Position measuring device

Abstract

An encoder apparatus comprises a reflective scale and a readhead. The readhead comprises 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, wherein the optical device forms an image of an illuminated area of ​​the reflective scale onto the sensor. The optical path of the system from the light emitting element to the sensor passes through the optical device on its way to the scale 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.

Description

[0001] The present invention relates to a position measuring device, also referred to as a position measuring encoder, a position encoder, an encoder apparatus or just an “encoder.” In particular, the present invention relates to a so-called absolute position measuring device / encoder apparatus.

[0002] Encoder devices / position measuring apparatus for measuring the relative position between two movable objects are well known. Typically, a series of scale markings are provided on one object and a readhead for reading the scale markings is provided on the other object. The scale markings may be integrally formed with the object or may be provided on a scale that may be fixed to the object.

[0003] Encoder devices are generally classified as incremental encoder devices or absolute encoder devices. In incremental encoder devices, a scale has a plurality of periodic marks that can be detected by a read head to provide incremental up / down counting. For example, such a scale is described in European Patent Application No. 0207121. Reference marks can be set adjacent to or embedded in the periodic marks to define reference points. For example, such a scale is disclosed in the published international patent application WO 2005 / 124282. Absolute position encoder devices typically measure relative displacement by detecting a unique series of marks (e.g., codes) by a read head and converting these codes into absolute positions. Such a scale is disclosed in International Patent Application No. PCT / GB2002 / 001629, and such an encoder is described in detail in WO 2010 / 116144.

[0004] Absolute encoder devices differ from incremental encoder devices in that they can determine the absolute position of the readhead relative to the scale at start-up without requiring relative movement of the readhead in the scale. In contrast, in incremental encoder devices the readhead must travel to a reference mark in order to determine the zero position.

[0005] Encoder devices can also be classified based on the primary means by which they detect features on the scale, such as optical, magnetic, inductive, or capacitive.

[0006] The present invention relates to an improved optical encoder device.

[0007] According to a first aspect of the invention there is provided 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, wherein the optical device forms an image of an illuminated area of ​​the reflective scale onto the sensor, and wherein the light path of the system from the light emitting element to the sensor passes through the optical device on its way towards the scale and after reflection from the scale.

[0008] The light path between the light emitting element and the optical device may be direct / non-reflected. The light path between the optical device and the sensor may be direct / non-reflected. In other words, the device (e.g., a read head) may include a non-reflected light path between the light emitting element and the optical device and a non-reflected light path between the optical device and the sensor.

[0009] Preferably, the optical path of light between the light emitting element and the scale is direct / non-reflected, and the optical path of light between the scale and the sensor is also direct / non-reflected.

[0010] For embodiments in which the readhead comprises a housing / casing and a window through which light (from the light emitting element) exits and through which light (reflected by the scale) enters the readhead (in other words, exits and enters the housing / casing), preferably the entire light path within the readhead (or within the housing / casing) is direct / unreflected.

[0011] The present invention enables a particularly compact readhead for an optical absolute position measurement device. For example, configuring the readhead so that light emitted from the light emitting element passes through the same optical device on both the outward path and the return path can reduce the number of optical components required. Furthermore, ensuring a direct / unreflected light path between the light emitting element and the optical device and a direct / unreflected light path between the optical device and the sensor means that reflective optical components (such as mirrors and / or beam splitters) are not required (e.g. the readhead can have no reflective optical components in its optical path). Accordingly, the number of optical components in the readhead can be further reduced, and the compactness and complexity of the readhead can be improved.

[0012] The light emitting element may be mounted to the readhead such that, in a dimension perpendicular to the plane of the sensor, the light emitting element is disposed / located between the sensor and the optical device. For example, the light emitting element may be located in the space (or "volume") between the sensor and the optical device (defined by the outer edges / outside of the sensor and the optical device).

[0013] The light emitting element may be substantially located at the focal plane of the optical device so that the light emitted thereby is collimated by the optical device. For example, preferably, the light emitting element is no more than 500 μm (micrometers) from the focal plane of the optical device, more preferably no more than 250 μm (micrometers) from the focal plane of the optical device, and particularly preferably no more than 100 μm (micrometers) from the focal plane of the optical device.

[0014] Optionally, light reflected by the scale and imaged by the optical device onto the sensor converges towards a point at a specific distance between the optical device and the sensor. Further, the light emitting element may be located substantially at said specific distance between the optical device and the sensor.

