Apparatus for detecting position of a rotating element using a two-grating moire pattern

Apparatus for detecting the position of a rotatable element comprising:

• mirror means connected to said element to rotate therewith;
• a source of light;
• a first fixed grating having a regular pattern of areas that are transparent and opaque to the radiation from said source having a first spatial frequency;
• a second fixed grating having two sections each including a pattern of areas that are transparent and opaque to the radiation of said light source said patterns having the same spatial frequency but having a spatial phase difference of ninety degrees with respect to one another;
• means for projecting an image of said first grating onto said second grating, said means including said mirror means and arranged such that the pattern from said first grating moves across said second grating at a rate proportional to the rate of motion of said mirror whereby the intensity of light passing through said two sections of said second grating varies in a regular pattern proportional to the rate of motion of said mirror;
• detector means located behind each of said two sections of the second grating such that light passing through said sections of said second grating falls thereon, each of said detector means being adapted to produce electrical signals proportional to the intensity of light passing through said sections of said second grating and means for processing said electrical signals to produce a mirror position and motion signal.