CIBSE Guide H: Building Control Systems

Appendix A2:     Tuning rules

A2.1        The PID control loop

The basic feedback controller is illustrated in Figure A2.1. The error signal x_e is processed by a controller which produces an output signal Y, which in turn drives the controlled device, e.g. actuator and fan coil, resulting in a change in the controlled variable x. In 3.3.3.1 a description was given of how the components of the controlled device, e.g. valve and heat emitter, may be matched to linearise the heat output, resulting in an approximately linear relation between the controller output Y and the heat output to the conditioned space. However, even if the change in the output is roughly linear with controller output, the gain of the system can change with operating conditions. ...