fig3 comprises the same capacitor 37b for generating the control signal for the actuator 13 during locking in and for the integrator of the pid controller after locking in this has the advantage that the number of capacitors is reduced another advantage is that at the closing instant the integrator has already been charged to the correct value so that no undesirable transient effect arises resulting in a very stable lock in behaviour moreover since the input signal of the differentiator 34a is already available during the locking in closing of the control loop will have no adverse effect on the differentiator fig4 shows another embodiment of the control circuit 30 elements corresponding to those in fig3 bearing the same reference numerals in this embodiment the circuits for obtaining the proportional and the differentiating action are combined in one circuit for this purpose a resistor 34c is arranged in parallel with the rc circuit 34b moreover a diode 50 is arranged in parallel across the switch 43 if the operating range b is traversed in the direction indicated by the arrow 12 this results in the output signal of the circuit comprising the elements 34a 34b and 34c being applied to the actuator 13 via the diode 50 indeed on account of the steep slope of the fe characteristic in the operating range b the differentiating term which then has a positive sign is predominant over the proportional term the transfer of the differentiating term via the diode 50 while the operating range b is traversed has the advantage that the rate at which the operating range b is traversed is limited so that.