Shaft angle encoder with rotating off-axis interference pattern

A shaft angle encoder for determining the rotation of a shaft relative to a frame, the shaft angle encoder comprising:

• an optical pattern concentrically mounted to the shaft;
• means for illuminating a portion of the optical pattern;
• phase plate means aligned so as to interfere with the portion of the optical pattern for providing a circular interference pattern which rotates about its optical axis as the shaft rotates with respect to the frame;
• and
• means for detecting the rotation of the rotating interference pattern.

A shaft angle encoder for determining the rotation of an inner gimbal with respect to an outer gimbal in a two axis gimbal system having two orthogonal rotational degrees of freedom of movement between the housing and the inner gimbal, the shaft angle encoder comprising:

• an outer gimbal bearing transparent along its axis;
• an optical pattern mounted on the inner gimbal;
• means for illuminating the optical pattern;
• phase plate means mounted to the outer gimbal for optically interfering with the image of a portion of the optical pattern and for providing a circular interference pattern which rotates about its optical axis in response to rotation of the inner gimbal with respect to the outer gimbal;
• detection means mounted to the housing for detecting the rotation of the rotating interference pattern;
• and
• optical path means for providing an optical path from the image through the phase plate to the detection means, the optical path passing through the transparent axis of the outer gimbal bearing.

A shaft angle encoder for determining the rotation of an inner gimbal with respect to an outer gimbal in a two axis gimbal system having two orthogonal rotational degrees of freedom of movement between the housing and the inner gimbal, the shaft angle encoder comprising:

• two inner gimbal bearings suspending the inner gimbal from the outer gimbal along a first axis;
• two outer gimbal bearings suspending the outer gimbal from the frame along a second axis orthogonal to the first axis, one outer gimbal bearing being transparent along its axis;
• an optical pattern mounted on the inner gimbal concentrically about the axis of rotation of the inner gimbal and optically aligned with the outer gimbal bearing;
• means for illuminating the optical pattern;
• phase plate means mounted to the outer gimbal so as to optically interfere with the image of the portion of the optical pattern optically aligned with the outer gimbal bearing for providing an interference pattern which rotates about its optical axis as the inner gimbal rotates with respect to the outer gimbal;
• and
• detection means mounted to the housing and aligned to receive the rotating interference pattern for detecting the rotation of the rotating interference pattern;
• wherein the optical path from the image through the phase plate to the detection means passes through the transparent axis of the outer gimbal bearing.