Patent GB-991907-A (GB)
Title: Apparatus for detecting and indicating the extent of relative movement
IPC: G01D5/20
Filed: 1961-02-08
Abstract: 991,907. Photo-electric position determination; measuring displacement electrically. C. R. COOKE. Feb. 8, 1962 [Feb. 8, 1961; March 20, 1961], Nos. 4732/61 and 10051/61. Headings G1A and G1N. In a device for determining the position or size of a body or the amount a body has moved a grating is arranged to be positioned and to move in association with the body and relative to a second grating to produce a pattern of interference fringes to give an output whose phase relative to a reference signal gives the desired information. The gratings may be optical and sensed photo-electrically or magnetic read by a magnetic field pattern sensitive head. The device is preferably used to give the last two decimal places of the dimension to be determined and other means provided to provide a coarser indication. Fig. 1 shows the principle of one optical form of device according to the invention using three lamps (e.g. neon tubes) 1, 2, 3 fed from the different phases of a three phase supply. The lamps are separated by screens and individually illuminate slots 4, 5, 6, adjacent which moves grating 8 positioned in accordance with the body. Close to grating 8 and in the path of light beams from the lamps 1, 2, and 3 is a fixed grating 9 and a set of three slots 11, 12, 13 similar, but at right angles, to slots 4, 5, 6. The light passing through slots 10, 11 and 12 falls on groups fo cells a<SP>1</SP>, a<SP>11</SP>, a<SP>111</SP>; b<SP>1</SP>, b<SP>11</SP>, b<SP>111</SP>; and c<SP>1</SP>, c<SP>11</SP>, c<SP>111</SP>. The outputs from the three cells in each group are combined vectorially and the rulings of the two gratings differ in inclination so as to produce an interference fringe pattern parallel to a diagonal of the photo-cell group pattern and having a fringe spacing equal to the length of a side of this square pattern measured along the side. Thus if one lamp of fixed intensity was used to illuminate slots 4, 5 and 6 the outputs of the three photo-cell groups would be the same and would remain constant regardless of any movement of fringe pattern. The use of three lamps coupled to a three-phase supply enables an output signal, derived by combining the three cell group outputs in a three-phase transformer, to be obtained whose phase relative to the lamp supply is dependent on the position of the fringe pattern and thus of grating 8. It may be necessary to double the frequency of the supply before application as a reference signal to the phasemeter. The three cells in each group may comprise one long cell if preferred. The angle of the reference grating may be automatically varied over a small range to compensate for the effect of temperature variations. To correct for any irregularities in the movement of scale 8 the system may be duplicated with the two sets of fringes moving parallel to crossed-diagonals to cancel errors out. Beam splitting arrangements maybe provided to use the same light sources and/ or group of cells for each system. An arrangement in which a three phase detector system output is converted to a single phase signal and then back to a three phase signal again in order to compensate for difference in the light output of the lamps in described with reference to Fig. 5 (not shown). An alternative way of obtaining "three-phase" illumination is suggested using a steady source behind a cylinder or disc carrying sinusoidally graduated bands printed on it. The quantity of light directed towards each cell it illuminates by the or each source should be the same and this may be achieved by the use of collimating lenses, by the insertion of stops of suitable sizes in the light paths and by controlling the voltages applied to the lamps. Alternatively compensation for any inequality can be arranged by modification of the cell output circuits. Variation in amplitude between one detector phase signal on another can be compensated by the use of limiters, automatic gain control or negative feedback in the amplifiers. The construction of a height gauge incorporating the invention is described with reference to Figs. 7 and 10 inclusive (all not shown). In the gauge the height determining probe is coupled to a moving grating arranged in a system similar to that of Fig. 1. The strip carrying the grating may also carry a digital code read photo-electrically to give an unambiguous coarse positional indication. In a modification (Figs. 11 to 13 not shown) the grating is wider and co-operates with two inspection systems to reduce errors as previously described. In this arrangement the probe is suspended by a steel tape passing over a pulley to a counterweight and the pulley incorporates the coarse position determining code marks. The pulley is coupled to a knob for manually positioning the probe through a spiral spring and means are provided for indicating the pressure of the probe on the test body. In the application of the invention to a dividing head the table 130 of the head carries a ruled stub tape 134 closely adjacent to transmissive index grating 135. Light from lamps 138 passes through slots and collimating lenses to prism 143. It illuminates the gratings and is reflected back from the steel tape through prism 143 and passes through lenses 141 to respective ones of cells 142.