Encoder Device

Abstract

1. A position measuring encoder device comprising: a scale including a series of position features readable by a readhead; and a readhead, the readhead including: a polarized light source that emits polarized light towards the scale; a first sensor configured to sense light filtered along a first polarization axis and positioned to sense light from the polarized light source that has interacted with the scale; and a polarization manipulator located in an optical path between the polarized light source and the first sensor, the polarization manipulator configured so that, regardless of the polarization state of 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.

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 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 several diffraction orders. The diffraction orders interfere / interact / recombine at the detector to produce the resultant field. Optionally, optical elements (e.g. diffraction gratings and / or lenses) may be provided in the readhead to redirect / deflect the diffraction orders so that they interfere / interact at the detector to produce the resultant field. As the scale and readhead move relative to each other, the resultant field changes. The readhead can record and / or report movement and position by monitoring the resulting field changes (e.g., movement of the interference fringe pattern / fringe field). Such a position encoder is described in U.S. Pat. No. 5,861,953.

[0004] FIG. 1a shows schematically 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. As will be understood, references to light herein include visible and invisible light ranging from ultraviolet to infrared. The diffraction orders 1001, 1002, 1003 are relayed to a sensor 3000 by a relay element 2000 (e.g., a lens, prism, or diffraction grating) (both the sensor and relay element are located within a readhead that is movable relative to the scale 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 relay elements), 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] The present invention relates to an improved optical encoder.

[0012] According to a first aspect of the present invention, there is provided a position measuring encoder apparatus comprising: a scale including a series of position features readable by a readhead, the readhead including a polarized light source that emits polarized light towards the scale, a first sensor configured to sense light filtered along a first polarization axis and arranged to sense light from the polarized light source that has interacted with the scale, and a polarization manipulator located in an optical path between the polarized light source and the first sensor, configured such that, regardless of the polarization state of 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.

[0013] As will be explained in more detail below, it may be beneficial for the first sensor to be configured to sense light that is filtered along a first polarization axis, for example, because this may provide at least some level of control over what hits the first sensor. This functionality may be utilized, for example, to improve (the quality of) the signal received by the first sensor.

[0014] At the same time, optical encoders typically employ unpolarized light sources. In fact, at the time of writing, all optical encoders sold by Renishaw plc utilize light-emitting diodes (LEDs) that emit unpolarized light. However, the present invention provides an alternative optical encoder that utilizes a polarized light source to emit light that illuminates the scale. Optionally, the light source emits linearly polarized light. For purposes of this patent application, polarized light source refers to a light source that emits light with a degree of polarization of at least 0.2. (In other words, optionally, the degree of polarization of light emitted from a polarized light source is 0.2 or greater.) Similarly, for purposes of this patent application, polarized light refers to light with a degree of polarization of at least 0.2. Light sources can include, for example, laser light sources, such as vertical cavity surface-emitting lasers (VCSELs). As will be understood by those skilled in the art, laser light sources, such as VCSELs, emit polarized light, typically having a degree of polarization of at least 0.3 (typically 0.3-0.7).

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

[0016] As will be 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 / purely 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:

[0017]

number

[0018] 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.

[0019] As will be understood, linearly polarized light and elliptically polarized light (e.g., elliptically polarized light with a DOP of at least 0.2) will have a (e.g., primary) polarization direction. Therefore, it can be said that the polarization manipulator (located in the optical path between the polarized light source and the first sensor) is configured such that, regardless of the (e.g., primary) polarization direction of the light emitted from the polarized light source, the light leaving the polarization manipulator toward the first sensor has a (e.g., primary) polarization direction that is at least partially resolvable along the first polarization axis.

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

[0021] Of course, due to the polarization-sensitive nature of the first sensor, and due to the polarization of the light from the light source, there can be problems with the first sensor not receiving enough (or any) light if the (e.g., primary) polarization direction of the light is angled too much with respect to the first polarization axis. Thus, the present invention provides a polarization manipulator located in the optical path between the polarized light source and the first sensor, and configured such that, regardless of the polarization state of the light emitted from the polarized light source, light leaving the polarization manipulator towards the first sensor has an at least partially resolvable polarization state along the first polarization axis (thereby ensuring that the first sensor receives at least some light).

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

[0023] Optionally, the apparatus is configured such that, all other factors being equal, the optical power of the light sensed by the first sensor is substantially the same for all possible (e.g., principal) polarization directions of the light emitted from the polarized light source;

[0024] The readhead may further include a second sensor configured to sense light filtered along a second polarization axis. The second sensor may be positioned to sense light from the polarized light source that has interacted with the scale. The polarization manipulator may be configured such that, regardless of the polarization state (e.g., (e.g.,) dominant polarization direction) of the light emitted from the polarized light source, light leaving the polarization manipulator toward the second sensor has a polarization state (e.g., (e.g.,) dominant polarization direction) that is at least partially resolvable along the second polarization axis. It may be beneficial for the second polarization axis to be orthogonal to the first polarization axis. For example, this means that each of the first and second sensors may be blind to the light sensed by the other sensor.

[0025] The apparatus may be configured such that, all other factors being equal (e.g., for a constant output of optical power by the light source), the optical power of the light sensed by the first sensor is substantially the same for all possible (e.g., primary) polarization directions of the light emitted from the polarized light source, and the optical power of the light sensed by the second sensor is substantially the same for all possible (e.g., primary) polarization directions of the light emitted from the polarized light source. In other words, all other factors being equal, the optical power of the light sensed by the first sensor and the optical power of the light sensed by the second sensor are independent of the (e.g., primary) polarization direction of the light emitted from the polarized light source. Substantially the same may mean that the change / fluctuation in the optical power of the light received by the first (or second) sensor is 25% or less (more preferably, 15% or less, e.g., 5% or less, e.g., 2% or less) if the optical power of the light received by the first (or second) sensor is measured for all possible (e.g., primary) polarization directions of the light emitted by the light source.

[0026] The polarization manipulator may include a diffuser. Advantageously, the polarization manipulator may include a waveplate (also known as a "retarder"). The waveplate may have a substantially uniform fast axis over its range (at least over its range through which light received by the first sensor and, if present, the second sensor passes). Advantageously, the effective full-wave retardation characteristic of the polarization manipulator is n+1 / 4, where n is an integer greater than or equal to 0. Thus, in embodiments in which light passes through the waveplate once (in its path from the light source to the first sensor and, if present, the second sensor), it may be advantageous for the waveplate to be a quarter (1 / 4) waveplate. In embodiments in which light passes through the waveplate twice (in its path from the light source to the first sensor and, if present, the second sensor), it may be advantageous for the waveplate to be an octadic (1 / 8) waveplate. Similarly, the polarization manipulator can include two octadic (1 / 8) wave plates in the light path from the light source to the first sensor (and the second sensor, if present), with the light passing through each of the two octadic (1 / 8) wave plates once.

[0027] It may be advantageous for the waveplate to be arranged with its fast axis at an angle of 30° to 60° (inclusive) to the first polarization axis, more preferably at an angle of 40° to 50° (inclusive) to the first polarization axis, particularly preferably at an angle of 43° to 47° (inclusive), for example at an angle of 44° to 46° (inclusive), for example at an angle of substantially 45°.

[0028] The readhead may further comprise a diffractive lens (e.g. a Fresnel lens) or a refractive lens (e.g. a cylindrical / spherical lens). It may be beneficial to position the lens in the optical path between the polarized light source and the polarization manipulator so that light from the light source only hits / passes the polarization manipulator after passing through the lens. This may avoid any adverse effects on the performance of the lens (e.g. reduced collimation if the lens is a collimator).

[0029] It may be beneficial to position a polarization manipulator in the light path before the scale. In other words, it may be beneficial for light from a polarized light source to pass through a polarization manipulator before reaching / hitting the scale. However, this does not necessarily have to be the case, and the polarization manipulator can also be positioned in the light path after the scale. As mentioned above and described in more detail below, the polarization manipulator can be positioned in the light path before or after the scale (i.e., so that light from the light source passes through the polarization manipulator (e.g., an optical retarder element) twice).

[0030] As described above, light from a polarized light source can pass through the polarization manipulator twice on its path from the light source to the first sensor. For example, light from the polarized light source can pass through the polarization manipulator a first time on its way to the scale, and then pass through the polarization manipulator a second (e.g., final) time after being reflected by the scale. This can be advantageous in view of the difficulty of manufacturing a position measurement encoder device in which light passes through the polarization manipulator only once on its path from the light source to the first sensor. This is particularly true in embodiments in which the device also includes other optical components, such as a diffractive lens (e.g., a Fresnel lens) or a refractive lens (e.g., a cylindrical / spherical lens), as described above. In embodiments in which light from a light source passes through the polarization manipulator twice on its path from the light source to the first sensor, it can be advantageous for the polarization manipulator / waveplate to include an octadic (1 / 8) waveplate.

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

[0032] The apparatus may be configured such that light sensed by a first sensor comprises a different diffraction order composition than light sensed by a second sensor, which may be due to i) polarization states of the diffraction orders (e.g., different polarization states, e.g., different diffraction orders having different (principal) polarization directions), ii) the first sensor being configured to sense light filtered along a first polarization axis, and iii) the second sensor being configured to sense light filtered along a second polarization axis.

