Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

Page: 137 / 512

ELECTRONIC CONTROL FUNDAMENTALS
ENGINEERING MANUAL OF AUTOMATIC CONTROL
127
Fig. 13. Quartz Crystal Relative Humidity Sensor.
PRESSURE SENSORS
An electronic pressure sensor converts pressure changes into
a signal such as voltage, current, or resistance that can be used
by an electronic controller.
A method that measures pressure by detecting changes in
resistance uses a small flexible diaphragm and a strain gage
assembly (Fig. 13). The strain gage assembly includes very
fine (serpentine) wire or a thin metallic film deposited on a
nonconductive base. The strain gage assembly is stretched or
compressed as the diaphragm flexes with pressure variations.
The stretching or compressing of the strain gage (shown by
dotted line in Fig. 14) changes the length of its fine wire/thin
film metal, which changes the total resistance. The resistance
can then be detected and amplified. These changes in resistance
are small. Therefore, an amplifier is provided in the sensor
assembly to amplify and condition the signal so the level sent
to the controller is less susceptible to external noise interference.
The sensor thus becomes a transmitter.
Fig. 15. Capacitance Type Pressure Transmitters.
A variation of pressure sensors is one that measures
differential pressure using dual pressure chambers (Fig. 16).
The force from each chamber acts in an opposite direction with
respect to the strain gage. This type of sensor can measure small
differential pressure changes even with high static pressure.
QUARTZ CRYSTAL
(ENLARGED VIEW)
POLYMER COATING
TO CONTROLLER
OSCILLATING
CIRCUIT
FREQUENCY
MEASURING
CIRCUIT
C3088
PRESSURE INLET
(E.G., AIR, WATER)
FLEXIBLE
DIAPHRAGM
AMPLIFIER
STRAIN GAGE ASSEMBLY
DOTTED LINE SHOWS
FLEXING (GREATLY EXAGGERATED)
C3102
AMPLIFIER
CONNECTION
FINE
(SERPENTINE)
WIRE/THIN
FILM METAL
FLEXIBLE
BASE
PRESSURE INLET
(E.G., AIR, WATER)
FLEXIBLE
DIAPHRAGM
FLEXIBLE PLATE
(TOP PORTION OF
CAPACITOR)
FIXED PLATE
(BOTTOM PORTION
OF CAPACITOR)
AMPLIFIER
C3103
CHAMBER B
FLEXIBLE DIAPHRAGM
C3104
PRESSURE A
CHAMBER A
FLEXIBLE DIAPHRAGM
STRAIN GAGE
PRESSURE B
Fig. 14. Resistance Type Pressure Sensor.
Another pressure sensing method measures capacitance
(Fig. 15). A fixed plate forms one part of the capacitor assembly
and a flexible plate is the other part of the capacitor assembly.
As the diaphragm flexes with pressure variations, the flexible
plate of the capacitor assembly moves closer to the fixed plate
(shown by dotted line in Fig. 14) and changes the capacitance.
Fig. 16. Differential Pressure Sensor.
CONTROLLER
The electronic controller receives a sensor signal, amplifies
and/or conditions it, compares it with the setpoint, and derives
a correction if necessary. The output signal typically positions
an actuator. Electronic controller circuits allow a wide variety
of control functions and sequences from very simple to multiple
input circuits with several sequential outputs. Controller circuits
use solid-state components such as transistors, diodes, and
integrated circuits and include the power supply and all the
adjustments required for proper control.
INPUT TYPES
Electronic controllers are categorized by the type or types
of inputs they accept such as temperature, humidity, enthalpy,
or universal.