ENGINEERING MANUAL OF AUTOMATIC CONTROL
ELECTRONIC CONTROL FUNDAMENTALS
120
INTRODUCTION
This section provides information about electronic control
systems used to control HVAC equipment. An electronic control
system comprises a sensor, controller, and final control element.
The sensors used in electronic control systems are simple, low-
mass devices that provide stable, wide range, linear, and fast
response. The electronic controller is a solid-state device that
provides control over a discrete portion of the sensor range and
generates an amplified correction signal to control the final
control element. Electronic controllers provide two-position,
proportional, or proportional plus integral (PI) control.
Features of electronic control systems include:
— Controllers can be remotely located from sensors and
actuators.
— Controllers can accept a variety of inputs.
— Remote adjustments for multiple controls can be located
together, even though sensors and actuators are not.
— Electronic control systems can accommodate complex
control and override schemes.
— Universal type outputs can interface to many different
actuators.
— Display meters indicate input or output values.
The sensors and output devices (e.g., actuators, relays) used
for electronic control systems are usually the same ones used
on microprocessor-based systems. The distinction between
electronic control systems and microprocessor-based systems
is in the handling of the input signals. In an electronic control
system, the analog sensor signal is amplified, then compared
to a setpoint or override signal through voltage or current
comparison and control circuits. In a microprocessor-based
system, the sensor input is converted to a digital form, where
discrete instructions (algorithms) perform the process of
comparison and control.
Electronic control systems usually have the following
characteristics:
Controller: Low voltage, solid state.
Inputs: 0 to 1V dc, 0 to 10V dc, 4 to 20 mA, resistance
element, thermistor, thermocouple.
Outputs: 2 to 10V dc or 4 to 20 mA device.
Control Mode: Two-position, proportional, proportional
plus integral (PI), step.
Circuits in this section are general. A resistance-temperature
input and a 2 to 10V dc output are used for purposes of
discussion. Electric circuits are defined in Electric Control
Fundamentals. A detailed discussion on control modes can be
found in the Control Fundamentals section.
DEFINITIONS
NOTE: For definitions of terms not in this section, see the
Control Fundamentals section.
Authority (Compensation Authority or Reset Authority): A
setting that indicates the relative effect a compensation
sensor input has on the main setpoint (expressed in
percent).
Compensation changeover: The point at which the
compensation effect is reversed in action and changes
from summer to winter or vice versa. The percent of
compensation effect (authority) may also be changed
at the same time.
Compensation control: A process of automatically adjusting
the control point of a given controller to compensate
for changes in a second measured variable such as
outdoor air temperature. For example, the hot deck
control point is reset upward as the outdoor air
temperature decreases. Also known as “reset control”.
Compensation sensor: The system element which senses a
variable other than the controlled variable and resets
the main sensor control point. The amount of this effect
is established by the authority setting.
Control Point: The actual value of a controlled variable
(setpoint plus or minus offset).
Deviation: The difference between the setpoint and the value
of the controlled variable at any moment. Also
called offset.
Direct acting: A direct acting controller increases its output
signal on an increase in input signal.
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,
although relays and switches are often included in the
circuit.
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. Electronic devices are
primarily used as sensors. The controller usually
furnishes fixed control routines based on the logic
of the solid-state components.
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