Patent GB-914970-A (GB)
Title: Improvements in or relating to position indicating or repeating devices
IPC: G08C19/46
Filed: 1960-05-03
Abstract: 914,970. Measuring electrically; electric selective signalling systems. BRYANS AEROQUIPMENT Ltd., and SCOTT, A. B. May 3, 1961 [May 3, 1960], No. 15606/60. Class 40 (1). In a device for measuring relative angular or linear displacement between a movable member and a stationary member, the latter comprises three similar segments or portions each having an equal number n of poles, or teeth (i.e. capacitor plates), which are equally energized by different phases of a 3-phase supply, and the movable member has 3nŒ1 poles or teeth coupled to a single winding, the output of which is compared in a phase-comparing device with one phase of the supply. The device may be used in a repeater or a servo-control system. In Fig. 1, an annular stator comprising three segments S1, S2, S3, each comprising a ferromagnetic yoke and three poles, co-operates with a ferromagnetic rotor R having ten poles and a single winding which supplies terminals T through slip-rings. As shown, the rotor signal is in phase with phase 2 of the supply and will shift through one complete cycle while the rotor rotates through an angle equal to the rotor-pole spacing; here 36 degrees. A higher number of poles, e.g. 99 stator and 100 rotor poles, enhances the resolution of the device. Fig. 2 shows an electrostatic equivalent of Fig. 1; each stator segment comprises 3 conductive teeth x mounted in non-conductive material and connected to one phase of the supply; the non- conductive rotor carries ten conductive teeth Y all connected to a slip-ring Z. Fig. 3 (not shown) depicts a device for measuring linear displacement in which the stator is in the form of a straight bar comprising three portions each having three poles, and a straight movable member has ten poles; the three portions of the stator are repeated along its length as far as is necessary. In Fig. 6, two stators as in Fig. 1, or preferably one such stator, co-operate(s) with two rotors R1, R2, mounted on the same shaft, R1 comprising eight poles and R2 ten poles. The output of R1 is compared with the reference phase in a phase-comparator P1 and with the output of R2 in a second phase-comparator P2. Both P1 and P2 produce outputs which vary in magnitude and sign with the phase-difference of their inputs, and may be connected to a counting device. A number of devices of the Fig. 1 type may be mounted on a shaft to produce a digital output indicative of shaft displacement. For example, to obtain a binary digital code, eight devices may be used, the numbers of rotor poles of which constitute a geometrical progression whose starting number is a power of 2 and whose argument is 2; the number of stator poles is one more or less than the number of corresponding rotor poles according to whether the latter number is an odd or even power of two. The output from each rotor is fed to a phase-sensitive relay which produces an " on " or " off " signal according to whether the phase of the output leads or lags a reference phase. The Provisional Specification discloses an arrangement similar to that shown in Fig. 6 except that P1 is omitted.