: Single pole, single throw relay. An electrically controlled mechanical switch that either opens or closes a circuit.
SPDT relay: Single pole, double throw relay. An electrically controlled mechanical switch that in one position closes a circuit, and in the second position closes another circuit, breaking the first.
SSR: Solid state relay. An electrically controlled switch that has no moving parts. The switching action is done through a transistor. Silent operation and longer lifetime than a SPDT relay. External SSRs are used for systems that have a high current draw, which might be too high for a relay that is built into the controller.
Pulse output for SSR: A logic signal that is used to actuate an external SSR. See page 1929 for our external SSRs.
Analog output: Usually a 4 to 20 mA signal or a voltage signal that is proportional to the temperature reading.
Powered output: Usually a three-prong receptacle that provides 115 VAC power (or 230 VAC, depending on controller) to a heater or to other control devices.