Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL FUNDAMENTALS
33
Applying the fluid jet principle allows the measurement of
very small changes in air velocity that a differential pressure
sensor cannot detect. A jet of air is emitted from a small tube
perpendicular to the flow of the air stream to be measured. The
impact of the jet on a collector tube a short distance away causes
a positive pressure in the collector. An increase in velocity of
the air stream perpendicular to the jet deflects the jet and
decreases pressure in the collector. The change in pressure is
linearly proportional to the change in air stream velocity.
Another form of air velocity sensor uses a microelectronic
circuit with a heated resistance element on a microchip as the
primary velocity sensing element. Comparing the resistance of
this element to the resistance of an unheated element indicates
the velocity of the air flowing across it.
PROOF-OF-OPERATION SENSORS
Proof-of-operation sensors are often required for equipment
safety interlocks, to verify command execution, or to monitor
fan and pump operation status when a central monitoring and
management system is provided. Current-sensing relays,
provided with current transformers around the power lines to
the fan or pump motor, are frequently used for proof-of-
operation inputs. The contact closure threshold should be set
high enough for the relay to drop out if the load is lost (broken
belt or coupling) but not so low that it drops out on a low
operational load.
Current-sensing relays are reliable, require less maintenance,
and cost less to install than mechanical duct and pipe devices.
TRANSDUCERS
Transducers convert (change) sensor inputs and controller
outputs from one analog form to another, more usable, analog
form. A voltage-to-pneumatic transducer, for example, converts
a controller variable voltage input, such as 2 to 10 volts, to a
linear variable pneumatic output, such as 20 to 100 kPa. The
pneumatic output can be used to position devices such as a
pneumatic valve or damper actuator. A pressure-to-voltage
transducer converts a pneumatic sensor value, such as 15 to
100 kPa, to a voltage value, such as 2 to 10 volts, that is
acceptable to an electronic or digital controller.
CONTROLLERS
Controllers receive inputs from sensors. The controller
compares the input signal with the desired condition, or setpoint,
and generates an output signal to operate a controlled device.
A sensor may be integral to the controller (e.g., a thermostat)
or some distance from the controller.
Controllers may be electric/electronic, microprocessor, or
pneumatic. An electric/electronic controller provides two-
position, floating, or modulating control and may use a
mechanical sensor input such as a bimetal or an electric input
such as a resistance element or thermocouple. A microprocessor
controller uses digital logic to compare input signals with the
desired result and computes an output signal using equations
or algorithms programmed into the controller. Microprocessor
controller inputs can be analog or on/off signals representing
sensed variables. Output signals may be on/off, analog, or
pulsed. A pneumatic controller receives input signals from a
pneumatic sensor and outputs a modulating pneumatic signal.
ACTUATORS
An actuator is a device that converts electric or pneumatic
energy into a rotary or linear action. An actuator creates a change
in the controlled variable by operating a variety of final control
devices such as valves and dampers.
In general, pneumatic actuators provide proportioning or
modulating action, which means they can hold any position in
their stroke as a function of the pressure of the air delivered to
them. Two-position or on/off action requires relays to switch
from zero air pressure to full air pressure to the actuator.
Electric control actuators are two-position, floating, or
proportional (refer to CONTROL MODES). Electronic
actuators are proportional electric control actuators that require
an electronic input. Electric actuators are bidirectional, which
means they rotate one way to open the valve or damper, and
the other way to close the valve or damper. Some electric
actuators require power for each direction of travel. Pneumatic
and some electric actuators are powered in one direction and
store energy in a spring for return travel.
Figure 54 shows a pneumatic actuator controlling a valve. As
air pressure in the actuator chamber increases, the downward force
(F1) increases, overcoming the spring compression force (F2),
and forcing the diaphragm downward. The downward movement
of the diaphragm starts to close the valve. The valve thus reduces
the flow in some proportion to the air pressure applied by the
actuator. The valve in Figure 54 is fully open with zero air pressure
and the assembly is therefore normally open.