Encoder device

Abstract

A position measurement encoder apparatus includes: a scale including a series of position features readable by a read head; and a read head comprising: a polarized light source for emitting polarized light toward the scale; a first sensor configured to sense light filtered along the first polarization axis, the first sensor arranged to sense light from the polarized light source that has interacted with the scale; and a polarization manipulator located on an optical path between the polarized light source and the first sensor, configured such that light exiting the polarization manipulator toward the first sensor will have a polarization state that is at least partially distinguishable along a first polarization axis regardless of the polarization state of light emitted from the polarized light source.

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 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 field / pattern ("fringe field"), modulated dots, or an image, at a detector in the readhead that changes with relative movement. For example, light from a source (within the readhead) can impinge on the scale, diffracting the light into a plurality of diffraction orders. The diffraction orders interfere / interact / recombine at the detector to generate the 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 generating the 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 US Pat. No. 5,861,953.

[0004] Figure 1a The optical operating principle of this common prior art incremental position encoder system is schematically illustrated. A scale 1000 has a series of periodic features of period "p" which, when illuminated by light, produce a set of diffraction orders 1001, 1002, 1003. As will be understood, the light referred to in this document includes visible and invisible light from the ultraviolet range to the infrared range. The diffraction orders 1001, 1002, 1003 are relayed to a sensor 3000 (both the sensor and the relay element are located in a read head that is movable relative to the scale 1000) via a relay element 2000 (e.g. a lens, prism or diffraction grating). The diffraction orders interfere at the sensor 3000, thereby producing an interference fringe pattern / field at the sensor 3000 (schematically illustrated by wave 1100) of period equal to Mp; where M is the magnification of the optical system and p is the scale period. In Figure 1aIn 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 2000 and sensor 3000 are all provided by / arranged in a readhead arrangement that is configured to move relative to the scale 1000.

[0005] 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 field / pattern at the detector (e.g., Figure 1b Schematically shown in ).

[0006] For illustrative purposes, Figure 1a and Figure 1b Only the 0th and + / - 1st orders are shown in FIG. As will be appreciated, higher diffraction orders are produced and can contribute to forming the fringe field at the sensor 3000, although their intensity, and therefore contribution to the fringe field, is typically much weaker than the 0th and + / - 1st diffraction orders (with higher orders gradually decreasing in intensity).

[0007] 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.

[0008] 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 relay element), whereas in reality at least one of these rays may be reflected.

[0009] As will be appreciated, reference marks may for example be provided adjacent to and / or embedded within the diffractive features of the scale in order to provide a defined reference position.Such a position encoder is described in US 7,659,992.

[0010] 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.

[0011] The present invention relates to an improved optical encoder.

[0012] According to a first aspect of the present invention, there is provided a position measuring encoder device comprising: a scale comprising a series of position features readable by a read head; a read head comprising: a polarized light source for emitting polarized light toward the scale; a first sensor configured to sense light filtered along a first polarization axis, the first sensor being arranged to sense light from the polarized light source that has interacted with the scale; a polarization manipulator located in an optical path between the polarized light source and the first sensor, configured such that, regardless of the polarization state of the light emitted from the polarized light source, the light leaving the polarization manipulator toward the first sensor will have a polarization state (e.g., a (e.g., predominant) polarization orientation) that is at least partially distinguishable along the first polarization axis.

[0013] As explained in more detail below, it may be beneficial for the first sensor to be configured to sense light filtered along the first polarization axis because, for example, the first sensor can provide at least some degree of control over the content that falls on the first sensor. For example, this functionality can be utilized to improve (the quality of) the signal received by the first sensor.

[0014] At the same time, optical encoders typically use non-polarized light sources. In fact, as of the time of writing, all optical encoders sold by Renishaw plc utilize light emitting diodes (LEDs) that emit non-polarized light. However, the present invention provides an alternative optical encoder that utilizes a polarized light source to emit light that illuminates the scale. Optionally, the light source emits linearly polarized light. For the purposes of this patent application, a polarized light source means a light source that emits light with a degree of polarization of at least 0.2. (In other words, optionally, the degree of polarization of light emitted from the polarized light source is not less than 0.2). Similarly, for the purposes of this patent application, polarized light means light with a degree of polarization of at least 0.2. The light source may include, for example, a laser light source, such as a vertical cavity surface emitting laser (VCSEL). As will be understood by those skilled in the art, a laser light source (such as a VCSEL) emits polarized light, for example, typically with a degree of polarization of at least 0.3 (typically between 0.3 and 0.7). Compared to LEDs, laser light sources can provide optical power advantages, which have been found to provide improved metrological performance, particularly reduced jitter due to reduced noise.

[0015] Optionally, the light source is a linearly polarized light source; in other words, optionally, the light source emits linearly polarized light.

[0016] As will be understood, the "degree of polarization" (or "DOP," as it will sometimes be referred to herein) of light is a simple metric that quantifies the degree to which the light is polarized. Completely unpolarized light has a DOP of 0 (or 0%), while completely / purely polarized light has a DOP of 1 (or 100%). The DOP of light can be easily measured experimentally, as will now be described. Light passes through a linear polarizer, and the exiting light that has passed through the linear polarizer falls on a photodetector. The linear polarizer is rotated 180 degrees, and a measurement is made of the optical power observed at the photodetector (as the linear polarizer is rotated). If the light has polarization, the optical power at the photodetector will vary in a sinusoidal waveform. The modulation depth of this sinusoidal wave can reveal the DOP. Accordingly,

[0017]

[0018] As is well known, in addition to linearly polarized light, light can be "circularly" polarized or "elliptically" polarized. As will be appreciated, by the above measurements, the DOP of circularly polarized light will be 0. The DOP of pure linearly polarized light will be 1, and the DOP of elliptically polarized light will be between 0 and 1 (excluding its extreme values), depending on the degree of ellipticity of the elliptically polarized light. As will be appreciated, linearly polarized light may not be "pure" or "purely" linearly polarized, and therefore the DOP of linearly polarized light may not be 1.

[0019] As will be understood, linearly polarized light and elliptically polarized light (particularly elliptically polarized light with a DOP of at least 0.2) will have a (e.g., predominant) polarization orientation. Thus, it can be said that the polarization manipulator (which is located in the optical path between the polarized light source and the first sensor) is configured such that, regardless of the (e.g., predominant) polarization orientation of the light emitted from the polarized light source, the light exiting the polarization manipulator toward the first sensor will have a (e.g., predominant) polarization orientation that is at least partially discernible along the first polarization axis.

[0020] As mentioned above, for the purposes of the present invention, a polarized light source can be any light source that emits light (e.g., linearly polarized or elliptically polarized) with a DOP of at least 0.2. However, the DOP of a polarized light source can, of course, be higher. In fact, the higher the DOP of the polarized light source, the greater the advantage of the polarization manipulator.

[0021] Of course, due to the polarization-sensitive nature of the first sensor, and due to the polarization of the light from the light source, if the (e.g., primary) polarization orientation of the light is too large an angle relative to the first polarization axis, a problem that may arise is that the first sensor does not receive enough light (or even receives no light at all). Therefore, the present invention provides a polarization manipulator located in the optical path between the polarized light source and the first sensor, and configured such that, regardless of the polarization state of the light emitted from the polarized light source, the light exiting the polarization manipulator toward the first sensor will have a polarization state that is at least partially resolvable along the first polarization axis (thereby ensuring that the first sensor will receive at least some light).

[0022] The device, and in particular the polarization manipulator, can be configured such that the ratio of: i) the optical power of light distinguishable along a first polarization axis to ii) the optical power of light distinguishable along a second orthogonal polarization axis leaving the polarization manipulator towards the first sensor is at least 2:3, more preferably at least 3:4, more preferably at least 4:5, for example at least 12:13.

[0023] Optionally, the device is configured such that, all other factors being equal, the optical power of the light sensed by the first sensor is substantially the same for all possible (eg, predominant) polarization orientations of light emitted from the polarized light source.

[0024] The readhead may additionally comprise: a second sensor configured to sense light filtered along a second polarisation axis. The second sensor may be arranged to sense light from the polarised light source that has interacted with the scale. The polarisation manipulator may be configured such that, regardless of the polarisation state (e.g. (e.g. predominant) polarisation orientation) of the light emitted from the polarised light source, light exiting the polarisation manipulator towards the second sensor will have a polarisation state (e.g. (predominant) polarisation orientation) that is at least partially distinguishable along the second polarisation axis. It may be beneficial that the second polarisation axis is orthogonal to the first polarisation axis. This means, for example, that each of the first and second sensors can ignore light to which the other sensor is sensitive.

[0025] The device can be configured such that, for all other factors being equal (e.g., for a constant optical power output of the light source), the optical power of the light sensed by the first sensor is substantially the same for all possible (e.g., predominant) polarization orientations of the light emitted from the polarized light source, and the optical power of the light sensed by the second sensor is substantially the same for all possible (e.g., predominant) polarization orientations of the light emitted from the polarized light source. In other words, for all other factors being equal, the optical power of the light sensed by the first sensor and the optical power of the light sensed by the second sensor do not depend on the (e.g., predominant) polarization orientation of the light emitted from the polarized light source. Substantially the same can mean that if the optical power of the light received by the first (or second) sensor is measured for all possible (e.g., predominant) polarization orientations of the light emitted by the light source, the change / variation in the optical power of the light received by the first (or second) sensor will be no more than 25% (more preferably no more than 15%, such as no more than 5%, such as no more than 2%).

[0026] The polarization manipulator may include a diffuser. Advantageously, the polarization manipulator may include a wave plate (also known as a "retarder"). The wave plate may have a substantially uniform fast axis over its range (at least over the range through which the light received by the first sensor and the second sensor (if present) passes). Advantageously, the effective total wave retardation of the polarization manipulator may be n+1 / 4, where n is an integer ≥ 0. Accordingly, in embodiments where the light passes through the wave plate once (on its path from the light source to the first sensor and the second sensor (if present)), it may be advantageous for the wave plate to be a quarter (1 / 4) wave plate. In embodiments where the light passes through the wave plate twice (on its path from the light source to the first sensor and the second sensor (if present)), it may be advantageous for the wave plate to be a one-eighth (1 / 8) wave plate. Similarly, the polarization manipulator may include two one-eighth (1 / 8) wave plates on the light path from the light source to the first sensor (and the second sensor (if present)), wherein the light passes through each of the two one-eighth (1 / 8) wave plates once.

[0027] It may be beneficial if the wave plate is arranged so that its fast axis is arranged at an angle of between 30° and 60° (inclusive) to the first polarization axis, more preferably at an angle of between 40° and 50° (inclusive), particularly preferably at an angle of between 43° and 47° (inclusive), for example at an angle of between 44° and 46° (inclusive), for example at an angle of substantially 45°.

[0028] The readhead may further include a diffractive lens (e.g., a Fresnel lens) or a refractive lens (e.g., a cylindrical / spherical lens). Advantageously, the lens is positioned in the optical path between the polarized light source and the polarization manipulator, such that light from the light source first strikes / passes through the polarization manipulator after having already passed through the lens. This can avoid adverse effects on lens performance (e.g., reduced collimation if the lens is a collimator).

