Encoder Device

Abstract

a scale readable by the readhead and including a series of features that diffract light into a plurality of diffraction orders so as to determine the relative position of the scale and the readhead; and a readhead including a light source for illuminating the scale, a first sensor, and a second sensor, wherein at least one diffraction order has an optical state that is different from the optical state of at least one other diffraction order, and the optical states of the diffraction orders are configured such that a signal sensed by the first sensor is formed from a diffraction order composition that is different from the diffraction order composition of a signal sensed by the second sensor.

Description

[Technical Field]

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

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

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

[0004] FIG. 1a shows a schematic of the optical operation of such a typical prior art incremental position encoder system. The scale 1000 has a periodic series of features with period "p" that, when illuminated by light, produces a set of diffraction orders 1001, 1002, 1003. It will be understood that references to light herein include visible and invisible light ranging from ultraviolet to infrared. In the particular embodiment described, near-infrared light is used. The diffraction orders 1001, 1002, 1003 are relayed to a sensor 3000 by an optical relay element 2000 (e.g., a lens, prism, or diffraction grating) (both the sensor and the optical relay element are located within a readhead that is movable relative to the scale 1000). The diffraction orders interfere at the sensor 3000, thereby producing an interference fringe pattern / field (schematically represented by wave 1100) at the sensor 3000 with a period equal to Mp, where M is the magnification of the optical system and p is the scale period. 1a shows the light as being transmitted through the scale, it will be appreciated that the light may be reflected from the scale and therefore may originate from a light source located on the same side of the scale as the sensor. It will be appreciated that typically the light source, relay element 2000 and sensor 3000 will all be provided by a readhead arrangement that is arranged to move relative to the scale 1000.

[0005] Figure 1a is a simplified diagram of the optical situation encountered within an encoder apparatus. In reality, the optical situation shown in Figure 1a is repeated many times along the length of the scale (i.e. across the area illuminated by the light source), producing a longer interference fringe field / pattern at the detector (e.g. as shown schematically in Figure 1b).

[0006] For illustrative purposes, only the 0th and ±1st orders are shown in Figures 1a and 1b. As will be appreciated, higher diffraction orders can be generated and contribute to the formation of the fringe field in sensor 3000, but their intensity, and therefore their contribution to the fringe field, is typically much weaker than the 0th and ±1st diffraction orders (higher orders being progressively weaker in intensity).

[0007] It will be appreciated that if the mark space ratio of the scale (i.e., the ratio of the width of the scale features to the spacing between marks) is exactly 1:1 (as is often the case with amplitude scales in encoder devices), no even diffraction orders (e.g., ±2nd, ±4th diffraction orders) will be produced, and only odd diffraction orders will be produced (e.g., ±3rd, ±5th diffraction orders). In practice, some slight manufacturing errors may mean that the mark space ratio of the scale is not exactly 1:1, and therefore even diffraction orders may be present (although they may be significantly less intense than adjacent odd diffraction orders). It will be appreciated that if the mark space ratio of the scale is intentionally formed so that it is not 1:1, significant even diffraction orders may be present.

[0008] For simplicity of illustration, the ray diagrams in Figures 1a and 1b are shown as transmitted ray diagrams (i.e., light is shown transmitted through each of the scale and optical element), although in practice at least one of these may be reflective.

[0009] As will be appreciated, reference marks may be provided, for example adjacent to and / or embedded within the diffractive features of the scale, to provide predetermined reference positions. Such a position encoder is described in US Patent No. 7,659,992.

[0010] So-called "absolute" position encoders are also known, which can determine the absolute position of a readhead relative to a scale without the need to count from a predetermined position, such as a reference mark or the end position of the scale. Absolute position encoders typically include a scale with unique position data formed thereon along the measuring length of the scale. The data can be in the form of a pseudorandom sequence or discrete code words, for example. By reading this data as the scale reader passes over the scale, the scale reader can determine its absolute position. Examples of absolute position encoders are described in U.S. Pat. No. 7,499,827, U.S. Pat. No. 10,132,657, and U.S. Patent Application Publication No. 2012 / 0072169. Some absolute encoders are known to use incremental scales in conjunction with the absolute scale. Optionally, the absolute scale may retain sufficient periodicity to allow the scale to be used as a periodic incremental scale (e.g., as described in U.S. Pat. No. 7,499,827). In either case, such incremental scales can be used, for example, to fine-tune the determined absolute position. Optionally, after starting up to determine absolute position, such a system can be used to subsequently measure the relative position of the readhead and scale by using the incremental scale to "count" changes in position. Such an incremental scale can be read in the same way as above, for example by analysing the resultant field generated (at a sensor in the readhead) by diffraction orders produced by the scale. Summary of the Invention

[0011] According to a first aspect of the present invention, there is provided a position measurement encoder apparatus comprising: a scale readable by a readhead, extending along a measurement dimension and including a series of features that diffract light into a plurality of diffraction orders; and the readhead including a light source for illuminating the scale and first and second sensors for detecting signals generated thereon by one or more diffraction orders produced by the positional features of the scale which can be used to determine the relative position of the scale and readhead along the measurement dimension, wherein at least one diffraction order has a different optical state (or in other words, different "optical property") than the optical state of at least one other diffraction order, and wherein the optical state (or "optical property") of the diffraction orders is configured such that the signal sensed by the first sensor is formed from a different diffraction order composition than the diffraction order composition of the signal sensed by the second sensor.

[0012] The encoder device according to the present invention can provide improved performance over existing encoder devices. While it is known (e.g., from German Patent Application Publication No. 102015209716, U.S. Patent Application Publication No. 2003 / 141441, EP Patent No. 0760932, and U.S. Patent No. 9651404) that suppressing the zeroth diffraction order from the scale can be beneficial in incremental systems, particularly when the scale includes a relatively fine-pitch scale (e.g., a scale pitch of less than 20 μm, especially less than 10 μm), the invention described herein relates to a system that can facilitate selective suppression of different diffraction orders of different sensors within an encoder readhead. This can be beneficial for both incremental and absolute position encoders. Also described herein is a particularly advantageous arrangement for suppressing the zeroth order term.

[0013] In particular, the encoder apparatus according to the present invention allows for appropriate adjustment of the signals sensed by the different sensors to provide improved / optimized signals for each of the different sensors. For example, it may be beneficial to suppress the zeroth diffraction order from contributing to the signal sensed by a first sensor (which may be, for example, an incremental sensor), but not to do so for a second sensor (which may be, for example, a reference mark sensor or an absolute sensor). Therefore, it is not desirable to completely block such diffraction orders from propagating towards the sensors of the readhead; instead, the present invention facilitates selective / customized suppression of diffraction orders of different sensors of the readhead based on their optical state.

[0014] The diffraction order composition may vary from sensor to sensor, in the sense that the degree to which different diffraction orders contribute to / influence the signal sensed by the sensor varies for different sensors. For example, both the signal sensed by a first sensor and the signal sensed by a second sensor may be formed from the zeroth diffraction order and ±1st diffraction orders, but due to their optical states, the relative influence of the zeroth diffraction order and ±1st diffraction orders in forming the signal sensed by the first sensor is different from the relative influence of the zeroth diffraction order and ±1st diffraction orders in forming the signal sensed by the second sensor. For example, the device may be configured so that the influence of the zeroth diffraction order in the signal sensed by the first sensor is 50% less than the influence of the zeroth diffraction order in the signal sensed by the second sensor.

[0015] While some benefit can be obtained from only partially attenuating the influence of a particular diffraction order (e.g., the zeroth or ±1st diffraction orders) on generating the signal formed at the sensor, for example by attenuating the influence of the particular diffraction order on signal generation by at least 50%, as described above, it is often preferable for the influence of a particular diffraction order on generating the signal sensed by the sensor to be substantially completely attenuated, for example by at least 90%, for example by at least 95%, for example by at least 98%. In particularly preferred embodiments, the readhead is configured so that one or more particular diffraction orders (e.g., the zeroth or ±1st diffraction orders) have substantially no effect on generating the signal sensed by one of the first and second sensors, while their effect on generating the signal sensed by the other sensor remains unaffected. In fact, it has been found that the present invention provides greatest benefit when selected diffraction orders are substantially completely suppressed for at least one of the sensors while remaining substantially unsuppressed for the other sensor.

[0016] Thus, the apparatus may be configured such that the signals sensed by the first and second sensors may be formed / composed of different subsets of diffraction orders based on their optical states. For example, in one embodiment, the signal sensed by the first sensor may be formed of one or more diffraction orders other than the zeroth diffraction order (i.e., the zeroth diffraction order does not contribute to the signal sensed by the first sensor), while the zeroth diffraction order contributes (optionally together with other diffraction orders) to the signal sensed by the second sensor.

[0017] In the field of encoder devices, the zeroth and ±1st diffraction orders often have the strongest influence on the signal generated by / sensed by a sensor. Therefore, in a preferred embodiment of the present invention, the zeroth diffraction order has a different optical state (e.g., a different polarization state and / or wavelength state) than the ±1st diffraction orders. This allows selective control of the extent to which the zeroth and / or ±1st diffraction orders influence or contribute to the signal sensed by the first and second sensors. While the +1st diffraction order can be encoded to have a different optical state than the −1st diffraction order, it is typically beneficial to configure the readhead so that they have the same optical state (e.g., the same polarization and / or wavelength). Therefore, in other words, it may be beneficial for diffraction orders of the same order (e.g., the + / −1st diffraction orders have the same order “1”) to have the same optical state.

[0018] The device may be configured such that the first sensor and / or the second sensor are essentially at least partially blind to certain optical conditions (and thus essentially at least partially blind to one or more diffraction orders). Thus, the composition of the signal as sensed by the sensor is determined by the extent to which the sensor is at least partially blind to the optical conditions. In preferred embodiments, the encoder device may include a first sensor filter configured to at least partially (e.g., substantially) filter light based on the optical conditions before the light impinges on the first sensor, and / or a second sensor filter configured to at least partially (e.g., substantially) filter light based on the optical conditions before the light impinges on the second sensor. Thus, diffraction orders may be selectively at least partially (e.g., substantially) filtered by the first sensor filter and / or the second sensor filter based on their optical conditions. Thus, light impinging on the first sensor may be filtered differently than light impinging on the second sensor. If both a first sensor filter and a second sensor filter are provided, they may be configured to filter at least in part (e.g., substantially) different diffraction orders based on their optical states, and thus the composition of the signal as sensed by the first and / or second sensors may be determined / controlled by the filters.

