Two-dimensional optical encoder with three gratings in each dimension

A two-dimensional optical encoder comprising a first member and a second member having a main scale and an index scale, respectively which are opposed to each other, and detecting a relative displacement between the first member and the second member in directions of an X-axis and a Y-axis, said optical encoder comprising:

• a first grating formed on the main scale and including two sub-gratings having grating lines arranged in the directions of the X- and Y-axis, respectively;
• a second grating formed on a central area of the index scale, and including a sub-grating extending along the X-axis and having grating lines arranged in the direction of the X-axis, and a sub-grating extending along the Y-axis and having grating lines arranged in the direction of the Y-axis;
• a third grating formed on the index scale, and having two sub-gratings formed outside the corresponding sub-grating of the second grating and having grating lines arranged in the direction of the X-axis with a phase difference between the two sub-gratings, and two sub-gratings formed outside the corresponding sub-grating of the second grating and having grating lines arranged in the direction of the Y-axis with a phase difference between the two sub-gratings;
• a light source fixed to the second member;
• a plurality of photodetectors fixed to the second member and arranged corresponding to the respective sub-gratings of the third grating, for receiving light emitted from the light source, transmitted through the second grating, reflected by the first grating, and transmitted through the third grating;
• and
• a computing circuit for producing signals indicating relative displacements in the directions of the X- and Y-axis by processing output signals of the plurality of photodetectors.