Patent Description:
Encoder apparatus/position measurement devices for measuring the relative position between two moveable objects are well known. Typically, a series of scale markings are provided on one object and a readhead for reading the scale markings on another. The scale markings can be formed integrally with the object or can be provided on a scale which can be secured to the object.

An encoder apparatus is commonly categorised as being either an incremental encoder apparatus or an absolute encoder apparatus. In an incremental encoder apparatus, the scale has a plurality of periodic markings which can be detected by the readhead so as to provide an incremental up/down count. For instance, such a scale is described in <CIT>. Reference marks can be provided, either next to or embedded in the periodic markings so as to define reference points. For example, such a scale is disclosed in Published International Patent Application <CIT>. An absolute position encoder apparatus typically measures relative displacement by a readhead detecting unique series of marks, e.g. codes, and translating those codes into an absolute position. Such a scale is disclosed in International Patent Application no. <CIT>, and such an encoder is described in detail in <CIT>.

An absolute encoder apparatus is distinct from an incremental encoder apparatus in that an absolute encoder apparatus can determine the absolute position of the readhead relative to the scale, on start-up, without requiring relative motion of the readhead in scale. In contrast, in an incremental encoder apparatus, the readhead has to travel to a reference mark in order to determine a zero position.

Encoder apparatus can also be categorised based on their primary means of detecting the features on the scale, e.g. optical, magnetic, inductive, capacitive.

<CIT> describes an optical distance measuring unit having a reflection-type scale and a sensor head. The sensor head is formed of a resin-molded block. The block has a hole opened to be opposed to the scale, and interconnection lines. In the hole, a light emitting device is buried so as to irradiate the scale. A device substrate, on which a light receiving IC chip is mounted for receiving the reflected light from the scale, is attached to the front surface of the block. The interconnection lines, to which the light emitting device and the IC chip are connected, are led to external circuits through an FPC substrate.

<CIT> describes a surface-emitting laser disposed on a photodetector disposed in parallel with a scale, so that an optical pattern surface of the scale can be irradiated with a light beam having a desired shape. A major axis of the light beam emitted from the surface-emitting laser is vertical to the photodetector, and forms a beam spot on the optical pattern surface of the scale. A light emission portion of the light source is disposed in a <NUM>-order diffracted light region from the scale and other than a region in which only a <NUM>-order diffracted light and 1st-order diffracted light or a <NUM>-order diffracted light and -1st-order diffracted light interfere. A light shield metal and/or a dummy light receiving portion are/is disposed to surround a light receiving portion outer periphery, and is disposed in a region between an inner periphery of the light receiving portion and the surface-emitting laser.

<CIT> describes an encoder apparatus comprising a scale and a readhead assembly comprising a scale signal receiver. The scale and the scale signal receiver are located within a protective housing which is configured to protect them from contamination located outside the protective housing and comprises a seal through which the scale signal receiver can be connected to a part outside the protective housing. The arrangement of the scale signal receiver inside the protective housing is independent of the scale and protective housing.

<CIT> describes a scanning unit for detection of optical measure embodiments. The unit has a light transmitter, a scanning receiver, an optical auxiliary scanning diaphragm and micro lens array. The optical measure embodiments have an optical position coding. An aperture diaphragm array is arranged in the picture-sided focal plane of the micro lens array, such that an aperture diaphragm opening is located in the picture-lateral focal point of each micro lens of the micro lens array. The scanning diaphragm, the aperture diaphragm array and the micro lens array are positioned on each other in the given series directly on the surface of the scanning receiver above the photo-sensitive receiver field.

<CIT> describes a user navigational apparatus including a dial, a coding element, and an encoder. The coding element is coupled to a dial. The coding element includes a track of alternating reflective and non-reflective sections, each having a substantially oblique leading edge relative to a direction of movement of the coding element. The encoder includes an emitter and a detector. The emitter generates a light signal incident on the track of the coding element. The detector detects a reflected light signal which corresponds to a portion of the incident light signal that is reflected off of the reflective sections of the track. The detector also generates a channel signal corresponding to the reflected light signal.