[0015] Optionally, the ratio of i) the distance between the center of the emission surface (or emission point) of the light emitting element and the sensing plane of the sensor in a direction perpendicular to the plane of the sensor to ii) the distance between the center of the emission surface (or emission point) of the light emitting element and the optical device in a direction perpendicular to the plane of the sensor is not less than 35:65, for example not less than 40:60, optionally not less than 50:50, preferably not less than 60:40, and for example not less than 65:35.

[0016] The light emitting element may be positioned such that it is offset from the optical axis of the optical device. For example, the light emitting element may be offset from the optical axis of the optical device (e.g. measured from the center of the emission area of ​​the light source) by no more than 1 mm, such as no more than 750 μm, such as no more than 500 μm. Optionally, the ratio of the offset to the focal length of the lens is no more than 1:2.5, such as no more than 0.5:2.5.

[0017] Optionally, the direction of the light path when it strikes the scale and / or is reflected from the scale is not perpendicular to the scale. For example, the angle between a line extending perpendicular to the scale (in the illuminated area) and the direction of the light path when it strikes the scale (and / or is reflected from the scale) is not less than 1° (degrees), such as not less than 2°, such as not less than 5°, and optionally not more than 20°, such as not more than 15°. In other words, optionally, there is an angle (i.e., greater than 0°) between the incident direction of light striking the scale and the reflected direction of light reflected from the scale, such as an angle of at least 2°, and such as at least 4°, optionally at least 10°, and such as not more than 40°, such as not more than 30°.

[0018] Accordingly, optionally, the shape of the light path when it impinges on and / or reflects from the scale is V-shaped. Optionally, the light path of the system from the light emitting element to the sensor is substantially rhombus-shaped.

[0019] The optical path through the optical device on the way to the scale and after reflection from the scale may be laterally offset. Accordingly, for example, for any given ray passing through the optical system, the point at which it leaves the optical device towards the scale and the point at which it re-enters the optical device after having reflected from the scale are different / laterally offset. As will be appreciated, the optical paths (e.g., light beams) on the way to the scale and after reflection from the scale may overlap (e.g., partially overlap, and optionally substantially but not completely overlap).

[0020] Optionally, the light emitting element and the sensor both face the optical device and the scale. The light emitting element and the sensor may both face the same direction. In other words, the light emitting element and the sensor may be mounted in the readhead such that the sensor plane is substantially parallel to the emitting surface of the light emitting element.

[0021] Optionally, the sensor, and the image of the scale formed by the optical device, are located behind (e.g. directly behind) the light emitting element. Optionally, the light emitting element is positioned so that rays from the light emitting element reflected by the scale converge to a certain point, thereby bypassing the light emitting element on the return path, and then diverge and form said image of the scale on the sensor (behind the light source).

[0022] As will be appreciated, an image of the scale is formed when the rays from any given point on the scale converge to substantially a unique common point at the image plane (where the sensor is located). (The point is "unique" because for different given points on the scale, the rays from that point will converge to substantially different common points.) This image may be a spatially filtered image.

[0023] A plurality of light emitting elements may be provided. In this case, the plurality of light emitting elements are preferably provided together to act as a single light source. Optionally, the read head comprises only one light emitting element.

[0024] Optionally, the light emitting element comprises an "uncapped", "unpackaged" or "lensless" semiconductor diode, such as a bare die semiconductor diode. The light emitting element may comprise, for example, a light emitting diode (LED) or a laser (eg, a vertical cavity surface emitting laser (VCSEL)).

[0025] The at least one sensor may comprise a plurality of sensor elements (eg photodiodes). For example, the read head may comprise an array of sensor elements. The array may be one-dimensional or two-dimensional.

[0026] As used herein, "light" refers to electromagnetic radiation (EMR) anywhere from ultraviolet to infrared. For example, the light can be ultraviolet light, visible light, infrared light, or a combination thereof.

[0027] The optical device may comprise a lens, such as a single lens. Alternatively, the optical device comprises a diffractive optical element, such as a Fresnel zone plate. Alternatively, the optical device comprises a holographic optical element, such as a hologram of a lens.

