1. Field of the Invention
The present invention relates generally to optical shaft encoders and, more specifically, to a method of manufacturing an optical shaft encoder with a minimum of precision machining.
2. Description of the Prior Art
Optical shaft encoders typically include a rotating code disk coupled to a shaft and convert the rotary position to a digital output signal. The code disk includes predetermined opaque and transparent patterns or lines disposed about the circumference. The code disk rotates between a light source and a photodetector. A mask is disposed between the code disk and the photodetector, such that as the code disk rotates, light from the light source is alternately blocked and transmitted through the code disk and mask. The light which transmitted through the code disk and mask strikes the photodetector. The photodetector generates electrical output signals indicative of the position of the rotating code disk and the shaft to which it is coupled.
The assembly and installation of optical shaft encoders must be done within precise tolerances. Otherwise misalignments cause distortions in the output signals of the optical shaft encoder. The alignment problem can be reduced by using more precisely machined components. Precision machining, however, greatly increases the manufacturing costs. Several patents have addressed the alignment problems associated with the manufacture and installation of optical shaft encoders.
U.S. Pat. No. 4,345,149 discloses an optical shaft encoder having a mounting support which carries the light source. The optical encoder assembly is centered on a motor casing with respect to the motor shaft by a disc-shaped centering fixture. The fixture has a central bore of a diameter corresponding to the dimater of the motor shaft, and an annular peripheral downwardly depending lip at a precise radial distance corresponding to the radial distance between the geometrical center of the mounting support and its circular periphery. The disc-shaped fixture can overlie the mounting support with its lip engaged about the periphery with its central bore receiving the motor shaft. The geometry is such that the mounting support is necessarily held in a precise concentric relationship with respect to the motor shaft by means of the centering fixture. The mounting support can then be secured to the motor casing itself and thereafter the centering fixture removed.
U.S. Pat. No. 4,184,071 discloses a modular encoder with a dimensionally stable housing which can be easily aligned for mounting on a motor housing. The encoder utilizes a monolithic, in-line, sensor array and a single light source to achieve improved long-term performance and includes improved alignment permitting rapid alignment of the encoder with a motor. Pre-alignment of the mask and sensor assembly simplifies the alignment performed by the user.
U.S. Pat. No. 4,075,478 discloses an optical encoder design which maintains a constant fixed air gap between the commutator and light sensors. The encoder head is mounted on a partially compressed hollow bellows or other spring member biasing snubber posts adjacent to the light sensors against tracks on the commutator. The height of the snubber posts above the light sensors defines the air gap.
All of above described patents are directed towards alignment problems, but none of the patents reduce the expensive and time consuming precision machining requirements for an optical shaft encoder. Accordingly, there is a need for a method of manufacturing an optical shaft encoder that reduces the precision machining requirements and simplifies the alignment problems.