[0019] In particularly preferred embodiments, the first sensor filter substantially prevents all zeroth order diffracted light from reaching the first sensor (i.e., substantially filters the zeroth diffraction order). In particularly preferred embodiments, the second sensor filter substantially prevents all ±1st order diffracted light from reaching the second sensor (i.e., substantially filters the ±1st diffraction orders). As described in more detail below in connection with FIG. 10 , substantially filtering the ±1st order terms can provide better rideheight clarity, e.g., remove incremental / higher frequency information from the signal.

[0020] As described in more detail below, in preferred embodiments, diffraction orders greater than the ±1 diffraction orders are substantially suppressed / filtered / prevented from reaching the first and / or second sensors. Thus, in preferred embodiments, the signal formed at the first sensor is formed primarily (e.g., exclusively) from the ±1 diffraction orders. In other preferred embodiments, the signal formed at the second sensor is formed primarily (e.g., exclusively) from the 0 diffraction order.

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

[0022] The optical states of the diffraction orders can be imparted / encoded by the scale. Thus, the scale can include / be a diffraction order encoder. For example, the scale can include a birefringent scale or a holographic scale, such as those described in U.S. Patent Application Publication No. 2003 / 0141441. The birefringent or holographic scale includes a polarization hologram recorded on the scale such that the polarization directions of the ±1 diffraction orders are rotated 90° relative to the 0 diffraction order.

[0023] Nevertheless, the scale (and in particular its manufacture) can be simplified if the encoding of the optical states of the diffraction orders is not performed by the scale. It may therefore be beneficial for the encoding of the optical states of the diffraction orders to be performed after the scale. It may therefore be beneficial for the readhead to include a diffraction order encoder that encodes at least one diffraction order with a different optical state (or in other words, different optical properties) than the optical state of at least one other diffraction order. Providing a diffraction order encoder in the readhead may be simpler and cheaper than providing a holographic scale. Further details of such a diffraction order encoder are provided below.

[0024] The light source can include a polarized light source. For 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. (In other words, optionally, the degree of polarization of the light emitted from the polarized light source is 0.2 or greater.) Similarly, for purposes of this patent application, a polarized light source refers to a light source that emits light with a degree of polarization that is at least 0.2.

[0025] The light source can include, 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, laser light sources such as VCSELs emit polarized light, typically having a degree of polarization of, for example, at least 0.3 (typically, 0.3 to 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 appreciated, the "degree of polarization" (or, as sometimes referred to herein, "DOP") of light is a simple metric that quantifies how polarized the light is. Perfectly unpolarized light has a DOP of 0 (or 0%), and perfectly polarized light has a DOP of 1 (or 100%). As will be explained herein, the DOP of light can be easily measured experimentally. Light is passed through a linear polarizer, and the exiting light from the linear polarizer strikes a photodetector. The linear polarizer is rotated 180 degrees (as is the rotation of the linear polarizer) and the optical power observed at the photodetector is measured. The optical power at the photodetector will vary in a sinusoidal pattern if the light has polarization. This sinusoidal modulation reveals the DOP. Thus:

[0028]

number

[0029] As is well known, light can be "circularly" polarized or "elliptically" polarized, just as it can be linearly polarized. As will be appreciated, circularly polarized light has a DOP of 0, per the above measurement. Pure linearly polarized light has a DOP of 1, and elliptically polarized light has a DOP between 0 and 1 (inclusive), depending on how elliptical the elliptical polarization is. As will be appreciated, linearly polarized light may not be "pure" or "purely" linearly polarized, and therefore, linearly polarized light may not have a DOP of 1.

[0030] The optical state can include a state of polarization. Thus, the first sensor can be configured to sense light filtered along a first polarization axis, and / or the second sensor can be configured to sense light filtered along a second polarization axis (which can be orthogonal to the first polarization axis). Thus, for example, the first sensor filter and / or the second sensor filter can include a polarization filter. Thus, the diffraction order encoder can be configured to encode at least one diffraction order with a polarization state different from the polarization state of at least one other diffraction order. For example, the diffraction order encoder can be configured to encode one diffraction order (e.g., the zeroth diffraction order) to be polarized (e.g., vertically polarized) (e.g., to have a degree of polarization of at least 0.5 or 50%, preferably at least 0.75 or 75%, more preferably at least 0.85 or 85%, e.g., at least 0.9 or 90%). The other diffraction orders may remain unpolarized, or one or more of the other diffraction orders (e.g., the ±1 diffraction orders) may be polarized differently (e.g., orthogonally) than the zeroth diffraction order. The diffraction order encoder may include at least one polarizer element configured to encode at least one diffraction order in a polarization state that is different from the polarization state of at least one other diffraction order. In particularly preferred embodiments, the diffraction order encoder comprises a first polarizer element configured to encode the zeroth diffraction order in a first polarization state and one or more additional (e.g., second and third) polarizer elements configured to encode the ±1 diffraction orders in second polarization states that are different from the first polarization state. The polarizer element includes a polarizer configured to polarize (e.g., increase the degree of polarization) at least one diffraction order. The polarizer element includes a polarization manipulator, e.g., a wave plate or retarder, specific to at least one diffraction order, configured to change the polarization state of at least one diffraction order, e.g., to rotate the polarization direction of the at least one diffraction order. In such a case, the diffraction orders would need to be polarized before the diffraction order encoder.Thus, for example, as described above, the light source can be a polarized light source and a polarizer can be placed in the light path before the waveplate.

[0031] In embodiments where the polarization direction of the light emitted from the light source is unknown or can vary (e.g., as in the case of 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 may be beneficial to provide a polarization manipulator located in the light path between the polarized light source and the first and second sensors (e.g., between the polarized light source and the diffraction order encoder), such that, regardless of the polarization state of the light emitted from the polarized light source, light leaving the polarization manipulator towards the first sensor has a polarization state that is at least partially resolvable along the first polarization axis and / or at least partially resolvable along the polarization axis of the second sensor filter. Thus, in embodiments in which a first sensor is configured to sense light filtered along a first polarization axis and a second sensor is configured to sense light filtered along a second polarization axis, it may be beneficial to provide a polarization manipulator located in the 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), such that, regardless of the polarization state of the light emitted from the polarized light source, light leaving the polarization manipulator toward the diffraction order encoder has 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, this polarization manipulator may be separate from, for example, any diffraction polarization manipulator of the diffraction order encoder (provided to encode specific diffraction orders with specific polarization states). Notably, in contrast to any polarization manipulator of the diffraction order encoder, this polarization manipulator is not configured to manipulate the polarization states of different diffraction orders in different ways. Rather, it is configured to manipulate the polarization states of all diffraction orders in the same way. Therefore, this polarization manipulator may be referred to as a "bulk" polarization manipulator.

[0032] The (bulk) polarization manipulator may include a diffuser. Preferably, the polarization manipulator includes a retarder (also known as a "wave plate"), such as a "patterned retarder" (or alternatively, a "non-uniform retarder" or "structured retarder"), as described in more detail below. The polarization manipulator may therefore be referred to as an optical retarder element.

[0033] It will be understood that the use of the terms "pattern" and "patterned" herein, particularly in relation to polarization manipulators, is not intended to imply the existence of any kind of repeating configuration. Rather, the terms "pattern" and "patterned" are used to refer to a polarization manipulator, e.g., a retarder, having a non-uniform design / shape, and in particular a non-uniform fast axis. As will be explained in more detail below, the (fast axis) design / shape / pattern of a polarization manipulator (e.g., a retarder) can be repeated, but this is not necessarily the case, and in some cases it may be preferable not to repeat.

[0034] A (bulk) polarization manipulator can include a spatial polarization manipulator or a temporal (in other words, "time-based") polarization manipulator. As will be appreciated, in the case of a spatial polarization manipulator, the polarization state of the light is spatially mixed (or, in other words, spatially "scrambled") by the polarization manipulator (i.e., the polarization state is mixed / varied across the footprint of the light exiting the polarization manipulator), whereas in the case of a temporal / time-based polarization manipulator, the polarization state of the light is time / temporally mixed (or, in other words, time / temporally scrambled).

[0035] The (bulk) polarization manipulator may include a waveplate with a uniform fast axis (in that the angle / direction of the fast axis is substantially the same at least over the area through which light from the light source passes). In embodiments where light passes through the waveplate once on the path between the light source and the sensor, the waveplate may be a quarter-wave plate. In embodiments where light passes through the waveplate twice on the path between the light source and the sensor, the waveplate may be an octadic (⅛) waveplate (in which case the effective total wavelength retardation of the polarization manipulator is ¼ due to the double pass therethrough). It may therefore be beneficial for the (bulk) polarization manipulator to include a waveplate where the effective total wave retardation characteristic of the polarization manipulator is n+¼, where n is an integer greater than or equal to 0. It may be beneficial for the angle between the fast axis of the waveplate and the first polarization axis to be between 30° and 60°, optionally between 35° and 55°, optionally between 40° and 50°, e.g., 45°. Similarly, it may be beneficial for the angle between the fast axis of the waveplate 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 can include a wavelength state. Thus, the first sensor filter and / or the second sensor filter can include a wavelength filter. For example, the diffraction order encoder can be configured to encode at least one diffraction order at a wavelength state that is different from the wavelength state of at least one other diffraction order. For example, the diffraction order encoder can be configured to encode one diffraction order (e.g., the zeroth diffraction order) so that it has light falling in a first band of wavelengths. The device can be configured so that the wavelengths of the other diffraction orders remain unchanged, or alternatively, the device can be configured so that the diffraction order encoder encodes one or more of the other diffraction orders (e.g., the ±1st diffraction orders) in a second band of wavelengths that is different from the first band of wavelengths.

[0037] In embodiments in which the readhead includes a diffraction order encoder, the light source and the diffraction order encoder may be provided on a single mounting member, in other words on a single substrate, for example a glass substrate.

[0038] The readhead may include at least one optical relay element for relaying the diffraction orders from the scale towards the first and second sensors. For example, the readhead may include at least one refractive and / or diffractive optical relay element for relaying the diffraction orders from the scale towards the first and second sensors. Suitable optical relay elements include lenses and / or diffraction gratings. The diffraction order encoder may be positioned before or after the optical relay element to interact with one or more diffraction orders. Optionally, the diffraction order encoder and the optical relay element may be one and the same (e.g., at least one optical relay element may include a diffraction order encoder integrated therein).

