Encoder device

Abstract

A position measurement encoder device includes a scale including a series of features readable by a read head to determine relative positions of the scale and the read head and diffract light into a plurality of diffraction orders; a read head including a light source for illuminating the scale, a first sensor, and a second sensor; wherein at least one diffraction order has an optical state that is different from an optical state of at least one other diffraction order, and is configured such that, by the optical state of the diffraction order, the signal sensed by the first sensor is formed from a diffraction order composition that is different from a diffraction order composition of the signal sensed by the second sensor.

Description

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

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

[0003] So-called "incremental" position encoders may work, for example, 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 positions are "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) may be directed onto the scale, thereby diffracting the light into a plurality of diffraction orders. The diffraction orders interfere / interact / recombine at the detector to produce the composite field. Optionally, an optical element may be provided within the readhead that redirects / deflects the diffraction orders so that they interfere / interact at the detector to produce the composite field. As the scale and readhead move relative to each other, the composite field changes. The readhead may 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 general prior art incremental position encoder system is schematically illustrated. A scale 1000 has a series of periodic features of period "p" and is illuminated by light to 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. In the specific embodiment described, near infrared light is used. The diffraction orders 1001, 1002, 1003 are relayed to a sensor 3000 (both the sensor and the optical relay element are located in a read head that is movable relative to the scale 1000) via an optical relay element 2000 (e.g., a lens, prism or diffraction grating). The diffraction orders interfere at the sensor 3000 to produce 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 element), when in reality at least one of the 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] According to a first aspect of the present invention, there is provided a position measuring encoder device comprising: a scale comprising a series of features readable by a read head, the series of features extending along a measuring dimension; a read head comprising a light source for illuminating the scale, and a first sensor and a second sensor for detecting signals generated thereat by one or more diffraction orders generated by position features of the scale, which position features can be used to determine the relative position of the scale and the read head along the measuring dimension; wherein at least one diffraction order has an optical state (or in other words, a different "optical characteristic") from the optical state of at least one other diffraction order, and is configured such that, through the optical state (or "optical characteristic") of the diffraction order, the signal sensed by the first sensor is formed by a diffraction order composition that is different from the diffraction order composition of the signal sensed by the second sensor.

[0012] An encoder device according to the present invention may provide improved performance over existing encoder devices. While it is known (e.g. from DE 102015209716, US 2003 / 141441, EP 0760932, US 9651404) that suppression of the 0th diffraction order from a scale may be beneficial in incremental systems, particularly where the scale comprises a relatively fine scale pitch (e.g. less than 20 μm, particularly for scale pitches less than 10 μm), the invention described herein relates to a system that may facilitate selective suppression of different diffraction orders for different sensors within an encoder readhead. This may be beneficial for both incremental and absolute position encoders. A particularly advantageous arrangement for suppressing the 0th order is also described herein.

[0013] In particular, in an encoder device according to the present invention, the signals sensed by the different sensors can be appropriately tailored 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 (e.g., which may be an incremental sensor), doing so may not be beneficial for a second sensor (e.g., which may be a reference mark sensor or an absolute sensor). Accordingly, it is undesirable to completely block such diffraction orders from propagating toward the sensors of the readhead, and therefore, instead, the present invention facilitates selective / customized suppression of diffraction orders of different sensors of the readhead based on their optical states.

[0014] 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, the signal sensed by a first sensor and the signal sensed by a second sensor can both be formed by the 0th diffraction order and the + / - 1st diffraction order, but due to their optical states, the relative influence of the 0th diffraction order and the + / - 1st diffraction order on the signal sensed by the first sensor is different from the relative influence of the 0th diffraction order and the + / - 1st diffraction order on the signal sensed by the second sensor. For example, the device can be configured such that the influence of the 0th diffraction order on the signal sensed by the first sensor is 50% of the influence of the 0th diffraction order on the signal sensed by the second sensor.

[0015] 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.

[0016] 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 optical states 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).

[0017] In the field of encoder devices, it is often the case that the 0th diffraction order and the + / -1st diffraction order have a significant influence on the signal formed at the sensor / the signal sensed by the sensor. Therefore, in a preferred embodiment of the present invention, the 0th diffraction order has an optical state that is different from the optical state of the + / -1st diffraction order (e.g., a different polarization state and / or wavelength state). 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 signals sensed by the first and second sensors, or whether they influence or contribute to the signals sensed by the first and second sensors. Although the +1st diffraction order can be encoded to have an optical state that is different from the -1st diffraction order, it is typically beneficial to configure the readhead so that they have the same optical state (e.g., the same polarization and / or wavelength). Therefore, in other words, it can be beneficial for diffraction orders of the same magnitude (e.g., the + / -1st diffraction order has the same magnitude of "1") to have the same optical state.

[0018] The device can be configured such that the first sensor and / or the second sensor are inherently at least partially blind to a particular optical state (and therefore, inherently at least partially blind to one or more diffraction orders). Accordingly, the composition of the signal sensed by the sensor is determined by the degree to which the sensor is at least partially blind to the optical state. In a preferred embodiment, the encoder device can include a first sensor filter configured to at least partially (e.g., substantially) filter light based on its optical state before it strikes the first sensor, and / or a second sensor filter configured to at least partially (e.g., substantially) filter light based on its optical state before it strikes the second sensor. Accordingly, 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 optical state. Thus, light striking the first sensor can be filtered differently than light striking 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 optical state. Accordingly, the composition of the signals sensed by the first sensor and / or the second sensor may be determined / controlled by the filter.

[0019] In a particularly preferred embodiment, the first sensor filter prevents substantially all of the 0th diffraction order light from reaching the first sensor (in other words, it substantially filters out the 0th diffraction order). In a particularly preferred embodiment, the second sensor filter prevents substantially all of the + / - 1st diffraction order light from reaching the second sensor (in other words, it substantially filters out the + / - 1st diffraction order). As explained in more detail below in conjunction with FIG. 10 , substantially filtering out the + / - 1st order can provide better working gap clarity and, for example, can remove incremental / higher frequency information from the signal.

[0020] As described in more detail below, in preferred embodiments, diffraction orders greater than the + / - 1st diffraction order are substantially suppressed / filtered out / prevented from reaching the first sensor and / or the second sensor. Accordingly, in preferred embodiments, the signal formed at the first sensor is primarily (e.g., solely) formed by the + / - 1st diffraction order. In other preferred embodiments, the signal formed at the second sensor is primarily (e.g., solely) formed by the 0th diffraction order.

[0021] The optical state of the diffraction orders can be controlled / assigned / encoded by a diffraction order encoder.

[0022] The optical states of the diffraction orders can be assigned / encoded by a scale. Accordingly, the scale can include / 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 No. US 2003 / 0141441, which includes a polarization hologram recorded on the scale such that the polarization orientation of the + / - 1st diffraction order is rotated 90° relative to the 0th diffraction order.

[0023] However, if the encoding of the optical state of the diffraction orders is not performed by a scale, the scale (in particular its manufacture) may be simplified. Thus, it may be advantageous if the encoding of the optical state of the diffraction orders is performed after the scale. Therefore, it may be advantageous for the readhead to include a diffraction order encoder that encodes at least one diffraction order having an optical state that is different from the optical state of at least one other diffraction order (or in other words, different optical properties). Providing the readhead with a diffraction order encoder may be simpler and less expensive than providing a holographic scale. Further details of such a diffraction order encoder are provided below.

[0024] The light source may include a polarized light source. For the purposes of this patent application, polarized light refers to light having 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, a polarized light source refers to a light source that emits light having a degree of polarization of at least 0.2.

[0025] The light source may comprise, for example, a laser light source, such as a vertical cavity surface emitting laser (VCSEL). As will be appreciated 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).

[0026] Optionally, the light source is a linearly polarized light source; in other words, optionally, the light source emits linearly polarized light.

[0027] 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. The DOP of completely unpolarized light is 0 (or 0%), while the DOP of completely polarized light is 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,

[0028]

[0029] 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.

