Method and apparatus for measuring displacement by using a diffracted inverted image projected on a diffraction grating

A method of measuring displacement, comprising the steps of:

• projecting a beam, having a focal point located between a first optical grating and a second optical grating displaced together with the first optical grating, onto the first optical grating, wherein the beam is split into overlapping diffracted light components;
• projecting the beam onto the second optical grating, wherein diffracted images resulting from overlapping of the diffracted light components are formed on said second optical grating and are shifted in a direction opposite to the displacing direction of said first optical grating;
• and
• detecting the beam having passed through the second optical grating.

A driving system having a displacement measuring apparatus, comprising:

• a driving mechanism;
• and
• a displacement measuring apparatus for measuring a drive state of said driving mechanism,
• said displacement measuring apparatus including:
• a scale mounted on a displacement portion of said driving mechanism;
• a first optical grating formed on said scale,
• a second optical grating formed on said scale,
• an optical system for projecting a beam, having a focal point located between said first and second optical gratings, onto said first optical grating, wherein the beam is split into overlapping diffracted light components forming diffracted images on said second optical grating shifted in a direction opposite to the displacing direction of said first optical grating;
• and
• means for measuring the relative displacement information between said optical system and said scale by detecting the beam having passed through the second optical grating.

An apparatus for detecting displacement information, including:

• an optical grating;
• a first optical member for projecting a beam, having a focal point located at an opposite side of said first optical member with respect to said optical grating, wherein the beam is split into diffracted light components;
• a second optical member for projecting the beam back onto said optical grating, wherein diffracted images as a result of overlapping of the diffracted light components are formed on said optical grating and are shifted in a direction opposite to the displacing direction of said optical grating;
• and
• means for detecting displacement information of said optical grating relative to said first optical member by detecting the diffracted images having passed through said optical grating.

An apparatus for detecting displacement information of an optical grating, comprising:

• a first optical member for projecting a beam, having a focal point located at an opposite side of said first optical member with respect to the optical gratings wherein the beam is split into diffracted light components;
• a second optical member for projecting the beam back onto the optical grating, wherein diffracted images as a result of overlapping of the diffracted light components are formed on the optical grating and are shifted in a direction opposite to the displacing direction of the optical grating;
• and
• means for detecting displacement information of the optical grating relative to said first optical member by detecting the diffracted images having passed through the optical grating.