Position detector

Abstract

The encoder apparatus includes 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 that, together with the scale, form an optical system that forms an image of an illuminated area of ​​the reflective scale on the sensor. An optical path of the optical system passes through the optical device from the light emitting element to the sensor on the way to the scale and after reflection from the scale, and includes a non-reflective optical path between the light emitting element and the optical device, and a non-reflective optical path between the optical device and the sensor.

Description

Technical Field

[0001] The present invention relates to a position measuring encoder, a position encoder, an encoder device, or a position measuring instrument also known simply as an "encoder". In particular, the present invention relates to a so-called absolute position measurement device / absolute position encoder apparatus.

Background Art

[0002] Encoder devices / position measuring instruments 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 reading head for reading the scale markings is provided on another object. The scale markings can be formed integrally with the object or provided on a scale that can be fixed to the object.

[0003] Encoder devices are generally classified into either incremental encoder apparatus or absolute encoder apparatus. In an incremental encoder apparatus, the scale has a plurality of periodic markings detectable by the reading head to provide an incremental up / down count. For example, such a scale is described in Patent Document 1. A reference mark can be provided adjacent to the periodic markings or embedded within the periodic markings to define a reference point. For example, such a scale is disclosed in Patent Document 2. An absolute position encoder apparatus typically measures relative displacement by a reading head that detects a unique series of marks, such as codes, and converts those codes into an absolute position. Such a scale is described in Patent Document 3, and such an encoder is described in detail in Patent Document 4.

[0004] An absolute encoder device differs from an incremental encoder device in that the absolute encoder device does not require relative movement of the reading head on the scale and can determine the absolute position of the reading head relative to the scale at startup. In contrast, in an incremental encoder device, the reading head must move to a reference mark to determine the zero position.

[0005] Encoder devices can also be classified based on their main means of detecting features on the scale, such as optical features, magnetic features, inductive features, and capacitive features.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

[0007] The present invention relates to an improved optical encoder device.

[0008] According to a first aspect of the present invention, there is provided an encoder device including a reflective scale and a reading head, wherein the reading head includes at least one light emitting element, at least one sensor, and at least one optical device, the at least one optical device forms an optical system together with the scale, the optical device forms an image of an irradiation region of the reflective scale on the sensor, and an optical path of the system passes through the optical device on the way to the scale and after reflection from the scale, from the light emitting element to the sensor.

[0009] The optical path between the light emitting element and the optical device may be direct / non-reflective. The optical path between the optical device and the sensor may be direct / non-reflective. In other words, the device (e.g., the reading head) can include a non-reflective optical path between the light emitting element and the optical device and a non-reflective optical path between the optical device and the sensor.

[0010] Preferably, the optical path of light between the light emitting element and the scale is direct / non-reflective, and the optical path of light from the scale towards the sensor is also direct / non-reflective.

[0011] In an embodiment where the reading head includes a shell / housing and a window through which light (from the light emitting element) exits the reading head and light (reflected by the scale) enters the reading head (i.e., exists and enters the shell / housing), it is preferable that the entire optical path within the reading head (or within the shell / housing) is direct / non-reflective.

[0012] The present invention enables a particularly compact reading head for an optical absolute position measuring device. For example, by configuring the reading head such that the light emitted from the light emitting element passes through the same optical device on the outward path and on the return path, the number of necessary optical components can be reduced. Further, ensuring a direct / non-reflective optical path between the light emitting element and the optical device, and a direct / non-reflective optical path between the optical device and the sensor means that reflective optical components (such as mirrors and / or beam splitters) are not necessary (for example, the reading head can be configured to not have reflective optical components in its optical path). Thus, the number of optical components of the reading head can be further reduced, improving the compactness of the reading head and reducing complexity.

[0013] The light emitting element can be attached to the reading head such that, in a dimension perpendicular to the plane of the sensor, the light emitting element is seated / located between the sensor and the optical device. For example, the light emitting element can be located in the space (or "volume") between the sensor and the optical device (depicted by the outer edges / sides of the sensor and the optical device).

[0014] The light emitting element can be substantially located in the focal plane of the optical device such that the light emitted by the light emitting element is collimated by the optical device. For example, preferably, the light emitting element is located at 500 μm (micrometers) or less, more preferably 250 μm (micrometers) or less, and particularly preferably 100 μm (micrometers) or less from the focal plane of the optical device.

[0015] Optionally, the light reflected by the scale and imaged onto the sensor by the optical device converges towards a point at a specific distance between the optical device and the sensor. Further, the light emitting element can be arranged at approximately the specific distance between the optical device and the sensor.

