POSITION MEASUREMENT DEVICE

Abstract

An encoder apparatus including a reflective scale and a readhead. The readhead includes at least one light emitting element, at least one sensor and at least one optical device, which together with the scale form an optical system in which the optical device forms an image of an illuminated region of the reflective scale onto the sensor. The system's optical path, from the light emitting element to the sensor, passes through the optical device on its way toward and after reflection from the scale. and includes an unreflected optical path between the light emitting element and the optical device and an unreflected optical path between the optical device and the sensor.

Claims

1. An encoder apparatus comprising a reflective scale and a readhead, the readhead comprising at least one light emitting element, at least one sensor and at least one optical device, which together with the scale form an optical system in which the optical device forms an image of an illuminated region of the reflective scale onto the sensor, and in which the system's optical path, from the light emitting element to the sensor, passes through the optical device on its way toward and after reflection from the scale, and comprises an unreflected optical path between the light emitting element and the optical device and an unreflected optical path between the optical device and the sensor.

2. An encoder apparatus as claimed in claim 1, in which the light emitting element is mounted to the readhead such that, in the dimension perpendicular to the plane of the sensor, the light emitting element is situated between the sensor and the optical device.

3. An encoder apparatus as claimed in claim 1, in which the light emitting element is located in the space between the sensor and the optical device.

4. An encoder apparatus as claimed in claim 1, in which the light emitting element is located substantially at the optical device's focal plane such that light emitted thereby is collimated by the optical device.

5. An encoder apparatus as claimed in claim 1, in which light reflected by the scale and imaged onto the sensor by the optical device converges toward a point at a particular distance between the optical device and the sensor, and in which the light emitting element is located approximately at said particular distance between the optical device and the sensor.

6. An encoder apparatus as claimed in claim 2, in which the ratio of i) the distance between the centre of the light emitting element's emission surface to the sensing plane of the sensor, in the direction perpendicular to the plane of the sensor, and ii) the distance between the centre of the light emitting element's emission surface to the optical device, in the direction perpendicular to the plane of the sensor, is not less than 35:65.

7. An encoder apparatus as claimed in claim 1, in which the light emitting element is positioned such that it is offset from the optical device's optical axis.

8. An encoder apparatus as claimed in claim 1, in which the direction of the optical path as it impinges on and/or reflects from the scale is not perpendicular to the scale.

9. An encoder apparatus as claimed in claim 1, in which the system's optical path, from the light emitting element to the sensor, is substantially diamond-shaped.

10. An encoder apparatus as claimed in claim 1, in which the light emitting element and the sensor both face the optical device and scale.

11. An encoder apparatus as claimed in claim 1, in which the sensor, and the image of the scale formed by the optical device, lies behind the light emitting element.

12. An encoder apparatus as claimed in claim 1, in which the optical device comprises a lens.

13. An encoder apparatus as claimed in claim 1, in which the scale comprises an absolute scale which comprises features defining a series of unique positions along its length, and in which the apparatus is configured to extract absolute position information from the image obtained by the sensor.

14. An encoder apparatus as claimed in claim 1, in which the sensor is mounted to a printed circuit board (PCB), and in which the light emitting element is electrically connected to the same PCB as the sensor, but is physically mounted to the readhead by a support member which holds the light emitting element away from the PCB so as to provide a gap between the light emitting element and the sensor.

15. A readhead for reading a reflective scale to determine their relative position, the readhead comprising a light emitting element, a sensor and an optical device, for illuminating and for forming onto the sensor an image of a region of a reflective scale located adjacent the readhead, and in which light from the light emitting element passes through the optical device on its way toward and after reflection from the scale, in which the path of the light between the light emitting element and the optical device is unreflected, and the path of the light between the optical device and the sensor is unreflected.