<CIT> describes a position measuring device comprising a scanning unit which has a detector unit having two detector arrangements provided on a support board. Above the detector unit having the two detector arrangements, a transparent carrier substrate, e.g. in the form of a plate-shaped glass carrier substrate, is disposed on the side of the scanning unit in a central partial area of the detector arrangements. In the example embodiment, this carrier substrate takes up only a smaller part of the total area of the detector arrangement(s) or the surface of detector unit, as illustrated in Figure 1b of <CIT>. On the upper side of carrier substrate, a light source is placed. A so-called point light source, for example, a so-called VCSEL light source, may be provided as the light source.

The present invention relates to an improved optical encoder apparatus.

According to a first aspect of the invention there is provided an encoder apparatus comprising a readhead for reading a reflective scale located adjacent the readhead, the readhead comprising: a circuit board; a sensor mounted on the circuit board, the sensor comprising one or more photodiodes for detecting light reflected from a scale located adjacent the readhead; at least one light emitting element; a light emitting element support structure; and at least one lens; characterised in that: the light emitting element is mounted to the circuit board via the light emitting element support structure, in which the light emitting element support structure comprises a folded sheet-material structure, folded to provide a three-dimensional frame, which holds the light emitting element away from the circuit board and the sensing plane of the sensor; at least a part of the light emitting element support structure extends over the sensor; and the light emitting element is located between the sensor and the lens.

Typically, the light emitting element(s) for a readhead is mounted directly on the same circuit board as the sensor, next to the sensor. The inventors have taken a novel approach whereby the light emitting element(s) is(are) mounted to/supported on (physically/structurally) the same circuit board as the sensor via a light emitting element support structure which holds the light emitting element(s) substantially away from the circuit board and the sensing plane of the sensor, for instance such that at least a part of the light emitting element support structure can extend over the sensor. Such a support configuration opens up a new range of optical configurations for readheads. In particular, it enables the light emitting element(s) to be placed substantially in-line with (for instance between) the sensor and other optical components, such as for example a lens and/or diffraction element. This can help to reduce the total number of optical components needed, thereby helping to reduce the size and/or cost of the readhead.

The light emitting element support structure holds the light emitting element(s) away from the circuit board and the sensing plane of the sensor. In other words, this can be such that in a dimension extending perpendicular to the plane of the circuit board (or the sensing plane of the sensor) the light emitting element is spaced apart from the circuit board and the sensor chip. In other words, the light emitting element(s) is located out-of-plane (e.g. in an "elevated position") with respect to the sensor on the circuit board. In other words, the light emitting element support structure suspends the light emitting element away from the circuit board and sensor. For instance, the distance between the light emitting element and the sensor, measured in the direction perpendicular to the plane of the sensor, can be at least <NUM>, for example at least <NUM>, for instance at least <NUM>. More particularly, the distance between the centre of the light emitting element's emission surface (or emission point) to the sensing plane of the sensor, in the direction perpendicular to the sensing plane of the sensor, can be at least <NUM>, for example at least <NUM>, preferably at least <NUM>, for example approximately <NUM>. Said distance can be between <NUM> and <NUM>, for example approximately <NUM>.

The light emitting element support structure could be described as being a "raised" light emitting element support structure, in that it is configured to extend, and hold the light emitting element(s), away from the circuit board and sensor. As will be understood, terms such as "raised" and "elevated" are used to aid the description of the relationship between various components, in particular their relative location, but they are not intended to restrict the orientation of the parts described. For example, the term "raised" can be used to describe that the light emitting element support structure rises from the circuit board, regardless of its direction. For instance, in this case, the term "raised" can be used even when the readhead is used in an orientation in which, relative to gravity/earth, the light emitting element will be held below the circuit board and sensor.

The sensor could be a sensor chip/component. Accordingly, as well as the sensor's photodiode(s), the sensor could comprise additional elements/parts, including a body/shell/casing/housing, for instance for the photodiodes and/or other electrical elements of the sensor.

The at least one light emitting element and the sensing plane of the sensor can be separated in the dimension which extends perpendicular to the sensing plane of the sensor. In particular, the light emitting element can be held directly over the sensor. This can be such that a line extending perpendicular to the sensing plane of the sensor passes through both the light emitting element and the sensor.

The light emitting element support structure can comprise a frame. The light emitting element support structure, e.g. the frame can be mounted to the circuit board separately from the sensor. In other words, optionally, the light emitting element support structure, e.g. the frame, is not mounted via the sensor, but rather is mounted directly to the circuit board. Accordingly, the apparatus can be configured such that the light emitting element support structure (e.g. the frame) does not directly touch/engage the sensor, e.g.. such that there is a gap between the light emitting element support structure (e.g. the frame) and the sensor.

