Object measuring method and apparatus

A method for measuring the movement of an object comprising the steps of:

• (a) producing an image of the object using an imaging device;
• (b) positioning a grating structure to receive the image;
• (c) providing a periodic relative movement between the grating structure and the image;
• (d) generating reference signals corresponding to the periodic relative movement;
• (e) positioning photoelectric receiving means to receive light from said grating structure;
• (f) generating electrical rotating field signals from photoelectric receiving means corresponding to the periodic relative movement of said image and grating structure;
• (g) generating control signals from said electrical rotating field signals and said reference signals;
• and
• (h) feeding said control signals to inputs of a bi-directional counter;
• whereby said counter provides a measure of the movement of said object.

A method for obtaining control signals for use with a bi-directional counter and an optical correlator having an image forming means, a grating for receiving the image, means for producing periodic relative movement between the image and grating, and photoelectric receiving means for generating electrical rotating field signals corresponding to the object movement in a direction at least substantially parallel to the relative movement comprising the steps of:

• (a) rectifying the fundamental and at least an odd harmonic component of the electrical rotating field signals generated by the photoelectric receiving means using phase-sensitive rectifiers;
• (b) generating reference signals from the relative periodic movement of the grating and image corresponding in frequency to the fundamental and odd harmonic component of the electrical rotating field signals;
• (c) phase displacing said generated harmonic reference signals;
• (d) controlling said rectifying step in response to said fundamental reference signals and said phase-displaced harmonic reference signals;
• (e) filtering the outputs of said phase-sensitive rectifiers;
• and
• (f) feeding said filtered outputs to inputs of said bi-directional counter.