[0030] The optical state may include a polarization state. Accordingly, the first sensor may be configured to sense light filtered along a first polarization axis and / or the second sensor may be configured to sense light filtered along a second polarization axis (the second polarization axis may be orthogonal to the first polarization axis). Thus, for example, the first sensor filter and / or the second sensor filter may include a polarization filter. Thus, the diffraction order encoder may be configured to encode at least one diffraction order to have a polarization state that is different from the polarization state of at least one other diffraction order. For example, the diffraction order encoder may be configured to encode one diffraction order (e.g., the 0th diffraction order) so that it is polarized (e.g., perpendicularly polarized) (e.g., so that it has a polarization degree of at least 0.5 or 50%, preferably at least 0.75 or 75%, more preferably at least 0.85 or 85%, for example at least 0.9 or 90%). The other diffraction orders may remain unpolarized, or one or more of the other diffraction orders (e.g., the + / - 1st diffraction orders) may be polarized differently (e.g., orthogonally) than the 0th diffraction order. The diffraction order encoder may comprise at least one polarizer element configured to encode at least one diffraction order having a polarization state that is different from the polarization state of at least one other diffraction order. In a particularly preferred embodiment, the diffraction order encoder comprises a first polarizer element configured to encode the 0th diffraction order having a first polarization state and one or more additional (e.g., second and third) polarizer elements configured to encode the + / - 1st diffraction orders having a second polarization state that is different from the first polarization state. Optionally, the polarizer element comprises a polarizer configured to polarize at least one diffraction order (e.g., increase its degree of polarization). Optionally, the polarizer element comprises at least one diffraction order-specific polarization manipulator (e.g., a wave plate or retarder) configured to change the polarization state of at least one diffraction order, for example, configured to rotate the polarization orientation of the at least one diffraction order. In this case, the diffraction orders will need to be polarized before the diffraction order encoder. Accordingly, for example, as described above, the light source may be a polarized light source, or a polarizer may be placed in the light path before the wave plate.

[0031] In embodiments where the polarization orientation of light emitted from the light source is unknown or can vary (e.g., as is the case with a VCSEL) and the first sensor is configured to sense light filtered along a first polarization axis and / or the second sensor is configured to sense light filtered along a second polarization axis, it can be beneficial to provide a polarization manipulator located in an optical path between the polarized light source and the first and second sensors (e.g., between the polarized light source and the diffraction order encoder) and configured such that, regardless of the polarization state of the light emitted from the polarized light source, light exiting the polarization manipulator toward the first sensor will have a polarization state that is at least partially distinguishable along the first polarization axis and / or at least partially distinguishable along the polarization axis of the second sensor filter. Accordingly, in embodiments where 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, it may be beneficial to provide a polarization manipulator located in an optical path between a polarized light source and the first and second sensors (e.g., between the polarized light source and the diffraction order encoder) and configured such that, regardless of the polarization state of the light emitted from the polarized light source, light exiting the polarization manipulator toward the diffraction order encoder will have a polarization state that is at least partially resolvable along the first polarization axis and at least partially resolvable along the polarization axis of the second sensor filter. Thus, as will be appreciated, the polarization manipulator may be separate from, for example, in addition to, any diffraction polarization manipulator of the diffraction order encoder that is provided to encode a particular diffraction order to have a particular polarization state. In particular, the polarization manipulator is not configured to manipulate the polarization state of different diffraction orders differently than any polarization manipulator of the diffraction order encoder. Rather, it is configured to manipulate the polarization state of all diffraction orders in the same manner. Thus, the polarization manipulator may be referred to as a "bulk" polarization manipulator.

[0032] The (bulk) polarization manipulator may comprise a diffuser. Preferably, the polarization manipulator comprises a retarder (also called a "wave plate"), such as a "patterned retarder" (in other words, a "non-uniform retarder" or a "structured retarder"), as explained in more detail below. Accordingly, the polarization manipulator may be referred to as an optical retarder element.

[0033] As will be understood, 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, 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 repeat, but this is not necessarily the case, and it may be preferred that it does not repeat.

[0034] (Volume) polarization manipulators may include spatial polarization manipulators or temporal (in other words, "time-based") polarization manipulators. As will be understood, in the case of spatial polarization manipulators, the polarization manipulator spatially mixes (or in other words, spatially "scrambles") the polarization states of light (i.e., the polarization states are mixed / changed over the footprint of the light exiting the polarization manipulator), whereas in the case of temporal / time-based polarization manipulators, the polarization states of the light are temporally / temporally mixed (or in other words, temporally / temporally scrambled).

[0035] The (bulk) polarization manipulator may comprise a wave plate having a uniform fast axis (because the angle / orientation of the fast axis is substantially the same at least over the region through which the light from the light source passes). In embodiments where the light is configured to pass through the wave plate once on the path between the light source and the sensor, the wave plate may be a quarter wave plate. In embodiments where the light is configured to pass through the wave plate twice on the path between the light source and the sensor, the wave plate may be an eighth (1 / 8) wave plate (in this case, the effective total retardation of the polarization manipulator is 1 / 4 due to the two passes). Therefore, it may be beneficial for the (bulk) polarization manipulator to comprise a wave plate where the effective total retardation of the polarization manipulator is n+1 / 4, where n is an integer ≥ 0. It may be beneficial for the angle between the fast axis of the wave plate and the first polarization axis to be between 30° and 60°, optionally between 35° and 55°, optionally between 40° and 50°, for example 45°. Similarly, it may be beneficial for the angle between the fast axis of the wave plate and the second polarization axis to be between 30° and 60°, optionally between 35° and 55°, optionally between 40° and 50°, for example 45°.

[0036] The optical state may comprise a wavelength state. Thus, the first sensor filter and / or the second sensor filter may comprise a wavelength filter. For example, the diffraction order encoder may be configured to encode at least one diffraction order to have a wavelength state that is different from the wavelength state of at least one other diffraction order. For example, the diffraction order encoder may be configured to encode one diffraction order (e.g., the 0th diffraction order) so that it has light that falls within a first wavelength band. The device may be configured so that the wavelengths of the other diffraction orders remain unchanged, or alternatively, the device may be configured so that the diffraction order encoder encodes one or more of the other diffraction orders (e.g., the + / - 1st diffraction orders) to have a second wavelength band that is different from the first wavelength band.

[0037] In embodiments where the readhead comprises a diffraction order encoder, the light source and the diffraction order encoder may be provided on a single mounting member (in other words, a single substrate, such as a glass substrate).

[0038] The readhead may comprise at least one optical relay element for relaying 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 for relaying diffraction orders from the scale towards the first sensor and the second sensor. 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 (for example, at least one optical relay element may include a diffraction order encoder integrated therein).

[0039] Optionally, the device is configured so that the diffraction orders converge onto corresponding / corresponding light spots (or "different convergence points") on the optical path between the optical relay element and the sensor. For example, there may be a light spot / convergence point for each diffraction order, such as a 0th diffraction order light spot / convergence point, a +1st diffraction order light spot / convergence point, a -1st diffraction order light spot / convergence point, and so on. Such light spots may be located at the focal plane (focal plane of the lens) of the optical relay element used to relay the diffraction order. 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 used to relay the diffraction order, 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.

[0040] The optical states of diffraction orders greater than the + / - 1st order can be configured to selectively control the extent to which, or whether at all, they affect or contribute to the signals sensed by the first and / or second sensors. Such diffraction orders may include, for example, the + / - 3rd and / or + / - 5th orders. Alternatively, in some embodiments, many of which are preferred, diffraction orders greater than the + / - 1st order are simply prevented / stopped / blocked from reaching the first and second sensors, such that they have no effect on the signals formed at the first and second sensors. Such diffraction orders may be stopped by absorption, deflection, scattering, and / or reflection. For example, an opaque material may be positioned at the aforementioned light spot / convergence point to absorb and block selected diffraction orders, or all diffraction orders greater than the + / - 1st 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 into an optical state such that these diffraction orders do not interact / interfere with the 0th and / or + / - 1st diffraction orders at at least the first sensor (and the second sensor (if present)), or such that these diffraction orders are at least partially (e.g., substantially) filtered out by appropriate filters (e.g., filtered out by the first sensor filter and / or the second sensor filter) before reaching at least the first sensor (and the second sensor (if present)).

[0041] The scale may be illuminated with collimated light (optionally, the diffraction orders of the scale may themselves be collimated). Optionally, the readhead comprises an optical collimator element for collimating the light from the light source. Optionally, the same optical element is used to collimate the light from the light source and to relay the diffraction orders towards the first and second sensors. Optionally, the same optical element is used to collimate the light from the light source and to focus the diffraction orders to corresponding / respective light spots.

[0042] The device may be configured such that the first sensor and the second sensor are located substantially at conjugate planes 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 the second sensor.

[0043] 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 its first and 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 smaller than 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.

[0044] 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.

[0045] 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.

[0046] The scale may be a transmissive scale. Alternatively, the scale may be a reflective scale. Accordingly, preferably, the light source of the read head and the first and second sensors are located on the same side of the scale.

[0047] 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.

[0048] 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.

[0049] 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 optical system and p is the period of the scale (this is / can be achieved when the 0th diffraction order has been substantially removed). The scale may comprise an incremental scale track comprising a series of periodic features defining the incremental scale track. One or more reference marks may be provided, embedded in and / or located adjacent to the incremental scale track. Such reference marks may comprise optical reference marks. Accordingly, optionally, the signal generated at the sensor may comprise a reference mark signal. The second sensor may be configured to detect a reference mark signal generated by the reference mark.