[0016] Optionally, the ratio of i) the distance in the direction perpendicular to the surface of the sensor between the center of the light-emitting surface (or light-emitting point) of the light-emitting element and the sensing surface of the sensor, and ii) the distance in the direction perpendicular to the surface of the sensor between the center of the light-emitting surface (or light-emitting point) of the light-emitting element and the optical device, is 35:65 or more, for example 40:60 or more, optionally 50:50 or more, preferably 60:40 or more, for example 65:35 or more.

[0017] The light-emitting element can be positioned so as to be offset from the optical axis of the optical device. For example, the light-emitting element can be offset by 1 mm or less, for example 750 μm or less, for example 500 μm or less from the optical axis of the optical device (for example, it can be measured from the center of the light-emitting zone of the light source). Optionally, the ratio of the offset to the focal length of the lens is 1:2.5 or less, for example, 0.5:2.5 or less.

[0018] Optionally, when the optical path collides with the scale and / or reflects from the scale, the direction of the optical path is not perpendicular to the scale. For example, the angle formed by a line extending perpendicular to the scale (in the irradiation region) and the direction of the optical path that collides with (and / or reflects from) the scale is 1° (degree) or more, for example 2° or more, for example 5° or more, and optionally 20° or less, for example 15° or less. In other words, optionally, there is an angle of reflection from the scale, such as at least 2°, for example at least 4°, optionally at least 10°, for example 40°, for example 30° or less, between the incident direction and the reflection direction of the light (i.e., greater than 0°).

[0019] Therefore, optionally, the shape of the optical path when the optical path collides with the scale and / or reflects from the scale is V-shaped. Optionally, the optical path of the system is substantially diamond-shaped / rhombic from the light-emitting element to the sensor.

[0020] The optical path through the optical device on the way towards the scale and after reflection from the scale can be offset laterally. Thus, for example, for any given ray of light passing through the optical system, the point at which the ray exits the optical device towards the scale and the point at which the ray re-enters the optical device after being reflected from the scale are offset laterally / different. As will be appreciated, the optical path (e.g., the light beam) on the way towards the scale and after reflection from the scale can overlap (e.g., partially and optionally, substantially, but not completely).

[0021] Optionally, the light-emitting element and the sensor face both the optical device and the scale. The light-emitting element and the sensor can both face the same direction. In other words, the light-emitting element and the sensor can be attached to the reading head such that the sensor plane is substantially parallel to the light-emitting surface of the light-emitting element.

[0022] Optionally, the sensor and the image of the scale formed by the optical device are behind (e.g., directly behind) the light-emitting element. Optionally, the light-emitting element is positioned such that the light rays from the light-emitting element reflected by the scale converge at a point, bypass the light-emitting element on the return path, and then branch off to form the image of the scale on the sensor (behind the light source).

[0023] As will be appreciated, the image of the scale is formed when the light rays from any given point on the scale substantially converge at a common unique point in the image plane (where the sensor is located). (This point is "unique" in that for different given points on the scale, the light rays from that point converge at substantially different common points). The image can be a spatially filtered image.

[0024] A plurality of light emitting elements may be provided. In such a case, the plurality of light emitting elements are preferably provided together so as to act as a single light source. Optionally, the reading head comprises only one light emitting element.

[0025] Optionally, the light emitting element comprises an "uncapped", "unpackaged", or "unlensed" semiconductor diode, for example a bare die semiconductor diode. The light emitting element may comprise, for example, a light emitting diode (LED) or a laser (e.g., a vertical-cavity surface-emitting laser (VCSEL)).

[0026] The at least one sensor can comprise a plurality of sensor elements (e.g., photodiodes). For example, the reading head can comprise an array of sensor elements. The array can be one-dimensional or two-dimensional.

[0027] References to "light" herein refer to electromagnetic radiation (EMR) anywhere in the range from ultraviolet to infrared. For example, the light can be ultraviolet, visible light, infrared, or a combination thereof.

[0028] The optical device can comprise the lens, for example a singlet lens. Optionally, the optical device comprises a diffractive optical element such as a Fresnel zone plate. Optionally, the optical device comprises a holographic optical element, for example a hologram of a lens.