[0033] Such an apparatus can provide improved performance over existing position measurement encoder apparatus. In particular, the signals sensed by different sensors can be tailored 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 toward the readhead's sensors; instead, the present invention facilitates selective / tailored suppression of diffraction orders for different sensors of the readhead based on their polarization states. For example, the zeroth diffraction order can have a (principal) polarization direction that is substantially orthogonal to the polarization directions of the ±1st diffraction orders.

[0034] The diffraction order composition may vary from sensor to sensor, in the sense that the degree to which different diffraction orders contribute / 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 polarization states, the relative influence of the zeroth diffraction order and ±1st diffraction orders on the formation of the signal sensed by the first sensor is different from the relative influence of the zeroth diffraction order and ±1st diffraction orders on the formation of the signal sensed by the second sensor. For example, the device may be configured so that the influence of the zeroth diffraction order on the signal sensed by the first sensor is 50% less than the influence of the zeroth diffraction order on the signal sensed by the second sensor.

[0035] 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, and remain uninfluenced on generating the signal sensed by the other sensor. Indeed, 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.

[0036] 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 polarization states / directions. 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.

[0037] In the field of encoder devices, the zeroth and ±1st diffraction orders often have the strongest influence on the signal formed at / sensed by a sensor. Accordingly, in preferred embodiments of the present invention, the zeroth diffraction order has a (dominant) polarization direction that is different (e.g., orthogonal) to that of 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 sensor. While the +1st diffraction order can be encoded to have a polarization state that is different from that of the −1st diffraction order, it is typically beneficial to configure the readhead so that they have the same polarization state.

[0038] The series of positional features on the scale can diffract light into multiple diffraction orders. The device can be configured so that at least one diffraction order has a polarization state that is different from the polarization state of at least one other diffraction order. The polarization 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 scale 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.

[0039] Advantageously, the readhead may include a diffraction order encoder that encodes at least one diffraction order produced by the series of positional features of the scale in a polarization state that differs from the polarization state of at least one other diffraction order produced by the series of positional features of the scale. 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.

[0040] The polarization manipulator can include, for example, a “patterned retarder” (or, in other words, a “non-uniform retarder” or “structured retarder”). Thus, in contrast to the above-described embodiments, a polarization manipulator can be configured so that light output therefrom contains a mix of polarization directions across its footprint / extent. To achieve this, the polarization manipulator (e.g., an optical retarder element) can include non-uniform fast axes and / or non-uniform retardation characteristics (or waveplate fractions) that vary along at least one axis / dimension. In other words, a polarization manipulator (e.g., an optical retarder element) according to the present invention can include non-uniform fast axes and / or retardation characteristics along at least one axis / dimension. For example, the polarization manipulator can include a series of waveplate pixels with different fast axes. It will be understood that the use of the terms “pattern” and “patterned” herein, particularly in relation to a polarization manipulator, is not intended to imply the presence 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, particularly a non-uniform fast axis. As described in more detail below, the (fast axis) design / shape / pattern of a polarization manipulator (e.g., retarder) can be repeated, but it may be beneficial not to repeat. A patterned retarder may be configured such that there is a large variation in the angle of the fast axis along at least one axis / dimension. However, this need not be the case. For example, for systems in which light passes twice through a patterned retarder with octadic (or ⅛) waveplate pixels (e.g., as in the optical schemes of Figures 8 or 14), it has been found beneficial for the fast axis angles to be within a very small range. For example, it has been found beneficial for the fast axes of the waveplate pixels to all be at an angle of approximately 45°±1°.

[0041] The device may be configured such that the sensors (e.g., the first sensor and / or the second sensor) are essentially at least partially blind to a particular polarization state / orientation (and thus essentially at least partially blind to one or more diffraction orders) (e.g., a polarizer integrated into the sensor having said first polarization axis may make the first sensor blind to polarized light having a polarization direction orthogonal to the first polarization axis). Thus, the composition of the signal as sensed by the sensor may be determined by the extent to which the sensor is at least partially blind to the polarization state / orientation. Optionally, the encoder device includes a first sensor polarizing filter configured to at least partially (e.g., substantially) filter light along the first polarization axis based on the polarization state / orientation of the light before it hits the first sensor. For example, in embodiments in which the readhead includes a first sensor and a second sensor, the readhead may include a first sensor polarizing filter (having a first polarization axis) located in front of the first sensor and / or a second sensor polarizing filter (having a second polarization axis) located in front of the second sensor. Thus, the diffraction orders can be selectively at least partially (e.g., substantially) filtered by the first sensor filter and / or the second sensor filter based on their polarization state / direction. Thus, light impinging on the first sensor can be filtered differently than light impinging on the second sensor. If both the first sensor filter and the second sensor filter are provided, they can be configured to filter at least partially (e.g., substantially) different diffraction orders based on their polarization state. Thus, the composition of the signal as sensed by the first and / or second sensors can be determined / controlled by the filters.

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

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

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

[0045] The above-described diffraction order encoder can include at least one polarizer element configured to encode at least one diffraction order in a polarization state 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 (first principal polarization direction) and one or more additional (e.g., second and third) polarizer elements configured to encode the ±1st diffraction orders in second polarization states (second principal polarization directions) different from the first polarization state (direction). Optionally, the polarizer element includes a polarizer configured to polarize (e.g., increase the degree of polarization) of at least one diffraction order. Optionally, the polarizer element includes at least one polarization manipulator, such as a wave plate or a retarder, configured to change the polarization state of at least one diffraction order, e.g., to rotate the polarization direction of the at least one diffraction order.

[0046] In embodiments in which the readhead includes a diffraction order encoder, the polarized 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.

[0047] The readhead may include at least one optical relay element for relaying light, e.g., the diffraction orders, from the scale to the first and / or second sensors. For example, the readhead may include at least one refractive optical relay element and / or diffractive optical relay element. Suitable optical relay elements include lenses and / or diffraction gratings. The diffraction order encoder may be positioned to interact with one or more diffraction orders before or after the optical relay element. 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).

[0048] Optionally, the device is configured so that the diffraction orders converge to corresponding / respective spots (or "different convergence points") in the optical path before the first and / or second sensors. For example, there may be a spot / convergence point for each diffraction order, e.g., a spot / convergence point for the 0th diffraction order, a spot / convergence point for the +1st diffraction order, a spot / convergence point for the -1st diffraction order, etc. Such spots may be located in the focal plane (focal plane of the lens) of an optical relay element for relaying the diffraction orders. A diffraction order encoder may be located substantially at these convergence points. For example, the diffraction order encoder may be located in the focal plane of the optical relay element, 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.

[0049] The polarization states (e.g., (principal) polarization directions) of diffraction orders greater than ±1 diffraction orders may be selectively controlled to what extent they affect or contribute to the signal sensed by the first and / or second sensors. Such diffraction orders may include, for example, ±3 diffraction orders and / or ±5 diffraction orders. Optionally, diffraction orders greater than ±1 diffraction orders may simply be prevented / stopped / blocked from reaching the first and / or second sensors so as not to affect the signal formed therein. Such diffraction orders may be stopped by absorption, deflection, scattering, and / or reflection. For example, an opaque material may be placed at the spot / convergence point for selecting all diffraction orders greater than ±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 polarization states at the sensor (e.g., at the first and second sensors) so that they do not interact / interfere with the 0th and / or ±1st diffraction orders, or so that they are at least partially (e.g., substantially) filtered by, for example, an appropriate filter (e.g., filtered by the first sensor polarization filter and / or the second sensor polarization filter) before reaching the sensor.

[0050] The scale may be illuminated with collimated light (and optionally the diffraction orders from the scale may themselves be collimated). Optionally, the readhead includes an optical collimator element for collimating the light from the polarized light source. Optionally, the same optical collimator element (for collimating the light from the polarized light source) and the optical relay element for relaying the light (e.g. the diffraction orders) towards the first and / or second sensors are one and the same optical element. Thus, optionally the same optical element is used for both collimating the light from the polarized light source and focusing the diffraction orders onto corresponding / respective spots.

[0051] The apparatus may be configured such that the first and / or 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 / or second sensors.

[0052] 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 the 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 scale features at different depths to less than a fraction of the wavelength of light. Typically, an amplitude scale produces a significant zeroth diffraction order, along with significant ±1st diffraction orders (with higher ±odd diffraction orders that decrease in intensity). This is in contrast to a phase scale, which does not produce a zeroth diffraction order.

[0053] 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.

[0054] 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.

[0055] The scale may be a transmissive scale. Optionally, the scale is a reflective scale. Thus, optionally, the light source and the first and / or second sensors of the readhead are located on the same side of the scale.

[0056] 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.

[0057] 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.

[0058] 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 (this can be achieved when the zeroth diffraction order is substantially eliminated).

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

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

[0061] Optionally, the signal generated by the first sensor includes an absolute position signal. Thus, the first sensor may include an absolute position sensor. Thus, the scale may include an absolute scale track including a series of features defining the absolute scale track. As will be appreciated, an absolute scale track differs from an incremental scale track (with or without reference marks) in that its features define a series of unique positions along the length of the scale. It may be read by the readhead such that the relative positions of the readhead and scale can be determined (e.g., upon start-up) at any position along the scale without requiring movement to a reference position (e.g., a reference mark). Examples of absolute scales include those described in U.S. Patent No. 7,499,827 and U.S. Patent No. 5,279,044.

