Encoder device

Abstract

A position measurement encoder device includes a scale including a series of features readable by a read head to determine relative positions of the scale and the read head and diffract light into a plurality of diffraction orders; a readhead comprising a light source for illuminating the scale, and at least a first sensor configured to detect a signal generated at the first sensor by one or more diffraction orders generated by the scale; wherein the read head further comprises a diffraction order encoder that encodes the at least one diffraction order to have a different optical state than the optical state of the at least one other diffraction order.

Description

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

[0002] A position measuring 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 includes a scale and a readhead, one disposed on one portion of the device and the other disposed on another portion of the device. The scale may include a series of features that the readhead can read in order to measure its position along the scale (e.g., the features of the scale may be disposed on a substrate that is secured to a portion of the device, or may even be integrally formed as part of the device).

[0003] So-called "incremental" position encoders can, for example, operate by "counting" the position of a scale along its length (e.g., from its starting position and / or from reference marks defined on the scale). As will be appreciated, the manner in which the readhead position is "counted" will vary between encoder devices. For example, one approach is to generate a composite field, such as an interference fringe pattern ("fringe field"), or a modulated light spot, at a detector in the readhead that changes with relative movement. For example, light from a source (within the readhead) can be directed onto the scale, diffracting the light into a plurality of diffraction orders. The diffraction orders interfere / interact / recombine at the detector to produce a composite field. Alternatively, an optical element (e.g., a diffraction grating and / or lens) can be provided within the readhead that redirects / deflects the diffraction orders so that they interfere / interact at the detector, thereby producing a composite field. As the scale and readhead move relative to each other, the composite field changes. The readhead can record and / or report movement and position by monitoring changes in the composite field (e.g., movement of the interference fringe pattern / fringe field). Such a position encoder is described in US5861953. As will be appreciated, reference marks may be provided, for example, adjacent to and / or embedded within the diffractive features of the scale to provide a defined reference position. Such a position encoder is described in US7659992.

[0004] So-called "absolute" position encoders are also known, which are capable of determining the absolute position of a read head relative to a scale to be determined without having to count from a predetermined position (such as a reference mark or the end position of the scale). An absolute position encoder typically comprises a scale on which unique position data is formed along its measured length. The data may be in the form of, for example, a pseudo-random sequence or a discrete codeword. 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 US 7499827, US 10132657 and US 2012 / 0072169. It is known that some absolute encoders use an incremental scale in addition to an absolute scale. It is also known (and described, for example, in US 7499827) that, optionally, an absolute scale retains sufficient periodicity so that the scale can be used as a periodic incremental scale. In either case, such an incremental scale can be used, for example, to fine-tune the absolute position that has been determined. Alternatively, such a system may be used so that, after start-up and having determined the absolute position, the relative position of the readhead and scale is then measured by "counting" changes in position using an incremental scale. Such an incremental scale may be read in the same manner as described above, for example by analysing the resultant field produced (at the readhead's sensor) by the diffraction orders produced by the scale.

[0005] The present invention relates to improvements to those types of encoder devices that rely on the diffraction of light to form 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 device comprising: a scale comprising a series of position features readable by a read head, the series of position features extending along a measuring dimension; a read head comprising a light source for illuminating the scale, and at least a first sensor, the first sensor being configured to detect signals generated at the first sensor by one or more diffraction orders generated by the series of position features of the scale, the series of position features being capable of being used to determine the relative position of the scale and the read head along the measuring dimension; wherein the read head further comprises a diffraction order encoder which encodes at least one diffraction order into an optical state (or in other words, a different optical characteristic) that is different from the optical state of at least one other diffraction order.

[0007] Providing a readhead with a diffraction order encoder can be useful in providing an improved signal formed at the sensor of the readhead, the diffraction order encoder encoding at least one diffraction order to have an optical state that is different from the optical state of at least one other diffraction order (or in other words, a different optical characteristic). For example, this can mean that diffraction orders having different optical states / characteristics have reduced / no optical interference with each other (even if they all reach the sensor), which can provide an improved signal, such as a signal with improved visibility and / or working gap tolerance and / or improved harmonic quality. This can also enable filtering and / or optical separation of different diffraction orders based on different optical states. Furthermore, such improvements can be achieved without having to design the scale so that it does not produce certain diffraction orders, or so that it produces diffraction orders having certain optical states, thereby simplifying scale design and / or manufacture, which can result in cost benefits and / or facilitate the easy production of longer scale lengths.

[0008] In those embodiments where there is a 0th diffraction order and + / -1st diffraction orders (e.g., when the scale comprises an amplitude-type scale), the diffraction order encoder can be configured to encode the 0th diffraction order to have a different optical state than the + / -1st diffraction orders. Typically, these are the orders with the greatest intensity and have the greatest impact on the signal formed at the sensor of the readhead, so it can be beneficial to encode them to have different optical states to reduce / avoid their interaction at the sensor and / or so that they can be selectively attenuated / blocked from reaching / or 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 than the -1st diffraction order, it is typically beneficial 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 or a rotary encoder device. Correspondingly, 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 a first sensor and a second sensor. For example, the second sensor may be further configured to detect a signal generated at the second sensor by the diffraction order from the scale. The first sensor and the second sensor may be configured to detect the same type of signal (for example, both may be incremental signals, or both may detect absolute signals) or different types of signals (for example, one may detect an incremental signal, while the other may detect a reference mark signal or an absolute signal). Optionally, the first sensor and the second sensor both detect signals formed by the one or more diffraction orders.

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

[0012] It may be preferable to configure the readhead such that at least one diffraction order is at least partially filtered out based on its optical state, so as to at least partially reduce its effect on the signal sensed by the first sensor (and / or the second sensor, if present). In particular, for example, if at least one diffraction order is substantially completely filtered out, such that it does not contribute to the signal sensed by the sensor at all, this can provide an even greater improvement in the quality of the signal sensed by the sensor. For example, the position measuring encoder device may be configured to i) at least partially (e.g., substantially) filter out the 0th diffraction order, so as to at least partially reduce (e.g., substantially completely reduce / eliminate) its effect on the signal sensed by the first sensor, or ii) at least partially (e.g., substantially) filter out the + / - 1st diffraction order, so as to at least partially reduce (e.g., substantially reduce / eliminate) its effect on the signal sensed by the first sensor. Option i) may be particularly preferred when the sensor (e.g., the first sensor) is configured to sense incremental position signals (e.g., an interference fringe pattern or a modulated light spot). Option ii) may be particularly preferred when the sensor (e.g., the second sensor) is configured to sense reference marks or absolute values ​​on a scale. If both a first sensor and a second sensor are present, the light that generates the signal at the first sensor may be different (e.g., filtered differently) than the light that generates the signal at the second sensor.

[0013] While some benefit may be gained from only partially attenuating the effect of a particular diffraction order (e.g., the 0th diffraction order, or the + / - 1st diffraction order) on the generation of the signal formed at the first sensor, for example by attenuating the effect of the particular diffraction order on the generation of the signal by at least 50%, it is preferred that the effect of the particular diffraction order on the generation of the signal sensed by the first sensor is substantially fully attenuated, for example by at least 90%, for example by at least 95%, for example by at least 98%. In a particularly preferred embodiment, the readhead is configured such that the particular diffraction order (e.g., the 0th diffraction order) has substantially no effect on the generation of the signal sensed by the first sensor.