[0029] The scale can comprise a series of features that can be detected by the sensor to determine the relative movement / relative position of the scale and the read head. Such features can be arranged periodically or aperiodically. As will be appreciated, there are many suitable ways to define the features on the scale. For example, the features can be defined by specific electromagnetic radiation (EMR) characteristics, such as markings having specific optical characteristics, such as specific optical transmittance or reflectance of a portion of the scale. Thus, the features can be defined, for example, by the portion of the scale having a minimum reflectance or transmittance value. Optionally, the features can be defined, for example, by the portion of the scale having a maximum reflectance or transmittance value. Optionally, the features can be defined, for example, by the way it reflects light (e.g., direction) (e.g., towards and away from the read head). These features can take the form of lines, dots, or other configurations detectable by the sensor. A preferred configuration for a one-dimensional scale can comprise a line extending across the width of a track in a dimension perpendicular to the dimension being measured.

[0030] The encoder device may be an incremental encoder device. Accordingly, the scale may comprise an incremental scale. The incremental encoder device may comprise 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 can determine the absolute position of the reading head relative to the scale without requiring relative movement of the reading head and the scale. The absolute encoder comprises an absolute scale having a function of defining a series of unique positions along its length. The series of unique absolute positions may be defined by features of a plurality of tracks, for example a plurality of adjacent tracks. Optionally, the series of unique absolute positions can be defined by features contained in only a single track. For example, the absolute position information can be determined from a combination of features obtained along the measured length of the scale. Accordingly, the encoder device may be configured to extract absolute position information from the image acquired by the sensor. Such extraction may be performed by the reading head or by a device external to the reading head.

[0031] Optionally, the read head is configured to read the scale by obtaining at least one discrete snapshot (i.e., a snapshot image) of the scale. This can be used, for example, instead of continuous phase measurement and phase counting. Thus, by taking a discrete snapshot of the scale by the read head, the scale image can be obtained. The snapshot can be taken instantaneously or constructed by quickly and continuously taking small read values of successive sections of the scale. The snapshot reading of the scale can offer several advantages. For example, the maximum operating speed of the scale reader with respect to the scale is not limited by the natural frequency limitations of a continuous phase measurement and phase counting system and can thus be greater. Further, in an optical system taking a snapshot, the light emitting element need only be turned on for a short time, which allows the light intensity to be increased compared to a continuous system without increasing the average power consumption or limiting the lifetime of the source. This increased light intensity can mean that more photons can be captured by the sensor and thus reduce the noise floor of the system and result in less positional noise.

[0032] The sensor can be attached to a Printed Circuit Board (PCB). The light-emitting element can be electrically connected to the same PCB as the sensor. The light-emitting element can be physically attached to the reading head (e.g., to the PCB) by a support member that holds the light-emitting element away from the substrate so as to provide a gap (e.g., in a direction perpendicular to the plane of the sensor) between the light-emitting element and the sensor. The support member can be configured to hold the light-emitting element in the space (volume) between the sensor and the optical device. For example, the distance between the light-emitting element and the sensor can be measured in a direction perpendicular to the plane of the sensor and can be at least 1 mm, for example at least 1.5 mm, for example at least 2 mm. More specifically, in a direction perpendicular to the sensing surface of the sensor, the distance from the center of the light-emitting surface (or light-emitting point) of the light-emitting element to the sensing surface of the sensor can be at least 1 mm, for example at least 1.5 mm, preferably at least 2 mm, for example about 2.5 mm. The distance can be from 2 mm to 3 mm, for example about 2.5 mm.

[0033] According to another aspect of the present invention, there is provided a reading head for reading a reflection scale and determining its relative position, the reading head comprising a light-emitting element, a sensor, and an optical device, the reading head being for irradiating and forming an image of a region of the reflection scale located adjacent to the reading head on the sensor, wherein light from the light-emitting element passes through the optical device on its way towards the scale and after reflection from the scale, the optical path of the light between the light-emitting element and the optical device is direct / non-reflected, and the optical path of the light between the optical device and the sensor is direct / non-reflected. The features described above in connection with the first aspect of the present invention are equally applicable to this aspect of the present invention.

[0034] According to another aspect of the present invention, there is provided a reading head for reading a reflective scale and determining its relative position, the reading head comprising at least one light-emitting element, at least one sensor, and at least one optical device (such as a lens, etc.). The light-emitting element can be attached to the reading head such that the light-emitting element is positioned 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 can be located in the space (or "volume") between the sensor and the optical device (e.g., delineated by the outer edge / sides of the sensor and the optical device). Such a configuration provides a very compact reading head. The optical device can be configured to form an image of the illuminated area of the reflective scale on the sensor. Optionally, the optical path of the system from the light-emitting element to the sensor can pass through the optical device on the way towards the scale and after reflection from the scale. Optionally, the optical path of the system can comprise an unreflected optical path between the light-emitting element and the optical device. Optionally, the optical path of the system can comprise an unreflected optical path between the optical device and the sensor. The features described above in connection with other aspects of the present invention are also applicable to this aspect of the present invention.

