Encoder Device

Abstract

a scale readable by the readhead and including a series of features that diffract light into a plurality of diffraction orders so as to determine the relative position of the scale and the readhead; and a readhead including a light source for illuminating the scale and at least a first sensor configured to detect signals produced thereon by one or more diffraction orders produced by the scale, the first sensor being configured to detect signals produced thereon by one or more diffraction orders produced by the scale, the readhead further comprising a diffraction order encoder that encodes at least one diffraction order in an optical state that is different from the optical state of at least one other diffraction order.

Description

[Technical Field]

[0001] The present invention relates to a position measuring encoder device. [Background technology]

[0002] A position measurement encoder device (hereinafter referred to as an "encoder device" or "position encoder") can be used to determine the relative movement of two relatively movable parts of a device. A position encoder typically comprises a scale and a readhead (one provided on one part of the device and the other provided on another part of the device). The scale may include a series of features that can be read by the readhead to measure its position along the scale (e.g., the scale features may be provided on a substrate that is fixed to one part of the device, or may be integrally formed as part of the device).

[0003] So-called "incremental" position encoders may, for example, function by "counting" positions along the length of the scale, for example from its starting position and / or from a predetermined reference marker on the scale. As will be appreciated, the way in which "counting" the readhead's position is done may vary from encoder device to encoder device. For example, one way is to generate a resultant field, such as an interference fringe pattern (a "fringe field") or a modulated spot (a modulated spot) at the detector of the readhead, that changes with relative movement. For example, light from a light source in the readhead may impinge on the scale which diffracts the light into several diffraction orders. The diffraction orders interfere / interact / recombine at the detector to produce the resultant field. Optionally, optical elements (e.g. diffraction gratings and / or lenses) may be provided in the readhead to redirect / deflect the diffraction orders so that they interfere / interact at the detector to produce the resultant field. As the scale and readhead move relative to each other, the resultant field changes. The readhead can record and / or report movement and position by monitoring the resulting field changes (e.g. movement of the interference fringe pattern / fringe field). Such a position encoder is described in US Patent No. 5,861,953. As will be appreciated, reference marks may be provided next to and / or embedded within diffractive features of the scale, for example to provide a predetermined reference position. Such a position encoder is described in US Patent No. 7,659,992.

[0004] So-called "absolute" position encoders are also known, which can determine the absolute position of a readhead relative to a scale without the need to count from a predetermined position, such as a reference mark or the end position of the scale. Absolute position encoders typically include a scale with unique position data formed thereon along the measuring length of the scale. The data can be in the form of a pseudorandom sequence or discrete code words, for example. By reading this data as the scale reader passes over the scale, the scale reader can determine its absolute position. Examples of absolute position encoders are described in U.S. Pat. No. 7,499,827, U.S. Pat. No. 10,132,657, and U.S. Patent Application Publication No. 2012 / 0072169. Some absolute encoders are known to use incremental scales in conjunction with the absolute scale. Optionally, the absolute scale may retain sufficient periodicity to allow the scale to be used as a periodic incremental scale (e.g., as described in U.S. Pat. No. 7,499,827). In either case, such incremental scales can be used, for example, to fine-tune the determined absolute position. Optionally, after starting up to determine absolute position, such a system can be used to subsequently measure the relative position of the readhead and scale by using the incremental scale to "count" changes in position. Such an incremental scale can be read in the same way as above, for example by analysing the resultant field generated (at a sensor in the readhead) by diffraction orders produced by the scale. Summary of the Invention

[0005] The present invention relates to improvements to those types of encoder devices that rely on the diffraction of light to create a resultant field on a sensor in a readhead.

[0006] According to a first aspect of the present invention, there is provided a position measurement encoder apparatus comprising: a scale readable by a readhead and extending along a measurement dimension, the readhead including a light source for illuminating the scale and at least a first sensor configured to detect signals produced by one or more diffraction orders produced by a series of positional features on the scale that can be used to determine the relative position of the scale and the readhead along the measurement dimension, the readhead further comprising a diffraction order encoder encoding at least one diffraction order with a different optical state (or in other words different optical properties) than the optical state of at least one other diffraction order.

[0007] Providing a readhead with a diffraction order encoder that encodes at least one diffraction order having a different optical state (or, in other words, different optical properties) from the optical state of at least one other diffraction order can help to provide an improved signal formed at the readhead's sensor. For example, diffraction orders having different optical states / properties may mean reduced / no optical interference with each other (even if they both reach the sensor), which can provide an improved signal, such as a signal with improved visibility and / or rideheight tolerance and / or improved harmonic quality. It may also allow different diffraction orders to be filtered and / or optically separated based on their different optical states. Furthermore, such improvements can be achieved without having to design the scale to not generate certain diffraction orders, or to generate diffraction orders having certain optical states, thereby simplifying scale design and / or manufacture, which can lead to cost benefits and / or ease of producing scales of longer lengths.

[0008] In embodiments in which there is a zeroth and ±1st diffraction orders (for example when the scale comprises an amplitude scale), the diffraction order encoder may be configured to encode the zeroth diffraction order in a different optical state to the ±1st diffraction orders. Typically, these are the orders that have the greatest intensity and have the greatest effect on the signal formed at the readhead's sensor, and it may therefore be useful to encode them in different optical states to reduce / avoid interaction at the sensor and / or so that they can be selectively attenuated / blocked from reaching and being sensed by the sensor (as described in more detail below). Although it is possible to encode the +1st diffraction order to have a different optical state from the -1st diffraction order, it is typically useful to configure the readhead so that they have the same optical state (e.g. the same polarization and / or wavelength).

[0009] The position measuring encoder device may be a linear encoder device or a rotary encoder device, and therefore the scale may be a linear scale or a rotary scale (in which case the scale may be a ring scale or a disk scale).

[0010] The readhead may include at least first and second sensors. For example, the second sensor is also configured to detect signals generated therein by diffraction orders from the scale. The first and second sensors may be configured to detect the same type of signal (e.g., both may be incremental signals or both may detect absolute signals), or different types of signals (e.g., one of them may detect incremental signals and one of them may detect reference mark signals or absolute signals). Optionally, both the first and second sensors detect signals formed from one or more diffraction orders.

[0011] As will be appreciated, the output of the first sensor (and second sensor, if present) may provide one or more signals that can be used to indicate relative position information (with respect to the readhead and scale). As will be explained in more detail later in this specification, such position information may be incremental or absolute position information. Such position information may be index (also known as "reference" or "datum") position information. The position information may be linear or angular position information (e.g., in the case of a rotary encoder device). The readhead may output raw, unprocessed signals from the sensors. Optionally, the readhead may process signals from the sensors and output one or more signals derived from the signals from the sensors. As with a position encoder, the readhead may output one or more signals, for example, quadrature signals (e.g., sine and cosine signals) that vary with changes in the relative position of the scale and readhead (i.e., vary with relative motion). The readhead may output analog or digital signals that represent the relative position of the scale and readhead. For example, the quadrature signals may be analog or digital quadrature signals. Optionally, the readhead may maintain and output an incremental "count" of the position of the readhead and the scale (e.g., it may count from an index position). Optionally, the readhead may output a digital codeword representing the relative position of the scale and readhead. For example, the readhead may output an absolute digital codeword representing the absolute relative position of the scale and readhead. It will also be appreciated that a controller may use the output of the readhead to determine how to control an apparatus / device to which the encoder apparatus is attached.

[0012] It may be preferable to configure the readhead such that at least one diffraction order is at least partially filtered, based on its optical state, to at least partially attenuate its influence on the generation of the signal sensed by the first sensor (and / or the second sensor, if present). Doing so can result in an even greater improvement in the quality of the signal sensed by the sensor, particularly if, for example, at least one diffraction order is substantially completely filtered so that it does not contribute at all to the signal sensed by the sensor. For example, a position measurement encoder apparatus may be configured to i) at least partially (e.g., substantially) filter the 0th diffraction order so as to at least partially attenuate (e.g., substantially completely attenuate / eliminate) its influence on the generation of the signal sensed by the first sensor, or ii) at least partially (e.g., substantially) filter the ±1st diffraction orders so as to at least partially attenuate (e.g., substantially attenuate / eliminate) their influence on the generation of the signal sensed by the first sensor. Option i) may be particularly preferable when the sensor (e.g., the first sensor) is configured to sense incremental position signals such as interference fringe patterns or modulated spots. Option ii) may be particularly preferred when a sensor (e.g., a second sensor) is configured to sense a reference mark or absolute code on the scale. If both a first and second sensor are present, the light that generates the signal at the first sensor may be different (e.g., filtered in a different way) from the light that generates the signal at the second sensor.