[0028] The scale may include a series of features that can be detected by the sensor to determine the relative movement / position of the scale and the readhead. Such features may be arranged periodically or aperiodically. As will be appreciated, there may be many suitable ways of defining features on the scale. For example, features may be defined by marks having specific electromagnetic radiation (EMR) properties (e.g. specific optical properties), for example by specific optical transmittance or reflectivity of a portion of the scale. Accordingly, features may be defined, for example, by portions of the scale having minimum reflectivity or transmittance values. Alternatively, features may be defined, for example, by portions of the scale having maximum reflectivity or transmittance values. Alternatively, features may be defined, for example, by the way (e.g. direction) in which they reflect light (e.g. towards and away from the readhead). These features may take the form of lines, dots or other configurations that can be detected by sensors. A preferred configuration for a one-dimensional scale may include a line extending across the entire width of the track in a dimension perpendicular to the measurement dimension.

[0029] The encoder device may be an incremental encoder device. Accordingly, the scale may include an incremental scale. The incremental encoder device may include one or more reference marks for defining one or more reference positions. Optionally, the encoder device is an absolute encoder device. As will be appreciated, in contrast to an incremental encoder, an absolute encoder device may determine the absolute position of a readhead relative to the scale without requiring relative movement of the readhead and the scale. An absolute encoder comprises an absolute scale comprising features for defining a series of unique positions along its length. The series of unique absolute positions may be defined by features in a plurality of tracks, such as a plurality of adjacent tracks. Alternatively, the series of unique absolute positions may be defined by features contained only in a single track. For example, the absolute position information may be determined based on a combination of features taken along a measured length of the scale. Accordingly, the encoder device may be configured to extract the absolute position information from an image obtained by a sensor. This extraction may be performed by the readhead or by a device external to the readhead.

[0030] Optionally, the readhead is configured to read the scale by obtaining at least one discrete snapshot (i.e. snapshot image) of the scale. This may be replaced, for example, by continuously measuring and counting phases. Accordingly, an image of the scale may be obtained by the readhead taking discrete snapshots of the scale. The snapshot may be taken at a certain moment, or established by taking smaller readings of successive sections of the scale in rapid succession. Snapshot readings of the scale may provide several advantages. For example, the maximum operating speed of the scale reader relative to the scale may be greater because it is not limited by the inherent frequency limits of the continuous phase measurement and counting system. Further, in an optical system that takes a snapshot, the light emitting element only needs to be turned on for a short amount of time, which allows the light intensity to be increased relative to a continuous system without increasing the average power consumption or limiting the service life of the light source. This increased light intensity may mean that the sensor can capture more photons, thereby reducing the background noise of the system and generating less position noise.

[0031] The sensor may be mounted to a printed circuit board (PCB). The light emitting element may be electrically connected to the same PCB as the sensor. The light emitting element may be physically mounted (to a read head (e.g. to a PCB)) by a support member that holds the light emitting element away from the board so as to provide a gap between the light emitting element and the sensor (e.g. in a direction perpendicular to the plane of the sensor). The support member may be configured to hold the light emitting element in a space (volume) between the sensor and the optical device. For example, the distance between the light emitting element and the sensor measured in a direction perpendicular to the plane of the sensor may be at least 1 mm, such as at least 1.5 mm, such as at least 2 mm. More specifically, the distance at the center of the emitting surface (or emitting point) of the light emitting element and the sensing plane of the sensor in a direction perpendicular to the sensing plane of the sensor may be at least 1 mm, such as at least 1.5 mm, preferably at least 2 mm, such as about 2.5 mm. The distance may be between 2 mm and 3 mm, such as about 2.5 mm.

[0032] According to another aspect of the invention there is provided a readhead for reading a reflective scale to determine their relative position, the readhead comprising a light emitting element, a sensor and optical means for illuminating an area of ​​the reflective scale adjacent the readhead and for forming an image of the area of ​​the reflective scale onto the sensor, and wherein light from the light emitting element passes through the optical means on its way to and after reflection from the scale, wherein the path of light between the light emitting element and the optical means is direct / unreflected and the path of light between the optical means and the sensor is direct / unreflected. Features described above in connection with the first aspect of the invention apply equally to this aspect of the invention.