[0050] Optionally, the signal generated at the second sensor includes an absolute position signal. Accordingly, the second sensor may include an absolute position sensor. Accordingly, the scale may include an absolute scale track, which includes a series of position features that define the absolute scale track. As will be understood, the difference between an absolute scale track and an incremental scale track (with or without reference marks) is that its features define a series of unique positions along the length of the scale, which 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 (for example, at startup) without having to move to a reference position (for example, a reference mark). Examples of absolute scales include those described in U.S. Patents US 7499827 and US 5279044. The scale may include separate incremental scale tracks and absolute scale tracks. Optionally, the incremental scale features and the absolute scale features are combined in one track. For example, the absolute scale features can be superimposed on the periodic incremental scale features. As is known, and as explained above in the prior art referred to in this paragraph, absolute position information can be encoded in a scale track by omitting selected position features from an otherwise periodic series of position features.

[0051] As will be appreciated, the scale may comprise a first series of position features and a second series of position features readable by a readhead. As will be appreciated, the first series of position features may produce a first set of diffraction orders and the second series of position features may produce a second set of diffraction orders (which produce the signals detected by the first and second sensors). These first and second sets of diffraction orders may be superimposed / spatially overlapped with each other. It may be that the position measuring encoder device is configured such that only one of the first and second sets of diffraction orders has diffraction orders that have different optical states. For example, the position encoder device may be configured such that at least one diffraction order in the first set of diffraction orders (e.g. the 0th diffraction order) has an optical state that is different from the optical state of at least one other diffraction order in the first set of diffraction orders (e.g. the + / - 1st diffraction orders), but the diffraction orders in the second set of diffraction orders (e.g. at least the 0th diffraction order and the + / - 1st diffraction orders) may have the same optical state. Alternatively, the position-measuring encoder device may be configured such that at least one diffraction order in the first group of diffraction orders has an optical state that is different from the optical state of at least one other diffraction order, and such that at least one diffraction order in the second group of diffraction orders has an optical state that is different from the optical state of at least one other diffraction order. Either way, as will be understood, the first group of diffraction orders and the second group of diffraction orders may be filtered differently based on their optical states.

[0052] 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.

[0053] As will be appreciated, the output of the first sensor and the output of the second sensor may provide one or more signals that may be used to indicate relative position information (with respect to the readhead and the scale). As explained in more detail later in this document, such position information may be incremental position information or absolute position information. Such position information may be index (also referred to as "reference" or "datum") position information. The position information may be linear position information or angular position information (for example, in the case of a rotary encoder device). The readhead may output raw, unprocessed signals from the sensors. Alternatively, the readhead may process the signals from the sensors and output one or more signals derived from the signals from the sensors. As is typically the case with position encoders, the readhead may output one or more signals, for example, quadrature signals (for example, sine and cosine signals), which vary as the relative position of the scale and readhead changes (i.e., as a function of relative motion). The readhead may output an analog signal or a digital signal representing the relative position of the scale and readhead. For example, the quadrature signal may be an analog quadrature signal or a digital quadrature signal. Alternatively, the readhead may maintain and output an incremental "count" of the position of the readhead and scale (e.g., it may count from an index position). Alternatively, the readhead may output a digital codeword representing the relative position of the scale and readhead. For example, the readhead may output an absolute digital codeword representing the absolute relative position of the scale and readhead. As will also be understood, a controller may use the output of the readhead to determine how to control the device / apparatus on which the encoder device is mounted.

[0054] The position measuring encoder device may be a linear encoder or a rotary encoder device. Correspondingly, the scale may be a linear scale or a rotary scale (in which case the scale may be a ring scale or a disk scale).

[0055] According to a second aspect of the present invention, there is provided a position measuring encoder apparatus comprising: a scale comprising a series of features readable by a readhead in order to determine the relative position of the scale and the readhead; and a readhead comprising:

[0056] - a light source for illuminating the scale,

[0057] - a primary position information sensor and a secondary position information sensor, the signal from the primary position information sensor being used to determine the relative position of the scale and the read head,

[0058] - a lens configured to relay light diffracted by the scale to the primary position information sensor and the auxiliary position information sensor,

[0059] - a primary sensor filter configured to filter light before it falls on the primary position information sensor; and / or

[0060] - an auxiliary sensor filter configured to filter light before it falls on the auxiliary position information sensor;

[0061] The readhead further comprises an optical diffraction order encoder located in the path of the diffracted light relayed to the primary position sensor and the auxiliary position sensor, encoding different diffraction orders of the light into different optical states, which are then selectively filtered by the at least one primary sensor filter and / or the at least one auxiliary sensor filter, so that the light falling on the primary position information sensor is filtered differently from the light falling on the auxiliary position information sensor. The statements made above in conjunction with the first aspect of the invention also apply to this second aspect of the invention.

[0062] Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which:

[0063] Figure 1a and Figure 1b is a schematic diagram illustrating the optical operating principle of a general prior art position encoder system;

[0064] Figure 2 An incremental scale and a read head of an encoder device according to the present invention are shown;

[0065] Figure 3 Schematically illustrates a first embodiment of the present invention. Figure 2 Various optical components in the read head;

[0066] Figure 4 and Figure 5 yes Figure 2 and Figure 3 A schematic optical diagram of an encoder device;

[0067] 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;

[0068] Figure 7 Demonstrated for use in Figures 2 to 5 Example electro-optical grating sensor used in a read head;

[0069] Figure 8a Shows from Figures 2 to 5 An example reference mark signal of the reference mark photodiode of the read head, and Figure 8b The differential signals obtained therefrom are shown;

[0070] Figure 9 Schematically illustrating an incremental scale and a readhead of an encoder device according to a second embodiment of the present invention;

[0071] Figures 10 and 11 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;

[0072] Figure 12 Schematically illustrating an incremental scale and a readhead of an encoder device according to a third embodiment of the present invention;

[0073] Figure 13 Schematically illustrating an incremental scale and a readhead of an encoder device according to a fourth embodiment of the present invention;

[0074] Figure 14 Schematically illustrating an absolute scale and a readhead of an encoder apparatus according to a fifth embodiment of the present invention;

[0075] Figure 15 Schematically illustrating an absolute scale and a readhead of an encoder apparatus according to a sixth embodiment of the present invention;

[0076] Figure 16 Schematically illustrating a scale and a read head of an encoder device according to a seventh embodiment of the present invention;

[0077] Figure 17 Schematically illustrating a scale and a read head of an encoder device according to an eighth embodiment of the present invention;

[0078] Figure 18 schematically illustrates a schematic optical diagram of an encoder device according to a ninth embodiment of the present invention;

[0079] Figure 19 Shown Figure 16 Example optical retarder elements used in embodiments of the present invention; and

[0080] Figure 20 The scale and readhead of an encoder device according to a tenth embodiment of the invention are schematically illustrated.

[0081] refer to Figure 2 , an encoder apparatus 2 according to the invention comprises a scale 4 and a readhead 6. The readhead 6 is movable in the X dimension relative to the scale 4 (but the scale may also be movable relative to the readhead, or indeed both the readhead and the scale may be movable). For example, the scale 4 may be mounted to a fixed part of a machine (not shown) and the readhead 6 may be mounted to a movable part of the machine (not shown). 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).

[0082] 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.

[0083] In an alternative embodiment, the scale 4 may be a phase-type scale, in which the projections and depressions in the scale modulate the phase of the light leaving the scale towards the readhead. However, the invention may be particularly useful when used in conjunction with an amplitude-type scale rather than a phase-type scale, due to the presence of a 0th diffraction order produced by an amplitude-type scale (whereas a 0th diffraction order is often not produced by a phase-type scale).

[0084] 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 11 is embedded within the incremental features 10. In the depicted embodiment, the reference mark comprises a single continuous block formed by omitting a plurality of consecutive reflective lines.

