Patent ID: 8251775

Claim:
A mechanism for detecting a shaft motion, comprising: a first shaft that rotates around a predetermined axis; a cylindrical second shaft that rotates around the predetermined axis along with the first shaft and is attached to the first shaft so as to be displaceable along the predetermined axis relative to the first shaft; a frame that rotatably supports the first shaft; a plurality of first projections or recesses that are formed on a surface of the second shaft so as to be aligned in a circumferential direction; a second projection or recess that is formed in a position on the surface of the second shaft and extends about the entire circumference thereof, the position being away from the plurality of first projections or recesses in a direction of the predetermined axis; and at least three proximity sensors that are disposed facing the surface of the second shaft and fixed to the frame so as to be aligned around the predetermined axis; wherein the at least three proximity sensors sense the plurality of first projections or recesses and the second projection or recess to detect a motion of the second shaft.