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

[0015] The optical state may include a polarization state. Thus, the diffraction order encoder may be configured to encode at least one diffraction order to have a polarization state that is different from the polarization state of at least one other diffraction order. For example, the diffraction order encoder may be configured to encode one diffraction order (e.g., the 0th diffraction order) so that it is polarized (e.g., vertically polarized) (e.g., so that it has a polarization degree of at least 0.5 or 50%, preferably at least 0.75 or 75%, more preferably at least 0.85 or 85%, for example at least 0.9 or 90%). The other diffraction orders may remain unpolarized, or one or more of the other diffraction orders (e.g., the + / - 1st diffraction order) may be polarized differently (e.g., orthogonally) than the 0th diffraction order. The diffraction order encoder may include at least one polarizer element configured to encode at least one diffraction order having a polarization state that is different 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 0th diffraction order having a first polarization state and one or more additional (e.g., second and third) polarizer elements configured to encode the + / - 1st diffraction orders having a second polarization state different from the first polarization state. Optionally, the polarizer element comprises a polarizer configured to polarize at least one diffraction order (e.g., increase its degree of polarization). Optionally, the polarizer element comprises at least one polarization manipulator (e.g., a wave plate or a retarder) configured to change the polarization state of at least one diffraction order, for example, configured to rotate the polarization orientation of the at least one diffraction order. In this case, the diffraction orders will need to be polarized before the diffraction order encoder. For example, the light source can be a polarized light source, or a polarizer can be placed in the light path before the wave plate.

[0016] The optical state may include a wavelength state. For example, the diffraction order encoder may be configured to encode at least one diffraction order to have a wavelength state that is different from the wavelength state of at least one other diffraction order. For example, the diffraction order encoder may be configured to encode one diffraction order (e.g., the 0th diffraction order) so that it has light that falls within a first wavelength band. The device may be configured so that the wavelengths of the other diffraction orders remain unchanged, or alternatively, the device may be configured so that the diffraction order encoder encodes one or more of the other diffraction orders (e.g., the + / - 1st diffraction order) to have a second wavelength band that is different from the first wavelength band.

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

[0018] The readhead may comprise at least one optical element for relaying diffraction orders from the scale towards at least a second sensor. For example, the readhead may comprise at least one refractive and / or diffractive optical element for relaying diffraction orders from the scale towards at least one sensor. Suitable optical elements include lenses and / or diffraction gratings. The diffraction order encoder may be positioned before the optical element for relaying the diffraction orders (e.g. according to Figure 2 ) or after the optical element for relaying the diffraction orders (e.g. according to FIG4 ) interacting with one or more diffraction orders. Alternatively, the diffraction order encoder and the optical element for relaying the diffraction orders may be one and the same object (e.g. Figure 2 The relay element 200 may include a diffraction order encoder 50 integrated therein).

[0019] Optionally, the device is configured so that the diffraction orders converge onto corresponding / corresponding light spots (or "different convergence points") on the optical path between the optical element and the sensor. For example, there may be a light spot / convergence point for each diffraction order, such as a 0th diffraction order light spot / convergence point, a +1st diffraction order light spot / convergence point, a -1st diffraction order light spot / convergence point, and so on. Such light spots may be located at the focal plane of the optical element (focal plane of the lens) used to relay the diffraction orders. The diffraction order encoder may be located substantially at those convergence points. For example, the diffraction order encoder may be located at the focal plane of the optical element used to relay the diffraction orders, such as at the focal plane of the optical element. The light spot may be formed at a conjugate plane of the light source. Accordingly, the light spot may be an image of the light source.

[0020] The position measurement encoder device can be configured to also reduce the influence of diffraction orders greater than the + / - 1st diffraction order on the generation of the composite field sensed by the first sensor (and / or the second sensor (if present)). Such diffraction orders may include, for example, the + / - 3rd diffraction order and / or the + / - 5th diffraction order. The position measurement encoder device can be configured to prevent diffraction orders greater than the + / - 1st diffraction order from reaching at least the first sensor (and the second sensor (if present)). Such diffraction orders can be stopped by absorption, deflection, scattering and / or reflection. For example, an opaque material can be located at the above-mentioned light spot for selected diffraction orders or all diffraction orders that are greater than the diffraction order of + / - 1st diffraction order than the diffraction order at the conjugate plane of the light source so as to absorb and block such diffraction orders. Optionally, the diffraction order encoder is configured to encode diffraction orders greater than the + / - 1st diffraction order into an optical state such that these diffraction orders do not interact / interfere with the 0th diffraction order and / or the + / - 1st diffraction order at at least the first sensor (and the second sensor (if present)), or such that these diffraction orders are at least partially (e.g., substantially) filtered out by appropriate filters (e.g., filtered out by the first sensor filter and / or the second sensor filter) before reaching at least the first sensor (and the second sensor (if present)).

[0021] The scale may be illuminated with collimated light (optionally, the diffraction orders of the scale may themselves be collimated). Optionally, the readhead comprises an optical element for collimating the 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 toward 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 to corresponding / respective light spots.

[0022] The device may be configured such that at least the first sensor is located substantially at a conjugate plane of the scale. Accordingly, the position measuring encoder device may be described as an imaging encoder device, wherein an image (or pseudo image) of the scale is formed at at least one sensor.

[0023] Preferably, the scale comprises what is commonly referred to as an amplitude-type or "Ronchi" scale. As will be appreciated, in an amplitude-type or "Ronchi" scale, features are configured to control the amplitude of light reflected (or transmitted in the case of a transmission-type scale) towards a sensor of the readhead, for example by selectively absorbing, scattering and / or reflecting light. An amplitude-type or "Ronchi" scale is to be contrasted with a phase-type scale, in which features are configured to control the phase of light reflected (or transmitted) towards a sensor of the readhead (e.g., by delaying the phase of the light). Typically, an amplitude-type scale produces a significant 0th diffraction order as well as significant + / - 1st diffraction orders (plus higher + / - odd diffraction orders of reduced intensity), in contrast to a phase-type scale which does not produce any 0th diffraction order.

[0024] Optionally, the period of the scale is no more than 40 μm, preferably no more than 20 μm, such as no more than 10 μm, for example no more than 8 μm.

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

[0026] The scale may be a transmissive scale. Alternatively, the scale may be a reflective scale. Accordingly, preferably, the light source of the read head and the at least one sensor are located on the same side of the scale.

[0027] Optionally, the position measuring encoder device is a single grating encoder system, wherein the scale comprises a sole diffraction grating in the optical path between the light source and the at least one sensor.

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

[0029] The signal generated at at least the first sensor may comprise an incremental position signal, such as an interference fringe pattern or a modulated light spot. Accordingly, at least the first sensor may comprise an incremental position sensor. The period of the fringes may be Mp / 2, where M is the magnification of the encoder optical system and p is the period of the scale (this is / can be achieved when the 0th diffraction order has been substantially removed). The scale may comprise an incremental scale comprising a series of periodic features defining an incremental scale track. One or more reference marks may be provided, embedded in and / or located adjacent to the incremental scale track. Such reference marks may comprise optical reference marks. The read head may comprise at least a second sensor configured to detect a reference mark signal generated by the reference mark.

[0030] Optionally, the signal generated at at least the first sensor comprises an absolute position signal. Accordingly, at least the first sensor may comprise an absolute position sensor. Accordingly, the scale may comprise an absolute scale track comprising a series of features defining an 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 that can be read by a readhead so that the relative position of the readhead and the scale can be determined at any position along the scale (e.g., at startup) without the need to move to a reference position (e.g., a reference mark). Examples of absolute scales include those described in U.S. Patents US 7,499,827 and US 5,279,044.