[0033] According to another aspect of the invention, there is provided a readhead for reading a reflective scale to determine their relative position, the readhead comprising at least one light emitting element, at least one sensor and at least one optical device (e.g. a lens). The light emitting element may be mounted to the readhead such that the light emitting element is located between the sensor and the optical device (e.g. in a dimension perpendicular to the plane of the sensor). For example, the light emitting element may be located in a space (or "volume") between the sensor and the optical device (e.g. defined by the outer edges / outside of the sensor and the optical device). This configuration provides a very compact readhead. The optical device may be configured to form an image of an illuminated area of ​​the reflective scale onto the sensor. Optionally, the optical path of the system from the light emitting element to the sensor may pass through the optical device on its way towards the scale and after reflection from the scale. Optionally, the optical path of the system may include an unreflected optical path between the light emitting element and the optical device. Optionally, the optical path of the system may include an unreflected optical path between the optical device and the sensor. Features described above in conjunction with other aspects of the invention are equally applicable to this aspect of the invention.

[0034] This document describes an encoder device comprising a readhead for reading a reflective scale located adjacent to the readhead, the readhead comprising: a sensor mounted thereon, the sensor comprising one or more photodiodes for detecting light reflected from a scale located adjacent to the readhead; and at least one light emitting element, wherein the light emitting element is held away from a sensing plane of the sensor such that the distance between the light emitting element and the sensor measured in a direction perpendicular to the sensing plane of the sensor is at least 1 mm. This document describes an encoder device comprising a readhead for reading a reflective scale located adjacent to the readhead, the readhead comprising: a circuit board having a sensor mounted thereon, the sensor comprising one or more photodiodes for detecting light reflected from a scale located adjacent to the readhead; and at least one light emitting element, wherein the light emitting element is mounted to the circuit board by a light emitting element support structure, the light emitting element support structure holding the light emitting element away from the circuit board and the sensing plane of the sensor such that the distance between the light emitting element and the sensor measured in a direction perpendicular to the sensing plane of the sensor is at least 1 mm. More specifically, the distance between the center of the emitting surface (or emitting point) of the light emitting element and the sensing plane of the sensor in a direction perpendicular to the sensing plane of the sensor may be at least 1 mm. The distance may be at least 1.5 mm, optionally at least 2 mm, for example at least 2.5 mm. The distance may be between 2 mm and 3 mm, for example about 2.5 mm.

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

[0036] Figure 1 shows a schematic isometric view of a position measuring device according to the invention;

[0037] Figure 2 yes Figure 1 A schematic diagram of the various optical and electronic components of a read head of a position measuring device;

[0038] Figure 3a and Figure 3b Shows Figure 1 and Figure 2 A schematic diagram of an optical arrangement of a position measuring device;

[0039] Figure 4 yes Figure 1 A cross-sectional view of a read head;

[0040] Figure 5 Shows Figure 1 An isometric view of the circuit board, light source, light source support structure and sensor of the read head;

[0041] Figure 6a and Figure 6b isometric front and rear views showing a support structure for a light source;

[0042] Figure 6c A top plan view showing a support structure for a light source;

[0043] Figure 7a and Figure 7b isometric front and rear views showing a support structure for bonding wires of a light source;

[0044] Figure 8 An alternative embodiment of a read head according to the invention is shown;

[0045] Figure 9a and Figure 9b A plan view of a readhead according to the invention is schematically illustrated, wherein the light source is positioned directly on the sensor.

[0046] Reference Figure 1 , shows an encoder apparatus 2 comprising a readhead 4, a scale 6 and a controller 7. The readhead 4 and scale 6 are mounted to first and second objects (not shown) respectively that are movable relative to each other. The speed of the relative motion may vary, but in the described embodiment the readhead 4 and scale 6 have a known maximum relative acceleration.

[0047] In some embodiments, the scale 6 is a linear scale. However, it will be appreciated that the scale 6 may be a non-linear scale, such as a rotating scale (e.g. a disk or annular scale). Furthermore, the scale 6 enables measurement in only a single dimension. However, this need not be the case, and for example, the scale may enable measurement in two dimensions.

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

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

[0050] like Figure 2 As shown, the readhead 4 includes a light emitting element / source 12, an optical arrangement 18, a sensor 20, and a window 22. In this embodiment, the light emitting element / source 12 includes a light emitting diode (LED). Also, in this embodiment, the optical arrangement includes a lens 18, although other optical arrangements may be used. For example, a diffractive optical element (such as a Fresnel zone plate) and / or a holographic optical element, such as a hologram of a lens, may be used. In this embodiment, the sensor 20 includes a complementary metal oxide semiconductor ("CMOS") sensor. As will be appreciated, other image sensors may be used in place of a CMOS sensor. For example, a CCD or a photodiode array may be used instead.

