Encoder device

Abstract

A position encoder read head for reading a scale member, the read head comprising: a primary sensor comprising a substantially continuous array of primary sensor elements for detecting a primary scale signal falling on the primary sensor, the array extending along a measurement dimension, the primary sensor configured to output a signal, the signal depends on the relative position of the read head and the scale along the measurement dimension; an auxiliary sensor for detecting an auxiliary scale signal falling on the auxiliary sensor, the auxiliary sensor comprising at least a first auxiliary sensor element positioned along the measurement dimension at a position between the ends of the main sensor; the primary sensor is configured such that: i) the influence of the primary sensor element on the position signal output by the primary sensor varies along the measurement dimension according to a multimodal distribution tapering towards its ends; and ii) a first imaginary line can be identified, which extends parallel to the measurement dimension and passes through the at least first auxiliary sensor element and passes through the at least one main sensor element on either side of the at least first auxiliary sensor element.

Description

[0001] The present invention relates to an encoder device, in particular to a position measuring encoder device comprising a scale and a readhead movable relative to each other.

[0002] As is well known, position measuring encoder devices typically comprise a scale having a series of features that a read head can read to determine and measure relative position (and its derivatives, such as velocity and / or acceleration). Encoders are typically classified as either incremental or absolute. For incremental encoders (such as the TONiC encoders available from Renishaw plc), the encoders are typically used to measure relative position (and its derivatives, such as velocity and / or acceleration). TM An absolute encoder (such as RESOLUTE® encoder) comprises a series of generally periodic features that are detected by a readhead to determine the relative position and movement of the scale and readhead. One or more reference marks may be provided on the scale to provide reference positions from which the relative position of the scale and / or readhead can be counted. TM An encoder) includes features that define unique positions (e.g., a series of unique absolute positions) along the length of the scale, and the scale enables a readhead to determine its absolute position at start-up without any relative motion.

[0003] WO 2017042570 discloses an example readhead for an absolute or incremental encoder, wherein the readhead includes a position sensor comprising an array of sensor elements that are weighted (e.g., shaped / sized) according to a predetermined window function configured to reduce specific types of errors, particularly subdivision error of the encoder device. As taught in WO 2017042570, the weighting is configured such that the effect of the sensor output on the determined relative position decreases primarily toward the ends of the sensor.

[0004] The present invention relates to an encoder readhead comprising a plurality of sensors, for example incremental sensors and reference sensors or absolute sensors, and a particularly advantageous arrangement thereof.

[0005] According to a first aspect of the invention, there is provided a position encoder readhead for reading a scale member, the position encoder readhead comprising: a main sensor comprising a substantially continuous array of main sensor elements for detecting a main scale signal falling thereon, the array extending along a measuring dimension; an auxiliary sensor comprising at least a first auxiliary sensor element positioned at a position between the ends of the main sensor along the measuring dimension for detecting an auxiliary scale signal falling thereon; the main sensor being configured such that: i) the influence of the main sensor element on the position signal output by the main sensor varies along the measuring dimension (in other words, varies depending on the position of the main sensor element along the measuring dimension) according to a multimodal distribution that tapers (in other words, gradually decreases) towards its ends; and ii) a first imaginary line can be identified that extends parallel to the measuring dimension and passes through at least the first auxiliary sensor element and passes through at least one main sensor element on either side of the at least first auxiliary sensor element.

[0006] Compared to the arrangements shown in WO 2017042570 (such as the embodiment of FIG. 21 comprising an incremental sensor and a reference mark sensor), the design of the present invention provides a more efficient use of the space occupied by the primary (e.g. incremental) sensor and the auxiliary (e.g. absolute reference mark) sensor. Although the primary sensor of the encoder readhead according to the present invention deviates from the teachings of WO 2017042570 and may therefore be more susceptible to the types of errors that WO 2017042570 seeks to suppress, the inventors have recognized that adopting this design for the sensor can help provide an otherwise improved readhead. In particular, adopting the arrangements of the present invention can help reduce the amount of noise present on the signal output by the primary sensor compared to those designs described in WO 2017042570, which in turn can help provide a readhead with reduced position jitter (or "jitter error").

[0007] As will be appreciated, noise appears as unwanted interference on a signal, which is a consequence of physics and thermodynamics. For example, noise can be measured as the root mean square (RMS) sum of all independent noise sources, examples of which include: thermal noise, amplifier noise and shot noise. Noise on the signal output by the primary sensor of the read head can result in "position jitter" (or "jitter error") in the position reported by the read head. Although position jitter / jitter error may exist and may be problematic in all circumstances, it may be most obvious to an observer when the scale and read head are held in a stationary / stable position relative to each other and the position reported by the read head is unstable. The effects of position jitter / jitter error may also be apparent when a position encoder is used in a servo mechanism to control the position of two relative members, and the servo mechanism uses the output of the position encoder to try to keep the two relative members in a stationary position - in this case, position jitter / jitter error may result in the servo mechanism causing the two relative members to change position rather than remain stationary. Another example of when position jitter / jitter errors can be a significant problem is when using a readhead to read a scale member having incremental position features and one or more reference marks - position jitter / jitter errors can result in inconsistent positional relationships between the incremental signals and the reference mark signals. As will be appreciated, noise and associated position jitter / jitter errors can be present in position encoder readheads that output analog position signals (e.g., analog sine and cosine signals) as well as position encoder readheads that output digital position signals (e.g., position encoder readheads that produce digital quadrature or digital serial outputs, such as digital words representing a unique position).