Description

[0035] Embodiments of the invention will now be described, by way of example only, with reference to the following drawings, in which:

[0036] FIG. 1 shows a schematic isometric view of a position measurement device according to the present invention;

[0037] FIG. 2 is a schematic diagram of the various optical and electronic components of the readhead of the position measurement device of FIG. 1;

[0038] FIGS. 3a and 3b show schematic diagrams of the optical arrangement of the position measurement device of FIGS. 1 and 2;

[0039] FIG. 4 shows a cross-sectional view of the readhead of FIG. 1;

[0040] FIG. 5 shows an isometric view of the circuit board, light source, light source support structure, and sensor of the readhead of FIG. 1;

[0041] FIGS. 6a and 6b show isometric front and rear views of the light source's support structure;

[0042] FIG. 6c shows a top plan view of the light source's support structure;

[0043] FIGS. 7a and 7b shows isometric front and rear views of the support structure for the light source's bond wire;

[0044] FIG. 8 shows an alternative embodiment of a readhead according to the present invention;

[0045] FIGS. 9a and 9b schematically illustrate plan views of a readhead according to the present invention wherein the light source is positioned directly over the sensor.

[0046] Referring to FIG. 1 there is shown an encoder apparatus 2 comprising a readhead 4, scale 6 and controller 7. The readhead 4 and scale 6 are mounted to first and second objects respectively (not shown) which are moveable relative to each other. The velocity of relative movement can vary, but in the described embodiment the readhead 4 and scale 6 have a known maximum relative acceleration.

[0047] In the embodiment described, the scale 6 is a linear scale. However, it will be understood that the scale 6 could be a non-linear scale, for example a rotary scale (e.g. disc or ring scale). Furthermore, the scale 6 enables measurement in a single dimension only. However, this need not be the case, and for example the scale could enable measurement in two dimensions.

[0048] In the described embodiment, the scale 6 is an absolute scale and comprises a series of reflective 8 and non-reflective 10 lines arranged to encode unique position data along its length. The data can be in the form of, for instance, a pseudorandom sequence or discrete codewords. In other embodiments, the scale could be an incremental scale (with or without a reference mark).

[0049] The width of the lines depends on the required positional resolution and is typically in the range of 1 μm to 100 μm, and more typically in the range of 5 μm to 50 μm, for instance in the range of 10 μm to 30 μm. In the described embodiment, the width of the lines is in the order of 15 μm. The reflective 8 and non-reflective 10 lines are generally arranged in an alternate manner at a predetermined period. However, select non-reflective lines 10 are missing from the scale 6 so as to encode absolute position data in the scale 6. For instance, the presence of a non-reflective line can be used to represent a “1” bit and the absence of a non-reflective line can represent a “0” bit.

[0050] As illustrated in FIG. 2 the readhead 4 comprises a light emitting element/source 12 an optical device 18, a sensor 20, and a window 22. In this embodiment, the light emitting element/source 12 comprises a light emitting diode (LED). Also, in this embodiment the optical device comprises a lens 18, but other optical devices could be used. For instance, a diffractive optical element, such as a Fresnel zone plate, and/or a holographic optical element could be used, for example a Hologram of a lens. In this embodiment, the sensor 20 comprises a Complementary Metal-Oxide-Semiconductor (“CMOS”) sensor. As will be understood, other image sensors could be used instead of a CMOS sensor. For instance, a CCD or a photodiode array could be used instead.

[0051] The readhead 4 also comprises a CPU 24, a memory device 25 (for example, Electrically Erasable Programmable Read-Only Memory (EEPROM) or Flash memory) and an interface 26. The readhead 4 may also include an analogue-to-digital converter to digitize the image data from the sensor 20. Optionally, the analogue-to-digital conversion could be performed within the sensor 20 or the CPU 24.

[0052] Light emitted from the LED 12 is collimated by the optical device 18, then passes through the window 22 and falls on the scale 6. The scale 6 reflects the light back through the window 22 which passes through the lens 18 which in turn forms a two-dimensional image of the scale onto the sensor 20 using the light reflected by the scale. Accordingly, the sensor 20 detects a two-dimensional image of a part of the scale 6 illuminated by the LED 12. The sensor could comprise a one or two dimensional array of pixels. For instance, the sensor could comprise a one-dimensional array of 256 elongate pixels, whose lengths extend parallel to the lengths of the reflective 8 and non-reflective lines 10 on the scale. Instead of the two-dimensional imaging arrangement described, a one-dimensional imaging arrangement could be used instead, in which a one-dimensional image of the scale is formed by the lens on the sensor.

