Online time delay and disturbance compensation for linear non minimum phase systems

Disturbances often occur in real systems and this has a negative effect on system stability and performance. In the past, a number of remedies have been proposed to enhance the stability and performance characteristics of feedback control systems. The classical disturbance observer estimates disturbances acting on the system utilizing a proper inverse model and eliminates the disturbance from the control channel. However, the model inversion for non minimum phase systems leads to unstable control loops, which require a special treatment for the right half plane zeros. This undesired situation narrows down both the simplicity and the capabilities of disturbance observer. Although researchers try to make the system robust by using more complex controllers due to restrictive effect of classical disturbance observers, the designed controllers often achieve one control target, making the system robust against disturbances yet sacrificing other control objectives. In addition to external disturbances, inherent time delays are also inevitable facts observed in dynamic systems, and similar to disturbances, they disrupt the system's stability and deteriorate its operation. Smith predictor is often used to restore the stability of such systems. In this approach, negative feedback is made from the controller output to input by using delay time model and the delay becomes a multiplier of the delay free closed loop transfer function. However, in order to design the delay time model, the actual delay time must be measured precisely, which is usually not possible in practice. In this study, both the disturbance observer for non minimum phase systems and the adaptive Smith predictor design for systems with time delay are proposed to eliminate the negative effects of disturbances and time delays, concurrently. According to the results, it is seen that the controller alone is not capable of maintaining the stability under time delay and disturbances. On the other hand, for non minimum phase and time delay systems, the response of the system is stable and it resembles the nominal system behavior with proposed time delay and disturbance estimation methods.