The light emitting element support structure (e.g. the frame) can sit astride the sensor. For example, light emitting element support structure (e.g. the frame) could be mounted to the circuit board on at least two (opposing) sides of the sensor, optionally three, for example four/all sides of the sensor.

The light emitting element can be mounted on a top surface of the light emitting element support structure (e.g. the frame). The top surface can be secured to the circuit board via one or more (side) supports extending between the circuit board and the top surface. Accordingly, the frame can comprise a table-like structure.

The top surface and one or more (side) supports of the light emitting element support structure (e.g. the frame) could comprise a single piece of material.

The light emitting element support structure (e.g. the frame) can comprise an opaque material. Accordingly, the light emitting element support structure (e.g. the frame) can be configured/arranged (e.g. shaped, sized and/or located) such that light returning from a scale can pass the light emitting element support structure (e.g. the frame) to reach the sensor. For instance, the light emitting element support structure (e.g. the frame) could comprise one or more holes/windows/openings/apertures through which the light returning from a scale can pass to reach the sensor.

Optionally, the light emitting element support structure can comprise a transparent material. Accordingly, light returning from a scale could pass through the material of the light emitting element support structure to reach the sensor.

As per the invention of claim <NUM>, the light emitting element support structure (e.g. the frame) comprises a sheet material structure, e.g. a sheet-metal structure. The use of a sheet material structure can provide significant cost benefits, can reduce the mass of the readhead, and can provide a more deformable structure than other structures (e.g. machined, moulded structures) which can be advantageous during assembly, e.g. when trying to set the height of the light source mounted thereon. As per the invention of claim <NUM>, the sheet material is a folded sheet-material structure, i.e. has been folded to provide the structure/a three-dimensional frame, which holds the light emitting element away from the circuit board, for instance, to provide the top surface and (side) supports. The sheet-material/metal could comprise one or more defined fold lines (e.g. lines of reduced thickness). Such lines could have been etched into the sheet material. Suitable metallic materials include brass, aluminium, tin, cadmium, gold, silver, etc. The light emitting element support structure (e.g. the frame) can be coated with another material. For example, the light emitting element support structure (e.g. the frame) could be coated with brass, aluminium, tin, cadmium, gold, silver, nickel-gold, etc. Preferably, the thickness of the sheet material is not more than <NUM>, for example not more than <NUM>, for instance not more than <NUM>.

The light emitting element support structure can be mounted to the circuit board via the sensor chip. The light emitting element support structure can comprise a transparent material and can cover (in other words "extend over") the one or more photodiodes. For instance, the light emitting element support structure could comprise a piece/block of transparent material (e.g. glass, plastic, sapphire, quartz).

A plurality of light emitting elements can be provided. In such a case, the plurality of light emitting elements are preferably provided together so as to act as a single source of light. Optionally, the readhead comprises only one light emitting element.

Optionally, the light emitting element comprises an "un-capped", "un-packaged" or "un-lensed" semiconductor diode, for example a bare-die semiconductor diode. The light emitting element could comprise, for example, a light emitting diode (LED) or a laser (e.g. a vertical-cavity surface-emitting laser (VCSEL)).

The light emitting element support structure could comprise an electrically conductive material. In this case, the light emitting element support structure could comprise the anode and/or cathode for the light emitting element.

Optionally, the apparatus comprises a bond wire support structure extending from the circuit board. A bond wire can extend between it and the light emitting element (so as to provide the anode and/or cathode for the light emitting element).

The bond wire support structure can comprise a frame. The bond wire support structure, e.g. the frame can be mounted to the circuit board separately from the sensor. In other words, optionally, the bond wire support structure, e.g. the frame, is not mounted via the sensor, but rather is mounted directly to the circuit board. Accordingly, the apparatus can be configured such that the bond wire support structure (e.g. the frame) does not directly touch/engage the sensor, e.g.. such that there is a gap between the bond wire support structure (e.g. the frame) and the sensor.

The bond wire support structure (e.g. the frame) can sit astride the sensor. For example, bond wire support structure (e.g. the frame) could be mounted to the circuit board on at least two (opposing) sides of the sensor, optionally three, for example four/all sides of the sensor.