[0013] Although some benefit may be obtained from only partially attenuating the influence of a particular diffraction order (e.g. the 0th or ±1st diffraction orders) on generating the signal formed at the first sensor, for example by attenuating the influence of the particular diffraction order on signal generation by at least 50%, it is preferred that the influence of the particular diffraction order on generating the signal sensed by the first sensor is substantially completely attenuated, for example by at least 90%, for example by at least 95%, for example by at least 98%. In particularly preferred embodiments, the readhead is configured so that a particular diffraction order (e.g. the 0th or ±1st diffraction orders) has substantially no effect whatsoever on generating the signal sensed by one of the first and second sensors, while its effect on generating the signal sensed by the other sensor remains unaffected.

[0014] Such filtering can be achieved by configuring the sensor to be blind or insensitive to certain optical conditions. In preferred embodiments, such filtering can be provided by a filter located in front of the sensor to at least partially block or filter diffraction orders from reaching the sensor. For example, a first sensor filter can be provided that is configured to filter light based on the optical condition of the light before it hits the first sensor. Optionally, a second sensor filter is provided that is configured to filter light based on the optical condition of the light, if present, before it hits the second sensor. Optionally, both a first sensor filter and a second sensor filter are provided. In such a case, the first sensor filter and the second sensor filter can be configured to filter light differently from each other based on the optical condition of the diffraction orders.

[0015] The optical state can include a state of polarization. Thus, the diffraction order encoder can be configured to encode at least one diffraction order with a polarization state that differs from the polarization state of at least one other diffraction order. For example, the diffraction order encoder can be configured to encode one diffraction order (e.g., the zeroth diffraction order) to be polarized (e.g., vertically polarized) (e.g., to have a degree of polarization of at least 0.5 or 50%, preferably at least 0.75 or 75%, more preferably at least 0.85 or 85%, e.g., at least 0.9 or 90%). The other diffraction orders can remain unpolarized, or one or more of the other diffraction orders (e.g., the ±1st diffraction orders) can be polarized differently (e.g., orthogonally) to the zeroth diffraction order. The diffraction order encoder can include at least one polarizer element configured to encode at least one diffraction order with a polarization state that differs from the polarization state of at least one other diffraction order. In a particularly preferred embodiment, the diffraction order encoder comprises a first polarizer element configured to encode the zeroth diffraction order in a first polarization state and one or more additional (e.g., second and third) polarizer elements configured to encode the ±1st diffraction orders in a second polarization state different from the first polarization state. Optionally, the polarizer element includes a polarizer configured to polarize (e.g., increase the degree of polarization) at least one diffraction order. Optionally, the polarizer element includes at least one polarization manipulator, such as a waveplate or retarder, configured to change the polarization state of at least one diffraction order, e.g., to rotate the polarization direction of the at least one diffraction order. In such a case, the diffraction orders would need to be polarized before the diffraction order encoder. For example, the light source can be a polarized light source, and the polarizer can be placed in the optical path before the waveplate.

[0016] The optical state can include a wavelength state. For example, the diffraction order encoder can be configured to encode at least one diffraction order at a wavelength state that is different from the wavelength state of at least one other diffraction order. For example, the diffraction order encoder can be configured to encode one diffraction order (e.g., the zeroth diffraction order) to have light falling within a first band of wavelengths. The device can be configured so that the wavelengths of the other diffraction orders remain unchanged, or alternatively, the device can be configured so that the diffraction order encoder encodes one or more of the other diffraction orders (e.g., the ±1st diffraction orders) at a second band of wavelengths that is different from the first band of wavelengths.

[0017] The light source and the diffraction order encoder may be provided on a single mounting member, in other words, on a single substrate, for example, a glass substrate.

[0018] The readhead may include at least one optical element for relaying diffraction orders from the scale to at least a first sensor. For example, the readhead may include at least one refractive and / or diffractive optical element for relaying diffraction orders from the scale to at least one sensor. Suitable optical elements include lenses and / or diffraction gratings. The diffraction order encoder may be positioned to interact with one or more diffraction orders before the optical element for relaying the diffraction orders (e.g., as in FIG. 2) or after the optical element for relaying the diffraction orders (e.g., as in FIG. 4). Optionally, the diffraction order encoder and the optical element for relaying the diffraction orders may be one and the same (e.g., relay element 200 of FIG. 2 may include diffraction order encoder 50 integrated therein).

[0019] Optionally, the device is configured so that the diffraction orders converge to corresponding / respective spots (or "different convergence points") in the optical path between the optical element and the sensor. For example, there may be a spot / convergence point for each diffraction order, e.g., a spot / convergence point for the 0th diffraction order, a spot / convergence point for the +1st diffraction order, a spot / convergence point for the -1st diffraction order, etc. Such spots may be located in a focal plane (focal plane of a lens) of an optical element for relaying the diffraction orders. A diffraction order encoder may be located substantially at these convergence points. For example, the diffraction order encoder may be located in a focal plane of an optical element for relaying the diffraction orders, e.g., the focal plane of the optical element. The spots may be formed in a conjugate plane of the light source. Thus, the spots may be images of the light source.

[0020] The position measurement encoder device may also be configured to attenuate the effect of diffraction orders greater than ±1 diffraction orders on the generation of the resulting field sensed by the first sensor (and / or the second sensor, if present). Such diffraction orders may include, for example, ±3 diffraction orders and / or ±5 diffraction orders. The position measurement encoder device may be configured to stop diffraction orders greater than ±1 diffraction orders from reaching at least the first sensor (and the second sensor, if present). Such diffraction orders may be stopped by absorption, deflection, scattering, and / or reflection. For example, an opaque material may be placed at the aforementioned spot for selection of all diffraction orders greater than ±1 diffraction orders at a conjugate plane of the light source to absorb and block such diffraction orders. Optionally, the diffraction order encoder is configured to encode diffraction orders greater than the ±1 diffraction orders in an optical state so as not to interact / interfere with the 0th and / or ±1st diffraction orders at at least the first sensor (and, if present, the second sensor), or so as to be at least partially (e.g., substantially) filtered by an appropriate filter (e.g., filtered by the first sensor filter and / or the second sensor filter) before reaching at least the first sensor (and, if present, the second sensor).

[0021] The scale may be illuminated with collimated light (and optionally the diffraction orders from the scale may themselves be collimated). Optionally, the readhead includes an optical element for collimating light from the light source. Optionally, the same optical element is used to collimate the light from the light source and to relay the diffraction orders towards at least the first sensor. Optionally, the same optical element is used to collimate the light from the light source and to focus the diffraction orders onto corresponding / respective spots.

[0022] The apparatus may be configured such that at least the first sensor lies substantially in a conjugate plane of the scale, and the position measurement encoder apparatus may therefore be described as an imaging encoder apparatus, where an image (or pseudo-image) of the scale is formed by the sensors (e.g. the first and second sensors).

[0023] Preferably, the scale comprises what is commonly referred to as an amplitude or "Ronchi" scale. As will be appreciated, in an amplitude or "Ronchi" scale, the features are configured to control the amplitude of light reflected (or transmitted in transmission scale embodiments) towards the readhead sensor, for example by selectively absorbing, scattering, and / or reflecting light. An amplitude or "Ronchi" scale should be arranged with a phase scale in which the features are configured to control the phase of light reflected (or transmitted) towards the readhead sensor (for example by delaying the phase of the light). Typically, an amplitude scale will produce a significant zeroth diffraction order, along with significant ±1st diffraction orders (the higher ±odd diffraction orders which decrease in intensity). This is in contrast to a phase scale, which does not produce a zeroth diffraction order.

[0024] Optionally, the period of the scale is 40 μm or less, preferably 20 μm or less, such as 10 μm or less, for example 8 μm or less.

[0025] Preferably, the scale has a feature space (or "mark" space) ratio of 1:1, so in other words the ratio of the width of the scale features to the spacing of the scale is 1:1.

