Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL FUNDAMENTALS
33
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 3 to 15 psi. 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 2 to 15
psi, 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.