Auto-tuning cascade loops subject to saturation

Auto-tuning of outer-loop (master) controllers for cascade feedback loops runs into considerable problems if the inner-loop (slave) controller is liable to saturate. We explain how these problems arise, and propose a modified auto-tuning procedure to overcome these problems. Two enhancements of the inner-loop controller are proposed, which prevent this controller from saturating, and a correspondingly modified method of analysing the resulting data and determining PID controller parameters is presented. The efficacy of these proposals is illustrated on a realistic example.