[0062] The scale may include a separate incremental track and an absolute scale track. Optionally, the incremental and absolute scale features are combined into one track. For example, an absolute scale feature may be superimposed on a periodic incremental scale feature.

[0063] In embodiments in which the readhead includes a first and second sensor, 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.

[0064] Optionally, the scale includes first and second series of position features readable by the readhead. 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 each other. The position measurement encoder device can be configured such that only one of the first and second sets of diffraction orders has a diffraction order with a different optical state. For example, the position encoder device can be configured such that at least one diffraction order (e.g., the zeroth diffraction order) of the first set of diffraction orders has an optical state that is different from the optical state of at least one other diffraction order (e.g., the ±1st diffraction orders) of the first set of diffraction orders, while some diffraction orders (e.g., at least the zeroth diffraction order and the ±1st diffraction orders) of the second set of diffraction orders can have the same optical state. Alternatively, the position measurement encoder apparatus may be configured such that at least one diffraction order of the first set has an optical state that is different from the optical state of at least one other diffraction order, and such that at least one diffraction order of the second set has an optical state that is different from the optical state of at least one other diffraction order. In any event, it will be appreciated that the first and second sets of diffraction orders may be filtered differently based on their optical states.

[0065] 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.

[0066] As will be appreciated, the output of the first sensor (and the output of the second sensor, if present) can provide one or more signals that can be used to indicate relative position information (with respect to the readhead and 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 quadrature signals 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.

[0067] The position measuring encoder device may be a linear encoder device or a rotary encoder device, and therefore the scale may be a linear scale or a rotary scale (in which case the scale may be a ring scale or a disk scale).

[0068] According to a second aspect of the present invention, there is provided a position measuring encoder apparatus comprising: a scale including a series of position features readable by a readhead, the readhead including: i) a polarized light source that emits polarized light towards the scale; ii) a first sensor configured to sense light filtered along a first polarization axis and arranged to sense light from the polarized light source that has interacted with the scale; and iii) a waveplate in an optical path between the polarized light source and the first sensor, the waveplate having an effective full wave retardation of n+1 / 4, where n is an integer greater than or equal to 0, and arranged so that its fast axis is arranged at an angle of between 30° and 60° to the first polarization axis.

[0069] The apparatus may include iv) a second sensor configured to sense light filtered along a second polarization axis orthogonal to the first polarization axis and positioned to sense light from the polarized light source that has interacted with the scale.

[0070] The waveplate may include an octadic waveplate, where light from the light source passes through the octadic waveplate twice on the path from the light source to the first sensor (and the second sensor, if present).

[0071] The scale can diffract light from the light source into a plurality of diffraction orders. The readhead can include a diffraction order encoder configured to encode the zeroth diffraction order with a polarization state aligned with the second polarization axis and to encode the ±1st diffraction orders with a polarization state aligned with the first polarization axis. This allows a first sensor to sense the ±1st diffraction orders but not the zeroth diffraction order, and a second sensor to sense the zeroth diffraction order but not the ±1st diffraction orders.

[0072] Features described above in relation to the first aspect of the invention are equally applicable to this second aspect of the invention (and vice versa) and will not be repeated here for the sake of brevity. [Brief explanation of the drawings]

[0073] Hereinafter, an embodiment of the present invention will be described by way of example with reference to the drawings.

[0074] [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 FIG. 3. [Figure 5] FIG. 4 is a schematic optical diagram of the encoder device of FIG. 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 8] 10 illustrates schematically an encoder device according to a second embodiment of the present invention. [Figure 9] FIG. 9 is a schematic optical diagram of the encoder device of FIG. 8. [Figure 10] FIG. 9 is a schematic optical diagram of the encoder device of FIG. 8. [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 12a] 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 12b] 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 12c] 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 13] 10 shows a schematic optical diagram of an encoder arrangement according to another embodiment of the present invention; [Figure 14] 10 shows a schematic diagram of an absolute scale and a readhead of an encoder device according to another embodiment of the present invention. [Figure 15] 10 shows a schematic diagram of an absolute scale and a readhead of an encoder device according to another embodiment of the present invention. [Figure 16] 10 shows a scale and a readhead of an encoder device according to another embodiment of the present invention. [Figure 17] 4 shows schematically various optical components of a readhead according to another embodiment of the invention; [Figure 18(a)] 1 shows an exemplary polarization manipulator for use in a readhead according to the present invention; [Figure 18(b)] 18(a) shows four different (i-iv) exemplary orientations of the orthogonal polarization axes (v, h) and a graph of the optical power along the orthogonal polarization axes of the light output from the polarization manipulator of FIG. 18(a) within the optical footprint. [Figure 19] 1 shows an exemplary polarization manipulator for use in a readhead according to the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0075] Referring to Figure 2, an encoder device 2 according to the present invention comprises a scale 4 and a readhead 6. The scale 4 and readhead 6 are movable relative to each other in the X dimension. For example, the scale 4 may be 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) (although this may be in opposite directions, or both may actually be movable). The scale 4 may take many different forms, including, for example, a linear scale as shown, or a rotary scale (e.g., provided on a ring with scale features provided on its periphery, or provided on a disk with scale features provided on its flat surface).

[0076] 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.

[0077] 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.

[0078] The incremental features 10 form an incremental scale that facilitates measurement along the X-axis (the "measurement dimension"). In the illustrated embodiment, a reference mark 111 is provided, and in particular, embedded within the incremental features 10. The reference mark 111 in this embodiment comprises a patterned reference mark, which in this embodiment comprises one thick dark band and two thin dark bands, each separated along the measurement direction by at least one incremental feature between them. The term "patterned" in the expression "patterned reference mark" is not used to mean that the reference mark has a repeating design / form, but rather is used to refer to the reference mark having a non-uniform design / form. Of course, a "patterned reference mark" can have a repeating form / design, or not have a repeating form / design. As will be appreciated, the reference mark does not necessarily have to be embedded within the incremental scale track. Instead, it can be provided in a separate track. In other embodiments, the reference mark is non-optical, or, for example, not provided at all.

[0079] 3-5 show various optical components located within the readhead 6 (for clarity, the body has been omitted and the relative sizes and positions of components are not to scale). In this embodiment, the readhead 6 includes a polarized light source 12, a polarization manipulator 14 (in this embodiment a quarter-wave plate, as described in more detail below), a refractive lens 109, a diffraction order encoder 116, a primary position information sensor 20 (later referred to herein as the "incremental sensor" 20), a polarizing filter 120 located in the optical path of the light traveling towards the incremental sensor 20, and a secondary position information sensor 122 (later referred to herein as the "reference mark sensor" 122). In the described embodiment, the light source 12 and the diffraction order encoder 116 are provided on an opaque substrate 115 in substantially the same plane. It will be appreciated that the substrate 115 need not actually be opaque, although being opaque means that other diffraction orders that may be useful are blocked (described in more detail below).

[0080] The polarizing filter 120 has a polarization axis along which it will filter light reaching the incremental sensor 20. In the described embodiment, for ease of reference and explanation, the angle / orientation of the polarization axis of the polarizing filter will be described as "horizontal."

[0081] The quarter wave plate 14 has a fast axis that is configured to be at a 45° angle with respect to the "horizontal" polarization axis of the polarizing filter 120 (the significance of which will be explained in more detail below). Unlike the embodiments of Figures 18 and 19, the quarter wave plate 14 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).

[0082] The polarized light source 12 emits polarized light, i.e., light having at least a dominant, if not only, polarization direction. Polarized light sources can emit linearly polarized or elliptically polarized light. In this embodiment, the polarized light source 12 comprises a laser light source, in particular a VCSEL, which emits linearly polarized light with a dominant polarization direction. It will be appreciated that the polarization direction and / or degree of polarization (DOP) of light from a laser light source such as a VCSEL can vary unpredictably from light source to light source. Furthermore, the polarization direction and / or DOP of light from a laser light source such as a VCSEL can change during operation of the readhead 6 due to, for example, changes in the operating environment, such as temperature or applied current (which can be changed by servo control). Typically, the DOP of the type of VCSEL used in the described embodiment is in the range of 0.3 to 0.7. It will be appreciated that a light source such as the VCSEL 12 can inherently emit polarized light, or can include an unpolarized emitter and an associated polarizer that polarizes the light emitted from the unpolarized emitter.

[0083] In the described embodiment, light from the VCSEL 12 is emitted towards the scale 4 through a quarter wave plate 14 (the significance of which will be explained below). Light output from the quarter wave plate 14 is relayed to the scale 4 via a lens 109 (through a clear / transparent window 117 in the readhead 106). The VCSEL 12 is positioned at a focal length f of the lens 109 so that when the light from the VCSEL 12 hits the scale 4, it is collimated by the lens 109. The light from the VCSEL 12 illuminates the footprint 24 on the scale 4. In Figures 4 and 5, thick dashed lines indicate schematically light traveling towards the scale, and thin solid lines indicate light reflected from / by the scale.

[0084] The lens 109 relays light reflected by the scale 4 to the incremental sensor 20 and the reference mark sensor 122 .