[0039] Optionally, the apparatus is configured to cause the diffraction orders to converge to corresponding / respective spots (or "different convergence points") in the optical path between the optical relay element and the sensor. For example, there may be a spot / convergence point for each diffraction order, e.g., a 0th diffraction order spot / convergence point, a +1st diffraction order spot / convergence point, a -1st diffraction order spot / convergence point, etc. Such spots may be located in a focal plane (a focal plane of a lens) of the optical relay element for relaying the diffraction orders. The diffraction order encoder may be located substantially at these convergence points. For example, the diffraction order encoder may be located in a focal plane of the optical relay element for relaying the diffraction orders, e.g., the focal plane of the optical relay element. The spots may be formed in a conjugate plane of the light source. Thus, the spots may be images of the light source.

[0040] The optical state of diffraction orders greater than the ±1 diffraction orders can be configured to selectively control the extent to which they affect or contribute to the signals sensed by the first and / or second sensors. Such diffraction orders can include, for example, the ±3 diffraction orders and / or the ±5 diffraction orders. Alternatively, in some embodiments, many of which are preferred, diffraction orders greater than the ±1 diffraction orders are simply prevented / stopped / blocked from reaching the first and second sensors so that they do not affect the signals formed therein. Such diffraction orders can be stopped by absorption, deflection, scattering, and / or reflection. For example, an opaque material can be placed at the spot / convergence point for selected or all diffraction orders greater than the ±1 diffraction orders at the conjugate plane of the light source to absorb and block such diffraction orders. Optionally, the diffraction order encoder is configured to encode diffraction orders greater than the ±1 diffraction orders in the optical state such that they do not interact / interfere with the 0th and / or ±1st diffraction orders at at least the first sensor (and, if present, the second sensor), or such that they are at least partially (e.g., substantially) filtered by an appropriate filter (e.g., filtered by the first sensor filter and / or the second sensor filter) before reaching at least the first sensor (and, if present, the second sensor).

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

[0042] The apparatus may be configured such that the first and second sensors lie substantially in conjugate planes of the scale, and the position measurement encoder apparatus may therefore be described as an imaging encoder apparatus, where an image (or pseudo-image) of the scale is formed by the first and second sensors.

[0043] Preferably, the scale comprises what is commonly referred to as an amplitude scale or "Ronchi" scale (see phase scale). As will be appreciated, in an amplitude or "Ronchi" scale, features are configured to control the amplitude of light reflected (or, in transmission scale embodiments, transmitted) towards the readhead (particularly towards its first and second sensors), for example by selectively absorbing, scattering, and / or reflecting light. In contrast, a phase scale is configured to control the phase of light reflected (or, in transmission scale embodiments, transmitted) towards the readhead (particularly towards its first and second sensors), for example by controlling the different depths of scale features to less than the wavelength of light. Typically, an amplitude scale produces a significant zeroth diffraction order, along with significant ±1st diffraction orders (the higher ±odd diffraction orders which decrease in intensity). This is in contrast to a phase scale, which does not produce a zeroth diffraction order.

[0044] Optionally, the period of the scale is 40 μm or less, preferably 20 μm or less, such as 10 μm or less, for example 8 μm or less.

[0045] Preferably, the scale has a feature space (or "mark" space) ratio of 1:1, so in other words the ratio of the width of the scale features to the spacing of the scale is 1:1.

[0046] The scale may be a transmissive scale. Preferably, the scale is a reflective scale. Therefore, the light source and the first and second sensors of the readhead are preferably located on the same side of the scale.

[0047] Optionally, the position measurement encoder device is a one-grating encoder system, and the scale includes only a diffraction grating in the optical path between the light source and the first and second sensors.

[0048] The series of position features of the scale may be provided in at least one track (a "scale track"). The scale may include one or more scale tracks.

[0049] The signal generated by the first sensor can include an incremental position signal, such as an interference fringe pattern or a modulated spot. Thus, the first sensor can include an incremental position sensor. The period of the interference fringes can be Mp / 2, where M is the magnification of the encoder's optics and p is the period of the scale (which can be achieved when the zeroth diffraction order is substantially eliminated). The scale can include an incremental scale track including a series of periodic features that define the incremental scale track. One or more reference marks can be provided within, embedded in, and / or positioned adjacent to the incremental scale track. Such reference marks can include optical reference marks. Thus, optionally, the signal generated by the sensor can include a reference mark signal. The second sensor can be configured to detect the reference mark signal generated by the reference mark.

[0050] Optionally, the signal generated by the second sensor includes an absolute position signal. Thus, the second sensor can include an absolute position sensor. Thus, the scale can include an absolute scale track including a series of position features defining an absolute scale track. As will be appreciated, an absolute scale track differs from an incremental scale track (with or without reference marks) in that its features define a series of unique positions along the length of the scale. It can be read by the readhead such that the relative positions of the readhead and scale can be determined (e.g., upon start-up) at any position along the scale without requiring movement to a reference position (e.g., reference mark). Examples of absolute scales include those described in U.S. Patent Nos. 7,499,827 and 5,279,044. The scale can include a separate incremental track and an absolute scale track. Optionally, the incremental and absolute scale features are combined into one track. For example, the absolute scale features can be superimposed on periodic incremental scale features. In embodiments in which the readhead includes first and second sensors, the first sensor may be configured to detect an incremental scale / signal (and therefore may also be referred to as an incremental sensor) and the second sensor may be configured to detect an absolute scale / signal (and therefore may also be referred to as an absolute sensor), or vice versa. As is known, and as described in the prior art referenced above in this paragraph, absolute position information can be encoded in the scale track by omitting selected position features from a periodic series of other position features.

[0051] As will be appreciated, the scale includes first and second series of position features readable by the readhead. As will be appreciated, the first series of position features can generate a first set of diffraction orders, and the second series of position features can generate a second set of diffraction orders (which generate signals detected by the first and second sensors). These first and second sets of diffraction orders can be superimposed / spatially overlapping with one another. The position measurement encoder device may be configured such that only one of the first and second sets of diffraction orders has a diffraction order with a different optical state. For example, the position encoder device may be configured such that at least one diffraction order (e.g., the zeroth diffraction order) of the first set of diffraction orders has an optical state that is different from the optical state of at least one other diffraction order (e.g., the ±1st diffraction orders) of the first set of diffraction orders, while some diffraction orders (e.g., at least the zeroth diffraction order and the ±1st diffraction orders) of the second set of diffraction orders can have the same optical state. Alternatively, the position measurement encoder apparatus may be configured such that at least one diffraction order of the first set has an optical state that is different from the optical state of at least one other diffraction order, and such that at least one diffraction order of the second set has an optical state that is different from the optical state of at least one other diffraction order. In any event, it will be appreciated that the first and second sets of diffraction orders may be filtered differently based on their optical states.

[0052] The first and / or second series of position features may be periodic (in other words, the scale may include a first periodic series of position features and a second periodic series of position features). The period of the first series of position features may be different from the period of the second series of position features. For example, the scale may include a scale track including a first series of position features having a first period (e.g., a relatively fine period) and a second series of position features having a second period (e.g., a relatively coarse period, i.e., coarser than the period of the first period). The first and / or second series of position features may be provided on the same track, i.e., embedded / superimposed on each other.

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

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

[0055] According to a second aspect of the present invention there is provided a position measuring encoder apparatus comprising a readhead and a scale including a series of features readable by the readhead so as to determine the relative position of the scale and readhead, the readhead comprising: A light source to illuminate the scale, a primary position information sensor and a secondary position information sensor whose signals can be used to determine the relative position of the scale and readhead; a lens configured to relay light diffracted by the scale to both the primary position information sensor and the secondary position information sensor; a primary sensor filter configured to filter light before it strikes the primary position information sensor; and / or a secondary sensor filter configured to filter the light before it hits the secondary position information sensor; Including, The readhead further includes an optical diffraction order encoder located in the path of the diffracted light relayed to the primary and secondary position sensors, the optical diffraction order encoder encoding different diffraction orders of the light in different optical states which are then selectively filtered by the at least one primary sensor filter and / or the at least one secondary sensor filter such that light impinging on the primary position information sensor is filtered differently from light impinging on the secondary position information sensor. Also, what was discussed above in relation to the first aspect of the invention also applies to this second aspect of the invention. [Brief explanation of the drawings]

[0056] Hereinafter, an embodiment of the present invention will be described by way of example with reference to the drawings.

[0057] [Figure 1a] FIG. 1 is a schematic diagram illustrating the optical operation of a typical prior art position encoder system. [Figure 1b] FIG. 1 is a schematic diagram illustrating the optical operation of a typical prior art position encoder system. [Figure 2] 1 shows an incremental scale and readhead of an encoder device according to the present invention. [Figure 3] 3 shows schematically various optical components located within the readhead of FIG. 2 according to a first embodiment of the invention; [Figure 4] FIG. 4 is a schematic optical diagram of the encoder device of FIGS. 2 and 3. [Figure 5] FIG. 4 is a schematic optical diagram of the encoder device of FIGS. 2 and 3. [Figure 6] 10 is a graph showing the effect of the zeroth order term on the visibility of interference fringes impinging on the sensor as the distance between the scale and readhead is changed. [Figure 7] 6 illustrates an exemplary electrical grid sensor for use in the readhead of FIGS. [Figure 8a] 6 shows an exemplary reference mark signal from a reference mark photodiode of the readhead of FIGS. [Figure 8b] The resulting differential signal is shown. [Figure 9] 10 is a schematic diagram illustrating an incremental scale and a readhead of an encoder device according to a second embodiment of the present invention. [Figure 10a] 10 is a graph illustrating the effect of filtering the ±1st diffraction orders on an image (or "pseudo image") at the detector plane of the readhead. [Figure 10b] 10 is a graph illustrating the effect of filtering the ±1st diffraction orders on an image (or "pseudo image") at the detector plane of the readhead. [Figure 10c] 10 is a graph illustrating the effect of filtering the ±1st diffraction orders on an image (or "pseudo image") at the detector plane of the readhead. [Figure 11a] 10 is a graph illustrating the effect of filtering the ±1st diffraction orders on an image (or "pseudo image") at the detector plane of the readhead. [Figure 11b] 10 is a graph illustrating the effect of filtering the ±1st diffraction orders on an image (or "pseudo image") at the detector plane of the readhead. [Figure 11c] 10 is a graph illustrating the effect of filtering the ±1st diffraction orders on an image (or "pseudo image") at the detector plane of the readhead. [Figure 12] 10 is a schematic diagram illustrating an incremental scale and a readhead of an encoder device according to a third embodiment of the present invention. [Figure 13] 10 is a schematic diagram illustrating an incremental scale and a readhead of an encoder device according to a fourth embodiment of the present invention. [Figure 14] 10 is a schematic diagram illustrating an absolute scale and a readhead of an encoder device according to a fifth embodiment of the present invention. [Figure 15] 13 is a schematic diagram illustrating an absolute scale and a readhead of an encoder device according to a sixth embodiment of the present invention. [Figure 16] 13 is a schematic diagram showing a scale and a readhead of an encoder device according to a seventh embodiment of the present invention. [Figure 17] 13 is a schematic diagram showing a scale and a readhead of an encoder device according to an eighth embodiment of the present invention. [Figure 18] FIG. 13 is a schematic optical diagram of an encoder device according to a ninth embodiment of the present invention; [Figure 19] 17 illustrates an exemplary optical retarder element for use with the embodiment of FIG. 16. [Figure 20] 16A and 16B are schematic diagrams illustrating a scale and a readhead of an encoder device according to a tenth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0058] Referring to Figure 2, an encoder device 2 according to the present invention comprises a scale 4 and a readhead 6. The readhead 6 is movable in the X dimension relative to the scale 4 (although this may be in the opposite direction, or indeed both may be movable). For example, the scale 4 may be attached to a stationary part of a machine (not shown) and the readhead 6 may be attached 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 scale features provided on its periphery, or provided on a disk with scale features provided on its flat surface).

