DEFINITIONS
The following terms are used in this manual. Figure 1 at the end of this list illustrates a typical control loop with the components identified using terms from this list.
Analog: Continuously variable (e.g., a faucet controlling water from off to full flow).
Automatic control system: A system that reacts to a change or imbalance in the variable it controls by adjusting other variables to restore the system to the desired balance.
Algorithm: A calculation method that produces a control output by operating on an error signal or a time series of error signals.
Compensation control: A process of automatically adjusting the setpoint of a given controller to compensate for changes in a second measured variable (e.g., outdoor air temperature). For example, the hot deck setpoint is normally reset upward as the outdoor air temperature decreases. Also called “reset control”.
Control agent: The medium in which the manipulated variable exists. In a steam heating system, the control agent is the steam and the manipulated variable is the flow of the steam.
Control point: The actual value of the controlled variable (setpoint plus or minus offset).
Controlled medium: The medium in which the controlled variable exists. In a space temperature control system, the controlled variable is the space temperature and the controlled medium is the air within the space.
Controlled Variable: The quantity or condition that is measured and controlled.
Controller: A device that senses changes in the controlled variable (or receives input from a remote sensor) and derives the proper correction output.
Corrective action: Control action that results in a change of the manipulated variable. Initiated when the controlled variable deviates from setpoint.
Cycle: One complete execution of a repeatable process. In basic heating operation, a cycle comprises one on period and one off period in a two-position control system.
Cycling: A periodic change in the controlled variable from one value to another. Out-of-control analog cycling is called “hunting”. Too frequent on-off cycling is called “short cycling”. Short cycling can harm electric motors, fans, and compressors.
Cycling rate: The number of cycles completed per time unit, typically cycles per hour for a heating or cooling system. The inverse of the length of the period of the cycle.
Deadband: A range of the controlled variable in which no corrective action is taken by the controlled system and no energy is used. See also “zero energy band”.
Deviation: The difference between the setpoint and the value of the controlled variable at any moment. Also called “offset”.
DDC: Direct Digital Control. See also Digital and Digital control.
Digital: A series of on and off pulses arranged to convey information. Morse code is an early example. Processors (computers) operate using digital language.
Digital control: A control loop in which a microprocessor- based controller directly controls equipment based on sensor inputs and setpoint parameters. The programmed control sequence determines the output to the equipment.
Droop: A sustained deviation between the control point and the setpoint in a two-position control system caused by a change in the heating or cooling load.
Enhanced proportional-integral-derivative (EPID) control: A control algorithm that enhances the standard PID algorithm by allowing the designer to enter a startup output value and error ramp duration in addition to the gains and setpoints. These additional parameters are configured so that at startup the PID output varies smoothly to the control point with negligible overshoot or undershoot.
Electric control: A control circuit that operates on line or low voltage and uses a mechanical means, such as a temperature-sensitive bimetal or bellows, to perform control functions, such as actuating a switch or positioning a potentiometer. The controller signal usually operates or positions an electric actuator or may switch an electrical load directly or through a relay.
Electronic control: A control circuit that operates on low voltage and uses solid-state components to amplify input signals and perform control functions, such as operating a relay or providing an output signal to position an actuator. The controller usually furnishes fixed control routines based on the logic of the solid- state components.
Final control element: A device such as a valve or damper that acts to change the value of the manipulated variable. Positioned by an actuator.
Hunting: See Cycling.
Lag: A delay in the effect of a changed condition at one point in the system, or some other condition to which it is
related. Also, the delay in response of the sensing element of a control due to the time required for the sensing element to sense a change in the sensed variable.
Load: In a heating or cooling system, the heat transfer that the system will be called upon to provide. Also, the work that the system must perform.
Manipulated variable: The quantity or condition regulated by the automatic control system to cause the desired change in the controlled variable.
Measured variable: A variable that is measured and may be controlled (e.g., discharge air is measured and controlled, outdoor air is only measured).
Microprocessor-based control: A control circuit that operates on low voltage and uses a microprocessor to perform logic and control functions, such as operating a relay or providing an output signal to position an actuator. Electronic devices are primarily used as sensors. The controller often furnishes flexible DDC and energy management control routines.
Modulating: An action that adjusts by minute increments and decrements.
Offset: A sustained deviation between the control point and the setpoint of a proportional control system under stable operating conditions.
On/off control: A simple two-position control system in which the device being controlled is either full on or full off with no intermediate operating positions available. Also called “two-position control”.
Pneumatic control: A control circuit that operates on air pressure and uses a mechanical means, such as a temperature-sensitive bimetal or bellows, to perform control functions, such as actuating a nozzle and flapper or a switching relay. The controller output usually operates or positions a pneumatic actuator, although relays and switches are often in the circuit.
Process: A general term that describes a change in a measurable variable (e.g., the mixing of return and outdoor air streams in a mixed-air control loop and heat transfer between cold water and hot air in a cooling coil). Usually considered separately from the sensing element, control element, and controller.
Proportional band: In a proportional controller, the control point range through which the controlled variable must pass to move the final control element through its full operating range. Expressed in percent of primary sensor span. Commonly used equivalents are “throttling range” and “modulating range”, usually expressed in a quantity of engineering units (degrees of temperature).
Proportional control: A control algorithm or method in which the final control element moves to a position proportional to the deviation of the value of the controlled variable from the setpoint.
Proportional-Integral (PI) control: A control algorithm that combines the proportional (proportional response) and integral (reset response) control algorithms. Reset response tends to correct the offset resulting from proportional control. Also called “proportional-plus- reset” or “two-mode” control.
Proportional-Integral-Derivative (PID) control: A control algorithm that enhances the PI control algorithm by adding a component that is proportional to the rate of change (derivative) of the deviation of the controlled variable. Compensates for system dynamics and allows faster control response. Also called “three- mode” or “rate-reset” control.
Reset Control: See Compensation control.
Sensing element: A device or component that measures the value of a variable.
Setpoint: The value at which the controller is set (e.g., the desired room temperature set on a thermostat). The desired control point.
Short cycling: See Cycling.
Step control: Control method in which a multiple-switch assembly sequentially switches equipment (e.g., electric heat, multiple chillers) as the controller input varies through the proportional band. Step controllers may be actuator driven, electronic, or directly activated by the sensed medium (e.g., pressure, temperature).
Throttling range: In a proportional controller, the control point range through which the controlled variable must pass to move the final control element through its full operating range. Expressed in values of the controlled variable (e.g., degrees Fahrenheit, percent relative humidity, pounds per square inch). Also called “proportional band”. In a proportional room thermostat, the temperature change required to drive the manipulated variable from full off to full on.
Time constant: The time required for a dynamic component, such as a sensor, or a control system to reach 63.2 percent of the total response to an instantaneous (or “step”) change to its input. Typically used to judge the responsiveness of the component or system.
Two-position control: See on/off control.
Zero energy band: An energy conservation technique that allows temperatures to float between selected settings, thereby preventing the consumption of heating or cooling energy while the temperature is in this range.
Zoning: The practice of dividing a building into sections for heating and cooling control so that one controller is sufficient to determine the heating and cooling
