This invention relates to optical position encoders and, more particularly, to arrangements of optical fibers or light guides that function as a readhead for such optical position encoders.
As is known in the art, optical encoders include a plurality of adjacent strip-like bands that are subdivided into regions or segments that exhibit first and second reflection coefficients (e.g., transmissive regions and highly reflective regions). When the optical encoder is illuminated along a linear path that traverses the optical encoder bands, light reflected from each band is detected with the two degrees of reflectivity corresponding to the "0" and "1" binary states. Collectively, the light reflected from the encoder bands thus forms a binary encoded optical signal that is representative of the position of the encoder at the time of illumination.
One type of prior art arrangement for illuminating the bands of an optical encoder and, in addition, receiving the reflected light includes a linear array of optical fibers that are arranged and positioned so that the end regions of the fibers are maintained substantially parallel to one another with the end faces of the fibers being in spaced apart parallel juxtaposition with the surface of the optical encoder. In such an arrangement, which commonly is called a reading head or readhead, each band of the optical encoder is spatially and operably associated with one or more specific optical fibers that direct light onto the surface of the associated encoder band and receive light reflected from the associated encoder band. In some arrangements, the associated optical fibers both transmit light to the surface of the optical encoder and receive reflected light. In other arrangements, separate transmitting and receiving fibers are used.
In operation, a pulse of light is transmitted along those optical fibers that are employed to transmit light to the associated encoder bands. Light, launched from the ends of the optical fibers impinges on the surface of the optical encoder with light that is reflected from each individual band being received either by the transmitting fibers or by dedicated receiving fibers. The reflected light then propagates along the associated receiving fibers for subsequent reception and processing to form a binary encoded optical or electrical signal that is representative of the encoder position relative to the fixed or reference position of the readhead.
For example, a rotary optical encoder for producing an N-bit Gray Code signal representative of the angular position of a shaft or other rotatable object generally is formed as a disk that includes N circumferential bands concentrically surrounding the axis of rotation. Each circumferential band is subdivided into interspersed highly reflective and highly transmissive equal angular radial segments with the two innermost circumferential bands each including a single reflective (and transmissive) segment that is subtended by an angle of .pi. radians (180.degree.). In this arrangement, the number of reflective (and transmissive) segments in the N-2 circumferential bands located outwardly of the two innermost bands sequentially increases in accordance with the sequence 2.sup.i-2 (i=3, 4, . . ., N); where i identifies the circumferential band (as counted from the center of the optical encoder). Thus, each reflective (and transmissive) segment of the outmost N-2 circumferential bands is subtended by an angle of .pi./2.sup.i-2. In addition, the reflective (and transmissive) regions of the two innermost circumferential bands are positioned relative to one another so that the reflective (and transmissive) segments overlap one another by .pi./2 (90.degree.) and each reflective segment of the outermost N-2 circumferential bands overlaps a contiguous reflective segment of the nextmost inner circumferential band by an angle of .pi./2.sup.i-1. When a suitably configured readhead is mounted at a fixed position that extends radially along the surface of such an encoder disk, an N-bit binary signal is formed by light that is reflected from the encoder bands to uniquely identify the radial position (angle of rotation) of the optical encoder. Since an encoder of the described type provides 2.sup.N binary encoded N-bit binary signals, an angular resolution of .pi./2.sup.N-1 radians is attained.
Although prior art readheads of the above-described configuration perform satisfactorily in many applications, disadvantages and drawbacks are encountered in situations that require optical encoders of relatively small size with an accompanying need for precise determination of angular position (high resolution). In particular, such applications require a relatively large number of small diameter optical fibers which must be mounted in precise relationship both with respect to one another and with respect to the surface of the optical encoder. Because of the small fiber size and positional constraints, small high resolution optical readheads are not well suited to automated large scale fiberoptics manufacturing techniques and are otherwise difficult to fabricate. Thus, in the prior art, such devices have been both costly and, in many cases, not as reliable as desired.
Moreover, when small high resolution optical readheads are fabricated with prior art fabrication techniques using discrete optical fibers, it often is difficult to achieve desired performance specifications. For example, manufacturing tolerances associated with the assembly of individual fibers may not closely align groups of fibers with the associated bands of the optical encoder. When proper alignment is not achieved, light that is launched from the end of the fibers may impinge upon an adjacent band of the optical encoder and/or light that is received by the optical fibers may have been reflected from an adjacent band of the optical encoder. If this occurs, an undesirable amount of "cross-talk" may be encountered (i.e., the optical signals provided to and received from the individual bands of the optical encoder may not be properly isolated from one another). In addition, using prior art discrete fiber fabrication techniques, it may be difficult to obtain low insertion loss and desired efficiency.