The bond wire can be connected to a top surface of the bond wire support structure (e.g. the frame). The top surface can be secured to the circuit board via one or more (side) supports extending between the circuit board and the top surface. Accordingly, the bond wire support structure (e.g. frame) can comprise a table-like structure.

The top surface and one or more (side) supports of the bond wire support structure (e.g. the frame) could comprise a single piece of material.

The bond wire support structure (e.g. the frame) can comprise an opaque material. Accordingly, the bond wire support structure (e.g. the frame) can be configured/arranged (e.g. shaped, sized and/or located) such that light returning from a scale can pass the bond wire support structure (e.g. the frame) to reach the sensor. For instance, the bond wire support structure (e.g. the frame) could comprise one or more holes/windows/openings/apertures through which the light returning from a scale can pass to reach the sensor.

Optionally, the bond wire support structure can comprise a transparent material. Accordingly, light returning from a scale could pass through the material of the bond wire support structure to reach the sensor.

The bond wire support structure (e.g. the frame) can comprise a sheet material structure, e.g. a sheet-metal structure. Optionally, the sheet material is a folded sheet-material structure, i.e. has been folded to provide the structure/a three-dimensional frame, which holds the bond wire away from the circuit board, for instance, to provide the top surface and (side) supports. The sheet-material/metal could comprise one or more defined fold lines (e.g. lines of reduced thickness), for example which could have been formed by etching. Suitable metallic materials include brass, aluminium, tin, cadmium, gold, silver, etc. The light emitting element support structure (e.g. the frame) can be coated with another material. For example, the light emitting element support structure (e.g. the frame) could be coated with brass, aluminium, tin, cadmium, gold, silver, nickel-gold, etc. Preferably, the thickness of the sheet material is not more than <NUM>, for example not more than <NUM>, for instance not more than <NUM>.

References herein to "light" refers to electromagnetic radiation (EMR) anywhere from the ultraviolet to the infrared range. For instance, the light might be ultraviolet light, visible light, infrared light, or a combination thereof.

The readhead can comprise an optical device. The optical device could comprise a 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. The light emitting element could be held substantially at the optical device's focal plane, whereas the sensor is held substantially away from the lens' focal plane.

The encoder apparatus could be an incremental encoder apparatus. Accordingly, the scale could comprise an incremental scale. The incremental encoder apparatus could comprise one or more reference marks for defining one or more reference positions. Optionally, the encoder apparatus is an absolute encoder apparatus. As will be understood, in contrast to an incremental encoder, an absolute encoder apparatus can determine the absolute position of the readhead relative to the scale without requiring relative movement of the readhead and scale. An absolute encoder comprises an absolute scale which comprises features defining a series of unique positions along its length. The series of unique absolute positions can be defined by features in a plurality of tracks, for example a plurality of adjacent tracks. Optionally, the series of unique absolute position can be defined by features contained in a single track only. For example, the absolute position information can be determined from the combination of features taken along the measuring length of the scale. Accordingly, the encoder apparatus could be configured to extract absolute position information from the image obtained by the sensor. Such extraction could be performed by the readhead or by a device external to the readhead.

Optionally, the readhead is configured to read the scale by obtaining at least one discrete snapshot of the scale (i.e. snapshot image). This can be instead of, for instance, continuously measuring and counting phase. Accordingly, an image of the scale can be obtained by the readhead taking a discrete snapshot of the scale. The snapshot could be taken at one instant in time, or be built up by taking a quick succession of smaller readings of consecutive sections of the scale. Snapshot reading of a scale can provide a number of advantages. For instance, the maximum operating velocity of the scale reader relative to the scale can be greater as it is not limited by the inherent frequency limits of the continuous phase measuring and counting system. Further, in optical systems taking snapshots, the light emitting element only has to be 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 life time of the source. This increased light intensity can mean that more photons can be captured by the sensor thus reducing the noise floor of the system giving less position noise.

The scale can comprise a series of features which the sensor can detect for determining relative motion/position of the scale and readhead. Such features can be periodically or aperiodically arranged. As will be understood, there are many suitable ways in which the features can be defined on a scale. For instance, features can be defined by markings having particular electromagnetic radiation (EMR) properties, for example particular optical properties, for instance by the particular optical transmissivity or reflectivity of parts of the scale. Accordingly, a feature could for example be defined by parts of the scale having a minimum reflectivity or transmissivity value. Optionally, a feature could for example be defined by parts of the scale having a maximum reflectivity or transmissivity value. Optionally, a feature could for example be defined by the way (e.g. direction) in which it reflects light (e.g. toward and away from the readhead). The features can take the form of lines, dots or other configurations which can be detected by the sensor. Preferred configurations for one-dimensional scales can comprise lines extending across the entire width of a track in a dimension perpendicular to the measuring dimension.

