GB2134341A - Position encoders - Google Patents

Abstract

A positional encoder including a marked scale mounted on a member movement of which results in movement of the scale past a scale sensing station, wherein the scale comprises three parallel tracks each provided with a binary pattern, the sensing station being provided with three sensor means one for each track, characterised in that the two inner tracks are marked to produce pairs of coincident markings forming cyclic sequences of n pairs and the third outer track is marked with a reference marking at the commencement of each sequence of n pairs and also with additional markings according to an incremental binary code wherein said code increments once for each sequence of n pairs. For a 360 DEG scale with 1 DEG increments the pair sequence is 11, 00, 01, 10, 00, 01, 10, 00, 01, 10. The three tracks provide a reference 111 combination every 10 DEG and the decades are coded according to a straight binary code on the third track. An initial maximum movement of 10 DEG is required to establish an unambiguous absolute position. <IMAGE>

Description

SPECIFICATION Position encoders This invention relates to position encoders such as are used for sensing and measuring angular movement of a shaft or linear movement of a member relative to another member.

Shaft encoders are well known and typically take the form of a circular scale marked according to a digital code which is read by a stationary sensor. Common scales in use include simple Binary, Gray, Binary Coded Decimal. The choice of code depends partially on the application, e.g. incremental or absolute position encoding, partly on the number of sensors required to read the code, and partly on the complexity of the electronics required to translate the code readings into useful information.

Consider for a moment a simple Binary code for a disc to provide a unique code output for every 10 of rotation. Such a code requires six concentric track markings and six sensors to provide binary counts from 000000(0") to 100011(3500). Another disc marked in a Gray code to provide a count every 1" from 000000000(00) to 111010100(3590) will require nine tracks and nine sensors. Whereas both these codes are not dependent on disc rotation to determine disc position they both need a considerable number of sensors.

A disc requiring only three sensors can be produced in which a count is given every 10 but which requires a movement of up to 40 before a coarse reading can be obtained and up to 1200 before an exact position can be determined. However, complicated electronics are required to decode the code signals read from the disc.

Conversely a three sensor arrangement with very simple electronics can be devised which gives an incremental reading every 1" but requires up to 3600 rotation to establish a reference position for the disc.

In general it may be remarked that linear position encoders are the equivalent of angular encoders in which the circumferential scale has been opened out into a straight scale, incremental readings now being taken at units of linear movement instead of at degrees of angular movement.

According to the present invention there is provided a positional encoder including a marked scale mounted on a member movement of which results in movement of the scale past a scale sensing station, wherein the scale comprises three parallel tracks each provided with a binary pattern, the sensing station being provided with three sensor means one for each track, characterised in that two of the tracks are marked to produce pairs of coincident markings forming cyclic sequences of n pairs and the third track is marked with a reference marking at the commencement of each sequence of n pairs and also with additional markings according to an incremental binary code wherein said code increments once for each sequence of n pairs.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates in section a rotary dial position encoder utilising a fibre-optic sensing means, Figure 2 illustrates a three-track scale, Figure 3 illustrates in section an alternative rotary position encoder, Figure 4 illustrates the construction of a fibre-optic sensing head, Figure 5 illustrates a duplex connector arrangement for a fibre-optic sensing means, and Figures 6-9 illustrate an optical circuit for a fibre-optic sensing means.

The dial position encoder illustrated in Figure 1 comprises a drive shaft 10 running in ball bearings 11 supported in a housing base 12. Mounted on the end of the shaft is a drum 13 carrying on its side wall 14 a reflective strip 15 which provides a marked scale (as described below). The end surface of the drum 13 has affixed to it a pointer 16 behind which there is a conventional dial 17 and the whole structure is enclosed in a cover 18 provided with a glass window 19. Inserted into an aperture in the side of the housing 12 is a sensing head 20 provided with optical fibres 21. Light from a remote monitor (not shown) passes down the optical fibres to illuminate the scale 15.Light is reflected from those parts of the scale not covered by black non-reflecting areas and the reflected light passes back along the optical fibres to the remote monitor where the optical signals are detected and converted into electrical pulses.

