Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL FUNDAMENTALS
6
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
operationg 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.