An automatic speed control system is disclosed in which an error signal Ve, representing the speed error of the vehicle, is added to a ramp voltage, ramping between fixed values, to produce a control signal Vc. Two comparators compare Vc with upper and lower limits, respectively, so as to produce a pulse train on one of two lines depending on the sign of the error and having a mark-space ratio depending on the magnitude of the error. A circuit responds to the first pulse to occur in the pulse train from one of the two comparators when the vehicle is accelerating towards and overshooting a new desired speed and, via a switching circuit, lowers the upper limit applied to that comparator so as to lengthen the first pulses and help to reduce overshoot of the desired speed. When the vehicle ceases accelerating, the upper limit is switched back to the desired value, and is also switched back to its original setting when the speed falls by a preset amount.
A circuit arrangement for generating a useful direct voltage signal associated with the frequency of an alternating current signal serves, in particular, to regulate the synchronism of an electric motor. It includes a comparator circuit (7) which converts the two opposite polarity half-waves of the alternating current signal of a tacho coil (2) into pulses whose edges trigger a measuring and control circuit (18) which generates direct voltages corresponding to the half-period durations of the alternating current signal measured from the respective zero passages. The comparison voltage input (9) of the comparator (7) is connected with the output of a differential amplifier (10) whose two inputs (29, 30) are each connected to a storage capacitor (27, 28) which can be connected, via associated first and second switches (80, 81), with the useful direct voltage signal (22), with the first switch (80) being used to periodically couple out the useful direct voltage signal associated with the first half-periods and the second switch (81) being used to periodically couple out the direct voltage signal associated with the second half-periods.
