GB2241125A

GB2241125A - Digital shaft-encoder

Info

Publication number: GB2241125A
Application number: GB9003274A
Authority: GB
Inventors: Peter Richard Milner
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-02-14
Filing date: 1990-02-14
Publication date: 1991-08-21
Also published as: GB9003274D0

Abstract

A digital shaft-encoder uses two detectors on each track, whilst retaining a Gray code type of digital output. By this means 2n-bit discrimination can be encoded on n tracks and (2n+1)-bit discrimination on n+1 tracks (n an integer). The system may be adapted to detect angular position within a full rotation, or over a limited arc, or linear position. <IMAGE>

Description

IMPROVED DIGITAL SHAFT-ENCODER TRACK PATTERNS This invention relates to detection of the angular position of a rotatable shaft by means of digital encoders.

Many mechanical devices, in a variety of fields, contain shafts whose angular positions are required to be detected or controlled to a specified accuracy. This is commonly achieved either by potentiometers or by digital shaft encoders which conventionally work on the principle of the 4-bit encoder shown in Fig.1. A patterned disc is attached to the rotating shaft. Each track of the pattern has a single fixed detector at positions A to D, typically an electrical pickup or optical sensor, which detects changes from conducting to non-conducting or white to black states of the track at various points during a complete rotation of the shaft. The inner track detects in which half of the rotation the shaft lies, the next which half of each half, and so on outwards.Hence each t,aci; increases the angular discrimination by a factor of two, and an n-track device has a discrimination of 360 /2n degrees. These devices are also required to have a 'Gray code' type of output in which only one bit switches at any instant, in order to avoid spurious outputs near the switching positions.

The invention described herein is a modified track pattern to be applied using the above principles, except that the present invention uses two detectors per track, thereby reducing by a factor of two the number of tracks required for a given precision. This permits a more compact design for a given precision, or a greatly enhanced precision for a given size of encoder.

Three examples are shown in Figures 2, 3 and 4. Fig 2 shows a 6-bit rotary encoder to the new design. With the exception of the innermost track, each track is of similar, but progressively finer pattern and the pattern can be extended to an 2n-bit encoder where n is any integer. The tracks have switching at intervals of 2, 3, 3, 2, 3 ...... times their discrimination interval and each track has detectors placed so that no switching occurs at intervals of 4 times the discrimination interval, thus avoiding the switching points of less discriminating tracks and thereby generating Gray code switching. The relative disposition of the detectors is not unique but the principle remains similar, wherever they are put.In this design, on the n th track (n > l) from the centre, one detector is placed on a line common to one of those on each other track, and one at an angle of 3600/411-1 clockwise from the common line of sensors.

Fig 3 shows how a 5-bit detector can be simplified to three tracks from the 5 tracks of fig 1. The outermost track has only one detector and is of similar pattern to that of fig.l. Any detector of 2n-1 bit discrimination can be constructed along similar lines to the principle of fig.3.

Fig 4 shows an example of how the technique may be applied to a shaft which rotates through a restricted arc. The slotted disc gives 6-bit discrimination over an arc of 960 by means of chopping infrared beams in optical switches disposed around its rim at the positions indicated. An arc length of 1200 is required for each sector and this design can also be adapted for linear positional encoding, using straight tracks of length 11/4 times the total movement to be monitored.

Claims

1. A system of track pattern design for digital rotary encoders which enables the measurement of angular position to a discrimination of 360 /4n) by means of n tracks with two detectors on each, or 360o/(22n+1) by means of n+1 tracks. (n is a positive integer).

2. The adaptation of the principle of the pattern design in claim 1 to the design of linear positional encoders and to angular encoders which operate only over a restricted arc.