[0008] The primary sensor may comprise a one-dimensional array of primary sensor elements (extending along the measurement dimension).The secondary sensor may comprise an array of secondary sensor elements (eg a one-dimensional array of secondary sensor elements, eg extending along the measurement dimension).

[0009] As will be understood, the sensor element array of the primary sensor is substantially continuous in that there are no significant gaps between immediately adjacent sensor elements in the array. As will be understood, due to manufacturing tolerances, the gaps between immediately adjacent sensor elements in the array may not be exactly the same for every pair of immediately adjacent sensor elements in the array. Furthermore, due to the specific design of the primary and / or auxiliary sensors, the gaps between immediately adjacent sensor elements may intentionally be different for each pair of immediately adjacent sensor elements, and / or there may be one or more locations where the gap between one pair of immediately adjacent sensor elements is larger than the gaps between most other pairs of immediately adjacent sensor elements. However, the present invention relates to primary sensors having no significant gaps between immediately adjacent sensor elements in the sensor element array. In particular, for the purposes of the present invention and this document, the sensor element array of the primary sensor is substantially continuous in that, for all pairs of immediately adjacent sensor elements in the array, the gaps between immediately adjacent sensor elements (measured along a measurement dimension) are no greater than 10% (preferably no greater than 5%) of the total length of the primary sensor measured along the measurement dimension.

[0010] The influence of the sensor elements of the primary sensor on the position signal output by the primary sensor can be controlled by electronic weighting. In a preferred embodiment, the influence of the sensor elements of the primary sensor on the position signal output by the primary sensor is controlled by the effective sensing length of the sensor elements (in a dimension perpendicular to the measurement direction). Accordingly, in a preferred embodiment, the primary sensor is configured such that the effective sensing area of ​​the primary sensor elements varies along the measurement dimension according to a multimodal distribution that tapers toward its ends. The effective sensing area of ​​the sensor elements can be determined by the physical size of the sensor elements and / or by one or more mask features configured to prevent light from reaching the entire physical sensing area of ​​the sensor elements. Such a mask can be configured such that the effective sensing area of ​​any individual sensing element is continuous, or alternatively, such a mask can be configured such that each individual sensing element has multiple discrete sensing areas along its length, separated by blocked / masked areas (e.g., such that the sensing areas are spread out along the length of the sensor element). It may be preferred that the width of the sensor elements (as measured along the measurement dimension) is substantially the same for all sensor elements in the array. The main sensor may be configured such that an effective sensing length of the main sensor element (as measured perpendicular to the measuring dimension) varies along the measuring dimension according to a multimodal distribution that tapers towards its ends.

[0011] According to these embodiments of the present invention, the sensing area of ​​the entire primary sensor can be shaped such that the effective sensing length of the primary sensor element (as measured in a direction perpendicular to the measuring dimension) varies along the measuring dimension according to a multimodal distribution that tapers toward its ends. In other words, the primary sensor can be shaped such that the effective sensing length of the primary sensor element (as measured in a direction perpendicular to the measuring dimension) varies depending on the position of the primary sensor element along the measuring dimension.

[0012] Preferably, the position encoder readhead is configured such that for at least 15%, for example at least 25%, optionally at least 50%, preferably at least 75% of the length (measured perpendicularly to the measuring dimension) of at least the first auxiliary sensor element, a first imaginary line can be identified, the first imaginary line extending parallel to the measuring dimension and passing through at least the first auxiliary sensor element and passing through at least the main sensor element on either side of the at least first auxiliary sensor element. It may be the case that the greater the proportion of the length of at least the first auxiliary sensor element for which the first imaginary line can be identified, the greater the benefit that can be obtained by the design method of the invention.

[0013] Although the first imaginary line may pass through only one sensor element of the primary sensor on either side of the at least first auxiliary sensor element, it is typically the case, particularly for configurations that benefit most from the design approach of the present invention, that the line will pass through a plurality of sensor elements of the primary sensor on either side of the at least first auxiliary sensor element. Accordingly, the position encoder readhead is optionally configured such that the first imaginary line passes through the at least first auxiliary sensor element and through a plurality of primary sensor elements on either side of the at least first auxiliary sensor element, wherein X is at least 5% of the total number of sensing elements in the sensing element array of the primary sensor, and more preferably, wherein X is at least 10% of the total number of sensing elements.