[0085] As will be appreciated, due to the well-known natural phenomenon of diffraction, light reflected by the scale 4 is diffracted due to the presence of the scale features thereon (i.e., in the same manner as described above in connection with FIGS. 1a and 1b). In the case of the incremental features 10, which are periodic, the light reflected thereby is diffracted into distinguishable diffraction orders. As described above in connection with FIGS. 1a and 1b, light is diffracted in the X dimension into the 0th and ±1st diffraction orders. 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. In either case, in the described embodiment, they are blocked by the opaque substrate 115 from proceeding to either the incremental sensor 20 or the reference mark sensor 122.

[0086] The diffracted orders are incident on lens 109, which focuses the corresponding diffracted orders to form spots 150 at a back focal plane fp of lens 109, each spot formed from a respective diffracted order. Spots 150 are images of the light source at the point where the back focal plane fp is a conjugate plane to the plane in which the light source is located.

[0087] In this embodiment, the diffraction order encoder 116 is coexistent with the spot 150. In particular, in this embodiment, the diffraction order encoder 116 includes a zeroth-term polarizer 116a (e.g., with a "vertical" polarization axis) coincident with the zeroth diffraction order spot 150a, and first and second non-polarizing transparent regions 116b, 116c coincident with the ±1st diffraction order spots 150b, 150c. Thus, the diffraction order encoder 116 encodes the zeroth diffraction order with only vertical polarization, while the polarizations of the ±1st diffraction orders remain unaffected (and thus include orthogonal polarizations, such as the output from the nonuniform / patterned retarder 14).

[0088] The zeroth and ±1st diffraction orders propagate toward the incremental sensor 20 and the reference mark sensor 122. As mentioned above, the polarizing filter 120 has a "horizontal" polarization axis, and thus a polarization axis orthogonal to the polarization axis of the zeroth-order polarizer 116a. Therefore, the zeroth-order diffracted light is blocked from reaching the incremental sensor 20. In contrast, the ±1st diffraction orders (despite being "horizontally" polarized during processing) are able to pass through the polarizing filter 120, thereby impinging on the incremental sensor 20. In particular, the ±1st diffraction orders propagate toward the incremental sensor 20, interacting (constructively and destructively interfering) and forming an interference fringe pattern (or "fringe field") that impinges on the incremental sensor 20. Movement of the readhead 6 relative to the scale 4 causes movement of the interference fringe pattern relative to the incremental sensor 20, thus enabling up / down counting by downstream electronics and thereby measuring relative displacement.

[0089] Blocking the zeroth diffraction order results in many improvements to the interference fringe pattern impinging on incremental sensor 20. In particular, fringe pattern visibility is improved. Additionally, removal of the zeroth diffraction order means that the interference fringe pattern produced by incremental sensor 20 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.

[0090] 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 modulation is done with a period of p / λ, where p is the scale period and λ is the wavelength of light. If the zeroth order term is eliminated, there is no longer any modulation. 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.

[0091] 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 20, effectively doubling the system resolution and significantly improving the rideheight tolerance of the readhead.

[0092] Furthermore, if diffraction orders higher than the first are also blocked (as is the case in this embodiment with an opaque substrate 115), higher harmonics are removed from the fringe field, providing a purer interference fringe pattern. It will be appreciated that in alternative embodiments, diffraction orders higher than the first need not be blocked; for example, the substrate 115 may be transparent (in which case the first and second non-polarizing transparent regions 116b, 116c may not be clearly distinguishable).

[0093] In the above-described embodiment, there is no polarizing filter in front of the reference mark sensor 122. Thus, the zeroth and ±1st diffraction orders all impinge on the reference mark sensor 122. Depending on various factors (discussed in more detail below), it may be preferable in some circumstances to not place a polarizing 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 122, then there is no advantage to having a corresponding filter in front of the reference mark sensor 122 (indeed, doing so may be harmful due to a degradation of photometric performance).

[0094] Therefore, as described above, the signal sensed by incremental sensor 20 is formed from only the ±1st diffraction orders, while the signal sensed by reference mark sensor 122 is formed from the 0th and ±1st diffraction orders. Therefore, the signal sensed by incremental sensor 20 is formed from a diffraction order composition that is different from the diffraction order composition of the signal sensed by reference mark sensor 122.

[0095] One thing to note is that in this embodiment, the zeroth and ±1st diffraction orders are unequally attenuated because the zeroth order term is polarized by the zeroth-order polarizer 116a. If desired, the unequally attenuated light can be avoided / reduced by replacing the first and second non-polarizing transparent regions 116b, 116c with polarizing filters having a “horizontal” polarization axis (i.e., a polarization axis orthogonal to the polarization axis of the zeroth-order polarizing filter 116a). In such an embodiment, the zeroth and ±1st diffraction orders impinging on the reference mark sensor 122 are attenuated, which may result in a better quality image impinging on the reference mark sensor. Furthermore, the ±1st diffraction orders still pass through the polarizing filter 120 and interfere to form an interference fringe pattern on the incremental sensor 20. In any event, it will be understood that the light impinging on the incremental sensor 20 is filtered differently than the light impinging on the reference mark sensor 122.

[0096] Light impinging on the incremental sensor 20 and reference mark sensor 122 can be described as an "image" of the scale 4, in that the incremental sensor 20 and reference mark sensor 122 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 115 blocks them) means that the "image" at the conjugate plane c p when it strikes the incremental sensor 20 has a purer 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 (which might rather 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 122, the "image" of the scale 4 on the reference mark sensor 122 will be affected to a lesser extent, particularly when the zeroth and ±1st diffraction orders are polarized by the diffraction order encoder 116 so that they both strike the reference mark sensor 122 to the same extent.

[0097] Without the quarter-wave plate 14, what strikes the incremental sensor 20 and reference mark sensor 122 will vary depending on the predominant polarization state / direction emitted by the VCSEL 12. In the worst case, this could mean that the incremental sensor 20 detects no light at all. For example, in a configuration identical to that of FIG. 3 but without the quarter-wave plate, if the VCSEL light source 12 emits only vertically polarized light, the incremental sensor 20 would detect no light at all. However, by including the quarter-wave plate 14 in the optical path between the light source 12 and the incremental sensor 20 and configuring its fast axis and the polarization axis of the polarizing filter 120 of the incremental sensor 20 so that they are angled relative to each other, the incremental sensor 20 can be assured of receiving a proper signal regardless of the polarization direction of the VCSEL light source 12. For example, placing the fast axis of quarter-wave plate 14 at 45° to the polarization axis of polarizing filter 120 of the incremental sensor means that, all other factors being equal (e.g., for a constant optical power output from VCSEL 12), the optical power of the signal impinging on incremental sensor 20 will be the same regardless of the polarization state of the light output by VCSEL 12. Another way to look at this is that, of the light leaving quarter-wave plate 14 toward incremental sensor 20 (in this embodiment, via lens 109, scale 4, again lens 190, diffraction order encoder 116, and polarizing filter 120), the ratio of i) the optical power of the light resolvable along the "horizontal" polarization axis of polarizing filter 120 to ii) the optical power of the light resolvable along the orthogonal "vertical" axis is 1:1. This will be true regardless of the polarization state of the light output by VCSEL 12 (due to the 45° angle between the fast axis of the quarter-wave plate and the polarization axis of polarizing filter 120 of incremental sensor 20).

[0098] It will be appreciated that it may be preferable, but not required, for the angle between the fast axis of the quarter-wave plate and the polarization axis of the polarizing filter 120 of the incremental sensor 20 to be 45° (to ensure, all other factors being equal, that the optical power of the signal impinging on the incremental sensor 20 is the same regardless of the polarization state of the light output by the VCSEL 12). It will be appreciated that a deviation from 45° between the fast axis of the quarter-wave plate and the polarization axis of the polarizing filter 120 of the incremental sensor 20 means that the optical power of the signal impinging on the incremental sensor 20 will not be the same regardless of the polarization state of the light output by the VCSEL 12. Nevertheless, it may be acceptable for the optical power of the signal impinging on the incremental sensor 20 to vary depending on the polarization state of the light output by the VCSEL 12. In such cases, the limits of the acceptable angle between the fast axis of the quarter-wave plate and the polarization axis of the polarizing filter 120 of the incremental sensor 20 will depend on the degree of acceptable variation in the optical power of the signal impinging on the incremental sensor 20. Nevertheless, the inventors have found that it is generally desirable for the angle between the fast axis of the quarter-wave plate and the polarization axis of the polarizing filter 120 of the incremental sensor 20 to be between 30° and 60°, more preferably between 35° and 55°, and particularly preferably between 40° and 50°.

[0099] In the described embodiment, incremental sensor 20 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 (schematically represented by wave 1100) in incremental sensor 20. FIG. 7 shows an example of an electrical grid. In FIG. 7, a portion of an electrical grid is shown, where the fingers / photodiodes of four sets of photodiodes (A, B, C, and D) are interdigitated / interleaved to form an array of sensor elements extending along the length "L" of the sensor.

[0100] 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'.

[0101] 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.

[0102] In the described embodiment, the reference mark sensor 122 includes two sets of photodiodes 122a, 122b, 122c of corresponding placement and shape / size located on opposite sides of the incremental sensor 20. Each set of photodiodes 122a, 122b, 122c of the reference mark sensor 122 is configured such that only when the readhead 6 and the reference mark 111 are aligned will an image (or the "pseudo image" discussed above) of the pattern of the reference mark 111 impinge on and align / correlate with the pattern of the three photodiodes 122a, 122b, 122c of the reference mark sensor 122, thereby causing a significant and sudden change in the intensity of light impinging thereon. In this embodiment, where the reference mark 111 includes dark bands, the intensity of light received by the reference mark sensor 122 drops significantly when the readhead 6 and the reference mark 111 are aligned. Signal processing electronics and / or software downstream of the reference mark sensor 122 may be configured to identify such changes in the intensity of light received at the reference mark sensor 122 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 122a, 122b, 122c function as one photodiode.