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

[0032] As will be appreciated, the scale may comprise a first series of position features and a second series of position features readable by a readhead. As will be appreciated, the first series of position features may produce a first set of diffraction orders and the second series of position features may produce a second set of diffraction orders (which produce the signals detected by the first and second sensors). These first and second sets of diffraction orders may be superimposed / spatially overlapped with each other. It may be that the position measuring encoder device is configured such that only one of the first and second sets of diffraction orders has diffraction orders that have different optical states. For example, the position encoder device may be configured such that at least one diffraction order in the first set of diffraction orders (e.g. the 0th diffraction order) has an optical state that is different from the optical state of at least one other diffraction order in the first set of diffraction orders (e.g. the + / - 1st diffraction orders), but the diffraction orders in the second set of diffraction orders (e.g. at least the 0th diffraction order and the + / - 1st diffraction orders) may have the same optical state. Alternatively, the position-measuring encoder device may be configured such that at least one diffraction order in the first group of diffraction orders 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 in the second group of diffraction orders has an optical state that is different from the optical state of at least one other diffraction order. Either way, as will be understood, the first group of diffraction orders and the second group of diffraction orders may be filtered differently based on their optical states.

[0033] The first series of position features and / or the second series of position features may be periodic (in other words, the scale may include a first series of periodic position features and a second series of periodic 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 comprising 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., a period that is coarser than the first period). The first series of position features and / or the second series of position features may be provided in the same scale track, i.e., embedded within each other / superimposed on each other.

[0034] According to another aspect of the present invention, there is provided a position measuring encoder device comprising: a scale comprising a series of periodic features configured to diffract light into a plurality of diffraction orders, including a 0th diffraction order; a read head comprising a light source for illuminating the scale, and at least one sensor configured to detect an interference fringe pattern / fringe field that moves with the relative movement of the scale and the read head, the interference fringe pattern / fringe field being generated at the at least one sensor by the diffraction orders from the scale; wherein the period of the fringes is Mp / 2, wherein M is the magnification of the optical system of the encoder, and p is the period of the scale.

[0035] According to another aspect of the present invention, there is provided a position measuring encoder device comprising: a scale comprising a series of periodic features configured to diffract light into a plurality of diffraction orders, including a 0th diffraction order; a read head comprising a light source for illuminating the scale, and at least one sensor configured to detect a composite field that changes with relative movement of the scale and the read head, the composite field being generated at the at least one sensor by the diffraction orders from the scale; wherein the read head is configured such that the 0th diffraction order does not contribute to the generation of the composite field at the at least one sensor.

[0036] Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which:

[0037] Figure 1a and Figure 1b is a schematic diagram illustrating the optical operating principle of a general prior art position encoder system;

[0038] Figure 2 is a schematic diagram illustrating the optical operation of a position encoder apparatus arranged in accordance with the present invention;

[0039] Figure 3is a graph showing the effect of the 0th diffraction order on the visibility of fringes falling on the sensor as the distance between the scale and the read head varies;

[0040] Figure 4a and Figure 4b is an optical diagram illustrating an alternative embodiment of the present invention;

[0041] Figure 5a and Figure 5b is an optical diagram illustrating another alternative embodiment of the present invention;

[0042] Figure 6a and Figure 6b is an optical diagram of yet another alternative embodiment of the present invention;

[0043] Figure 7 Schematic diagram showing the combination of Figure 6a and Figure 6b an optically arranged read head and an associated scale;

[0044] Figure 8 Schematically shows Figure 6a and Figure 6b The parts in Figure 7 The arrangement within the read head;

[0045] Figure 9 Shown Figure 8 A plan view of a glass substrate;

[0046] Figure 10 is an optical diagram of yet another alternative embodiment of the present invention;

[0047] Figure 11 An example electro-grating sensor is shown;

[0048] Figure 12 Schematically illustrates an encoder device according to another embodiment of the present invention;

[0049] Figure 13 An incremental scale and a readhead of an encoder device according to another embodiment of the present invention are shown;

[0050] Figure 14 An incremental scale and readhead illustrating an encoder apparatus according to another embodiment of the present invention; and

[0051] Figure 15 An absolute scale and readhead showing an encoder apparatus according to another embodiment of the present invention; and

[0052] Figure 16 An absolute scale and a readhead of an encoder apparatus according to another embodiment of the invention are schematically illustrated.

[0053] With reference to the accompanying drawings, Figure 1a The optical working of a general prior art position encoder system is schematically illustrated wherein a scale 100 having a periodic series of features with a period "p" is illuminated by light to produce a set of diffraction orders 101, 102, 103. As will be appreciated, reference to light in this document includes visible and invisible light from the ultraviolet range to the infrared range. In the particular embodiment described, near infrared light is used. The diffraction orders 101, 102, 103 are relayed to a sensor 300 (both the sensor and the optical element being located in a read head movable relative to the scale 100) via an optical element 200 (e.g. a lens, prism or diffraction grating). In particular, the diffraction orders interfere at the sensor 300 to produce an interference fringe pattern / fringe field (schematically illustrated by wave 110) at the sensor 300 having a period equal to Mp; where M is the magnification of the optical system and p is the scale period. In Figure 1a In the diagram, light is shown as being transmitted through the scale, although as will be appreciated, the light may be reflected from the scale and may therefore originate from a light source located on the same side of the scale as the sensor. As will be appreciated, typically the light source, relay element 200 and sensor 300 are all provided by / arranged in a readhead arrangement that is configured to move relative to the scale 100.

[0054] As will be understood, Figure 1a is a simplified illustration of the optical situation encountered in an encoder device. In practice, Figure 1a The optical situation shown in is repeated multiple times along the length of the scale (ie over the area illuminated by the light source), resulting in a longer interference fringe pattern / fringe field pattern at the detector (e.g. Figure 1b Schematically shown in ).

[0055] Out of Figure 1a and Figure 1b For illustrative purposes in FIG, only the 0th and + / - 1st diffraction orders are shown. As will be appreciated, higher diffraction orders are produced and can contribute to forming the interference fringe pattern / fringe field at the sensor 300, although their intensity, and therefore contribution to the interference fringe pattern / fringe field, is typically much weaker than the 0th and + / - 1st diffraction orders (with higher orders gradually decreasing in intensity).

[0056] As will be appreciated, where the mark-space ratio of the scale (i.e. the ratio of the width of the scale features to the spacing between the marks) is strictly 1:1 (as is common with amplitude-type scales of encoder devices), no even diffraction orders (e.g. + / - 2nd, + / - 4th) will be produced, and only odd diffraction orders (e.g. + / - 3rd, + / - 5th) will be produced. In practice, some minor manufacturing errors may mean that the mark-space ratio of the scale is not strictly 1:1, and therefore even diffraction orders may be present (although the intensity may be significantly lower than the adjacent odd diffraction orders). As will be appreciated, where the mark-space ratio of the scale is deliberately set so that it is not 1:1, then significant even diffraction orders may be present.

[0057] For the sake of simplicity, Figure 1a and Figure 1b The ray diagrams are shown as transmitted ray diagrams (ie, light is shown as being transmitted through each of the scale and the optical element), when in reality at least one of the rays may be reflected.

[0058] Figure 2 is a schematic diagram showing the Figure 1b The optical operation of a position encoder device similar to that of a position encoder device but modified according to the present invention. In this case, the read head includes a diffraction order encoder 50 that encodes the 0th diffraction order to have an optical state different from the optical state of the + / - 1st diffraction orders. In this embodiment, the diffraction order encoder 50 includes a first polarizer portion 54a and two second polarization portions 54b, 54c, the first polarizer portion being configured to encode the 0th diffraction order to have a first polarization state, and the two second polarization portions 54b, 54c being configured to encode the + / - 1st diffraction orders to have a second polarization state orthogonal to the polarization state of the 0th diffraction order. Accordingly, because the 0th diffraction order and the + / - 1st diffraction order are encoded to have orthogonal polarization states, they do not interfere with each other (constructively or destructively) when they fall on the sensor. This means that the 0th diffraction order does not contribute to the modulated portion of the interference fringe pattern formed on the sensor. This provides a number of improvements to the interference fringe pattern / fringe field 110' that falls on the sensor 300.