The scale pattern is illustrated in Figure 2, although for simplicity of illustration the actual scale shown is a version used in the alternative construction of Figure 3. The scale has three parallel tracks, inner, middle and outer. The inner and middle tracks are marked to provide a cyclic sequence of pairs of coincident markings (again for simplicity of illustration the reflecting areas are shown in black whilst the non-reflecting areas are represented by the plain paper). Thus the two inner tracks provide a repeated sequence of 11, 00, 01, 10, 00, 01, 10, 00,01, 10. The middle track provides the first marking of each pair and the inner track the second marking of each pair. There are a total often pairs in the sequence and the angular spacing of the markings is equal to 10 for each pair.The ten-pair sequence is thus repeated every 10 round the scale. The outer track has one reference marking every 10 and, located between the reference markings, markings arranged according to a natural binary code such that the code value is incremented once every 10 . The following table shows the complete scale arrangement for the sector 0 -30 .

Degrees Inner Middle Outer Swept Inner and Middle Tracks Track Track Track Code Clockwise Anticlockwise (Outer) Sweep Sweep 0 1 1 1 REF 0011 1 0 0 0 1100 1000 2 1 0 0 0010 0110 3 0 1 0 1001 0001 4 0 0 0 01 0100 1000 5 1 0 0 0010 0110 6 0 1 0 1001 0001 7 0 0 0 0100 1000 8 1 0 1 E / 0010 0110 9 0 1 0 1 1001 1101 10 1 1 1 REF 0111 0011 11 0 0 0 1100 1000 12 1 0 0 0010 0110 13 0 1 0 1001 0001 14 0 0 0 0100 1000 15 1 0 0 10 0010 0110 16 0 1 0 | 1001 0001 17 0 0 1 | 0100 1000 18 1 0 0 / 0010 0110 19 0 1 0 1001 1101 20 1 1 1 REF 0111 0011 21 0 0 0 1100 1000 22 1 0 0 0010 0110 23 0 1 0 1001 0001 24 0 0 0 0100 1000 25 1 0 0 11 0010 0110 26 0 1 0 1001 0001 27 0 0 1 0100 1000 28 1 0 1 \ g 0010 0110 29 0 1 0 1001 1101 30 1 1 1 REF 0111

Note that the direction of rotation can be determined unambiguously from the sequence in which successive pairs of inner and middle track markings are presented to the sensing head. Thus a sequence of 11 00 can only occur during a clockwise rotation whilst a sequence of 10 00 can only occur during an anticlockwise rotation.Once the scale has passed through a reference point, i.e. when a combination 111 occurs on all three tracks, then the succeeding pairs of markings on the inner and middle tracks will give unambiguous incrementing or decrementing at 10 intervals through the adjacent decades of 10 . Once the scale has moved through a maximum of 100 not only is the position of the scale determinable to an accuracy of 1" relevant to a decade but the decade code will also be determined as the outer track will also have presented a binary count to the sensing head.Thus, using only three tracks and requiring a maximum initial movement of 10 it is possible to encode the shaft rotation at 1" intervals over a full 3600. The electronic logic circuits required to decode the signals received from the sensing head can be readily designed in accordance with the above encoding rules by those skilled in the art and do not therefore form part of the present invention.

Figure 3 shows an alternative construction for a shaft position encoder without a dial and pointer. In this construction the shaft 11 running in bearings 11 supported in a housing 12 carries a disc 22 the surface of which carries a face mounted scale 23, illustrated in Figure 2. The housing is closed by a cover 24 which is provided with an apertured boss 25. The boss 25 forms a mounting for a sensing head 20 carrying three optical fibres 21.

Figure 4 illustrates the construction of a fibre optic sensing head. A plastic moulding 30 of a transparent material has recesses 31 each formed with tapering walls and a rounded or hemispherical base. The ends of the optical fibres 32 are inserted into the recesses and a quantity of index matching fluid 33 is inserted whereby the core of the fibre is intimately optically coupled to the transparent plastic material, the fluid in the hemispherical base of the recess forming a focussing lens, depending on the relative indices of the fluid and the plastics material, Also, the use of a coupling fluid makes polishing of the ends of the fibres unnecessary.