[0103] An alternative embodiment of an encoder apparatus 202 according to the present invention is shown in Figures 8, 9 and 10. This embodiment shares some parts that are the same as the embodiment of Figures 3 to 7, and similar parts share the same reference numbers.

[0104] In this embodiment, the polarization manipulator 214 is different from the previous embodiment. First, in this embodiment, the polarization manipulator 214 is located in the optical path between the lens 109 and the scale 4 (i.e., between the lens 109 and the window 117 of the readhead 206). The inventors have discovered that it may be beneficial to provide the polarization manipulator 214 after the lens 109. For example, after collimation, all incident angles are the same, which may mean that the polarization manipulator 214 has better performance. Also, the polarization manipulator 214 may have optical aberrations that may interfere with collimation. Furthermore, from a manufacturing standpoint, it may be easier to place the polarization manipulator here.

[0105] 9 and 10, in contrast to the embodiment of Figures 3-7, the light from the VCSEL 12 passes through the polarization manipulator 214 twice: once on the way to the scale 4 and a second time on the return from the scale 4 towards the sensor (20, 122). In practice, the degree of overlap at the polarization manipulator 214 of the beams of light to and from the scale is likely to be greater than is shown schematically in Figure 9.

[0106] In this embodiment, polarization manipulator 214 comprises an octadic (i.e., "1 / 8") waveplate rather than the quarter-waveplate of the single-pass embodiment of FIGS. 3-7. Nevertheless, it will be understood that due to a double pass of light through octadic waveplate 214 (and according to the embodiment of FIG. 3), the effective full-wave retardation characteristic of polarization manipulator 214 is n+1 / 4 of the wavelength of light emitted by light source 12, where n is an integer greater than or equal to 0 (in the described embodiment, n=0). According to the embodiment of FIGS. 3-7, the fast axis of octadic waveplate 214 is also at a 45° angle with respect to the polarization axis of first polarizing filter 120 (and thus, in this embodiment, will also be at a 45° angle with respect to the polarization axis of second polarizing filter 223, as described in more detail below). Also, according to the embodiment of Figures 3-7, it may be preferable for the fast axis of the octadic waveplate to be oriented at an angle of 45° to the polarization axis of the first polarizing filter 120, although this is not required.

[0107] 3-7. In this embodiment, diffraction order encoder 216 includes a zeroth-order term polarizer 216a aligned with zeroth-order diffraction order spot 150a, a +first-order term polarizer 216b aligned with +first-order diffraction order spot 150b, and a −first-order term polarizer 216c aligned with −first-order diffraction order spot 150c. The polarization axes of +first-order term polarizing filter 216b and −first-order term polarizing filter 216c are configured to have the same direction as each other such that ±first-order diffraction orders are polarized by +first-order term polarizing filter 216b and −first-order term polarizing filter 216c with the same (e.g., “horizontal”) polarization direction as each other. The polarization axis of zeroth-order term polarizer 216a is configured to be different, preferably orthogonal to the polarization axes of +1st-order term polarizing filter 216b and −1st-order term polarizing filter 216c, so that the zeroth-order diffraction order is polarized in a different (e.g., “perpendicular”) polarization direction relative to the ±1st-order diffraction orders. Thus, diffraction order encoder 216 encodes the +1st-order and −1st-order diffraction orders in a first polarization direction and encodes the zeroth-order diffraction order in a second polarization direction different from the first polarization direction. As shown, zeroth-order term polarizer 216a, +1st-order term polarizer 216b, and −1st-order term polarizer 216c are spaced apart from one another so as not to be in direct contact with one another, although this is not necessarily the case.

[0108] The polarized diffraction orders propagate toward the incremental sensor 20 and the reference mark sensor 122. In the same manner as the embodiment described above in connection with the embodiments of Figures 3-7, a first polarizing filter 120 is positioned in the optical path of the diffraction orders traveling toward the incremental sensor 20. The first polarizing filter 120 has a polarization axis parallel to the polarization axes of the +1st-order term polarizer 216b and the -1st-order term polarizer 216c (and therefore orthogonal to the polarization axis of the 0th-order term polarizer 216a). Thus, the 0th-order diffracted light is blocked from reaching the incremental sensor 20. In contrast, the ±1st-order diffracted light passes through the first polarizing filter 120, thereby allowing it to impinge on the incremental sensor 20. In particular, the ±1st-order diffracted orders propagate toward the incremental sensor 20 and interact (constructively and destructively interfere) to form fringe fields that impinge on the incremental sensor 20. The benefits of preventing the zeroth diffraction order from contributing to the optical signal impinging on incremental sensor 20 were discussed above in connection with the embodiment of FIGS.

[0109] In contrast to the embodiments of FIGS. 3-7, the second polarizing filter 223 is located in the optical path of the diffraction orders traveling toward the reference mark sensor 122. In particular, the second polarizing filter 223 includes a polarizing filter having a polarization axis parallel to the polarization axis of the zeroth-order polarizer 216a (and therefore orthogonal to the polarization axes of the +1st-order polarizing filter 216b and the −1st-order polarizing filter 216c). The second polarizing filter 223 will therefore block the ±1st-order diffraction orders, thereby preventing them from contributing to the optical signal impinging on the reference mark sensor 122. Therefore, only the zeroth-order diffracted light will reach the reference mark sensor 122 and contribute to the optical signal impinging on the reference mark sensor 122. Such an arrangement has been found to be advantageous, particularly in embodiments with incremental features located within the reference mark.

[0110] For example, referring to Figures 11 and 12, the effect of filtering the ±1st diffraction orders on an image (or "pseudo image") at the detection plane dp of the readhead is shown. In particular, Figures 11a, 11b, and 11c respectively 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 ride height, b) +75 μm from the nominal ride height, and c) +150 μm from the nominal ride height. As shown, at the nominal ride height, the image of the reference mark (highlighted by the circle in Figures 11a-11c) 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, but can also be an issue (albeit to a lesser extent) for reference marks that do not contain incremental features. Such image corruption causes the signal output by the reference mark sensor 122 to become wider and less distinct as the readhead passes over the reference mark, which can cause problems with reference mark reliability (e.g. the signal is too weak to detect the reference mark) and / or reference mark repeatability (e.g. the reference mark signal is too broad and not repeatable within one incremental period).

[0111] Figures 12a, 12b, and 12c 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 12, the image of the reference mark reconstructed at the readhead's detection plane dp / conjugate plane cp (highlighted by a circle in Figures 12a-12c) maintains good structure and similarity to the reference mark, providing a clear and strong reference mark signal from the reference mark sensor 122, even at ride heights far from the nominal ride height. We have therefore found it beneficial to include a secondary sensor filter 223 in front of the reference mark sensor 122 that filters out the ±1st diffraction orders so that they do not contribute to the image of the scale reconstructed at the readhead's detection plane dp / conjugate plane cp. In particular, such a configuration provides the readhead with better ride height tolerance.

[0112] As will be appreciated, other reference mark sensor arrangements can be used. For example, reference mark sensor 122 can include a split detector, where photodiodes 122a, 122b, 122c are duplicated and laterally offset in the X dimension, and outputs from the duplicate photodiodes are connected to form a second reference mark output that can be used to provide a differential signal (e.g., as described in WO 2005 / 124282). Also, for example, reference mark sensor 122 need not include sensor elements on either side of the incremental sensor. Further, for example, one or more of the reference mark sensor elements can be partially or completely embedded within the incremental sensor (e.g., as described in WO 2005 / 124282).

[0113] In an alternative embodiment, the first polarizing filter 120 can be omitted so that only the reference mark sensor 122 has a polarizing filter in front of it (thereby obtaining the above-mentioned benefit of filtering selected diffraction orders so that they do not reach the reference mark sensor 122).

[0114] Therefore, as described above, the signal sensed by incremental sensor 20 is formed from only the ±1st diffraction orders, while the signal sensed by reference mark sensor 122 is formed from only the 0th diffraction order. Thus, the signal sensed by incremental sensor 20 is formed from a diffraction order composition that is different from the diffraction order composition of the signal sensed by reference mark sensor 122.