[0029] It may be beneficial to position the polarization manipulator in the optical path before the scale. In other words, it may be beneficial for light from the polarized light source to pass through the polarization manipulator before reaching / irradiating the scale. However, this need not necessarily be the case, and the polarization manipulator may be located in the optical path after the scale. As described above and in detail below, the polarization manipulator may be located in the optical path before and after the scale (i.e., such that light from the light source passes through the polarization manipulator twice).

[0030] As described above, light from the polarized light source may pass through the polarization manipulator twice on its path from the light source to the first sensor. For example, light from the polarized light source may be configured to pass through the polarization manipulator for a first time on its way to the scale, and also pass through the polarization manipulator a second time (e.g., a final time) after it is reflected by the scale. This may be beneficial from a point of view because it may be difficult to manufacture a position measuring encoder device in which the light only passes through the polarization manipulator once on its path from the light source to the first sensor. This is particularly the case in those embodiments where the device also includes other optical components, such as the above-mentioned diffractive (e.g., Fresnel lens) or refractive (e.g., cylindrical / spherical) lenses. In embodiments where the light from the light source passes through the polarization manipulator twice on its path from the light source to the first sensor, it may be beneficial that the polarization manipulator / wave plate comprises a one-eighth (1 / 8) wave plate.

[0031] The positional features of the scale may be configured such that they diffract light into a plurality of diffraction orders. Advantageously, at least one of the diffraction orders may have a polarization state (e.g., a (predominant) polarization orientation) that is different from the polarization state of at least one other diffraction order. The apparatus may be configured such that, since the first sensor is configured to sense light filtered along the first polarization axis, the signal sensed by the first sensor is primarily formed by the selected subset of diffraction orders.

[0032] The device can be configured such that the light sensed by the first sensor includes a diffraction order composition that is different from the diffraction order composition of the light sensed by the second sensor. This can be achieved by: i) the polarization states of the diffraction orders (e.g., different diffraction orders with different polarization states (e.g., different (primary) polarization orientations)); ii) the first sensor is configured to sense light filtered along a first polarization axis; and iii) the second sensor is configured to sense light filtered along a second polarization axis.

[0033] Such a device may provide improved performance over existing position measurement encoder devices. In particular, the signals sensed by the different sensors may be suitably tailored so as to provide an improved / optimized signal for each of the different sensors. For example, while it may be beneficial to suppress the contribution of the 0th diffraction order to the signal sensed by a first sensor (which may be an incremental sensor, for example), it may not be beneficial to do so for a second sensor (which may be a reference mark sensor or an absolute sensor, for example). Accordingly, it is not desirable to completely block such diffraction orders from propagating towards the sensors of the readhead, and so instead the present invention facilitates selective / customized suppression of diffraction orders of different sensors of the readhead based on their polarization state. For example, the 0th diffraction order may have a different polarization state than the + / - 1st diffraction orders. For example, the 0th diffraction order may have a (primary) polarization orientation that is substantially orthogonal to the polarization orientation of the + / - 1st diffraction orders.

[0034] The diffraction order composition can vary between sensors because the extent to which different diffraction orders contribute to / influence the signals sensed by the sensors is different for different sensors. For example, a signal sensed by a first sensor and a signal sensed by a second sensor may both be formed by the 0th diffraction order and the + / - 1st diffraction order, but due to their polarization states, the relative contributions of the 0th diffraction order and the + / - 1st diffraction order to the signal sensed by the first sensor are different from the relative contributions of the 0th diffraction order and the + / - 1st diffraction order to the signal sensed by the second sensor. For example, the device can be configured such that the contribution of the 0th diffraction order to the signal sensed by the first sensor is 50% of the contribution of the 0th diffraction order to the signal sensed by the second sensor.

[0035] While some benefit can 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 a signal formed at a sensor, for example by attenuating the effect of the particular diffraction order on the generation of the signal as described above by at least 50%, it is generally preferred that the effect of the particular diffraction order on the generation of the signal sensed by the sensor be substantially completely 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 one or more particular diffraction orders (e.g., the 0th diffraction order, or the + / - 1st diffraction order) have substantially no effect on the generation of the signal sensed by one of the first sensor and the second sensor, while their effect on the generation of the signal sensed by the other sensor is unaffected. In practice, it has been found that the present invention provides the greatest benefit when the selected diffraction order is substantially completely suppressed for at least one of the sensors, while being substantially unsuppressed for the other sensor.

[0036] Accordingly, the device can be configured such that the signals sensed by the first sensor and the second sensor can be formed / composed of different subsets of diffraction orders based on the polarization states / orientations of the diffraction orders. For example, in one embodiment, the signal sensed by the first sensor can be formed by one or more diffraction orders other than the 0th diffraction order (i.e., the 0th diffraction order does not contribute to the signal sensed by the first sensor), while the 0th diffraction order does contribute to the signal sensed by the second sensor (optionally together with other diffraction orders).

[0037] In the field of encoder devices it is often the case that the 0th diffraction order and the + / - 1st diffraction orders have a significant influence on the signal formed at the sensor / the signal sensed by the sensor. Accordingly, in a preferred embodiment of the present invention, the 0th diffraction order has a polarization orientation that is different (e.g. orthogonal) (predominantly) from the polarization orientation of the + / - 1st diffraction orders. This can enable selective control of the extent to which the 0th diffraction order and / or the + / - 1st diffraction order influence or contribute to the signal sensed by the sensor, or whether they influence or contribute to the signal sensed by the sensor. Although the +1st diffraction order can be encoded to have a different polarization state than the -1st diffraction order, typically it is beneficial to configure the read head so that they have the same polarization state.

[0038] A series of positional features of the scale can diffract light into a plurality of diffraction orders. The device can be configured so that at least one diffraction order has a polarization state that is different from the polarization state of at least one other diffraction order. The polarization state of the diffraction order can be assigned / encoded by the scale. Accordingly, the scale can include / can be a diffraction order encoder. For example, the scale can include a birefringent scale or a holographic scale, such as the scale described in U.S. Patent Application Publication US2003 / 0141441, which includes a polarization hologram recorded on the scale so that the polarization orientation of the + / - 1st diffraction order is rotated 90° relative to the 0th diffraction order.

[0039] Advantageously, the readhead may include a diffraction order encoder that encodes at least one diffraction order produced by a series of positional features of the scale, wherein the polarisation state of the at least one diffraction order is different from the polarisation state of at least one other diffraction order produced by the series of positional features of the scale. Providing a diffraction order encoder for the readhead may be simpler and less expensive than providing a holographic scale. Further details of such a diffraction order encoder are provided below.

[0040] The polarization manipulator may comprise, for example, a "patterned retarder" (in other words, a "non-uniform retarder" or a "structured retarder"). Thus, in contrast to the embodiments described above, the polarization manipulator may be configured such that the light output therefrom comprises a mixture of polarization orientations across its coverage area / range. To achieve this, the polarization manipulator (e.g., an optical retarder element) may comprise a non-uniform fast axis and / or a non-uniform retardation amount (or wave plate fraction) that varies along at least one axis / dimension. In other words, a polarization manipulator (e.g., an optical retarder element) according to the present invention may comprise a fast axis and / or retardation amount that is non-uniform along at least one axis / dimension. For example, a polarization manipulator may comprise a series of wave plate pixels with different fast axes. As will be understood, the use of the terms "pattern" and "patterned" in this document, particularly in conjunction with a polarization manipulator, is not intended to imply the presence of any kind of repetitive configuration. Instead, the terms "pattern" and "patterned" are used to indicate that the polarization manipulator (e.g., a retarder) has a non-uniform design / form, in particular a non-uniform fast axis. As explained in more detail below, the design / form / pattern of the (fast axis) of the polarization manipulator (e.g., retarder) may be repeating, or it may be beneficial for it not to be repeating. The patterned retarder may be configured such that there is a large variation in the angle of the fast axis along at least one axis / dimension. However, this need not necessarily be the case. For example, it has been found that for light to pass twice through a patterned retarder having one-eighth (or 1 / 8) wave plate pixels (e.g., according to Figure 8 or Figure 14For systems with optical schemes such as those described above, it has been found beneficial that the angular range of the fast axis is very small. For example, it has been found beneficial that the fast axes of the wave plate pixels are all angled such that they are at an angle of approximately 45° + / - 1°.

[0041] The device can be configured such that a sensor (e.g., a first sensor and / or a second sensor) is inherently at least partially blind to a particular polarization state / orientation (and thus inherently at least partially blind to one or more diffraction orders) (e.g., by having an integrated polarizer on the sensor having the first polarization axis, the first sensor can be blind to polarized light having a polarization orientation orthogonal to the first polarization axis). Accordingly, the composition of the signal sensed by the sensor can be determined by the degree to which the sensor is at least partially blind to the polarization state / orientation. Optionally, the encoder device includes a first sensor polarization filter configured to at least partially (e.g., substantially) filter light along the first polarization axis based on its polarization state / orientation before it falls on the first sensor. For example, in those embodiments where the readhead includes the first sensor and the second sensor, the readhead can include a first sensor polarization filter (having the first polarization axis) positioned before / in front of the first sensor and / or a second sensor polarization filter (having the second polarization axis) positioned before / in front of the second sensor. Accordingly, the diffraction orders can be selectively at least partially (e.g., substantially) filtered out by the first sensor filter and / or the second sensor filter based on their polarization state / orientation. Thus, light falling on the first sensor can be filtered differently than light falling on the second sensor. If both a first sensor filter and a second sensor filter are provided, they can be configured to at least partially (e.g., substantially) filter out different diffraction orders based on their polarization states. Accordingly, the composition of the signals sensed by the first sensor and / or the second sensor can be determined / controlled by the filters.

[0042] The first sensor can be configured to sense light filtered along a first polarization axis such that (substantially all) 0th diffraction order light does not contribute to a signal sensed by the first sensor and / or output by the first sensor. For example, the 0th diffraction order can be prevented from reaching the first sensor (e.g., the 0th diffraction order can be filtered out by the first sensor polarization filter). For example, optionally, the first sensor polarization filter prevents (substantially all) 0th diffraction order light from reaching the first sensor (in other words, it substantially filters out the 0th diffraction order).

[0043] Optionally, the second sensor is configured to sense light filtered along the second polarization axis such that (substantially all) + / -1st diffraction order light does not contribute to a signal sensed and / or output by the second sensor. For example, the + / -1st diffraction order can be prevented from reaching the second sensor (e.g., the + / -1st diffraction order can be filtered out by the second sensor polarization filter). For example, optionally, the second sensor polarization filter prevents (substantially all) + / -1st diffraction order light from reaching the second sensor (in other words, it substantially filters out the + / -1st diffraction order).

[0044] As described in more detail below, diffraction orders greater than the + / - 1st diffraction order can be substantially suppressed / filtered out / prevented from contributing to the signal sensed and / or output by the first sensor and / or the second sensor (e.g., can be prevented from reaching the first sensor and / or the second sensor). Accordingly, the signal formed at the first sensor can be primarily (e.g., solely) formed by the + / - 1st diffraction order. In other embodiments, the signal formed at the (e.g., second) sensor can be primarily (e.g., solely) formed by the 0th diffraction order.

