Shaft encoders, or what is sometimes known as position encoders, are well known in the art. Numerous designs have been developed and utilized in order to transduce the position of a rotatable shaft to an electrical output by means of a coded output. Tomasulo et al. U.S. Pat. No. 3,171,034 issued on Feb. 23, 1965 and entitled "Electro-Optical Control," illustrates an early design utilizing photo-cells which sense light transmitted through tracks varying from opacity to transparency. This is typical of the early uses of light sensitive photo-cells to effect an output corresponding directly to the change of shaft position. Leonard U.S. Pat. No. 4,259,570 issued Mar. 31, 1981 and entitled "Optical Comparator" and the related Epstein et al. U.S. Pat. No. 4,266,125 issued May 5, 1981 and entitled "Optical Shaft Angle Encoder," disclose and illustrate a shaft encoder that utilizes single phase differences to transmit the shaft position. The encoder has two tracks which generate square waves whose phase relationship is detected so that the position of the shaft is determined by the number of pulses received. This is an incremental shaft encoder device which utilizes a reference point or datum that must be established in order for the position of the shaft to be determined. If power to the device is momentarily interrupted, or deactivated for a period of time, the position of the shaft is not known without first establishing a datum and then counting incrementally to the present position.
Swenson U.S. Pat. No. 4,284,885 issued Aug. 18, 1981 and entitled "Optical Potentiometer" illustrates a device using phototransistors to sense the intensity of light transmitted through windows varying from opaque to transparent. Brienza et al. U.S. Pat. No. 4,338,518 issued July 6, 1982 and entitled "Position Encoder with Column Scanning and Row Sensing" discloses light transmitting optical fibers having portions of some of the fibers selectively masked at predetermined linear positions of the device so that a nine bit code is effected by the linear displacement of device components.
There has long been a need for a miniature shaft encoder or position encoder which is inexpensive, requires a minimum number of components, is easily assembled, and which may be used for a wide variety of applications. The prior art has not provided a device having an eight bit parallel code output which may be interfaced directly with a microprocessor such as those used in automotive applications. Such a device must be small in size, have a long functional life span, be inexpensive, and preferably comprise an absolute angle position encoder.