(Note that only two fibres are shown). Light sent down the fibre from a remote monitor will be directed through the transparent plastics material onto the reflecting scale 34, e.g. polished aluminium with black anodised markings. Light reflected from the polished areas 34a is sent back down the fibre to the remote monitor, whilst no light is reflected from the black areas 34b.

The fibres are conveniently used in a duplex mode by means of the coupling arrangement shown in Figure 5 at the remote monitor. Incident light from a light source, e.g. a light emitting diode (not shown) is coupled into the fibre core and reflected light from the scale is coupled out of the fibre primary cladding to a photodetector (not shown). An alternative construction is shown in Figures 6-9 in which an integrated optical circuit is used. A monomer doped polymer film 40 (Figure 7) is first prepared and coated with a mask 41 to define light guiding channels 42. After exposure of the masked film to ultraviolet light 43 the unreacted monomer is evaporated away and surface cladding layers 44 are formed. The result is a totally enclosed light guiding channel which is shaped as required by the marking operation.As shown in Figure 6 the light guiding channels are shaped to provide Y-couplers. The stem of each coupler is coupled to an optical fibre 45 and the two branches are coupled to a light emitting diode 46 and a photodetector 47 respectively.

It is clear that there are other applications for position encoders according to the invention, e.g. they can be used to provide a combined speed and position sensor for a motor shaft, monitors for multi-position switches, temperature gauges when used in conjunction with temperature responsive device which produce movement of a member in response to temperature changes, likewise pressure gauges, accelerometers, in plotting tables, pump metering, liquid level measurement, tape control and machine tool position control.

The use of a remote monitoring circuit coupled to the encoder via fibre optics make the invention particularly suitable for safe use in hazardous environments, such as inflammable or explosive atmospheres.

Claims (9)

1. A positional encoder including a marked scale mounted on a member movement of which results in movement of the scale past a scale sensing station, wherein the scale comprises three parallel tracks each provided with a binary pattern, the sensing station being provided with three sensor means one for each track, characterised in that two of the tracks are marked to produce pairs of coincident markings forming cyclic sequences of n pairs and the third track is marked with a reference marking at the commencement of each sequence of n pairs and also with additional markings according to an incremental binary code wherein said code increments once for each sequence of n pairs.

2. An encoder according to claim 1 wherein said sensing station comprises a fibre-optic sensing head carrying three optical fibres each coupled at its end remote from the sensing head to a light source and a respective photodetector, the fibre ends in the sensing head being arranged so that each fibre illuminates a respective scale track and light reflected from the tracks is passed back along the respective fibres to the photodetector coupled thereto.

3. An encoder according to claim 1 or 2 wherein said member comprises a rotatable disc, said scale comprising concentric circular scale tracks on one face of the disc.

4. An encoder according to claim 1 or 2 wherein said member comprises a circumferential surface, said marked scale comprising parallel tracks on said circumferential surface.

5. An encoder according to claim 4 wherein said circumferential surface is provided by a drum affixed to the end of a rotatably mounted shaft, the end of said drum remote from the shaft cooperating with a stationary housing to provide a dial/pointer mechanism.

6. An encoder according to any preceding claim wherein said sequence ofn pairs comprises 11,00,01, 10,00,01, 10,00,01,10 with said reference marking being a 1 coincident with the 11 pair of the sequence.

7. An encoder according to claim 2 wherein the fibre optic sensing head comprises a block of transparent plastics material formed with recesses each with tapering walls and a rounded base, each recess having a fibre end inserted therein together with a quantity of index matching fluid forming a lens structure between the fibre end and the plastics material.

8. An encoder according to claim 1 or claim 6 when dependent on claim 1 wherein said scale is a straight scale for linear movement measurement.

9. A positional encoder substantially as described with reference to Figures 1 and 2 or Figures 2 and 3 of the accompanying drawings.