[0045] The above-mentioned diffraction order encoder may include at least one polarizer element, which is configured to encode at least one diffraction order, the polarization state of which is different from the polarization state of at least one other diffraction order. In a particularly preferred embodiment, the diffraction order encoder includes a first polarizer element and one or more additional (e.g., second and third) polarizer elements, the first polarizer element being configured to encode the 0th diffraction order having a first polarization state (first principal polarization orientation), and the one or more additional polarizer elements being configured to encode the + / -1st diffraction order having a second polarization state (second principal polarization orientation) different from the first polarization state (orientation). Optionally, the polarizer element includes a polarizer, which is configured to polarize at least one diffraction order (e.g., increase its polarization degree). Optionally, the polarizer element includes at least one polarization manipulator (e.g., a wave plate or a retarder), which is 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.

[0046] In embodiments where the readhead comprises a diffraction order encoder, the polarised 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).

[0047] The readhead may comprise at least one optical relay element for relaying light (e.g. said diffraction orders) from the scale towards the first sensor and / or the second sensor. For example, the readhead may comprise at least one refractive and / or diffractive optical relay element. Suitable optical relay 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 relay element or after the optical relay element. Optionally, the diffraction order encoder and the optical relay element may be the same item (e.g. at least one optical relay element may include a diffraction order encoder integrated therein).

[0048] Optionally, the device is configured so that the diffraction orders converge onto corresponding / corresponding light spots (or "different convergence points") on the optical path before the first sensor and / or the second 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 an optical relay element (focal plane of a lens) for relaying 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 relay element, such as at the focal plane of the optical relay 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.

[0049] The polarization states (e.g., (predominant) polarization orientations) of diffraction orders greater than the + / - 1st diffraction order can be configured so that the extent to which they influence or contribute to the signals sensed by the first and / or second sensors, or whether they influence or contribute to the signals sensed by the first and / or second sensors, can be selectively controlled. Such diffraction orders can include, for example, the + / - 3rd and / or + / - 5th diffraction orders. Optionally, diffraction orders greater than the + / - 1st diffraction order are simply prevented / stopped / blocked from reaching the first and / or second sensors, such that they have no effect on the signals formed at the first and / or second sensors. Such diffraction orders can be stopped by absorption, deflection, scattering, and / or reflection. For example, an opaque material can be located at the aforementioned light spot / convergence point to absorb and block selected diffraction orders or all diffraction orders greater than the diffraction order at the conjugate plane of the light source. Optionally, the diffraction order encoder is configured to encode diffraction orders greater than the + / - 1st diffraction order having a polarization state such that they do not interact / interfere with the 0th diffraction order and / or the + / - 1st diffraction order at the sensor (e.g., at the first sensor and the second sensor), or such that they are at least partially (e.g., substantially) filtered out by, for example, an appropriate filter before reaching the sensor (e.g., filtered out by the first sensor polarization filter and / or the second sensor polarization filter).

[0050] The scale can be illuminated with collimated light (optionally, the diffraction orders of the scale can themselves be collimated). Optionally, the readhead includes an optical collimator element for collimating the light from the polarized light source. Optionally, the same optical collimator element (for collimating the light from the polarized light source) and the optical relay element for relaying the light (e.g., the diffraction orders) toward the first sensor and / or the second sensor are the same object / optical element. Accordingly, optionally, the same optical element is used to collimate the light from the polarized light source and to focus the diffraction orders to corresponding / respective light spots.

[0051] The device may be configured such that the first sensor and / or the second sensor are 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 the first sensor and / or the second sensor.

[0052] Preferably, the scale comprises what is commonly referred to as an amplitude-type scale or "Ronchi" scale (see phase-type scale). As will be appreciated, in an amplitude-type scale or "Ronchi" scale, features are configured to control the amplitude of light reflected towards the readhead (particularly towards the first and / or second sensors) (or transmitted in a transmission-type scale embodiment), for example by selectively absorbing, scattering and / or reflecting light. In contrast, a phase-type scale is configured to control the phase of light reflected towards the readhead (particularly towards its first and second sensors) (or transmitted in a transmission-type scale embodiment), for example by controlling scale features at different depths to be less than a fraction of the wavelength 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.

[0053] 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.

[0054] 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 its spacing is 1 :1.

[0055] The scale may be a transmissive scale. Alternatively, the scale may be a reflective scale. Accordingly, optionally, the light source of the read head and the first and second sensors are located on the same side of the scale.

[0056] Optionally, the position measuring encoder device is a single grating encoder system, wherein the scale comprises a sole diffraction grating in an optical path between the light source and the first and second sensors.

[0057] 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.

[0058] The signal generated at the first sensor may comprise an incremental position signal, such as an interference fringe pattern or a modulated light spot. Accordingly, 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 optics and p is the period of the scale (this is / can be achieved when the 0th diffraction order has been substantially removed).

[0059] The scale may include an incremental scale track comprising a series of periodic features defining the incremental scale track. One or more reference marks may be provided, embedded within and / or located adjacent to the incremental scale track. Such reference marks may include optical reference marks.

[0060] Optionally, the signal generated at the first sensor comprises a reference mark signal.In those embodiments where the readhead comprises a first sensor and a second sensor, the second sensor may be configured to detect the reference mark signal generated by a reference mark.

[0061] Optionally, the signal generated at the first sensor comprises an absolute position signal. Accordingly, 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 read head so that the relative position of the read head 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.

[0062] The scale may comprise separate incremental and absolute scale tracks. Alternatively, the incremental and absolute scale features may be combined in a single track. For example, the absolute scale features may be superimposed on the periodic incremental scale features.

[0063] In those embodiments where the read head comprises a first sensor and a second sensor, the first sensor may be configured to detect an incremental scale / signal (and may therefore be referred to as an incremental sensor), and the second sensor may be configured to detect an absolute scale / signal (and may therefore be referred to as an absolute sensor), or vice versa.

[0064] Optionally, the scale comprises a first series of position features and a second series of position features readable by the readhead. 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 sensor and the second sensor). These first and second sets of diffraction orders may be superimposed / spatially overlapped with each other. The position measurement encoder device may be configured such that only one of the first and second sets of diffraction orders has diffraction orders with 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 order), but the diffraction orders in the second set of diffraction orders (e.g. at least the 0th diffraction order and the + / - 1st diffraction order) 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.

[0065] 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.

[0066] As will be appreciated, the output of the first sensor (and the output of the second sensor, if present) can provide one or more signals that can 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 can be incremental position information or absolute position information. Such position information can be indexed (also known as "reference" or "datum") position information. The position information can be linear position information or angular position information (for example, in the case of a rotary encoder device). The readhead can output raw, unprocessed signals from the sensor. Alternatively, the readhead can 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 can 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 can output analog signals or digital signals representing the relative position of the scale and readhead. For example, the quadrature signals can be analog quadrature signals or digital quadrature signals. 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.

[0067] The position measuring encoder device may be a linear encoder device 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).

[0068] According to a second aspect of the present invention, there is provided a position measuring encoder device comprising: a scale comprising a series of position features readable by a read head; a read head comprising: i) a polarized light source for emitting polarized light toward the scale; ii) a first sensor configured to sense light filtered along a first polarization axis, the first sensor being arranged to sense light from the polarized light source that has interacted with the scale; and iii) a wave plate located in an optical path between the polarized light source and the first sensor, wherein the wave plate has an effective total wave delay of n+1 / 4, wherein n is an integer ≥0, and is arranged so that its fast axis is arranged at an angle of between 30° and 60° to the first polarization axis.

[0069] The apparatus may comprise: iv) a second sensor configured to sense light filtered along a second polarisation axis orthogonal to the first polarisation axis, the second sensor being arranged to sense light from the polarised light source that has interacted with the scale.

[0070] The wave plate may comprise an eighth wave plate, and wherein light from the light source passes through the eighth wave plate twice on its way from the light source to the first sensor (and the second sensor if present).

[0071] The light from the light source may be diffracted into a plurality of diffraction orders. The read head may include a diffraction order encoder configured to encode the 0th diffraction order with a polarization state aligned with the second polarization axis and to encode the + / - 1st diffraction orders with a polarization state aligned with the first polarization axis. This may cause the first sensor to sense the + / - 1st diffraction orders but not the 0th diffraction order, and cause the second sensor to sense the 0th diffraction order but not the + / - 1st diffraction orders.

[0072] Features described above in connection with the first aspect of the invention are equally applicable to this second aspect of the invention (and vice versa), but are not repeated here for the sake of brevity.

[0073] Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which:

[0074] Figure 1a and Figure 1b is a schematic diagram illustrating the optical operating principle of a general prior art position encoder system;

[0075] Figure 2 An incremental scale and a read head of an encoder device according to the present invention are shown;

[0076] Figure 3 Schematically illustrates a first embodiment of the present invention. Figure 2 Various optical components in the read head;

[0077] Figure 4 and Figure 5 yes Figure 3 A schematic optical diagram of an encoder device;

[0078] Figure 6 is 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;

[0079] Figure 7 Demonstrated for use in Figures 2 to 5 Example electro-optical grating sensor used in a read head;

[0080] Figure 8 schematically illustrates an encoder device according to a second embodiment of the present invention;

[0081] Figure 9 and Figure 10 yes Figure 8 A schematic optical diagram of an encoder device;

[0082] Figures 11 and 12 are graphs illustrating the effect of filtering out the + / - 1st diffraction order on the image (or "pseudo-image") at the detector plane of the readhead;

[0083] Figure 13 schematically illustrates a schematic optical diagram of an encoder device according to another embodiment of the present invention;

[0084] Figure 14 Schematically illustrates an absolute scale and a readhead of an encoder device according to another embodiment of the present invention;

[0085] Figure 15 Schematically illustrates an absolute scale and a readhead of an encoder device according to another embodiment of the present invention;

[0086] Figure 16 A scale and a read head of an encoder device according to another embodiment of the present invention are shown;

[0087] Figure 17 Schematically illustrates various optical components of a read head according to another embodiment of the present invention;

[0088] FIG18( a ) shows an example polarization manipulator for use in a read head according to the present invention;

[0089] FIG18( b ) shows four different (i-iv) example orientations of orthogonal polarization axes (v, h) and a graph of the optical power of light output from the polarization manipulator of FIG18( a ) along those orthogonal polarization axes within the footprint of the light; and

[0090] Figure 19 An example polarization manipulator according to the present invention is shown for use in a read head.

[0091] refer to Figure 2, an encoder apparatus 2 according to the invention comprises a scale 4 and a readhead 6. The scale 4 and readhead 6 are movable relative to each other in the X dimension. For example, the scale 4 may be mounted to a fixed part of the machine (not shown) and the readhead 6 may be mounted to a movable part of the machine (not shown) (although this could be the other way around, or indeed both may be movable). The scale 4 may take many different forms, including, for example, a linear scale (as shown) or a rotary scale (e.g. provided on a ring with the scale features provided on its circumferential edge, or on a disc with the scale features provided on its planar face).

