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Oct
30

Control of Deadtime Dominant Processes

A small fraction of the control loops in industry are characterized by the process deadtime being dominant i.e. greater than the process time constant. In most cases the source of the process deadtime is associated with transport delay or analyzer sample time for the process measurement. In many cases the loop directly impacts final product and thus can have a significant influence on the process efficiency and product quality. For such a process, the loop response to load disturbances and setpoint changes may be slower than desired since the dominant deadtime limits the amount of reset and gain that may be applied in the loop tuning. One approach that may be taken to improve the control of a deadtime dominant process is to utilize deadtime compensation with the PID. The Smith predictor is one of the best known techniques for deadtime compensation. Also, the Dahlin algorithm has been successfully applied by the pulp and paper industry in the control of deadtime dominant processes such as the paper machine. Having confronted some difficult applications in which process deadtime was a limiting factor in the loop performance, I took some time to look into the different implementations of the Smith predictor and to compare these with the Dahlin algorithm.

Interesting enough, it turns out that mathematically the Dahlin algorithm is identical to a Smith predictor applied to a PI controller if the PI tuning is set in a specific manner. This specific tuning of the PI controller is based on the loop period of execution, process gain, time constant, deadtime and the desired closed loop time constant. Through a sight modification of the Smith predictor, it is possible to extend the use of the Smith predictor to address processes that are characterized by unmeasured disturbances that modify the process gain. Also, it is possible to structure the Smith predictor to allow control to be done using sampled process measurements e.g. composition from a gas chromatograph. These modifications of the Smith predictor are the basis of the Provox deadtime compensation PCA and the DeltaV PID_DEADTIME module template. If you have an interest in this area of control, then the mathematical analysis and derivation of the tuning to provide the response of the Dahlin algorithm using a Smith Predictor and details on the modifications used to extend the applicability of the Smith predictor can be found in the paper Modifying the Smith Predictor for an Application Software Package, T.L. Blevins, ISA National Conference, 1979.