[0059] In particular, Figure 3The effect of encoding the 0th and + / -1st diffraction orders to have orthogonal polarization states on the visibility of fringes falling on the sensor 300 is shown as the distance between the scale 100 and the optical element 200 readhead (commonly referred to as the "working gap" of the readhead relative to the scale) is changed. As shown, when the 0th and + / -1st diffraction orders have the same polarization state (including, for example, that they are both unpolarized) so that the 0th and + / -1st diffraction orders interfere with each other and so that the 0th diffraction order contributes entirely to the interference fringe pattern / fringe field, the visibility of the interference fringe pattern / fringe field is modulated to have p 2 / λ period, where p is the scale period and λ is the wavelength of the light. If the 0th and + / -1st diffraction orders are encoded with orthogonal polarization states so that they do not interfere with each other at the sensor plane, the modulation no longer exists. This is beneficial because it allows encoder devices with a scale that produces a 0th diffraction order to have an operating gap tolerance that is independent of the scale period.

[0060] Another advantage of encoding the 0th and + / -1st diffraction orders to have orthogonal polarization states is that (e.g. Figure 2 The interference fringe pattern / fringe field 110′ produced at the sensor 300 (schematically shown in FIG) has a period equal to Mp / 2, which is half the period of a system in which the 0th and + / -1st diffraction orders have the same polarization state (e.g., the system of FIG1 ). Accordingly, encoding the 0th and + / -1st diffraction orders to have orthogonal polarization states effectively doubles the resolution of the system.

[0061] Furthermore, if diffraction orders greater than the 1st are also encoded to have different optical states than the + / - 1st diffraction orders (according to the embodiments described below), higher order harmonics are removed from the interference fringe pattern / fringe field, thereby providing a purer interference fringe pattern.

[0062] Figure 4a An optical diagram illustrating an embodiment of the present invention is shown which uses a lens 40 to form an image of the light source 10. Figure 4a , light from the light source 10 is collimated by the first lens 20 before being irradiated to the scale 100, and the 0th (101st), -1st (102nd), and +1st (103th) diffraction orders are incident on the second lens 40. The second lens 40 converges the corresponding diffraction orders to form light spots 150 at the back focal plane (fp2) of the second lens 40; each light spot 150 is formed by a corresponding diffraction order. The light spots 150 are images of the light source 10 because the back focal plane fp2 is a conjugate plane of the plane where the light source 10 is located.

[0063] With Figure 2In the same manner as illustrated, the diffraction order encoder 50 can encode the 0th diffraction order (101) from the scale to have a different optical state than the + / -1st diffraction orders (102, 103) from the scale. The + / -1st diffraction orders 102, 103 propagate toward the sensor 300 and interact (interfere constructively and destructively) to form an interference fringe pattern / fringe field (schematically illustrated by wave 110') that falls on the sensor 300. The 0th diffraction order also propagates toward the sensor 300 and falls on the sensor. It does not interfere with the + / -1st diffraction order and therefore does not affect (or interfere with) the interference fringe pattern / fringe field 110'. However, the 0th diffraction order is sensed by the sensor 300 and therefore the output of the sensor 300 is affected by the 0th diffraction order. However, the intensity of the 0th diffraction order falling on the sensor will be substantially constant and therefore appear as a constant background signal on the output of the sensor, rather than a varying signal that will interfere with the varying signal representing the detected interference fringe pattern and used for position measurement.

[0064] The interference fringe pattern / fringe field 110' can be described as an image of the scale 100 because the sensor 300 is positioned at a conjugate plane cp of the scale plane sp (so that, for example, light from point A on the scale is imaged onto point A' on the sensor, light from point B on the scale is imaged onto point B' on the sensor, and light from point C on the scale is imaged onto point C' on the sensor). However, as will be appreciated, the interference fringe pattern / fringe field 110' at the sensor 300 (i.e., the so-called "image") will have a sinusoidal form, whereas a perfect image of the scale would have a square wave form. Furthermore, preventing the 0th diffraction order (and diffraction orders greater than the 1st diffraction order, either by blocking them or by encoding them in the same manner as the 0th diffraction order, for example) from contributing to the modulated portion of the interference fringe pattern / fringe field formed on the sensor means that the image at the sensor 300 has an even more purely sinusoidal form, and therefore, the image on the sensor is not actually a "true" or "perfect" image of the scale 100 (but rather what can be described as a pseudo-image of the scale). Since the encoder 50 effectively blocks all diffraction orders except the + / - 1st diffraction order from contributing to the interference fringe pattern / fringe field 110', the interference fringe pattern / fringe field 110' will (but for lens aberrations) be purely sinusoidal, and the period of the interference fringe pattern / fringe field 110' will be 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 spacing between the scale 100 and the lens 40 changes.

[0065] Figure 4b Alternative embodiments of the present invention are presented. Figure 4b Examples and Figure 4a, except that the polarizing element 60 is located in front of the sensor 300 with its polarization direction arranged to be orthogonal to the polarization direction of the 0th diffraction order so as to suppress / block the 0th diffraction order, but allow the + / - 1st diffraction orders to pass through the sensor. In this case, the 0th diffraction order will not fall on the sensor at all (and similarly, if the diffraction orders greater than the + / - 1st diffraction order are polarized to have the same polarization orientation as the 0th diffraction order, they will not fall on the sensor either). Blocking the 0th diffraction order from reaching the sensor greatly improves the interference fringe pattern sensed by the sensor 300. In fact, as described above in conjunction with the other embodiments, and as Figure 3 As shown, when the 0th diffraction order and the + / -1st diffraction order both fully contribute to the interference fringe pattern / fringe field, the interference fringe pattern / fringe field is modulated to have a visibility of p 2 / λ, where p is the scale period and λ is the wavelength of the light. If the 0th diffraction order is blocked so that it does not fall on the sensor, the modulation no longer exists. Moreover, as Figure 2 As schematically illustrated in FIG1 , removing the 0th diffraction order means that the interference fringe pattern / fringe field 110 ′ generated at the sensor 300 has a period equal to Mp / 2 (which is half that of the system of FIG1 ). Accordingly, blocking the 0th diffraction order effectively doubles the resolution of the system. These are the same effects as described above that would be experienced when encoding the 0th and + / - 1st diffraction orders to have different optical states so that these diffraction orders do not interact / interfere at the sensor. However, as Figure 3 As shown, blocking the 0th diffraction order so that it does not reach the sensor at all has the additional benefit that fringe visibility is significantly improved compared to the case where the 0th diffraction order is still present on the sensor, but has a different optical state than the + / - 1st diffraction orders.

[0066] Figure 4a and Figure 4b The embodiment is a transmissive optical system in which light from the light source 10 is transmitted through the scale 100 . Figure 5a and Figure 5b as well as Figure 6a and Figure 6b The same optical scheme is shown, but for a reflective (or "folded") optical system, where the 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 a conjugate between the scale 100 and the sensor 300. Figure 5a / Figure 6a In the view shown, the portion of the diffraction order encoder encoding the 0th diffraction order appears to coincide with the light source 10, but as shown in FIG. Figure 5b / Figure 6b As shown, it is actually offset along the y-axis.

[0067] In the embodiments of Figures 4, 5 and 6, the optical system is such that an image of the light source 10 is formed at the plane fp2 where the diffraction order encoder 50 is located. However, this need not necessarily be the case. Figure 2 This is not the case in the embodiment of FIG.