[0059] In the described embodiment, the scale 4 is an amplitude scale (in that it controls the amplitude of light that passes towards and away from the scale) and includes a substrate having incremental features 10 in the form of periodic dark / relatively non-reflective lines made on an otherwise relatively reflective substrate, so as to provide relative reflection between the incremental features 10 and the scale. Of course, the scale incremental features 10 could be made in other ways, for example by forming relatively reflective lines on an otherwise relatively non-reflective substrate, or even by forming both relatively reflective and non-reflective lines on a substrate. It will also be appreciated that the incremental features 10 could be provided in other ways, for example they could be provided in the form of reflective surfaces or lines that reflect light towards and away from the readhead. In the described embodiment, the scale substrate is metal, but it will be appreciated that other materials could be used, such as glass, for example.

[0060] In an alternative embodiment, the scale 4 may be a phase scale, in which peaks and pits in the scale modulate the phase of light leaving the scale towards the readhead. However, the present invention may be particularly useful when used in conjunction with an amplitude scale rather than a phase scale, due to the presence of the zeroth diffraction order produced by an amplitude scale (whereas the zeroth diffraction order does not tend to be produced by a phase scale).

[0061] The incremental features 10 form an incremental scale that facilitates measurements along the X axis (the "measurement dimension"). In the illustrated embodiment, a reference mark 11 is embedded within the incremental features 10. In the described embodiment, the reference mark comprises a single contiguous block formed by the omission of multiple contiguous reflective lines.

[0062] Figure 3 shows various optical components located within the readhead 6 (the main body of which has been omitted from Figure 3 for clarity). In this embodiment, the readhead includes a light source 12, 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 "primary") position information sensor 18, a second (hereinafter referred to as "secondary") position information sensor 20, and a first ("primary") sensor filter 22. The light source 12 and the diffraction order encoder 16 are provided substantially in the same plane on an opaque substrate 15. It will be appreciated that the substrate 15 need not actually be opaque, although opacity means that other diffraction orders which may be useful are blocked. As will be explained in more detail below, in this embodiment the primary position information sensor is configured to detect signals / resulting fields generated by the incremental features 10 and output signals from which the incremental position can be determined / monitored, and the secondary position information sensor is configured to detect signals / resulting fields generated by the reference marks 11 and output signals indicative of the presence of the reference marks. Thus, the primary position information sensor 18 may be (and is hereinafter referred to as) the incremental sensor 18, and the secondary position information sensor 20 may be (and is hereinafter referred to as) the reference mark sensor 20.

[0063] The optical arrangement of the encoder device 2 will now be described in more detail with reference to Figures 3, 4 and 5. Light from the LED 12 is emitted towards the scale 4 via a lens 14. The LED 12 is positioned substantially in the focal plane fp of the lens 14 such that the light from the LED 12 is substantially collimated by the lens 14. The light from the LED 12 illuminates a footprint 24 on the scale. In Figures 4 and 5, light traveling towards the scale is shown schematically by a thick dashed line, while light returning from / reflected by the scale is shown by a thin solid line.

[0064] The lens 14 relays light reflected by the scale 4 to an incremental sensor 18 and a reference mark sensor 20 .

[0065] As will be appreciated, due to the well-known natural phenomenon of diffraction, light reflected by the scale 4 will be diffracted due to the presence of the scale features thereon. In the case of the incremental features 10, which are periodic, the light reflected thereby will be diffracted into distinguishable diffraction orders. As discussed above in connection with Figures 1a and 1b, light will be diffracted primarily into the 0th and ±1st diffraction orders in the X dimension. Higher diffraction orders (e.g., ±3rd and ±5th orders) also exist, but are not shown because they have substantially lower intensities than the 0th and ±1st diffraction orders and, in any case, are blocked from traveling in either direction by the opaque substrate 15.

[0066] The diffracted orders are incident on lens 14, which focuses corresponding diffracted orders to form spots 250, each formed from a respective diffracted order, at the back focal plane fp of lens 14. Spots 250 are images of the light source in that the back focal plane fp is a conjugate plane to the plane in which the light source is located.

[0067] In this embodiment, the diffraction order encoder 16 coexists with the spot 250. In particular, in this embodiment, the diffraction order encoder 16 includes a polarizing filter 16a (e.g., with a "vertical" polarization axis) that coincides with the zeroth diffraction order spot 250a and first and second non-polarizing transparent regions 16b, 16c that coincide with the ±1st diffraction order spots 250b, 250c. Thus, the diffraction order encoder 16 encodes the zeroth diffraction order with vertical polarization while leaving the ±1st diffraction orders unpolarized. Thus, the zeroth diffraction order has an optical state that is different from the optical state of the ±1st diffraction orders.

[0068] The zeroth and ±1st diffraction orders propagate toward the incremental sensor 18 and the reference mark sensor 20. As described above, the primary sensor filter 22 is located in the optical path of the diffraction orders traveling toward the incremental sensor 18. The primary sensor filter 22 includes a polarizing filter having a "horizontal" polarization axis, i.e., an axis orthogonal to the polarization axis of the first polarizing filter 16a. Thus, the zeroth diffraction order light is substantially blocked from reaching the primary position information sensor. In contrast, the ±1st diffraction orders are able to pass through the primary sensor filter 22 (albeit with "horizontal" polarization in the process) and thereby impinge on the incremental sensor 18. In particular, the ±1st diffraction orders propagate toward the incremental sensor 18 and interact (constructively and destructively interfere) to form an interference fringe pattern (or "fringe field") that impinges on the incremental sensor 18.

[0069] Blocking the zeroth diffraction order results in many improvements to the interference fringe pattern impinging on incremental sensor 18. In particular, the visibility of the interference fringe pattern is improved. Also, the elimination of the zeroth diffraction order means that the interference fringe pattern produced at incremental sensor 18 has a period equal to Mp / 2 (half that of a system that does not block the zeroth diffraction order). Thus, blocking the zeroth diffraction order effectively doubles the system resolution.

[0070] Figure 6 illustrates another benefit of removing the zeroth diffraction order. In particular, Figure 6 shows how blocking the zeroth diffraction order affects the visibility of fringe patterns impinging on the incremental sensor 20 as the distance between the scale 4 and the readhead 106 (commonly known as the "rideheight") varies. As shown, if the zeroth diffraction order contributes fully to the fringe field, the visibility of the fringe field will be reduced by p 2The zeroth order is modulated in the propagation direction with a period of p / λ, where p is the scale period and λ is the wavelength of the light. If the zeroth order term is eliminated, the modulation no longer exists. This is beneficial because the encoder device that produces the zeroth diffraction order can have a rideheight tolerance that is independent of the scale period.

[0071] So, in summary, blocking the zeroth diffraction order produced by the scale improves the absolute fringe visibility of the fringe field impinging on the incremental sensor 18, effectively doubling the system resolution and significantly improving the rideheight tolerance of the readhead.

[0072] Furthermore, if diffraction orders greater than the first are also blocked (in this embodiment by an opaque substrate 15, but this can also be achieved by encoding them in the same optical state as the zeroth diffraction order, e.g., the same polarization state), higher harmonics will be removed from the fringe field, providing a purer interference fringe pattern. It will be appreciated that in alternative embodiments, diffraction orders greater than the first need not be blocked; for example, substrate 15 could be transparent (in which case first and second non-polarizing transparent regions 16b and 16c may not be clearly distinguishable).

[0073] In the described embodiment, there is no corresponding filter in front of the reference mark sensor 20. Thus, the zeroth and ±1st diffraction orders all impinge on the reference mark sensor 20. Depending on various factors (discussed in more detail below), it may be preferable in some circumstances to not place a corresponding filter in front of the reference mark sensor. Indeed, if the presence of both the zeroth and ±1st diffraction orders impinging on the reference mark sensor does not have a deleterious effect on the optical signal, e.g., the “image” or “pseudo image” (discussed below), formed at the reference mark sensor 20, then there is no advantage to having a corresponding filter in front of the reference mark sensor 20 (indeed, doing so may be detrimental due to reduced photometric performance).

[0074] Therefore, as described above, the signal sensed by incremental sensor 18 is formed from only the ±1st diffraction orders, while the signal sensed by reference mark sensor 20 is formed from the 0th and ±1st diffraction orders. Therefore, the signal sensed by incremental sensor 18 is formed from a diffraction order composition that is different from the diffraction order composition of the signal sensed by reference mark sensor 20.

[0075] One thing to note is that in this embodiment, because only the zeroth order term is polarized by the first polarizer 116a, the zeroth and ±1st diffraction orders are attenuated unevenly. If necessary, this uneven attenuation can be avoided by replacing the first and second non-polarizing transparent regions 16b and 16c with second and third polarizing filters 16b and 16c having "horizontal" polarization axes (i.e., polarization axes orthogonal to the polarization axis of the first polarizing filter 16a). In such an embodiment, the zeroth and ±1st diffraction orders impinging on the reference mark sensor 20 are attenuated equally, which may result in a better-quality image impinging on the reference mark sensor. Furthermore, the ±1st diffraction orders still pass through the primary sensor filter 22 and interfere to form an interference fringe pattern on the incremental sensor 18.