[0026] The scale may be a transmissive scale, but preferably the scale is a reflective scale, and therefore preferably the light source and the at least one sensor of the readhead are located on the same side of the scale.

[0027] Optionally, the position measurement encoder device is a one-grating encoder system, and the scale includes only a diffraction grating in the optical path between the light source and the at least one sensor.

[0028] The series of position features of the scale may be provided in at least one track (a "scale track"). The scale may include one or more scale tracks.

[0029] The signal generated by the at least first sensor may include an incremental position signal, e.g., an interference fringe pattern or a modulated spot. Thus, the at least first sensor may include an incremental position sensor. The period of the interference fringes may be Mp / 2, where M is the magnification of the encoder's optics and p is the period of the scale (which may be achieved when the zeroth diffraction order is substantially eliminated). The scale may include an incremental scale track including a series of periodic features defining the incremental scale track. One or more reference marks may be provided within, embedded in, and / or positioned adjacent to the incremental scale track. Such reference marks may include optical reference marks. The readhead may include at least a second sensor configured to detect a reference mark signal generated by the reference mark.

[0030] Optionally, the signal generated by at least the first sensor includes an absolute position signal. Thus, the at least first sensor can include an absolute position sensor. Thus, the scale can include an absolute scale track including a series of features defining the absolute scale track. As will be appreciated, an absolute scale track differs from an incremental scale track (with or without reference marks) in that its features define a series of unique positions along the length of the scale. It can be read by the readhead such that the relative positions of the readhead and scale can be determined (e.g., upon start-up) at any position along the scale without requiring movement to a reference position (e.g., a reference mark). Examples of absolute scales include those described in U.S. Patent No. 7,499,827 and U.S. Patent No. 5,279,044.

[0031] The scale may include a separate incremental track and an absolute scale track. Optionally, the incremental and absolute scale features are combined into one track. For example, absolute scale features may be superimposed on periodic incremental scale features. As is known, and as described in the prior art referenced above in this paragraph, absolute position information may be encoded in a scale track by omitting selected position features from an otherwise periodic series of position features.

[0032] As will be appreciated, the scale can include first and second series of position features readable by the readhead. As will be appreciated, the first series of position features can generate a first set of diffraction orders, and the second series of position features can generate a second set of diffraction orders (which generate signals detected by the first and second sensors). These first and second sets of diffraction orders can be superimposed / spatially overlapping with one another. The position measurement encoder device can be configured such that only one of the first and second sets of diffraction orders has a diffraction order with a different optical state. For example, the position encoder device can be configured such that at least one diffraction order (e.g., the zeroth diffraction order) of the first set of diffraction orders has an optical state that is different from the optical state of at least one other diffraction order (e.g., the ±1st diffraction orders) of the first set of diffraction orders, while some diffraction orders (e.g., at least the zeroth diffraction order and the ±1st diffraction orders) of the second set of diffraction orders can have the same optical state. Alternatively, the position measurement encoder apparatus may be configured such that at least one diffraction order of the first set has an optical state that is different from the optical state of at least one other diffraction order, and such that at least one diffraction order of the second set has an optical state that is different from the optical state of at least one other diffraction order. In any event, it will be appreciated that the first and second sets of diffraction orders may be filtered differently based on their optical states.

[0033] The first and / or second series of position features may be periodic (in other words, the scale may include a first periodic series of position features and a second periodic series of position features). The period of the first series of position features may be different from the period of the second series of position features. For example, the scale may include a scale track including a first series of position features having a first period (e.g., a relatively fine period) and a second series of position features having a second period (e.g., a relatively coarse period, i.e., coarser than the period of the first period). The first and / or second series of position features may be provided on the same track, i.e., embedded / superimposed on each other.

[0034] According to another aspect of the present invention there is provided a position measuring encoder apparatus comprising: a scale including a series of periodic features arranged to diffract light into a plurality of diffraction orders including the zeroth diffraction order; and a readhead including a light source for illuminating the scale, wherein at least one sensor is arranged to detect a fringe pattern / fringe field that moves with relative movement of the scale and the readhead, the fringe pattern / fringe field being produced by the diffraction orders from the scale at the at least one sensor, wherein the fringe pattern period is Mp / 2, where M is the magnification of the encoder optics and p is the period of the scale.

[0035] According to another aspect of the present invention there is provided a position measuring encoder apparatus comprising: a scale including a series of periodic features arranged to diffract light into a plurality of diffraction orders including a zeroth diffraction order; and a readhead including a light source for illuminating the scale, wherein at least one sensor is arranged to detect a resultant field that varies with relative movement of the scale and the readhead, the resultant field being generated at the at least one sensor by diffraction orders from the scale, wherein the readhead is arranged such that the zeroth diffraction order does not contribute to generating the resultant field at the at least one sensor. [Brief explanation of the drawings]

[0036] Hereinafter, an embodiment of the present invention will be described by way of example with reference to the drawings.

[0037] [Figure 1a] FIG. 1 is a schematic diagram illustrating the optical operation of a typical prior art position encoder system. [Figure 1b] FIG. 1 is a schematic diagram illustrating the optical operation of a typical prior art position encoder system. [Figure 2] 1 is a schematic diagram illustrating the optical operation of a position encoder device according to the present invention; [Figure 3] 10 is a graph showing the effect of the zeroth order term on the visibility of interference fringes impinging on the sensor as the distance between the scale and readhead is changed. [Figure 4a] FIG. 10 is an optical diagram illustrating an alternative embodiment of the present invention. [Figure 4b] FIG. 10 is an optical diagram illustrating an alternative embodiment of the present invention. [Figure 5a] FIG. 10 is an optical diagram illustrating another alternative embodiment of the present invention. [Figure 5b] FIG. 10 is an optical diagram illustrating another alternative embodiment of the present invention. [Figure 6a] FIG. 10 is an optical diagram of a further alternative embodiment of the present invention. [Figure 6b] FIG. 10 is an optical diagram of a further alternative embodiment of the present invention. [Figure 7] Figure 6 shows a schematic diagram of a readhead incorporating the optical arrangement of Figures 6a and 6b, together with an associated scale; [Figure 8] 8 shows schematically the arrangement of the components of FIGS. 6a and 6b within the readhead of FIG. 7; [Figure 9] FIG. 9 shows a plan view of the glass substrate of FIG. [Figure 10] FIG. 10 is an optical diagram of yet another alternative embodiment of the present invention. [Figure 11] 1 illustrates an exemplary electrical grid sensor. [Figure 12] 10 illustrates schematically an encoder device according to another embodiment of the present invention; [Figure 13] 10 shows an incremental scale and readhead of an encoder device according to another embodiment of the present invention. [Figure 14] 10 shows an incremental scale and readhead of an encoder device according to another embodiment of the present invention. [Figure 15] 10 shows an absolute scale and a readhead of an encoder device according to another embodiment of the present invention. [Figure 16] 10 shows a schematic diagram of an absolute scale and a readhead of an encoder device according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0038] Referring to the drawings, FIG. 1a shows a schematic representation of the optical operation of a typical prior art incremental position encoder system. Here, a scale 100, having a periodic series of features with period “p”, is illuminated with light, thereby generating a set of diffraction orders 101, 102, 103. As will be understood, references to light herein include visible and invisible light ranging from ultraviolet to infrared. In the particular embodiment described, near-infrared light is used. The diffraction orders 101, 102, 103 are relayed to a sensor 300 by an optical element 200 (e.g., a lens, prism, or diffraction grating) (both the sensor and relay element are located within a readhead that is movable relative to the scale 100). In particular, the diffraction orders interfere at the sensor 300, thereby generating an interference fringe pattern / fringe field (schematically represented by wave 110) at the sensor 300 with a period equal to Mp, where M is the magnification of the optical system and p is the scale period. 1a shows the light as being transmitted through the scale, it will be appreciated that the light may be reflected from the scale and therefore may originate from a light source located on the same side of the scale as the sensor. It will be appreciated that typically the light source, relay element 200 and sensor 300 will all be provided by a readhead arrangement that is arranged to move relative to the scale 100.

[0039] It will be appreciated that Figure 1a is a simplified illustration of the optical situation encountered within the encoder apparatus. In reality, the optical situation shown in Figure 1a will be repeated many times along the length of the scale (i.e. across the area illuminated by the light source) to produce a longer interference fringe pattern / fringe field pattern at the detector (e.g. as shown schematically in Figure 1b).