[0035] In this book, an encoder device is described that includes a reading head for reading a reflection scale positioned adjacent to the reading head, a sensor including one or more photodiodes for detecting light reflected from the scale positioned adjacent to the reading head, and at least one light-emitting element, and is held away from a sensing surface of the sensor such that a distance between the light-emitting element and the sensor is measured in a direction perpendicular to the sensing surface of the sensor. In this book, a circuit board including a reading head for reading a reflection scale positioned adjacent to the reading head and a sensor including one or more photodiodes for detecting light reflected from the scale positioned adjacent to the reading head, and at least one light-emitting element are provided, and the light-emitting element is attached to the circuit board via a light-emitting element support structure that holds the light-emitting element at a position away from the circuit board such that a distance between the light-emitting element and the sensor measured in a direction perpendicular to the sensing surface of the sensor is at least 1 mm. More specifically, a distance in a direction perpendicular to the sensing surface of the sensor from the center of a light-emitting surface (or a light-emitting point) of the light-emitting element to the sensing surface of the sensor can be at least 1 mm. The distance can be at least 1.5 mm, optionally at least 2 mm, for example at least 2.5 mm. The distance can be 2 mm to 3 mm, for example, about 2.5 mm.

Brief Description of the Drawings

[0036] Here, embodiments of the present invention are described by way of example only with reference to the following drawings.

Figure 1

Figure 2

Figure 3a

Figure 3b

Figure 4

Figure 5

Figure 6a

Figure 6b

Figure 6c

Figure 7a

Figure 7b

Figure 8

Figure 9a

Figure 9b

DETAILED DESCRIPTION OF THE INVENTION

[0037] Referring to FIG. 1, an encoder device 2 including a read head 4, a scale 6, and a controller 7 is shown. The read head 4 and the scale 6 are respectively mounted on first and second objects (not shown) that are movable relative to each other. The speed of the relative movement can vary, but in the embodiments described herein, the read head 4 and the scale 6 have a known maximum relative acceleration.

[0038] In the embodiments described herein, the scale 6 is a linear scale. However, it will be understood that the scale 6 can be a non-linear scale, for example, a rotary scale (e.g., a disk scale or a ring scale). Further, the scale 6 enables measurement in only one dimension. However, this is not necessarily the case, and for example, the scale can enable measurement in two dimensions.

[0039] In the embodiments described herein, 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 can be in the form of, for example, a pseudo-random array or a discrete codeword. In other embodiments, the scale can be an incremental scale (regardless of the presence or absence of reference marks).

[0040] The width of the lines depends on the required position resolution and typically ranges from 1 μm to 100 μm, more typically from 5 μm to 50 μm, for example from 10 μm to 30 μm. In the described embodiments, the width of the lines is about 15 μm. The reflective lines 8 and non-reflective lines 10 are generally arranged alternately over a given period. However, selected non-reflective lines 10 are missing from scale 6 in order to encode the absolute position data within scale 6. For example, the presence of a non-reflective line can be used to represent a "1" bit and the absence of a non-reflective line can be used to represent a "0" bit.

[0041] As shown in FIG. 2, the reading head 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, although other optical devices can be used. For example, diffractive optical elements such as Fresnel zone plates, and / or holographic optical elements, such as holograms of lenses, can be used. In this embodiment, the sensor 20 comprises a Complementary Metal-Oxide-Semiconductor (CMOS) sensor. As will be appreciated, other image sensors can be used instead of the CMOS sensor. For example, a CCD or a photodiode array can be used instead.

[0042] The reading head 4 also includes a CPU 24, a memory device 25 (e.g., an Electrically Erasable Programmable Read-Only Memory (EEPROM) or a flash memory), and an interface 26. The reading head 4 may also include an analog-to-digital converter for digitizing the image data from the sensor 20. Optionally, the analog-to-digital conversion may be performed within the sensor 20 or the CPU 24.

[0043] The light emitted from the LED 12 is collimated by the optical device 18 and then passes through the window 22 and is projected onto the scale 6. The scale 6 reflects the light through the window 22, and the window 22 passes through the lens 18. The lens 18 uses the light reflected by the scale to form a two-dimensional image of the scale on the sensor 20. Thus, the sensor 20 detects a two-dimensional image of a part of the scale 6 irradiated by the LED 12. The sensor can comprise a one-dimensional or two-dimensional array of pixels. For example, the sensor can comprise a one-dimensional array of 256 elongated pixels, the length of which extends parallel to the lengths of the reflective lines 8 and non-reflective lines 10 on the scale. Instead of the two-dimensional imaging device described herein, a one-dimensional imaging device can be used, and instead, a one-dimensional image of the scale is formed by a lens on the sensor.