[0051] The read head 4 also includes a CPU 24, a memory device 25 (e.g., an electrically erasable programmable read-only memory (EEPROM) or flash memory), and an interface 26. The read head 4 may also include an analog-to-digital converter to digitize image data from the sensor 20. Alternatively, the analog-to-digital conversion may be performed within the sensor 20 or the CPU 24.

[0052] The 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 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 the portion of the scale 6 illuminated by the LED 12. The sensor may include a one-dimensional or two-dimensional array of pixels. For example, the sensor may include a one-dimensional array of 256 elongated pixels, the lengths of which extend parallel to the lengths of the reflective lines 8 and non-reflective lines 10 on the scale. Instead of the two-dimensional imaging arrangement described, a one-dimensional imaging arrangement may be used instead, in which a one-dimensional image of the scale is formed on the sensor by a lens.

[0053] The LED 12 is connected to the CPU 24 so that the LED 12 can be operated as required by the CPU 24. The sensor 20 is connected to the CPU 24 so 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 as required by the CPU 24 to take a snapshot of the intensity falling across the sensor. 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 information from, for example, the controller 7 (such as the controller 7) via line 40. Figure 1 The line 40 also includes a power line, and the reading head 4 is powered by the power line.

[0054] As will be appreciated, absolute position data may be encoded in the scale 6 by the absence of reflective lines 8 and, or alternatively, by the absence of non-reflective lines 10. Furthermore, absolute position data may be embedded in the scale 6 without adding or removing reflective lines 8 or non-reflective lines 10. For example, the width of the lines, the distance between the lines or the colour of the lines may be varied in order to embed absolute position data in the scale 6. Furthermore, rather than the scale defining an absolute position by a unique combination of features taken along the measured length of the scale, the scale may have features defining an absolute position by a unique combination of features taken along the width of the scale. For example, the scale may include a plurality of "barcodes" whose lengths extend across the scale, for example substantially perpendicular to the measured length of the scale. Alternatively, the scale may include a plurality of tracks, wherein at least one, optionally at least two and possibly all of these tracks may include a plurality of features regularly spaced apart (i.e., the tracks may substantially include incremental scale features of different fundamental frequencies), wherein the scale periods of the tracks differ from one another so that the combination of features across the width of the scale is unique at any point along the measured length of the scale.

[0055] A series of groups of marks can be used to encode a series of unique binary codewords along the length of the scale that define unique (i.e. absolute) position information, while still having enough information to enable phase information to be extracted from the series of marks to enable fine position information (e.g. position information with a resolution finer than the period of the scale marks) to be determined. Accordingly, in such a system, the position information can consist of a coarse absolute position (determined from the codewords extracted from the image) and a fine position (determined by looking at the phase offset of the roughly periodic marks). Further details of this so-called hybrid incremental and absolute scale are described in International Patent Application No. PCT / GB2002 / 001629 (Publication No. WO 2002 / 084223), the contents of which are incorporated into this specification by reference.

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

[0057] Will refer to Figure 3a and Figure 3b Describe in more detail Figure 1 and Figure 2 The optical system of the pickup head 4. Figure 3a and Figure 3b The path taken by light from the light source 12 to the sensor 20 through the optical system which forms an image of the scale 6 onto the sensor 20 is schematically illustrated.

[0058] As shown, the optical device 18 includes 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 substantially located at the focal plane fp of the lens 18, but slightly offset from the optical axis OA of the lens 18. For example, the light source 12 is offset from the optical axis OA of the lens 18 by approximately 450 μm (measured from the center of the emission area of ​​the light source). In particular, the ratio of the offset to the focal length of the lens is approximately 0.45:2.5. Positioning the light source 12 substantially at the focal plane of the lens 18 helps to ensure that the light emitted from the light source is substantially collimated by the lens 18 as it travels toward the scale 6. Accordingly, the light reflected by the scale 6 is then focused by the lens 18 to a point at the focal plane fp of the lens 18 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, the image of the light source 12 will be formed at the focal plane fp. Positioning light source 12 at focal plane fp of lens 18 but offset from optical axis OA of lens 18 means that light source 12 may be located in the space (or "volume") between sensor 20 and lens 18 (defined by Figure 3b This helps to make the readhead compact but does not obstruct the path of 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 (as well as the window 22 and the scale 6). (In other words, the emitting surface of the light source 12 and the sensing surface of the sensor 20 face the lens 18). Moreover, there is an unreflected (in other words, "straight") light path between the light source 12 and the lens 18, and there is also an unreflected (i.e., "straight") light path between the lens 18 and the sensor 20. Therefore, no reflective optical components are needed or used to turn or redirect 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 both for collimating light from the light source 12 and for forming an image of the scale 6 onto the sensor. Accordingly, the optical arrangement of the readhead 4 described uses only one optical device / lens 18 and is therefore particularly compact and inexpensive. In the embodiment described, the lens 18 is a single lens, but may be a different type of lens (e.g. a double lens, a compound lens or a gradient refractive index (GRIN) lens). As will be appreciated, the optical device need not be a lens, but may be another type of optical device, such as a Fresnel zone plate or a holographic optical element (HOE), e.g. a hologram of a lens.