[0040] 1a and 1b, only the 0th and ±1st orders are shown. As will be appreciated, higher diffraction orders can be generated and contribute to the formation of the interference fringe pattern / fringe field at sensor 300, but their intensity, and therefore their contribution to the interference fringe pattern / fringe field, is typically much weaker than the 0th and ±1st diffraction orders (higher orders being progressively weaker in intensity).

[0041] It will be appreciated that if the mark space ratio of the scale (i.e., the ratio of the width of the scale features to the spacing between marks) is exactly 1:1 (as is often the case with amplitude scales in encoder devices), no even diffraction orders (e.g., ±2nd, ±4th diffraction orders) will be produced, and only odd diffraction orders will be produced (e.g., ±3rd, ±5th diffraction orders). In practice, some slight manufacturing errors may mean that the mark space ratio of the scale is not exactly 1:1, and therefore even diffraction orders may be present (although they may be significantly less intense than adjacent odd diffraction orders). It will be appreciated that if the mark space ratio of the scale is intentionally formed so that it is not 1:1, significant even diffraction orders may be present.

[0042] For simplicity of illustration, the ray diagrams in Figures 1a and 1b are shown as transmitted ray diagrams (i.e., light is shown transmitted through each of the scale and optical element), although in practice at least one of these may be reflective.

[0043] FIG. 2 is a schematic diagram illustrating the optical operation of a position encoder device similar to that shown in FIG. 1b, but modified in accordance with the present invention. In this case, the readhead includes a diffraction order encoder 50 that encodes the zeroth diffraction order in an optical state different from the optical states of the ±1st diffraction orders. In this embodiment, the diffraction order encoder 50 includes a first polarizer section 54a configured to encode the zeroth diffraction order in a first polarization state and two second polarizer sections 54b, 54c configured to encode the ±1st diffraction orders in a second polarization state that is orthogonal to the zeroth diffraction order. Therefore, because the zeroth and ±1st diffraction orders are encoded in orthogonal polarization states, they do not interfere with each other (constructively or destructively) when they strike the sensor. This means that the zeroth order term does not contribute to a modulated portion of the interference fringe pattern formed on the sensor. This provides several improvements to the interference fringe pattern / fringe field 110′ striking the sensor 300.

[0044] In particular, Figure 3 shows how encoding the zeroth and ±1st diffraction orders with orthogonal polarization states affects the visibility of the fringe pattern impinging on the sensor 300 as the distance between the scale 100 and the readhead optics 200 (commonly known as the "rideheight" of the readhead relative to the scale) varies. As shown, when the zeroth and ±1st diffraction orders have the same polarization state (including, for example, they are both unpolarized) so that the zeroth and ±1st diffraction orders interfere with each other and the zeroth diffraction order contributes fully to the interference fringe pattern / fringe field, the visibility of the interference fringe pattern / fringe field is modulated with a period of p / λ, where p is the scale period and λ is the wavelength of the light. If the zeroth and ±1st diffraction orders are encoded with orthogonal polarization states so that they do not interfere with each other at the sensor plane, there is no longer any modulation. This is beneficial because an encoder device having a scale that produces the zeroth diffraction order can have a rideheight tolerance that is independent of the scale period.

[0045] Another advantage of encoding the zeroth and ±1st diffraction orders with orthogonal polarization states is that the interference fringe pattern / fringe field 110′ produced by sensor 300 (as shown generally in FIG. 2) has a period equal to Mp / 2, half that of a system in which the zeroth and ±1st diffraction orders have the same polarization states, such as the system of FIG. 1. Thus, encoding the zeroth and ±1st diffraction orders with orthogonal polarization states effectively doubles the system resolution.

[0046] Furthermore, if diffraction orders higher than 1 are also encoded in a different optical state than the ±1 diffraction orders (as in the embodiments described below), higher harmonics are removed from the fringe pattern / fringe field, providing a purer fringe pattern.

[0047] FIG. 4a shows an optical diagram illustrating one embodiment of the present invention using a lens 40 to form an image of a light source 10. In FIG. 4a, before light from the light source 10 strikes the scale 100, the light is collimated by a first lens 20, and the 0th (101), −1st (102), and +1st (103) diffraction orders are shown incident on a second lens 40. The second lens 40 focuses the corresponding diffraction orders to form spots 150 at the back focal plane (fp2) of the second lens 40. Each spot 150 is formed from a respective diffraction order. The spots 150 are images of the light source 10 at the point where the back focal plane fp2 is a conjugate plane of the plane in which the light source 10 is located.

[0048] In the same manner as shown in FIG. 2 , the diffraction order encoder 50 can encode the zeroth diffraction order (101) coming from the scale in a different optical state than the ±1st diffraction orders (102, 103) coming from the scale. The ±1st diffraction orders 102, 103 propagate toward the sensor 300 and interact (constructively and destructively interfere) to form an interference fringe pattern / fringe field (schematically represented by wave 110′) that strikes the sensor 300. The zeroth diffraction order also propagates toward and strikes the sensor 300. It does not interfere with the ±1st diffraction orders and therefore does not contribute to (or interfere with) the interference fringe pattern / fringe field 110′. However, the zeroth diffraction order is sensed by the sensor 300, and therefore the output of the sensor 300 is affected by the zeroth diffraction order. However, the intensity of the zeroth order term striking the sensor is substantially constant and therefore appears as a constant background signal on the output of the sensor, in contrast to the changing signal that represents the detected interference fringe pattern and interferes with the changing signal used for position measurement.

[0049] The fringe pattern / fringe field 110' can be described as an image of the scale 100 in that the sensor 300 is located in a conjugate plane c p to the scale surface sp (e.g. so that light from point A on the scale is imaged to point A' on the sensor, light from point B on the scale is imaged to point B' on the sensor, and light from point C on the scale is imaged to point C' on the sensor). However, it will be appreciated that the fringe pattern / fringe field 110' (i.e. the so-called "image") at the sensor 300 has a sinusoidal waveform, whereas a perfect image of the scale would have a square waveform. Furthermore, by preventing the zeroth diffraction order (and diffraction orders greater than the 1st diffraction order) from contributing to a modulated portion of the fringe pattern / fringe field formed at the sensor, either by blocking them or, for example, by encoding them in the same way as the zeroth diffraction order, the image at the sensor 300 has a more pure sinusoidal waveform and so the image on the sensor is not in fact a "true" or "complete" image of the scale 100 (but rather may be referred to as a pseudo-image of the scale). With the encoder 50 effectively preventing all diffraction orders other than the ±1st diffraction orders from contributing to the fringe pattern / fringe field 110', the fringe pattern / fringe field 110' becomes a pure sinusoidal wave (excluding lens aberrations) and the period of the fringe pattern / fringe field 110' is Mp / 2 (where M is the magnification of the optical system and p is the scale period). In this embodiment, M does not change as the distance between the scale 100 and the lens 40 changes.

[0050] FIG. 4b illustrates an alternative embodiment of the present invention. The embodiment of FIG. 4b is identical to the embodiment of FIG. 4a, except that a polarizing element 60 is positioned in front of the sensor 300, with its polarization direction orthogonal to that of the zeroth diffraction order, so as to suppress / block the zeroth diffraction order but allow the ±1st diffraction orders to pass to the sensor. In this case, the zeroth diffraction order does not strike the sensor at all (similarly, if diffraction orders greater than the ±1st diffraction orders were polarized in the same polarization direction as the zeroth diffraction order, they would also not strike the sensor). Blocking the zeroth diffraction order from reaching the sensor significantly improves the fringe pattern perceived by the sensor 300. In fact, as discussed above in connection with other embodiments, if both the zeroth and ±1st diffraction orders fully contribute to the fringe pattern / fringe field, as shown in FIG. 3, the visibility of the fringe pattern / fringe field would be modulated with a period of p / λ, where p is the scale period and λ is the wavelength of light. If the zeroth order is blocked from reaching the sensor, modulation is no longer present. Also, as shown schematically in FIG. 2, removing the zeroth diffraction order means that the fringe pattern / fringe field 110′ generated at the sensor 300 has a period equal to Mp / 2 (half that of the system in FIG. 1). Therefore, blocking the zeroth diffraction order effectively doubles the system resolution. These are the same effects experienced when encoding the zeroth and ±1st diffraction orders in different optical states so that they do not interact / interfere with the sensor. However, as shown in FIG. 3, blocking the zeroth diffraction order from reaching the sensor at all has the added benefit of significantly improving fringe visibility compared to the situation where the zeroth diffraction order is still present on the sensor, albeit in a different optical state than the ±1st diffraction orders.