[0092] In the described embodiment, the scale 4 is an amplitude-type scale (because it controls the amplitude of light leaving the scale towards the readhead) and comprises a substrate having incremental features 10 in the form of periodic dark / relatively low reflective lines formed on an otherwise relatively highly reflective substrate, such that between the incremental features 10 the scale is relatively highly reflective. Of course, the incremental features 10 of the scale may be formed in other ways, for example by forming relatively highly reflective lines on an otherwise relatively low reflective substrate, or even by forming both relatively highly reflective and relatively low reflective lines on the substrate. As will also be understood, the incremental features 10 may be arranged in other ways, for example, the incremental features 10 of the scale may be arranged in the form of reflective facets or lines that reflect light both towards and away from the readhead. In the described embodiment, the substrate of the scale is metal, but as will be understood, other materials, such as glass, may be used.

[0093] In an alternative embodiment, the scale 4 may be a phase-type scale, where the peaks and valleys in the scale modulate the phase of light leaving the scale towards the readhead.

[0094] The incremental features 10 form an incremental scale that facilitates measurement along the X-axis (the "measurement dimension"). In the embodiment shown, a reference mark 111 is provided, in particular, embedded within the incremental feature 10. The reference mark 111 of this embodiment comprises a patterned reference mark, which in this embodiment comprises a thick dark band and two thin dark bands, each separated along the measurement direction by at least one incremental feature between them. The term "patterned" in the expression "patterned reference mark" is not intended to imply that the reference mark has a repetitive design / form, but rather to give the reference mark a non-uniform design / form. Of course, a "patterned reference mark" may have a repetitive form / design, or may not have a repetitive form / design. As will be understood, the reference mark does not necessarily have to be embedded within the incremental scale track. Instead, the reference mark may be provided in a separate scale track. In other embodiments, the reference mark is non-optical, or, for example, no reference mark is provided at all.

[0095] Figures 3 to 5 Various optical components located within the readhead 6 are shown (the main body has been omitted for clarity, and the relative sizes and positions of the components are not drawn to scale). In this embodiment, the readhead 6 includes a polarized light source 12, a polarization manipulator 14 (which is a 1 / 4 (quarter) wave plate, as explained in more detail below in this embodiment), a refractive lens 109, a diffraction order encoder 116, a primary position information sensor 20 (hereinafter referred to as the "incremental sensor" 20), a polarization filter 120 located in the optical path of light proceeding toward the incremental sensor 20, and an auxiliary position information sensor 122 (hereinafter referred to as the "reference mark sensor" 122). In the embodiment depicted, the light source 12 and the diffraction order encoder 116 are disposed in substantially the same plane, on an opaque substrate 115. As will be appreciated, the substrate 115 need not actually be opaque, but opaque means that other diffraction orders are blocked, which may be beneficial (as described in more detail below).

[0096] Polarization filter 120 has a polarization axis and will filter light along its polarization axis that reaches incremental sensor 20. In the described embodiment, the angle / orientation of the polarization axis of the polarization filter will be described as "horizontal" for ease of reference and explanation.

[0097] The 1 / 4 (quarter) wave plate 14 has a fast axis that is arranged at an angle of 45° to the "horizontal" polarization axis of the polarization filter 120 (the significance of which will be explained in more detail below). Figure 19 Unlike the embodiment of FIG. 1 , the quarter-wave plate 14 has a uniform fast axis (in that the angle / orientation of the fast axis is substantially the same at least across the region through which light from the light source passes).

[0098] The polarization light source 12 emits polarized light, i.e. light having at least a predominant polarization orientation (if not a sole polarization orientation). The polarization light source may emit linearly polarized light or elliptically polarized light. In this embodiment, the polarization light source 12 comprises a laser light source, in particular a VCSEL, which emits linearly polarized light having a predominant polarization orientation. As will be appreciated, the polarization orientation and / or degree of polarization (DOP) of light from a laser light source, such as a VCSEL, may vary unpredictably between light sources. Moreover, the polarization orientation and / or DOP of light from a laser light source, such as a VCSEL, may vary during operation of the read head 6, for example due to changes in the operating environment, such as changes in temperature and applied current (which may vary due to a servo). Typically, the DOP of the VCSEL type used in the described embodiments will be in the range of 0.3 to 0.7. As will be appreciated, the light source may emit inherently polarized light, such as the VCSEL 12, or may comprise an unpolarized light emitter and an associated polarizer which polarizes light emitted from the unpolarized light emitter.

[0099] In the embodiment described, light from the VCSEL 12 is emitted toward the scale 4 through the quarter wave plate 14 (the significance of which will be explained in more detail below). The light output from the quarter wave plate 14 is relayed to the scale 4 via the lens 109 (through the clear / transparent window 117 in the read head 106). The VCSEL 12 is positioned at the focal length f of the lens 109 so that the light from the VCSEL 12 is collimated by the lens 109 when it is irradiated to the scale 4. The light from the VCSEL 12 illuminates the footprint 24 on the scale 4. Figure 4 and Figure 5 Light travelling towards the scale is schematically illustrated by a thick dashed line in FIG, while light returning from / reflected by the scale is illustrated by a thin solid line.

[0100] The lens 109 relays light reflected by the scale 4 to the incremental sensor 20 and the reference mark sensor 122 .

[0101] As will be appreciated, due to the well-known phenomenon of natural diffraction, light reflected from the scale 4 will be diffracted by the presence of scale features thereon (ie, in conjunction with the above Figure 1a 、 Figure 1b In the same manner as described above). In the case of periodic incremental features 10, the light reflected thereby will be diffracted into identifiable diffraction orders. Figure 1a and Figure 1b As explained, light will be diffracted in the X dimension into the 0th and + / - 1st diffraction orders. Higher diffraction orders (e.g., + / - 3rd and + / - 5th) will also exist, but these are not shown because they are much less intense than the 0th and + / - 1st diffraction orders and, in any case, in the depicted embodiment, are blocked by the opaque substrate 115 from advancing to the incremental sensor 20 or the reference mark 122 sensor.

[0102] The diffraction orders are incident on lens 109, which causes the corresponding diffraction orders to converge to form light spots 150 at the back focal plane fp of lens 109; each light spot is formed by a corresponding diffraction order. Light spots 150 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.

[0103] In this embodiment, the diffraction order encoder 116 is co-located with the light spot 150. Specifically, in this embodiment, the diffraction order encoder 116 includes a 0th order polarizer 116a (e.g., having a "vertical" polarization axis) that coincides with the 0th diffraction order light spot 150a, and a first non-polarized transparent area 116b and a second non-polarized transparent area 116c that coincide with the + / -1st diffraction order light spots 150b and 150c. Thus, the diffraction order encoder 116 encodes the 0th diffraction order using only vertically polarized light, while leaving the polarization of the + / -1st diffraction orders unaffected.

[0104] The 0th and + / -1st diffraction orders propagate toward the incremental sensor 20 and reference mark sensor 122. As described above, the polarization filter 120 has a "horizontal" polarization axis and, therefore, has a polarization axis that is orthogonal to the polarization axis of the 0th-order polarizer 116a. Accordingly, the 0th-order diffracted light is blocked from reaching the incremental sensor 20. In contrast, the + / -1st-order diffracted light can pass through the polarization filter 120 (but becomes "horizontally" polarized in the process) and thereby impinge on the incremental sensor 20. In particular, the + / -1st-order diffraction orders propagate toward the incremental sensor 20 and interact (interfering constructively and destructively) to form an interference fringe pattern (or "fringe field") that impinges on the incremental sensor 20. Movement of the readhead 6 relative to the scale 4 causes movement of the interference fringe pattern relative to the incremental sensor 20, thereby enabling up / down counting by the downstream electronics, thereby enabling measurement of relative displacement.

[0105] Blocking the 0th diffraction order provides numerous improvements to the interference fringe pattern falling on incremental sensor 20. In particular, interference fringe pattern visibility is improved. Furthermore, removing the 0th diffraction order means that the interference fringe pattern generated at incremental sensor 20 has a period equal to Mp / 2 (half that of a system that does not block the 0th diffraction order). Accordingly, blocking the 0th diffraction order effectively doubles the system resolution.

[0106] Figure 6 Another advantage of removing the 0th diffraction order is demonstrated. In particular, Figure 6 The effect of blocking the 0th diffraction order on the visibility of fringes falling on the incremental sensor 20 is shown as the distance between the scale 4 and the read head 106 (commonly referred to as the "working gap") is varied. As shown, when the 0th diffraction order fully contributes to the fringe field, the visibility of the fringe field is modulated to have a p 2 The period of the scale is determined by the scale period, where p is the scale period and λ is the wavelength of the light. If the 0th order is eliminated, there is no modulation. 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.

[0107] Accordingly, overall, the 0th diffraction order produced by the barrier scale improves the absolute fringe visibility of the fringe field falling on the incremental sensor 20, effectively doubling the system resolution and significantly improving the working gap tolerance of the readhead.

[0108] Furthermore, if diffraction orders greater than the first diffraction order are also blocked (as occurs in this embodiment due to the opaque substrate 115), higher-order harmonics are removed from the fringe field, thereby providing a purer interference fringe pattern. As will be appreciated, in alternative embodiments, blocking diffraction orders greater than the first diffraction order is not necessary, and, for example, the substrate 115 may be transparent (in which case the first non-polarization transparent region 116b and the second non-polarization transparent region 116c may not be clearly identifiable).

[0109] In the described embodiment, no polarization filter is located in front of reference mark sensor 122. Accordingly, both the 0th and + / -1st diffraction orders fall on reference mark sensor 122. Depending on various factors (described in more detail below), in some cases, it may be advantageous not to place a polarization 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 122, then there is no advantage in providing a corresponding filter in front of reference mark sensor 122 (and in fact, it may be disadvantageous due to the reduced light throughput).

[0110] Accordingly, as described above, the signal sensed by incremental sensor 20 is formed only of the + / - 1st diffraction orders, while the signal sensed by reference mark sensor 122 is formed of the 0th diffraction order and the + / - 1st diffraction orders. Accordingly, the signal sensed by incremental sensor 20 is formed of a diffraction order composition different from the diffraction order composition of the signal sensed by reference mark sensor 122.

[0111] It should be noted that in this embodiment, since the 0th diffraction order has already been polarized by the 0th polarizer 116a, the 0th and + / -1st diffraction orders are non-uniformly attenuated. If desired, the non-uniform attenuation can be avoided / reduced by replacing the first and second non-polarization transparent regions 116b, 116c with polarization filters having a "horizontal" polarization axis (i.e., having a polarization axis orthogonal to the polarization axis of the 0th-order polarizer 116a). In such an embodiment, the 0th and + / -1st diffraction orders that fall on the reference mark sensor 122 will be attenuated, which can improve the image quality of the reference mark sensor. In addition, the + / -1st diffraction orders will still pass through the polarization filter 120 and interfere to form an interference fringe pattern on the incremental sensor 20. In either case, as will be appreciated, the light that falls on the incremental sensor 20 is filtered differently than the light that falls on the reference mark sensor 122.