[0044] The LED 12 is connected to the CPU 24 so as to be operable in response to a request from 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 projected across the image sensor 20. The sensor 20 is also directly connected to the CPU 24 because the sensor 20 can be operated to take a snapshot of the intensity of light projected across the sensor 20 in response to a request from the CPU 24. The CPU 24 is connected to the memory 25 and can store and retrieve data for processing. The interface 26 is connected to the CPU 24 so that the CPU 24 can receive requests from external devices such as the controller 7 (shown in FIG. 1) via the line 40 and output results. The line 40 also comprises a power line to which the read head 4 is powered.

[0045] As will be appreciated, the absolute position data can be encoded on the scale 6 by omitting the reflective lines 8 as well as the non-reflective lines 10, or alternatively. Further, the absolute position data can be embedded in the scale 6 without adding or removing the reflective lines 8 or the non-reflective lines 10. For example, the width of the lines, the distance between them, or their color can be varied to embed the absolute position data in the scale 6. Further, rather than a scale that defines the absolute position by a unique combination of features photographed along the measured length of the scale, it can have features that define the absolute position by a unique combination of features photographed along the width of the scale. For example, the scale can comprise a plurality of "barcodes" whose length extends across the scale, for example, substantially perpendicular to the measured length of the scale. Optionally, the scale can have a plurality of tracks, where at least one, optionally at least two, and in some cases all of these tracks have different track scale periods from each other, and the combination of features across the width of the scale is unique at any one point along the measured length of the scale (i.e., the tracks can essentially comprise incremental scale features of different fundamental frequencies).

[0046] A group of markings can still have enough information to enable extraction of phase information from the series of markings in order to enable determination of fine position information (e.g., position information having a resolution finer than the period of the scale markings), while being used to encode a series of unique binary codewords along a scale length that defines unique, i.e., absolute position information. Thus, in such a system, the position information can be composed 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 substantially periodic markings). Further details of such so-called hybrid incremental and absolute scales can be found in Patent Document 5, the contents of which are incorporated herein by reference.

[0047] In an alternative embodiment, the scale can comprise an absolute track with features that define absolute position information and a separate incremental track with features of regular spacing.

[0048] Referring to FIGS. 3a and 3b, the optical system of the read head 4 of FIGS. 1 and 2 will be described in more detail. FIGS. 3a and 3b schematically show the path of light through the optical system that forms an image of the scale 6 on the sensor 20 from the light source 12 to the sensor 20.

[0049] 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 on the focal plane fp of the lens 18, but is 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 about 450 μm (measured from the center of the light-emitting zone of the light source). In particular, the ratio of the offset to the focal length of the lens is about 0.45:2.5. Positioning the light source 12 substantially on the focal plane 18 of the lens helps ensure that the light radiated therefrom is substantially aligned by the lens 18 when the lens 18 faces the scale 6. Thus, the light reflected by the scale 6 diverges after being focused by the lens 18 at a point on the focal plane fp of the lens 18, and forms a two-dimensional image of the scale 6 on the sensor 20 behind the light source 12. As understood, the image of the light source 12 is formed on the focal plane fp. Positioning the light source 12 on the focal plane fp of the lens 18 but offset from the optical axis OA of the lens 18 allows the light source 12 to be located in the space (or "volume") between the sensor 20 and the lens 18 (shown by the hatched area in FIG. 3b), which helps to make the reading head compact, but means that it does not interfere with the light reflected by the scale on the return path to the sensor 20.

[0050] 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 light-emitting surface of the light source 12 and the sensing surface of the sensor 20 face the lens 18). Also, there is an unreflected (i.e., "direct") optical path between the light source 12 and the lens 18, and there is an unreflected (i.e., "direct") optical path between the lens 18 and the sensor 20. Thus, no reflective optical components are required or used to turn or manipulate the light. Avoiding the use of reflective optical components such as mirrors and beam splitters can help significantly reduce the size of the read head.