[0014] It may be preferred that the position encoder readhead is configured such that the multimodal distribution tapers between its peak and valley regions. For example, the sensing area of ​​the primary sensor may be shaped such that the multimodal distribution tapers between its peak and valley regions. Accordingly, such taper may provide a smooth transition between the peak and valley regions. This may be advantageous because it helps reduce systematic / positional errors, particularly when the signal falling on the sensor is non-optimal (e.g., distorted).

[0015] The above-mentioned taper (eg, taper towards the end of the main sensor and / or taper between the peak region and the valley region) may be a linear taper. Alternatively, the taper may be non-linear.

[0016] It may be preferred that at least the first auxiliary sensor element is positioned substantially centrally relative to the main sensor along the measurement dimension. This may be beneficial, for example, when the pattern falling on the sensor plane is non-uniform (e.g., due to the signal being formed by a point light source). In this case, for example, positioning at least the first auxiliary sensor element substantially centrally relative to the main sensor along the measurement dimension provides minimal sensitivity to errors caused by poor construction and / or installation of the readhead.

[0017] At least the first auxiliary sensor element may be arranged beside the main sensor, at least predominantly (optionally completely) on a first side of a notional center line of the main sensor extending parallel to the measuring dimension.

[0018] According to the invention, at least the first auxiliary sensor element can be located at least partially (optionally completely) within a valley-shaped section of the main sensor. In the case where at least the first auxiliary sensor element is positioned substantially centrally relative to the main sensor along the measuring dimension, at least the first auxiliary sensor element can be located at least partially (optionally completely) within the centrally positioned valley-shaped section of the main sensor.

[0019] The auxiliary sensor may include at least a second auxiliary sensor element. The at least second auxiliary sensor element may be positioned along the measurement dimension at a position between the ends of the primary sensor. Optionally, the at least second auxiliary sensor element is arranged alongside the primary sensor, at least predominantly (optionally entirely) on a second side of a notional centerline of the primary sensor extending parallel to the measurement dimension.

[0020] Preferably, the readhead is configured such that a second imaginary line can be identified that extends parallel to the measurement dimension and passes through the second auxiliary sensor element and passes through at least one of the primary sensing elements on either side of at least the second auxiliary sensor element. As with the first auxiliary sensor element and the first imaginary line (described above), it may be preferred that the readhead be configured such that, for at least 50%, optionally at least 75%, and preferably at least 95% of the length (measured perpendicular to the measurement dimension) of at least the second auxiliary sensor element, a second imaginary line can be identified that extends parallel to the measurement dimension and passes through at least the second auxiliary sensor element and passes through at least one of the primary sensing elements on either side of at least the second auxiliary sensor element. As with the first auxiliary sensor element and the first imaginary line (described above), it will typically be the case, particularly for configurations that are intended to benefit most from the present invention, that the second imaginary line will pass through a group of sensor elements of the primary sensor on either side of at least the second auxiliary sensor element. Accordingly, optionally, the position encoder read head is configured such that the second imaginary line passes through at least a second auxiliary sensor element and through a group of at least X primary sensor elements on either side of the at least second auxiliary sensor element, wherein X is at least 5% of the total number of sensing elements in the sensing element array of the primary sensor, more preferably, wherein X is at least 10% of the total number of sensing elements.

[0021] As with the at least first auxiliary sensor element, it may be preferred that the at least second auxiliary sensor element is positioned substantially centrally relative to the main sensor along the measuring dimension. Accordingly, the read head may be configured such that the at least second auxiliary sensor element is at least partially (optionally completely) located within the centrally located valley-shaped section of the main sensor.

[0022] The primary sensor can be configured such that the contribution of the primary sensor element to the position signal output by the primary sensor varies along the measurement dimension according to a bimodal distribution that tapers toward its ends. Alternatively, the sensing area of ​​the entire primary sensor can be shaped such that the effective sensing length of the sensor element (as measured in a direction perpendicular to the measurement dimension) varies along the measurement dimension according to a bimodal distribution that tapers toward its ends.

[0023] The variation in the effect of the primary sensor on the position signal output by the primary sensor may be substantially symmetrical about a conceptual centre line of the primary sensor extending perpendicular to the measurement dimension. This may be preferred to help reduce errors caused by phase shifts in the signals output by the primary and auxiliary sensors, which phase shifts are caused by relative yaw of the scale and readhead (i.e. rotation about an axis normal to the scale). Accordingly, the shape of the sensing area of ​​the entire primary sensor may be substantially symmetrical about a conceptual centre line of the primary sensor extending perpendicular to the measurement dimension. As explained in more detail below, the shape of the sensing area of ​​the entire primary sensor may be asymmetrical about a conceptual centre line of the primary sensor extending parallel to the measurement dimension, and in some circumstances this may be beneficial, for example if the footprint of light falling at the plane of the primary sensor is not symmetrical.