[0112] The light that falls on the incremental sensor 20 and the reference mark sensor 122 can be described as an "image" of the scale 4 because the incremental sensor 20 and the reference mark sensor 122 are positioned at a detector plane dp, which, in the embodiment shown and described, is coplanar with a conjugate plane cp of the scale plane sp (such 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, suppressing / blocking selective diffraction orders from reaching the sensor will have an impact on the "image" seen at the conjugate plane cp. For example, removing the 0th diffraction order (and removing diffraction orders greater than the 1st diffraction order due to blocking by the opaque substrate 115) means that the "image" falling on the incremental sensor 20 at the conjugate plane cp has a purer sinusoidal waveform rather than a square waveform, and therefore, the image on the sensor is not actually a "true" or "perfect" image of the scale 4 (but rather can be referred to as a pseudo-image of the scale). As will be appreciated, due to the presence of the 0th diffraction order at the reference mark sensor 122, the "image" of the scale 4 at the reference mark sensor 122 is affected to a lesser extent, particularly as both the 0th diffraction order and the + / - 1st diffraction orders have been polarized by the diffraction order encoder 116 so that they fall on the reference mark sensor 122 to the same extent.

[0113] Without the quarter wave plate 14, what falls on the incremental sensor 20 and the reference mark sensor 122 will vary depending on the dominant polarization state / orientation emitted by the VCSEL 12. In the worst case, this could mean that the incremental sensor 20 sees no light at all. Figure 3In the same setup without the quarter-wave plate, if the VCSEL light source 12 emitted only vertically polarized light, the incremental sensor 20 would see no light at all. However, by placing a quarter-wave plate 14 in the optical path between the light source 12 and the incremental sensor 20, and configuring it so that the fast axis of the wave plate and the polarization axis of the polarization filter 120 of the incremental sensor 20 are angled relative to each other, it is ensured that the incremental sensor 20 will receive an appropriate signal regardless of the polarization orientation of the VCSEL light source 12. For example, arranging the fast axis of the quarter-wave plate 14 at 45° to the polarization axis of the polarization filter 120 of the incremental sensor means that, all other factors being equal (e.g., for a constant optical power output from the VCSEL 12), the optical power of the signal falling on the incremental sensor 20 will be the same, regardless of the polarization state of the light output by the VCSEL 12. Another way to look at this is that for light exiting the quarter-wave plate 14 toward the incremental sensor 20 (which, in this embodiment, will pass through lens 109, scale 4, further lens 190, diffraction order encoder 116, and polarization filter 120), the ratio of the optical power i) resolvable along the "horizontal" polarization axis of the polarization filter 120 to the optical power ii) resolvable along the orthogonal "vertical" axis is 1:1. This holds true regardless of the polarization state of the light output by the VCSEL 12 (due to the 45° angle between the fast axis of the quarter-wave plate and the polarization axis of the polarization filter 120 of the incremental sensor 20).

[0114] As will be appreciated, while it may be preferred that the angle between the fast axis of the quarter-wave plate and the polarization axis of the polarization filter 120 of the incremental sensor 20 be 45° (to ensure that, all other factors being equal, the optical power of the signal falling on the incremental sensor 20 is the same, regardless of the polarization state of the light output by the VCSEL 12), this is not required. As will be appreciated, a 45° deviation in the angle between the fast axis of the quarter-wave plate and the polarization axis of the polarization filter 120 of the incremental sensor 20 would mean that the optical power of the signal falling on the incremental sensor 20 would be different, regardless of the polarization state of the light output by the VCSEL 12. However, it is acceptable that the optical power of the signal falling on the incremental sensor 20 varies depending on the polarization state of the light output by the VCSEL 12. In this case, the limits of the acceptable angle between the fast axis of the quarter-wave plate and the polarization axis of the polarization filter 120 of the incremental sensor 20 will depend on the acceptable range of variation in the optical power of the signal falling on the incremental sensor 20. However, the inventors have found that it is generally desirable that the angle between the fast axis of the quarter wave plate and the polarization axis of the polarization filter 120 of the incremental sensor 20 be between 30° and 60°, more preferably between 35° and 55°, and even more preferably between 40° and 50°.

[0115] In the depicted embodiment, the incremental sensor 20 is in the form of an electrical grating, in other words, a light sensor array 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 a fringe pattern / field (schematically represented by wave 1100) at the incremental sensor 20. Figure 7 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.

[0116] 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'.

[0117] As will be appreciated, other types of sensors can 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 readhead rather than an interference fringe pattern, a bulk sensor photodiode can be used to detect the intensity of the modulated light spot (e.g., as described in US Pat. No. 4,776,701). Similar to systems that generate fringe patterns, in systems that generate 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 diffraction orders above + / -1st with an optical state different from that of the + / -1st diffraction order reduces / eliminates harmonics in the modulated light spot intensity.

[0118] In the depicted embodiment, reference mark sensor 122 includes two sets of correspondingly arranged and shaped / sized photodiodes 122a, 122b, 122c located on opposite sides of incremental sensor 20. Each set of photodiodes 122a, 122b, 122c of reference mark sensor 122 is configured such that, when and only when readhead 6 and reference mark 111 are aligned, an image of the pattern of reference mark 111 (or the "pseudo-image" explained above) falls on the three photodiodes 122a, 122b, 122c of reference mark sensor 122 and aligns / correlates with the pattern of these photodiodes, causing a significant and dramatic change in the intensity of light falling on these photodiodes. In this embodiment, in which reference mark 111 includes a dark band, the intensity of light received at reference mark sensor 122 will drop significantly when readhead 6 and reference mark 111 are aligned. Signal processing electronics and / or software downstream of the reference mark sensor 122 can be configured to recognize this change in the intensity of light received at the reference mark sensor 122 and output a signal indicative of the presence of the reference mark. As far as the downstream signal processing electronics and / or software are concerned, the two sets of correspondingly arranged photodiodes 122a, 122b, 122c act as one photodiode.

[0119] Figure 8 、 Figure 9 and Figure 10 An alternative embodiment of an encoder device 202 according to the present invention is shown. Figures 3 to 7 Some parts of the embodiments are the same, and similar parts share the same reference numerals.

[0120] In this embodiment, the polarization manipulator 214 is different from the polarization manipulator of the aforementioned embodiment. First, in this embodiment, the polarization manipulator 214 is located in the optical path between the lens 109 and the scale 4 (i.e., between the lens 109 and the window 117 of the read head 206). The inventors have found that it may be beneficial to arrange the polarization manipulator 214 after the lens 109. For example, after collimation, all incident angles are the same and may mean that the polarization manipulator 214 has better performance. Moreover, the polarization manipulator 214 may have optical aberrations that may hinder collimation. In addition, from a manufacturing point of view, positioning the polarization manipulator here may be easier.

[0121] like Figure 9 and Figure 10 What is shown, and Figures 3 to 7 Compared to the embodiment of FIG. 1 , light from the VCSEL 12 passes through the polarization manipulator 214 twice; once on its way toward the scale 4 and once on its return from the scale 4 toward the sensor ( 20 , 122 ). In practice, the overlap of the light beams toward and away from the scale 4 at the polarization manipulator 214 will likely be greater than Figure 9 The overlapping range is schematically shown in FIG.

[0122] In this embodiment, the polarization manipulator 214 includes a one-eighth (ie, "1 / 8") wave plate, rather than Figures 3 to 7 However, as will be appreciated, since light passes through the eighth wave plate 214 twice (and according to Figure 3 The effective total retardation of the polarization manipulator 214 is n+1 / 4 of the wavelength of the light emitted by the light source 12, where n is an integer ≥ 0 (n=0 in the described embodiment). Figures 3 to 7 In the embodiment of FIG. 1 , the fast axis of the eighth wave plate 214 is arranged to form an angle of 45° with the polarization axis of the first polarization filter 120 (and in this embodiment, will also form an angle of 45° with the polarization axis of the second polarization filter 223, which will be described in more detail below). Figures 3 to 7 In the embodiment, although it may be preferred that the fast axis of the eighth wave plate is arranged at an angle of 45° to the polarization axis of the first polarization filter 120, this is not required.

[0123] The diffraction order encoder 216 of this embodiment is different from Figures 3 to 7 In this embodiment, the diffraction order encoder 216 includes a 0th-order polarizer 216a that coincides with the 0th diffraction order spot 150a, a +1st-order polarizer 216b that coincides with the +1st diffraction order spot 150b, and a -1st-order polarizer 216c that coincides with the -1st diffraction order spot 150c. The polarization axis of the +1st-order polarization filter 216b and the polarization axis of the -1st-order polarization filter 216c are configured to have the same orientation as each other, so that the + / -1st diffraction order is polarized by the +1st-order polarization filter 216b and the -1st-order polarization filter 216c, so that they have the same (e.g., "horizontal") polarization orientation as each other. The polarization axis of the 0th order polarizer 216a is configured to be in a different, preferably orthogonal, orientation than the polarization axes of the +1st polarization filter 216b and the -1st polarization filter 216c, so that the 0th diffraction order is polarized to have a different (e.g., "perpendicular") polarization orientation than the + / -1st diffraction orders. Accordingly, the diffraction order encoder 216 encodes the +1st diffraction order and the -1st diffraction order having a first polarization orientation, and encodes the 0th diffraction order having a second polarization orientation different from the first polarization orientation. As shown, the 0th polarizer 216a, the +1st order polarizer 216b, and the -1st order polarizer 216c are spaced apart from each other so that they are not in direct contact with each other, but this need not be the case.

[0124] The polarized diffraction orders propagate toward the incremental sensor 20 and the reference mark sensor 122. Figures 3 to 7In the same manner as the embodiment described in the embodiment of , the first polarization filter 120 is located in the optical path of the diffraction orders heading toward the incremental sensor 20. The first polarization filter 120 has a polarization axis that is parallel to the polarization axes of the +1st order polarizer 216b and the -1st order polarizer 216c (and is therefore orthogonal to the polarization axis of the 0th order polarizer 216a). Accordingly, the 0th order diffraction light is blocked and cannot reach the incremental sensor 20. In contrast, the + / -1st order diffraction light can pass through the first polarization filter 120 and thereby fall on the incremental sensor 20. In particular, the + / -1st diffraction orders propagate toward the incremental sensor 20 and interact with each other (interfere constructively and destructively) to form a fringe field that falls on the incremental sensor 20. The above description is in conjunction with Figures 3 to 7 The embodiment of FIG. 4 illustrates the benefit of preventing the 0th diffraction order from contributing to the optical signal falling on the incremental sensor 20 .

[0125] and Figures 3 to 7 In contrast to the embodiment of the present invention, a second polarization filter 223 is positioned in the optical path of the diffraction orders that proceed toward the reference mark sensor 122. Specifically, the second polarization filter 223 comprises a polarization filter whose polarization axis is parallel to the polarization axis of the 0th-order polarizer 216a (and therefore orthogonal to the polarization axes of the +1st polarization filter 216b and the -1st polarization filter 216c). Accordingly, the second polarization filter 223 will block the + / -1st diffraction orders, thereby preventing them from contributing to the optical signal falling on the reference mark sensor 122. Thus, only the 0th diffraction order light will reach the reference mark sensor 122 and contribute to the optical signal falling on the reference mark sensor. This arrangement has been found to be advantageous, particularly in those embodiments in which there are incremental features located within the reference mark.