[0061] like Figure 3a As shown, the light source is between the sensor 20 and the lens 18, closer to the lens. This configuration is different from conventional encoder designs, in which the light source is typically mounted to the same board as the sensor and is generally coplanar with the sensor. Figure 3a As shown, in this embodiment, the read head is configured so that the ratio of i) the distance between the center of the emission surface (or emission point) of the light emitting element and the sensing plane of the sensor in a direction perpendicular to the plane of the sensor (D1) and ii) the distance between the center of the emission surface (or emission point) of the light emitting element and the optical device in a direction perpendicular to the plane of the sensor (D2) is about 70:30. In absolute terms, the distance between the center of the emission surface (or emission point) of the light emitting element and the sensing plane of the sensor in a direction perpendicular to the sensing plane of the sensor is about 2.5 mm, for example 2.6 mm.

[0062] like Figure 3b As schematically shown by the thick black lines in FIG. 1 , due to the configuration of the light source 12, the lens 18 and the sensor 20, the light path from the light source to the sensor is roughly rhombus / square, while the light path between the lens 18 and the scale 6 is roughly V-shaped. In the described embodiment, the line extending perpendicular to the scale ( Figure 3b The angle θ between the dotted line in FIG. 1 and the direction of the light path when it hits the scale is approximately 10°.

[0063] As shown, the sensor 20 can be tilted so that its sensing surface / plane is not perpendicular to the optical axis of the lens. Such tilting can help compensate for any trapezoidal distortion in the image formed on the sensor, which may be caused by the image being formed by the off-axis portion of the lens 18. In the illustrated embodiment, the sensors 20 are tilted so that the angle α between a plane extending parallel to their sensing surface (e.g., their sensing plane) and a plane extending perpendicular to the optical axis is about 3°. However, this need not be the case, and the sensor can be configured so that its sensing surface can extend perpendicular to the optical axis of the lens (i.e., so that the angle α is less than 1°). As described in more detail below, such tilting of the sensor 20 can be achieved by mounting the PCB 32 (to which the sensor is mounted) at an inclined angle. Accordingly, any other components mounted to the sensor 20 or PCB 12 (including, for example, the light source 12) can also be tilted for mechanical convenience; but this need not be the case. As will be appreciated, other ways of compensating for trapezoidal distortion are available, such as by appropriately shaping the sensor element, such as "keystone correction" of the sensor element itself.

[0064] Now refer to Figure 4 and Figure 5 , it will now be described how a readhead may be constructed to implement an example embodiment of the optical arrangement described above.

[0065] like Figure 4 As shown, the reading head 4 includes a body 30, to which the lens 18, the window 22 and the printed circuit board (PCB) 32 are mounted (e.g., by adhesive, mechanical and / or friction means). The sensor 20, the LED 12 and other electronic components (such as the above-mentioned CPU 24, the memory 25 and the interface 26, Figure 4 or Figure 5 ) is mechanically and electrically mounted to 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," i.e., it is mounted to the PCB 32, but is mounted by a raised support structure 34 that holds the LED 12 away from the PCB 32. In particular, the support structure 34 extends beyond the sensor 20 to hold the LED 12 further away from the PCB 32 than the sensor 20.

