Disturbance Supervision in Feedback Loops

This paper treats the problem of disturbance supervision in feedback loops. Disturbances in feedback loops a,re unavoidable, but there are certain disturbances that can and should, be eliminated. These disturbances can be introduced to the control loop from an external source, but they can also be generated inside the loop by friction in the valves or actuators. A procedure to detect these disturbances is presented, as well as those actions that should be taken to remove them. The detection procedure does not have any parameters to be tuned by the operator. It is intended to be included on a supervisory level in both adaptive and constant parameter controllers. 1 fntroduction There are always disturbances in feedback loops. If not, there would not be any reason to use feedback, but the process input signal could be given a consta,nt value once and for all. The controller is normally not able to eliminate the effects of the disturbances completely. The disturba,nces introduce control errors and consequently deteriorate the control. It is therefore of interest to try to eliminate the disturbances, if possible. This can be done in different ways depending on the origin of the disturbances. If the disturbances are introduced from an external source one should of course try to remove them at the source. If this is not possible, feedforward could be tried. If the disturbances are generated by friction inside the loop, valve maintenance should be performed. The problem of load disturba¡rces is of particular importance in adaptive control. Oscillating disturbances with frequencies in the neighbourhood of the ultimate frequency is one of the most common reasons for bad performance in adaptive control. See Hägglund (1991). These disturba¡rces will detune most adaptive controllers, since the adaptive controllers interpret the oscillations as a result of a too high loop gain. It is therefore of interest to detect this situation automatically and stop the adaptation to avoid the detuning. The paper is organized as follows. In the next section, the problem of load disturbances is further discussed, and the need for disturbance detection procedures is demonstrated. Section 3 treats the problem of friction in feedback loops, and it is shown that friction can conveniently be handled in the context ofload disturba¡rces. Section 4 presents a detection procedure, which provides a detection of oscillating disturbances. The diagnosis problem, i.e. how to isolate and eliminate the disturbances, is treated in Section 5. The detection procedure is analysed in Section 6, a"nd conclusions are finally given is Section 7.