[0126] For example, referring to Figures 11 and 12, the effect of filtering out the + / - 1st diffraction order on the image (or "pseudo-image") at the detector plane dp of the readhead is shown. In particular, Figure 11a 、 Figure 11b and Figure 11c The scale image reconstructed from both the 0th and + / -1st diffraction orders at the detector plane dp / conjugate plane cp of the read head is shown when the read head is located at: a) the nominal working gap of the read head; b) +75 μm from the nominal working gap; and c) +150 μm from the nominal working gap. As shown in the figure, at the nominal working gap, the image of the reference mark (given by Figures 11a to 11c) is good, but the image of the reference mark is significantly corrupted when the readhead moves away from its nominal working gap. This problem has been found to be particularly pronounced when the reference mark includes periodic incremental features, but it can also exist (albeit to a lesser extent) even for reference marks that do not include incremental features. This image corruption causes the signal output by the reference mark sensor 122 to be broader and less distinct as the readhead passes over the reference mark, which can lead to issues with the reliability of the reference mark (e.g., the signal is too weak to detect the reference mark) and / or issues with the repeatability of the reference mark (e.g., the reference mark signal is too broad to repeat within one incremental period).

[0127] Figure 12a 、 Figure 12b and Figure 12c The scale image reconstructed from the 0th diffraction order at the detector plane dp / conjugate plane cp of the read head is shown when the read head is located at the following positions: a) the nominal working gap of the read head; b) +75 μm from the nominal working gap; and c) +150 μm from the nominal working gap. As shown in Figure 12, even at the working gap far from the nominal working gap, the image of the reference mark reconstructed from the detector plane dp / conjugate plane cp of the read head ( Figures 12a to 12c ) also maintains a good structure and a good resemblance to the reference mark, and thus provides a significantly strong reference mark signal from the reference mark sensor 122. Accordingly, it has been found beneficial to provide a second polarization filter 223 in front of the reference mark sensor 122 that filters out the + / - 1st diffraction orders so that they do not contribute to the scale image reconstructed at the detector plane dp / conjugate plane cp of the readhead. In particular, this configuration provides a readhead with better working gap tolerance.

[0128] As will be appreciated, other reference mark sensor arrangements can be used. For example, the reference mark sensor 122 can include a split detector in which the photodiodes 122a, 122b, 122c are replicated and laterally offset in the X dimension, and the outputs of the replicated photodiodes are connected to form a second reference mark output that can be used to provide a difference signal (e.g., as described in WO 2005 / 124282). Furthermore, for example, the reference mark sensor 122 need not include sensing elements on both sides of the incremental sensor. Furthermore, for example, one or more of the reference mark sensing elements can be partially or completely embedded within the incremental sensor (e.g., as described in WO 2005 / 124282).

[0129] As an alternative embodiment, the first polarization filter 120 may be omitted so that only the reference mark sensor 122 has a polarization filter in front of it (thereby obtaining the above-mentioned benefit of filtering out selected diffraction orders so that they do not reach the reference mark sensor 122).

[0130] Accordingly, as described above, the signal sensed by incremental sensor 20 is formed only of the + / - 1st diffraction order, while the signal sensed by reference mark sensor 122 is formed only of the 0th diffraction order. Accordingly, the signal sensed by incremental sensor 20 is formed of a diffraction order composition different from the diffraction order composition of the signal sensed by reference mark sensor 122.

[0131] Without eighth-wave plate 214, what falls on incremental sensor 20 and reference mark sensor 122 can vary significantly depending on the orientation of the dominant polarization emitted by VCSEL 12. In the worst case, this could mean that incremental sensor 20 or reference mark sensor 122 sees no light at all. Figure 8 In the same setup without the eighth-wave plate 214, if the VCSEL light source 12 only emits vertically polarized light, the incremental sensor 20 will see no light at all, while if the VCSEL light source only emits horizontally polarized light, the reference mark sensor 122 will see no light at all. However, by providing the eighth-wave plate 214 and configuring it so that its fast axis is angled relative to both the polarization axis of the first polarization filter 120 and the polarization axis of the second polarization filter 223, it is ensured that both the incremental sensor 20 and the reference mark sensor 122 will receive appropriate signals regardless of the polarization orientation of the light emitted by the VCSEL light source 12. For example, arranging the fast axis of the eighth-wave plate 214 at 45° to the polarization axis of the first polarization filter 120 and also at 45° to the polarization axis of the second polarization filter 223 means that, all other factors being equal, the optical power of the signals falling on the incremental sensor 20 and the reference mark sensor 122 will not vary, regardless of the polarization state of the light output by the VCSEL 12. Another way to view this is that for light exiting the eighth-wave plate 214 toward the incremental sensor 20 and the reference mark sensor 122 (i.e., after the light has passed through the eighth-wave plate 214 a second time): the ratio of the optical power of light i) resolvable along the "horizontal" polarization axis (i.e., along the polarization axis of the first polarization filter 120) to the optical power of light ii) resolvable along the "vertical" polarization axis (i.e., along the polarization axis of the second polarization filter 223) is 1:1. This holds true regardless of the polarization state of the light output by the VCSEL 12 (due to the 45° angle between the fast axis of the eighth-wave plate and the polarization axes of the first and second polarization filters 120, 223).

[0132] 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 an interference fringe pattern, a bulk sensor photodiode may be used to detect the intensity of the modulated light spot.

[0133] Figure 3 and Figure 8 The above-described embodiment uses a refractive lens 109 to relay the diffraction orders toward the sensor. However, it will be appreciated that this need not necessarily be the case. For example, Figure 13 Schematically illustrates an optical system of an encoder device comprising an incremental scale 104 and a read head 206' according to another embodiment of the present invention, the encoder device being similar to Figure 8 1 (and like parts share like reference numerals), except that it does not rely on a refractive lens 109 to relay the diffraction orders toward the sensor. In this embodiment, the 0th and + / -1st diffraction orders are produced by light from a light source striking and being reflected by the scale 104. As shown, the 0th and + / -1st diffraction orders are located in a first plane pl 1 The diffraction order encoder 216 is located on the first plane pl 1 According to the embodiment of FIG11 , the diffraction order encoder includes a 0th order polarizer 216a (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 +1st order polarizer 216b and a -1st order polarizer 216c that coincide with the +1st diffraction order and the -1st diffraction order. The +1st order polarizer 216b and the -1st order polarizer 216c 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 polarization of the + / -1st diffraction order. The 0th diffraction order is not deflected. Accordingly, the 0th diffraction order and the + / -1st diffraction order are in the plane p1 in which the incremental sensor 20 and the reference mark sensor 122 are located. 2 According to Figure 8 In an embodiment, the first polarizer 120 can be located in front of the incremental sensor 20 and can be configured to filter out the 0th diffraction order so that the diffraction order does not reach the incremental sensor 20, and / or the second polarizer 223 can be located in front of the reference mark sensor 122 and can be configured to filter out the + / - 1st diffraction orders so that these diffraction orders do not reach the reference mark sensor 122. For clarity of explanation, Figure 13 The light source 12 and the non-uniform / patterned retarder are omitted.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.

[0134] The embodiments described so far have been used in conjunction with incremental encoders comprising an incremental scale with (or optionally without) one or more reference marks. The invention may also be used with absolute encoders comprising an absolute scale, such as in Figure 14 and Figure 15 The absolute encoder depicted in . Figure 14 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 read head 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 read head 506 comprises a VCSEL light source 12, a lens 109, a polarization manipulator 214 (e.g. Figure 8 The eighth wave plate of the embodiment of the present invention), the diffraction order encoder 216, the incremental detector 20 and the first polarization filter 120, which are combined with the above Figure 8 The embodiment of the invention is configured and arranged in the same manner as described above. Accordingly, the 0th diffraction order is blocked so that it does not contribute to the formation of the signal detected by the incremental sensor 20. Figure 8 As in the embodiment of the present invention, the first polarization filter 120 has a polarization axis perpendicular to the polarization axis of the 0th order polarizer 216a of the diffraction order encoder 216. In particular, in this embodiment, the 0th order polarizer 216a has a vertical polarization axis, and the first polarization filter 120 has a horizontal polarization axis. Figure 8 In the embodiment of FIG. 5 , the fast axis of the eighth wave plate 214 (which light passes through twice on its way from the light source 12 to the sensor 20 , 520 ) is arranged at a 45° angle to the polarization axis of the first polarizer 120 .

[0135] In this embodiment, the read head 506 further includes an absolute sensor 520, which includes a photodiode array (in this embodiment a one-dimensional photodiode array, but it can also be a two-dimensional photodiode array) onto which the image (or "pseudo-image" - see above) falls. As is known, and described, for example, in US7499827, US5279044 and US10989567, the image of the scale can be processed to extract the absolute / unique code and thus determine the absolute position. The absolute position can be combined with the incremental position determined from the incremental detector 20 to provide a fine-pitch absolute position. Alternatively, once the absolute position has been determined, subsequent positions can be determined simply by monitoring the output from the incremental detector 20.

[0136] 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 520. Furthermore, similar to the reference mark embodiment described above, it may be beneficial to prevent the + / -1st diffraction orders from contributing to the signal falling on the absolute sensor 520. Accordingly, a second polarization filter 523 may be provided in front of the absolute sensor 520, which has a polarization axis that is orthogonal to the polarization axes of the +1st order polarizer 216b and the -1st order polarizer 216c of the diffraction order encoder 216. In particular, in this embodiment, the 0th order polarizer 216a has a vertical polarization axis, the +1st order polarizer 216b and the -1st order polarizer 216c have horizontal polarization axes, and the second polarization filter 523 has a vertical polarization axis.

[0137] Figure 15 Another embodiment of an absolute encoder 600 including an absolute scale 604 is shown, which is combined with the above Figure 14The 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 0th order polarizer 216a of the diffraction order encoder 216, and the +1st / -1st diffraction orders in the first group of diffraction orders pass through the +1st order polarizer 216b and the -1st order polarizer 216c, respectively, while the 0th diffraction order and the + / -1st diffraction orders in the second group of diffraction orders both pass through the 0th order polarizer 216a of the diffraction order encoder 216. The higher diffraction orders (e.g., the + / -3rd diffraction order, the + / -5th diffraction order) in the first group of diffraction orders are blocked by the opaque substrate 115. The +3rd and -3rd diffraction orders in the second group of diffraction orders pass through the +1st order polarizer 216b and the -1st order polarizer 216c, respectively, of the diffraction order encoder 216. Diffraction orders greater than the + / - 3rd order in the second set of diffraction orders have essentially insignificant power, but those that do exist will be blocked by the opaque substrate 115. As will be appreciated, which diffraction orders pass through which filter is system dependent and may depend on various factors, including the size and / or position of the polarization filter.

[0138] As shown in the figure, Figure 15 The sensor arrangement of the embodiment is Figure 14 The sensor arrangement is slightly different in that (similar to Figure 3 and Figure 8 (Embodiment 1) The read head of the absolute sensor 620 is divided into two parts, with the incremental sensor 22 located between the two parts. 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 20 and the absolute sensor 620 can be configured according to Figure 14 arrangement of embodiments (i.e., two sensors side by side).