[0024] According to the invention, the output of the primary sensor depends on the relative position of the readhead and the scale along the measuring dimension. The readhead may be configured such that the signal output by the auxiliary sensor depends on the relative position of the readhead and the scale (e.g. in any one or more of the six relative degrees of freedom (i.e. three linear degrees of freedom and three rotational degrees of freedom)). The readhead may be configured such that the signal output by the auxiliary sensor depends on the relative position of the readhead and the scale along the measuring dimension and / or along different degrees of freedom (e.g. in one or two dimensions perpendicular to the measuring dimension, and / or around an axis of rotation). In an alternative embodiment, the output of the auxiliary sensor is used to determine other information in addition to information about the relative position of the scale and readhead, for example the auxiliary sensor may be configured to sense a scale type identifier which may, for example, be used to program the readhead (e.g. as described in WO 2010 / 116145).

[0025] The main sensor may include an incremental position sensor. Accordingly, the substantially continuous array of main sensor elements may include a substantially continuous array of incremental position sensor elements. Accordingly, the main scale signal may include an incremental scale signal. The main scale signal may include a fringe field, such as interference fringes. The array of sensor elements of the main sensor may form an electrical grating comprising two or more groups of interdigitated auxiliary sensor elements, each group being configured to detect a different phase of the fringe field. The readhead may include one or more diffraction gratings for generating the interference fringes.

[0026] The auxiliary sensor may include a reference mark sensor. Accordingly, at least the first auxiliary sensor element may include at least a first auxiliary reference mark sensor element. Accordingly, the auxiliary scale signal may include a reference mark signal. The auxiliary sensor may include an absolute position sensor. Accordingly, the auxiliary scale signal may include an absolute position signal.

[0027] The main scale signal and the auxiliary scale signal can be substantially the same / identical signals. The main scale signal and the auxiliary scale signal can be a portion of the same signal formed by the scale member, and the read head is configured to read the portion (for example, by the same scale features on the scale member, the read head is configured to read these scale features). For example, as explained in more detail below, the position feature that produces the auxiliary scale signal can be at least partially (for example, completely) embedded in the series of position features that produce the main scale signal. Accordingly, and in particular where the position feature that produces the auxiliary scale signal is completely embedded in the series of position features that produce the main scale signal, the main signal and the auxiliary signal falling on the main sensor and the auxiliary sensor can be the same / identical. Of course, the main signal and the auxiliary signal do not necessarily have to be the same / identical, even if they are formed by the same features on the scale (for example, even when the position feature that produces the auxiliary scale signal is completely embedded in the series of position features that produce the main scale signal). For example, the optics within the readhead (e.g., diffractive optics, such as a diffraction grating, and / or refractive optics, such as a lens) may be configured to manipulate the signal from the scale so that the primary signal falling on the primary sensor is different from the auxiliary signal falling on the auxiliary sensor. Indeed, it may be preferred that the primary signal falling on the primary sensor is different from the auxiliary signal falling on the auxiliary sensor in order to provide primary and auxiliary signals that are tailored to their specific purposes.

[0028] The sensor element of the primary sensor may include a light-sensitive element. At least a first auxiliary sensor element of the auxiliary sensor may include a light-sensitive element. As will be appreciated, the sensor elements of the primary sensor and / or the auxiliary sensor may be different types of sensors, such as magnetic sensors, inductive sensors, or capacitive sensors. The sensor elements of both the primary sensor and the auxiliary sensor may be the same type of sensor (e.g., they may both be light-sensitive elements), but this need not necessarily be the case.

[0029] The main sensor and the auxiliary sensor can be provided on different substrates (e.g., different semiconductor substrates, such as silicon). Alternatively, the main sensor and the auxiliary sensor are provided on the same substrate (e.g., the same semiconductor substrate), which can facilitate the manufacture / assembly of the read head.

[0030] The position encoder readhead can be configured to read a linear scale.The position encoder readhead can be configured to read a rotary scale.

[0031] According to a second aspect of the invention, there is provided a position encoder device comprising: a read head according to any one of the above or subsequent embodiments; and a scale comprising at least one series of (e.g., primary) position features that produce the primary scale signal. The series of (e.g., primary) position features may comprise a series of generally periodically arranged features. The series of (e.g., primary) position features may define an incremental scale. The series of (e.g., primary) position features may define an absolute scale. As is known, the series of position features may define a hybrid incremental and absolute scale (e.g., as described in WO 2002 / 084223). As described in WO 2002 / 084223, such a scale is essentially an incremental scale in which absolute position information is embedded. Depending on the particular implementation, the absolute position features may form the primary scale signal, or the absolute position features may form the auxiliary scale signal.

[0032] The scale may include one or more (eg, auxiliary) (eg, position) features that generate auxiliary scale signals. Auxiliary scale signals may be generated from (eg, primary) position features that are also used to generate the primary scale signal.

