Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual page 44

CONTROL FUNDAMENTALS
DIAPHRAGM
AIR
PRESSURE
SPRING
FLOW
Fig. 54. Typical Pneumatic Valve Actuator.
A pneumatic actuator similarly controls a damper. Figure 55
shows pneumatic actuators controlling normally open and
normally closed dampers.
NORMALLY
OPEN DAMPER
AIR
PRESSURE
ACTUATOR
SPRING
PISTON
Fig. 55. Typical Pneumatic Damper Actuator.
ACTUATOR
CHAMBER
F1
F2
VALVE
C2086
NORMALLY
CLOSED DAMPER
AIR
PRESSURE
ACTUATOR
ROLLING
DIAPHRAGM
C2087
Electric actuators are inherently positive positioning. Some
pneumatic control applications require accurate positioning of
the valve or damper. For pneumatic actuators, a positive
positioning relay is connected to the actuator and ensures that
the actuator position is proportional to the control signal. The
positive positioning relay receives the controller output signal,
reads the actuator position, and repositions the actuator
according to the controller signal, regardless of external loads
on the actuator.
Electric actuators can provide proportional or two-position
control action. Figure 56 shows a typical electric damper
actuator. Spring-return actuators return the damper to either
the closed or the open position, depending on the linkage, on a
power interruption.
ACTUATOR
CRANK ARM
Fig. 56. Typical Electric Damper Actuator.
AUXILIARY EQUIPMENT
Many control systems can be designed using only a sensor,
controller, and actuator. In practice, however, one or more
auxiliary devices are often necessary.
Auxiliary equipment includes transducers to convert signals
from one type to another (e.g., from pneumatic to electric),
relays and switches to manipulate signals, electric power and
compressed air supplies to power the control system, and
indicating devices to facilitate monitoring of control system
activity.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
34
DAMPER
PUSH ROD
C2721