HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New MillenniumFigure 2-2. Digital control closed-loop system.Proportional Zone ControlConsider proportional zone temperature control. The zone temperatureis sensed and compared to a setpoint. If the temperature is notat the setpoint, a control action is taken to add heat or cooling to thezone. Then, the temperature is sensed again.A thermostat with electronic or digital circuitry provides the necessaryinterface for digital control. It may be a microprocessor-baseddevice that implements a sophisticated control loop, or sequence, and iscapable of communications over a local area control network (LACN).The control sequences can expand on the basic proportional temperaturecontrol and include integral derivative control. In this case, theintegral is used to calculate the amount that the temperature is from thesetpoint. The control action is limited to avoid overshooting the setpointand the oscillations that cause delays in control response. These delaysoften occur with proportional control.Derivative control is often used with dynamic applications such aspressure control. Derivative control is used to measure the change ofspeed in the controlled condition and it adjusts the action of the controlalgorithm to respond to this change.©2001 by The Fairmont Press, Inc. All rights reserved.