[0033] The (e.g., auxiliary) (e.g., position) features that produce the auxiliary scale signal may be at least partially embedded within a series of position features that produce the primary scale signal. The (e.g., auxiliary) (e.g., position) features that produce the auxiliary scale signal may extend across the entire width of the scale track that contains the (e.g., primary) position features that produce the primary scale signal (the width of the scale track being in a dimension perpendicular to the measurement dimension / perpendicular to the extent of the series of features). (Accordingly, as will be appreciated, the concept of a scale track comprising two basic scale periods (e.g., 8μm and 32μm) is not limited to multi-track embodiments). However, this need not necessarily be the case, and the (e.g., auxiliary) (e.g., position) features that produce the auxiliary scale signal may extend only partially across the width of the scale track that contains the (e.g., primary) position features that produce the primary scale signal. Optionally, the (e.g., auxiliary) (e.g., position) features that generate the auxiliary scale signal are contained in at least two rows extending along the measurement dimension (along the range of the series of (e.g., primary) position features that generate the primary scale signal). The at least two rows may be laterally separated (e.g., spaced apart in a dimension perpendicular to the measurement dimension).

[0034] The one or more reference features may be embedded within a series of position features. The scale may include one or more features defining absolute scale position information. The features defining absolute scale position information may be embedded within the series of position features.

[0035] According to a third aspect of the present invention, a scale for a position encoder apparatus is provided, the scale comprising: an incremental scale track comprising a series of periodically arranged incremental scale features extending along a measurement dimension; and one or more auxiliary position features at least partially embedded within the incremental scale track, the one or more auxiliary position features being divided into at least two parts perpendicular to the measurement dimension, wherein an uninterrupted series of periodically arranged incremental scale features is provided in the space between the at least two parts. Accordingly, this configuration means that, although the auxiliary scale features are at least partially embedded within the incremental scale track, an uninterrupted series of incremental position features extending continuously along the measurement dimension is present in the space between the two parts of the auxiliary scale features. This can help to increase the strength of the incremental signal generated by the scale track of the incremental sensor.

[0036] The scale may comprise a series of auxiliary position features extending along the same measurement dimension as the series of periodically arranged incremental scale features, wherein the features in the series of auxiliary position features are divided into at least two parts in a dimension perpendicular to the measurement dimension, so that the scale comprises two rows of scale features comprising both incremental scale features and auxiliary position features and a row of uninterrupted periodically arranged incremental scale features located between the two rows of scale features (i.e. a row of only incremental scale features located between the two rows of scale features comprising both incremental scale features and auxiliary position features).

[0037] The auxiliary position feature may define a reference mark. Alternatively, the auxiliary position feature defines a series of unique absolute positions along the measurement dimension. Accordingly, the auxiliary position feature may be an absolute scale position feature.

[0038] The one or more auxiliary position features may be contained in at least two rows extending along the measuring dimension, the at least two rows being separated laterally in a dimension perpendicular to the measuring dimension.

[0039] Optionally, the one or more auxiliary position features are fully embedded within the incremental scale track.

[0040] Aspects of the ruler described above and below in conjunction with other aspects of the invention are equally applicable to this aspect of the invention, and vice versa.

[0041] According to a fourth aspect of the present invention, there is provided a position encoder apparatus comprising a scale as described above (or below) and a readhead, the readhead comprising at least one incremental sensor for sensing a feature of the incremental scale, and at least first and second auxiliary position sensors located on opposite sides of the at least one incremental sensor in a dimension perpendicular to the measurement dimension. Aspects of the position encoder apparatus described above and below in conjunction with other aspects of the invention are equally applicable to this aspect of the invention, and vice versa.

[0042] The position encoder device may be a linear position encoder device. The position encoder device may be a rotary position encoder device.

[0043] Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which:

[0044] Figure 1 A first example position encoder apparatus comprising a readhead according to the present invention is shown;

[0045] Figure 2 Schematically illustrates a first embodiment of the present invention. Figure 1 Optical components within the read head;

[0046] Figure 3 An example electro-grating sensor is shown;

[0047] Figure 4a Shows from Figure 1 and Figure 2 An example reference mark signal of the reference mark photodiode of the read head, and Figure 4b The differential signals obtained therefrom are shown;

[0048] Figure 5a Shown Figure 1 and Figure 2 an arrangement of the incremental sensor elements and the reference mark sensor elements of the read head;

[0049] Figure 5b It shows the relationship between the various sensor elements Figure 5a A graph showing how the position signal I output by the incremental sensor changes along the sensor element array of the incremental sensor;

[0050] Figure 6 Schematically illustrates the additional sensing area achieved by adopting the method of the present invention;

[0051] Figure 7a A first alternative configuration of an incremental sensor element and a reference mark sensor element according to the invention is shown;

[0052] Figure 7b It shows the relationship between the various sensor elements Figure 7a A graph showing how the position signal I output by the incremental sensor changes along the sensor element array of the incremental sensor;

[0053] Figure 8a A second alternative configuration of incremental sensor elements and reference mark sensor elements according to the invention is shown;

[0054] Figure 8b It shows the relationship between the various sensor elements Figure 8a A graph showing how the position signal I output by the incremental sensor changes along the sensor element array of the incremental sensor;