[0051] The embodiment of Figures 4a and 4b is a transmissive optical system, where light from the light source 10 is transmitted through the scale 100. Figures 5a and 5b and 6a and 6b show the same optical scheme, but for reflective (or "folded") optical systems, light from the light source 10 is reflected by the scale 100. In this case, only a single lens 40 is used to collimate the light from the light source 10 and form conjugates between the scale 100 and the sensor 300. In the diagrams shown in Figures 5a / 6a, the portion of the diffraction order encoder that encodes the zeroth diffraction order appears to be coincident with the light source 10, but as shown in Figures 5b / 6b, it is actually offset along the y-axis.

[0052] In the embodiments of Figures 4, 5 and 6, the optical system is such that it forms an image of the light source 10 at the plane fp2 where the diffraction order encoder 50 is located. However, this is not necessarily the case, for example, in the embodiment of Figure 2.

[0053] Figure 7 shows schematically a readhead 400 incorporating an optical arrangement according to the invention arranged to read a scale 100. As will be appreciated, the scale 100 is fixed to a first part of a machine (not shown) and the readhead 400 is fixed to a second part of the machine (not shown), the first and second parts of the machine being moveable relative to each other, in this case along the x-axis.

[0054] As shown, the scale 100 includes a series of scale markings that define an incremental track. In this embodiment, the encoder device is an optical diffraction-based encoder. The incremental track therefore includes a series of periodically arranged features that form a diffraction grating. The incremental track is what is commonly referred to as an amplitude scale. As will be appreciated, in an amplitude scale, the features are configured to control the amplitude of light reflected (or transmitted, in transmission scale embodiments) towards a sensor in the readhead (e.g., by selectively absorbing, scattering, and / or reflecting the light). This is in contrast to a phase scale in which features are configured to control the phase of light reflected (or transmitted) towards a sensor in the readhead (e.g. by delaying the phase of the light). Although not shown, as is commonly known in the field of position measurement encoders, one or more reference marks may be provided in adjacent tracks (e.g. as described in US Pat. No. 10,281,301) or embedded within an incremental track (e.g. as described in US Pat. No. 7,659,992). In such cases, the readhead will have at least one sensor for detecting the reference marks.

[0055] Figure 8 shows a schematic representation of the arrangement of the components of Figures 5a and 5b within the readhead of Figure 7. As shown, the sensor 300 is mounted on a printed circuit board member (PCB) 410, and the diffraction order encoder 50 (which, in the orientation shown, is behind the diffraction order encoder 50) and light source 10 are mounted on a substrate, in this case a glass substrate 420. Figure 9 shows a plan view of the glass substrate 420, and also shows the arrangement of the light source 10 and diffraction order encoder 50. In the embodiment shown in Figure 9, the diffraction order encoder 50 comprises an opaque area on the glass (shown as dotted area 52 in Figure 9) having first, second and third elongated transparent slots 54a, 54b and 54c. The first transparent slot 54a contains a polarizer having a first polarization direction, and the second and third transparent slots 54b, 54c contain polarizers having second polarization directions arranged so that light passing therethrough is polarized in a polarization direction orthogonal to that of the light passing through the first transparent slot 54a. The diffraction order encoder 50 is configured, as shown in the optical diagrams of FIGS. 4, 5, and 6, such that the zeroth diffraction order from the scale passes through the first elongated transparent slot 54a (and is thereby encoded in the first polarization direction) and the ±1st diffraction orders pass through the second and third elongated transparent slots 54b, 54c (and is thereby encoded in the second polarization direction orthogonal to the first polarization direction) toward the sensor 300. Other diffraction orders (i.e., diffraction orders greater than the ±1st diffraction orders) are stopped by the opaque areas 52. The opaque areas 52 can comprise a very thin (e.g., here 200 nm thick) metal layer on the glass substrate 420. Alternatively, the opaque areas 52 can comprise a thin metal shim with chemically etched slots. As will be appreciated, the opaque areas 52 do not need to be actually opaque, but being opaque means that other diffraction orders that may be beneficial are blocked.

[0056] In the described embodiment, the light source 10 is a light emitting diode ("LED"), in particular an infrared LED. However, it will be appreciated that other types of light sources can be used instead of an LED, such as a surface emitting laser (SEL), a vertical cavity surface-emitting laser (VCSEL), an edge-emitting laser, etc.

[0057] In the described embodiment, an interference fringe pattern is formed on the sensor 300. However, this does not necessarily have to be the case. For example, the optical system could be configured so that the diffraction orders produce several modulated spots at the sensor (the intensity of the modulated spots varies / modulates with changes in the relative position of the readhead and scale). Similar to systems that produce interference fringe patterns, in systems that produce modulated spots, encoding the 0th and ±1st diffraction orders so that they do not interfere with each other at the sensor effectively doubles the resolution of the encoder system (in that the intensity of the spots modulates at a frequency of p / 2). Similarly, encoding diffraction orders higher than the ±1st diffraction orders in a different optical state to the ±1st diffraction orders reduces / eliminates harmonics in the modulated spot intensity.

[0058] In the described embodiment, sensor 300 is in the form of an electrical grid, or in other words, an optical sensor array including two or more sets of interdigitated / interlaced / interleaved light-sensitive sensor elements (also referred to herein as "photodetectors" or "fingers"). Each set can, for example, detect a different phase of the interference fringe pattern / fringe field (schematically represented by wave 110') in sensor 300. FIG. 11 shows an example of an electrical grid. FIG. 11 shows a portion of an electrical grid, with four sets of photodiode fingers / photodiodes (A, B, C, and D) interdigitated / interleaved to form an array of sensor elements extending along the length "L" of the sensor.

[0059] The outputs from each finger / photodiode in a set are combined to provide a single output, resulting in four channel outputs: A', B', C', and D'. These outputs are then used to obtain quadrature SIN and COS signals. In particular, A'-C' are used to provide a first signal (SIN), and B'-D' are used to provide a second signal (COS) that is 90 degrees out of phase with the first signal. In certain embodiments, the electrical grid includes four sets of photodiodes providing four channels A', B', C', and D', although this need not be the case. For example, the electrical grid may include two sets of photodiodes providing only two channels, A' and B'.

[0060] It will be appreciated that other types of sensors can be used instead of the electrical gratings described above, for example in embodiments where the modulated spot is produced by the readhead optics instead of an interference fringe pattern, a bulk sensor photodiode can be used to detect the intensity of the modulated spot (e.g. as described in U.S. Pat. No. 4,776,701).

[0061] In the above-described embodiment, the zeroth and ±1st diffraction orders are encoded with orthogonal polarization directions, meaning that they do not interact / interfere with each other at all in sensor 300. This need not necessarily be the case. For example, depending on the particular apparatus configuration and / or desired performance enhancement, it may be preferable that the effect of the zeroth diffraction order on the modulated portion of the signal sensed by the sensor (e.g., the interference fringe pattern) is attenuated by at least 90%, such as by at least 95%, such as by at least 98%, although sufficient benefit may be obtained from simply partially reducing the extent to which the zeroth diffraction order can interact / interfere with the ±1st diffraction orders, e.g., by specifying the extent to which the zeroth diffraction order can interact / interfere with the ±1st diffraction orders such that the effect of the zeroth diffraction order on the modulated portion of the signal sensed by the sensor (e.g., the interference fringe pattern) is attenuated by at least 50%. Thus, in such an alternative embodiment, the diffraction order encoder 50 may be configured to polarize only the zeroth diffraction order (thus regions 54b, 54c may be left empty with no polarizer present, leaving the ±1st diffraction orders unpolarized). In such an embodiment, the zeroth diffraction order has an optical state (i.e., polarization state) that is different from the optical state (i.e., unpolarized state) of the ±1st diffraction orders. Doing so reduces the intensity of the zeroth diffraction order, thereby reducing its interaction with the ±1st diffraction orders and providing some improvement in the signal sensed by the sensor.

