Device for determining the exact position of two members linearly displaceable relative to each other

Claim 1

Apparatus for determining the position of one of a pair of linearly displaceable members relative to the other, said members having contiguous horizontal surfaces, comprising (a) a horizontal scale (3) secured to one of said members adjacent said horizontal surfaces, said scale having a horizontal scale surface (15) adjacent the other of said members; (b) electrooptical scanning head means (13); (c) holder means (9) pivotally connected with the other of said members for supporting said scanning head means adjacent said scale horizontal surface, said holder means being pivotable about a horizontal pivot axis (8) which is normal to a vertical plane which contains the axis of linear displacement of said members; (d) spring means (12) pivotally biasing said holder means in the direction of said scale; and (e) correction means for compensating for variations in the scale measurement, comprising (1) a correction ruler (5) having a generally horizontal correction guide edge (6); (2) means (4, 7) adjustably connecting said correction ruler with said scale at a generally horizontal position at which said correction guide edge is adjacent and spaced from said scale horizontal surface; and (3) follower means (10) connected with said holder means for abutting engagement with said ruler correction guide edge, whereby upon displacement of one of said members relative to the other, said follower means pivots said holder means to vary the area of said horizontal scale surface viewed by said scanning head means in accordance with the configuration of said correction ruler, thereby to vary the number of graduations read off said scale in accordance with the direction of pivotal movement of said holder means.