Patent GB-847158-A (GB)
Abstract: 847,158. Electric measuring systems. SOC. D'ELECTRONIQUE ET D'AUTOMATISME. Jan. 24, 1958 [Jan. 25, 1957; June 8 1957; Nov. 13, 1957], No. 2416/58. Class 40 (1). Apparatus for measuring relative displacement comprises two magnetic members, one of the members having magnetic discontinuities at uniform intervals along its length defining steps in a scale of measurement, and the other member comprising a body having two polepieces, each provided with two sets of teeth, the teeth on one polepiece being spaced from the teeth on the other polepiece by a fraction of a scale step more than an integral number of steps, the body carrying an energizing winding, and each set of teeth being provided with a pick-up coil. As shown, Figs. 1 and 3, the magnetic members comprise a rule 19 provided with two parallel rows 20, 21, of magnetic discontinuities, the rows being mutually displaced by half a scale step, and a U-shaped, laminated measuring head 1 having two legs 2, 3, each bifurcated and carrying sets of teeth 7, 15, and 8, 17, respectively, the latter sets being displaced relative to the former sets by an integral number of scale steps plus a quarter of a step. The magnetizing winding 4 on the head 1 is supplied with A.C. and the displacement of the head relative to the rule is sensed by connecting in series opposition pick-up windings 5, 13, associated with sets of teeth 7, 15 respectively, the magnitude of the resultant signal varying substantially sinusoidally, and similarly connecting, windings 6, 14, associated with sets of teeth 8, 17, to produce a second resultant signal which lags by 90 degrees on the first, these two signals being fed via transformers 22, 23 (Fig. 6), to a computer 24, to derive a D.C. signal representative of the displacement. Preferably, the teeth on head 1 are in contact with the face of rule 19. The legs may be disposed transversely on the head 1, in which case the rule 19 is provided with four rows of magnetic discontinuities. Construction of rule.-The magnetic discontinuities may, for. operation at frequencies of about 1 kc/s. and above, comprise thin, electrically-conductive screens, depressions in the rule being filled with powdered metal which is subsequently fixed by the addition of thermosetting or polymerizing resin; alternatively, the screens may be printed on the rule, powdered magnetic material being deposited between the screens to maintain the level. For lower frequency operation, the screening is replaced by the provision of air-gaps of adequate depth in the face of the rule. Fig. 9 shows part of a rule constructed of sheets 26, the top edge of each of which is profiled, as at 29, to enable gaps to be formed in the face of the rule which conduce to a sinusoidal variation of flux. The sheets 26 are embedded in a channel 27, being accurately located therein by the engagement of notches (not shown) in the lower edges of the sheets with pins in the channel 27, and then fixed by thermo-setting. resin. Sheets with a common profile may be used to provide cuboid gaps, as in Fig. 11; in this case, to achieve an approximately sinusoidal output, the teeth of each set are arranged in two groups, the spaces 39, 40 between the groups of each set being one-quarter of a step wider than the normal spacing between teeth. Modifications.-Figs. 13 and 14 (not shown), depict arrangements in which the rule has a single row of magnetic discontinuities, the sets of teeth on each leg being arranged in alignment with a space between the sets one half step wider than an integral number of steps. The teeth of each set may be arranged in groups spaced as shown in Fig. 11. Inaccuracy of indication due to interaction between the flux circuits associated with the legs 2, 3 is avoided by the arrangement shown in Fig. 15, wherein each leg has its own magnetizing winding 4<SP>1</SP>, 4<SP>2</SP> and its own flux circuit, which is closed by branches 103 on each side of the sets of teeth.