[0062] In another alternative embodiment, the diffraction order encoder 50 may be configured to polarize only the zeroth diffraction order (and therefore regions 54b, 54c may be left empty with no polarizer present, leaving the ±1st diffraction orders unpolarized), and a polarizing filter may be provided in the readhead suitably configured to block the polarized zeroth diffraction order from reaching the sensor 300.

[0063] In other embodiments, the polarizer 54a for the 0th diffraction order and the polarizers 54b, 54c for the ±1st diffraction orders do not need to have their polarization axes arranged orthogonally, which may result in some improvement in the signal sensed by the sensor compared to when both remain unpolarized (but of course the more orthogonal they are, the more advantage there is that can be gained due to less interaction between the 0th diffraction order and the ±1st diffraction orders).

[0064] In the embodiments of Figures 4a and 5, the zeroth diffraction order is sensed by sensor 300, and the signal output by sensor 300 includes zeroth diffraction order signal information. The embodiments of Figures 4b and 6 prevent the zeroth diffraction order from reaching the sensor by a polarizing filter placed in front of the sensor. In alternative embodiments, the sensor itself can be configured so that it does not sense the zeroth diffraction order (e.g., by a polarizer integrated on the sensor), thereby negating the need for a separate polarizing filter placed in front of the sensor.

[0065] In the embodiment described above, light source 10 emits electromagnetic radiation (EMR) in the near-infrared range. However, it will be appreciated that this need not be the case and that EMR can be emitted in other ranges, for example, any range from infrared to ultraviolet. It will be appreciated that the selection of an appropriate wavelength for light source 10 can depend on many factors, including the availability of suitable gratings and detectors operating at EMR wavelengths.

[0066] In the above-described embodiment, the light source 10 emits unpolarized light, the light striking the scale is unpolarized, and the diffraction orders coming from the scale 100 are unpolarized. However, this need not be the case. For example, the light source 10 can emit polarized light, which can be located at a point between the light source 10 and the diffraction order encoder 50. In such a case, the diffraction order encoder can include one or more optical devices (e.g., one or more wave plates) for changing the polarization direction of one or more of the diffraction orders. For example, the first transparent region 54a of the diffraction order encoder 50 can include a half-wave plate that rotates the polarization direction of the zeroth diffraction order by 90°, while the second and third transparent regions 54b, 54c can be configured to be clear and not change the polarization direction of the ±1st diffraction orders (thereby resulting in the zeroth and ±1st diffraction orders having orthogonal polarization directions).

[0067] In the above-described embodiments, the primary focus is on encoding the zeroth and ±1st diffraction orders to have different optical states. However, the present invention can be used to encode combinations of different diffraction orders. For example, the diffraction orders can be encoded such that the zeroth and ±1st diffraction orders have the same optical state, but other diffraction orders (e.g., the third and / or fifth diffraction orders) have optical states that are different from the optical states of the zeroth and ±1st diffraction orders.

[0068] The above-described embodiments of Figures 4 to 9 use a refractive lens 40 to relay the diffracted orders towards the sensor. However, it will be appreciated that this need not be the case. For example, Figure 12 shows a schematic representation of the optical system of an encoder apparatus including an incremental scale 100 and a readhead 400' according to another embodiment of the present invention. It is similar to that of Figure 6 (like parts share like reference numerals), except that it does not rely on a refractive lens 40 to relay the diffracted orders towards the sensor. In this embodiment, the zeroth and ±1st diffraction orders are generated from light from a light source that strikes and is reflected by the scale 100. As shown, the zeroth and ±1st diffraction orders are reflected by a first plane pl 1 The diffraction order encoder 50 is located in a first plane pl 1 According to the embodiment of FIG. 6, the diffraction order encoder includes a polarizing filter 54a (e.g., with a "vertical" polarization axis). The polarizing filter 54a is matched to the zeroth diffraction order so as to encode the zeroth diffraction order in a first (e.g., vertical) polarization direction. The diffraction order encoder further includes second and third polarizing filters 54b, 54c matched to the +1st and -1st diffraction orders. The second and third polarizing filters 54b, 54c are configured with the same orientation as each other so that the ±1st diffraction orders have the same (e.g., "horizontal") polarization direction as each other but different from the polarization direction of the zeroth diffraction order. The diffraction grating G2 is positioned to focus the polarized ±1st diffraction orders. The zeroth diffraction order is not polarized. Therefore, the zeroth and ±1st diffraction orders are polarized in a plane pl in which the sensor 300 is located. 2 6, a first polarizer 60 may be located in front of the sensor 300 and configured to filter the zeroth diffraction order so that it does not reach the sensor 300. For clarity of illustration, the light source 10 has been omitted from FIG. 12. As will be appreciated, if necessary, another refractive lens or grating may be used to collimate the light from the light source towards the scale.

[0069] In the above-described embodiments, the scale includes an incremental scale without any reference marks. Of course, the scale can include one or more reference marks, for example, one or more optical reference marks. Accordingly, the readhead can include one or more reference mark sensors. Encoding the diffraction orders as described above can provide benefits for reference mark detection. For example, encoding the diffraction orders as described above can change the form of the reconstructed signal, which can be beneficial for reference mark detection. If desired, one or more filters can be provided to block one or more of the encoded diffraction orders from reaching the incremental sensor and / or the reference mark sensor. For example, a filter can be placed before the incremental sensor configured to block the encoded 0th diffraction order from reaching the incremental sensor as described above in connection with FIGS. 4b and 6, but no filter is used before the reference mark sensor. Additionally or alternatively, a filter can be placed before the reference mark sensor and configured to block the encoded ±1st diffraction orders.

[0070] 13 shows an exemplary encoder apparatus 800 including a reference mark and an associated reference mark sensor. In this embodiment, a readhead 806 (the body of which is omitted for clarity) includes a light source 812, in this embodiment in the form of a light emitting diode (LED) (which in this embodiment emits infrared light), a lens 814, a diffraction order encoder 816 (which may also be referred to as an “optical spatial encoder”), a first (hereinafter referred to as a “primary”) position information sensor 818, a second (hereinafter referred to as a “secondary”) position information sensor 820, and a first (hereinafter referred to as a “primary”) sensor filter 822. The light source 812 and the diffraction order encoder 816 are provided substantially in the same plane on an opaque substrate 815. It will be appreciated that areas of this substrate 815 need not actually be opaque, although being opaque means that other diffraction orders that may be useful are blocked. The primary position information sensor is configured to detect signals / resulting fields generated by the incremental feature 810 and output signals by which the incremental position can be determined / monitored, and the second position information sensor is configured to detect signals / resulting fields generated by the reference mark 811 and output signals indicative of the presence of the reference mark. Accordingly, the primary position information sensor 818 may be (and will be) referred to as the incremental sensor 818, and the secondary position information sensor 820 may be (and will be) referred to as the reference mark sensor 820. Light from the LED 812 is emitted toward the scale 804 via a lens 814. The LED 812 is positioned substantially in the focal plane fp of the lens 814 such that the light from the LED 812 is substantially collimated by the lens 814. The light from the LED 812 illuminates a footprint 824 on the scale. The lens 814 relays light reflected by the scale 804 to the incremental sensor 818 and the reference mark sensor 820. The diffracted orders are incident on lens 814, which focuses the corresponding diffracted orders to form spots 850 at the back focal plane fp of lens 814. Each spot is formed from a respective diffracted order. Spots 850 are images of the light source in that the back focal plane fp is a conjugate plane to the plane in which the light source is located.

[0071] The diffraction order encoder 816 coexists with the spot 850. In particular, in this embodiment, the diffraction order encoder 816 includes a polarizing filter 816 (e.g., with a “vertical” polarization axis) that coincides with the zeroth diffraction order spot 250a and first and second non-polarizing transparent regions 816b, 816c that coincide with the ±1st diffraction order spots 850b, 850c. Thus, the diffraction order encoder 816 encodes the zeroth diffraction order with vertical polarization while leaving the ±1st diffraction orders unpolarized. Thus, the zeroth diffraction order has an optical state that is different from the optical state of the ±1st diffraction orders. As shown, the first, second, and third polarizing filters 816a, 816b, 816c are spaced apart so that they do not directly contact one another, although this need not be the case.