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

[0068] In the depicted embodiment, the support structure 34 also forms / provides an electrical connection between the LED 12 and the PCB 32. Accordingly, in the depicted embodiment, the support structure 34 used to hold the LED 12 away from the PCB 32 is a cathode 34 between the LED 12 and the PCB 32. Accordingly, the cathode 34 comprises a rigid conductive support structure for the LED 12 that rises from the PCB 32. Figure 4 and Figure 5 As shown, the support structure / cathode 34 includes 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, anode 36 also comprises a rigid conductive structure that rises from PCB 32 and is wire bonded to LED 12 via bonding wires 38, such as Figure 4 and Figure 5 In other words, the read head includes a raised bond wire support structure extending from the PCB 32, and wherein the bond wires 38 extend between the bond wire support structure 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 may also have an opening / window through which light emitted from the LED 12 may pass toward the lens 18 / scale 6, and light reflected by the scale 6 may pass through the opening / window to reach the sensor 20.

[0070] As will be appreciated, the rigid structure of anode 36 may be omitted, and LED 12 may be wire bonded via bond wires extending between LED 12 and PCB 32. However, it may be beneficial to minimize the length of the bond wires as much as possible, since bond wires may be fragile, and the longer a bond wire is, the more likely it is to break.

[0071] In this particular embodiment, the LED support structure / cathode 34 and the bonding wire support structure / anode 36 each include a sheet portion, each of which is 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 brass plated with nickel gold. As shown in Figures 6 and 7, fold lines 37 have been chemically etched into the sheet to assist in folding. Once folded, each of the support structures 34, 36 includes a top surface 31 and a plurality of side supports (or "legs") 33 soldered to the PCB 32. As will be understood, the support structure / cathode 34 can be formed in other ways, for example, it can be machined / cut to shape and / or stamped / pressed to shape. The bare die LED 12 is directly mounted on the support structure / cathode 34 by conductive epoxy, and the wire bonds extend between the top surface 31 of the LED 12 and the anode 36.

[0072] like Figure 6c As shown, the top surface 31 of the LED support structure 34 extends over / partially covers the sensor 20 (the outline of the sensor is in Figure 6c 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 support structure 34. This configuration enables the LED 12 to be placed very close to the sensor 20, and if desired, on 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 is crimped to hold the lens 18 in place (although other means of securing the lens 18 to the body may be used, such as by epoxy and / or by pushing the lens 18 into a flexure that holds the lens). The PCB 32, including the LEDs 12 already mounted thereon, is then mounted to the body 30, for example by gluing and / or mechanical means such as crimping. If desired, an alignment process may be used to align the PCB (and therefore the sensors and LEDs thereon) relative to the lens. Such an alignment process may include using a camera to view the position of the PCB / components thereon and making adjustments based on the output of the camera, and / or connecting the PCB / components thereon and making adjustments using the output of the sensors. Once assembled, the cover 46 is secured to the body 30, for example by gluing, crimping and / or welding.

[0074] In the above described embodiments, the bond wire support structure 34 also forms a cathode, but as will be appreciated, this need not be the case and the support structure 34 may, for example, form an anode instead.

[0075] In the depicted embodiment, the LED 12 is mechanically mounted to the PCB 32 via the electrodes 34, but as will be appreciated, this need not be the case. For example, the LED 12 may be mechanically mounted directly to the PCB 32 via one or more non-conductive members, and electrically connected to the PCB 32 via a separate member, such as one or more leads (e.g., via wire bonds). Furthermore, the LED 12 need not be directly mounted to the PCB 32. For example, the LED 12 may be mechanically mounted directly to the body 30, and electrically connected to the PCB 32 via one or more leads (e.g., via wire bonds). In another embodiment, the LED 12 may be electrically connected to a different PCB (i.e., different from the PCB 32 to which the sensor is connected).

[0076] Figure 8 A read head 4' according to another embodiment of the invention is shown. Figure 8 The read head 4' is shared with Figure 1 Many of the parts are identical to those of the embodiment of FIG. 7 , and identical parts share the same reference numerals. Figure 8 In the embodiment of the present invention, the LED 12 is mounted to the circuit board through the sensor 20 via a transparent support structure 50 (e.g., a glass block 50). In particular, the glass block 50 is fixed to the sensor 20 by an adhesive epoxy. The LED 12 is then placed on a conductive pad 52 that has been deposited on the side of the glass block 50 that faces the lens 18 and away from the sensor 20. The LED 12 is electrically connected to the circuit board 32 via an anode 36' and a cathode 34', which in this embodiment each include a raised bond wire support structure extending from the PCB 32, and bond wires extending between the bond wire support structure and the LED 12 / conductive pad 52. As will be appreciated, in variations of this embodiment, the LED 12 may be connected to the circuit board in other ways, such as by anodes and cathodes deposited on and laid along the surface / side of the glass block 50, or even by anodes and cathodes laid through the glass block 50.