[0085] Figure 3 The various optical components located within the read head 6 are shown (the body of the read head has been removed for clarity). Figure 3). In this embodiment, the readhead includes a light source 12 (in this embodiment in the form of a light emitting diode (LED) (which in this embodiment emits infrared light)), a lens 14, a diffraction order encoder 16 (which may also be referred to as an "optical spatial encoder"), a first (hereinafter referred to as the "primary") position information sensor 18, a second (hereinafter referred to as the "auxiliary") position information sensor 20, and a first ("primary") sensor filter 22. The light source 12 and the diffraction order encoder 16 are arranged in substantially the same plane, on an opaque substrate 15. As will be appreciated, the substrate 15 need not actually be opaque, but opaque means that other diffraction orders are blocked, which may be beneficial. As will be explained in more detail below, in this embodiment, the primary position information sensor is configured to detect the signal / synthetic field generated by the incremental feature 10 and output a signal from which the incremental position can be determined / monitored, and the second position information sensor is configured to detect the signal / synthetic field generated by the reference mark 11 and output a signal indicative of the presence of the reference mark. Accordingly, the primary position information sensor 18 can be (and will be hereinafter) referred to as an incremental sensor 18 , and the auxiliary position information sensor 20 can be (and will be hereinafter) referred to as a reference mark sensor 20 .

[0086] Now refer to Figure 3 、 Figure 4 and Figure 5 The optical configuration of the encoder device 2 is described in more detail. Light from the LED 12 is emitted towards the scale 4 via the lens 14. The LED 12 is positioned substantially at the focal plane fp of the lens 14 so that the light from the LED 12 is substantially collimated by the lens 14. The light from the LED 12 illuminates the footprint 24 on the scale. 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.

[0087] The lens 14 relays light reflected by the scale 4 to an incremental sensor 18 and a reference mark sensor 20 .

[0088] 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. In the case of periodic incremental features 10, the light reflected thereby will be diffracted into identifiable diffraction orders. As discussed above in conjunction with Figure 1a and Figure 1bAs explained, light will primarily be diffracted in the X dimension into the 0th and + / - 1st diffraction orders. Higher diffraction orders (e.g., + / - 3rd and + / - 5th) will also be present, but these are not shown because they are much less intense than the 0th and + / - 1st diffraction orders and, in any case, are blocked by the opaque substrate 15 from reaching either the incremental sensor 18 or the reference mark 20 sensor.

[0089] The diffraction orders are incident on lens 14, which causes the corresponding diffraction orders to converge to form light spots 250 at the back focal plane fp of lens 14; each light spot is formed by a corresponding diffraction order. Light spots 250 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.

[0090] In this embodiment, the diffraction order encoder 16 is co-located with the light spot 250. Specifically, in this embodiment, the diffraction order encoder 16 includes a polarization filter 16a (e.g., having a "vertical" polarization axis) that coincides with the 0th diffraction order light spot 250a, and a first non-polarized transparent region 16b and a second non-polarized transparent region 16c that coincide with the + / -1st diffraction order light spots 250b and 250c. Accordingly, the diffraction order encoder 16 encodes the 0th diffraction order using vertically polarized light while leaving the + / -1st diffraction orders unpolarized. Accordingly, the 0th diffraction order has an optical state that is different from the optical state of the + / -1st diffraction orders.

[0091] The 0th and + / -1st diffraction orders propagate toward the incremental sensor 18 and the reference mark sensor 20. As described above, the main sensor filter 22 is located in the optical path of the diffraction orders traveling toward the incremental sensor 18. The main sensor filter 22 comprises a polarization filter having a "horizontal" polarization axis (i.e., a polarization axis orthogonal to the polarization axis of the first polarization filter 16a). Accordingly, the 0th-order diffraction light is substantially blocked and does not reach the primary position information sensor. In contrast, the + / -1st-order diffraction light can pass through the main sensor filter 22 (but becomes "horizontally" polarized in the process) and thereby falls upon the incremental sensor 18. In particular, the + / -1st-order diffraction orders propagate toward the incremental sensor 18 and interact (interfering constructively and destructively) to form an interference fringe pattern (or "fringe field") that falls upon the incremental sensor 18.

[0092] Blocking the 0th diffraction order provides numerous improvements to the interference fringe pattern falling on incremental sensor 18. In particular, interference fringe pattern visibility is improved. Furthermore, removing the 0th diffraction order means that the interference fringe pattern generated at incremental sensor 18 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.

[0093] 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 18 is shown as the distance between the scale 4 and the read head 6 (commonly referred to as the "working gap") is varied. As shown, when the 0th diffraction order contributes completely to the fringe field, the visibility of the fringe field in the direction of propagation is modulated to have a value of 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.

[0094] Accordingly, overall, blocking the 0th diffraction order produced by the scale improves the absolute fringe visibility of the fringe field falling on the incremental sensor 18, effectively doubling the system resolution and significantly improving the working gap tolerance of the readhead.

[0095] Furthermore, if diffraction orders greater than the 1st diffraction order are also blocked (which is the case in this embodiment due to the opaque substrate 15, but can also be accomplished by encoding these diffraction orders to have the same optical state (e.g., the same polarization state) as the 0th diffraction order), higher-order harmonics are removed from the fringe field, thereby providing a purer interference fringe pattern. As will be appreciated, in alternative embodiments, it is not necessary to block diffraction orders greater than the 1st diffraction order, and, for example, the substrate 15 can be transparent (in which case the first non-polarization transparent area 16b and the second non-polarization transparent area 16c may not be clearly identifiable).

[0096] In the described embodiment, there is no corresponding filter in front of the reference mark sensor 20. Accordingly, both the 0th diffraction order and the + / - 1st diffraction order fall on the reference mark sensor 20. Depending on various factors (described in more detail below), in some cases it may be advantageous not to place a corresponding filter in front of the reference mark sensor. In fact, if the presence of both the 0th diffraction order and the + / - 1st diffraction order falling on the reference mark sensor has no adverse effect on the optical signal (e.g., "image" or "pseudo-image" (see later)) formed at the reference mark sensor 20, then there is no advantage in providing a corresponding filter in front of the reference mark sensor 20 (and in fact, it may be disadvantageous due to the reduced light throughput).

[0097] Accordingly, as described above, the signal sensed by incremental sensor 18 is formed only of the + / - 1st diffraction orders, while the signal sensed by reference mark sensor 20 is formed of the 0th diffraction order and the + / - 1st diffraction orders. Accordingly, the signal sensed by incremental sensor 18 is formed of a diffraction order composition different from the diffraction order composition of the signal sensed by reference mark sensor 20.

[0098] It should be noted that in this embodiment, because the 0th diffraction order has already been polarized by the first polarizer 16a, the 0th diffraction order and the + / -1st diffraction orders are non-uniformly attenuated. If desired, the non-uniform attenuation can be avoided by replacing the first non-polarization transparent area 16b and the second non-polarization transparent area 16c with a second polarization filter 16b and a third polarization filter 16c having a "horizontal" polarization axis (i.e., having a polarization axis orthogonal to the polarization axis of the first polarization filter 16a). In such an embodiment, the 0th diffraction order and the + / -1st diffraction order falling on the reference mark sensor 20 will also be attenuated, which can improve the image quality falling on the reference mark sensor. In addition, the + / -1st diffraction order will still pass through the main sensor filter 22 and interfere to form an interference fringe pattern on the incremental sensor 18.

[0099] According to this embodiment of the invention, the light falling on the incremental sensor 18 is therefore already filtered differently than the light falling on the sensor of the reference mark 20 .

[0100] The light that falls on the incremental sensor 18 and the reference mark sensor 20 can be described as an "image" of the scale 4 because the incremental sensor 18 and the reference mark sensor 20 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 so that they do not reach the sensor will have an impact on the "image" as 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 15) means that the "image" falling on the incremental sensor 18 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 20, the "image" of the scale 4 at the reference mark sensor 20 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 16 so that they fall on the reference mark sensor 20 to the same extent.

[0101] In the described embodiment, the incremental sensor 18 is in the form of an electric grating, in other words, an optical sensor array comprising two or more groups of interdigitated / staggered / interwoven optical sensor elements (also referred to herein as "photodetectors" or "fingers"). For example, each group can detect a different phase of the interference fringe pattern at the incremental sensor 18. 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.

[0102] 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 does not necessarily have to be the case. For example, the electric grating may include two groups of photodiodes that provide only two channels, A' and B'.

[0103] 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 by 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 interference 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 higher than the + / -1st diffraction order to have an optical state different from that of the + / -1st diffraction order reduces / eliminates harmonics in the modulated light spot intensity.

[0104] In the depicted embodiment, the reference mark sensor 20 comprises a split detector including a first photodiode 20a and a second photodiode 20b. Figure 8a The outputs (S1, S2) from the first photodiode 20a and the second photodiode 20b are schematically shown as the read head 6 moves along the scale 4 and passes over the reference mark 11. The signals S1 and S2 from the first photodiode 20a and the second photodiode 20b are subtracted to form Figure 8b The difference signal S3 is shown. The position of the reference mark can be determined by identifying where the falling edge of signal S3 crosses a known threshold in the reading direction shown. As will be appreciated, although in this embodiment a split detector is used to detect the reference mark, other types of detectors and / or other techniques for identifying the presence of a reference mark may be used.