[0115] Without the octadic waveplate 214, what strikes the incremental sensor 20 and the reference mark sensor 122 can vary significantly depending on the predominant polarization direction emitted by the VCSEL 12. In the worst case, this could mean that the incremental sensor 20 or the reference mark sensor 122 detects no light at all. For example, in a configuration identical to that of FIG. 8 but without the octadic waveplate 214, if the VCSEL light source 12 emits only vertically polarized light, the incremental sensor 20 would detect no light at all, and if the VCSEL light source emits only horizontally polarized light, the reference mark sensor 122 would detect no light at all. However, by providing the octadic waveplate 214 and configuring it so that its fast axis is angled with respect to both the polarization axis of the first polarizing filter 120 and the polarization axis of the second polarizing filter 223, both the incremental sensor 20 and the reference mark sensor 122 can be assured of receiving the appropriate signals regardless of the polarization direction of the light emitted by the VCSEL light source 12. For example, placing the fast axis of octadic waveplate 214 at 45° to the polarization axes of first polarizing filter 120 and second polarizing filter 223 means that, all other factors being equal, the optical power of the signal impinging on incremental sensor 20 and reference mark sensor 122 will not change regardless of the polarization state of the light output by VCSEL 12. Another way to look at this is that for light leaving octadic waveplate 214 towards incremental sensor 20 and reference mark sensor 122 (i.e., after the light has passed through octadic waveplate 214 twice), the ratio of i) the optical power of light resolvable along the “horizontal” polarization axis (i.e., along the polarization axis of first polarizing filter 120) to ii) the optical power of light resolvable along the “vertical” polarization axis (i.e., along the polarization axis of second polarizing filter 223) is 1:1. This will be true regardless of the polarization state of the light output by the VCSEL 12 (due to the 45° angle between the fast axis of the octadic waveplate and the polarization axes of the first and second polarizing filters 120, 223).

[0116] It will be appreciated that other types of sensors can be used instead 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.

[0117] The embodiments of Figures 3 and 8 described above use a refractive lens 109 to relay the diffracted orders towards the sensor. However, as will be appreciated, this is not necessarily the case. For example, Figure 13 shows a schematic of the optical system of an encoder device including an incremental scale 104 and a readhead 206' according to another embodiment of the present invention. This embodiment is similar to Figure 8 (like parts share like reference numerals), except that it does not rely on a refractive lens 109 to relay the diffracted orders towards the sensor. In this embodiment, the zeroth and ±1st diffraction orders are generated from light from a light source that strikes and is reflected by the scale 104. As shown, the zeroth and ±1st diffraction orders are reflected by a first plane pl 1 The diffraction order encoder 216 is located in a first plane pl 1 According to the embodiment of FIG. 11, the diffraction order encoder includes a zeroth-order term polarizer 216a that is aligned with the zeroth diffraction order (e.g., has a "vertical" polarization axis) to encode the zeroth diffraction order in a first (e.g., vertical) polarization direction. The diffraction order encoder further includes a +1st-order term polarizer 216b and a -1st-order term polarizer 216c that are aligned with the +1st and -1st diffraction orders. The +1st-order term polarizer 216b and the -1st-order term polarizer 216c are configured with the same orientation as each other such 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. Thus, the zeroth and ±1st diffraction orders are polarized relative to the plane pl where the incremental sensor 20 and the reference mark sensor 122 are located. 28, a first polarizer 120 may be positioned in front of the incremental sensor 20 and configured to filter the 0th diffraction order from reaching the incremental sensor 20, and / or a second polarizer 223 may be positioned in front of the reference mark sensor 122 and configured to filter the ±1st diffraction orders from reaching the reference mark sensor 122. For clarity of illustration, the light source 12 and non-uniform / patterned retarder have been omitted from FIG. 13. 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.

[0118] 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 (the body of which 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 VCSEL light source 12, a lens 109, a polarization manipulator 214 (e.g., an octadic waveplate in the embodiment of FIG. 8), a diffraction order encoder 216, an incremental detector 20, and a first polarization filter 120, which are configured and arranged in the same manner as described above in connection with the embodiment of FIG. 8. Thus, the zeroth order term is blocked from contributing to the formation of the signal detected by the incremental sensor 20. As with the embodiment of FIG. 8, the first polarization filter 120 includes a polarization filter having a polarization axis perpendicular to the polarization axis of the zeroth order term polarizer 216a of the diffraction order encoder 216. Notably, in this embodiment, the zeroth order term polarizer 216a has a vertical polarization axis, and the first order sensor filter 120 has a horizontal polarization axis. Also similar to the embodiment of FIG. 8, the fast axis of the octadic waveplate 214 (where light passes twice from the light source 12 to the sensor 20, 520) is oriented at a 45° angle with respect to the polarization axis of the first polarizer 120.

[0119] 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 20 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 20.

[0120] Similar to 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 520. Additionally, similar to 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. Accordingly, a second polarizing filter 523 may be provided in front of the absolute sensor 520 having a polarization axis orthogonal to the polarization axes of the +1st order term polarizer 216b and the −1st order term polarizer 216c of the diffraction order encoder 216. In particular, in this embodiment, the zeroth order term polarizer 216a has a vertical polarization axis, the +1st order term polarizer 216b and the −1st order term polarizer 216c have horizontal polarization axes, and the second polarizing filter 523 has a vertical polarization axis.

[0121] 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). Indeed, in this embodiment, the zeroth diffraction order of the first set of diffraction orders passes through the zeroth-term polarizer 216a of the diffraction order encoder 216, and the +1st and −1st diffraction orders of the first set of diffraction orders pass through the +1st-term polarizer 216b and the −1st-term polarizer 216c, respectively, while the zeroth diffraction order and ±1st diffraction orders of the second set of diffraction orders all pass through the zeroth-term polarizer 216a of the diffraction order encoder 216. The higher diffraction orders of the first set of diffraction orders (e.g., the ±3rd and ±5th diffraction orders) are blocked by the opaque substrate 115. The +3rd and −3rd diffraction orders of the second set of diffraction orders pass through the +1st-term polarizer 216b and the −1st-term polarizer 216c, respectively, of the diffraction order encoder 216. Diffraction orders higher than the ±3 orders of the second set of diffraction orders have substantially insignificant power, but what is present will be blocked by the opaque substrate 115. As will be appreciated, which diffraction orders pass through which filters is system dependent and may depend on various factors, including the size and / or position of the polarizing filters.

[0122] 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. 3 and 8). 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 pure incremental scale track 605 between the two absolute tracks 603). Rather, as in the embodiment of FIG. 14, the absolute scale features can be fully embedded throughout the incremental features. In this case, the above comments regarding the two sets of diffraction orders still apply because in this embodiment the incremental and absolute position features have different nominal periods. If desired, an alternative configuration could arrange the incremental sensor 20 and absolute sensor 620 according to the embodiment of FIG. 14 (i.e., with two sensors arranged side by side).

[0123] The above-described embodiments utilize a lensless or single-lens system through which light from the light source passes on its way to and from the scale. It will be appreciated that other optical configurations are possible, such as the one shown schematically in FIG. 16 (which illustrates an absolute encoder apparatus including an absolute scale). The encoder 300 of FIG. 16 includes an absolute scale 304 that includes conceptually periodically arranged features, with selected features removed to encode unique / absolute position data along the measuring length of the scale. The data may be in the form of, for example, a pseudo-random sequence or discrete code words. Details of such scales are described in more detail in U.S. Pat. Nos. 7,499,827 and 5,279,044. The readhead 306 (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 306 includes a polarized VCSEL light source 12 and a quarter-wave plate 14. A first lens 309 is provided to collimate the light from the VCSEL 12. An optical beam splitter 307 is provided which allows light from the VCSEL 12 to pass through and illuminate the scale 304 and redirect light reflected by the scale to a sensor 320 (in this case a one-dimensional array of photodiodes, e.g. a complimentary metal-oxide-semiconductor (CMOS) sensor). The sensor 320 is configured to sense an image of the scale formed in situ by a second lens 310. In this embodiment, the sensor 320 has an integrated polarising filter (not shown) so that the sensor 320 senses light filtered along the polarising axis of the integrated polarising filter. The readhead is configured so that the angle between the fast axis of the quarter wave plate 14 and the polarising axis of the integrated polarising filter is 45°. As with the previous embodiment, the angle does not necessarily have to be 45°, although such an angle may be preferred. The image obtained by the sensor 320 may be processed by a processor device to determine the relative positions of the readhead 306 and the scale 304 in a known manner (e.g. as described in U.S. Pat. No. 1,098,9567).

[0124] FIG. 17 illustrates another exemplary embodiment of the present invention. In this embodiment, except for the use of a polarized light source 12, a polarizing filter 120, and a polarization manipulator 14 (a quarter-wave plate in this embodiment), the configuration and operation of this encoder are substantially identical to those described in WO2005124282. Thus, with the polarization manipulator 14 set aside, light from the light source 12 illuminates an area 24 of the scale 4. Due to the periodic arrangement of the incremental features 10, light reflected by the incremental features 10 is diffracted into diffraction orders (i.e., in the same manner as described above in connection with FIGS. 1a and 1b). The diffracted light strikes a diffraction grating 16, where the light is diffracted into further diffraction orders and recombines at the incremental sensor 20 to form an interference fringe pattern (or "fringe field") thereon. Movement of the readhead 6 relative to the scale 4 causes movement of the interference fringes relative to the incremental sensor 20, thus generating an up / down count that allows for measurement of the variation. As with the previous embodiment, the incremental sensor 20 is in the form of an electrical grating and the light source 12 is a VCSEL.

[0125] As the readhead 6 passes the reference mark 11, changes in the intensity of light reflected by the scale 4 are imaged by the optical imager lens 18 onto a reference mark sensor 22 (hereinafter referred to as the "reference mark sensor"). In this embodiment, the reference mark sensor 22 comprises a split detector including first and second photodiodes 22a, 22b onto which light from the scale 4 is imaged. The optical imager 18 shown in FIG. 17 is a Fresnel zone plate. However, other types of optical imagers having the same optical function may be used, such as refractive lenses. As described in WO2005124282, the reference mark sensor 22 may be partially or completely embedded within the incremental sensor 20 (similarly, the optical imager 18 may be partially or completely embedded within the diffraction grating 16).