[0076] Therefore, in accordance with the present invention, in this embodiment the light impinging on the incremental sensor 18 is filtered differently than the light impinging on the reference mark sensor 20 .

[0077] Light impinging on the incremental sensor 18 and reference mark sensor 20 can be described as an "image" of the scale 4, in that the incremental sensor 18 and reference mark sensor 20 lie at a detection plane dp, which in the embodiment shown and described is coplanar with the conjugate plane cp of the scale surface sp (e.g. light from point A on the scale is imaged to point A' on the sensor, light from point B on the scale is imaged to point B' on the sensor and light from point C on the scale is imaged to point C' on the sensor). However, it will be appreciated that suppressing / blocking selective diffraction orders such that they do not reach the sensor will affect the "image" as seen at the conjugate plane cp. For example, the removal of the zeroth diffraction order (and diffraction orders greater than the first diffraction order because the opaque substrate 15 blocks them) means that the "image" at the conjugate plane c p when it strikes the incremental sensor 18 has a more purely sinusoidal waveform rather than a square waveform, and so the image on the sensor is not in fact a "true" or "perfect" image of the scale 4 (rather, it might be referred to as a pseudo-image of the scale). As will be appreciated, due to the presence of the zeroth order term in the reference mark sensor 20, the "image" of the scale 4 on the reference mark sensor 20 will be affected to a lesser extent, particularly when the zeroth and ±1st diffraction orders are polarized by the diffraction order encoder 16 so that they both strike the reference mark sensor 20 to the same extent.

[0078] In the described embodiment, incremental sensor 18 is in the form of an electrical grid, or in other words, an optical sensor array including two or more sets of interdigitated / interlaced / interleaved light-sensitive sensor elements (also referred to herein as "photodetectors" or "fingers"). Each set can, for example, detect a different phase of the interference fringe pattern field in incremental sensor 18. FIG. 7 shows an example of an electrical grid. In FIG. 7, a portion of the electrical grid is shown, with four sets of photodiode fingers / photodiodes (A, B, C, and D) interdigitated / interleaved to form an array of sensor elements extending along the length "L" of the sensor.

[0079] The outputs from each finger / photodiode in a set are combined to provide a single output, resulting in four channel outputs: A', B', C', and D'. These outputs are then used to obtain quadrature SIN and COS signals. In particular, A'-C' are used to provide a first signal (SIN), and B'-D' are used to provide a second signal (COS) that is 90 degrees out of phase with the first signal. In certain embodiments, the electrical grid includes four sets of photodiodes providing four channels A', B', C', and D', although this need not be the case. For example, the electrical grid may include two sets of photodiodes providing only two channels, A' and B'.

[0080] It will be appreciated that other types of sensors can be used in place of the electrical gratings described above. For example, in embodiments where a modulated spot is created by the readhead optics instead of an interference fringe pattern, a bulk sensor photodiode can be used to detect the intensity of the modulated spot (e.g., as described in U.S. Pat. No. 4,776,701). In systems that generate modulated spots, similar to systems that generate interference fringe patterns, encoding the 0th and ±1st diffraction orders so that they do not interfere with each other at the sensor effectively doubles the resolution of the encoder system (in that the intensity of the spot modulates at a frequency of p / 2). Similarly, encoding diffraction orders higher than the ±1st diffraction orders in an optical state different from that of the ±1st diffraction orders reduces / eliminates harmonics in the modulated spot intensity.

[0081] In the described embodiment, the reference mark sensor 20 comprises a split detector including first and second photodiodes 20a, 20b. FIG. 8a shows a schematic representation of the outputs (S1, S2) from the first and second photodiodes 20a, 20b as the readhead 6 moves along the scale 4 and passes the reference mark 11. The signals S1, S2 from the first and second photodiodes 20a, 20b are subtracted to form the difference signal S3 shown in FIG. 8b. By identifying where the falling edge of signal S3 crosses a known threshold in the reading direction shown, the position of the reference mark can be determined. It will be appreciated that although the present embodiment uses a split detector to detect the reference mark, other types of detectors and / or other techniques for identifying the presence of a reference mark may be used.

[0082] In the above-described embodiment, the zeroth diffraction order is completely filtered by the polarizing filter 22. However, while it may be most preferable for the zeroth diffraction order to be completely filtered, this is not necessary and it may be beneficial to at least partially filter the zeroth diffraction order, for example so 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).

[0083] Referring now to FIG. 9, another embodiment of an encoder apparatus 2′ according to the present invention is shown. This embodiment is similar to that shown in and described above with reference to FIG. 3, with like parts sharing like reference numerals. Differences include that, unlike the continuous reference mark 11 of FIG. 3, reference mark 11′ comprises a patterned reference mark, which in this embodiment includes 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 phrase “patterned reference mark” is not used to mean that the reference mark has a repeating design / form, but rather to refer to the reference mark having a non-uniform design / form. Of course, a “patterned reference mark” may or may not have a repeating form / design. Reference mark sensor 20′ includes two sets of photodiodes 21a, 21b, and 21c of corresponding arrangement and shape / size, 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 only when the readhead 6 and the reference mark 11 are aligned will an image (or "pseudo image") of the pattern of the reference mark 11' strike and align / correlate with the pattern of the three photodiodes 21a, 21b, 21c of the reference mark sensor 20', thereby causing a significant and sudden change in the intensity of the light impinging thereon. In this embodiment, where the reference mark 11' includes a dark band, when the readhead 6 and the reference mark 11' are aligned, the intensity of the light received by the reference mark sensor 20' drops significantly. Signal processing electronics and / or software downstream of the reference mark sensor 20' may be configured to identify such a change in the intensity of the light received by the reference mark sensor 20' and output a signal indicative of the presence of a reference mark. As far as the downstream signal processing electronics and / or software are concerned, the two sets of correspondingly positioned photodiodes 21a, 21b, 21c function as one photodiode.

[0084] In this embodiment, the diffraction order encoder 16' includes first, second, and third polarizing filters 16a', 16b', and 16c' that are aligned with the 0th, +1st, and −1st diffraction order spots 250a, 250b, and 250c, respectively, to polarize the 0th, +1st, and −1st diffraction orders. The polarization axes of the second and third polarizing filters 16b', 16c' are configured in the same direction as each other so that the ±1st diffraction orders are polarized by the second and third polarizing filters 16b', 16c' and have the same (e.g., "horizontal") polarization direction as each other. The polarization axis of the first polarizing filter 16a' is configured in a different, preferably orthogonal, direction from the polarization axes of the second and third polarizing filters 16b', 16c' so that the 0th diffraction order is polarized in a different (e.g., "vertical") polarization direction from the polarization direction of the ±1st diffraction orders. Thus, diffraction order encoder 16' encodes the +1 and -1 diffraction orders in a first polarization direction and encodes the 0th diffraction order in a second polarization direction different from the first polarization direction. As shown, first, second, and third polarization filters 16a', 16b', 16c' are spaced apart so that they are not in direct contact with each other, although this need not be the case.

[0085] The polarized diffraction orders propagate toward the incremental sensor 18 and the reference mark sensor 20′. In the same manner as in the embodiment described above in connection with the embodiment of FIG. 3, a primary sensor filter 22 is positioned in the optical path of the diffraction orders traveling toward the incremental sensor 18. The primary sensor filter 22 includes a polarizing filter having a polarization axis parallel to the polarization axes of the second and third polarizing filters 16b′, 16c′ (and thus orthogonal to the polarization axis of the first polarizing filter 16a′). Thus, the zeroth diffraction order light is blocked from reaching the primary position information sensor. In contrast, the ±1st diffraction orders pass through the primary sensor filter 22, thereby allowing them to impinge on the incremental sensor 18. In particular, the ±1st diffraction orders propagate toward the incremental sensor 18 and interact (constructively and destructively interfere) to form a fringe field that impinges on the incremental sensor 18. The benefits of preventing the zeroth diffraction order from contributing to the optical signal impinging on the incremental sensor 18 were described above in connection with the embodiment of FIG. 3.

[0086] In contrast to the embodiment of FIG. 3, a second (“secondary”) sensor filter 23 is located in the optical path of the diffraction orders traveling toward the reference mark sensor 20′. In particular, the secondary sensor filter 23 includes a polarizing filter having a polarization axis parallel to the polarization axis of the first polarizing filter 16a′ (and thus orthogonal to the polarization axes of the second and third polarizing filters 16b′, 16c′). The secondary sensor filter 23 will therefore filter / block the ±1st diffraction orders, thereby preventing them from contributing to the optical signal impinging on the reference mark sensor 20′. Therefore, only the 0th order diffracted light will reach the reference mark sensor 122 and contribute to the optical signal impinging on the reference mark sensor 20′. Therefore, in this embodiment, the signal sensed by the incremental sensor 18 is formed only from the ±1st diffraction orders, while the signal sensed by the reference mark sensor 20 is formed from the 0th diffraction order. Therefore, the signal sensed by incremental sensor 18 is formed from a different diffraction order composition than the diffraction order composition of the signal sensed by reference mark sensor 20 .

[0087] Blocking the ±1st order terms from forming the reference mark signal on the reference mark sensor 20' has been found to be advantageous, particularly in embodiments where there are incremental features located within the reference mark. In particular, blocking the ±1st order terms from contributing to the reference mark signal can provide better rideheight clarity, for example, removing incremental / higher frequency information from the signal. For example, with reference to Figures 10 and 11, the effect of filtering the ±1st diffraction orders on an image (or "pseudo image") at the detection plane of the readhead is shown. In particular, Figures 10a, 10b, and 10c show images of the scale reconstructed at the detection plane dp / conjugate plane cp of the readhead from both the 0th and ±1st diffraction orders when the readhead is positioned a) at the readhead's nominal rideheight, b) +75 μm from the nominal rideheight, and c) +150 μm from the nominal rideheight, respectively. As shown, at the nominal ride height, the image of the reference mark (highlighted by a circle in Figures 10a-10c) is good, but as the readhead moves away from the nominal ride height, the image of the reference mark becomes substantially corrupted. This has been found to be a particular problem when the reference mark contains periodic incremental features. Such image corruption causes the signal output by the reference mark sensor 20' to become wider and less distinct as the readhead passes the reference mark. This can cause issues with reference mark reliability (e.g., the signal is too weak to detect the reference mark) and / or reference mark accuracy (e.g., the reference mark signal is too wide and not accurate to within one incremental period).