[0051] Furthermore, in the specific embodiments described, the same optical device / lens 18 is used to collimate the light from the light source 12 and form an image of the scale 6 on the sensor. Thus, the optical arrangement of the reading head 4 described above is particularly compact and inexpensive because it uses only one optical device / lens 18. In the embodiments described, the lens 18 is a single lens, but it could be a different type of lens (e.g., a doublet, a compound lens, or a gradient-index (GRIN) lens). As will be understood, the optical device does not necessarily have to be a lens, but could 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.

[0052] As shown in FIG. 3a, the light source is much closer to the lens 18 than to the sensor 20. Such a configuration deviates from conventional encoder designs where the light source is typically mounted on the same substrate as the sensor, which is substantially planar with the sensor. As shown in FIG. 3a, in this embodiment, the reading head is configured such that i) the ratio of the distance (D1) from the center of the light-emitting surface (or point) of the light-emitting element to the sensing surface of the sensor in a direction perpendicular to the surface of the sensor, and ii) the ratio of the distance (D2) from the center of the light-emitting surface (or point) of the light-emitting element to the optical device in a direction perpendicular to the surface of the sensor is approximately 70:30. In absolute terms, the distance in the direction perpendicular to the sensing surface of the sensor from the center of the light-emitting surface (or point) of the light-emitting element to the sensing surface of the sensor is approximately 2.5 mm, e.g., 2.6 mm.

[0053] As schematically shown by the thick black line in FIG. 3b, due to the configuration of the light source 12, the lens 18, and the sensor 20, the optical path from the light source to the sensor is substantially diamond-shaped / rhombus-shaped, and the optical path between the lens 18 and the scale 6 is substantially V-shaped. In the embodiments described above, the angle θ between the line extending perpendicular to the scale (the dotted line in FIG. 3b) and the direction of the optical path impinging on the scale is approximately 10°.

[0054] As shown, sensor 20 can be tilted such that its sensing surface / plane is not perpendicular to the optical axis of the lens. Such a tilt can serve to compensate for keystone distortion in the image formed on the sensor, which can be caused by the image formed by the off-axis portion of lens 18. In the embodiment shown, sensor 20 is tilted such that the angle α between the plane extending parallel to their sensing surface (e.g., its sensing surface) and the plane extending perpendicular to the optical axis is about 3°. However, this is not necessarily the case, and the sensing surface of the sensor can be configured to extend perpendicular to the optical axis of the lens (i.e., such that the angle α is less than 1°). As will be explained in more detail below, such a tilt of sensor 20 can be achieved by mounting the PCB 32 (to which the sensor is attached) at the tilt angle. Thus, for example, sensor 20 including light source 12 or any other component attached to PCB 12 can be titled for mechanical convenience, but this is not necessarily the case. As will be appreciated, other methods of compensating for keystone distortion are available, such as appropriately shaping the sensor element, e.g., "keystoning" the sensor element itself.

[0055] Referring now to FIGS. 4 and 5, an exemplary embodiment of how a read head can be configured to achieve the above optical layout is described herein.

[0056] As shown in FIG. 4, read head 4 includes a body 30 to which lens 18, window 22, and printed circuit board (PCB) 32 are attached (e.g., via adhesive, mechanical, and / or friction means). Sensor 20, LED 12, and other electronic components (such as CPU 24, memory 25, and interface 26 described above (not shown in FIGS. 4 or 5)) are mechanically and electrically attached to PCB 32.

[0057] As shown, LED 12 is attached to PCB 32, but in the sense that LED 12 is attached via a raised support structure 34 that holds it away from PCB 32, it is attached to the circuit board "off-board". In particular, support structure 34 extends beyond sensor 20 so as to hold LED 12 further away from PCB 32 than sensor 20. Thus, as shown, sensor 20 is attached relatively close to PCB 32, while LED 12 is attached relatively far from PCB 32. As shown in FIG. 4, LED 12 is much closer to lens 18 than to PCB 32, while sensor 20 is much closer to PCB 32 than to lens 18. Thus, as shown, the light-emitting element and the sensor are separated by a dimension extending perpendicular to the plane of the sensor / circuit board. In particular, there is a (free) space between the light-emitting element and the sensor in the dimension extending perpendicular to the plane of the sensor / circuit board. In this example, the ratio of i) the distance in the direction parallel to the optical axis OA of the imaging member between the light-emitting surface (or light-emitting point) of LED 12 and the sensing surface of sensor 20, and ii) the distance in the direction parallel to the optical axis OA of the imaging member between the light-emitting surface (or light-emitting point) of LED 12 and lens 18 is approximately 70:30.