[0055] Figure 9a A third alternative configuration of an incremental sensor element and a reference mark sensor element according to the invention is shown;

[0056] Figure 9b It shows the relationship between the various sensor elements Figure 9a A graph showing how the position signal I output by the incremental sensor changes along the sensor element array of the incremental sensor;

[0057] Figure 10a A fourth alternative arrangement of incremental sensor elements and reference mark sensor elements according to the invention is shown;

[0058] Figure 10b It shows the relationship between the various sensor elements Figure 10a A graph showing how the position signal I output by the incremental sensor changes along the sensor element array of the incremental sensor;

[0059] Figure 11a and Figure 11b An embodiment of the present invention is shown, except for the reference mark arrangement on the scale, with Figure 1 and Figure 2 The same as the embodiment;

[0060] Figure 12a and Figure 12b An alternative embodiment of the invention implemented for use with an absolute scale is shown;

[0061] Figures 13a to 13d Schematic illustration of the effect of the reference mark on the position signal due to the mismatch between the scale signal period and the incremental sensor period;

[0062] Figure 14 A fifth alternative arrangement of incremental sensor elements and reference mark sensor elements according to the invention is shown;

[0063] Figure 15 is a graph schematically showing erroneous changes in the position signal caused by reference marks and / or contaminants on the scale when the read head passes over the reference marks / contaminants on the scale;

[0064] Figure 16 A first embodiment illustrating another aspect of the present invention; and

[0065] Figure 17 A second embodiment presents another aspect of the present invention.

[0066] refer to Figure 1 and Figure 2 , shows a first example encoder device 2 according to the invention comprising a readhead 6. The encoder device comprises a scale 4 and a readhead 6. Although not shown, in use the readhead 6 may be fastened to one part of a machine and the scale 4 to another part of the machine so that the parts are movable relative to each other along a degree of freedom. The readhead 6 is used to move along Figure 1 The dimension shown by arrow A in FIG. 2 (i.e., along the X dimension in the orientation shown) measures its relative position relative to the scale 4 and can therefore be used to provide a measure of the relative position of two movable parts of the machine along the X dimension. In this embodiment, the two parts of the machine (and therefore the scale 4 and the readhead 6) are configured to move relative to each other along a linear degree of freedom, in particular along the direction shown by arrow A / along the X dimension. Accordingly, in the embodiment described, the encoder device 2 is what is generally referred to as a linear position encoder device. However, as will be appreciated, the present invention is equally applicable to the readhead of a rotary position encoder device; for example, a ring encoder device or a disc encoder device, which is configured to measure the relative rotational position of the scale and readhead about an axis of rotation.

[0067] The read head 6 communicates with a processing device, such as a controller (not shown), via a wired communication channel and / or a wireless communication channel. As will be understood, the processing device may include a custom processor configured for a specific application (e.g., a field programmable gate array "FPGA") as well as a more general processor that can be programmed (e.g., via software) according to the needs of the application in which it is used. The read head 6 can report signals from its sensors (described in more detail below) to the controller, which then processes these signals to determine position information, and / or the read head 6 itself can process the signals from its sensors and send the position information to the controller. In another embodiment, an intermediate unit (e.g., an interface unit) can be positioned between the read head 6 and the controller. The interface unit can facilitate communication between the read head 6 and the controller. For example, the interface unit can be configured to process the read head signals and provide position information to the controller.

[0068] The scale 4 includes a plurality of scale / position features. In the depicted embodiment, the scale 4 is an incremental scale comprising a single graduated track including a generally periodic arrangement of incremental position features 10. In the depicted embodiment, the scale 6 also includes reference marks 11 embedded within the same graduated track as the incremental features 10.

[0069] In this embodiment, the encoder device is a diffraction-based optical encoder. The incremental position features 10 include a series of periodic scale features that form a diffraction grating. The scale 4 can be a scale that is commonly referred to as an amplitude-type scale or a phase-type scale. As will be understood, if the scale is an amplitude-type scale, the incremental position features 10 (and optionally the reference marks 11) are configured to control the amplitude of light transmitted toward the read head 6 (for example, by selectively absorbing, scattering and / or reflecting the light), while if the scale is a phase-type scale, the incremental position features 10 (and optionally the reference marks 11) are configured to control the phase of light transmitted toward the read head (for example, by delaying the phase of the light). In this embodiment, the incremental position features 10 and the reference marks 11 are amplitude-type scale features, and light interacts with the incremental position features 10 to generate diffraction orders. As will be understood, the incremental features and the reference mark features need not both be amplitude features or both be phase features, for example, the incremental position features can be phase features and the reference marks can be amplitude features. Furthermore, neither the incremental feature nor the reference mark feature need be optical features. For example, the incremental feature can be an optical feature and the reference mark feature can be a magnetic feature, in which case the primary (incremental) sensor 24 can include an optical sensor / photosensitive sensor and the auxiliary (reference mark) sensor 26 can include a magnetic sensor.