As mentioned above, the readhead can comprise at least one optical device. The at least one light emitting element, at least one sensor and at least one optical device can together with a reflective scale, form an optical system in which the optical device forms an image of an illuminated region of the reflective scale onto the sensor. Preferably, the system's optical path, from the light emitting element to the sensor, passes through the optical device on its way toward and after reflection from the scale. Preferably, the optical path between the light emitting element and the optical device is direct/unreflected. Preferably, the optical path between the optical device and the sensor is direct/unreflected. In other words, the apparatus (e.g. the readhead) can comprise 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.

Preferably, the optical path of light between the light emitting element and the scale is direct/unreflected and the optical path of light between the scale and the toward the sensor is also direct/unreflected.

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

Such an arrangement can enable a particularly compact readhead for an optical absolute position measurement device. For example, configuring the readhead such that the light emitted from the light emitting element passes through the same optical device on the outward and return paths, can reduce the number of optical components needed. Also, ensuring a direct/unreflected optical path between the light emitting element and the optical device and a direct/unreflected optical path between the optical device and the sensor, means that reflective optical components (such as mirrors and/or beam-splitters) are not needed, (for instance, the readhead can be without a reflective optical component in the its optical path). Accordingly, the number of optical components in the readhead can be further reduced and the compactness of the readhead can be improved and the complexity reduced.

The apparatus (e.g. the readhead, in particular for example the light emitting support structure) can be configured such that, in the dimension perpendicular to the plane of the sensor, the light emitting element is situated/is located between the sensor and the optical device (e.g. such as a lens). For instance, the light emitting element could be located in the space (or the "volume") between the sensor and the optical device (e.g. delineated by the outer edges/sides of the sensor and optical device).

The light emitting element can be located substantially at the optical device's focal plane such that light emitted thereby is collimated by the optical device. For example, preferably, the light emitting element is located not more than <NUM> (microns) from the optical device's focal plane, more preferably not more than <NUM> (microns) from the optical device's focal plane, especially preferably not more than <NUM> (microns) from the optical device's focal plane.

Optionally, light reflected by the scale and imaged onto the sensor by the optical device converges toward a point at a particular distance between the optical device and the sensor. Further, the light emitting element could be located approximately at said particular distance between the optical device and the sensor.

Optionally, the ratio of: i) the distance between the centre of the light emitting element's emission surface (or emission point) to the sensing plane of the sensor, in the direction perpendicular to the plane of the sensor, and ii) the distance between the centre of the light emitting element's emission surface (or emission point) to the optical device, in the direction perpendicular to the plane of the sensor, is not less than <NUM>:<NUM>, for example not less than <NUM>:<NUM>, optionally not less than <NUM>:<NUM>, preferably not less than <NUM>:<NUM>, and for instance not less than <NUM>:<NUM>.

The light emitting element could be positioned such that it is offset from the optical device's optical axis. For instance, the light emitting element could be offset (e.g. measured from the centre of the light source's emission zone) by not more than <NUM>, for example not more than <NUM>, for instance not more than <NUM>, from the optical device's optical axis. Optionally, the ratio of the offset to the focal length of the lens is not more than <NUM>:<NUM>, for example not more than <NUM>:<NUM>.

Optionally, the direction of the optical path as it impinges on and/or reflects from the scale is not perpendicular to the scale. For instance, the angle between a line extending perpendicular to the scale (at the illuminated region) and the direction of the optical path as it impinges on (and/or reflects from) the scale is not less than <NUM>° (degrees), for example not less than <NUM>°, for instance not less than <NUM>°, and optionally is not more than <NUM>°, for instance not more than <NUM>°. In other words, optionally there is an angle between the directions of incidence and reflection (i.e. greater than <NUM>°) of light hitting and reflected from the scale, for example an angle of at least <NUM>°, and for instance at least <NUM>°, optionally at least <NUM>°, and for example not more than <NUM>°, for instance not more than <NUM>°.

