Self-tuning

The self-tuning algorithm is based on the classic inflectional tangents method. This method was first developed by Ziegler and Nichols. It is assumed that a linear P-T1 loop with a dead time is being examined. The maximum rate of change is determined following an experimental step. This is achieved through examining the differences over a number of samples. A tangent is constructed to the point where the rate of change is a maximum, and its intersection with the time axis is found. The delay time, Tu, is the time from the start of the measurement up to the intersection of the inflection tangent and the time axis. Knowing Tu and Vmax, the Chien, Hrones and Reswick formula yields the controller parameters for suppression of disturbances with 20% overshoot. The parameters for the pre-controller can easily be derived from the parameters for the main controller with the aid of heuristic formulas. After completion of the self-tuning these parameters are used in an automatic switch to closed loop operation.