[0126] In contrast to the embodiment of WO2005124282, a polarizing filter is used to filter the light reaching the incremental sensor. Therefore, as with the above-described embodiment, without the quarter-wave plate 14, what strikes the incremental sensor 20 and reference mark sensor 22 would vary depending on the predominant polarization state / direction emitted by the VCSEL 12. In the worst case, this could mean that the incremental sensor 20 detects no light at all. However, by providing the quarter-wave plate 14 in the optical path between the light source 12 and the incremental sensor 20 and configuring its fast axis and the polarization axis of the polarizing filter 120 of the incremental sensor 20 so that they are angled relative to each other, the incremental sensor 20 can be guaranteed to receive an appropriate signal regardless of the polarization direction of the VCSEL light source 12. For example, placing the fast axis of the quarter-wave plate 14 at 45° to the polarization axis of the polarizing filter 120 of the incremental sensor means that, all other factors being equal (e.g., for a constant optical power output from the VCSEL 12), the optical power of the signal impinging on the incremental sensor 20 will be the same regardless of the polarization state of the light output by the VCSEL 12.

[0127] The above-described embodiments use waveplates 14, 214 as polarization manipulators. Instead of waveplates, other types of optical elements, such as diffusers, non-uniform / patterned retarders, or time-based polarization manipulators, can be used. As will be appreciated, for time-based polarization manipulators, the modulation rate will need to be faster than the acquisition bandwidth of the encoder's sensor. Typically, a rate of at least 1 MHz should be sufficient. Fiber-coupled time-based polarization frequency band converters, such as those available from FIBREPRO Inc., exist that are suitable for fiber-optic position encoders. Because time-based manipulators require electronics that are not suitable for compact encoders, and because diffusers, in some embodiments, can provide significant adverse ray deflections that adversely affect the formation of interference fringe patterns, spatial polarization manipulators, particularly non-uniform / patterned retarders, have been found to be preferred.

[0128] As mentioned, a non-uniform / patterned retarder can be used as a polarization manipulator in place of the quarter wave plate or octadic wave plate described above. Figure 18 shows an example suitable non-uniform / patterned retarder 414 for use in place of the quarter wave plate 14 used in the embodiment of Figure 17.

[0129] In this embodiment, the patterned retarder 414 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 "patterned retarder"). In particular, the patterned retarder 414 includes an array of discrete elongated half-wave plate pixels / columns 19, where the array extends in the Y dimension and the elongated length of the pixels / columns extends in the X dimension. Thus, the array / series of pixels extends perpendicular to the measuring dimension (X) of the scale 4. Thus, in this embodiment, the patterned retarder 414 does not include structure in the measuring dimension (X). This may be advantageous to prevent the patterned retarder 14 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 20, 22. However, as will be appreciated, in other embodiments / applications, such diffraction may be acceptable and therefore the non-uniform / patterned retarder may comprise a structure in the X dimension, for example it may be arranged such that an array / series of retarder pixels extend along the Y dimension, or the non-uniform / patterned retarder may comprise a two-dimensional array of retarder pixels.

[0130] As shown in FIG. 18( a), the half-wave plate pixels 19 are arranged with alternating fast axes at 0° and 45°. Such a configuration ensures that, regardless of the predominant input polarization direction of the footprint 21 of light impinging on the patterned retarder 414, the light output from the patterned retarder 414 has a mix of polarization directions across its extent / footprint (in the Y dimension) such that the DOP of the light leaving the patterned retarder 414 is smaller than the DOP of the light impinging on the patterned retarder 414. In particular, in this embodiment, the light output from the patterned retarder 414 always includes alternating sections / portions / columns (in the Y dimension) of light having two different polarization directions that are orthogonal to each other, regardless of the predominant input polarization direction. Thus, regardless of the polarization state of the light emitted from the polarized light source 12, the light leaving the patterned retarder 414 towards the incremental sensor is guaranteed to have a polarization state that is at least partially resolvable along the polarization axis of the first polarizing filter 120. Furthermore, in the described embodiment, the light output from the patterned retarder 414 contains a balanced mix of polarization directions (e.g., in the present embodiment, there are substantially equal amounts of two different orthogonal polarization directions).

[0131] Thus, the patterned retarder 414 is configured such that the optical power (e.g., milliwatts, “mW”) of the light within the light footprint 21 output from the patterned retarder 414 is substantially equal along the orthogonal polarization axes, regardless of the orientation of the orthogonal polarization axes (thus, the DOP of the light footprint 21 output from the optical retarder element is close to 0, e.g., 0.1 or less, and preferably 0.02 or less). This can be beneficial for reasons of balance and consistency in encoder performance. For example, if the optical power along one polarization axis is substantially greater than another orthogonal polarization axis, especially if the respective powers change with changes in input polarization direction, the effects of polarization on encoder performance may not be as suppressed as in a configuration in which the optical power of the light along the orthogonal polarization axes is substantially the same.

[0132] Referring to FIG. 18(b), four different (i-iv) exemplary orientations of the orthogonal polarization axes (v, h) are shown, along with a graph of the optical power along the orthogonal polarization axes of light output from the patterned retarder 414 within the optical footprint 21. As shown, due to the substantially balanced mix of polarization directions output by the patterned retarder 414 within the optical footprint 21, for any pair of orthogonal polarization axes, the optical power along each polarization axis is substantially the same regardless of the orientation of the orthogonal polarization axes. In other words, within the optical footprint 21 output from the patterned retarder 414, if the light output from the patterned retarder 414 is polarized along a first polarization axis, the optical power of the polarization will be substantially the same as the optical power of the light if it were instead polarized along a second polarization axis that is orthogonal to the first polarization axis.

[0133] The patterned retarder 414 is configured such that, within at least a region of the footprint of the light output from the patterned retarder 414 that forms the optical signal at the sensor, regardless of the predominant input polarization state of the light striking the optical retarder element, the difference in optical power along the orthogonal polarization axis of the output beam is at least half that of the input beam, regardless of the orientation of the orthogonal polarization axis.

[0134] In the embodiments described herein, substantially equal (or “substantially the same”) optical power along orthogonal polarization axes means that the difference in optical power along the orthogonal polarization axes varies by no more than 2% of the total power. Thus, in such cases, the DOP of the light output from the patterned retarder 414 within the optical footprint 21 is 0.02 or 2% or less. However, it will be appreciated that in other embodiments, such tight tolerances may not be required, and thus substantially equal (or “substantially the same”) optical power of the light along orthogonal polarization axes may mean that the difference in optical power along the orthogonal polarization axes varies by 5% of the total power, e.g., by no more than 10% of the total power, e.g., by no more than 20% of the total power. In other words, the patterned retarder 414 may be configured such that the DOP of the light output from the optical retarder element within the optical footprint 21 is 0.05 (or 5%) or less, e.g., 0.1 (or 10%) or less, e.g., 0.2 (or 20%) or less.

[0135] As shown in FIG. 18( a), the pitch p of the half-wave plate pixels 19 is substantially smaller than the width of the patterned retarder 414 in the Y dimension. In particular, although the patterned retarder 414 may contain only two pixels (so that the first half of the light output therefrom has one polarization direction and the second half of the light output therefrom has a different, orthogonal polarization direction), it has been found to be beneficial to provide substantially more half-wave plate pixels 19 in the patterned retarder 414 to spread different polarization directions (rather than the same or similar polarization directions all grouped to one side) across the footprint of the light output from the patterned retarder 414, and therefore across the scale features on the scale 4. This can be beneficial for a number of reasons, including providing greater freedom for where a sensor can be placed along the Y dimension and still receive the benefits of the present invention. In the present embodiment, this is achieved by providing multiple half-wave plate pixels 19 of alternating fast axes in the patterned retarder 414. In particular, the pitch p1 of the half-wave plate pixels 19 (in the dimension along which the series of pixels extends, in this embodiment along the Y dimension) and the width W of the patterned retarder 414 r The ratio of is at least 1:4, more preferably at least 1:10, particularly preferably at least 1:20, for example at least 1:30.

[0136] It should be noted that in this embodiment, the arrangement of half-wave plate pixels 19 is such that the above statements regarding the optical power along orthogonal polarization axes of the light output from patterned retarder 414 in light footprint 21 being substantially the same (and below a certain level regarding the DOP of the light output from patterned retarder 414 in light footprint 21) also apply to region 21 a of the footprint of the light output from patterned retarder 414 towards incremental sensor 20, and also apply to region 21 b of the footprint of the light output from patterned retarder 414 towards reference mark sensor 22. Thus, it is ensured that the light impinging on incremental sensor 20 and reference mark sensor 22 contains a balanced mix of polarization directions.

