WO1982000728A1 - Optical positiontransducer - Google Patents

Abstract

An optical position transducer comprises a code disc (1) and a lighting device (2) which can move one another. The code disc comprises a code track (4) on an electrically insulating support (3), this code track comprising a plurality of electrical conductors (6, 7) with interposed elements (8) sensitive to light. By placing the electrical conductors parallel or concentrically and arranging the interposed elements sensitive to light according to a binary code configuration and by producing a ray of light in the form of a line extending over all the conductors, a digital transducer is obtained. absolute position.

Description

OPTICAL POSITION TRANSDUCER . Technical Field

The present invention relates to an optical position transducer comprising a code disc and a lighting device which are mutually movable, sa id code disc comprising a support of electrically insulating material on which electrical conductors with interposing light sensitive elements are provided, wherein the conductors are substantially concentric, and the interposed light sensitive elements are arranged in a binary code pattern , and wherein the lighting device is arranged so as to provide a narrow line-shaped light beam extending over all the conductors substandially radially with respect thereto. Background Art

In an optical angular motion transducer known from US patent No. 3,815,126 a phosphor light source is mounted on an arm which rotates with a shaft carrying a pointer of a utility meter, and the angular position of which is to be detected for remote indication . The light source provides radiation wh ich selectively activates l ight sensitive sensor elements on a stationary code disc as a function of the shaft position . The known code disc comprises ten sensor elements each comprising a couple of conductors provided on an insulating support and separated from each other by a photo res istive material forming an electric connection between the conductors when illuminated. In order to achieve a good electrical connection between the conductors and to enable detection of an interposition between each two of said ten sensor elements the conductors are positioned in an interlacing zig-zag pattern . As to its function this known construction is equivalent to a transducer arrangement of relatively extensive discrete photo cells requiring individual termina l connections for reading out the position output signals of the transducer for transfer to signal processing circuits connected thereto. in particular it is a disadvantage of this prior construction that - within given small dϊmensions - it only enables the use of a sma ll number of sensor elements and that said elements require separate reading and signal processing means. Consequently, only a rough pos ition detection with a poor resolution can be achieved . In another known optical angular position transducer the code disc is positioned on or connected to the shaft, the angular position of which is to be detected. The code disc has several ring shaped concentric information tracks of alternate clear and opaque areas arranged in a certain coded pattern . The code disc is positioned between a light source and a l ight sensitive unit, such as a set of photo cells which are selectively activated of light from the light source dependent on the angular position of the disc and consequently of the shaft, in order to provide binary output signals.

Such optical transducers are disadvantageous in that they require comp lex and heavy optical systems to parallelize and focus the light from a light source to a narrow beam to be directed through the clear areas of the code disc on to the photo cells. Thereby the trans ducer necessarily gets a considerable extent in the direction of the beam. As the code disc is often made of g lass and consequently relatively heavy, a secure support is required in order to prov ide and ma intain the necessary very precise position of the light source, code disc, and photo cells with respect to each other. Because of this necessary high accuracy the system, besides, is sensitive to temperature f luctuations, variations of light level and mechan ical changes. Also diffusion and deflection phenomena occur with i llumination through a disc of a certain dimension in the direction of the beam. Therefore, if a high aceuracy and resolution is to be obtained it is required that this prior construction is manufactured with high mechanical accuracy and the entire transducer gets a considerable size which causes limitations in its use.

From US patent specification No. 3,303,490 an optical position transducer of the kind mentioned initial ly is known , wherein a stationary code disc is provided with concentric tracks each consisting of alternate light sensitive elements and insulating elements positioned between an inner and an outer conducting ring to be connected to a voltage source. Dependent on the fact whether a light sensitive element is illuminated or not a large or small voltage drop between the rings is achieved which voltage drop is used as the output signa l of the track. The individual tracks are radially spaced from one another in order to provide the necessary insulating clearance between the outer conducting ring of one track and the inner conducting ring of the track located radially outside thereof . These necessary insulating clearances contribute to a resulting relatively great diameter of the total set of concentric tracks and consequently of the entire disc. Furthermore, as the conducting rings - which in this prior art construction are formed by the printed circuit board techniques - extend through 360°, the connecting leads of the rings are to be arranged on the back side of the disc or to be laid under some of the other rings so that some conductors must cross each other, which as a whole results in a relatively complex manufacturing technique . Furthermore, in the prior art construction it is disadvantageous that the lamp for illumination of the light sensitive elements rotates with the shaft, the position of which is to be detected, as in that case the lamp both contributes to increase the weight of the rotating system and requires slip rings for its power supply which increases the required torque of the shaft. Disclosure of Invention

