GB2201508A

GB2201508A - Optical encoder

Info

Publication number: GB2201508A
Application number: GB08405287A
Authority: GB
Inventors: Graham Leonard Adams
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1984-02-29
Filing date: 1984-02-29
Publication date: 1988-09-01
Also published as: GB2201508B; GB8405287D0

Abstract

Rotation of an actuating ring 33 having a multi-lobed internal cam surface 34, such as by movement of a member whose movement is to be sensed, causes movement of a plunger 8 having a central web with an aperture 12 therein. In dependence on the position of the aperture 12 light can be transmitted from an optical fibre (13) via a lens (14) to a reflector element (11) and back down the optical fibre, or prevented from being so transmitted, thus encoding movement of the member as an optical signal. More than one aperture in the plunger web may be employed, for example, to increase the count resolution. The reflector element may be replaced by a return optical fibre which bypasses the plunger through a bore in the body member (1). Instead of using optical fibres, electrical conductors may be used and light generated adjacent the plunger on one side thereof by means of a light emitting diode, and transmission through the plunger aperture determined by a photodetector on the opposite side thereof. The optics in the shutter area are completely sealed from the environment by diaphragms (24) and the resulting cavity filled with silicone oil, thus enabling the encoder to operate in dirty or pressurised environments. <IMAGE>

Description

OPTICAL ENCODER This invention relates to an optical encoder that is to say an encoding device in which an optical signal is affected in dependence on a variable parameter, for example the position of a member when the encoder is employed as a position monitor.

According to the present invention there is provided an optical encoder including an optical path disposed along a first axis, a plunger slidable along an axis intersecting said first axis, an actuating ring disposed around said first axis and provided with an internal multi-lobed cam, with an odd number of lobes, engaging both ends of said plunger such that rotation of the ring about the first axis causes the plunger to slide along its axis and to act as a shutter for light transmitted along said path whereby for each complete rotation of the ring the transmission of light along said path is interrupted a plurality of times.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figs. 1 and 2 illustrate, respectively, longitudinal and transverse sections through part of one embodiment of encoder; Fig. 3 illustrates, on a larger scale, the lens and shutter arrangement of the embodiment of Figs. 1 and 2; Figs. 4 and 5 illustrate, respectively, loniudinal and transverse sections through a second embodiment of encoder; Fig. 6 illustrates a plan view of the connector assembly of the Figs. 4 and 5 embodiment, and Fig. 7 illustrates, on a larger scale, the lens and shutter arrangement of the embodiment of Figs. 4 and 5.

Referring firstly to the embodiment of Figs. 1 to 3, the encoder illustrated therein has a central body member 1 which has a threaded cylindrical end 2 and two shoulder portions 3 and 4 which are linked by a flat web 5. The body member 1 has a through hole 6, extending along its axis, which is intersected centrally of the web 5 by a second through hole 7 extending in a direction normal to the web and the body member axis. A plunger 8 slides in this second through hole 7 and is provided with a pair of slots 9 which are engaged, respectively, by an end of an optical fibre termination ferrule 10 inserted in the axial through hole 6 via threaded cylindrical end 2 and bv the end of a reflector element 11 inserted in the axial through hole 6 from the opposite end of the body member 1.The engagement of the ferrule 10 and the element 11 in the slots 9 prevents the plunger from turning about its axis. An aperture 12 in the web of material of the plunger between the slots can be brought into line with the axis of the body member so that the plunger is operable as a simple shutter permitting or preventing optical coupling between an optical fibre 13 of ferrule 10 and the reflector element 11.

The optical fibre 13 is coupled to an optical fibre cable (not shown) which serves to supply an optical signal to the encoder from a source remote from the encoder. In use, for sensing or monitoring applications, the encoder can thus be located remotely from a central monitoring station and connected thereto merely by the optical fibre cable.