[0137] Furthermore, in the described embodiment, the patterned retarder 414 is configured such that the different polarization directions output within region 21a are substantially evenly distributed in the Y dimension (i.e., perpendicular to the measuring dimension X), such that, to conceptually divide the region into a one-dimensional array of two equal-width columns (21aa and 21ab) extending parallel to the Y dimension, the optical power along the orthogonal polarization axes of the light output from the patterned retarder 414 within each of the two columns is substantially equal, regardless of the orientation of the orthogonal polarization axes. Note that this is also true for region 21b of the footprint of the light output from the patterned retarder 414 that forms the optical signal at the reference mark sensor 22 (e.g., to conceptually divide the region into a one-dimensional array of two equal-width columns 21ba and 21bb extending parallel to the Y dimension, the optical power along the orthogonal polarization axes of the light output from the patterned retarder 414 within each of the n columns is substantially equal, regardless of the orientation of the orthogonal polarization axes). Thus, graphs (i-iv) of FIG. 6(b) (and to some extent the description of the DOP of the light output from the optical retarder elements within the optical footprint 21) apply equally to each row 21aa and 21ab of the optical footprint region 21a output from the patterned retarder 414 forming the optical signal at the incremental sensor 20, and equally to each row 21ba and 21bb of the optical footprint region 21b output from the patterned retarder 414 forming the optical signal at the reference mark sensor 22. Such a configuration can help suppress errors that might otherwise be caused by imperfections in the scale features along their length. Additionally, uniform illumination across the sensor can be beneficial. For example, the sensor may be shaped / windowed / weighted to improve the signal output by the sensor, as described, for example, in U.S. Pat. No. 1,067,0431, and uniform illumination may be required to maintain the benefits / effects of the shaping / windowing / weighting.

[0138] The patterned retarder 414 described above is suitable for use in place of a quarter-wave plate used in embodiments in which light from a light source passes through the polarization manipulator once (such as the embodiments of FIGS. 3, 16, and 17), but is not suitable for use in place of an octadic wave plate used in embodiments in which light from a light source passes through the polarization manipulator twice (such as the embodiments of FIGS. 8, 14, and 15). Indeed, it has been found that the polarization direction of light returning through the patterned retarder 414 of FIG. 18(a) the second time is changed the second time, "reverting" the polarization of many sections / portions of the light. This causes the light output from the patterned retarder 414 to have a non-uniform mix of polarization directions after the second pass, which may be undesirable, such that the light after the second pass still has a dominant polarization direction that may or may not be at least partially resolvable along the polarization axis of the sensor's polarizer (e.g., first polarizing filter 120).

[0139] The inventors have identified that for double-pass systems, the mix of polarization directions leaving the patterned retarder twice can be more evenly balanced (thus ensuring that light leaving the patterned retarder towards the incremental sensor 20 has a polarization state that is at least partially, and more preferably substantially, resolvable along the polarization axis of the first polarizing filter 120) by providing a patterned retarder with half-wave plate pixels arranged with their different fast axes in a non-periodic arrangement. While the inventors have found that a random non-periodic arrangement typically provides an improvement over a periodic arrangement, the inventors have found that some non-periodic arrangements of fast axes are better than others in providing a balanced mix of polarization directions after two passes through the patterned retarder, regardless of the predominant input polarization direction of the footprint of the light that initially strikes the patterned retarder from the VCSEL 12.

[0140] 19 shows an example patterned retarder 514 suitable for use in a double-pass configuration and capable of providing a well-balanced mix of polarization directions after light passes through it twice. The table below lists the fast axis directions of each of the 60 pixels that make up the non-uniform / patterned retarder 514:

[0141] [Table 1]

[0142] Thus, as shown, in this embodiment, the patterned retarder 514 includes a plurality (60 in this embodiment) of half-wave plate pixels 119 having three or more different fast axis directions, which are not arranged periodically.

[0143] Of course, the above fast axis arrangement is not the only arrangement that can provide a substantially balanced mix of polarization directions output from the non-uniform / patterned retarder 514 after the light has passed through the non-uniform / patterned retarder 514 twice. Other arrangements are possible and can be selected by the designer of the optical encoder system.

[0144] The use of the terms "pattern" and "patterned" herein, particularly in relation to polarization manipulators, is not intended to imply that there is any repeating configuration. Rather, the terms "pattern" and "patterned" are used to refer to the polarization manipulator, e.g., retarder, having a non-uniform design / formation.

[0145] 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).

[0146] In the described embodiment, light source 12 emits electromagnetic radiation (EMR) in the near-infrared range. However, it will be appreciated that this is not necessarily the case and light source 12 can emit EMR in other ranges, for example, any range from infrared to ultraviolet. It will be appreciated that the selection of an appropriate wavelength for light source 12 can depend on many factors, including the availability of suitable gratings and detectors operating at EMR wavelengths.

[0147] The waveplates of the above-described embodiments may include birefringent materials such as quartz, mica, liquid crystal, etc. Optionally, the waveplates of the above-described embodiments may include structures configured to provide birefringence-like properties such as metamaterials, etc. For example, it is known that nanocasting lithography can be used to provide waveplates.

[0148] 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.

Claims

1. a scale including a series of position features readable by the readhead; The read head and Equipped with The read head i) a polarized light source that emits polarized light toward the scale; ii) a first sensor configured to sense light filtered along a first polarization axis and positioned to sense light from the polarized light source that has interacted with the scale; and iii) a polarization manipulator located in the optical path between the polarized light source and the first sensor, configured so that, regardless of the polarization state of 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; Including, Position measuring encoder device.

2. configured such that the ratio of the optical power of the light leaving the polarization manipulator towards the first sensor between i) the optical power of the light resolvable along the first polarization axis and ii) the optical power of the light resolvable along a second, orthogonal polarization axis is at least 2:3; 2. The position measuring encoder device of claim 1.

3. configured such that, all other factors being equal, the optical power of light sensed by the first sensor is substantially the same for all possible polarization directions of light emitted from the polarized light source.

3. A position measuring encoder device according to claim 1 or 2.

4. The read head iv) a second sensor configured to sense light filtered along a second polarization axis and positioned to sense light from the polarized light source that has interacted with the scale; further comprising the polarization manipulator is configured such that, regardless of the polarization state of light emitted from the polarized light source, light leaving the polarization manipulator towards the second sensor has a polarization state that is at least partially resolvable along the second polarization axis. A position measuring encoder device according to any one of claims 1 to 3.

5. the second polarization axis is orthogonal to the first polarization axis; 5. A position measuring encoder device according to claim 4.

6. all other factors being equal, the optical power of the light sensed by the first sensor is substantially the same for all possible polarization directions of light emitted from the polarized light source, and the optical power of the light sensed by the second sensor is substantially the same for all possible polarization directions of light emitted from the polarized light source.

6. A position measuring encoder device according to claim 4 or 5.

7. the polarization manipulator includes a wave plate, and the effective full-wave retardation characteristic of the polarization manipulator is n+1 / 4, where n is an integer greater than or equal to 0; A position measuring encoder device according to any one of claims 1 to 6.

8. the waveplate is oriented with its fast axis at an angle of 30° to 60° relative to the first polarization axis, more preferably at an angle of 40° to 50° relative to the first polarization axis; 8. A position measuring encoder device according to claim 7.

9. the light from the polarized light source passes through the polarization manipulator twice on the path from the polarized light source to the first sensor; A position measuring encoder device according to any one of claims 1 to 8.

10. the polarization manipulator includes an octadic waveplate; A position measuring encoder arrangement according to claims 7 and 9.

11. the scale features diffract the light into multiple diffraction orders; at least one of the diffraction orders has a polarization state that is different from the polarization state of at least one other diffraction order; A position measuring encoder device according to any one of claims 1 to 10.

12. the first sensor is configured to primarily sense light filtered along a first polarization axis, such that a signal sensed by the first sensor is primarily formed from a selected subset of diffraction orders; 12. A position measuring encoder device according to claim 11.

13. the polarization states of the diffraction orders, said first sensor configured to sense light filtered along said first polarization axis; a second sensor configured to sense light filtered along a second polarization axis, configured such that the light sensed by the first sensor comprises a diffraction order composition that is different from the diffraction order composition of the light sensed by the second sensor; 13. A position measuring encoder device according to claim 3 and 11 or 12.

14. a scale including a series of position features readable by the readhead; The read head and Equipped with The read head i) a polarized light source that emits polarized light toward the scale; ii) a first sensor configured to sense light filtered along a first polarization axis and positioned to sense light from the polarized light source that has interacted with the scale; and iii) a waveplate located in an optical path between the polarized light source and the first sensor, the waveplate having an effective full-wave retardation of n+¼, where n is an integer greater than or equal to 0, and oriented such that its fast axis is at an angle between 30° and 60° relative to the first polarization axis; Including, Position measuring encoder device.

15. iv) a second sensor configured to sense light filtered along a second polarization axis orthogonal to the first polarization axis, the second sensor being positioned to sense light from the polarized light source that has interacted with the scale; 15. A position measuring encoder device according to claim 14.

16. the wave plate includes an octadic wave plate, and the light from the polarized light source passes through the octadic wave plate twice on a path from the polarized light source to the first sensor.

16. A position measuring encoder device according to claim 14 or 15.

17. the wave plate includes an octadic wave plate, and the light from the light source passes through the octadic wave plate twice on a path from the light source to the first sensor and the second sensor; 17. A position measuring encoder device according to claim 16.

18. the scale diffracts the light into a plurality of diffraction orders; The read head the first sensor senses ±1st diffraction orders but does not sense the 0th diffraction order, and the second sensor senses the 0th diffraction order but does not sense the ±1st diffraction orders; encoding said zeroth diffraction order in a polarization state aligned with a second polarization axis; encoding said ±1st diffraction orders with polarization states aligned with said first polarization axis; a diffraction order encoder configured to 18. A position measuring encoder device according to claim 15 or 17.

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