[0068] Figure 7 There is schematically illustrated a readhead 400 incorporating an optical arrangement according to the invention, the readhead being arranged to read a scale 100. As will be appreciated, the scale 100 will be fixed to a first part of a machine (not shown) and the readhead 400 will be fixed to a second part of the machine (not shown), wherein the first and second parts of the machine are moveable relative to each other, in this case along the x-axis.

[0069] As shown, the scale 100 includes a series of scale markings defining an incremental scale track. In this embodiment, the encoder device is a diffraction-based optical encoder. Therefore, the incremental scale track includes a series of periodically arranged features that form a diffraction grating. Incremental scale tracks are commonly referred to as amplitude-type scales. As will be understood, in amplitude-type scales, the features are configured to control the amplitude of light reflected (or transmitted in the case of a transmission-type scale) toward a sensor of a readhead, for example by selectively absorbing, scattering, and / or reflecting light. This is in contrast to phase-type scales, in which the features are configured to control the phase of light reflected (or transmitted) toward a sensor of a readhead (for example, by delaying the phase of the light). Although not shown, as is known in the art of position measurement encoders, one or more reference marks may be provided in adjacent scale tracks (for example, as described in US Pat. No. 1,028,1301), or embedded within the incremental scale track (for example, as described in US Pat. No. 7,659,992). In this case, the readhead will have at least one sensor for detecting the reference mark.

[0070] Figure 8 Schematically shows Figure 5a and Figure 5b The parts in Figure 7 As shown, the sensor 300 is mounted on a printed circuit board assembly (PCB) 410, and the diffraction order encoder 50 and the light source 10 (which is located behind the diffraction order encoder 50 in the orientation shown) are mounted on a substrate (in this case a glass substrate 420). Figure 9 A plan view of the glass substrate 420 is shown in FIG, which also illustrates the arrangement of the light source 10 and the diffraction order encoder 50. Figure 9 In the embodiment shown, the diffraction order encoder 50 comprises an opaque area on the glass (at Figure 94 , 5 , and 6 , the diffraction order encoder 50 is configured such that, according to the optical diagrams of FIG. 4 , FIG. 5 , and FIG. 6 , the 0th diffraction order from the scale passes through the first elongated transparent slit 54a (and is thereby encoded to have the first polarization orientation), and the + / -1st diffraction order passes through the second elongated transparent slit 54b and the third elongated transparent slit 54c (and is thereby encoded to have the second polarization orientation orthogonal to the first polarization orientation). Other diffraction orders (i.e., orders greater than + / -1st) are blocked by the opaque region 52. The opaque region 52 may comprise a very thin (e.g., 200 nm thick) metal layer on the glass substrate 420. Alternatively, the opaque region 52 may comprise a thin metal spacer with chemically etched slits. As will be appreciated, the opaque region 52 need not actually be opaque, but opaque means that other diffraction orders are blocked, which may be beneficial.

[0071] In the embodiment described, the light source 10 is a light emitting diode ("LED"), in particular an infrared LED. However, as will be appreciated, other types of light sources, such as a surface emitting laser (SEL), a vertical cavity surface emitting laser (VCSEL), or an edge emitting laser, may be used instead of an LED.

[0072] In the described embodiment, an interference fringe pattern is formed at the sensor 300. However, this need not necessarily be the case. For example, the optical system can be configured so that the diffraction orders produce multiple modulated light spots at the sensor (whose intensities vary / are modulated as the relative positions of the readhead and scale change). Similar to the system that produces the interference fringe pattern, in the system that produces the modulated light spot, the 0th and + / -1st diffraction orders are encoded so that they do not interfere with each other at the sensor, effectively doubling the resolution of the encoder system (because the intensity of the light spot is modulated to have a frequency of p / 2). Similarly, encoding the diffraction orders above the + / -1st diffraction order to have an optical state different from the + / -1st diffraction order reduces / eliminates harmonics in the modulated light spot intensity.

[0073] In the depicted embodiment, sensor 300 is in the form of an electrical grating, in other words, an array of light sensors comprising two or more groups of interdigitated / staggered / interwoven light-sensitive sensor elements (also referred to herein as "photodetectors" or "fingers"). For example, each group can detect a different phase of an interference fringe pattern / fringe field (schematically represented by wave 110) at sensor 300. Figure 11 An example of an electric grating is shown in FIG, wherein a portion of an electric grating is shown and wherein the fingers / photodiodes in four groups of photodiodes (A, B, C and D) are interdigitated / interleaved to form an array of sensor elements extending along the length “L” of the sensor.

[0074] The outputs from each finger / photodiode in a group 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 signals SIN and COS. 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. Although in a particular embodiment, the electric grating includes four groups of photodiodes that provide four channels A', B', C', and D', this need not necessarily be the case. For example, the electric grating may include two groups of photodiodes that provide only two channels, A' and B'.

[0075] As will be appreciated, other types of sensors may be used in place of the electrical grating described above. For example, in embodiments where the modulated light spot is created by the optical system of the read head rather than by an interference fringe pattern, a bulk sensor photodiode may be used to detect the intensity of the modulated light spot (e.g., as described in US 4776701).

[0076] In the above-described embodiment, the 0th and + / -1st diffraction orders are encoded to have orthogonal polarization orientations, meaning that they do not interact / interfere at all at the sensor 300. However, this need not necessarily be the case. For example, depending on the particular device configuration and / or desired performance enhancement, sufficient benefit may be obtained by only partially reducing the extent to which the 0th diffraction order can interact / interfere with the + / -1st diffraction orders, such as by reducing the extent to which the 0th diffraction order can interact / interfere with the + / -1st diffraction orders such that the effect of the 0th diffraction order on the modulated portion of the signal sensed by the sensor (e.g., the interference fringe pattern) is reduced by at least 50%, but preferably the effect of the 0th diffraction order on the modulated portion of the signal sensed by the sensor (e.g., the interference fringe pattern) is reduced by at least 90%, such as at least 95%, or at least 98%. Accordingly, in such an alternative embodiment, the diffraction order encoder 50 can be configured to polarize only the 0th diffraction order (correspondingly, regions 54b and 54c can be left empty, without a polarizer, so that the + / -1st diffraction order is not polarized). In such an embodiment, the 0th diffraction order has an optical state (i.e., a polarization state) that is different from the optical state (i.e., a non-polarized state) of the + / - 1st diffraction orders. Doing so will reduce the intensity of the 0th diffraction order, thereby reducing the extent to which it interacts with the + / - 1st diffraction orders, providing some improvement in the signal sensed by the sensor.

[0077] In another alternative embodiment, the diffraction order encoder 50 can be configured to polarize only the 0th diffraction order (correspondingly, regions 54b and 54c can be left blank, with no polarizer present, so that the + / - 1st diffraction orders are not polarized), and a polarization filter can be provided in the read head, which is appropriately configured so that it blocks the polarized 0th diffraction order from reaching the sensor 300.

[0078] 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 polarization axes arranged orthogonally, which will provide some improvement to the signal sensed by the sensor compared to the case where neither is polarized (but of course, since there is less interaction between the 0th diffraction order and the + / -1st diffraction orders, the greater the benefit that can be obtained the greater their orthogonality).

[0079] exist Figure 4a 5 , the 0th diffraction order is sensed by the sensor 300 , and the signal output by the sensor 300 includes the 0th diffraction order signal information. Figure 4b 6 prevents the 0th diffraction order from reaching the sensor by a polarization filter placed in front of the sensor. In an alternative embodiment, the sensor itself can be configured so that it does not sense the 0th diffraction order (e.g., by an integrated polarizer on the sensor), thereby eliminating the need for a separate polarization filter placed in front of the sensor.

