Arrangement for preventing ambiguity in digital displacement measuring apparatus

Displacement measuring apparatus for measuring displacement in a predetermined direction of a first element relative to a second element, comprising, in combination, code display means coupled to said first element, said code display means having a plurality of digital code markings each representing a predetermined unit of displacement spaced at predetermined distances from each other in said predetermined direction, said code display means further having control markings arranged in predetermined positions relative to said code markings;

• read-out means coupled to said second element and comprising code read-out means and control read-out means mounted in predetermined positions relative to each other, for respectively reading out code markings and control markings passing in operative proximity thereto and furnishing corresponding data and control signals;
• means for providing said control signals only when said code readout means are positioned relative to the central portion of said code markings;
• output circuit means connected to said read-out means for furnishing output signals corresponding to said data signals only upon receipt of said control signal and for storing said output signals until receipt of the next-subsequent one of said control signals thereby furnishing stored output signals;
• a plurality of light sources connected to said output circuit means for energization by said stored output signals;
• camera means for photographing said light sources, said camera means having a shutter and secondary optics for recording said light from said light sources;
• means for furnishing a shutter signal when said shutter is travelling across said secondary optics;
• and gating means interconnected between said means for furnishing a shutter signal, said control read-out means, and said output circuit means for furnishing said control signals to said output circuit means only in the presence of said control signals and the absence of said shutter signals.