Linear reset providing adaptive response and control action reversal of pid loops

Abstract

An HVAC system for controlling the temperature and humidity of air supplied to a comfort zone includes a heater and a cooler operating concurrently. In response to a supply air temperature sensor, a PID control loop with substantially constant gain controls the cooler to maintain the zone air at a certain comfortable temperature. The heater, also under PID control, is controlled in response to a humidistat. To prevent the heater from overloading the cooler during periods of high cooling demand, a variable gain multiplier decreases the heater's gain when the cooler's output exceeds a predetermined limit; otherwise, the heater's gain remains substantially constant. When the cooler is operating above the predetermined limit, the multiplier varies linearly between positive-one and negative-one and does so inverse-proportionally with the cooling load to smoothly change the heater's PID control from direct acting to reverse acting as the cooler approaches its maximum capacity.