Patent GB-1325583-A (GB)
Title: Apparatus for sensing a quantity
IPC: G01R25/00
Filed: 1970-10-14
Abstract: 1325583 Measuring phase shift PYE Ltd 1 Oct 1971 [14 Oct 1970] 48846/70 Heading G1U In a system for measuring the phase shift produced in a reference signal by a transducing arrangement, pulses are produced whose durations depend upon both the frequency of the reference signal and the magnitude of the phase shift, these pulses are integrated to produce a corresponding amplitude significant voltage which is used to control the frequency of the reference signal whereby the system eventually assumes a stable condition in which the frequency of the reference signal is proportional to the phase shift produced by the transducing arrangement. As described a reference signal from an oscillator 30 has its frequency divided by ten, 33, and then by four, 39, 40 to produce, via two resistors R1, R2, quadrature signals which are applied to a transducing arrangement comprising flux-gates X, Y and an adder 45. The flux-gates are arranged to attenuate the input signals applied to them by factors cos # and sin#, where # is a parameter to be measured, and the resulting signals are summed 45 to produce, on a lead 48, a signal of the same frequency as that on a line 43 but shifted in phase by an amount #. This signal is appropriately filtered 49 and amplified 51, and applied to a monostable device 53 to produce short pulses whose leading edges are adapted to reset a trigger 63. The reference signal from the oscillator 30, after division by ten, 33, and by four, 39, 40, is divided 55 by a further factor n, having the value sixteen in the preferred embodiment, and then applied to a monostable device 61 which produces a pulse of a fixed duration the trailing edge of which is adapted to turn the trigger 63 on. Thus, once within, and at a fixed time after the beginning of, a cycle equal to sixteen cycles of the signal on line 43, the trigger 63 is turned on (by the trailing edge of the pulse from the monostable 61) and it stays on until the next pulse is produced by monostable 53. Thus the 'on' time of trigger 65 depends inversely upon the frequency of oscillator 30 and directly upon the phase shift #. This time is integrated 65, C3, to produce a signal ranging from 0 to 4V on a line 31 which determines the frequency of the oscillator 30, and hence eventually a stable condition is attained. The frequency of the oscillator may then be determined by any suitable means to provide a measure of the phase shift #. In the preferred embodiment the pulse produced by monostable 61 is 6À3 msecs long, which is the duration of nine cycles of the signal on line 43 when oscillator 30 receives a zero volt control signal on lead 31 and therefore has its lowest frequency. Thus, trigger 63 is turned on 6À3 msecs after each pulse emitted from divider 55, and it stays on until the tenth pulse from monostable 53 occurs after the pulse from divider 55. It can therefore be seen that trigger 63 is on for a duration of (400/f - 0 - 6À3) msecs following each pulse from the divider 55, where f is the frequency of the oscillator in kc/s. The circuit described also includes gates 66, 67 and voltage clamps 69, 71 which are so arranged that when the duration for which trigger 63 is on approaches zero the line 31 is momentarily clamped at 4v, and when the duration approaches a value which would drive the line 31 to 4v the line is momentarily clamped at 0V. This feature is of use when the flux-gates X, Y are determining the direction of the earth's magnetic field, since a rotation of the direction of the field through an angle of (2# + #) is clearly equivalent to one of #.