Optical encoder comprising a plurality of encoder wheels

A rotary shaft position indicator comprising:

• four rotatably mounted encoder wheels, each encoder wheel including at least one code sequence extending concentrically around the encoder wheel, whereby said four encoder wheels in combination define a plurality of code sequences, wherein said plurality of code sequences comprise a predetermined number of absolute code sequences and at least one incremental code sequence;
• three pinions, each pinion being provided between two encoder wheels for rotating the second encoder wheel a predetermined amount following a predetermined amount of rotation of the first encoder wheel, whereby a first pinion is between a first encoder wheel and a second encoder wheel, a second pinion is between said second encoder wheel and a third encoder wheel, and a third pinion is between said third encoder wheel and a fourth encoder wheel;
• sensing means including light emitting means for illuminating a defined region of a predetermined number of said plurality of code sequences and detector means for identifying light illuminated by said light emitting means, said sensing means further including a plurality of substantially aligned towers for housing said light emitting means and said detector means, each tower defining substantially aligned upper and bottom surfaces at spaced separation, with the upper and bottom surfaces extending from a base to a terminating end defining a cavity into which at least one said light emitting means and said detector means are received, wherein at least one of the upper and bottom surfaces of each tower includes at least one aperture therethrough provided at a distance from said base toward the terminating end for providing a passage for said light emitting means and said detector means, wherein each encoder wheel is positioned between a pair of towers, with the tower aperture being positioned adjacent the defined region of said code sequence;
• and
• control means for identifying a position of said rotary shaft position indicator, wherein said rotary shaft position indicator operates in at least first and second modes, said control means communicating with said sensing means for monitoring said defined region of each of said absolute code sequences to identify a position of said rotary shaft position indicator when said rotary shaft position indicator is operating in said first mode, said control means communicating with said sensing means for monitoring at least during a defined first time interval only said defined region of said at least one incremental code sequence, to identify a position of said rotary shaft position indicator when said rotary shaft position indicator is operating in said second mode.