[0139] The above embodiments utilize a lensless system or a single lens system through which light from the light source passes on its way to and from the scale. As will be appreciated, other optical configurations are possible, such as Figure 16 The optical arrangement schematically shown in (which also happens to show an absolute encoder device including an absolute scale). Figure 16 The encoder 300 comprises an absolute scale 304 comprising features that are conceptually periodically arranged, but in which 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 read head 306 (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 read head 306 comprises a polarised VCSEL light source 12 and a quarter wave plate 14. A first lens 309 is provided which collimates the light from the VCSEL 12. Beam splitting member 307 is configured to allow light from VCSEL 12 to pass through it to scale 304 for illumination, and to redirect light reflected by the scale back to sensor 320 (in this case, a one-dimensional array of photodiodes, such as a complementary metal oxide semiconductor (CMOS) sensor). This sensor is configured to sense an image of the scale formed at the sensor by second lens 310. In this embodiment, sensor 320 includes an integrating polarization filter (not shown) such that sensor 320 senses light filtered along the polarization axis of the integrating polarization filter. The readhead is configured such that the angle between the fast axis of quarter wave plate 14 and the polarization axis of the integrating polarization filter is 45°. This angle need not be 45°, but may be preferred, as described in the above-described embodiment. The image obtained by sensor 320 can be processed by a processor device to determine the relative position of readhead 306 and scale 304 in a known manner (e.g., as described in US Pat. No. 10,989,567).

[0140] Figure 17Another exemplary embodiment according to the present invention is shown. In this embodiment, the configuration and operation of the encoder according to this embodiment are substantially the same as those described in WO 2005124282, except that a polarized light source 12, a polarization filter 120, and a polarization manipulator 14 (a quarter wave plate in this embodiment) are used. Accordingly, disregarding the polarization manipulator 14 for the moment, light from the light source 12 illuminates the area 24 of the scale 4. Due to the periodic arrangement of the incremental features 10, the light reflected by the incremental features 10 is diffracted into diffraction orders (i.e., in accordance with the above description). Figure 1a 、 Figure 1b The diffracted light strikes the diffraction grating 16, where it is diffracted into additional diffraction orders. These orders are recombined at the incremental sensor 20 to form an interference fringe pattern (or "fringe field") thereon. Movement of the readhead 6 relative to the scale 4 causes the interference fringes to move relative to the incremental sensor 20, thereby generating an up / down count that can measure displacement. According to the previously described embodiment, the incremental sensor 20 is in the form of an electric grating, and the light source 12 is a VCSEL.

[0141] When the read head 6 passes the reference mark 11, the change in the intensity of the light reflected by the scale 4 is imaged by the optical imaging element lens 18 onto the reference mark sensor 22. In this embodiment, the reference mark sensor 22 includes a segmented detector including a first photodiode 22a and a second photodiode 22b, onto which the light from the scale 4 is imaged. Figure 17 The optical imaging element 18 shown in FIG2 is a Fresnel zone plate. However, other types of optical imaging elements having the same optical function, such as a refractive lens, may be used. As described in WO2005124282, the reference mark sensor 22 may be partially or completely embedded within the incremental sensor 20 (and similarly, the optical imaging element 18 may be partially or completely embedded within the diffraction grating 16).

[0142] In contrast to the embodiment of WO 2005124282, a polarization filter is used to filter the light reaching the incremental sensor. Therefore, according to the above embodiment, without a quarter-wave plate 14, the light falling on the incremental sensor 20 and reference mark sensor 22 would vary depending on the dominant polarization state / orientation emitted by the VCSEL 12. In the worst case, this could mean that the incremental sensor 20 would see no light at all. However, by placing a quarter-wave plate 14 in the optical path between the light source 12 and the incremental sensor 20, and configuring it so that the fast axis of the wave plate and the polarization axis of the polarization filter 120 of the incremental sensor 20 are angled relative to each other, it is ensured that the incremental sensor 20 will receive the appropriate signal regardless of the polarization orientation of the VCSEL light source 12. For example, arranging the fast axis of the quarter wave plate 14 at 45° to the polarization axis of the polarization filter 120 of the incremental sensor means that, for all other factors being equal (e.g., for a constant optical power output from the VCSEL 12), the optical power of the signal falling on the incremental sensor 20 will be the same regardless of the polarization state of the light output by the VCSEL 12.

[0143] The above-described embodiments use wave plates 14, 214 as polarization manipulators. Other types of optical elements may be used in place of wave plates, such as diffusers, non-uniform / patterned retarders, or time-based polarization manipulators. As will be appreciated, in the case of time-based polarization manipulators, a modulation rate faster than the acquisition bandwidth of the encoder's sensor will be required. Typically, a rate of at least 1 MHz should be sufficient. Fiber-coupled time-based polarization scramblers exist, such as those available from FIBREPRO, which would be suitable for fiber-type position encoders. Spatial polarization manipulators, particularly non-uniform / patterned retarders, have been found to be preferred because time-based manipulators require electronics that may be detrimental to compact encoders, and, depending on the embodiment, diffusers may provide undesirably significant light deflection, which adversely affects the formation of the interference fringe pattern.

[0144] As mentioned above, instead of the above-mentioned quarter wave plate or eighth wave plate, a non-uniform / patterned retarder can be used as a polarization manipulator. Figure 17 A suitable example of a non-uniform / patterned retarder 414 is used with the quarter wave plate 14 used in the embodiments of FIG.

[0145] In this embodiment, the patterned retarder 414 includes a non-uniform fast axis along the Y dimension, which is orthogonal to the measurement dimension (X) of the encoder (hence why it can be described as a "non-uniform retarder" or a "patterned retarder"). In particular, the patterned retarder 414 includes an array of discrete elongated half-wave plate pixels / rows 19, wherein the array extends in the Y dimension and the elongated length of the pixels / rows extends in the X dimension. Accordingly, the pixel array / pixel series extends perpendicular to the measurement dimension (X) of the scale 4. Therefore, in this embodiment, the patterned retarder 414 does not include any structure in the measurement (X) dimension. This can be advantageous in order to prevent the patterned retarder 414 from becoming a source of light diffraction in the X dimension, which could otherwise adversely interfere with the generation of optical signals at the sensors 20, 22. However, as will be appreciated, in other embodiments / applications, such diffraction may be tolerable and thus the non-uniform / patterned retarder may comprise a structure in the X dimension, for example, which may be arranged such that the array / series of retarder pixels extend along the Y dimension, or the non-uniform / patterned retarder may comprise a two-dimensional array of retarder pixels.

[0146] As shown in FIG18( a ), the half-wave plate pixels 19 are arranged to have alternating fast axes of 0° and 45°. This configuration ensures that, regardless of the dominant input polarization orientation of the footprint 21 of the light irradiated to the patterned retarder 414, the light output from the patterned retarder 414 will have a mixture of polarization orientations across its extent / footprint (in the Y dimension), such that the DOP of the light exiting the patterned retarder 414 will be less than the DOP of the light irradiated to the patterned retarder 414. In particular, in this embodiment, the light output from the patterned retarder 414 will include alternating light segments / portions / rows (in the Y dimension) having two different polarization orientations, which are always orthogonal to each other regardless of the dominant input polarization orientation. Thus, it is ensured that, regardless of the polarization state of the light emitted from the polarized light source 12, the light exiting the patterned retarder 414 toward the incremental sensor will have a polarization state that is at least partially resolvable along the polarization axis of the first polarization filter 120. Furthermore, in the described embodiment, the light output from the patterned retarder 414 will comprise a balanced mix of polarization orientations (eg, in this embodiment, there will be substantially equal amounts of two different orthogonal polarization orientations).

[0147] Accordingly, the patterned retarder 414 is configured such that the optical power (e.g., in milliwatts, "mW") of the light within the footprint 21 of the light output from the patterned retarder 414 will be substantially equal along the orthogonal polarization axes, regardless of the orientation of the orthogonal polarization axes (correspondingly, the DOP of the footprint 21 of the light output from the optical retarder element will be close to 0, e.g., no greater than 0.1, preferably no greater than 0.02). This may be beneficial for reasons of balance and consistency in encoder performance. For example, if the optical power along one polarization axis is significantly greater than the optical power along another orthogonal polarization axis, the effect of polarization on encoder performance may not be as suppressed as in a configuration where the optical power of the light along the orthogonal polarization axes is substantially the same, especially if the corresponding power of the light varies as the input polarization orientation changes.

[0148] Referring to FIG18( b ), four different (i-iv) example orientations of orthogonal polarization axes (v, h) are shown, along with a graph of the optical power of the light output from the patterned retarder 414 within the light's footprint 21 along those orthogonal polarization axes. As shown, due to the substantially balanced mix of polarization orientations output by the patterned retarder 414 within the light's footprint 21, for any pair of orthogonal polarization axes, the optical power along each polarization axis is substantially the same, regardless of the orientation of the orthogonal polarization axes. In other words, within the light's footprint 21 output from the patterned retarder 414, if the light output from the patterned retarder 414 is polarized along a first polarization axis, the optical power of the polarized light will be substantially the same as if the light were polarized along a second polarization axis that is orthogonal to the first polarization axis.

[0149] The patterned retarder 414 is configured such that, within at least the area of ​​the footprint of light output from the patterned retarder 414 where an optical signal is formed at the sensor, and regardless of the dominant input polarization state of the light irradiating the optical retarder element, the optical power difference along the orthogonal polarization axes of the output light beam is at least half of the optical power difference of the input light beam, regardless of the orientation of the orthogonal polarization axes.

[0150] In the embodiments described herein, substantially equal optical power (or "substantially the same") along the orthogonal polarization axes means that the optical power difference along the orthogonal polarization axes does not vary by more than 2% of the total power. Accordingly, in this case, the DOP of the light output from the patterned retarder 414 within the optical footprint 21 does not exceed 0.02% or 2%. However, as will be appreciated, in other embodiments, such tight tolerances may not be required, and thus substantially equal optical power along the orthogonal polarization axes may mean that the optical power difference along the orthogonal polarization axes does not vary by more than 5% of the total power, e.g., by less than 10% of the total power, e.g., by less than 20% of the total power. In other words, the patterned retarder 414 may be configured such that the DOP of the light output from the optical retarder element within the optical footprint 21 does not exceed 0.05 (or 5%), e.g., by less than 0.1 (or 10%), e.g., by less than 0.2 (or 20%).

[0151] As shown in FIG18( a ), the pitch p of the half-wave plate pixels 19 is significantly smaller than the width of the patterned retarder 414 in the Y dimension. In particular, while the patterned retarder 414 may include only two pixels (such that the first half of the light output from the optical retarder element has one polarization orientation and the second half of the light output from the optical retarder element has a different orthogonal polarization orientation), it has been found beneficial to provide the patterned retarder 414 with a significantly higher number of half-wave plate pixels 19 in order to disperse the different polarization orientations over the footprint of the light output from the patterned retarder 414, and therefore over the scale features on the scale 4 (rather than grouping the same or similar polarization orientations all on one side). This can be beneficial for a number of reasons, including providing greater freedom in where the sensor can be positioned along the Y dimension and still achieve the benefits of the present invention. In this embodiment, this is achieved by providing the patterned retarder 414 with multiple half-wave plate pixels 19 having alternating fast axes. In particular, the ratio of the pitch p1 of the half-wave plate pixels 19 to the width Wr of the patterned retarder 414 (in the dimension along which the series of pixels extends - in this embodiment along the Y dimension) is at least 1:4, more preferably at least 1:10, especially preferably at least 1:20, for example at least 1:30.