Inside the ferrule 10 the optical fibre 13 is terminated with an 0.375 pitch graded-index cylindrical lens 14. This pitch value ensures that the light from the fibre 13 is focussed, when the shutter is open (see Fig. 3), on a second graded-index cylindrical lens 15, which is of 0.25 pitch and has a reflective coating 16 (Fig. 3) applied to its far end face. The 0.25 pitch of lens 15 ensures that light reflected by coating 16-is directed back along the same optical path, through the aperture 12, and thus back down the fibre optic cable and the reflected signal can be separated from the input signal by means of a branching coupler at the end of the fibre optic cable remote from the encoder.The lens 15 and coating 16, comprising the reflector element 11, are disposed in a protective sleeve assembly 17 which includes a collar 18 that serves to axially position the reflector element. To ensure that light is not reflected from the web of the plunger when the aperture is not aligned with the body member axis, and thus spuriously indicating the state of the plunger, it may be necessary to provide the plunger web with a non-reflective coating or surface finish. The space between the ferrule 10 and the reflector element 11 is filled with a liquid 19, such as a silicone oil, thus enabling operation in dirty environments and the withstanding of high external pressures. The use of such a liquid necessitates the use of cylindrical lens 14 and 15 since lenses with convex surfaces would have little refractive effect.

- A seal is effected between ferrule 10 and the body member 1 and between the reflector element 11 and the body member 1. The ferrule 10 is housed in an intermediate mounting 20 and a seal 21 is provided between this mounting and the ferrule, while a further seal 22 is provided between the mounting and the body member. A seal 23 is interposed between the collar 18 of the reflector element, 11 and the body member 1. Seals are effected between the plunger 8 and both major faces of the body member web 5 by means of diaphragms 24. In this way a sealed volume is defined containing the liquid 19. Sets of holes (not shown) through the web 5 within the area defined by the diaphragms 24 so as to allow the liquid to vent from side to side when the plunger is moved.The plunger 8 is secured to each diaphragm 24 by means of detachable end caps 25 screwed on to threaded studs (not shown) of the plunger which extend through a respective hole in each diaphragm. A bead 26 at the periphery of each diaphragm is held against the web 5 by means of a clamping member 27. Each clamping member has the cross-section of a minor segment of a circle, and together they fit against the major surfaces of the body member web 2 to form a circular cross-section whole.

Each clamping member is penetrated bya hole 28 large enough to accommodate movement of the central region of its associated diaphragm, and the flat face of the clamping member is provided with an annular groove 29 to receive and locate the diaphragm bead 26.

The clamping members 27 are held in position against the body member web 5 by a collar 30 screwed on to body member end 2 and a collar member 31 secured such as by bolt-headed screws 32 to shoulder portion 4. After fitting of one collar portion 30 or 31, an actuating ring 33 is engaged over the web portion of the body member to be trapped in position by fitting of the other collar portion 30 or 31. A washer 38 serves to retain the clamping members 27 in position prior to fitting of collar portions 30 and 31. The actuating ring 33 is provided with an internal cam surface 34 which engages both end caps 25 of the plunger 8 which therefore act as cam followers. The cam surface 34 illustrated has five lobes. The cam space 35 is preferably packed with grease which is retained by H-ring seals 36.

Rotation of the actuating ring 33 with respect to the rest of the assembly causes the plunger 8 to move back and forth thro the web 5. With the five lobe cam illustrated and a single aperture 12 in the plunger-web the encoder gives twenty counts (ten ON and ten OFF) per revolution of ring 33, that is there are ten possible positions of the cam surface with corresponding extreme movement (stroke) positions of the plunger and between each of the cam surface positions the plunger moves from a position in which input light from the fibre 13 is prevented from reaching the reflector element (an OFF position), to a position in which input light is permitted to reach the reflector element (an ON position) and thence to another OFF position, this providing two counts, ON and OFF.The plunger acts as a three-way switch which is OFF in the "up" and "down" positions, but ON in the centre position.

The rotation of the actuating ring 33 may be effected by, for example, a gear (not shown) meshing with external teeth of the ring 33 or a push rod provided with a rack engaging external teeth of the ring 33. The device or parameter to be monitored will, in general, determine the rotation causing means.

If two such encoders are mounted relative to a device to be monitored such that they are 900 out of phase then the direction of rotation of the actuating ring can be determined as with incremental electronic encoders enabling determination of which way a device is moving, for example, as well as indicating that it is moving. If a third encoder is also employed to provide a marker then the extent of movement can also be determined. The use of two encoders also doubles the count resolution. Other methods of changing the count resolution include changing the number of cam lobes, there must however be an odd number, which is at least three, increasing the number of apertures in the plunger, and by gearing up or down between the encoder and the device being monitored.

