Engineering Manual o.. - HVAC.Amickracing

CONTROL FUNDAMENTALSThe start value EPID setpoint sets the output to a fixed valueat startup. For a VAV air handling system supply fan, a suitablevalue might be twenty percent, a value high enough to get thefan moving to prove operation to any monitoring system and toallow the motor to self cool. For a heating, cooling, andventilating air handling unit sequence, a suitable start valuewould be thirty-three percent, the point at which the heating,ventilating (economizer), and mechanical cooling demands areall zero. Additional information is available in the Air HandlingSystem Control Applications section.The error ramp time determines the time duration duringwhich the PID error (setpoint minus input) is slowly ramped,linear to the ramp time, into the PID controller. The controllerthus arrives at setpoint in a tangential manner without overshoot,undershoot, or cycling. See Figure 41.ACTUATOR POSITIONPERCENT OPEN100SETPOINTSTARTVALUE0ERRORRAMPTIMEOFFSETCONTROLPOINTT1 T2 T3 T4 T5 T6ELAPSED TIMEFig. 41. Enhanced Proportional-Integral-Derivative (EPID) Control.ADAPTIVE CONTROLM13038Adaptive control is available in some microprocessor-basedcontrollers. Adaptive control algorithms enable a controller toadjust its response for optimum control under all loadconditions. A controller that has been tuned to control accuratelyunder one set of conditions cannot always respond well whenthe conditions change, such as a significant load change orchangeover from heating to cooling or a change in the velocityof a controlled medium.An adaptive control algorithm monitors the performance ofa system and attempts to improve the performance by adjustingcontroller gains or parameters. One measurement ofperformance is the amount of time the system requires to reactto a disturbance: usually the shorter the time, the better theperformance. The methods used to modify the gains orparameters are determined by the type of adaptive algorithm.Neural networks are used in some adaptive algorithms.An example of a good application for adaptive control isdischarge temperature control of the central system cooling coilfor a VAV system. The time constant of a sensor varies as afunction of the velocity of the air (or other fluid). Thus the timeconstant of the discharge air sensor in a VAV system isconstantly changing. The change in sensor response affects thesystem control so the adaptive control algorithm adjusts systemparameters such as the reset and rate settings to maintainoptimum system performance.T7T8Adaptive control is also used in energy management programssuch as optimum start. The optimum start program enables anHVAC system to start as late as possible in the morning and stillreach the comfort range by the time the building is occupied forthe lease energy cost. To determine the amount of time requiredto heat or cool the building, the optimum start program uses factorsbased on previous building response, HVAC systemcharacteristics, and current weather conditions. The algorithmmonitors controller performance by comparing the actual andcalculated time required to bring the building into the comfortrange and tries to improve this performance by calculating newfactors.PROCESS CHARACTERISTICSAs pumps and fans distribute the control agent throughoutthe building, an HVAC system exhibits several characteristicsthat must be understood in order to apply the proper controlmode to a particular building system.LOADProcess load is the condition that determines the amount ofcontrol agent the process requires to maintain the controlledvariable at the desired level. Any change in load requires achange in the amount of control agent to maintain the samelevel of the controlled variable.Load changes or disturbances are changes to the controlledvariable caused by altered conditions in the process or itssurroundings. The size, rate, frequency, and duration ofdisturbances change the balance between input and output.Four major types of disturbances can affect the quality ofcontrol:— Supply disturbances— Demand disturbances— Setpoint changes— Ambient (environmental) variable changesSupply disturbances are changes in the manipulated variableinput into the process to control the controlled variable. Anexample of a supply disturbance would be a decrease in thetemperature of hot water being supplied to a heating coil. Moreflow is required to maintain the temperature of the air leaving thecoil.Demand disturbances are changes in the controlled mediumthat require changes in the demand for the control agent. In thecase of a steam-to-water converter, the hot water supplytemperature is the controlled variable and the water is thecontrolled medium (Fig. 42). Changes in the flow or temperatureof the water returning to the converter indicate a demand loadchange. An increased flow of water requires an increase in theflow of the control agent (steam) to maintain the watertemperature. An increase in the returning water temperature,however, requires a decrease in steam to maintain the supplywater temperature.26ENGINEERING MANUAL OF AUTOMATIC CONTROL