Output Devices - Honeywell AUTOMATIC CONTROL Engineering Manual

ELECTRONIC CONTROL FUNDAMENTALS
Temperature Controllers
Temperature controllers typically require a specific type or
category of input sensors. Some have input circuits to accept
RTD sensors such as BALCO or platinum elements, while
others contain input circuits for thermistor sensors. These
controllers have setpoint and throttling range scales labeled in
degrees F or C.
Relative Humidity Controllers
The input circuits for relative humidity controllers typically
receive the sensed relative humidity signal already converted
to a 0 to 10V dc voltage or 4 to 20 mA current signal. Setpoint
and scales for these controllers are in percent relative humidity.
Enthalpy Controllers
Enthalpy controllers are specialized devices that use specific
sensors for inputs. In some cases, the sensor may combine
temperature and humidity measurements and convert them to
a single voltage to represent enthalpy of the sensed air. In other
cases, individual dry bulb temperature sensors and separate wet
bulb or relative humidity sensors provide inputs and the
controller calculates enthalpy. In typical applications, the
enthalpy controller provides an output signal based on a
comparison of two enthalpy measurements, indoor and outdoor,
rather than on the actual enthalpy value. In other cases, the
return air enthalpy is assumed constant so that only outdoor air
enthalpy is measured. It is compared against the assumed
nominal return air value.
Universal Controllers
The input circuits of universal controllers can accept one or
more of the standard transmitter/transducer signals. The most
common input ranges are 0 to 10V dc and 4 to 20 mA. Other
input variations in this category include a 2 to 10V dc and a 0
to 20 mA signal. Because these inputs can represent a variety
of sensed variables such as a current of 0 to 15 amperes or
pressure of 0 to 3000 psi, the settings and scales are often
expressed in percent of full scale only.
CONTROL MODES
The control modes of some electronic controllers can be
selected to suit the application requirements. Control modes
include two-position, proportional, and proportional-integral.
Other control features include remote setpoint, the addition
of a compensation sensor for reset capability, and override or
limit control.
ENGINEERING MANUAL OF AUTOMATION CONTROL
OUTPUT CONTROL
Electronic controllers provide outputs to a relay or actuator
for the final control element. The output is not dependent on
the input types or control method. The simplest form of output
is two-position where the final control element can be in one of
two states. For example, an exhaust fan in a mechanical room
can be turned either on or off. The most common output form,
however, provides a modulating output signal which can adjust
the final control device (actuator) between 0 and 100 percent
such as in the control of a chilled water valve.

OUTPUT DEVICES

Actuator, relay, and transducer (Fig. 2) are output devices
which use the controller output signal (voltage, current, or relay
contact) to perform a physical function on the final control
element such as starting a fan or modulating a valve. Actuators
can be divided into devices that provide two-position action
and those that provide modulating action.
TWO-POSITION
Two-position devices such as relays, motor starters, and
solenoid valves have only two discrete states. These devices
interface between the controller and the final control element.
For example, when a solenoid valve is energized, it allows steam
to enter a coil which heats a room (Fig. 17). The solenoid valve
provides the final action on the controlled media, steam. Damper
actuators can also be designed to be two-position devices.
ELECTRONIC
CONTROLLER
OUTPUT
TWO-POSITION
SOLENOID VALVE
Fig. 17. Two-Position Control.
MODULATING
Modulating actuators use a varying control signal to adjust
the final control element. For example, a modulating valve
controls the amount of chilled water entering a coil so that cool
supply air is just sufficient to match the load at a desired
setpoint (Fig. 18). The most common modulating actuators
accept a varying voltage input of 0 to 10 or 2 to 10V dc or a
current input of 4 to 20 mA. Another form of actuator requires
a pulsating (intermittent) or duty cycling signal to perform
modulating functions. One form of pulsating signal is a Pulse
Width Modulation (PWM) signal.
128
120V
STEAM SUPPLY
TO COIL
C3080