Selecting PID Control Gains for Nonlinear HVAC&R Systems

While the dynamics of most HVAC&R processes are inherently nonlinear, a majority of HVAC&R systems utilize simple linear controllers, such as the proportional-integral-derivative (PID) controller. Although PID controllers are a time-tested and effective control strategy, most are improperly tuned or tuned for only one operating condition. As a result, these systems operate inefficiently and often exhibit poor performance. This study addresses the challenges of tuning PID controllers for HVAC&R systems. This paper presents recent advances in PID control design and extends and adapts these techniques for nonlinear HVAC&R systems. Two examples-namely temperature control of a variable air volume (VAV) unit, and evaporator superheat control using an electronic expansion valve-illustrate the efficacy of the techniques presented here.