The collar 30 may be such as to provide a means of mounting the encoder to a structure adjacent a device to be monitored. The input optical fibre cable is preferably connected to fibre 13 within an extension of the intermediate mounting 20, the joint thus formed being protected by tapered polyethylene moulding 37 (only part of which is shown) provided over the intermediate mounting. A heat shrunk sleeve (not shown) may be employed to secure the moulding to the collar 30. Fig. 4 indicates a similar polyethylene moulding and heat shrunk sleeve on another embodiment of encoder.

The embodiment of encoder illustrated in Figs. 4 to 7 whilst also employing an optical signal and a shutter for affecting it differs from the embodiment of Figs. 1 to 3 in that the optical signal is generated within the encoder, rather than being coupled thereto from a remote source via an optical fibre, and that the optical signal is not reflected. This encoder whilst still optically encoding information may be termed an opto-electronic encoder, since the input signal is electrical and the encoded information is transmitted to a monitoring station remote from the encoder electrically rather than optically.

The same reference numerals are employed in Figs. 4 to 7 as those in Figs. 1 to 3 for similar parts, and only those parts which differ will be specifically described in the following.

An electrical cable 40 having a minimum of four insulated conductors 41 extends into a connector assembly 42 (Figs. 4 and 6) which comprises a cup portion 43 with an axially extending cable guide 44, a cap portion 45 with an axially extending portion 46 having a stepped bore housing a light emitting diode 47 and an 0.29 pitch graded index cylindrical lens 48. Two of the conductors 41 are employed to supply drive current for the light emitting diode 47 whereas each of the other two conductors 41 extends through a respective cable support tube 49 (Fig. 6) carried by cap portion 43 and comprises a return co. s r as will be X parent from the following. The cable support tubes 49 are disposed in respective bores 50 (Fig. 5) of the body member 1 extending parallel to the through hole 6 and on opposite sides thereof.For reasons of clarity the conductors 41 are not all shown fully in Fig. 4.

The plunger 8 and its sealing arrangement differ from that in the Figs. 1 to 3 embodiment in that two apertures 51 are provided in the plunger web 52 rather than a single aperture 12. The cylindrical lens 48 focusses the light from the light emitting diode 47 to a point on the plunger web (shutter) 52 centre line (Fig.

7) this enables sufficient resolution to use more than one aperture in the plunger web, hence the two apertures 51. The light path is indicated by dashed lines in Fig.

7. ben light focussed by lens 48 passes through an aperture 51 it is affected by a second 0.29 pitch graded index cylindrical lens 53 which focusses it on a photo detector such as a phototransistor or photodiode 54. The conductors 41 extending through the support tubes 49 are connected to the detector and serve to provide an electrical signal to a monitoring station via the cable 40 whenever an aperture 51 is disposed to permit light two be transmitted therethrough. If necessary the detector output may be amplified by an amplifier such as 55 disposed within the encoder and powered from the monitoring station via suitable electrical conductors 56 (Fig. 5) which also extend through the support tubes 49, in which case the electrical cable 40 will comprise six insulated conductors.A polyethylene moulding 57 extends over the cable guide 44 and an adjacent portion of the cable 40 extending therefrom. The axially extending portion 46 of the cap portion 45, and the light emitting diode 47 and the lens 48 comprise a unit equivalent to the optical fibre termination ferrule 10 of the Figs. 1 to 3 embodiment. The photodetector 54 and lens 53 are disposed within a sleeve assembly 58 equivalent to sleeve assembly 17 of the Figs. 1 to 3 embodiment. The sleeve assembly 58 and the shoulder portion 4 are adapted to house amplifier and the connection wires. After insertion of the connector assembly 42 into the body member 1 and fitting of collar 30, a retaining member 59 for the connector assembly is screwed into a threaded bore of the collar 30 until a shoulder 60 thereof abuts the collar 30.A heat shrinkable sleeve 61 is disposed to extend over the axially extending portion of the retaining member 59 and an adjacent portion of the polyethylene moulding 57 and heat shrunk thereon.