[0070] like Figure 2As shown, in this embodiment, the readhead 6 (whose body / physical structure is not shown for ease of illustration) includes a light source 12 (which is configured to illuminate a portion of the scale 4 - shown by the footprint outline 25'), an optical device 16 and a sensor unit 20. Reflected light from within the footprint on the scale forms a scale light footprint 25" on the optical device 16, and the light within the footprint 25" is relayed to the sensor unit 20. In particular, the optical device 16 includes a diffraction grating 18 and a first optical focusing (e.g., imaging) element 22a and a second optical focusing (e.g., imaging) element 22b. In the embodiment shown, the optical focusing (e.g., imaging) elements 22a, 22b include diffractive lenses, such as Fresnel zone plates, but may alternatively include refractive lenses, such as cylindrical lenses or Fresnel lenses. One or more additional lenses, beam splitters, mirrors or other optical elements (not shown) may be used to direct, control and / or focus the light to and from the scale 4. For example, a lens (not shown) may be present between the light source 12 and the scale so that light therefrom is substantially collimated and then emitted towards the scale 4 .

[0071] like Figure 2 As shown, a footprint 25'' of light reflected from the scale 4 falls upon the sensor unit 20. The sensor unit 20 comprises a primary sensor 24 and an auxiliary sensor 26. As explained in more detail below, the primary sensor comprises a substantially continuous array of primary sensor elements extending along the measuring dimension of the readhead (i.e., along the X dimension in the orientation shown). In the depicted embodiment, the sensors / sensor elements are light-sensitive ("optical") sensors / sensor elements, but this need not necessarily be the case. For example, the primary and / or auxiliary sensors / sensor elements could alternatively be capacitive, magnetic, or inductive sensors / sensor elements.

[0072] In the depicted embodiment, the primary sensor 24 is an incremental sensor configured to detect incremental signals generated by incremental features 10 of the scale, while the auxiliary sensor 26 is a reference mark sensor configured to detect the reference mark 11 as it passes relative to the readhead 6. In this embodiment, the reference mark sensor 26 includes four sensor elements: a first reference mark sensor element 26a', a second reference mark sensor element 26a", a third reference mark sensor element 26b', and a fourth reference mark sensor element 26b". The outputs of the first reference mark sensor element 26a' and the second reference mark sensor element 26a" are combined so that downstream electronics can view their outputs as one signal (e.g., a first reference mark signal output). The outputs of the third reference mark sensor element 26b' and the fourth reference mark sensor element 26b" are combined so that downstream electronics can view their outputs as one signal (e.g., a second reference mark signal output). Accordingly, the reference mark sensor 26 comprises a so-called “split detector” comprising a first half formed by the first reference mark sensor element 26 a′ and the second reference mark sensor element 26 a″ and a second half formed by the third reference mark sensor element 26 b′ and the fourth reference mark sensor element 26 b″. However, as will be understood, the reference mark sensor need not comprise a split sensor and may, for example, comprise only a single reference mark sensor element.

[0073] As is well known and understood in the art of diffraction-based encoders (and as described, for example, in WO 2005 / 124282 and WO 2017 / 042570), the diffraction orders reflected by the scale 4 interact with the diffraction grating 18 to form an interference fringe pattern at the plane of the sensor unit 20 (and therefore fall on the incremental sensor 24). Relative movement of the readhead 6 and scale 4 causes modulation / movement of the interference fringes relative to the readhead / incremental sensor 24, the output of which can be processed to provide incremental up / down counts that enable incremental measurement of displacement. As will be appreciated, the encoder can typically be configured to provide two signals that are orthogonal (90 degrees out of phase with each other), and are typically labeled SIN and COS signals (even though they may not actually be sine or cosine signals). The quadrature signals can be interpolated to provide accurate measurement of the position of the readhead to less than one period of the repeating scale pattern. It is well known to provide such quadrature signals from encoder devices in order to provide an indication of the orientation and relative movement of the readhead and scale.

[0074] In the described embodiment, the incremental sensor 24 is in the form of an electric grating. In other words, the electric grating is a photosensor array comprising two or more groups of interdigitated / staggered / interwoven photosensor elements (also referred to herein as "photodetectors" or "fingers"). For example, each group can detect a different phase of interference fringes at the incremental sensor 24. Figure 3 An example of a known electric grating of standard / rectangular shape is shown in FIG, wherein the sensor elements of four groups of sensor elements (A, B, C and D) are interdigitated / staggered to form an array of sensor elements extending along the measuring dimension of the readhead (in this embodiment, along the X dimension). Groups of photodiodes are arranged in a repeating arrangement with a period "p" (and therefore a frequency "f" of 1 / "p").

[0075] As shown, in the depicted embodiment, the array of individual sensor elements extends substantially parallel to the measuring dimension X of the incremental detector 24. Furthermore, the individual fingers / photodiodes / sensor elements are substantially rectangular in shape, with their long sides extending perpendicular to the measuring dimension X. As will be appreciated, the present invention is also applicable to sensor elements of other shapes and arrangements.

