Engineering Manual o.. - HVAC.Amickracing

ELECTRONIC CONTROL FUNDAMENTALSELECTRONICCONTROLLER+OUTPUT-TRANSDUCERFig. 18. Modulating Control.In some applications, a transducer converts a controller outputto a signal that is usable by the actuator. For example, Figure 19shows an Electronic-to-Pneumatic (E/P) transducer: electronicto-pneumaticthat converts a modulating 2 to 10V dc signal fromthe electronic controller to a pneumatic proportional modulating3 to 13 psi signal for a pneumatic actuator.ELECTRONICCONTROLLER+OUTPUT–POWER2-10V dcMODULATINGVALVE ACTUATORCHILLED WATERSUPPLY TO COILC308120 PSIAIR SUPPLYE/PTRANSDUCER3-13 PSIPNEUMATICVALVEACTUATORVARIABLE RESTRICTION CONTROLLEDBY E/P TRANSDUCER ELECTRONICSC3082Fig. 19. Electric-to-Pneumatic Transducer.MINDICATING DEVICEAn electronic control system can be enhanced with visualdisplays that show system status and operation. Many electroniccontrollers have built-in indicators that show power, input signal,deviation signal, and output signal. Figure 20 shows some typesof visual displays. An indicator light can show on/off status or,if driven by controller circuits, the brightness of a light canshow the relative strength of a signal. If a system requires ananalog or digital indicating device and the electronic controllerdoes not include this type of display, separate indicating devicescan be provided.LED PANEL1 2 3 . 4DIGITALDISPLAY02040 60ANALOG METERFig. 20. Indicating Devices.80100C3089INTERFACE WITH OTHER SYSTEMSIt is often necessary to interface an electronic control deviceto a system such as a microprocessor-based building managementsystem. An example is an interface that allows a buildingmanagement system to adjust the setpoint or amount of reset(compensation) for a specific controller. Compatibility of the twosystems must be verified before they are interconnected.ELECTRONIC CONTROLLER FUNDAMENTALSGENERALThe electronic controller is the basis for an electronic controlsystem. Figure 21 shows the basic circuits of an electroniccontroller including power supply, input, control, and output.For greater stability and control, internal feedback correctioncircuits also can be included, but are not discussed. The circuitsdescribed provide an overview of the types and methods ofelectronic controllers.INPUTPOWERSENSORSREMOTESETPOINTPOWERSUPPLYCIRCUITINPUTCIRCUITRESET-COMPENSATIONSENSORREGULATEDVOLTAGESINPUTDEVIATIONSIGNALCONTROLCIRCUITHIGH/LOWLIMITSENSORSCONTROLSIGNAL(INTERNAL)OVERRIDECONTROLOUTPUTCIRCUITFig. 21. Electronic Controller Circuits.OUTPUT TOACTUATOR ORACCESSORYDEVICEC3087POWER SUPPLY CIRCUITThe power supply circuit of an electronic controller providesthe required voltages to the input, control, and output circuits.Most voltages are regulated dc voltages. The controller designdictates the voltages and current levels required.All power supply circuits are designed to optimize both lineand load regulation requirements within the needs and constraintsof the system. Load regulation refers to the ability ofthe power supply to maintain the voltage output at a constantvalue even as the current demand (load) changes. Similarly,line regulation refers to the ability of the power supply to maintainthe output load voltage at a constant value when the input(ac) power varies. The line regulation abilities or limitations ofa controller are usually part of the controller specificationssuch as 120V ac +10%, –15%. The degree of load regulationinvolves the end-to-end accuracy and repeatability and is usuallynot explicitly stated as a specification for controllers.ENGINEERING MANUAL OF AUTOMATIC CONTROL 123