[0077] As described above, the light emitting element support structure 34 and / or the light emitting element 12 may be held directly on the sensor 20 such that a line extending perpendicular to the plane of the circuit board / sensor passes through both the light emitting element support structure 34 and the sensor 20 and / or passes 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 may include at least one photosensitive element 21 (e.g., a photosensitive element array) and other subcomponents and packages that make up the sensor 20. In other words, the sensor 20 may be a chip or component that includes at least one photosensitive element 21 (e.g., a photosensitive element array). For example, Figure 9aAs shown, the light emitting element 12 can be located directly on the sensor chip 20 in such a way that it is not directly arranged on the light sensitive element 21. Alternatively, Figure 9b As shown, the light-emitting element 12 can be located directly on the sensor chip 20 in that the light-emitting element 12 is placed directly on the photosensitive element 21 , for example so that a line extending perpendicular to the circuit board / sensor plane (i.e. parallel to the Y axis) passes through both the light-emitting element 12 and the photosensitive element 21 .

[0078] In the illustrated embodiment, the LED 12 is mounted "off-board" by the support structure 34. While this may be beneficial (e.g., to place the LED 12 at the focal plane of the lens 18 to achieve collimation while enabling the sensor 20 to capture an image of the scale), it need not be so. For example, the LED 12 may be mounted on the PCB 32 such that it is substantially coplanar with the sensor 20 (in other words, at substantially the same height as the sensor 20).

[0079] In the embodiment shown, the light impinging on the scale is collimated, but this need not be the case. Furthermore, even if the light impinging on the scale is collimated, the light reflected by the scale need not be collimated. For example, if the scale is curved, such as if the scale is annular, the light reflected by the scale will not be collimated.

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 including a lens, the at least one light emitting element, the at least one sensor and the at least one optical device together with the scale forming an optical system, wherein: the lens forms an image of the illuminated area of ​​the reflective scale onto the sensor, wherein the light emitting element is located in the space between the sensor and the lens and is positioned so that it is offset from the optical axis of the lens, and wherein the optical path of the system from the light emitting element to the sensor passes through the lens on its way to and after reflection from the scale and includes an unreflected optical path between the light emitting element and the lens and an unreflected optical path between the lens and the sensor, It is characterized in that The at least one sensor is tilted such that a sensing plane of the at least one sensor is not perpendicular to an optical axis of the lens.

2. An encoder device as claimed in any preceding claim, wherein The light emitting element is located substantially at a focal plane of the optical device, such that light emitted thereby is collimated by the optical device.

3. An encoder device as claimed in any preceding claim, wherein: The light reflected by the scale and imaged onto the sensor by the optical device converges toward a point located between the optical device and the sensor, and wherein the light emitting element is also located between the optical device and the sensor, and the distance between the convergence point and the optical device is approximately the same as the distance between the light emitting element and the optical device, and the distance between the convergence point and the sensor is approximately the same as the distance between the light emitting element and the sensor.

4. The encoder device of claim 1, wherein: The ratio of the following two items is not less than 35:65: i) the distance between the center of the emitting surface of the light emitting element and the sensing plane of the sensor in a direction perpendicular to the plane of the sensor; and ii) the distance between the center of the emitting surface of the light emitting element and the optical device in the direction perpendicular to the plane of the sensor.

5. The encoder device of claim 1, wherein: The direction of the light path is not perpendicular to the scale when it impinges on and / or reflects from the scale.

6. The encoder device of claim 1, wherein: The optical path of the system from the light emitting element to the sensor is roughly diamond-shaped.

7. The encoder device of claim 1, wherein: The light emitting element and the sensor both face the optical device and the scale.

8. The encoder device of claim 1, wherein: The sensor, and the image of the scale formed by the optical device, are located behind the light emitting element.

9. The encoder device of claim 1, wherein: The scale comprises an absolute scale including features defining a series of unique positions along its length, and wherein the apparatus is configured to extract absolute position information from an image obtained by the sensor.

10. The encoder device of claim 1, wherein: The sensor is mounted to a printed circuit board, and wherein the light emitting element is electrically connected to the same printed circuit board as the sensor but is physically mounted to the readhead by a support member which holds the light emitting element away from the printed circuit board so as to provide a gap between the light emitting element and the sensor.

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