[0088] Figures 11a, 11b, and 11c show images of the scale reconstructed from only the zeroth diffraction order when the readhead is positioned a) at the readhead's nominal ride height, b) +75 μm from the nominal ride height, and c) +150 μm from the nominal ride height, respectively. As shown in Figure 11, the image of the reference mark reconstructed at the readhead's detection plane dp / conjugate plane cp (highlighted by a circle in Figures 11a-11c) maintains good structure and maintains good likeness of the reference mark, providing a clear and strong reference mark signal from the reference mark sensor 20', even at ride heights away from the nominal ride height. It has therefore been found that it can be beneficial to provide a secondary 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 image of the scale reconstructed at the readhead's detection plane dp / conjugate plane cp. In particular, for this configuration, providing a secondary sensor filter 23 in front of the reference mark sensor 20' that filters out the ±1st diffraction orders provides the readhead with better rideheight tolerance.

[0089] It will be appreciated that, if desired, the reference mark sensor 20' can include a split detector similar to that of Figure 3, where photodiodes 21a, 21b, 21c are duplicated and laterally offset in the X dimension, and the outputs from the duplicate photodiodes are connected to form a second reference mark output which can be used to provide a differential signal as in Figure 8b, from which the presence of a reference mark can be detected.

[0090] In an alternative embodiment, shown in Figure 9, the primary sensor filter 22 can be omitted. In this case, both the 0th and ±1st diffraction orders will impinge on the incremental sensor 18. However, because they have different polarization states, they will not interfere with each other. The effect this has on fringe visibility is shown in Figure 6. As shown, the p of fringe visibility due to changes in ride height is similar to the embodiment that prevents the 0th diffraction order from reaching the incremental sensor. 2λ / λ modulation is removed. However, the absolute visibility of the fringes is less than in the embodiment that prevents the 0th diffraction order from reaching the incremental sensor.

[0091] 12, there is shown another embodiment of an encoder apparatus 2" in accordance with the present invention. This embodiment is substantially the same as the embodiment shown in and described above in connection with FIG. 9, and like parts share like reference numerals. In this embodiment, the light in the diffraction orders that strike the diffraction order encoder 16" is already polarized. In this particular embodiment, this is achieved via the presence of a polarizing filter 13 through which the light emitted from the light source 12 passes before reaching the lens 14. For example, the polarizing filter 13 may have a "horizontal" polarization axis, such that the light is therefore "horizontally" polarized. The light in the diffraction orders from the scale will therefore be horizontally polarized as well. This embodiment also differs in that the diffraction order encoder 16″ includes one or more wave plates instead of a polarizing filter. For example, the diffraction order encoder 16″ includes a half-wave plate 16a″ positioned to coincide with the zeroth diffraction order spot 250a, and non-polarization-changing transparent regions 16b and 16c positioned to coincide with the zeroth diffraction order spot 250a. Thus, following the diffraction order encoder 16″, the polarization direction of the zeroth diffraction order light is rotated 90° to become “vertically” polarized, while the polarization directions of the ±1st diffraction orders light are unchanged (thus, the polarization direction of the zeroth diffraction order is orthogonal to the polarization directions of the ±1st diffraction orders). Thus, similar to the embodiment of FIG. 9 , the zeroth and ±1st diffraction orders can be selectively filtered as desired. In particular, the zeroth diffraction order is selectively filtered by the primary sensor filter 22 (which includes a polarizing filter with a horizontal polarization axis) to prevent it from contributing to the optical signal impinging on the incremental sensor, and the secondary sensor filter 23 includes a polarizing filter with a vertical polarization axis to selectively filter the ±1st diffraction orders to prevent them from contributing to the optical signal impinging on the reference mark sensor 20′. As will be appreciated, in alternative embodiments, the diffraction order encoder 16″ can include half-wave plates (16b, 16c) positioned to coincide with the ±1st diffraction order spots 250b, 250c, and a non-polarization-altering transparent region (16a″) can be positioned to coincide with the zeroth diffraction order spot 250a.In a further alternative embodiment, the scale may polarise light instead of using a polarising filter 13 in the readhead (e.g. the scale 4 may be coated with a polarising film as described in US Patent Application Publication No. 2003 / 0141441 and a polarising hologram may be recorded on the scale).

[0092] It will be appreciated that polarized light sources and waveplates can be used in other embodiments, such as those described above in connection with Figures 3 and 9 or below in connection with Figures 13-15.

[0093] The above-described embodiments utilize polarizing filters positioned immediately before the incremental sensor 18 and the reference mark sensor 20 / 20′. FIG. 13 shows an embodiment substantially the same as that of FIG. 3 described above, except that the primary 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 allows only (predominantly) vertically polarized light to pass straight through to the reference mark sensor 20, and only (predominantly) horizontally polarized light to be deflected toward the incremental sensor 18. As a result, the zeroth diffraction order (polarized vertically by the first polarizing filter 16a of the diffraction order encoder 16) does not (substantially) contribute to the signal formed at the incremental sensor 18 (because it is substantially blocked by the polarizing beam splitter 70), while both the zeroth and ±1st diffraction orders contribute to the signal formed at the reference mark sensor 20. As will be appreciated, the polarizing beam splitter 70 may be used in place of the primary and secondary sensor filters of other embodiments described herein. Furthermore, the exact location of the polarizing beam splitter is not important, as long as it is positioned between the diffraction order encoder 16 and the detection plane. Thus, in some applications, it may be advantageous to place a diffraction order encoder substrate in front of the polarizing beam splitter and / or place the detectors 18, 20 at the exit plane.

[0094] The embodiments described thus far have related to incremental encoders that include an incremental scale with one or more reference marks (or, optionally, without). The present invention can also be used with absolute encoders that include absolute scales, such as those shown in FIGS. 14 and 15. In the embodiment of FIG. 14, the absolute scale 504 includes conceptually periodically arranged features, with selected features removed to encode unique / absolute position data along the measuring length of the scale. The data can be in the form of, for example, a pseudo-random sequence or discrete code words. Details of such scales are described in U.S. Pat. Nos. 7,499,827 and 5,279,044. The readhead 506 (whose body has been omitted for clarity) shares several parts identical to those described above in connection with other embodiments of the present invention, and therefore, similar parts share the same reference numerals. For example, the readhead 506 includes a light source 12 (in this embodiment, an unpolarized LED light source), a lens 14, an optical diffraction order encoder 216, an incremental detector 18, and a primary sensor filter 22. 3-12. Thus, the zeroth order term is blocked from contributing to the formation of the signal detected by the incremental sensor 18. As with the other embodiments, the primary sensor filter 22 includes a polarizing filter having a polarization axis perpendicular to the polarization axis of the first polarizing filter 516a of the diffraction order encoder 16. In particular, in this embodiment, the first polarizing filter 516a has a vertical polarization axis, and the primary sensor filter 22 has a horizontal polarization axis.

[0095] In this embodiment, the readhead 506 further includes an absolute sensor 520. The absolute sensor 520 includes a photodiode array (in this embodiment, a one-dimensional photodiode array, but could also be two-dimensional) that is impinged by an image (or "pseudo-image," see above). As is known and described, for example, in U.S. Pat. Nos. 7,499,827, 5,279,044, and 10,989,567, the image of the scale can be processed to extract an absolute / unique code and thereby 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. Optionally, once the absolute position is determined, subsequent positions can be determined independently by monitoring the output from the incremental detector 18.

[0096] As with the reference mark embodiment described above, it may be beneficial to allow the zeroth order term to contribute to the signal impinging on the absolute sensor. Additionally, as with the reference mark embodiment described above, it may be beneficial to prevent the ±1st diffraction orders from contributing to the signal impinging on the absolute sensor 520. Therefore, a secondary sensor filter 23 may be provided in front of the absolute sensor 520, having a polarization axis orthogonal to the polarization axes of the second and third polarizing filters 516b, 516c of the diffraction order encoder 16. Notably, in this embodiment, the second and third polarizing filters 516b, 516c have horizontal polarization axes, and the secondary sensor filter 23 has a vertical polarization axis.

[0097] FIG. 15 illustrates another embodiment of an absolute encoder 600 including an absolute scale 604. The absolute scale 604 is substantially identical to that described above in connection with FIG. 14 (like parts share the same reference numerals), except that in this embodiment, the scale includes a multi-track arrangement, with absolute code features provided in an absolute scale track 603 provided on either side of a pure incremental track 605. As shown, the absolute scale track 603 also includes finer pitch / period incremental features in spaces between the coarser pitch / period features of the absolute scale track 603. By way of example, the coarse features of the absolute scale track that encode absolute position information may have a nominal period of approximately 32 μm, and the finer pitch / period incremental features may have a period of approximately 8 μm. As a result, two sets of diffraction orders are effectively generated: a first set of diffraction orders generated by a first series of position features (e.g., 8 μm incremental scale features) and a second set of diffraction orders generated by a second series of position features (e.g., absolute scale features). In fact, in this embodiment, the zeroth diffraction order of the first set of diffraction orders passes through the first polarizing filter 516a of the diffraction order encoder 16, the ±1st diffraction orders of the first set of diffraction orders pass through the second and third polarizing filters 516b, 516c, while the zeroth diffraction order and the ±1st diffraction orders of the second set of diffraction orders all pass through the first polarizing filter 516a of the diffraction order encoder 16. The higher diffraction orders of the first set of diffraction orders (e.g., the ±3rd diffraction orders, the ±5th diffraction orders) are blocked by the opaque substrate 15. The ±3rd diffraction orders of the second set of diffraction orders pass through the second and third polarizing filters 516b, 516c of the diffraction order encoder 16. Diffraction orders higher than the ±3rd diffraction orders of the second set of diffraction orders have substantially insignificant power, but what is present will be blocked by the opaque substrate 15. As will be appreciated, which diffraction orders pass through which filters is system dependent and may depend on a variety of factors, including the size and / or position of the polarizing filters.

