One of the strengths of direct digital control in HVAC applications is that such features as the integral mode can be incorporated in software rather than using more expensive hardware. When proportional integral (PI) control is used, it is necessary to select both the proportional and integral constants in a combination that provides responsive, yet stable, control. This paper, reports the behavior, of an experimental system with various combinations of these two constants.

Valve and damper actuators used in HVAC practice typically experience hysteresis and the integral mode would ultimately compensate for this non-linearity, but this study suggests the advisability of hysteresis compensation. This process makes a step change in the control signal when the direction of control action reverses.