[0053] The LED 12 is connected to the CPU 24 so that the LED 12 can be operated on demand by the CPU 24. The sensor 20 is connected to the CPU 24 such that the CPU 24 can receive an image of the intensity of light falling across the image sensor 20. The sensor 20 is also directly connected to the CPU 24 so that the sensor 20 can be operated to take a snapshot of intensity falling across it on demand by the CPU 24. The CPU 24 is connected to the memory 25 so that it can store and retrieve data for use in its processing. The interface 26 is connected to the CPU 24 so that the CPU 24 can receive demands from and output results to an external device such as a controller 7 (shown in FIG. 1) via line 40. The line 40 also comprises power lines via which the readhead 4 is powered.

[0054] As will be understood, absolute position data could be encoded in the scale 6 by missing reflective lines 8, as well as, or instead of, missing non-reflective lines 10. Furthermore, absolute position data could be embedded in the scale 6 without the addition or removal of reflective 8 or non-reflective lines 10. For instance, the width of lines, the distance between them or their colour could be varied in order to embed the absolute position data in the scale 6. Furthermore, rather than the scale defining absolute position by the unique combinations of features taken along the scale's measuring length, the scale could have features defining absolute position by the unique combination of features taken along the width of the scale. For instance, the scale could comprise a plurality of “barcodes” the length of which extend across the scale, e.g. substantially perpendicular to the scale's measuring length. Optionally, the scale could comprise a plurality of tracks, in which at least one, optionally at least two and possibly all of these tracks could comprise a plurality of regularly spaced features (i.e. the tracks could essentially comprise incremental scale features of different fundamental frequencies) in which the scale period of the tracks differ from each other such that the combination of features across the scale's width is unique at any one point along the scale's measuring length.

[0055] A series of groups of markings can be used to encode a series of unique binary codewords along the scale length defining unique, i.e. absolute, position information, whilst still having sufficient information in order to enable phase information to be extracted from the series of markings to enable fine position information to be determined (e.g. position information with a resolution finer than the period of the scale markings). Accordingly, in such systems, the position information can be made up from a coarse absolute position (determined from the codeword extracted from the image) as well as a fine position (determined by looking at the phase offset of the substantially periodic markings). Further details of such a so-called hybrid incremental and absolute scale is described in International Patent Application no. PCT/GB2002/001629 (publication no. WO 2002/084223), the content of which is incorporated in this specification by this reference.

[0056] In an alternative embodiment, the scale could comprise an absolute track comprising features defining absolute position information and a separate incremental track comprising regularly spaced features.

[0057] The optical system of the readhead 4 of FIGS. 1 and 2 will be described in more detail with reference to FIGS. 3a and 3b. FIGS. 3a and 3b schematically illustrate the path light takes through the optical system which forms the image of the scale 6 onto the sensor 20, from the light source 12 to the sensor 20.

[0058] As shown, the optical device 18 comprises a lens 18 having an optical axis OA, a focal length f and a focal plane fp. As shown, the point light source 12 is located substantially at the lens' 18 focal plane fp, but slightly offset from the lens' 18 optical axis OA. For instance, the light source 12 is offset (measured from the centre of the light source's emission zone) by approximately 450 μm, from the lens' 18 optical axis OA. In particular, the ratio of the offset to the focal length of the lens is approximately 0.45:2.5. Locating the light source 12 substantially at the lens' focal plane 18 helps to ensure that light emitted therefrom is substantially collimated by the lens 18 as it heads toward the scale 6. Accordingly, the light reflected by the scale 6 is then focused by the lens 18 to a point at the lens' 18 focal plane fp before diverging and forming a two-dimensional image of the scale 6 at the sensor 20 behind the light source 12. As will be understood, an image of the light source 12 will be formed at the focal plane fp. Locating the light source 12 at lens' 18 focal plane fp, but offset from the lens' 18 optical axis OA, means that the light source 12 can be located in the space (or “volume”) between the sensor 20 and the lens 18 (illustrated by the hatched area shown in FIG. 3b), helping to make the readhead compact, but not be in the way of the light reflected by the scale on its return path to the sensor 20.