[0098] As shown, the sensor arrangement of the embodiment of FIG. 15 differs slightly from the embodiment of FIG. 14 in that the readhead of the absolute sensor 620 is split in two with the incremental sensor 22 located between them (similar to the embodiments of FIGS. 9 and 12). A variation of the embodiment of FIG. 15 is for the absolute scale features to extend completely across the incremental scale features (i.e., so that there is no purely incremental scale track 605 between the two absolute tracks 603). Rather, as in the embodiment of FIG. 14, the absolute scale features can be completely embedded throughout the incremental features. Therefore, it will be appreciated that the concept of a scale track including two basic scale periods (e.g., 8 μm and 32 μm) is not limited to multi-track embodiments. In this case, the comments above regarding the two sets of diffraction orders still apply because, in this embodiment, the incremental and absolute position features have different nominal periods. If desired, in an alternative configuration, the incremental sensor 18 and absolute sensor 620 can be arranged according to the embodiment of FIG. 14 (i.e., two sensors arranged side-by-side).

[0099] Referring now to FIG. 16, another embodiment of an encoder apparatus 2000 according to the present invention is shown. This embodiment is similar to the embodiment shown in and described above in connection with FIGS. 3, 9, and 12, and like parts share like 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, e.g., a VCSEL, can offer optical power advantages over non-laser light sources such as LEDs, which in turn can help provide an improved optical signal at the readhead sensor, e.g., an optical signal with reduced noise, and therefore an improved position measurement signal (e.g., a position measurement signal with less jitter). As will be appreciated by those skilled in the art, laser light sources, e.g., a VCSEL, emit polarized light. However, the inventors have determined that simply replacing the light source 12 of the encoder apparatus of FIG. 12 with a VCSEL would not provide a satisfactory and viable encoder apparatus, even though a VCSEL emits polarized light (which is what the LED and polarizing filter configuration of FIG. 12 provides). This is because the direction of the polarization axis of the light emitted from a VCSEL varies from one VCSEL to another, and is affected by external influences such as temperature and applied current (which can be changed by servo control), and therefore changes during operation of the encoder. Therefore, if a VCSEL were simply used in place of the LED 12 and polarizing filter 13 of Figure 12, the polarization direction of the light impinging on the waveplate of the diffraction order encoder 16' would be unknown and / or would change over time, which would result in potentially significantly unbalanced and / or varying intensities of light impinging on the incremental sensor 18 and reference mark sensor 20 (because in this embodiment, the primary and secondary sensor filters 22, 23 are configured to sense only light filtered by specific (first and second) polarization axes).This problem can be mitigated / 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, which is configured to ensure that, regardless of the polarization state of the light emitted from the polarized light source, the light leaving the polarization manipulator (e.g., optical retarder element 17) towards the first sensor has a polarization state that is at least partially resolvable along the polarization axes of the primary and secondary sensor filters 22, 23.

[0100] In one embodiment, the optical retarder element 17 may be configured to reduce the degree of polarization of light passing through it, regardless of the polarization of the light received / passed through it. As will be appreciated, the "degree of polarization" of light (or "DOP," as sometimes referred to herein) is a simple metric that quantifies how light is polarized. Perfectly unpolarized light has a DOP of 0 (or 0%), and perfectly polarized light has a DOP of 1 (or 100%). The DOP of light can be easily measured experimentally, as described herein. Light is passed through a linear polarizer, and the exiting light that passes through the linear polarizer strikes a photodetector. The linear polarizer is rotated 180 degrees, and the optical power observed at the photodetector (while the linear polarizer is rotated) is measured. The optical power at the photodetector will vary in a sinusoidal pattern if the light has polarization. This sinusoidal modulation reveals the DOP. Thus:

[0101]

number

[0102] A suitable optical retarder element can include a half-wave plate having a non-uniform fast axis along at least one dimension. FIG. 19 shows an exemplary optical retarder element 17 in more detail. In this embodiment, the optical retarder element 17 includes a non-uniform fast axis along the Y dimension, which is orthogonal to the measuring dimension (X) of the encoder, and thus may be described as a “non-uniform retarder” or a “patterned retarder.” As will be understood, the use of the terms “pattern” and “patterned” herein, particularly in relation to polarization manipulators, is not intended to imply the presence of any repeating configuration. Rather, the terms “pattern” and “patterned” are used to refer to a polarization manipulator, e.g., a retarder, having a non-uniform design / form. Of course, the design / form / pattern of a polarization manipulator (e.g., a retarder) can be repeated, but this is not necessarily the case, and it may be preferable not to repeat.

[0103] In particular, the optical retarder element 17 includes an array of discrete, elongated half-wave plate pixels / columns 19, with the array extending in the Y dimension and the elongated length of the pixels / columns extending in the X dimension. Thus, the array / series of pixels extends perpendicular to the measuring dimension (X) of the scale 4. Therefore, in this embodiment, the optical retarder element (also referred to as a non-uniform / patterned retarder) 17 does not include structure in the measuring dimension (X). This may be advantageous to prevent the non-uniform / patterned retarder 17 from being a source of diffraction of light in the X dimension, which may otherwise adversely affect the generation of an optical signal at the sensors 18, 21. However, as will be appreciated, in other embodiments / applications, such diffraction may be tolerable, and thus the non-uniform / patterned retarder may include structure in the X dimension, e.g., it may be arranged such that the array / series of retarder pixels extends along the Y dimension, or the non-uniform / patterned retarder may include a two-dimensional array of retarder pixels. The following table lists the fast axis directions of each of the 60 pixels that make up the non-uniform / patterned retarder 17:

[0104] [Table 1]

[0105] Thus, as shown, in this embodiment, the non-uniform / patterned retarder 17 includes a plurality (in this embodiment, 60) of half-wave plate pixels 19 having three or more different fast axis directions, which are not arranged periodically.

[0106] In an alternative embodiment, instead of using a non-uniform / patterned retarder, an alternative solution to address the problem of a light source emitting polarized light is to provide a waveplate in the optical path between the light source and the sensor (before each polarizing filter), which is positioned with its fast axis at an angle relative to the polarization axis of the primary sensor filter 22 and also at an angle relative to the polarization axis of the secondary sensor filter 23. Thus, in this embodiment, rather than an optical retarder element 17 having an array of discrete, elongated half-waveplate pixels / columns 19, the optical retarder element 17 has a uniform fast axis (the angle / orientation of the fast axis is substantially the same at least over the area through which light from the light source passes). In embodiments where light is configured to pass through the waveplate once on its path between the light source and the sensor, the waveplate can be a quarter-wave plate. In embodiments where light is configured to pass through the waveplate twice on its path between the light source and the sensor, the waveplate can be an octadic (⅛) waveplate (in which case the effective full-wave retardation of the polarization manipulator is ¼ due to the double pass therethrough). Therefore, it may be beneficial for the polarization manipulator to include a waveplate, and for the polarization manipulator to have an effective full-wave retardation characteristic of n+1 / 4, where n is an integer greater than or equal to 0.

[0107] Thus, in situations where the optical retarder element 17 is replaced with a waveplate having a uniform fast axis, the waveplate would be an octadic (⅛) waveplate. It may be beneficial for the angle between the fast axis of the waveplate of the primary sensor filter 22 and the polarization axis to be 45° (in this embodiment, this means that the angle between the fast axis of the waveplate of the secondary sensor filter 23 and the polarization axis is 45°). This is because it helps ensure that, all other factors being equal, the optical power of the signal impinging on the incremental sensor 18 is the same regardless of the polarization state of the light output by the VCSEL 12′ (and in this embodiment, it also means that, all other factors being equal, the optical power of the signal impinging on the reference mark sensor 20′ is the same regardless of the polarization state of the light output by the VCSEL 12′). As will be appreciated, a deviation of the angle between the fast axis and the polarization axis of the waveplate of primary sensor filter 22 of incremental sensor 18 from 45° would mean that the optical power of the signal impinging on incremental sensor 18 would not be the same regardless of the polarization state of the light output by VCSEL 12. Nevertheless, it may be acceptable for the optical power of the signal impinging on incremental sensor 18 (and / or reference mark sensor 20′) to vary depending on the polarization state of the light output by VCSEL 12. In such cases, the limit of the acceptable angle between the fast axis and the polarization axis of the waveplate of primary sensor filter 22 (and secondary sensor filter 23) depends on the degree of acceptable variation in the optical power of the signal impinging on incremental sensor 18 (and reference mark sensor 20′). Nevertheless, the inventors have found that it is generally desirable for the angle between the fast axis of the quarter-wave plate of the primary sensor filter 22 and the polarization axis to be between 30° and 60°, more preferably between 35° and 55°, and particularly preferably between 40° and 50° (the same applies to the secondary sensor filter 23).

[0108] As will be appreciated, in other embodiments (e.g., those in which the scale has a relatively coarse pitch, e.g., 20 μm or greater), blocking the zeroth order term from reaching the primary (e.g., incremental) sensor may not be desirable or substantially beneficial, but blocking one or more diffraction orders from reaching the secondary (e.g., reference mark or absolute) sensor may still be beneficial. Thus, in these cases, in contrast to the other embodiments described above, the primary sensor filter 22 may be omitted to allow the zeroth and ±1st diffraction orders to impinge on the primary (e.g., incremental) sensor, but the secondary sensor filter 23 may be provided to block the ±1st diffraction orders (along with the higher diffraction orders blocked by the opaque substrate 15) from reaching the secondary (e.g., reference mark or absolute) sensor. In such cases, the diffraction order encoder may be configured so that the zeroth diffraction order is not polarized at all, which would result in an imbalance in the optical power of the zeroth and ±1st diffraction orders impinging on the incremental sensor. Thus, the polarization axes of the diffraction order encoder 16 and the primary and secondary sensor filters 22, 23 can be selected so that the ±1 diffraction orders do not reach the secondary sensor, e.g., a reference mark / absolute sensor, but an equal amount of the zeroth and ±1st diffraction orders reach the primary sensor, e.g., an incremental sensor. This can be achieved, for example, by configuring the diffraction order encoder 16 so that the zeroth diffraction order is encoded with a polarization direction of 0° and the ±1st diffraction orders are encoded with a polarization direction of 45°; configuring the primary sensor filter 22 to have a polarization axis of 22.5° (whereby an equal amount of the zeroth and ±1st diffraction orders reach the primary sensor, e.g., an incremental sensor); and configuring the secondary sensor filter 23 to have 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 secondary sensor, e.g., a reference / absolute sensor.

