Control loop

Claim 1

A control loop for controlling a control quantity, said control loop comprising a detection system for supplying an error signal which is indicative of the difference between the instantaneous value and the desired value of the control quantity, and a control unit having an input terminal, which is coupled to the detection system, for receiving the error signal, and an output terminal, said control unit having a transfer characteristic with a number of peaks at a fundamental frequency and harmonics thereof and including a memory device with a plurality of memory locations for storing digitally coded signal values, a digital-analog converter, coupled to the memory device, for converting the digitally coded signal values supplied by the memory device into analog signal values, means for applying said analog signal values to said output terminal, comparison means for comparing the value of a signal value read from the memory device with the value of said error signal supplied by the detection system, correction means, coupled to the memory device and to the comparison means, for correcting the signal values obtained from the memory device in dependence on the signal supplied by the comparison means, the correction applied having a maximum value which is small relative to the maximum attainable signal value, and means for controlling read out and storage of said signal values in the memory device so that a number of signal values are read out from the memory device in accordance with a recurrent cycle and, after each read out of a signal value, the corrected signal value supplied by the correction means is stored in the memory instead of the signal value previously read, the cycle having a period equal to that associated with the fundamental frequency, said loop further comprising a control device, coupled to said output terminal of said control unit, for changing the control quantity in response to said signal values.