[0072] The zeroth and ±1st diffraction orders propagate toward the incremental sensor 818 and the reference mark sensor 820. As described above, the primary sensor filter 822 is located in the optical path of the diffraction orders traveling toward the incremental sensor 818. The primary sensor filter 822 includes a polarizing filter having a "horizontal" polarization axis, i.e., an axis orthogonal to the polarization axis of the first polarizing filter 16a. Thus, the zeroth diffraction order light is substantially blocked from reaching the primary position information sensor. In contrast, the ±1st diffraction orders (despite being "horizontally" polarized during processing) are able to pass through the primary sensor filter 822, thereby impinging on the incremental sensor 818. In particular, the ±1st diffraction orders propagate toward the incremental sensor 818, interact (constructively and destructively interfere), and form an interference fringe pattern (or "fringe field") that impinges on the incremental sensor 818.

[0073] In the described embodiment, there is no corresponding filter in front of the reference mark sensor 820 (which in this embodiment comprises a split detector including first and second photodiodes 820 a, 820 b). Thus, the zeroth and ±1st diffraction orders all impinge on the reference mark sensor 820. Depending on various factors (discussed in more detail below), it may be preferable in some circumstances to not place a corresponding filter in front of the reference mark sensor. Indeed, if the presence of both the zeroth and ±1st diffraction orders impinging on the reference mark sensor does not adversely affect the optical signal, e.g., “image” or “pseudo image” (discussed below), formed at the reference mark sensor 820, there is no advantage to having a corresponding filter in front of the reference mark sensor 820 (indeed, doing so may be detrimental due to reduced photometric performance and / or may provide a clearer reference mark image).

[0074] One thing to note is that in this embodiment, the zeroth and ±1st diffraction orders are unequally attenuated because the zeroth order term is polarized by the first polarizer 816a. If necessary, this unequally attenuated behavior can be avoided by replacing the first and second non-polarizing transparent regions 816b, 816c with second and third polarizing filters 816b, 816c having “horizontal” polarization axes (i.e., polarization axes orthogonal to the polarization axis of the first polarizing filter 816a). In such an embodiment, the zeroth and ±1st diffraction orders impinging on the reference mark sensor 820 are equally attenuated, which may result in a better quality image impinging on the reference mark sensor. Furthermore, the ±1st diffraction orders still pass through the primary sensor filter 822 and interfere to form an interference fringe pattern on the incremental sensor 18. Furthermore, in an optional embodiment, a second (“secondary”) sensor filter can be provided before the second / reference mark sensor 820 to filter one or more diffraction orders. For example, in some circumstances (e.g., if the reference mark is structured / patterned), it may be beneficial to filter the ±1st diffraction orders (and, optionally, higher diffraction orders) so that the reference mark signal formed at the reference mark sensor is formed from the 0th diffraction order only.

[0075] 10 shows an embodiment substantially the same as that of FIG. 13 described above, except that the primary sensor filter 822 has been replaced with a polarizing beam splitter 870 that splits the light from the diffraction order encoder 16 into two beams of orthogonal polarization states. In particular, the polarizing beam splitter 870 allows only (predominantly) vertically polarized light to pass straight through to the reference mark sensor 820, and only (predominantly) horizontally polarized light to be deflected toward the incremental sensor 818. As a result, the zeroth diffraction order (polarized vertically by the first polarizing filter 16a of the diffraction order encoder 16) does not propagate toward the incremental sensor 818 and does not contribute to the signal formed therein, while both the zeroth and ±1st diffraction orders contribute to the signal formed therein (because they are substantially blocked by the polarizing beam splitter 870). The exact location of the polarizing beam splitter is not important, as long as it is positioned between the diffraction order encoder 816 and the detection plane. Therefore, in some applications it may be advantageous to place a diffraction order encoder substrate in front of the polarizing beam splitter and / or place detectors 818, 820 at the exit face.

[0076] In another alternative embodiment, the scale may include an absolute scale. A readhead for an absolute scale, which may be configured as described above, may be configured, for example, so that the diffraction orders from the scale are encoded with different optical states. This may ensure that the differently encoded diffraction orders do not interfere / interact with each other, thereby improving the signal sensed by the sensor. For example, in such a case, it may be beneficial to encode the diffraction orders of the absolute encoder so that the ±1st and 0th diffraction orders have different optical states. Again, this may mean that they do not interact, thereby improving the signal of interest on the sensor. In a particularly preferred embodiment of the absolute encoder, the ±1st diffraction orders (and higher orders, if present to a significant extent) are substantially attenuated / blocked from reaching the readhead sensor (or the sensor is blind to the optical states of the ±1st diffraction orders) so that substantially only the 0th diffracted order strikes the readhead sensor. For example, a suitably configured polarizer may be positioned from the sensor to filter out the ±1st diffraction orders.

[0077] All of the above-described embodiments rely on polarization to encode different diffraction orders in different optical states. However, this is not necessarily the case. For example, different diffraction orders could be encoded at different wavelengths. FIG. 14 illustrates such an embodiment. The encoder 900 of this embodiment has some similarities to the embodiment of FIG. 13, with similar parts sharing the same reference numerals. In this embodiment, the reference mark 911 includes a structured / patterned reference mark, and therefore the reference mark sensor 920 includes a corresponding pattern of photodiodes 920a, 920b, and 920c such that when the readhead passes the reference mark 911, an image of the reference mark 911 aligns with the photodiodes, causing an abrupt change in the intensity of the light impinging thereon. Instead of the single LED 812 of the embodiment of FIG. 13, this embodiment includes red LEDs 912 and 913, the light from which is collimated by lens 814 and illuminates footprint 824 on scale 904. In this embodiment, a secondary sensor filter 923 is provided before the reference mark sensor 920. In this embodiment, the diffraction order encoder 916 and the primary and secondary sensor filters 922, 923 include wavelength (e.g., colored) filters, as opposed to polarization filters, to selectively control which diffraction orders contribute and which do not contribute to the signals formed by the respective incremental sensor 818 and reference mark sensor 920. In particular, with respect to the diffraction order encoder 916, the first wavelength filter 916a includes a blue filter that encodes the zeroth order term only in blue wavelengths, and the second and third wavelength filters 916b, 916c include red filters that encode the ±1st diffraction orders only in red wavelengths. Thus, to prevent the zeroth diffraction order from contributing to the signal formed by the incremental sensor 818, the primary sensor filter 922 includes a red wavelength filter that allows only light having a red wavelength to pass. Similarly, to prevent the ±1st diffraction orders from contributing to the signal formed by the reference mark sensor 920, the secondary sensor filter 923 includes a blue wavelength filter that allows only light having a blue wavelength to pass.As discussed above in connection with other embodiments, the primary or secondary sensor filters 922, 923 can be omitted if desired. It will be appreciated that although red and blue wavelengths are used in this example, other wavelengths can be used. Also, a single polychromatic light source can be used instead of two different monochromatic light sources.

[0078] The embodiments described thus far have related to incremental encoders that include an incremental scale with one or more reference marks (or, optionally, without). The present invention can also be used with absolute encoders that include an absolute scale, such as that shown in FIG. 15. In the embodiment of FIG. 15, absolute scale 504 includes conceptually periodically arranged features, with selected features removed to encode unique / absolute position data along the measuring length of the scale. The data can be in the form of, for example, a pseudo-random sequence or discrete code words. Details of such scales are described in U.S. Pat. Nos. 7,499,827 and 5,279,044. Readhead 506 (whose body has been omitted for clarity) shares several parts identical to those described above in connection with other embodiments of the present invention, and therefore, similar parts share the same reference numerals. For example, readhead 506 includes light source 10 (in this embodiment, an unpolarized LED light source), lens 14, and diffraction order encoder 50.

[0079] In this embodiment the readhead 506 includes an absolute sensor 520 which comprises a photodiode array (in this embodiment a one dimensional photodiode array, but could also be two dimensional) onto which an image (or "pseudo image") impinges. The image of the scale can be processed to extract an absolute / unique code and thereby determine absolute position, as is known and described, for example, in US Pat. No. 7,499,827, US Pat. No. 5,279,044 and US Pat. No. 1,098,9567.

