The design of autotune controllers based on frequency domain identification

An alternative approach to the automatic tuning of PID controller parameters, in which closed-loop operation is maintained throughout, is investigated. The approach uses frequency-domain system identification techniques to estimate the open-loop frequency response of the process from data obtained while the loop is closed. Periodic, multiharmonic excitation signals are used; these allow automated identification to be carried out, with only modest calculations required by the controller hardware. Controller tuning is on the basis of a nonparametric process model, namely a frequency response. Stochastic disturbances within the loop can be countered somewhat with amplitude selection of the perturbation signal, and with nonparametric smoothing. The validity of the model at the frequency (or frequencies) of interest is checked by estimating the coherence function.