Displacement measuring device utilizing an incremental code

A device for measuring displacement comprising a coding member carried by an object to be measured and having a coding section capable of causing sensing elements to generate signals which can be discriminated as "0" and "1", and first and second sensing elements arranged to sense the coding section of the coding member and adapted to generate respectively first and second detection signals with a predetermined phase difference therebetween in response to displacement of the code member, so that the direction of displacement is determined on the basis of the first and second detection signals and at least one of the first and second detection signals is counted for measurement of the displacement, characterized in that it comprises:

• first and second rectangular wave-converting circuits for receiving respectively the first and second detection signals to generate respectively first and second rectangular wave signals;
• a clock pulse generator for generating clock pulses repetition-frequency higher than the repetition-frequency of the first and second detection signals;
• an edge detector for receiving the first and second rectangular wave signals and the clock pulses to output, as output pulses, clock pulses substantially coinciding with the leading and trailing edges of the first and second rectangular wave signals;
• and
• a phase difference detecting device which includes a phase difference detector for receiving the first and second rectangular wave signals to detect the phase difference therebetween, a discriminator for determining whether the phase difference detected by the phase difference detector is within a predetermined range, and a display unit for making a display when the discriminator has determined that the difference is out of the predetermined range;
• and
• wherein the phase difference detector is a counter adapted to count the clock pulses and be cleared by the output pulse, and the discriminator is a comparator for comparing a counted value outputted from the counter with a set value from a phase difference setting unit to output a signal when the counted value has exceeded the set value.