[0105] In the above-described embodiment, the 0th diffraction order is completely filtered out by the polarization filter 22. However, while it may be most preferred to completely filter out the 0th diffraction order, this need not be the case, and benefits may be obtained by at least partially filtering out the 0th diffraction order, for example such that its effect on the signal sensed by the first sensor is reduced by 50% (e.g., compared to its effect on the signal sensed by the reference mark sensor 20).

[0106] Now refer to Figure 9 , shows another embodiment of the encoder device 2 ' according to the present invention. This embodiment is similar to Figure 3 The embodiments shown in and described in conjunction with the drawings are the same, and similar parts share similar reference numerals. Figure 3Unlike the continuous reference mark 11, reference mark 11' comprises a patterned reference mark. In this embodiment, the patterned reference mark comprises one thick dark band and two thin dark bands, each separated by at least one incremental feature between them along the measurement direction. The term "patterned" in the expression "patterned reference mark" is not intended to imply a repetitive design / form, but rather to connote a non-uniform design / form. Of course, a "patterned reference mark" may or may not have a repetitive form / design. Reference mark sensor 20 includes two sets of correspondingly arranged and shaped / sized photodiodes 21a, 21b, and 21c located on opposite sides of incremental sensor 18. Each set of photodiodes 21a, 21b, 21c of the reference mark sensor 20 is configured such that, when and only when the readhead 6 and the reference mark 11 are aligned, an image (or "pseudo-image") of the reference mark 11 pattern falls on the three photodiodes 21a, 21b, 21c of the reference mark sensor 20 and is aligned / correlated 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 the reference mark 11' includes a dark band, the intensity of the light received at the reference mark sensor 20' will drop significantly when the readhead 6 and the reference mark 11' are aligned. The signal processing electronics and / or software downstream of the reference mark sensor 20' can be configured to recognize this change in the intensity of the light received at the reference mark sensor 20' and output a signal indicating 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 21a, 21b, 21c act as one photodiode.

[0107] In this embodiment, the diffraction order encoder 16' includes a first polarization filter 16a', a second polarization filter 16b', and a third polarization filter 16c' that respectively coincide with the 0th diffraction order spot 250a, the +1st diffraction order spot 250b, and the -1st diffraction order spot 250c to polarize the 0th diffraction order, the +1st diffraction order, and the -1st diffraction order. The polarization axis of the second polarization filter 16b' and the polarization axis of the third polarization filter 16c' are configured to have the same orientation as each other, so that the + / -1st diffraction order is polarized by the second polarization filter 16b' and the third polarization filter 16c' so that these diffraction orders have the same (e.g., "horizontal") polarization orientation as each other. The polarization axis of the first polarization filter 16a' is configured to have a different (preferably orthogonal) orientation than the polarization axes of the second polarization filter 16b' and the third polarization filter 16c', so that the 0th diffraction order is polarized to have a different (e.g., "vertical") polarization orientation than the + / -1st diffraction order. Accordingly, the diffraction order encoder 16' 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 first polarization filter 16a', the second polarization filter 16b', and the third polarization filter 16c' are spaced apart from each other so that they are not in direct contact with each other, but this need not necessarily be the case.

[0108] The polarized diffraction orders propagate towards the incremental sensor 18 and the reference mark sensor 20'. Figure 3 In the same manner as the embodiment described in the embodiment of , the main sensor filter 22 is located in the optical path of the diffraction order advancing toward the incremental sensor 18. The main sensor filter 22 includes a polarization filter, the polarization axis of which is parallel to the polarization axis of the second polarization filter 16b' and the polarization axis of the third polarization filter 16c' (and therefore orthogonal to the polarization axis of the first polarization filter 16a'). Accordingly, the 0th order diffraction light is blocked and cannot reach the main position information sensor. In contrast, the + / -1st order diffraction light can pass through the main sensor filter 22 and thereby fall on the incremental sensor 18. In particular, the + / -1st diffraction orders propagate toward the incremental sensor 18 and interact (interfere constructively and destructively) to form a fringe field that falls on the incremental sensor 18. The above in combination Figure 3 The embodiment of FIG. 1 illustrates the benefit of preventing the 0th diffraction order from contributing to the optical signal falling on the incremental sensor 18 .

[0109] and Figure 3Compared to the embodiment of FIG1 , a second ("auxiliary") sensor filter 23 is positioned in the optical path of the diffraction orders heading toward the reference mark sensor 20. Specifically, auxiliary sensor filter 23 comprises a polarization filter whose polarization axis is parallel to that of first polarization filter 16a' (and therefore orthogonal to the polarization axes of second polarization filter 16b' and third polarization filter 16c'). Accordingly, auxiliary sensor filter 23 filters out / blocks the + / -1st diffraction orders, thereby preventing them from contributing to the optical signal falling on reference mark sensor 20'. Consequently, only the 0th diffraction order light reaches reference mark sensor 20' and contributes to the optical signal falling on the reference mark sensor. Accordingly, in this embodiment, the signal sensed by incremental sensor 18 is formed only by the + / -1st diffraction orders, while the signal sensed by reference mark sensor 20 is formed by the 0th diffraction order. Accordingly, the signal sensed by incremental sensor 18 is formed by a diffraction order composition that is different from the diffraction order composition of the signal sensed by reference mark sensor 20.

[0110] It has been found that blocking the + / - 1st diffraction order from contributing to the reference mark signal on the reference mark sensor 20' is advantageous, particularly in those embodiments where there are incremental features located within the reference mark. In particular, blocking the + / - 1st diffraction order contribution to the reference mark signal can provide better working gap clarity and, for example, can remove incremental / higher frequency information from the signal. For example, referring to Figures 10 and 11, the effect of filtering out the + / - 1st diffraction order on the image (or "pseudo-image") at the detector plane of the readhead is illustrated. In particular, Figure 10a 、 Figure 10b and Figure 10c 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 10a to 10c The reference mark image (highlighted by the circle in the image) is good, but the image of the reference mark is significantly corrupted when the readhead moves away from its nominal working gap. It has been found that this is particularly problematic when the reference mark includes periodic incremental features. This corruption of the image causes the signal output by the reference mark sensor 20 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 the accuracy of the reference mark (e.g., the reference mark signal is too broad to be accurate within one incremental period).

[0111] Figure 11a 、 Figure 11b and Figure 11c 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 11, 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 (reconstructed from the 0th diffraction order) Figures 11a to 11c ) also maintains a good structure and a good likeness of the reference mark, and thus provides a significantly strong reference mark signal from the reference mark sensor 20'. Accordingly, it has been found beneficial to provide an auxiliary sensor filter 23 in front of the reference mark sensor 20' 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, for this configuration, placing the auxiliary sensor filter 23 in front of the reference mark sensor 20', which filters out the + / - 1st diffraction orders, provides the readhead with a better working gap tolerance.

[0112] As will be appreciated, the reference mark sensor 20 may include a sensor that is compatible with Figure 3 A similar split detector is described in which the photodiodes 21a, 21b, 21c 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, such as Figure 8b A difference signal is generated from which the presence of the reference mark can be detected.

[0113] exist Figure 9 In an alternative embodiment to the one shown, the main sensor filter 22 may be omitted. In this case, both the 0th and + / - 1st diffraction orders will fall on the incremental sensor 18. However, since they have different polarization states, they will not interfere with each other. Figure 6 As shown in the figure, the effect of this on fringe visibility is eliminated as in the embodiment where the 0th diffraction order is prevented from reaching the incremental sensor. 2 However, the absolute visibility of the fringes is less than in those embodiments that prevent the 0th diffraction order from reaching the incremental sensor.