[0080] In the above-described embodiment, the light source 10 emits electromagnetic radiation (EMR) in the near-infrared range. However, as will be appreciated, this need not be the case, and EMR in other ranges (e.g., anywhere from the infrared to the ultraviolet) may be emitted. As will be appreciated, the selection of a suitable wavelength for the light source 10 may depend on a number of factors, including the availability of suitable gratings and detectors that operate at the EMR wavelength.

[0081] In the above embodiment, the light source 10 emits unpolarized light, the light irradiated to the scale is unpolarized, and the diffraction orders emerging from the scale 100 are unpolarized. However, this need not necessarily be the case. For example, the light source 10 may emit polarized light, or a polarization device may be placed between the light source 10 and the diffraction order encoder 50. In this case, the diffraction order encoder may include one or more optical devices (e.g., one or more wave plates) for changing the polarization orientation of one or more of the diffraction orders. For example, the first transparent region 54a of the diffraction order encoder 50 may include a half-wave plate that rotates the polarization orientation of the 0th diffraction order by 90°, and the second transparent region 54b and the third transparent region 54c may be transparent and configured not to change the polarization orientation of the + / -1st diffraction orders (thereby causing the 0th diffraction order and the + / -1st diffraction orders to have orthogonal polarization orientations).

[0082] In the above embodiments, the primary focus is on encoding the 0th and + / -1st diffraction orders so that they have different optical states. However, the present invention can be used to encode different combinations of diffraction orders. For example, the diffraction orders can be encoded so that the 0th and + / -1st diffraction orders have the same optical state, but other diffraction orders (e.g., the 3rd and / or 5th diffraction orders) have different optical states than the 0th and + / -1st diffraction orders.

[0083] Figure 4 to Figure 9 The above embodiment of uses a refractive lens 40 to relay the diffraction orders towards the sensor. However, it will be appreciated that this need not necessarily be the case. For example, Figure 12 The optical system of an encoder device comprising an incremental scale 100 and a readhead 400' according to another embodiment of the present invention is schematically illustrated, the encoder device being similar to the embodiment of FIG6 (and similar parts sharing similar reference numerals) except that it does not rely on a refractive lens 40 to relay the diffraction orders towards the sensor. In this embodiment, the 0th and + / -1st diffraction orders are generated by light from a light source impinging on and being reflected by the scale 100. As shown in the figure, the 0th and + / -1st diffraction orders are generated in a first plane pl 1 The diffraction order encoder 50 is located on the first plane pl1 According to the embodiment of FIG6 , the diffraction order encoder includes a polarization filter 54a (e.g., having a “vertical” polarization axis) that coincides with the 0th diffraction order so as to encode the 0th diffraction order having a first (e.g., vertical) polarization orientation. The diffraction order encoder further includes a second polarization filter 54b and a third polarization filter 546c that coincide with the +1st diffraction order and the -1st diffraction order. The second polarization filter 54b and the third polarization filter 54c are configured to have the same orientation as each other, so that the + / -1st diffraction orders have the same (e.g., “horizontal”) polarization orientation as each other, but are different from the polarization orientation of the 0th diffraction order. The diffraction grating G2 is positioned to converge the polarizations of the + / -1st diffraction orders. The 0th diffraction order is not deflected. Accordingly, the 0th diffraction order and the + / -1st diffraction order are in the plane pl where the sensor 300 is located. 2 According to the embodiment of FIG6 , the first polarizer 60 may be located in front of the sensor 300 and may be configured to filter out the 0th diffraction order so that it does not reach the sensor 300. For clarity of illustration, Figure 12 The light source 10 is omitted in FIG. As will be appreciated, a separate refractive lens or grating may be used to collimate the light from the light source towards the scale if desired or necessary.

[0084] In the above embodiments, the scale comprises an incremental scale without any reference marks. Of course, the scale may comprise one or more reference marks, such as one or more optical reference marks. Thus, the read head may comprise one or more reference mark sensors. The encoding of the diffraction orders as described above may provide benefits for detecting the reference marks. For example, encoding the diffraction orders as described above may change the form of the reconstructed signal, which may be beneficial for reference mark detection. If desired, one or more filters may 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, the filter may be placed in front of the incremental sensor and configured as described above in conjunction with Figure 4b 6 , but without using a filter in front of the reference mark sensor. Additionally or alternatively, a filter can be placed in front of the reference mark sensor and configured to block the encoded + / -1st diffraction orders.

[0085] Figure 13An example encoder device 800 including a reference mark and an associated reference mark sensor is shown. In this embodiment, a readhead 806 (whose body is omitted for clarity) includes a light source 812 (in this embodiment, in the form of a light emitting diode (LED) (which emits infrared light in this embodiment)), a lens 814, a diffraction order encoder 816 (also referred to as an "optical spatial encoder"), a first (hereinafter referred to as "primary") position information sensor 818, a second (hereinafter referred to as "auxiliary") position information sensor 820, and a first ("primary") sensor filter 822. The light source 812 and the diffraction order encoder 816 are disposed in substantially the same plane, on an opaque substrate 815. As will be appreciated, this region of substrate 815 need not actually be opaque, but opacity, meaning that other diffraction orders are blocked, may be beneficial. The primary position information sensor is configured to detect the signal / resultant field generated by the incremental feature 810 and output a signal from which the incremental position can be determined / monitored, and the secondary position information sensor is configured to detect the signal / resultant field generated by the reference mark 811 and output a signal indicating the presence of the reference mark. Accordingly, primary position information sensor 818 can be (and will be hereinafter) referred to as incremental sensor 818, and auxiliary position information sensor 820 can be (and will be hereinafter) referred to as reference mark sensor 820. Light from LED 812 is emitted toward scale 804 via lens 814. LED 812 is positioned substantially at focal plane fp of lens 814, such that the light from LED 812 is substantially collimated by lens 814. Light from LED 812 illuminates footprint 824 on scale. Lens 814 relays light reflected from scale 804 to incremental sensor 818 and reference mark sensor 820. Diffraction orders are incident on lens 814, which causes the corresponding diffraction orders to converge to form light spots 850 at the back focal plane fp of lens 814; each light spot is formed by a corresponding diffraction order. Light spots 850 are images of the light source because the back focal plane fp is a conjugate plane to the plane in which the light source lies.

[0086] The diffraction order encoder 816 is located at the same location as the light spot 850. Specifically, in this embodiment, the diffraction order encoder 816 includes a polarization filter 816a (e.g., having a "vertical" polarization axis) that overlaps with the 0th diffraction order light spot 250a, and a first non-polarized transparent region 816b and a second non-polarized transparent region 816c that overlap with the + / -1st diffraction order light spots 850b and 850c. Accordingly, the diffraction order encoder 816 encodes the 0th diffraction order using vertically polarized light while leaving the + / -1st diffraction orders unpolarized. Accordingly, the 0th diffraction order has an optical state that is different from the optical state of the + / -1st diffraction orders. As shown, the first polarization filter 816a, the second polarization filter 816b, and the third polarization filter 816c are spaced apart from each other so that they do not directly contact each other, but this does not necessarily have to be the case.

[0087] The 0th and + / -1st diffraction orders propagate toward incremental sensor 818 and reference mark sensor 820. As described above, primary sensor filter 822 is located in the optical path of the diffraction orders traveling toward incremental sensor 818. Primary sensor filter 822 comprises a polarization filter having a "horizontal" polarization axis (i.e., a polarization axis orthogonal to the polarization axis of first polarization filter 16a). Accordingly, the 0th-order diffraction light is substantially blocked from reaching the primary position information sensor. In contrast, the + / -1st-order diffraction light can pass through primary sensor filter 822 (but becomes "horizontally" polarized in the process) and thereby impinge upon incremental sensor 818. In particular, the + / -1st-order diffraction orders propagate toward incremental sensor 818 and interact (interfering constructively and destructively) to form an interference fringe pattern (or "fringe field") that impinges upon incremental sensor 818.