[0059] As shown, both the light source 12 and the sensor 20 face the lens 18 (and the window 22 and the scale 6). (In other words, the emission surface of the light source 12, and the sensing surface of the sensor 20, face the lens 18). Also, there is an unreflected (in other words “direct”) optical path between the light source 12 and the lens 18, and also an unreflected (in other words “direct”) optical path between the lens 18 and the sensor 20. No reflective optical components are therefore needed or used to turn or steer the light. Avoiding the use of reflective optical components, such as mirrors and beam-splitters, can help to significantly reduce the size of the readhead.

[0060] Furthermore, in the particular embodiment described, the same optical device/lens 18 is used to both collimate the light from the light source 12 and to form an image of the scale 6 onto the sensor. Accordingly, the optical arrangement of the described readhead 4 only uses one optical device/lens 18 and so is particularly compact and inexpensive. In the embodiment described, the lens 18 is a singlet lens, but could be a different type of lens (e.g. a doublet lens, compound lens or gradient-index (GRIN) lens. As will be understood, the optical device need not necessarily be a lens, but could be another type of optical device such as a Fresnel Zone Plate or a holographic optical element (HOE), for instance a hologram of a lens.

[0061] As illustrated in FIG. 3a, the light source is much closer to the lens 18 than it is to the sensor 20. Such a configuration departs from a traditional encoder design where the light source would normally be mounted to the same board as the sensor, approximately in plane with the sensor. As shown in FIG. 3a, in this embodiment, the readhead is configured such that the ratio of i) the distance (D1) between the centre of the light emitting element's emission surface (or emission point) to the sensing plane of the sensor, in the direction perpendicular to the plane of the sensor, and ii) the distance (D2) between the centre of the light emitting element's emission surface (or emission point) to the optical device, in the direction perpendicular to the plane of the sensor, is approximately 70:30. In absolute terms, the distance between the centre of the light emitting element's emission surface (or emission point) to the sensing plane of the sensor, in the direction perpendicular to the sensing plane of the sensor, is approximately 2.5 mm, for instance 2.6 mm.

[0062] As schematically illustrated by the heavy black line in FIG. 3b, due to the configuration of the light source 12, lens 18 and sensor 20, the optical path from the source to the sensor is substantially diamond/rhombus-shaped, and the optical path between the lens 18 and scale 6 is substantially V-shaped. In the embodiment described, the angle θ between a line extending perpendicular to the scale (the dotted line in FIG. 3b) and the direction of the optical path as it impinges on the scale is approximately 10°.

[0063] As shown, the sensor 20 can be tilted such that it's sensing surface/plane is not perpendicular to the lens' optical axis. Such tilting can help to compensate for any keystone distortion in the image formed on the sensor, which can be formed due to the image being formed by an off-axis part of the lens 18. In the embodiment shown, the sensor 20 is tilted such that the angle α between a plane extending parallel to their sensing surface (e.g. its sensing plane) and a plane extending perpendicular to the optical axis, is about 3°. However, this does not necessarily have to be the case, and the sensor could be configured such that its sensing surface can extend perpendicular to the lens' optical axis (i.e. such that the angle α is less than 1°). As described in more detail below, such tilting of the sensor 20 can achieved by mounting the PCB 32 (to which the sensor is mounted) at a tilted angle. Accordingly, any other components mounted to the sensor 20 or PCB 12, including for example the light source 12, can also be titled for mechanical convenience; although this need not necessarily be the case. As will be understood, other ways of compensating for the keystone distortion are available, such as by appropriately shaping the sensor elements, e.g. “keystoning” the sensor elements themselves.

[0064] Referring now to FIGS. 4 and 5, an example embodiment of how the readhead can be constructed to achieve the above described optical layout will now be described.

[0065] As shown in FIG. 4, the readhead 4 comprises a body 30 to which the lens 18, window 22 and a printed circuit board (PCB) 32 are mounted (e.g. via gluing, mechanical and/or frictional means). The sensor 20, LED 12 and other electronic components (such as the above-mentioned CPU 24, memory 25 and interface 26—not shown in FIG. 4 or 5) are mechanically and electrically mounted to the PCB 32.

[0066] As illustrated, although the LED 12 is mounted to the PCB 32, the LED 12 is mounted to the circuit board “off-board”, in that it is mounted to the PCB 32, but it is mounted via a raised support structure 34 which holds the LED 12 away from the PCB 32. In particular, the support structure 34 extends beyond the sensor 20 so as to hold the LED 12 further away from the PCB 32 than the sensor 20.