[0058] In the embodiments described herein, the support structure 34 described above also forms / provides an electrical connection between LED 12 and PCB 32. Thus, in the embodiments described, the support structure 34 for holding LED 12 away from PCB 32 is the cathode 34 between LED 12 and PCB 32. Thus, cathode 34 comprises a rigid conductive support structure for LED 12 rising from PCB 32. As shown in FIGS. 4 and 5, the support structure / cathode 34 comprises an opening / window 35 through which light reflected by scale 6 can pass to reach sensor 20.

[0059] In this embodiment, the anode 36 also comprises a rigid conductive structure that rises from the PCB 32 and is wire-bonded to the LED 12 via a bonding wire 38, as shown in FIGS. 4 and 5. In other words, the read head comprises a raised bond wire support structure extending from the PCB 32, and the 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 can also have an opening / window through which light emitted from the LED 12 can pass towards the lens 18 / scale 6 and through which light reflected by the scale 6 can pass to reach the sensor 20.

[0060] As can be understood, the rigid structure of the anode 36 can be omitted, and the LED 12 can be bonded via a bonding wire extending between the LED 12 and the PCB 32. However, the bonding wire can be fragile, and the longer the bonding wire, the higher the likelihood of breakage. Therefore, it can be beneficial to make the length of the bonding wire as short as possible.

[0061] In this particular embodiment, the LED support structure / cathode 34 and the bonding wire support structure / anode 36 each comprise a sheet material 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 bass, plated with nickel-gold. As shown in FIGS. 6 and 7, the fold lines 37 are chemically etched into the sheet material to assist folding. When folded, each of the support structures 34, 36 comprises a top surface 31 and a plurality of lateral supports (or "legs") 33 soldered to the PCB 32. As can be understood, the support structure / cathode 34 can be formed in other ways, for example, it can be machined / cut and / or stamped / compressed into shape. The bare-die LED 12 is directly mounted on the support structure / cathode 34 via a conductive epoxy, and the wire bond extends between the LED 12 and the top surface 31 of the anode 36.

[0062] As shown in FIG. 6c, the upper surface 31 of the LED support structure 34 extends over / partially covers the sensor 20 (its outline is schematically shown in FIG. 6c by a phantom line). In other words, a line extending vertically through the plane of the sensor 20 (and the PCB 32) also passes through the upper surface 31 of the LED support structure 34. With such a configuration, the LED 12 can be placed very close to the sensor 20 and, if necessary, on top of the sensor 20.

[0063] The read head 4 is assembled by dead-reckoning the lens 18 within the body 30 of the read head 4, and the body 30 is crimped to hold the lens 18 in a predetermined position (however, other methods of fixing the lens 18 to the body can be used, such as by epoxy and / or by pressing the lens 18 into a bent portion that holds the lens). The PCB 32 with the LED 12 already mounted thereon is then attached to the body 30 by mechanical means such as, for example, adhesion and / or crimping. If desired, an alignment process can be used to align the PCB (and thus the sensor and the LED thereon) with respect to the lens. Such an alignment process can comprise 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 to the PCB / components thereon and making adjustments using the output of the sensor. Once assembled, the lid 46 is fixed to the body 30 via, for example, adhesion, crimping, and / or welding.

[0064] In the above-described embodiment, the bonding 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 can, for example, alternatively form the anode.

[0065] In the described embodiment, the LED 12 is mechanically attached to the PCB 32 via the electrodes 34, but as will be understood, this need not necessarily be the case. For example, the LED 12 can be mechanically attached directly to the PCB 32 via one or more non-conductive members and can be electrically connected to the PCB 32 via a separate member, such as one or more wires (e.g., via wire bonding). Further, the LED 12 need not necessarily be attached directly to the PCB 32. For example, the LED 12 can be mechanically attached directly to the body 30 and can be electrically connected to the PCB 32 via one or more wires (e.g., via wire bonding). In another embodiment, the LED 12 can be electrically connected to a different PCB (i.e., not the same PCB 32 to which the sensor is connected).

[0066] FIG. 8 shows a read head 4' according to another embodiment of the present invention. The read head 4' of FIG. 8 shares many of the same parts as the embodiments of FIGS. 1-7, and like parts share like reference numerals. In the embodiment of FIG. 8, the LED 12 is attached to the circuit board via 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 via an adhesive epoxy. Next, the LED 12 faces the lens 18 and sits on a conductive pad 52 deposited on a side surface of the glass block 50 distal to the sensor 20. The LED 12 is electrically connected to the circuit board 32 via an anode 36' and a cathode 34' each having a raised bond wire support structure extending from the PCB 32 and a bond wire extending between them and the LED 12 / conductive pad 52. As will be understood, in a variation of this embodiment, the LED 12 can be connected to the circuit board in other ways, such as via an anode and a cathode deposited on and running along the surface / sides of the glass block 50 or via an anode and a cathode running through the glass block 50.