[0088] In the described embodiment, no corresponding filter is placed in front of reference mark sensor 820 (which, in this embodiment, includes a segmented detector comprising first photodiode 820a and second photodiode 820b). Accordingly, both the 0th and + / - 1st diffraction orders fall on reference mark sensor 820. Depending on various factors (described in more detail below), in some cases, it may be advantageous not to place a corresponding filter in front of the reference mark sensor. In fact, if the presence of both the 0th and + / - 1st diffraction orders falling on the reference mark sensor has no adverse effect on the optical signal (e.g., "image" or "pseudo-image" (see later)) formed at reference mark sensor 820, then there is no advantage in placing a corresponding filter in front of reference mark sensor 820 (and, in fact, may be disadvantageous due to reduced light throughput and / or providing a clearer reference mark image).

[0089] It should be noted that in this embodiment, because the 0th diffraction order has already been polarized by the first polarizer 816a, the 0th and + / -1st diffraction orders are non-uniformly attenuated. If desired, the non-uniform attenuation can be avoided by replacing the first non-polarization transparent region 816b and the second non-polarization transparent region 816c with a second polarization filter 816b and a third polarization filter 816c having a "horizontal" polarization axis (i.e., having a polarization axis orthogonal to the polarization axis of the first polarization filter 816a). In such an embodiment, the 0th and + / -1st diffraction orders falling on the reference mark sensor 820 will also be attenuated, which can improve the image quality falling on the reference mark sensor. In addition, the + / -1st diffraction orders will still pass through the main sensor filter 822 and interfere to form an interference fringe pattern on the incremental sensor 18. In another alternative embodiment, a second ("auxiliary") sensor filter can be placed before the second / reference mark sensor 820 to filter out one or more diffraction orders. For example, in some cases (e.g. if the reference mark is structured / patterned) it may be beneficial to filter out the + / - 1st diffraction orders (and optionally higher diffraction orders) so that the reference mark signal formed at the reference mark sensor is formed only by the 0th diffraction order.

[0090] Figure 10 Shows the above Figure 13 8, except that the main sensor filter 822 has been replaced by 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 (primarily) allows only vertically polarized light to pass directly toward the reference mark sensor 820, and (primarily) allows only horizontally polarized light to be deflected toward the incremental sensor 818. Thus, the 0th diffraction order (which has been vertically polarized 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 (because it has been substantially blocked by the polarizing beam splitter 870), while both the 0th diffraction order and the + / - 1st diffraction orders contribute to the signal formed at the reference mark sensor 20. The exact position of the polarizing beam splitter is not critical, as long as it is disposed between the diffraction order encoder 816 and the detector plane. Accordingly, in some applications it may be advantageous to place the diffraction order encoder substrate on the front face of the polarizing beam splitter and / or to place the detectors 818, 820 on the exit face.

[0091] In another alternative embodiment, the scale may comprise an absolute scale. A readhead for an absolute scale may be configured as described above, for example so that the diffraction orders from the scale are encoded to have different optical states. This may prevent the differently encoded diffraction orders from interfering / interacting with each other, thereby improving the signal sensed by the sensor. For example, in this case, it may be beneficial to encode the diffraction orders in the absolute encoder so that the + / -1st diffraction order and the 0th diffraction order have different optical states. Again, this may mean that they do not interact with each other, thereby improving the signal of interest on the sensor. In a particularly preferred embodiment of the absolute encoder, the + / -1st diffraction order (and any further diffraction orders, if present to a significant extent)) is substantially attenuated / blocked from reaching the sensor of the readhead (or the sensor ignores the optical state of the + / -1st diffraction order) so that substantially only the 0th diffraction order light falls on the sensor of the readhead. For example, a suitably configured polarizer may be located in the sensor to filter out the + / -1st diffraction order.

[0092] All of the above embodiments rely on polarization to encode different diffraction orders into different optical states. However, this does not necessarily have to be the case. For example, different wavelengths can be used to encode different diffraction orders. Figure 14 This embodiment is shown. The encoder 900 of this embodiment is Figure 13 The embodiments of have some similarities and similar parts share the same reference numerals. In this embodiment, reference mark 911 comprises a structured / patterned reference mark and therefore reference mark sensor 920 comprises a corresponding pattern of photodiodes 920a, 920b, 920c that are configured such that when the read head passes over reference mark 911, the image of reference mark 911 registers with the photodiodes, causing the intensity of light falling thereon to change dramatically. Instead Figure 138. The single LED 812 of an embodiment of the present invention includes a red LED 912 and a blue LED 913, the light from which is collimated by a lens 814 and illuminates a footprint 824 on the scale 904. In this embodiment, an auxiliary sensor filter 923 is provided in front of the reference mark sensor 920. In this embodiment, the diffraction order encoder 916 and the main sensor filter 922 and the auxiliary sensor filter 923 include wavelength (e.g., color) filters as opposed to polarization filters to selectively control which diffraction orders contribute / do not contribute to the signals formed at the respective incremental sensor 818 and the 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 0th order using only blue wavelengths, and the second wavelength filter 916b and the third wavelength filter 916c include red filters that encode the + / - 1st diffraction orders using only red wavelengths. Accordingly, to prevent the 0th diffraction order from contributing to the signal formed at incremental sensor 818, primary sensor filter 922 includes a red wavelength filter to allow only light having a red wavelength to pass through. Similarly, to prevent the + / - 1st diffraction order from contributing to the signal formed at reference mark sensor 920, auxiliary sensor filter 923 includes a blue wavelength filter to allow only light having a blue wavelength to pass through. As described above in connection with other embodiments, primary sensor filter 922 or auxiliary sensor filter 923 may be omitted if desired. As will be appreciated, while red and blue wavelengths are used in this example, other wavelengths may be used. Furthermore, a single polychromatic light source may be used rather than two different monochromatic light sources.

[0093] The embodiments described so far have been in conjunction with incremental encoders comprising an incremental scale with (or optionally without) one or more reference marks. The invention can also be used with absolute encoders comprising an absolute scale, such as in Figure 15 The absolute encoder depicted in Figure 15 In an embodiment of the invention, the absolute scale 504 comprises features that are conceptually periodically arranged, but where selected features have been removed to encode unique / absolute position data along the measured length of the scale. The data may be in the form of, for example, a pseudo-random sequence or discrete codewords. Details of such a scale are described in more detail in US 7499827 and US 5279044. The readhead 506 (the body of which has been omitted for clarity) shares some of the same parts as those described above in connection with other embodiments of the invention, and similar parts therefore share the same reference numerals. For example, the readhead 506 comprises a light source 10 (in this embodiment a non-polarized LED light source), a lens 14 and a diffraction order encoder 50.

[0094] In this embodiment, the readhead 506 includes an absolute sensor 520 comprising a photodiode array (in this embodiment a one-dimensional photodiode array, but it could also be a two-dimensional photodiode array) onto which an image (or "pseudo-image") falls. As is known, and as described, for example, in US 7499827, US 5279044, and US 10989567, the image of the scale can be processed to extract the absolute / unique code and thereby determine the absolute position.

[0095] Similar to the reference mark embodiment described above, it may be beneficial to allow the 0th diffraction order to contribute to the signal falling on the absolute sensor. In addition, similar to the reference mark embodiment described above, it may be beneficial to prevent the + / - 1st diffraction order (and optionally higher diffraction orders) from contributing to the signal falling on the absolute sensor 520. Accordingly, an auxiliary sensor filter 523 can be provided in front of the absolute sensor 520, which has a polarization axis that is orthogonal to the polarization axis of the second polarization filter 54b and the polarization axis of the third polarization filter 54c of the diffraction order encoder 50. In particular, in this embodiment, the second polarization filter 54b and the third polarization filter 54c have horizontal polarization axes, and the auxiliary sensor filter 523 has a vertical polarization axis.