The use of two apertures 51 in the plunger web 52 with a five lobe cam surface 34 means that the count resolution of 20 for the Figs. 1 to 3 embodiment is doubled to 40 for the Figs. 4 to 7 embodiment. The two 0.29 pitch cylindrical lenses were employed in order to keep the shutter stroke to a minimum by focussing the light to a point on the centre line of the shutter. With a lmm wide shutter web and a 4mm shutter stroke as employed in a particular practical embodiment it is considered that a maximum count resolution of 100 is probably obtainable using five apertures.

A further embodiment of encoder using fibre optics but not operating on the reflective principle as in the Figs. 1 to 3 embodiment can be envisaged. In such an embodiment of encoder the input light signal is supplied to the shutter via first fibre and a lens, this lens being such that when the input light signal can be transmitted through an aperture in the shutter it is collected by a further lens and focussed into a return optical fibre which extends through the body member in a cable support tube similar to tube 49 as in Fig. 6. Thus two optical fibres are required in an optical fibre cable connected between the encoder and a monitoring station and a suitable connector assembly with a corresponding bore in the body member for the return optical fibre cable S nort tube are required.

In all of the various embodiments described the optics in the shutter area are completely isolated from the environment in which the encoder is disposed by diaphragms, and the resulting cavity is filled with liquid, such as silicone oil, so that the encoder will operate in pressurised environments, such as deep water.

The use of a shutter (plunger) in a multi-lobed internal cam ring avoids the problems associated with sealing a rotating shaft.

Claims

CLAIMS:

1. An optical encoder including an optical path disposed along a first axis, a plunger slidable along an axis intersecting said first axis, an actuating ring disposed around said first axis and provided with an internal multi-lobed cam, with an odd number of lobes, engaging both ends of said plunger such that rotation of the ring about the first axis causes the plunger to slide along its axis and to act as a shutter for light transmitted along said path whereby for each complete rotation of the ring the transmission of light along said path is interrupted a plurality of times.

2. An optical encoder as claimed in claim 1 including a body member the optical path being disposed along an axis of the body member, the plunger being arranged in the body member and the actuating ring being disposed around the body member and rotatable relative thereto.

3. An optical encoder as claimed in claim 2, wherein the plunger is sealed to the body member whereby to form a sealed chamber which incorporates the intersection of the body member axis and the plunger axis.

4. An optical encoder as claimed in claim 3, wherein the plunger is sealed to the body member by a pair of diaphragms, whereby the plunger is enabled to slide in the body member without incurring any sliding movement of its seals relative either to the plunger or to the body member.

5. An optical encoder as claimed in claim 3 or claim 4, wherein said sealed chamber is liquid filled.

6. An optical encoder as claimed in any one of the preceding claims, wherein the actuating ring is externally toothed.

7. An optical encoder as claimed in any one of the preceding claims, wherein the plunger includes one or more apertures via which light can be transmitted along slid path between opposite sides of the plunger.

8. An optical encoder as claimed in claim 7, including light input means and means for reflecting light, transmitted between opposite sides of the plunger from said light input means, back towards said light input means.

9. An optical encoder as claimed in claim 8 wherein said light input means includes an optical fibre and a first graded index cylindrical lens and wherein said light reflecting means includes a second graded index cylindrical lens with a reflective coating applied to one end thereof, the first graded index lens being such as to focus light from said optical fibre on the other end of the second graded index when light is transmitted along said path.

10. An optical encoder as claimed in claim 7 including light input means disposed on one side of the plunger, and on the other side of the plunger means for detecting light transmitted along the optical path, and - means coupling the detecting means to a position on the one side of the plunger via a separate path.

11. An optical encoder as claimed in claim 10 as appendant to claim 2 wherein the light input means includes a first optical fibre and a first graded index cylindrical lens, wherein the detecting means comprises a second graded index cylindrical lens and wherein the coupling means comprises a second optical fibre, the second optical fibre extending in a respective bore of the body member comprising said separate path.

12. An optical encoder is claimed in claim 10 as appendant to claim 2, wherein the light input means includes a light emitting diode and a first graded index cylindrical lens and a photodetector, and wherein the coupling means comprises at least one electrical conductor coupled to the photodetector and extending through a respective bore of the body member comprising said separate path.

13. An optical encoder substantially as herein described with reference to and as illustrated' in Figs. 1 to 3 or 4 to 7 of the accompanying drawings.