[0067] Accordingly, as shown, the sensor 20 is mounted relatively close to the PCB 32 whereas the LED 12 is mounted relatively far from the PCB 32. As illustrated in FIG. 4, the LED 12 is much closer to the lens 18 than the PCB 32, whereas the sensor 20 is much closer to the PCB 32 than the lens 18. Accordingly, as shown, the light emitting element and the sensor are separated in the dimension which extends perpendicular to the plane of the sensor/circuit board. In particular, in the dimension which extends perpendicular to the plane of the sensor/circuit board, there is (free) space between the light emitting element and the sensor. In this example, the ratio of: i) the distance between the LED 12 emission surface (or emission point) to the sensor's 20 sensing plane, in the direction parallel to the imaging member's optical axis OA; and ii) the distance between the LED's 12 emission surface (or emission point) to lens 18, in the direction parallel to the imaging member's optical axis OA, is approximately 70:30.

[0068] In the embodiment described, the above-mentioned support structure 34 also forms/provides the electrical connection between the LED 12 and the PCB 32. Accordingly, in the embodiment described the support structure 34 for holding the LED 12 away from the PCB 32 is the cathode 34 between the LED 12 and the PCB 32. Accordingly, the cathode 34 comprises a rigid, electrically-conductive, support structure for the LED 12, which rises from the PCB 32. As shown in FIGS. 4 and 5, the support structure/cathode 34 comprises an opening/window 35 through which light reflected by the scale 6 can pass in order to reach the sensor 20.

[0069] In this embodiment, the anode 36 also comprises a rigid, electrically-conductive structure which rises from the PCB 32, and which is wire-bonded to the LED 12 via a bond wire 38 as shown in FIGS. 4 and 5. In other words, the readhead comprises a raised bond wire support structure which extends from the PCB 32, and wherein a bond wire 38 extends between it and the light emitting element 12. Although not necessary in this embodiment due to the shape and size of the anode, in other embodiments the anode 36 could also have an opening/window through which light emitted from the LED 12 can pass toward the lens 18/scale 6 and through which light reflected by the scale 6 can pass in order to reach the sensor 20.

[0070] As will be understood, the anode's 36 rigid structure could be omitted, and the LED 12 could be wire bonded via a bond wire which extends between the LED 12 and the PCB 32. However, it can be beneficial to reduce the length of the bond wire as much as possible because bond wires can be fragile, and the longer the bond wire the more likely it is to break.

[0071] In this particular embodiment, the LED's support structure/cathode 34 and the bond wire support structure/anode 36 each comprise a sheet material part, each of which have been folded to provide a three-dimensional frame, and soldered to the PCB 32. In the particular embodiment described, the cathode 34 is brass, and the anode is bass, plated with nickel-gold. As indicted in FIGS. 6 and 7, fold-lines 37 have been chemi-etched into the sheet material in order to aid folding. Once folded, each of the support structures 34, 36 comprise a top surface 31 and a plurality of side supports (or “legs”) 33 which are soldered to the PCB 32. As will be understood, the support structure/cathode 34 could be formed in other ways, for example it could be machined/cut into shape and/or stamp/pressed into shape. The bare-die LED 12 is mounted directly on the support structure/cathode 34 via conductive epoxy and the wire bond extends between the LED 12 and the top surface 31 of the anode 36.

[0072] As illustrated in FIG. 6c, the top surface 31 of the LED's support structure 34 extends over/partially covers the sensor 20 (the outline of which is schematically illustrated in FIG. 6c by the phantom line). In other words, a line extending through and perpendicular to the plane of the sensor 20 (and PCB 32) also passes through the top surface 31 of the LED's support structure 34. Such a configuration enables the LED 12 to be placed very close to, and if desired, over, the sensor 20.

[0073] The readhead 4 is assembled by dead-reckoning the lens 18 within the body 30 of the readhead 4, and the body 30 being crimped in order to hold the lens 18 in place (although other ways of securing the lens 18 to the body can be used, such as by epoxy and/or by pushing the lens 18 into flexures which hold the lens). The PCB 32 comprising the LED 12 already mounted thereon, is then mounted to the body 30, e.g. by gluing and/or mechanical means such as crimping. If desired, an alignment process can be used to align the PCB (and hence the sensor and LED thereon) relative to the lens. Such an alignment process could comprise using a camera to look at the position of the PCB/components thereon and make adjustments based on the output of the camera, and/or connect to the PCB/components thereon and use the output of the sensor to make adjustments. Once assembled, a lid 46 is secured to the body 30, e.g. via gluing, crimping and/or welding.

