Robust PI and PID controllers including Smith predictor structure

In this paper PI and PID controllers augmented by a Smith predictor structure are compared to ordinary PI and PID controllers, for plants with medium and great time delays. The comparison is based an a systematic evaluation method, where both robustness and performance aspects in different frequency regions are taken into account. The results show that a Smith predictor increases performance somewhat for a PID controller when the time delay is moderate. For great time delays, however, the improvement is insignificant. An important remark is that it is necessary to include the predictor structure in the design to get acceptable robustness properties. For a PI controller the benefits from a predictor are mostly small or negligible independent of time delay. Most important is, however that a derivative part obviously will bring significantly greater improvement than a Smith predictor to a system with a PI controller. This result is somewhat surprising, since Smith predictors are normally used together with PI and not with PID controllers. Furthermore, it is sometimes argued that it is no use to include derivative action for plants with significant time delay.