Digital controllers - Intelligent Buildings and Building Automation - page 25

￿ Control signal from the controller, where 0-10 V and 4-20 mA DC are
the most common.
The actuator may be fully modulating, where the position of the actuator
is proportional to the control signal, or tristate, where the motor may be
driven in either direction or stopped. Most actuators exhibit some degree
of hysteresis, and the relation between control signal and actuator position
depends on the direction of travel. Some actuators have the facility to provide
a positional feedback signal, indicating the actual position of the actuator.
2.7.2 Pneumatic actuators
Pneumatic actuators comprise a piston or diaphragm to which air pressure
is applied to provide a linear displacement. A mechanical linkage is required
where it is desired to produce a rotary movement, e.g. for damper control.
The construction of the actuator and its method of connection to the valve
or damper determine the direction of operation. Most pneumatic actuators
are of the single-action type where the force on the diaphragm is opposed
by a spring and the net force applied to the valve or damper is the differ-
ence between them. When the air pressure is removed the spring will return
the valve to the selected extreme position and this may be used for fail-safe
requirements.
Pneumatic controllers provide reliable and fast operation and are still used
in the HVAC industry. For new installations dominated by direct digital
control (DDC), pneumatic systems are now installed only in special situ-
ations. Where an existing pneumatic control system is being upgraded to
DDC control, it is possible to retain pneumatic operation of the actuators by
using hybrid electro-pneumatic transducers which use pneumatic power to
provide the operating force, but whose position is controlled by a standard
electronic signal.
References
Borer, J. R. and Reynolds, A. J. (1994) Building Management and Communication
Systems , Uxbridge: Brunel University.
Butcher, K. and Yarham, R. (2000) CIBSE Guide H - building control systems .
Oxford: Butterworth-Heinemann.
Dorf, R. C. and Bishop, R. H. (1998) Modern Control Systems , 8th edn, Menlo Park,
California: Addison-Wesley.