[0074] In the embodiment described above, the bond wire support structure 34 also forms the cathode, but as will be understood, this need not necessarily be the case, and the support structure 34 could form the anode instead, for example.

[0075] In the embodiment described, the LED 12 is mechanically mounted to the PCB 32 via an electrode 34, but as will be understood this need not necessarily be the case. For instance, the LED 12 could be mechanically mounted directly to the PCB 32 via one or more non-electrically conductive members, and electrically connected to the PCB 32 via separate members, e.g. one or more wires (for instance, via wire bonding). Furthermore, the LED 12 need not necessarily be mounted directly to the PCB 32. For instance, the LED 12 could be mechanically mounted directly to the body 30, and electrically connected to the PCB 32 via one or more wires (e.g. via wire bonding). In another embodiment, the LED 12 could be electrically connected to a different PCB (i.e. not the same PCB 32 to which the sensor is connected).

[0076] FIG. 8 illustrates a readhead 4′ according to another embodiment of the invention. The readhead 4′ of FIG. 8 shares many parts which are the same as that of the embodiment of FIGS. 1 to 7 and like parts share like reference numerals. In the embodiment of FIG. 8, the LED 12 is mounted to the circuit board via the sensor 20, by way of a transparent support structure 50 (e.g. a glass block 50). In particular, the glass block 50 is secured to the sensor 20 via adhesive epoxy. The LED 12 then sits on a conductive pad 52 which has been deposited on the side of the glass block 50 which faces the lens 18 and which is distal the sensor 20. The LED 12 is electrically connected to the circuit board 32 via an anode 36′ and cathode 34′ which in this embodiment each comprise raised bond wire support structures extending from the PCB 32, and bond wires which extend between them and the LED 12/conductive pad 52. As will be understood, in variations of this embodiment, the LED 12 could be connected to the circuit board in other ways, e.g. via an anode and cathode which are deposited on and run along the surface/side of the glass block 50, or even via an anode and cathode which run through the glass block 50.

[0077] As described above, the light emitting element's support structure 34 and/or the light emitting element 12 can be held directly over the sensor 20 such that a line extending perpendicular to the plane of the circuit board/sensor passes through both the light emitting element's support structure 34 and the sensor 20 and/or through both the light emitting element 12 and the sensor 20. As will be understood, and as schematically illustrated in FIG. 9 the sensor 20 could comprise at least one, and for example an array of, photosensitive elements 21, as well as other sub-components and packaging that make up the sensor 20. In other words, the sensor 20 could be a chip or component which comprises at least one, and for example an array of, photosensitive elements 21. For example, as illustrated in FIG. 9a, the light emitting element 12 can be located directly over the sensor chip 20 in a way which in which it does not sit directly over the photosensitive elements 21. Alternatively, as illustrated in FIG. 9b, the light emitting element 12 can be located directly over the sensor chip 20 in a way which in which the light emitting element 12 sits directly over the photosensitive elements 21, e.g. such that a line extending perpendicular to the plane of the circuit board/sensor (i.e. parallel to the Y-axis) passes through both the light emitting element 12 and the photosensitive elements 21.

[0078] In the embodiments shown, the LED 12 is mounted “off-board” by the support structure 34. Whilst this can be beneficial (e.g. so as to place the LED 12 at the lens' 18 focal plane, so as to achieve collimation, whilst enabling an image of the scale to be captured by the sensor 20), this need not necessarily be the case. For example, the LED 12 could be mounted on the PCB 32 such that it sits substantially in-plane with the sensor 20 (in other words, at substantially the same height as the sensor 20).

[0079] In the embodiments shown, the light impinging on the scale is collimated, but this need not necessarily be the case. Furthermore, even if the light impinging on the scale is collimated, the light reflected by the scale need not necessarily be collimated. For example, if the scale is curved, for instance if the scale is a ring scale, then the light reflected by the scale will not be collimated.