Fault detection in HVAC systems using model-based feedforward control

This paper describes how a model-based feedforward control scheme can improve control performance over traditional PI(D) and detect faults in the controlled process. The scheme uses static simulation models of the system under control to generate feedforward control action, which supplements a conventional PI(D) feedback loop. The feedforward action reduces the effect of plant non-linearity on control performance and provides more consistent disturbance rejection as operating conditions change. In addition to generating feedforward control action, the models act as a reference of correct operation. Faults that occur in the system under control cause the PI(D) loop to provide a greater than normal control action to compensate for fault-induced discrepancies between the feedforward models and the controlled process. By monitoring the level of feedback compensation, faults can be detected in the controlled process. The paper presents results from testing the controller with a simulated dual-duct air-handling unit. We also discuss recent experiences of implementing the control scheme in a real building using the BACnet communication protocol.