[0114] Now refer to Figure 12 , shows another embodiment of the encoder device 2" according to the present invention. Figure 9The embodiments shown in and described in conjunction with the same are substantially identical and like parts share like reference numerals. In this embodiment, the light of the diffraction orders that impinge on the diffraction order encoder 16" has been polarized. In this particular embodiment, this is achieved via the presence of a polarization filter 13, through which the light emitted from the light source 12 passes before reaching the lens 14. For example, the polarization filter 13 may have a "horizontal" polarization axis, so that the light is therefore "horizontally" polarized. Therefore, the light of the diffraction orders from the scale is similarly horizontally polarized. The difference in this embodiment is that the diffraction order encoder 16" includes one or more wave plates instead of a polarization filter. For example, the diffraction order encoder 16" includes a half-wave plate 16a" arranged to coincide with the 0th diffraction order spot 250a, and non-polarization-changing transparent areas 16b and 16c arranged to coincide with the 0th diffraction order spot 250a. Accordingly, after the diffraction order encoder 16", the polarization orientation of the 0th diffraction order light is rotated 90° so that it becomes "vertical" polarization, while the polarization orientation of the + / -1st diffraction order light remains unchanged (and therefore the polarization orientation of the 0th diffraction order is orthogonal to the polarization orientation of the + / -1st diffraction order). Accordingly, similar to Figure 9 In an embodiment, the 0th diffraction order and the + / -1st diffraction order can be selectively filtered out as needed. In particular, the 0th diffraction order is selectively filtered out by the main sensor filter 22 (which includes a polarization filter with a horizontal polarization axis) to prevent it from contributing to the optical signal falling on the incremental sensor, and the auxiliary sensor filter 23 includes a polarization filter with a vertical polarization axis, so that the + / -1st diffraction orders are selectively filtered out to prevent these diffraction orders from contributing to the optical signal falling on the reference mark sensor 20'. As will be understood, in an alternative embodiment, the diffraction order encoder 16" can include a half-wave plate (16b, 16c) arranged to coincide with the + / -1st diffraction order light spots 250b, 250c, and the non-polarization-changing transparent area (16a") can be arranged to coincide with the 0th diffraction order light spot 250a. In another alternative embodiment, the scale may polarize light rather than using a polarization filter 13 inside the readhead (for example, the scale 4 may be coated with a polarizing film, or a polarization hologram may be recorded on the scale as described in US Patent Application Publication No. US2003 / 0141441).

[0115] As will be appreciated, the use of polarized light sources and wave plates can be used in other embodiments, such as those described above in conjunction with Figure 3 and Figure 9 Said or combined with the following Figures 13 to 15 Those embodiments described.

[0116] The above described embodiment utilizes a polarization filter placed directly in front of the incremental sensor 18 and the reference mark 20 / 20 ′ sensor. Figure 13Shows the above Figure 3 The embodiment of the invention is substantially the same as that of the invention, except that the main sensor filter 22 has been replaced with a polarizing beam splitter 70 that splits the light from the diffraction order encoder 16 into two beams of orthogonal polarization states. In particular, the polarizing beam splitter 70 (primarily) allows only vertically polarized light to pass directly toward the reference mark sensor 20, and (primarily) allows only horizontally polarized light to be deflected toward the incremental sensor 18. As a result, the 0th diffraction order (which has been vertically polarized by the first polarizing filter 16a of the diffraction order encoder 16) does not contribute (primarily) to the signal formed at the incremental sensor 18 (because it has been substantially blocked by the polarizing beam splitter 70), while both the 0th diffraction order and the + / -1st diffraction orders contribute to the signal formed at the reference mark sensor 20. As will be understood, the polarizing beam splitter 70 can be used in place of the main and auxiliary sensor filters of the other embodiments described herein. Furthermore, the precise location of the polarizing beam splitter is not critical, as long as it is disposed between the diffraction order encoder 16 and the detector plane. Accordingly, in some applications it may be advantageous to arrange the diffraction order encoder substrate on the front face of the polarizing beam splitter and / or to arrange the detectors 18, 20 on the exit face.

[0117] 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 readhead 506 (the body of which has been omitted for clarity) shares some of the same parts as those described above in connection with other embodiments of the invention, and similar parts therefore share the same reference numerals. For example, the readhead 506 comprises a light source 12 (in this embodiment a non-polarized LED light source), a lens 14, a diffraction order encoder 16, an incremental detector 18 and a main sensor filter 22, which are the same components as those described above in connection with other embodiments of the invention. Figures 3 to 1216. The primary sensor filter 22 is configured and arranged in the same manner as described in the embodiment of FIG. Accordingly, the 0th diffraction order is blocked so that it does not contribute to the formation of the signal detected by the incremental sensor 18. As with the other embodiments, the primary sensor filter 22 includes a polarization filter having a polarization axis perpendicular to the polarization axis of the first polarization filter 516a of the diffraction order encoder 16. In particular, in this embodiment, the first polarization filter 516a has a vertical polarization axis, and the primary sensor filter 22 has a horizontal polarization axis.

[0118] 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 18 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 18.

[0119] 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. 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, the auxiliary sensor filter 23 may be disposed in front of the absolute sensor 520, which has a polarization axis that is orthogonal to the polarization axes of the second polarization filter 516b and the third polarization filter 516c of the diffraction order encoder 16. In particular, in this embodiment, the second polarization filter 516b and the third polarization filter 516c have horizontal polarization axes, and the auxiliary sensor filter 23 has a vertical polarization axis.

[0120] 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 first polarization filter 516a of the diffraction order encoder 16, and the + / - 1st diffraction order in the first group of diffraction orders passes through the second polarization filter 516b and the third polarization filter 516c, while the 0th diffraction order and the + / - 1st diffraction order in the second group of diffraction orders both pass through the first polarization filter 516a of the diffraction order encoder 16. Higher diffraction orders in the first group of diffraction orders (e.g., the + / - 3rd diffraction order, the + / - 5th diffraction order) are blocked by the opaque substrate 15. The + / - 3rd diffraction order in the second group of diffraction orders passes through the second polarization filter 516b and the third polarization filter 516c of the diffraction order encoder 16. Diffraction orders greater than the + / - 3rd diffraction order in the second group of diffraction orders have substantially insignificant power, but the diffraction orders that exist therein will be blocked by the opaque substrate 15. 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 polarisation filter.

[0121] 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 9 and Figure 12 (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 14Thus, as will be appreciated, the concept of a scale track comprising two basic scale periods (e.g., 8 μm and 32 μm) is not limited to the multi-track embodiment. In this case, since the incremental position features and the absolute position features in this embodiment have different nominal periods, the above comments regarding the two sets of diffraction orders still apply. If desired, in an alternative configuration, the incremental sensor 18 and the absolute sensor 620 may be configured in accordance with Figure 14 arrangement of embodiments (i.e., two sensors side by side).

[0122] Now refer to Figure 16 , shows another embodiment of an encoder device 2000 according to the present invention. This embodiment is similar to Figure 3 、 Figure 9 and Figure 12 The embodiments shown in and described above in conjunction with these embodiments are similar, and like parts share similar reference numerals. In this embodiment, the light source 12' comprises a laser light source, in particular a vertical cavity surface emitting laser (VCSEL) light source. A laser light source (such as a VCSEL) can provide an optical power advantage over a non-laser light source (such as an LED), which in turn can provide an improved optical signal at the sensor of the read head, such as an optical signal with reduced noise, and thus help provide an improved position measurement signal (e.g., a position measurement signal with less jitter). As will be understood by those skilled in the art, laser light sources and, for example, VCSELs emit polarized light. However, the inventors have found that although VCSELs emit polarized light (which is Figure 12 LED and polarization filter configuration provided), but only with VCSEL replacement Figure 12 The light source 12 of the encoder device will not provide a satisfactorily effective encoder device. This is because the orientation of the polarization axis of the light emitted from the VCSEL varies between VCSELs, and also because the orientation of the polarization axis of the light emitted from the VCSEL is affected by external influences such as temperature and applied current (which may change due to servo), and therefore changes during operation of the encoder. Therefore, if only VCSELs are used instead of Figure 12If the LED 12 and polarization filter 13 are configured, the polarization orientation of the light impinging on the waveplate of the diffraction order encoder 16' is unknown and / or also varies with time, which will result in potentially significant imbalances and / or variations in the intensity of the light falling on the incremental sensor 18 and the reference mark sensor 20' (because these sensors are configured to only sense light filtered along specific (first and second) polarization axes by the primary sensor filter 22 and the auxiliary sensor filter 23 in this embodiment). This problem can be reduced / overcome by placing a (e.g., bulk) polarization manipulator (such as an optical retarder element 17) in the path between the light source and the diffraction order encoder 16, the diffraction order encoder being configured to ensure that regardless of the polarization state of the light emitted from the polarized light source, the light exiting the polarization manipulator (e.g., optical retarder element 17) toward the first sensor will have a polarization state that is at least partially resolvable along the polarization axes of the primary sensor filter 22 and the auxiliary sensor filter 23.

[0123] In one embodiment, the optical retarder element 17 can be configured to reduce the degree of polarization of light passing through it, regardless of the polarization of the orientation of the polarized light it receives / passes through it. As will be understood, the "degree of polarization" or "DOP" of light, as it will sometimes be referred to herein, is a simple measure that quantifies the degree of polarization of the light. The DOP of completely unpolarized light is 0 (or 0%), while the DOP of completely polarized light is 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 outgoing 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,

[0124]

[0125] Suitable optical retarder elements may include half-wave plates having a non-uniform fast axis along at least one dimension. Figure 19An example optical retarder element 17 is shown in more detail in . In this embodiment, the optical retarder element 17 includes a non-uniform fast axis along the Y dimension, which is orthogonal to the measurement dimension (X) of the encoder, and can therefore be described as a "non-uniform retarder" or a "patterned retarder". As will be understood, the use of the terms "pattern" and "patterned" in this document, especially in conjunction with a polarization manipulator, is not intended to imply that there is any kind of repeating configuration. Rather, the terms "pattern" and "patterned" are used to indicate that the polarization manipulator (e.g., retarder) has a non-uniform design / form. Of course, the design / form / pattern of the polarization manipulator (e.g., retarder) may repeat, but this is not necessarily the case, and it may be preferred that it does not repeat.

