Smith predictor based control of multi time-delay processes

In the process industry, the presence of time-delays is unavoidable and these delays may happen at the input, output or at any internal point of the process. As a result, the output/input relationship may present multiple time-delays. The Smith Predictor is well recognized as the best approach to deal with time delays but its use is mainly limited to process models where the time delay is separated from the rest of the model. In this paper, a comprehensive review of the control of processes with multi time-delays has been done, identifying key research gaps in the literature. In particular, the case of multi time-delays in a single forward/feedback path is an area that remains insufficiently addressed. By means of precompensators, the initial process model is transformed and reduced to single time-delay models. Then, the Smith Predictor approach can be applied being proposed to design the final control structure. Some simplifications in the join implementation of the predictor and the precompensator are suggested. The procedure is also extended to deal with MIMO plants presenting multi time-delays in the transfer matrix elements. In this way, a general solution to design the control for multi time-delayed processes is presented and some examples illustrate the procedure. Some conclusions are drafted along with open problems for future research.