The object of the present invention is to provide an optica l position transducer of the kind stated in the introductory part which transducer is particularly compact and simple to manufacture, and by which the disadvantages of the prior constructions are avoided, and according to the invention this is achieved by the fact that the light sensitive elements are positioned in pairs of tracks between a central supply voltage conductor and outer reading conductors and that the code pattern on the code disc is shaped as a spiral , a circle, or a circular arc . Thereby only one supply voltage conductor is required for each two information tracks. Consequently, half of the insulation clearances between code tracks as wel l as half of the supply voltage conductors which were required in the last mentioned prior construction are eliminated, so that the total code pattern and consequently the entire code disc can be carried out with smaller dimensions. As a result of the spiral shaped code pattern angles of rotation of more than 360 may be detected, and the connecting leads to all supply voltage conductors and reading conductors may be carried out in a simple way without crossings on the same side of the code disc as the said conductors. By the measures stated a particularly compact and easily performed construction is achieved.

The code pattern of a photo sensitive material can be provided in any way but according to the invention it has proved appropriate that the code pattern cons ists of non-etched away or non -covered parts of a light sensitive coating applied on to the entire code disc because this as far as the manufacturing technique is concerned is particularly simple and, besides, enables the manufacture thereof in smaller dimensions as the printed circuit board techniques .

According to the invention it is advantageous that the device for providing the light line comprises a stationary light source and a light conducting member which is rotatab le around said source by means of the shaft, the position of which is to be detected, said light conducting member ending in a narrow slit located radially with respect to the code disc over the code pattern thereof. Thereby it is achieved that the rotatable part of the transducer may be carried out with an extremely low weight and without slip rings or other electrica l connections so that it wil l form no considerable load for the driving shaft, the angular position of which is to be detected .

According to the invention a particu larly low constructional height of the entire transducer may be achieved when the light conductor has a central bore for the light source and a bore extending radially therefrom, the outer end of wh ich is covered by an oblique mirror for directing the light from the light source down through the underlying radial sl it . In a further simp le embodiment accord ing to the invention the device for providing the light line comprises a stationary lamp and a light proof cover plate with a light permeable slit over the code pattern, said cover plate being rotatable with the shaft and positioned between the lamp and the code disc. As the signals provided by the code disc are low-level binary signals which normally require amplification and other kinds of processing, a light proof housing In which the code disc is mounted may advantageously also contain an electrical processing circuit for the signals provided by the code disc. Brief Description of Drawings

The invention will now be explained in more detail with reference to the drawings in which

Fig . 1 shows an embodiment of an optical position transducer according to the invention, Fig . 2 at a larger scale and seen from above a section of a code disc in the transducer of fig. 1 , Fig. 3 a section of the code disc of fig. 1 at a still larger scale , Fig. 4 another embodiment of a code disc for use in an opti cal position transducer accord ing to the invention,

Fig. 5 a perspective view of a transducer with a rotatable cover plate for providing a light line, Fig. 6 the same as in fig. 5 but showing an axial section, Fig. 7a-7c a light conductor seen from below, in section , and from above, respectively, and Fig. 8 a further embodiment of a position transducer according to the invention in which the light conductor in Figs. 7a-7c is used . Modes for Carrying Out the Invention

