Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1993-12-15
1995-08-01
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250237G, 341 13, G01D 534
Patent
active
054381932
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an absolute encoder for outputting an absolute positional signal in the form of a parallel bit code representative of an angular position of an encoder plate such as a rotary disc even though the rotary disc is held stationary. The present invention particularly relates to a synchronous control technology between a higher order bit signal and a lower order bit signal, and further relates to an optical detection structure of a slit pattern formed on the encoder plate such as the rotary disc.
BACKGROUND TECHNOLOGY
The encoder is widely utilized for a positional detection of a robot arm or else, and is constructed to read a scale of the encoder plate attached to a rotation shaft of a motor, by means of a detection element. The positional detection encoder includes an incremental type and an absolute type. The former type is constructed such as to read a position of the rotary disc by counting an incremental pulse with reference to an origin of the rotary disc. The latter type undergoes the positional detection by reading a code formed on the rotary disc regardless of a mechanical state thereof. Therefore, the incremental type needs to rotate at most one cycle to restore the origin when restarting the encoder operation after a power source has been shut down, whereas the absolute type advantageously does not need the origin restoring operation because the position can be instantly read without moving the rotary disc when the power source is recovered.
FIG. 13 shows a typical structure of the conventional absolute encoder. A rotary disc 101 is formed thereon with a plurality of concentric tracks 102-105. Each track is comprised of an annular slit pattern which is bit-coded according to a digital code system indicative of an absolute position of the rotary disc 101. A photodetector array 106 is opposed to one side of the rotary disc 101 , and a. photoemitter such as an LED 108 is opposed to the other side of the disc through a stationary slit plate 107. The slit pattern formed in the rotary disc 101 selectively passes and cuts off a light beam from the LED 108 so that the photodetector array 106 outputs detection signals according to a light intensity variation on respective tracks of the array 106. These detection signals are processed so as to read the angular absolute position or angular address of the rotary disc 101. Namely, this address is represented by the aforementioned digital code.
There have been known various digital code systems for representing an address. FIG. 14 schematically shows a slit pattern formed according to a regular binary code which is one example of the digital code systems. The illustrated pattern diagram indicates track numbers in a left column and addresses in a top row. The slit pattern of each track is binarily coded, and is composed of bright and dark sections. In this example, there are provided four tracks corresponding to four bits so as to represent 2.sup.4 =16 number of absolute addresses. Such a regular binary code is a basic arrangement in the digital process. However, at a transition from one address to another address, switching between adjacent bright and dark sections may occur concurrently at two or more tracks. It is quite difficult to just concurrently detect respective transitions, thereby causing the drawback that a reading error may be generated by transitional fluctuation of detection timings.
FIG. 15 shows Gray code which is designed to remove the above noted drawback. As seen from the pattern diagram, the Gray code is characterized in that the switching between adjacent bright and dark sections occurs only on one track at every transition of the addresses in contrast to the regular binary code, thereby effectively avoiding the reading error. However, the Gray code requires a multiple of tracks corresponding to a number of bits likewise the regular binary code. Therefore, as the bit number is increased to multiply addresses for achieving higher resolution power, a multiple of tracks are arranged in parallel
REFERENCES:
patent: 3487400 (1969-12-01), Ludewig, Jr. et al.
patent: 4152579 (1979-05-01), Feinland
patent: 4636079 (1987-01-01), Rieder et al.
patent: 4945231 (1990-07-01), Ohya et al.
patent: 5091643 (1992-02-01), Okutani et al.
Fukuda Takumi
Takagi Masaaki
Copan Company Limited
Nelms David C.
Nichols Steven L.
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