[0109] All of the above-described embodiments rely on polarization to encode different diffraction orders in different optical states. However, this need not be the case. For example, different diffraction orders could be encoded at different wavelengths. FIG. 17 illustrates such an embodiment. 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 embodiments, this embodiment includes a red LED 612 and a blue LED 613, the light from which is collimated by a lens 14 to illuminate the footprint 24 on the scale 4. In this embodiment, the diffraction order encoder 716 and primary and secondary sensor filters 722, 723 include wavelength (e.g., colored) filters, as opposed to polarization filters, to selectively control which diffraction orders contribute to the signals formed by 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 zeroth order term only in blue wavelengths, and the second and third wavelength filters 716b, 716c include red filters that encode the ±1st diffraction orders only in red wavelengths. Thus, to prevent the zeroth diffraction order from contributing to the signal generated by the incremental sensor 18, the primary sensor filter 722 includes a red wavelength filter that allows only light having a red wavelength to pass. Similarly, to prevent the ±1st diffraction orders from contributing to the signal generated by the reference mark sensor 720, the secondary sensor filter 723 includes a blue wavelength filter that allows only light having a blue wavelength to pass. As discussed above in connection with other embodiments, the primary or secondary sensor filters 722, 723 can be omitted, if desired. It should be understood that, although red and blue wavelengths are used in this example, other wavelengths can be used. Also, instead of two different monochromatic light sources, a single polychromatic light source can be used.

[0110] It will be appreciated that other types of sensors may be used instead of the electrical gratings described above, for example in embodiments where the modulated spot is produced by the readhead optics instead of an interference fringe pattern, a bulk sensor photodiode may be used to detect the intensity of the modulated spot, as described, for example, in US Pat. No. 4,776,701.

[0111] In the above-described embodiment, the zeroth diffraction order is (substantially) completely blocked from reaching the incremental sensor 18, and / or the ±1st diffraction orders are completely blocked from reaching the reference mark sensor 20 / 20′. This need not necessarily be the case. For example, depending on the particular apparatus configuration and / or desired performance improvement, it may be preferable to attenuate the zeroth order term by at least 90%, e.g., by at least 95%, e.g., by at least 98%, although sufficient benefit may be obtained by only partially attenuating the zeroth and / or ±1st diffraction orders, e.g., by at least 50%, rather than completely blocking them. Such partial blocking / attenuation may be achieved by appropriate selection of the relative orientations of the polarization axes of the sensor filters 22, 23 and the polarization axes of the polarizing filters 16a, 16b, 16c of the diffraction order encoder 16, i.e., so that they are not strictly orthogonal (or, for example, in the embodiment of FIG. 17 , by appropriate selection of the wavelengths filtered by the wavelength filters). Similar considerations apply to the ±1st diffraction orders and to second-order (e.g., reference mark or absolute) sensors.

[0112] In the above described embodiments, the scale is a reflective scale. However, this does not have to be the case. For example, the scale could be a transmissive scale, with the light source and sensor located on opposite sides / faces of the scale.

[0113] The above-described embodiments use filters 22, 23, 70, 722, 723 located in front of the sensors to block / attenuate selected diffraction orders from reaching the incremental sensor 18 and / or reference mark sensor 20, 20′. In alternative embodiments, the apparatus may be configured such that the sensor 18, 20, 20′, 720 itself is essentially at least partially blind to one or more diffraction orders (e.g., by an integrated polarizer on the sensor). For example, in the embodiment of FIG. 3, the incremental sensor 18 may be configured to be insensitive to the zeroth diffraction order (and, optionally, diffraction orders greater than the ±1st diffraction orders) even if they impinge on the incremental sensor 18 (because filter 22 has been removed). For example, the incremental sensor 18 may be configured to be sensitive only to a particularly oriented polarization (e.g., "vertical" polarization), and the diffraction order encoder 16 may be configured so that the zeroth diffraction order (and, optionally, orders greater than the ±1st diffraction orders) reaching the incremental sensor 18 include only (or substantially only) light polarized in different directions (e.g., include "horizontal" polarization).

[0114] The above-described embodiment uses a refractive lens 14 to relay the diffraction orders towards the sensor. However, it will be appreciated that this is not necessarily the case. For example, Figure 18 shows a schematic diagram of the optical system of an encoder device including an incremental scale 4 and readhead 6' according to another embodiment of the present invention that does not rely on a refractive lens. In this embodiment, the zeroth and ±1st diffraction orders are generated from light from a light source that strikes and is reflected by the scale 104. As shown, the zeroth and ±1st diffraction orders are reflected by a first plane pl 1 The diffraction order encoder is located in the first plane pl 1The diffraction order encoder includes a polarizing filter 16a" that matches the zeroth diffraction order (e.g., has a "vertical" polarization axis) so as to encode the zeroth diffraction order in a first (e.g., vertical) polarization direction. The diffraction order encoder further includes second and third polarizing filters 16b" and 16c" that match the +1st and -1st diffraction orders. The second and third polarizing filters 16b" and 16c" are configured with the same orientation as each other so that the ±1st diffraction orders have the same (e.g., "horizontal") polarization direction as each other but different from the polarization direction of the zeroth diffraction order. The diffraction grating G2 is positioned to focus the polarized ±1st diffraction orders. The zeroth diffraction order is not polarized. Therefore, the zeroth and ±1st diffraction orders are polarized in a plane pl in which the incremental sensor 18 and the reference mark sensor 20 are located. 2 As in many of the above-described embodiments, a first polarizer 22 may be positioned in front of the incremental sensor 18 and configured to filter the 0th diffraction order from reaching the incremental sensor 18, and / or a second polarizer 23 may be positioned in front of the reference mark sensor 22 and configured to filter the ±1st diffraction orders from reaching the reference mark sensor 22. For clarity, the light source has been omitted from FIG. 18. As will be appreciated, a separate refractive lens or grating may be used to collimate the light from the light source towards the scale, if desired.

[0115] Many of the above-described embodiments utilize a lensless or single-lens system through which light from a light source passes on its way to and from the scale. It will be appreciated that other optical configurations are possible, such as that shown schematically in FIG. 20 (which shows an absolute encoder apparatus including an absolute scale). The encoder 3000 of FIG. 20 is similar to that of FIG. 14 in that it includes an absolute scale 504 that includes conceptually periodically arranged features, but in that selected features are removed to encode unique / absolute position data along the measuring length of the scale (and similar parts share the same reference numerals). The data may be in the form of a pseudo-random sequence or discrete code words, for example. Details of such scales are described in more detail in U.S. Pat. Nos. 7,499,827 and 5,279,044. The readhead 3006 (its body omitted for clarity) comprises a light source 12, a first lens 3009 which collimates light from the light source 12, and an optical beam splitting element 3007 which passes light from the light source 12 through the scale 504 to illuminate the scale 504 and redirects light reflected by the scale towards a sensor 18, 520 which is configured to sense an image of the scale formed by a second lens 3010.

[0116] The present invention has been described above in relation to a linear encoder device, however, the present invention is equally applicable to rotary encoder devices including both ring encoders (which form scale features on the outer cylindrical surface of a ring member) and disk encoders (which form scale features on the planar surface of a disk member).

Claims

1. a scale readable by the readhead, the scale extending along the measurement dimension and including a series of positional features that diffract light into a plurality of diffraction orders; a readhead including: i) a light source for illuminating the scale; and ii) first and second sensors for detecting signals generated therein by one or more of the diffraction orders which can be used to determine the relative position of the scale and the readhead along the measurement dimension; Equipped with at least one of the diffraction orders is configured to have an optical state that is different from an optical state of at least one other diffraction order, the optical state of the diffraction order being configured such that a signal sensed by the first sensor is formed from a diffraction order composition that is different from a diffraction order composition of a signal sensed by the second sensor; Position measuring encoder device.

2. the 0th diffraction order has an optical state different from the optical states of the ±1st diffraction orders; 2. The position measuring encoder device of claim 1.

3. a first sensor filter configured to filter the light based on its optical state before the light hits the first sensor, and / or a second sensor filter configured to filter the light based on its optical state before the light hits the second sensor; 3. A position measuring encoder device according to claim 1 or 2.

4. the first sensor filter is configured to prevent substantially all zeroth order diffracted light from reaching the first sensor.

4. The position measuring encoder device of claim 3.

5. the second sensor filter is configured to prevent substantially all ±1st order diffracted light from reaching the second sensor; 5. A position measuring encoder device according to claim 3 or 4.

6. The optical state includes a state of polarization.

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

7. the first sensor filter and / or the second sensor filter include a polarizing filter; 7. A position measuring encoder device according to claim 3 or 6.

8. the optical state includes the wavelength of the diffraction order; 6. A position measuring encoder device according to any one of claims 1 to 5.

9. the first sensor filter and / or the second sensor filter include a wavelength filter; 9. A position measuring encoder device according to claim 8.

10. different diffraction orders of the light relayed to the first sensor and the second sensor are configured to converge to different convergence points in an optical path between the scale and the first and second sensors. A position measuring encoder device according to any one of claims 1 to 9.

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

11. A position measuring encoder device according to claim 10.

12. the readhead includes a diffraction order encoder located in a path of the diffracted light relayed to the first sensor and the second sensor, the diffraction order encoder encoding diffraction orders of the light in the different optical states; A position measuring encoder arrangement according to any one of claims 1 to 11.

13. the diffraction order encoder is located at the convergence point such that at least one diffraction order is encoded in an optical state that is different from the optical state of at least one other diffraction order; 13. A position measuring encoder device according to claim 10 or 11 and 12.

14. the first sensor includes an incremental position sensor configured to sense an incremental position signal; A position measuring encoder arrangement according to any preceding claim.

15. the second sensor comprises an absolute sensor or a reference mark sensor configured to sense an absolute signal or a reference mark signal; A position measuring encoder arrangement according to any preceding claim.

16. configured such that diffraction orders greater than ±1st diffraction orders are not sensed by the first and / or second position information sensor; A position measuring encoder arrangement according to any preceding claim.

17. a scale including a series of positional features extending along the measurement dimension that diffract light into multiple diffraction orders; a readhead configured to detect the position feature of the scale and to output one or more signals that can be used to determine the relative position of the scale and readhead; Equipped with The read head includes: a light source for illuminating the scale; a primary position information sensor and a secondary position information sensor whose signals can be used to determine the relative position of the scale; a primary sensor filter configured to filter light before it strikes the primary position information sensor; and / or a secondary sensor filter configured to filter the light before it strikes the secondary position information sensor; Including, the readhead further includes an optical diffraction order encoder located in a path of diffracted light relayed to the primary position information sensor and the secondary position information sensor, the optical diffraction order encoder encoding different diffraction orders of the light in different optical states which are then selectively filtered by at least one primary sensor filter and / or at least one secondary sensor filter such that light impinging on the primary position information sensor is filtered differently from light impinging on the secondary position information sensor; Position measuring encoder device.

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