Accordingly, optionally, the shape of the optical path as it impinges on and/or reflects from the scale is V-shaped. Optionally, the system's optical path, from the light emitting element to the sensor, is substantially diamond/rhombus-shaped.

The optical paths through the optical device on the way toward and after reflection from the scale could be laterally offset. Accordingly, for example, for any given ray through the optical system, the point at which it exits the optical device toward the scale and the point at which it re-enters the optical device after it has been reflected from the scale is different/laterally offset. As will be understood, the optical paths (e.g. the optical beam) on the way toward and after reflection from the scale could overlap (e.g. partially, and optionally substantially, but not completely).

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

Optionally, the sensor, and the image of the scale formed by the optical device, lies behind (e.g. directly behind) the light emitting element. Optionally, the light emitting element is positioned such that rays from the light emitting element reflected by the scale converge to a point so as to by-pass the light emitting element on the return path, and subsequently diverge and form said image of the scale on the sensor (behind the light source).

As will be understood, an image of the scale is formed when light rays from any given point on the scale substantially converge to a common, unique point at an image plane (where the sensor is located). (The point is "unique" in that for a different given point on the scale, rays from that point will substantially converge to different common point). The image could be a spatially filtered image.

This document describes an encoder apparatus comprising a readhead for reading a reflective scale located adjacent the readhead, the readhead comprising a sensor comprising one or more photodiodes for detecting light reflected from a scale located adjacent the readhead is mounted, and at least one light emitting element, in which the light emitting element is held away the sensing plane of the sensor, such that the distance between the light emitting element and the sensor, measured in the direction perpendicular to the sensing plane of the sensor, is at least <NUM>. This document describes an encoder apparatus comprising a readhead for reading a reflective scale located adjacent the readhead, the readhead comprising a circuit board on which a sensor comprising one or more photodiodes for detecting light reflected from a scale located adjacent the readhead is mounted, and at least one light emitting element, in which the light emitting element is mounted to the circuit board via a light emitting element support structure which holds 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 the direction perpendicular to the sensing plane of the sensor, is at least <NUM>. More particularly, the distance between the centre of the light emitting element's emission surface (or emission point) to the sensing plane of the sensor, in the direction perpendicular to the sensing plane of the sensor, can be at least <NUM>. Said distance can be at least <NUM>, optionally at least <NUM>, for example at least <NUM>. Said distance can be between <NUM> and <NUM>, for example approximately <NUM>.

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

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

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

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

The width of the lines depends on the required positional resolution and is typically in the range of <NUM> to <NUM>, and more typically in the range of <NUM> to <NUM>, for instance in the range of <NUM> to <NUM>. In the described embodiment, the width of the lines is in the order of <NUM>. The reflective <NUM> and non-reflective <NUM> lines are generally arranged in an alternate manner at a predetermined period. However, select non-reflective lines <NUM> are missing from the scale <NUM> so as to encode absolute position data in the scale <NUM>. For instance, the presence of a non-reflective line can be used to represent a "<NUM>" bit and the absence of a non-reflective line can represent a "<NUM>" bit.

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

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

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

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

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

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

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

The optical system of the readhead <NUM> of <FIG> will be described in more detail with reference to <FIG> and <FIG>. <FIG> and <FIG> schematically illustrate the path light takes through the optical system which forms the image of the scale <NUM> onto the sensor <NUM>, from the light source <NUM> to the sensor <NUM>.

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

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

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

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

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

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

Referring now to <FIG>, an example embodiment of how the readhead can be constructed to achieve the above described optical layout will now be described.

As shown in <FIG>, the readhead <NUM> comprises a body <NUM> to which the lens <NUM>, window <NUM> and a printed circuit board (PCB) <NUM> are mounted (e.g. via gluing, mechanical and/or frictional means). The sensor <NUM>, LED <NUM> and other electronic components (such as the above-mentioned CPU <NUM>, memory <NUM> and interface <NUM> - not shown in <FIG>) are mechanically and electrically mounted to the PCB <NUM>.