[0126] In particular, the optical retarder element 17 comprises 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 non-uniform patterned retarder 17 does not include any structure in the measurement dimension (X). This can be advantageous in order to prevent the non-uniform / patterned retarder 17 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 18, 21. However, as will be understood, in other embodiments / applications, such diffraction may be tolerable and the patterned retarder may therefore include structure in the X dimension, for example, it may be arranged so that the retarder pixel array / series extends along the Y dimension, or the patterned retarder may include a two-dimensional array of retarder pixels. The following table lists the fast axis orientation of each of the sixty pixels that make up the non-uniform / patterned retarder 17:

[0127]

[0128] Accordingly, as shown in the figure, in this embodiment, the non-uniform / patterned retarder 17 includes a plurality (in this embodiment, sixty) of half-wave plate pixels 19 having more than two different fast-axis orientations, and they are not periodically arranged.

[0129] In an alternative embodiment, rather than using a non-uniform / patterned retarder, an alternative solution to addressing the issue of light sources emitting polarized light is to provide a wave plate in the optical path between the light source and the sensor (before their respective polarization filters), with the wave plate arranged so that its fast axis is angled with the polarization axis of the primary sensor filter 22 and also with the polarization axis of the auxiliary sensor filter 23. Thus, in this embodiment, rather than having an array of discrete elongated half-wave plate pixels / rows 19, the optical retarder element 17 has a uniform fast axis (because the angle / orientation of the fast axis is substantially the same, at least across the area through which the light from the light source passes). In embodiments where the light is configured to pass through the wave plate once on its path between the light source and the sensor, the wave plate can be a quarter-wave plate. In embodiments where the light is configured to pass through the wave plate twice on its path between the light source and the sensor, the wave plate can be a one-eighth (1 / 8) wave plate (in which case the effective total wave retardation of the polarization manipulator is 1 / 4 due to the two passes). Accordingly, it may be beneficial for the polarization manipulator to comprise a wave plate, wherein the effective total wave delay of the polarization manipulator is n+1 / 4, where n is an integer ≥0.

[0130] Accordingly, when optical retarder element 17 is replaced with a wave plate having a uniform fast axis, the wave plate will be a one-eighth (1 / 8) wave plate. A 45° angle between the wave plate's fast axis and the polarization axis of primary sensor filter 22 (in this embodiment, this would mean a 45° angle between the wave plate's fast axis and the polarization axis of auxiliary sensor filter 23) can be beneficial because it can help ensure that, all other factors being equal, the optical power of the signal falling on incremental sensor 18 will be the same regardless of the polarization state of the light output by VCSEL 12' (and in this embodiment, it would also mean that, all other factors being equal, the optical power of the signal falling on reference mark sensor 20' will be the same regardless of the polarization state of the light output by VCSEL 12'). As will be appreciated, a 45° deviation in the angle between the wave plate's fast axis and the polarization axis of primary sensor filter 22 of incremental sensor 18 will mean that the optical power of the signal falling on incremental sensor 18 will be different regardless of the polarization state of the light output by VCSEL 12. However, it is acceptable that the optical power of the signal falling on incremental sensor 18 (and / or reference mark sensor 20') varies depending on the polarization state of the light output by VCSEL 12. In this case, the limit of the acceptable angle between the fast axis of the wave plate and the polarization axis of main sensor filter 22 (and auxiliary sensor filter 23) will depend on the acceptable range of variation in the optical power of the signal falling on incremental sensor 18 (and reference mark sensor 20'). However, the inventors have found that it is generally desirable that the angle between the fast axis of the 1 / 4 wave plate and the polarization axis of main sensor filter 22 be between 30° and 60°, more preferably between 35° and 55°, and even more preferably between 40° and 50° (same for auxiliary sensor filter 23).

[0131] As will be appreciated, in other embodiments (e.g., in embodiments where the scale has a relatively coarse pitch (e.g., 20 μm or more)), blocking the 0th diffraction order from reaching the main (e.g., incremental) sensor may not be preferred or significantly beneficial, however, blocking one or more diffraction orders from reaching the auxiliary (e.g., reference mark or absolute) sensor may still be beneficial. Thus, in those cases, and in contrast to the other embodiments described above, the main sensor filter 22 may be omitted, such that the 0th and + / -1st diffraction orders may fall on the main (e.g., incremental) sensor, while the auxiliary sensor filter 23 may be arranged to block the + / -1st diffraction order (where higher diffraction orders are blocked by the opaque substrate 15) from reaching the auxiliary (e.g., reference mark or absolute) sensor. In this case, the diffraction order encoder may be configured such that the 0th diffraction order is not polarized at all, but this will result in an imbalance in the optical power of the 0th and + / -1st diffraction orders falling on the incremental sensor. Therefore, the polarization axes of the diffraction order encoder 16 and the main and auxiliary sensor filters 22, 23 can be selected so that no + / -1st diffraction orders reach the auxiliary (e.g., reference mark / absolute) sensor, but equal amounts of the 0th and + / -1st diffraction orders reach the main (e.g., incremental) sensor. This can be achieved, for example, by configuring the diffraction order encoder 16 so that the 0th diffraction order is encoded with a polarization orientation of 0° and the + / -1st diffraction orders are encoded with a polarization orientation of 45°, configuring the main sensor filter 22 with a polarization axis of 22.5° (whereby equal amounts of the 0th and + / -1st diffraction orders reach the main (e.g., incremental) sensor), and configuring the auxiliary sensor filter 23 with a polarization axis of 315° so that its polarization axis is orthogonal to the polarization axis of the + / -1st diffraction orders, thereby preventing the + / -1st diffraction orders from reaching the auxiliary (e.g., reference / absolute) sensor.

[0132] All of the above embodiments rely on polarization to encode different diffraction orders into different optical states. However, this does not necessarily have to be the case. For example, different wavelengths can be used to encode different diffraction orders. Figure 17Such an embodiment is shown. The encoder 700 of this embodiment shares many of the same parts as the above-described embodiments, and similar parts share the same reference numerals. Instead of the single LED 12 of the above-described embodiment, this embodiment includes a red LED 612 and a blue LED 613, the light from which is collimated by a lens 14 and illuminates a footprint 24 on the scale 4. In this embodiment, the diffraction order encoder 716 and the main sensor filter 722 and the auxiliary sensor filter 723 include wavelength (e.g., color) filters as opposed to polarization filters to selectively control which diffraction orders contribute to / do not contribute to the signals formed at the respective incremental sensor 18 and reference mark sensor 720. In particular, with respect to the diffraction order encoder 716, the first wavelength filter 716a includes a blue filter that encodes the 0th order using only blue wavelengths, and the second wavelength filter 716b and the third wavelength filter 716c include red filters that encode the + / - 1st diffraction orders using only red wavelengths. Accordingly, to prevent the 0th diffraction order from contributing to the signal formed at incremental sensor 18, main sensor filter 722 includes a red wavelength filter to allow only light having a red wavelength to pass. Similarly, to prevent the + / - 1st diffraction order from contributing to the signal formed at reference mark sensor 720, auxiliary sensor filter 723 includes a blue wavelength filter to allow only light having a blue wavelength to pass. As described above in connection with other embodiments, main sensor filter 722 or auxiliary sensor filter 723 may be omitted if desired. As will be appreciated, while red and blue wavelengths are used in this example, other wavelengths may be used. Furthermore, a single polychromatic light source may be used instead of two different monochromatic light sources.

[0133] As will be appreciated, other types of sensors may be used in place of the electrical grating described above. For example, in embodiments where the modulated light spot is created by the optical system of the read head rather than by an interference fringe pattern, a bulk sensor photodiode may be used to detect the intensity of the modulated light spot, as described, for example, in US 4,776,701.

