Relay Self-tuning of Industrial PID Temperature Controller with Set-Point Weighting

Relay self-tuning of PID loops with on-off PWM control and noovershoot plus short settling time requirements is considered. Temperature loops in electric heating and cooling are typical examples. No-overshoot can be provided by I-PD structure of PID controller and short settling time by set-point weighting. No user parameters are allowed. The approach is restricted to lag plants, whose dynamics is evaluated by the shift between relay control and plant output during ultimate oscillation. Gain and phase margins for calculation of PID [1-3], as well as set-point weight, are selected in terms of this shift. Asymmetry of relay control is compensated by modification of the oscillation period. The self-tuner has been implemented in low-cost temperature controllers.