As illustrated, although the LED <NUM> is mounted to the PCB <NUM>, the LED <NUM> is mounted to the circuit board "off-board", in that it is mounted to the PCB <NUM>, but it is mounted via a raised support structure <NUM> which holds the LED <NUM> away from the PCB <NUM>. In particular, the support structure <NUM> extends beyond the sensor <NUM> so as to hold the LED <NUM> further away from the PCB <NUM> than the sensor <NUM>. Accordingly, as shown, the sensor <NUM> is mounted relatively close to the PCB <NUM> whereas the LED <NUM> is mounted relatively far from the PCB <NUM>. As illustrated in <FIG>, the LED <NUM> is much closer to the lens <NUM> than the PCB <NUM>, whereas the sensor <NUM> is much closer to the PCB <NUM> than the lens <NUM>. Accordingly, as shown, the light emitting element and the sensor are separated in the dimension which extends perpendicular to the plane of the sensor/circuit board. In particular, in the dimension which extends perpendicular to the plane of the sensor/circuit board, there is (free) space between the light emitting element and the sensor. In this example, the ratio of: i) the distance between the LED <NUM> emission surface (or emission point) to the sensor's <NUM> sensing plane, in the direction parallel to the imaging member's optical axis OA; and ii) the distance between the LED's <NUM> emission surface (or emission point) to lens <NUM>, in the direction parallel to the imaging member's optical axis OA, is approximately <NUM>:<NUM>.

In the embodiment described, the above-mentioned support structure <NUM> also forms/provides the electrical connection between the LED <NUM> and the PCB <NUM>. Accordingly, in the embodiment described the support structure <NUM> for holding the LED <NUM> away from the PCB <NUM> is the cathode <NUM> between the LED <NUM> and the PCB <NUM>. Accordingly, the cathode <NUM> comprises a rigid, electrically-conductive, support structure for the LED <NUM>, which rises from the PCB <NUM>. As shown in <FIG>, the support structure/cathode <NUM> comprises an opening/window <NUM> through which light reflected by the scale <NUM> can pass in order to reach the sensor <NUM>.

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

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

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

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

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

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

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

<FIG> illustrates a readhead <NUM>' according to another example not belonging to the invention. The readhead <NUM>' of <FIG> shares many parts which are the same as that of the embodiment of <FIG> and like parts share like reference numerals. In the example of <FIG>, the LED <NUM> is mounted to the circuit board via the sensor <NUM>, by way of a transparent support structure <NUM> (e.g. a glass block <NUM>). In particular, the glass block <NUM> is secured to the sensor <NUM> via adhesive epoxy. The LED <NUM> then sits on a conductive pad <NUM> which has been deposited on the side of the glass block <NUM> which faces the lens <NUM> and which is distal the sensor <NUM>. The LED <NUM> is electrically connected to the circuit board <NUM> via an anode <NUM>' and cathode <NUM>' which in this example each comprise raised bond wire support structures extending from the PCB <NUM>, and bond wires which extend between them and the LED <NUM>/conductive pad <NUM>. As will be understood, in variations of this example, the LED <NUM> could be connected to the circuit board in other ways, e.g. via an anode and cathode which are deposited on and run along the surface/side of the glass block <NUM>, or even via an anode and cathode which run through the glass block <NUM>.

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

In the embodiments shown, the LED <NUM> is mounted "off-board" by the support structure <NUM>. This is beneficial (e.g. so as to place the LED <NUM> at the lens' <NUM> focal plane, so as to achieve collimation, whilst enabling an image of the scale to be captured by the sensor <NUM>). In other examples not belonging to the invention, the LED <NUM> could be mounted on the PCB <NUM> such that it sits substantially in-plane with the sensor <NUM> (in other words, at substantially the same height as the sensor <NUM>).

Claim 1:
An encoder apparatus (<NUM>) comprising a readhead (<NUM>) for reading a reflective scale (<NUM>) located adjacent the readhead, the readhead comprising:
a circuit board (<NUM>);
a sensor (<NUM>) mounted on the circuit board, the sensor comprising one or more photodiodes (<NUM>) for detecting light reflected from a scale located adjacent the readhead;
at least one light emitting element (<NUM>);
a light emitting element support structure (<NUM>); and
at least one lens (<NUM>);
characterised in that:
the light emitting element is mounted to the circuit board via the light emitting element support structure, in which the light emitting element support structure comprises a folded sheet-material structure, folded to provide a three-dimensional frame (<NUM>), which holds the light emitting element away from the circuit board and the sensing plane of the sensor;
at least a part of the light emitting element support structure extends over the sensor; and
the light emitting element is located between the sensor and the lens.