GB2084416A

GB2084416A - Position detecting apparatus

Info

Publication number: GB2084416A
Application number: GB8127576A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1980-09-23
Filing date: 1981-09-11
Publication date: 1982-04-07

Abstract

An optical shaft encoder consists of a multiple element LED light source with very narrow width emitting windows which are focussed on to one side of a very finely encoded disc driven by the shaft. Only one of the source elements is on at a time, each element being labelled by its position in time. A single large area detector located on the other side of the disc collects the transmitted light. The detector therefore receives a sequence of light pulses, the sequence representing the position of the disc relative to the LED light source and hence the position of the shaft. <IMAGE>

Description

SPECIFICATION Improvements in or relating to position detecting apparatus This invention relates to position detecting apparatus, and more particularly to optical position detecting apparatus for detecting the position of a movable member such as a rotary shaft.

Optical position detectors for rotary shafts are known which consist of a disc having a predetermined arrangement of slots or holes or a transparent disc with an arrangement of nontransparent portions secured to the shaft, a source or a number of sources of light on one side of the disc, and a detector array located on the opposite side of the disc to detect the coded light beams passing through the disc. The position of the disc is then determined by decoding the output from the detector array.

For high resolution the windows in the disc must be small, and hence high brightness small area light sources must be used if sufficient light is to pass through these windows. Also the light passing through the disc windows will diverge rapidly so that large area detectors are necessary to collect the light.

To overcome this problem some position detectors use a detector array located very close to and imaged at the encoded disc.

This is wasteful of light and demands a narrow multiple element detector array for high resolution.

It is an object of the present invention to provide position detecting apparatus which will reduce or substantially overcome these disadvantages.

According to an aspect of the present invention position detecting apparatus comprises a movable member having a plurality of light transmitting portions, the light transmitting portions being arranged in a plurality of different series, each series being arranged along the path of movement of the member, a plurality of light sources located adjacent to one side of the movable member, each light source being aligned with one of the series of light transmitting portions, the light sources in operation of the apparatus being energised one at a time, light detecting means located adjacent to the opposite side of the movable member for detecting light from all the plurality of light sources, whereby the light detecting means receives a plurality of sequences of light pulses, each sequence repesenting a particular arrangement of the plurality of series of light transmitting portions and indicating the position of the movable member relative to the plurality of light sources.

In this specification the term "light" includes parts of the spectrum which are invisible to the eye, such as ultra-violet and infra-red.

Further aspects of the invention will become apparent from the following description of embodiments of the invention given by way of example only in which; Figure 1 is a schematic layout illustrating the operation of the invention, Figure 2 is a view of part of a disc shown in Figure 1 and Figure 3 is a cross-sectional view of a practical embodiment of the invention.

In Figure 1 there is illustrated a shaft 10 on which is mounted a transparent disc 11. The disc is coated one side with a layer 1 2 of non-light transmitting material. A plurality of openings 13 are formed in the layer 12 which form windows in the disc and allow light to pass through the disc.

These openings are formed in series, four series being shown in Figures 1 and 2 although many more series could be used i.e. 1 6. The openings 13 in each series are arranged in a predetermined manner so that at any angular position of the disc the arrangement of the openings in a radial direction is unique and thus the disc is coded so that an angular position on the disc and hence the shaft 10 can be detected by the arrangement of openings.

The accuracy of the apparatus depends upon how many openings are formed and how many series of openings there are. The coding may for example be in the form of the binary code. The openings are very small in the radial direction, for example, 30ELm, and circumferentially, for example 4cm., A multiple element L.E.D. light source 14 (in this case four elements 15,) with very narrow width emitting windows is mounted one side of the disc 11 , the light from each element being imaged on to the surface of the disc in line with each series of openings 13 by a lens 16.

A large area detector 1 7 is located on the other side of the disc, large enough to receive the rapidly diverging light from the openings 1 3 in the layer 12.

Only one of the elements 1 5 is energised at a time, the elements being energised one after another and being labelled by their position in time t2, t2, t3 and t4. It will be seen in Figures 1 and 2 that only two of the openings 13 are in line with the light source 14 when the disc is in the position shown, and so the signal received by the detector 17 is as shown in Figure 1A, the light pulses at times t2 and t3 not being received by the detector and thus being shown by broken lines. This signal is decoded by a suitable decoder (not shown) and if the binary code is being used could be 1001 or a position 9 on the disc.

By having for example 16 series of openings 13 and using the binary code, it will be seen that the disc could be calibrated to give extremely accurate angular position detection, having 65,536 coded positions (counting O as one position).

Alternative ways of encoding each source element could involve making each element emit a different wavelength of light, and decoding using detectors sensitive to the different wavelengths.

Furthermore the method could be used to detect the position of various movable members instead of a shaft, by fitting the member with an encoded strip having a series of transparent portions which pass a plurality of light sources.

Figure 3 shows a practical embodiment of the invention. In this case a shaft 30 is mounted in a housing 32 in a journal bearing 33.

The shaft 30 is connected by a cylindrical diaphragm 43 to a further shaft 35 also mounted in a journal bearing 34. A transparent disc 31 carrying an annular code pattern 40 as shown in Figures 1 and 2 is mounted on the end of the shaft 35. Held rigidly at a distance from the disc and focussed onto the code pattern is a multiple element L.E.D. array 39. A detector 42 is located on the opposite side of the disc in the housing 32.

The angular position of the shaft 30 can therefore be very accurately determined at any time and the shaft can be rotated to any other predetermined position and accurately positioned.

Claims

1. Position detecting apparatus comprising a movable member provided with a plurality of light transmitting portions, the light transmitting portions being arranged in a plurality of different series, each series being arranged along the locus of movement of the movable member, a plurality of light sources located adjacent to one side of the movable member, each light source being aligned with one of the series of light transmitting portions, the light sources in operation of the apparatus being energised one at a time, light detecting means located adjacent to the opposite side of the movable member for detecting light from all the plurality of light sources, whereby the light detecting means, in operation of the apparatus, receives a plurality of sequences of light pulses each sequence representing a particular arrangement of the plurality of series of light transmitting portions and indicating the position of the movable member relative to the plurality of light sources.

2. Position detecting apparatus as claimed in claim 1 in which the position of each of the plurality of light sources is determined by its position in time.

3. Position detecting apparatus as claimed in claim 1 or claim 2 in which the movable member is made of a transparent material and coated with non light transmitting material, portions of the transparent material being left uncoated to form a plurality of series of light transmitting portions.

4. Position detecting apparatus as claimed in any preceding claim in which the plurality of series of light transmitting portions are arranged in a binary code.

5. Position detecting apparatus as claimed in any preceding claim in which the plurality of light sources comprise light emitting diodes.

6. Position detecting apparatus as claimed in claim 5 in which a lens is positioned between each light emitting diode and the movable member to image light emitted from the diode on to one of the plurality of series of light transmitting portions.

7. Position detecting apparatus as claimed in claim 5 or claim 6 in which each light emitting diode emits a different wavelength of light whereby its position is determined by its wavelength.

8. Position detecting apparatus as claimed in any preceding claim in which the movable member comprises an elongate strip.

9. Position detecting apparatus as claimed in any of claims 1 to 7 in which the movable member comprises a rotatable disc.

10. Position detecting apparatus as claimed in claim 9 in which the plurality of series of light transmitting portions is arranged concentrically with the axis of rotation of the disc.

11. Position detecting apparatus constructed and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.