Development of Auto tuning technique for outer loop in cascaded PI system

Abstract

Auto tuning is used because user don't know the system parameters and we are tuning the system. Here cascaded system is used where outer loop controller's output will become inner loop controller's input or a command. For this system which is Variable frequency Drive with Induction motor has an inner loop controller as current controller and outer loop controller as speed controller. For inner loop, it's very easy to tune the kp and ki because it will become a first order system, for this time constant is the only thing which is required. For inner loop controller kp; ki depends on the motor parameters. For outer loop, it's difficult to tune the PI controller because it will become second order system, for this so many parameters are there which needs to be maintained, like peak overshoot, settling time, peak time, damping factor, natural frequency. For outer loop controller kp; ki depends on the damping factor and a feedback filter coeficient. The estimated PI controller tuning is good option for starting but to obtain better dynamic performance the kp; ki terms need to be tuned based on the whole mechanical system.