Rotary encoder having absolute angle patterns and relative angle

Details Rotary encoder having absolute angle patterns and relative angle Rotary encoder having absolute angle patterns and relative angle

1991-01-14

1993-05-25

Williams, Howard L.

Coded data generation or conversion

Digital pattern reading type converter

Optical

341 11, 250237G, G01B 534, H03M 130

Patent

active

052144267

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a rotary encoder for detecting a rotational angle.


BACKGROUND ART

Conventionally, a rotary encoder has been used for detecting the rotational angle of a rotor or a rotating member to be measured. A prior art rotary encoder generally used is provided with a rotating disc having optical patterns formed thereon and with an optical sensor disposed in the vicinity of the rotating disc for reading the optical patterns. When a rotational angle of the rotating member is desired to be detected in the form of an absolute value, optical patterns are provided on the rotating disc in the form of a digital code indicative of an absolute angle or corresponding to the angle.


PROBLEMS TO BE SOLVED BY THE INVENTION

With such conventional rotary encoders for detecting the absolute rotational angle of a rotating disc, the pitch of digital code patterns must be decreased and the number of bits thereof be increased in order to enhance resolution with the rotational angle to be measured. The requirements result in a large-sized device including the digital code patterns. If the diameter of a code plate is required to be made the same, the digital code patterns must be formed densely in a circumferential direction of the code plate, which demands a high accuracy in machining the code plate and in positioning a light source and an optical sensor.
In order to solve problems arising in enhancing resolution in measurement of rotational angles, there has been proposed a rotary encoder in the patent publication of an unexamined Japanese patent application No. 62-54122. The rotary encoder proposed here is arranged in such a way that helical (spiral) patterns are provided on a rotating disc and a sensor is provided to detect a position at which the sensor crosses the pattern to obtain an angle data. But, with the the construction of the rotary encoder proposed, an angle is detected by reading a so-called analog pattern and hence the degree of resolution in measurement of rotational angles depends largely on preciseness of the helical analog pattern. Therefore, high accuracy machining is required in order to obtain a desired resolution in measurement of rotational angles over the whole circumference.
An object of the present invention is to provide a rotary encoder which is capable of improving resolution in measurement of rotational angles with ease by ensuring a medium degree of resolution by means of digital codes and attaining a higher degree of resolution by means of analog patterns.
With the prior art rotary encoders for detecting absolute angles of a rotating disc as explained in the foregoing, code patterns are formed with an opaque material on a transparent plate made of glass or transparent resin and the codes are read by detecting the presence or absence of transmitted light. The transparent material such as a glass plate or transparent resin has caused light to be refracted or scattered, which has resulted in false readings of codes. Further, with the prior art rotary encoders, the process of forming patterns is complicated and production thereof is costly.
Although it is possible to provide code patterns by forming slits in an opaque member such as a metal plate, there are presented the following drawbacks.
FIG. 24 shows the relationship between angles and corresponding codes. In the figure, each angle (of 0 through 359 degrees) is expressed by the BCD code system wherein a digit in hundreds place, a digit in tens place and a digit in ones place are expressed by two bits, four bits and four bits respectively. If one slit is to be formed at a portion corresponding to the presence of a bit "1", for example, each angle of 100 degrees through 199 degrees requires one slit to be formed at each corresponding one of successive positions of 2.degree. bit of the digits in hundreds place, each angle of 200 degrees through 299 degrees requires one slit to be formed at each corresponding one of successive positions of 2.sup.1 bit of the digits in hundreds place, and further all the

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patent: 4465373 (1984-08-01), Tamaki et al.
patent: 4602242 (1986-07-01), Kimura
patent: 4757196 (1988-07-01), Yamada et al.
patent: 4965446 (1990-10-01), Vyse

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