Frequency-analog speed control

An electronic method of control that has been known since the early thirties is that of the phase-lock-loop technique. In this method the phase angle of a periodic signal whose frequency corresponds to the value of the quantity to be controlled is continuously compared with the phase angle of an applied signal whose frequency represents the specified value. A change in the phase difference between the two produces an error signal that counteracts the change; in this way the two frequencies are permanently synchronized and the quantity to be controlled is kept to the required value. A control system of this type, in which the quantities are represented in the form of a frequency, is referred to as frequency-analog control at Philips Forschungslaboratorium Hamburg [11. The principle of frequency control is convenient for the control of many kinds of quantity, but is particularly suitable for controlling the speed of electric motors, where the quantity to be controlled is essentially periodic in nature. We shall start by way of introduetion with such a system; see the diagram of fig. J. A voltage Vrer of sinusoidal waveform and angular frequency Wrer (or the sum of a number of signals of different frequencies) is multiplied by a voltage of sinusoidal waveform and angular frequency W in a multiplier circuit M. The output voltage of the multiplier is applied through a lowpass filterLP to an oscillator VCO whose frequency can be controlled by a voltage; this in turn supplies the voltage V of angular frequency w. The multiplication produces a voltage Va at the output of the multiplier, given by: