Engineering Manual o.. - HVAC.Amickracing

ELECTRONIC CONTROL FUNDAMENTALSPOLYMER COATINGOSCILLATINGCIRCUITFREQUENCYMEASURINGCIRCUITTO CONTROLLERPRESSURE INLET(E.G., AIR, WATER)FLEXIBLEDIAPHRAGMQUARTZ CRYSTAL(ENLARGED VIEW)Fig. 13. Quartz Crystal Relative Humidity Sensor.PRESSURE SENSORSC3088An electronic pressure sensor converts pressure changes intoa signal such as voltage, current, or resistance that can be usedby an electronic controller.A method that measures pressure by detecting changes inresistance uses a small flexible diaphragm and a strain gageassembly (Fig. 13). The strain gage assembly includes veryfine (serpentine) wire or a thin metallic film deposited on anonconductive base. The strain gage assembly is stretched orcompressed as the diaphragm flexes with pressure variations.The stretching or compressing of the strain gage (shown bydotted line in Fig. 14) changes the length of its fine wire/thinfilm metal, which changes the total resistance. The resistancecan then be detected and amplified. These changes in resistanceare small. Therefore, an amplifier is provided in the sensorassembly to amplify and condition the signal so the level sentto the controller is less susceptible to external noise interference.The sensor thus becomes a transmitter.AMPLIFIERFLEXIBLE PLATE(TOP PORTION OFCAPACITOR)FIXED PLATE(BOTTOM PORTIONOF CAPACITOR)C3103Fig. 15. Capacitance Type Pressure Transmitters.A variation of pressure sensors is one that measuresdifferential pressure using dual pressure chambers (Fig. 16).The force from each chamber acts in an opposite direction withrespect to the strain gage. This type of sensor can measure smalldifferential pressure changes even with high static pressure.PRESSURE ACHAMBER AFLEXIBLE DIAPHRAGMSTRAIN GAGEFINE(SERPENTINE)WIRE/THINFILM METALFLEXIBLEBASECHAMBER BFLEXIBLE DIAPHRAGMPRESSURE BC3104Fig. 16. Differential Pressure Sensor.AMPLIFIERPRESSURE INLET(E.G., AIR, WATER)FLEXIBLEDIAPHRAGMAMPLIFIERCONNECTIONSTRAIN GAGE ASSEMBLYDOTTED LINE SHOWSFLEXING (GREATLY EXAGGERATED)Fig. 14. Resistance Type Pressure Sensor.C3102Another pressure sensing method measures capacitance(Fig. 15). A fixed plate forms one part of the capacitor assemblyand a flexible plate is the other part of the capacitor assembly.As the diaphragm flexes with pressure variations, the flexibleplate of the capacitor assembly moves closer to the fixed plate(shown by dotted line in Fig. 14) and changes the capacitance.CONTROLLERThe electronic controller receives a sensor signal, amplifiesand/or conditions it, compares it with the setpoint, and derivesa correction if necessary. The output signal typically positionsan actuator. Electronic controller circuits allow a wide varietyof control functions and sequences from very simple to multipleinput circuits with several sequential outputs. Controller circuitsuse solid-state components such as transistors, diodes, andintegrated circuits and include the power supply and all theadjustments required for proper control.INPUT TYPESElectronic controllers are categorized by the type or typesof inputs they accept such as temperature, humidity, enthalpy,or universal.ENGINEERING MANUAL OF AUTOMATIC CONTROL 127