[0152] Note that in this embodiment, the arrangement of the half-wave plate pixels 19 is such that the above statement that the optical power of the light output from the patterned retarder 414 within the light coverage area 21 is substantially the same along the orthogonal polarization axes (and the statement that the DOP of the light output from the patterned retarder 414 within the light coverage area 21 is below a certain level) is also true for the area 21a of the light coverage area output from the patterned retarder 414 that is directed toward the incremental sensor 20, and is also true for the area 21b of the light coverage area output from the patterned retarder 414 that is directed toward the reference mark sensor 22.

[0153] Furthermore, in the described embodiment, the patterned retarder 414 is configured such that the different polarization orientations output within region 21a are substantially uniformly distributed in the Y dimension (i.e., perpendicular to the measurement dimension X), such that for a one-dimensional array conceptually dividing the region into two rows (21aa and 21ab) of equal width, which extend parallel to the Y dimension, the optical power of the light output from the patterned retarder 414 within each of the two rows is substantially equal along the orthogonal polarization axes, regardless of the orientation of the orthogonal polarization axes. Note that this is also the case for region 21b of the footprint of the light output from the patterned retarder 414 that forms the optical signal at the reference mark sensor 22 (e.g., for a one-dimensional array conceptually dividing the region into two rows 21ba and 21bb of equal width, which extend parallel to the Y dimension, the optical power of the light output from the patterned retarder 414 within each of the n rows is substantially equal along the orthogonal polarization axes, regardless of the orientation of the orthogonal polarization axes). Accordingly, Figure 6 Graphs (i to iv) of (b) (and the above statement regarding the DOP of light output from the optical retarder element being below a certain level within the light footprint 21) apply equally to each of rows 21aa and 21ab in region 21a of the light footprint output from the patterned retarder 414 that forms the optical signal at the incremental sensor 20, and also apply equally to each of rows 21ba and 21bb in region 21b of the light footprint output from the patterned retarder 414 that forms the optical signal at the reference mark sensor 22. This configuration can help suppress errors that might otherwise be caused by imperfections in scale features along their length. Furthermore, uniform illumination across the sensor can be beneficial. For example, the sensor may be shaped / windowed / weighted to improve the signal output by the sensor, such as described in US Pat. No. 10,670,431, and uniform illumination may be required to maintain the benefits / effects of the shaping / windowing / weighting.

[0154] The patterned retarder 414 described above has been found to be suitable for use in place of those embodiments where the light from the light source passes through the polarization manipulator once (e.g. Figure 3 、 Figure 16 and Figure 17), but is less suitable for replacing the quarter wave plate used in embodiments where the light from the light source passes through the polarization manipulator twice (e.g. Figure 8 、 Figure 14 and Figure 15 In practice, it has been found that the polarization orientation of the light returning from the patterned retarder 414 of FIG. 18( a ) will be altered a second time, resulting in the polarization of many segments / portions of the light being “cancelled.” This can result in the light output from the patterned retarder 414 having an uneven mix of polarization orientations after its second pass, which may be undesirable, and in turn, the light still having a primary polarization orientation after its second pass, which may or may not be at least partially resolvable along the polarization axis of the sensor's polarizer (e.g., the first polarization filter 120).

[0155] The inventors have determined that, for a two-pass system, by providing a patterned retarder having half-wave plate pixels arranged with their different fast axes in a non-periodic arrangement, the mix of polarization orientations exiting the patterned retarder a second time can be more evenly balanced (and thereby ensuring that light exiting the patterned retarder toward the incremental sensor 20 will have polarization states that are at least partially resolvable, and more desirably substantially resolvable, along the polarization axes of the first polarization filter 120). While random non-periodic arrangements have typically been found to provide improvements over periodic arrangements, the inventors have found that certain non-periodic arrangements of the fast axes are better than other arrangements at providing a balanced mix of polarization orientations after the second pass, regardless of the dominant input polarization orientation of the footprint of the light from the VCSEL 12 initially impinging on the patterned retarder.

[0156] Figure 19 An example patterned retarder 514 is shown that is suitable for use in a two-pass configuration and can provide a properly balanced mixture of polarization orientations after the light passes through it a second time. The following table lists the fast axis orientation of each of the sixty pixels that make up the non-uniform / patterned retarder 514:

[0157]

[0158]

[0159] Accordingly, as shown, in this embodiment, the patterned retarder 514 includes a plurality (sixty in this embodiment) of half-wave plate pixels 119 having more than two different fast-axis orientations, and they are not periodically arranged.

[0160] Of course, the fast axis arrangement described above is not the only arrangement that can provide a substantially balanced mix of polarization orientations output from the non-uniform / patterned retarder 514 after light passes twice through the non-uniform / patterned retarder 514. Other arrangements are possible and can be selected by the designer of the optical encoder system.

[0161] The use of the terms "pattern" and "patterned" in this document, especially in conjunction with a polarization manipulator, is not intended to imply the presence of any kind of repetitive configuration. Rather, the terms "pattern" and "patterned" are used to indicate that the polarization manipulator (e.g., retarder) has a non-uniform design / form.

[0162] 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).

[0163] In the described embodiment, the light source 12 emits electromagnetic radiation (EMR) in the near-infrared range. However, as will be appreciated, this need not necessarily be the case, and the light source 12 may emit EMR in other ranges (e.g., anywhere from the infrared to the ultraviolet). As will be appreciated, the selection of a suitable wavelength for the light source 12 may depend on a number of factors, including the availability of suitable gratings and detectors that operate at the EMR wavelength.

[0164] The wave plates of the above embodiments may comprise birefringent materials, such as quartz, mica, or liquid crystal. Alternatively, the wave plates of the above embodiments may comprise structures configured to provide birefringent-like properties, such as metamaterials. For example, nanocast lithography is known to be used to provide the wave plates.

[0165] In the above embodiments, the scale is a reflective scale. However, this need not necessarily be the case. For example, the scale may be a transmissive scale, where the light source and sensor are located on opposite sides / faces of the scale.

Claims

1. A position measurement encoder device comprising: a ruler comprising a series of position features readable by a readhead; as well as A reading head, the reading head comprising: i) a polarized light source, configured to emit polarized light toward the scale; ii) a first sensor configured to sense light filtered along a first polarisation axis, the first sensor being arranged to sense light from the polarised light source that has interacted with the scale; and iii) a polarization manipulator located in an optical path between the polarized light source and the first sensor and configured such that, regardless of the polarization state of the light emitted from the polarized light source, the light leaving the polarization manipulator toward the first sensor will have a polarization state that is at least partially distinguishable along the first polarization axis.

2. A device as described in claim 1, wherein the device is configured so that the following ratio of the light leaving the polarization manipulator toward the first sensor is at least 2:3: i) the ratio of the optical power of the light distinguishable along the first polarization axis to ii) the optical power of the light distinguishable along the second orthogonal polarization axis.

3. The device of claim 1 or 2, configured such that, all other factors being equal, the optical power of the light sensed by the first sensor is substantially the same for all possible polarization orientations of the light emitted from the polarized light source.

4. The apparatus according to any one of claims 1 to 3, wherein The read head further comprises: iv) a second sensor configured to sense light filtered along a second polarization axis, the second sensor being arranged to sense light from the polarized light source that has interacted with the scale; Wherein the polarization manipulator is configured such that, regardless of the polarization state of light emitted from the polarized light source, light exiting the polarization manipulator towards the second sensor will have a polarization state that is at least partially resolvable along the second polarization axis.

5. The apparatus of claim 4, wherein: The second polarization axis is orthogonal to the first polarization axis.

6. A device as described in any one of claims 4 or 5, wherein the device is configured so that, with all other factors being equal, the optical power of the light sensed by the first sensor is substantially the same for all possible polarization orientations of the light emitted from the polarized light source, and the optical power of the light sensed by the second sensor is substantially the same for all possible polarization orientations of the light emitted from the polarized light source.

7. An apparatus as claimed in any preceding claim, wherein The polarization manipulator comprises a wave plate, and wherein the effective total wave delay of the polarization manipulator is n+1 / 4, wherein n is an integer ≥0.

8. The apparatus of claim 7, wherein: The wave plate is arranged such that the fast axis of the wave plate is arranged at an angle between 30° and 60° to the first polarization axis, more preferably between 40° and 50° to the first polarization axis.

9. Apparatus as claimed in any preceding claim, wherein: Light from the light source passes through a polarization manipulator twice on its way from the light source to the first sensor.

10. The device according to claims 7 and 9, wherein The polarization manipulator includes an eighth-wave plate.

11. Apparatus as claimed in any preceding claim, wherein Positional features of the scale diffract the light into a plurality of diffraction orders; The polarization state of at least one of the diffraction orders is different from the polarization state of at least one other diffraction order.

12. The apparatus of claim 11, the apparatus being configured such that, because the first sensor is configured to sense primarily light filtered along a first polarization axis, a signal sensed by the first sensor is primarily formed by a selected subset of diffraction orders.

13. The device of claim 3 and 11 or 12, wherein the device is configured so that: - the polarization state of the diffraction order; - the first sensor is configured to sense light filtered along a first polarization axis; and - the second sensor is configured to sense light filtered along a second polarization axis, The light sensed by the first sensor includes a diffraction order composition different from a diffraction order composition of the light sensed by the second sensor.

14. A position measuring encoder device comprising: a ruler comprising a series of position features readable by a readhead; A reading head, the reading head comprising: i) a polarized light source, configured to emit polarized light toward the scale; ii) a first sensor configured to sense light filtered along a first polarization axis, the first sensor being arranged to sense light from the polarized light source that has interacted with the scale; iii) a wave plate located in an optical path between the polarized light source and the first sensor, wherein the wave plate has an effective total wave delay of n+1 / 4, where n is an integer ≥ 0, and is arranged such that the fast axis of the wave plate is arranged at an angle between 30° and 60° with respect to the first polarization axis.

15. The position measuring encoder device according to claim 14, comprising: iv) a second sensor configured to sense light filtered along a second polarisation axis orthogonal to the first polarisation axis, the second sensor being arranged to sense light from the polarised light source that has interacted with the scale.

16. The position measuring encoder device according to claim 14 or 15, wherein: The wave plate comprises an eighth wave plate, and wherein light from the light source passes through the eighth wave plate twice on its path from the light source to the first sensor.

17. The position measuring encoder device according to claim 16, wherein: The wave plate comprises an eighth wave plate, and wherein light from the light source passes through the eighth wave plate twice on its path from the light source to the first sensor and the second sensor.

18. The position measuring encoder device according to claim 15 or 17, wherein: The scale diffracts the light into a plurality of diffraction orders; and The readhead comprises a diffraction order encoder configured to: - encoding the 0th diffraction order to have a polarization state aligned with said second polarization axis, and - encoding the + / - 1st diffraction order to have a polarization state aligned with said first polarization axis; The first sensor is caused to sense the + / - 1st diffraction order but not the 0th diffraction order, and the second sensor is caused to sense the 0th diffraction order but not the + / - 1st diffraction order.

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