[0096] In this embodiment, the readhead 506 further includes an optional incremental detector 300 and a main sensor filter 60, which are configured and arranged in the same manner as described above in conjunction with the embodiment of FIG. 6 . The absolute position can be combined with the incremental position determined from the incremental detector 300 to provide a finer-pitch absolute position. Alternatively, once the absolute position has been determined, subsequent positions can be determined solely by monitoring the output from the incremental detector 300. According to the embodiment of FIG. 6 , the 0th diffraction order is blocked so that it does not contribute to the formation of the signal detected by the incremental sensor 18. As with the other embodiments, the main sensor filter 60 includes a polarization filter having a polarization axis perpendicular to the polarization axis of the first polarization filter 54 a of the diffraction order encoder 50. In particular, in this embodiment, the first polarization filter 54 a has a vertical polarization axis, and the main sensor filter 60 has a horizontal polarization axis.

[0097] Figure 16 Another embodiment of an absolute encoder 600 including an absolute scale 604 is shown, which is combined with the above Figure 15The described absolute encoder is essentially the same (and similar parts share the same reference numerals), except that in this embodiment, the scale comprises a multi-track arrangement, wherein absolute encoding features are provided in an absolute scale track 603, which is provided on either side of a purely incremental scale track 605. As shown, the absolute scale track 603 also includes finer pitch / period incremental features in the spaces between the coarser pitch / period features of the absolute scale track 603. As an example, the coarse features in 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. Thus, effectively two sets of diffraction orders are 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). When this occurs, in this embodiment, the 0th diffraction order in the first group of diffraction orders passes through the first polarization filter 54a of the diffraction order encoder 50, and the + / - 1st diffraction order in the first group of diffraction orders passes through the second polarization filter 54b and the third polarization filter 54c, while the 0th diffraction order and the + / - 1st diffraction order in the second group of diffraction orders both pass through the first polarization filter 54a of the diffraction order encoder 50. The higher diffraction orders in the first group of diffraction orders (e.g., the + / - 3rd diffraction order, the + / - 5th diffraction order) are blocked by the opaque substrate 15. The + / - 3rd diffraction order in the second group of diffraction orders passes through the second polarization filter 54b and the third polarization filter 54c of the diffraction order encoder 50. The diffraction orders greater than the + / - 3rd diffraction order in the second group of diffraction orders have substantially insignificant power, but the diffraction orders that exist in them will be blocked by the opaque substrate 15. As will be appreciated, which diffraction orders pass through which filter depends on the system and can depend on various factors, including the size and / or position of the polarization filters.

[0098] As shown in the figure, Figure 15 The sensor arrangement of the embodiment is Figure 14 The sensor arrangement is slightly different in that the absolute sensor 620 read head is split into two parts, with the incremental sensor 300 located in between. Figure 15 A variation of the embodiment is that the absolute scale feature extends completely over the incremental scale feature (i.e. such that there is no purely incremental scale track 605 between two absolute scale tracks 603). Instead, the absolute scale feature may be completely embedded over the entire incremental feature, e.g. Figure 14In this case, since the incremental position feature and the absolute position feature have different nominal periods in this embodiment, the above comments about the two sets of diffraction orders still apply. If desired, in an alternative configuration, the incremental sensor 300 and the absolute sensor 620 can be configured according to Figure 14 arrangement of embodiments (i.e., two sensors side by side).

[0099] The invention has been described above in conjunction with linear encoder devices. However, the invention is equally applicable to rotary encoder devices, including both ring encoders (where the scale features are formed on the cylindrical outer surface of a ring member) and disc encoders (where the scale features are formed on the planar surface of a disc member).

Claims

1. A position measurement encoder device comprising: a scale comprising a series of position features readable by a readhead, the series extending along a measurement dimension and diffracting light into a plurality of diffraction orders; a readhead comprising a light source for illuminating the scale, and at least a first sensor configured to detect a signal produced at the first sensor by one or more diffraction orders produced by a series of positional features of the scale, the series of positional features being usable to determine the relative position of the scale and readhead along the measurement dimension; In which, the read head further includes a diffraction order encoder, which encodes at least one diffraction order generated by a series of position features of the scale, wherein the optical state of the at least one diffraction order is different from the optical state of at least one other diffraction order generated by the series of position features of the scale.

2. The device according to claim 1, wherein The diffraction order encoder encodes the 0th diffraction order to have a different optical state than the + / - 1st diffraction orders.

3. The apparatus of claim 1, comprising at least partially filtering out at least one diffraction order based on its optical state so as to at least partially reduce its influence on the generation of the signal sensed by the first sensor.

4. The device of claims 2 and 3, wherein the device is configured to: i) substantially filter out the 0th diffraction order such that the 0th diffraction order has substantially no effect on the generation of the signal sensed by the first sensor, or ii) substantially filter out the + / -1st diffraction order such that the + / -1st diffraction order has substantially no effect on the generation of the signal sensed by the first sensor.

5. An apparatus as claimed in any preceding claim, wherein The diffraction order encoder encodes the at least one diffraction order to have a polarization state that is different from a polarization state of at least one other diffraction order.

6. An apparatus as claimed in any preceding claim, configured such that different diffraction orders of light relayed to the at least first sensor converge to corresponding different convergence points on the optical path between the scale and the at least first sensor.

7. The apparatus of claim 6, wherein: The convergence point is located at a conjugate plane of the light source.

8. The apparatus according to claim 6 or 7, wherein: The diffraction order encoder is located at the convergence point.

9. Apparatus as claimed in any preceding claim, wherein: The first sensor includes an incremental position sensor configured to sense incremental position signals.

10. An apparatus as claimed in any preceding claim, configured such that diffraction orders greater than the + / - 1st diffraction order are not sensed by the at least first sensor.

11. Apparatus as claimed in any preceding claim, further comprising a second sensor configured to detect a signal produced at the second sensor by the diffraction order from the scale.

12. The apparatus of any preceding claim, comprising a first sensor filter configured to filter light based on its optical state before it falls on the first sensor.

13. The device of claims 11 and 12, comprising a second sensor filter configured to filter the light based on the optical state of the light before it falls on the second sensor, the first sensor filter and the second sensor filter being configured to filter the light in different ways from each other.

14. Apparatus as claimed in any preceding claim, wherein The optical state includes a polarization state.

15. The apparatus according to claim 12 or 13 and 14, wherein The first sensor filter and / or the second sensor filter comprises a polarization filter.

16. A position measuring encoder device comprising: a ruler comprising a series of periodic features configured to diffract light into a plurality of diffraction orders, including a 0th diffraction order; a readhead comprising a light source for illuminating the scale, and at least one sensor configured to detect an interference fringe pattern / fringe field that moves with relative motion of the scale and readhead, the interference fringe pattern / fringe field being produced at the at least one sensor by diffraction orders from the scale; The period of the fringes is Mp / 2, where M is the magnification of the optical system of the encoder device and p is the period of the scale.

17. A position measurement encoder device comprising: a ruler comprising a series of periodic features configured to diffract light into a plurality of diffraction orders, including a 0th diffraction order; a readhead comprising a light source for illuminating the scale, and at least one sensor configured to detect a composite field that varies with relative motion of the scale and readhead, the composite field being generated at the at least one sensor by diffraction orders from the scale; Therein, the read head is configured such that the 0th diffraction order does not contribute to the generation of the composite field at the at least one sensor.

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