Patent GB-1472876-A (GB)
Title: Position resolver
IPC: G01D5/36
Filed: 1974-05-13
Abstract: 1472876 Measuring displacement electrically photo-electrically CONTRAVES AG 13 May 1974 [14 May 1973] 21162/74 Headings G1A and G1N Displacement measuring apparatus comprises a scale and a scale reader relatively movable, and means for algebraically counting and storing the number of uniform-increment graduations representing the scale displacement; the scale further comprising absolute value marks each characterized in respect of its absolute position by the number of increments separating it from the next absolutevalue mark; means for detecting the passage of an absolute-value mark; and means to determine the absolute displacement by means of the number of increments counted since the previous passage of an absolute-value mark, and thereby to set or correct the counter. An optical grid 11, Fig. 1 marked on a displaceable scale carrier 10 co-operates with a fixed grid (not shown) to produce a MoirÚ pattern detectable as phase-variable signals by photo-cells 111-113 which transmit currents to a transformation circuit 13 arranged to signal the number and sense ŒN a  of displacement increments E to a reversible counter 16 in a known manner. Other forms of the incremental measurement system may include, for example, a uniformly-spaced array of magnetic elements with an electro-magnetic reader. The phase-variable signals from the reader may also be used for fine interpolation between incremental divisions. Absolute-value marks MA alternating with semitransparent areas on a further scale 12 of the carrier 10 comprise identical patterns of bright and dark areas, each pattern co-operable with a corresponding pattern 120 of photo-cells 121-127 to produce a positive pulse I ao  in a differential amplifier 17 only when the pattern of the mark MA and that of the photo-cells 120 coincide, for the avoidance of false operation or failure due, e.g. to dirt on the scale. The point of recognition BZ n  of an absolute mark is separated from that of its neighbour BZ n   +   1  by a number D a  of increments which varies from mark to mark and which can therefore be used to identify the absolute position of that mark. Thus, e.g., for spanning a scale of 500 increments a total of 15 major divisions could be defined by marks arranged at incremental intervals of 26, 41, 27, 40 ... ... 32, 34, 33, the intervals uniquely characterizing absolute positions of 26, 67, 94, 134 ... ... 433, 467, 500 respectively. If a given scale of N t  increments could be divided so as to include every possible integral interval once only, the number of such intervals could approach #2N t . In Fig. 2 the counter 16 displays the number of increments ZA signalled by the transformation circuit 13, which number is compared at 22 with an absolute value in a store, 21, determined at the next previous passage of an absolute mark. The output D a  of the comparator 22 at the consecutive passage of an absolute mark is gated at 23 by a pulse Ia 1  into an input store 240 of an association device 24 comprising a read-only memory which, for every such interval D a  emits a new absolute-value signal N a . The new signal N a , via a set line S x  on a reset line R x  is gated at 26 or 27 by a delayed pulse Ia2 to update the store 21 and to set or reset the counter 16. Thus, e.g., if an incremental graduation is accidentally missed, or if the count is erased by a temporary power failure, the counter 16 will be corrected, at the latest, after the next correct counting of an interval separating consecutive marks. If, after the passage of an absolute mark, positive and negative displacements combine to return the scale to the same mark, the net value of D a  will return to zero, and no change of N a  will be signalled. In a simpler embodiment Fig. 3 (not shown) the store 21 and comparator 22 are omitted, the counter (16) being reset to zero at the passage of the first absolute mark following power restoration. The passage of the next absolute mark, if different from the first, then gates the counter (16) contents, equal to D a , into an association device (24) which feeds the new absolute position N' a  back to the counter (16). With this embodiment no further corrections are possible unless a further power interruption occurs.