Displacement detecting apparatus

Apparatus for detecting the relative displacement x of a member with respect to first and second transducer means, said member being provided with uniformly spaced indicia thereon of a predetermined wavelength .lambda., said apparatus comprising:

• first and second transducer means for reproducing said indicia from said member as a periodic function of (2.pi./.lambda.)x;
• means for supplying energizing pulse signals to said first and second transducer means, the repetition rate of said pulse signals being equal to (f.sub.c /2), whereby each transducer means generates a modulated pulse signal of repetition rate f.sub.c and having a modulation component that is a function of (2.pi./.lambda.)x;
• adding means for adding the modulated pulse signals generated by said first and second transducer means to produce a phase modulated pulse signal having a fundamental frequency f.sub.c and harmonics thereof, the fundamental and harmonic components having a pulse modulation that is a function of (2.pi./.lambda.)x;
• harmonic selecting means for selecting the nth harmonic of said phase modulated pulse signal, said nth harmonic being represented as E.sub.n sin (2.pi.nf.sub.c t+(2.pi./.lambda.)x), where E.sub.n is an amplitude and where n is greater than one;
• wave shaping means for shaping said selected nth harmonic of said phase modulated pulse signal to produce a further phase modulated pulse signal of repetition rate nf.sub.c and having a phase modulated component that is a function of (2.pi./.lambda.)x;
• phase detecting means including a source of timing pulses that is a multiple of the repetition rate f.sub.c and means connected to receive both said further phase modulated pulse signal of repetition rate nf.sub.c and said timing pulses to produce a pulse phase modulated rectangular wave of repetition rate f.sub.c and pulse phase variations that are a function of x;
• and
• sensing means for sensing if said pulse phase variations exceed a predetermined incremental amount in the positive and negative directions to produce first and second output pulses, respectively, representing that said displacement x is a positive and negative displacement of predetermined amount.