[0134] In the above described embodiments the 0th diffraction order is (substantially) completely blocked from reaching the incremental sensor 18 and / or the + / - 1st diffraction order is completely blocked from reaching the reference mark sensors 20 / 20'. However, this need not necessarily be the case. For example, depending on the particular device configuration and / or the desired performance enhancement, sufficient benefit may be obtained by only partially attenuating the 0th diffraction order and / or the + / - 1st diffraction order rather than blocking it / them completely (e.g. attenuating it by at least 50%), but preferably the 0th diffraction order is attenuated by at least 90%, e.g. at least 95%, e.g. at least 98%. Such partial blocking / attenuation may be achieved by appropriately choosing the relative orientation of the polarization axes of the sensor filters 22, 23 and the polarization axes of the polarization filters 16a, 16b, 16c of the diffraction order encoder 16, i.e. such that these polarization axes are not strictly orthogonal (or e.g. attenuated in the example of FIG. 1 ). Figure 17 In an embodiment, by appropriately choosing the wavelengths filtered out by the wavelength filter). Similar considerations apply to the + / - 1st diffraction order and auxiliary (e.g. reference mark or absolute) sensors.

[0135] In the above embodiment, the scale is a reflective scale. However, this need not necessarily be the case. For example, the scale could be a transmissive scale, where the light source and sensor are located on opposite sides / faces of the scale.

[0136] The above embodiments use filters 22, 23, 70, 722, 723 located in front of the sensor to block / attenuate selected diffraction orders from reaching the incremental sensor 18 and / or the reference mark sensor 20, 20'. In alternative embodiments, the device can be configured so that the sensor 18, 20, 20', 720 itself is inherently at least partially blind to one or more diffraction orders (e.g., by an integrated polarizer on the sensor). For example, in Figure 3 In an embodiment of the present invention, the incremental sensor 18 can be configured such that the 0th diffraction order (and optionally diffraction orders greater than + / - 1st diffraction order) is not sensed even if the 0th diffraction order (and optionally diffraction orders greater than + / - 1st diffraction order) impinges on the incremental sensor 18 (because the filter 22 has been removed). For example, the incremental sensor 18 can be configured to be sensitive only to light polarized in a particular orientation (e.g., "vertically" polarized light), and the diffraction order encoder 16 can be configured such that the 0th diffraction order (and optionally diffraction orders greater than + / - 1st diffraction order) that reaches the incremental sensor 18 only (or substantially only) includes light polarized in a different orientation (e.g., includes "horizontally" polarized light).

[0137] The above embodiment uses a refractive lens 14 to relay the diffraction orders towards the sensor. However, it will be appreciated that this need not necessarily be the case. For example, Figure 18The optical system of an encoder device according to another embodiment of the present invention is schematically shown. The optical system includes an incremental scale 4 and a readhead 6' that does not rely on a refractive lens. In this embodiment, the 0th diffraction order and the + / -1st diffraction order are generated by light from a light source impinging on the scale 4 and being reflected by the scale. As shown in the figure, the 0th diffraction order and the + / -1st diffraction order are located in the first plane pl 1 The diffraction order encoder is located on the first plane pl 1 The diffraction order encoder includes a polarization filter 16a" (e.g., having a "vertical" polarization axis) that coincides with the 0th diffraction order so as to encode the 0th diffraction order to have a first (e.g., vertical) polarization orientation. The diffraction order encoder further includes a second polarization filter 16b" and a third polarization filter 16c", which coincide with the +1st diffraction order and the -1st diffraction order. The second polarization filter 16b" and the third polarization filter 16c" 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 different from the polarization orientation of the 0th diffraction order. The diffraction grating G2 is positioned to converge the polarizations of the + / -1st diffraction orders. The 0th diffraction order is not deflected. Accordingly, the 0th diffraction order and the + / -1st diffraction order are in the plane pl in which the incremental sensor 18 and the reference mark sensor 20 are located. 2 As with many of the embodiments described above, a first polarizer 22 may be located in front of the incremental sensor 18 and may be configured to filter out the 0th diffraction order so that it does not reach the incremental sensor 18, and / or a second polarizer 23 may be located in front of the reference mark sensor 22 and may be configured to filter out the + / - 1st diffraction orders so that they do not reach the reference mark sensor 22. For clarity, Figure 18 As will be appreciated, a separate refractive lens or grating may be used to collimate light from the light source towards the scale if desired or necessary.

[0138] Many of the embodiments described above use 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 20 The optical arrangement schematically shown in (which also happens to show an absolute encoder device including an absolute scale). Figure 20 The encoder 3000 is similar to Figure 14The encoder is of the type described herein (and like parts share like reference numerals) in that it comprises an absolute scale 504 comprising features which 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 3006 (the body of which has been omitted for clarity) comprises a light source 12, a first lens 3009 which collimates light from the light source 12, a beam splitting member 3007 which enables light from the light source 12 to pass through to the scale 504 to illuminate the scale and redirects light reflected from the scale back to sensors 18, 520 which are configured to sense an image of the scale formed thereat by a second lens 3010.

[0139] The invention has been described above in conjunction with linear encoder devices. However, the invention is equally applicable to rotary encoder devices, including both ring encoders (where the scale features are formed on the cylindrical outer surface of a ring member) and disc encoders (where the scale features are formed on the planar surface of a disc member).

Claims

1. A position measurement encoder device comprising: a scale comprising a series of position features readable by a readhead, the series extending along a measurement dimension and diffracting light into a plurality of diffraction orders; a readhead comprising: i) a light source for illuminating the scale, and ii) first and second sensors for detecting signals produced at the first and second sensors by one or more of the diffraction orders, the diffraction orders being usable to determine the relative position of the scale and readhead along the measurement dimension; The read head is configured so that at least one of the diffraction orders has an optical state different from the optical state of at least one other diffraction order, and is configured so that through the optical state of the diffraction order, the signal sensed by the first sensor is formed by a diffraction order composition different from the diffraction order composition of the signal sensed by the second sensor.

2. The device according to claim 1, wherein The 0th diffraction order has an optical state that is different from the optical states of the + / - 1st diffraction orders.

3. A device as claimed in any preceding claim, comprising a first sensor filter, the first sensor filter being configured to filter the light based on its optical state before it falls on the first sensor, and / or a second sensor filter being configured to filter the light based on its optical state before it falls on the second sensor.

4. The apparatus of claim 3, configured such that the first sensor filter prevents substantially all 0th diffraction order light from reaching the first sensor.

5. A device as claimed in claim 3 or 4, configured such that the second sensor filter prevents substantially all + / - 1st diffraction order light from reaching the second sensor.

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

7. The apparatus as claimed in claims 3 and 6, wherein: The first sensor filter and / or the second sensor filter comprises a polarization filter.

8. The apparatus according to any one of claims 1 to 5, wherein The optical state includes the wavelength of the diffraction order.

9. The apparatus of claim 8, wherein: The first sensor filter and / or the second sensor filter comprises a wavelength filter.

10. Apparatus as claimed in any preceding claim, wherein The different diffraction orders of the light relayed to the first and second sensors are configured to converge to correspondingly different convergence points on an optical path between the scale and the first and second sensors.

11. The apparatus of claim 10, wherein: The convergence point is located at a conjugate plane of the light source.

12. Apparatus as claimed in any preceding claim, wherein The read head includes a diffraction order encoder located on a path of diffracted light relayed to the first sensor and the second sensor, the diffraction order encoder encoding the diffraction orders of the light to have the different optical states.

13. The device according to claim 10 or 11 and 12, wherein The diffraction order encoder is located at the convergence point such that at least one diffraction order is encoded to have an optical state that is different from an optical state of at least one other diffraction order.

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

15. Apparatus as claimed in any preceding claim, wherein The second sensor includes an absolute sensor or a reference mark sensor configured to sense an absolute signal or a reference mark signal.

16. An apparatus as claimed in any preceding claim, configured such that diffraction orders greater than the + / - 1st diffraction order are not sensed by the first position information sensor and / or the second position information sensor.

17. A position measurement encoder device comprising: a scale comprising a series of features extending along a measurement dimension and diffracting light into a plurality of diffraction orders; as well as a readhead configured to detect the scale feature and output one or more signals, the one or more signals being usable to determine the relative position of the scale and the readhead, the readhead comprising: - a light source for illuminating the scale, a primary position information sensor and an auxiliary position information sensor, the signals from which can be used to determine the relative position of the scale, - a primary sensor filter configured to filter light before it falls on the primary position information sensor, and / or - an auxiliary sensor filter configured to filter light before it falls on the auxiliary position information sensor; In which, the read head further includes an optical diffraction level encoder, which is located on the path of the diffracted light relayed to the main position sensor and the auxiliary position sensor, and encodes different diffraction levels of the light into different optical states. The diffraction levels are then selectively filtered by the at least one main sensor filter and / or the at least one auxiliary sensor filter, so that the light falling on the main position information sensor has been filtered differently from the light falling on the auxiliary position information sensor.

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