Fig. 1 shows an optical position transducer in the form of an angular motion transducer having a stationary code disc 1 and a lighting device 2 for providing a light line on the code disc. The code disc 1 comprises an electrically insulating support 3, e. g. of a ceramic material , in which a code track 4 with connecting leads 5 is arranged in an outer peripheral area. The code track 4 shown at an enlarged scale in the sectional views in figs. 2 and 3 comprises a plurality of electrically conducting rings, supply voltage conductors 6 and reading conductors 7, respectively, and light sensitive material 8 positioned therebetween , e. g . CdSe-, Ge- or Si-material , which , when i lluminated, becomes electrically conducting in order to transfer the electrical voltage from a supply voltage conductor 6 to a reading conductor 7. The voltage taken off from the reading conductors is preferably transferred via signal processing circuits which appropri ately are arranged in the transducer on a separate plate or on the code disc itself. The signal processing does not form part of the present invention and will not be further explained. The signals produced are used in a normal way for supervision, control, remote indication etc. The electrically conducting rings 6 and 7 consisting of e.g. gold, silver, copper are - as will appear from figs. 2 and 3 - arranged with a reading ring 7 on each side of a supply voltage ring 6, although the rings 6 and 7 may, of course, a lso be arranged in pairs. The width and mutual distance of the rings may be e. g . 10-100μ. The light sensitive material is preferably arranged in accordance with a normal binary code pattern, particularly in accordance with the Gray-code, but also other arrangements of the light sensitive material are possible for providing trigonometrical and other functions.

The pattern of the light sens itive material may be performed by coating, such as vapour deposition or spray deposition of a light sensitive material on the total surface of the disc and a following etching-away or covering with a light proof material of those portions of the coating not forming part of the code pattern desired .

An embodiment of a code disc with a spiral shaped code track is shown in fig. 4 in which only four information tracks corresponding to a four bit transducer are illustrated for the sake of clarity. Here two supply voltage rings 6 are connected to a common termina l 15 while the reading rings 7 are connected to separate terminals 16. The position of the light sensitive material accord ing to the Gray-code is apparent from the drawing.

As only small dimensions of the conducting rings and the light sensitive material therebetween are required it is possible to arrange e.g. 8-12 code tracks corresponding to 2⁸ to 2¹² positions on a code disc having a diameter of about 3 cm.

The light line for illuminating the light sensitive material 8 between the supply voltage rings and the reading rings 6 and 7 may be provided in different ways. The width of the light line must be smaller than half of the shortest length of an area 8 in order to determine the position exactly. In the embodiment of fig. 1 the lighting device 2 consists of a stationarϊly mounted light source 9 arranged within a hollow shaft 10 which is connected to the shaft (not shown), the position of which is to be detected. The light from the lamp is directed on to the code track 4 through a tube 11 , the lower end of which is performed as a narrow slit positioned radially with respect to the code disc. It is to be understood that the transducer is built into a light proof housing (not shown) so that only the light sensitive material positioned immediately under the light slit is conducting .

A further embodiment of the lighting device is shown in figs. 5 and 6. In this case a cover plate 12, e.g. an opaque plate or a glass sheet with an opaque covering, connected to the rotating shaft, is provided with a light slit 13 through which the light from a stationary light source 14 is directed onto the code track 4.

While the embodiments of the lighting device stated above result in a relatively large constructional height, said height may be considerably reduced by the use of a light conductor system as shown in figs . 7a-7c.

The light conductor 17 shown consists of a member of e.g. light metal having a hub part 18 in wh ich a shaft 19 may be mounted. A cylindrical bore 20 is arranged for accomodating a light source 21 . A bore 22 extends from the bore 20 through one half of the l ight conductor, and at the outer end the bore 22 is covered by an oblique mirror 23. The outermost part of the light conductor in the area under the mirror is, as will appear from figs. 7a-7c, provided with a slit 24 extending from outside and into the bore 22 so that the light emitted from the l ight source has a beam path through the light conductor as shown by arrows in fig. 7b. The width of the slit may by e.g. about 50μm. The position transducer shown in fig . 8, in which transducer the light conductor 17 is used, comprises a housing of e.g. aluminium, having an upper part 25 which is light proof assembled with a lower part 26. A light source 21 is firmly mounted in the upper part 20 and extends downwards into the bore 20 in the light conductor 17 arranged in a cavity 27 in the housing, the shaft 19 of said light conductor being rotatably mounted in bearing bushings 28 in the lower part 26. Below the lower part 26 the shaft 19 supports a coupling device 29, e.g. a coupling fork, for connection with a shaft or an instrument pointer, the rotational position of which is to be detected . In the housing 25, 26 a code disc 1 is mounted together with a plate 30 supporting an electrical signal processing circuit for the signals received from the code disc 1 . On the lower surface of the housing 26 means 31 for mounting the position transducer is shown .