[0080] As with the reference mark embodiment described above, it may be beneficial to allow the zeroth order term to contribute to the signal impinging on the absolute sensor. Furthermore, as with the reference mark embodiment described above, it may be beneficial to prevent the ±1st diffraction orders (and, optionally, higher orders) from contributing to the signal impinging on the absolute sensor 520. Therefore, a secondary sensor filter 523 may be provided in front of the absolute sensor 520, having a polarization axis orthogonal to the polarization axes of the second and third polarizing filters 54b, 54c of the diffraction order encoder 50. Notably, in this embodiment, the second and third polarizing filters 54b, 54c have horizontal polarization axes, and the secondary sensor filter 523 has a vertical polarization axis.

[0081] In this embodiment, the readhead 506 further includes an optional incremental detector 300 and a primary sensor filter 60, which are configured and arranged in the same manner as described above in connection with the embodiment of FIG. 6. An absolute position can be combined with the incremental position determined from the incremental detector 300 to provide a fine-pitch absolute position. Optionally, once the absolute position is determined, subsequent positions can be determined independently by monitoring the output from the incremental detector 300. According to the embodiment of FIG. 6, the zeroth-order term is blocked so that it does not contribute to forming the signal detected by the incremental sensor 18. As with the other embodiments, the primary sensor filter 60 includes a polarizing filter having a polarization axis perpendicular to the polarization axis of the first polarizing filter 54a of the diffraction order encoder 50. Notably, in this embodiment, the first polarizing filter 54a has a vertical polarization axis, and the primary sensor filter 60 has a horizontal polarization axis.

[0082] FIG. 16 illustrates another embodiment of an absolute encoder 600 including an absolute scale 604. The absolute scale 604 is substantially identical to that described above in connection with FIG. 15 (like parts share the same reference numerals), except that in this embodiment, the scale includes a multi-track arrangement, with absolute code features provided in an absolute scale track 603 provided on either side of a pure incremental track 605. As shown, the absolute scale track 603 also includes finer pitch / period incremental features in spaces between the coarser pitch / period features of the absolute scale track 603. By way of example, the coarse features of the absolute scale track that encode absolute position information may have a nominal period of approximately 32 μm, and the finer pitch / period incremental features may have a period of approximately 8 μm. As a result, two sets of diffraction orders are effectively generated: a first set of diffraction orders generated by a first series of position features (e.g., 8 μm incremental scale features) and a second set of diffraction orders generated by a second series of position features (e.g., absolute scale features). In fact, in this embodiment, the zeroth diffraction order of the first set of diffraction orders passes through the first polarizing filter 54a of the diffraction order encoder 50, the ±1st diffraction orders of the first set of diffraction orders pass through the second and third polarizing filters 54b, 54c, while the zeroth diffraction order and the ±1st diffraction orders of the second set of diffraction orders all pass through the first polarizing filter 54a of the diffraction order encoder 50. The higher diffraction orders of the first set of diffraction orders (e.g., the ±3rd diffraction orders, the ±5th diffraction orders) are blocked by the opaque substrate 15. The ±3rd diffraction orders of the second set of diffraction orders pass through the second and third polarizing filters 54b, 54c of the diffraction order encoder 50. Diffraction orders higher than the ±3rd diffraction orders of the second set of diffraction orders have substantially insignificant power, but what is present will be blocked by the opaque substrate 15. As will be appreciated, which diffraction orders pass through which filters is system dependent and may depend on a variety of factors, including the size and / or position of the polarizing filters.

[0083] As shown, the sensor arrangement for the embodiment of FIG. 15 differs slightly from the embodiment of FIG. 14 in that the readhead of the absolute sensor 820 is split in two with the incremental sensor 300 located between them. A variation on the embodiment of FIG. 15 is for the absolute scale features to extend completely across the incremental scale features (i.e., so that there is no pure incremental scale track 605 between the two absolute tracks 603). Rather, as in the embodiment of FIG. 14, the absolute scale features can be fully embedded throughout the incremental features. In this case, the above comments regarding the two sets of diffraction orders still apply because in this embodiment the incremental and absolute position features have different nominal periods. If desired, an alternative configuration could arrange the incremental sensor 300 and absolute sensor 620 according to the embodiment of FIG. 14 (i.e., two sensors arranged side by side).

[0084] The present invention has been described above in relation to a linear encoder device, however, the present invention is equally applicable to rotary encoder devices including both ring encoders (which form scale features on the outer cylindrical surface of a ring member) and disk encoders (which form scale features on the planar surface of a disk member).

Claims

1. a scale readable by the readhead, the scale extending along the measurement dimension and including a series of positional features that diffract light into a plurality of diffraction orders; a readhead including a light source that illuminates the scale and at least a first sensor configured to detect signals produced by one or more diffraction orders produced by the series of positional features of the scale that can be used to determine the relative position of the scale and the readhead along the measurement dimension; Equipped with the readhead further comprising a diffraction order encoder configured to encode at least one diffraction order produced by the series of position features of the scale in an optical state different from an optical state of at least one other diffraction order produced by the series of position features of the scale; Position measuring encoder device.

2. the diffraction order encoder encodes the zeroth diffraction order in a different optical state than the ±1st diffraction orders.

2. The position measuring encoder device of claim 1.

3. at least partially filtering at least one diffraction order based on its optical state to at least partially attenuate its effect on generating a signal sensed by the first sensor.

2. The position measuring encoder device of claim 1.

4. configured to: i) substantially filter the zeroth diffraction order so as to have substantially no effect on the generation of a signal sensed by the first sensor; or ii) substantially filter the ±1st diffraction orders so as to have substantially no effect on the generation of a signal sensed by the first sensor; 4. A position measuring encoder device according to claim 2 or 3.

5. the diffraction order encoder encodes the at least one diffraction order with a polarization state that is different from a polarization state of at least one other diffraction order.

5. A position measuring encoder device according to any one of claims 1 to 4.

6. different diffraction orders of light relayed to the at least first sensor are configured to converge to different respective convergence points in an optical path between the scale and the at least first sensor.

6. A position measuring encoder device according to any one of claims 1 to 5.

7. The convergence point is located in a conjugate plane of the light source.

7. A position measuring encoder device according to claim 6.

8. the diffraction order encoder is located at the convergence point; 8. A position measuring encoder device according to claim 6 or 7.

9. the first sensor includes an incremental position sensor configured to sense an incremental position signal; A position measuring encoder device according to any one of claims 1 to 8.

10. configured such that diffraction orders greater than ±1 diffraction orders are not sensed by the at least first sensor; A position measuring encoder device according to any one of claims 1 to 9.

11. a second sensor configured to detect signals generated thereat by the diffraction orders from the scale; A position measuring encoder device according to any one of claims 1 to 10.

12. a first sensor filter configured to filter the light based on its optical state before the light hits the first sensor; A position measuring encoder arrangement according to any one of claims 1 to 11.

13. a second sensor filter configured to filter the light based on an optical state of the light before it hits the second sensor, wherein the first sensor filter and the second sensor filter are configured to filter the light in different ways from each other; 13. A position measuring encoder device according to claim 11 or 12.

14. The optical state includes a state of polarization. A position measuring encoder arrangement according to any preceding claim.

15. the first sensor filter and / or the second sensor filter include a polarizing filter; 15. A position measuring encoder device according to claim 12 or 13 and 14.

16. 1. A position measurement encoder device, comprising: a scale including a series of periodic features configured to diffract light into a plurality of diffraction orders, including a zeroth diffraction order; a readhead including a light source for illuminating the scale, wherein at least one sensor is configured to detect an interference fringe pattern / fringe field that moves with relative movement of the scale and the readhead, the interference fringe pattern / fringe field being generated by the diffraction orders from the scale at the at least one sensor; Equipped with The period of the fringe pattern is M.p / 2, where M is the magnification of the encoder optics and p is the period of the scale. Position measuring encoder device.

17. a scale including a series of periodic features configured to diffract light into a plurality of diffraction orders, including a zeroth diffraction order; a readhead including a light source to illuminate the scale, at least one sensor configured to detect a resultant field that varies with relative movement of the scale and the readhead, the resultant field being generated by the diffraction orders from the scale at the at least one sensor; Equipped with the readhead is configured such that the zeroth diffraction order does not contribute to generating the resultant field at the at least one sensor. Position measuring encoder device.

Images