[0067] As described above, the support structure 34 of the light-emitting element and / or the light-emitting element 12 can 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 support structure 34 of the light-emitting element and the sensor 20, and / or through both the light-emitting element 12 and the sensor 20. As will be appreciated, and as schematically shown in FIG. 9, the sensor 20 can comprise at least one, 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 can be a chip or component that includes at least one, for example, an array of photosensitive elements 21. For example, as shown in FIG. 9a, the light-emitting element 12 can be positioned directly on the sensor chip 20 in a manner such that it is not directly above the photosensitive element 21. Alternatively, as shown in FIG. 9b, the light-emitting element 12 can be positioned directly above the sensor chip 20 such that, for example, a line (i.e., parallel to the Y-axis) extending perpendicular to the plane of the circuit board / sensor passes through both the light-emitting element 12 and the photosensitive element 21, so that the light-emitting element 12 is directly above the photosensitive element 21.

[0068] In the embodiment shown, the LED 12 is mounted "off-board" by the support structure 34. While this can be beneficial (e.g., it allows the LED 12 to be positioned in the focal plane of the lens 18 and enables the sensor 20 to capture an image of the scale to achieve collimation), it is not necessarily so. For example, the LED 12 can be mounted on the PCB 32 such that it sits substantially in the plane (i.e., at substantially the same height) as the sensor 20.

[0069] In the embodiment shown, the light that impinges on the scale is collimated, but this is not necessarily so. Also, it is not necessary to orthogonalize the light incident on the scale or the light reflected by the scale. For example, if the scale is curved, e.g., if the scale is a ring scale, the light reflected by the scale will not be collimated.

Claims

1. An encoder device comprising a reflective scale and a reading head, wherein the reading head comprises at least one light-emitting element, at least one sensor, and at least one optical device comprising a lens, the at least one optical device forms an optical system together with the scale, the lens forms an image of an irradiation region of the reflective scale on the sensor, the light-emitting element is positioned in a space between the sensor and the lens and offset from an optical axis of the optical device, an optical path of the optical system passes through the lens on the way to the scale from the light-emitting element and after reflection from the scale on the way to the sensor, the optical path of the optical system comprises a non-reflective optical path between the light-emitting element and the lens and a non-reflective optical path between the lens and the sensor, and the sensor is tilted such that a sensing plane of the sensor is not perpendicular to an optical axis of the lens. An encoder device characterized by this.

2. The encoder device according to claim 1, wherein the light-emitting element is substantially positioned on a focal plane of the lens such that light emitted by the light-emitting element is collimated by the lens.

3. The light reflected by the scale and imaged on the sensor by the lens converges toward a point at a specific distance between the lens and the sensor, and the light-emitting element is arranged at approximately the specific distance between the lens and the sensor. The encoder device according to claim 1 or claim 2, characterized by this.

4. The ratio of i) a distance in a direction perpendicular to the surface of the sensor between the center of the light-emitting surface of the light-emitting element and the sensing surface of the sensor, and ii) a distance in a direction perpendicular to the surface of the sensor between the center of the light-emitting surface of the light-emitting element and the lens, is 35:65 or more. The encoder device according to claim 1, characterized by this.

5. The encoder device according to any one of claims 1 to 4, wherein a direction of the optical path when the optical path collides with the scale and / or when reflected from the scale is not perpendicular to the scale.

6. The encoder device according to any one of claims 1 to 5, wherein the optical path of the optical system is substantially diamond-shaped from the light-emitting element to the sensor.

7. The encoder device according to any one of claims 1 to 6, wherein the light-emitting element and the sensor face both the lens and the scale.

8. The encoder device according to any one of claims 1 to 7, wherein the image of the scale formed by the sensor and the lens is behind the light-emitting element.

9. The scale includes an absolute scale having a function of defining a series of unique positions along its length, and the device is configured to extract absolute position information from an image acquired by the sensor. The encoder device according to any one of claims 1 to 8, characterized in that.

10. The sensor is attached to a printed circuit board (Printed Circuit Board: PCB), and the light-emitting element is electrically connected to the same PCB as the sensor. However, the light-emitting element is physically attached to the reading head by a support member that holds the light-emitting element away from the PCB so as to provide a gap between the light-emitting element and the sensor. The encoder device according to any one of claims 1 to 9, characterized in that.

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