A specific use of the transducer according to the invention may be achieved by performing the code track as a program track in a program device for controlling machines etc. A further use of the transducer may be achieved by providing the rotatable shaft with a plurality of fixed rotational positions corresponding to the positions of the code disc. Thereby the transducer may operate as a kind of change-over switch enabling an extremely large number of positions.

While the embodiments stated above , are concerned with optical angular position transducers for detecting circular movements it is obvious that the code track may also be rectilinearly performed and the light line may be moved over the code track in a longitudinal motion.

A stated above optical position transducers of mechanically small dimensions may be provided by the invention , said transducers simultaneously having a high resolution, e. g. 8-12 bits. Moreover, the mechanical construction is simple and does not require an accurate mutual positioning of a plurality of components opposite each other. Furthermore, the movable part, such as the light conductor between the light source and the code track, can be performed with a very low weight and, consequently, with a minimum torque and inertia which enables the use of the transducer in connection with mechanical precision measuring instruments. All in all the construction can be performed at relatively low costs but nevertheless provides a high reliability, among other things because of the relatively few component parts of the transducer.

Claims

C la im s :

1. An optical position transducer comprising a code disc (1) and a lighting device (2) which are mutually movable, said code disc comprising a support (3) of electrically insulating material on which electrical conductors (6, 7) with interposing light sensitive elements (8) are provided, wherein the conductors (6, 7) are substantially concentric, and the interposing light sensitive elements (8) are arranged in a binary code pattern, and wherein the lighting device (2) is arranged so as to provide a narrow line shaped light beam extending over all the conductors (6, 7) substantially radially with respect thereto, c h a r a c t e r i z e d in that the light sensitive elements (8) are arranged in pairs of tracks between a central supply voltage conductor (6) and outer reading conductors (7) and that the code pattern on the code disc (1) is shaped as a spiral, a circle or a circular arc.

2. Position transducer according to claim 1, c h a r a c t e r i z e d in that the pattern of the code disc (1) is formed of non-etched or non-covered parts (8) of a light sensitive coating on the entire code disc.

3. Position transducer according to claims 1 or 2, c h a r a c t e r i z e d in that the lighting device (2) for providing the line shaped light beam comprises a stationary light source (9, 21) and a light conducting member (10, 11, 17) which is rotatable around said light source by means of the shaft, the position of which is to be detected, said light conducting member (11, 17) ending in a narrow slit (24) located radially with respect to the code disc (1) over the code pattern (4) thereof.

4. Position transducer according to claim 3, c h a r a c t e r i z e d in that the light conductor (17) has a central bore (20) for the light source (21) and a bore (22) extending radially therefrom, the outer end of which is covered by an oblique mirror (23) provided for directing the light from the light source down through the underlying radial slit (24).

5. Position transducer according to claims 1 or 2, c h a r a c t e r i z e d in that the lighting device (2) for providing the line shaped light beam comprises a stationary light source (14) and a light proof cover plate (12) with a light permeable slit (13) over the code pattern (4), said cover plate (12) being rotatable with the shaft and positioned between the light source (14) and the code disc (1).

6. Position transducer according to any of the preceding claims, c h a r a c t e r i z e d in that the light proof housing (25, 26) in which the code disc (1) is mounted, also comprises an electrical signal processing circuit (30) for the signals provided by the code disc (1).