Table of Contents
Advertisement
Electronic controller: A solid-state device usually consisting
of a power supply, a sensor amplification circuit, a
process/comparing circuit, an output driver section,
and various components that sense changes in the
controlled variable and derive a control output which
provides a specific control function. In general,
adjustments such as setpoint and throttling range
necessary for the process can be done at the controller
via potentiometers and/or switches.
Final control element: A device such as a valve or damper
that changes the value of the manipulated variable.
The final control element is positioned by an actuator.
Integral action (I): An action in which there is a continuous
linear relationship between the amount of increase (or
decrease) on the output to the final control element
and the deviation of the controlled variable to reduce
or eliminate the deviation or offset.
Limit sensor: A device which senses a variable that may be
other than the controlled variable and overrides the
main sensor at a preset limit.
Main sensor: A device or component that measures the variable
to be controlled.
Negative (reverse) compensation: A compensating action
where a decrease in the compensation variable has
the same effect as an increase in the controlled
variable. For example, in a heating application as the
outdoor air temperature decreases, the control point
of the controlled variable increases. Also called
"winter compensation or reset".
Offset: A sustained deviation between the control point and
the setpoint of a proportional control system under
stable operating conditions. Also called Deviation.
Positive (direct) compensation: A compensating action where
an increase in the compensation variable has the same
effect as an increase in the controlled variable. For
example, in a cooling application as the outdoor air
temperature increases, the control point of the
controlled variable increases. Also called "summer
compensation or reset".
Proportional band (throttling range): In a proportional
controller, the control point range through which the
controlled variable must pass to drive the final control
element through its full operating range. Proportional
band is expressed in percent of the main sensor span.
A commonly used equivalent is "throttling range"
which is expressed in values of the controlled variable.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
ELECTRONIC CONTROL FUNDAMENTALS
Proportional control (P): 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 or deviation control algorithms. Integral action
tends to correct the offset resulting from proportional
control. Also called "proportional plus reset" or "two-
mode" control.
Remote setpoint: A means for adjusting the controller setpoint
from a remote location, in lieu of adjusting it at the
controller itself. The means of adjustment may be
manual with a panel or space mounted potentiometer,
or automatic when a separate device provides a signal
(voltage or resistive) to the controller.
Reset control: See Compensation Control.
Reset sensor: See Compensation Sensor.
Reverse acting: A reverse acting controller decreases its output
signal on an increase in input signal.
Setpoint: The value on the controller scale at which the
controller is set such as the desired room temperature
set on a thermostat. The setpoint is always referenced
to the main sensor (not the reset sensor).
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. Throttling range is expressed in values
of the controlled variable such as temperature in
kelvins, relative humidity in percent, or pressure in
pascals or kilopascals. A commonly used equivalent
is "proportional band" which is expressed in percent
of sensor or transmitter span.
Transducer: A device that converts one energy form to another.
It amplifies (or reduces) a signal so that the output of
a sensor or transducer is usable as an input to a
controller or actuator. A transducer can convert a
pneumatic signal to an electric signal (P/E transducer)
or vice versa (E/P transducer), or it can convert a
change in capacitance to an electrical signal.
Transmitter: A device that converts a sensor signal to an input
signal usable by a controller or display device. See
also Transducer.
121
Advertisement
Chapters
Table of Contents
