Engineering Manual o.. - HVAC.Amickracing

CONTROL FUNDAMENTALSFEEDBACKSETPOINTCONTROLLERCORRECTIVESIGNALSECONDARYINPUTSelf-powered systems are a comparatively minor but stillimportant type of control. These systems use the power of themeasured variable to induce the necessary corrective action.For example, temperature changes at a sensor cause pressureor volume changes that are applied directly to the diaphragmor bellows in the valve or damper actuator.FINAL CONTROLELEMENTPROCESSMANIPULATEDVARIABLEDISTURBANCESMany complete control systems use a combination of theabove categories. An example of a combined system is thecontrol system for an air handler that includes electric on/offcontrol of the fan and pneumatic control for the heating andcooling coils.SENSINGELEMENTCONTROLLEDVARIABLEC2072Various control methods are described in the followingsections of this manual:Fig. 19. Feedback in a Closed-Loop System.In this example, the sensing element measures the dischargeair temperature and sends a feedback signal to the controller.The controller compares the feedback signal to the setpoint.Based on the difference, or deviation, the controller issues acorrective signal to a valve, which regulates the flow of hotwater to meet the process demand. Changes in the controlledvariable thus reflect the demand. The sensing element continuesto measure changes in the discharge air temperature and feedsthe new condition back into the controller for continuouscomparison and correction.Automatic control systems use feedback to reduce themagnitude of the deviation and produce system stability asdescribed above. A secondary input, such as the input from anoutdoor air compensation sensor, can provide information aboutdisturbances that affect the controlled variable. Using an inputin addition to the controlled variable enables the controller toanticipate the effect of the disturbance and compensate for it,thus reducing the impact of disturbances on the controlledvariable.CONTROL METHODSGENERALAn automatic control system is classified by the type ofenergy transmission and the type of control signal (analog ordigital) it uses to perform its functions.The most common forms of energy for automatic controlsystems are electricity and compressed air. Systems maycomprise one or both forms of energy.Systems that use electrical energy are electromechanical,electronic, or microprocessor controlled. Pneumatic controlsystems use varying air pressure from the sensor as input to acontroller, which in turn produces a pneumatic output signal toa final control element. Pneumatic, electromechanical, andelectronic systems perform limited, predetermined controlfunctions and sequences. Microprocessor-based controllers usedigital control for a wide variety of control sequences.— Pneumatic Control Fundamentals.— Electric Control Fundamentals.— Electronic Control Fundamentals.— Microprocessor-Based/DDC Fundamental.See CHARACTERISTICS AND ATTRIBUTES OFCONTROL METHODS.ANALOG AND DIGITAL CONTROLTraditionally, analog devices have performed HVAC control.A typical analog HVAC controller is the pneumatic type whichreceives and acts upon data continuously. In a pneumaticcontroller, the sensor sends the controller a continuouspneumatic signal, the pressure of which is proportional to thevalue of the variable being measured. The controller comparesthe air pressure sent by the sensor to the desired value of airpressure as determined by the setpoint and sends out a controlsignal based on the comparison.The digital controller receives electronic signals from sensors,converts the electronic signals to digital pulses (values), andperforms mathematical operations on these values. Thecontroller reconverts the output value to a signal to operate anactuator. The controller samples digital data at set time intervals,rather than reading it continually. The sampling method is calleddiscrete control signaling. If the sampling interval for the digitalcontroller is chosen properly, discrete output changes provideeven and uninterrupted control performance.Figure 20 compares analog and digital control signals. Thedigital controller periodically updates the process as a functionof a set of measured control variables and a given set of controlalgorithms. The controller works out the entire computation,including the control algorithm, and sends a signal to an actuator.In many of the larger commercial control systems, an electronicpneumatictransducer converts the electric output to a variablepressure output for pneumatic actuation of the final controlelement.ENGINEERING MANUAL OF AUTOMATIC CONTROL19