[0076] The output of each sensor element in a group is combined to provide a single output, resulting in four channel outputs: A', B', C' and D'. These outputs are then used to obtain orthogonal signals SIN and COS. In particular, A'-C' are used to provide a first signal (SIN) and B'-D' are used to provide a second signal (COS) that is 90 degrees out of phase with the first signal. Although in a particular embodiment, the electric grating includes four groups of photodiodes providing four channels A', B', C' and D', this need not be the case. For example, the electric grating may include two groups of photodiodes providing only two channels A' and B'. Furthermore, in this embodiment, the incremental detector 24 is non-aliasing. However, as will be appreciated, the present invention is equally applicable to aliasing sensors (e.g., sensors in which the distance between sensor elements in each group is greater than one fringe period).

[0077] exist Figure 3 , the interference fringes are represented by line 27, which schematically illustrates the varying intensity of an ideal interference fringe across the incremental detector 24. As illustrated, the encoder apparatus is configured so that, ideally, at any one moment in time, all incremental sensor elements in any group detect the same phase of the interference fringes (if the fringe period p' and the sensor period p are the same).

[0078] Turning now to the detection of the reference mark 11, in the described embodiment, the reference mark 11 is formed by removing (or adding additional) incremental features. Alternatively, the reference mark may comprise any pattern that can be distinguished from the incremental position features 10. For example, the reference mark may have structure in the measurement dimension X and / or in a dimension perpendicular to the measurement dimension. For example, the reference mark may comprise an autocorrelator, wherein the reference mark forms a pattern that interleaves with the incremental position features 10, and a matching mask is provided in the read head, for example, as described in WO 2002 / 065061.

[0079] The optical focusing (e.g., imaging) elements 22a, 22b focus light (e.g., an image) reflected by the top and bottom portions of the scale onto the plane of the sensor unit 20, so that when the read head 6 passes over the reference mark 11, changes in the intensity of the light reflected by the scale 4 are sensed by the reference mark sensor 26. As explained above, in the described embodiment, the reference mark sensor 26 is a so-called "split detector" (comprising a first half formed by a first reference mark sensor element 26a' and a second reference mark sensor element 26a", and a second half formed by a third reference mark sensor element 26b' and a fourth reference mark sensor element 26b"). Figure 4a The outputs S1, S2 from the two halves of the split detector are shown as the readhead moves along the scale. The signals S1 and S2 from the two halves of the split detector can be subtracted to form Figure 4b The position of the reference mark can be determined based on signal S3, for example, based on the position at which a falling edge of signal S3 crosses a known threshold in the reading direction. Although a segmented reference mark sensor is used to detect the reference mark in this embodiment, other types of reference mark sensors can be used. For example, the outputs of three or more individual detector elements can be combined. In another alternative embodiment, only the output of a single reference mark sensor element can be used (for example, a reference mark can be identified as the position at which the output of the single reference mark sensor element crosses a predetermined threshold).

[0080] The signals from the main / incremental sensor 24 and the auxiliary / reference mark sensor 26 are processed by electronics (not shown) within the readhead 6 and / or within devices external to the readhead 6 to determine / monitor position information therefrom, as is well known in the encoder art.

[0081] As taught in WO 2017 / 042570, it can be beneficial to weight the outputs of the incremental sensor elements according to a window function, particularly in such a way that the influence of the sensor output on the determined relative position decreases primarily towards the ends of the sensor, in order to reduce segmentation errors caused by unwanted frequencies in the signal falling on the incremental sensor. As also taught in WO 2017 / 042570, the reference mark sensor can be located alongside the windowed incremental sensor, midway between the ends of the incremental sensor (e.g., see FIG. 21 of WO 2017 / 042570). However, this configuration results in a large portion of the light returning from the scale 4 not being sensed by the main (e.g., incremental) sensor. The inventors have found that in certain circumstances, it may be beneficial to deviate from the configuration shown in WO 2017 / 042570, for example, to help the main (e.g., incremental) sensor sense a larger proportion of the light returning from the scale 4.

[0082] For example, the inventors have found that, while it may still be beneficial for: i) the influence of the main sensor elements / incremental sensor elements to generally decrease (i.e. taper) towards the ends of the incremental sensor (but as explained in more detail below, for reasons other than those identified in WO 2017 / 042570), and ii) the reference mark sensor elements to be located between the ends of the incremental sensor along the measuring dimension, it may be beneficial if some of the incremental sensor elements are sufficiently long in a direction perpendicular to the measuring dimension so that they are located in the space on either side of the reference mark sensor element.

[0083] exist Figure 2 It is shown in Figure 5a An example embodiment of such a sensor is shown in more detail in FIG. 1 , where the incremental sensor 24 comprises a substantially continuous array of incremental sensor elements extending along the measuring dimension X. In this embodiment, the incremental sensor 24 is an electric grating and, according to the above description, Figure 3 As described above, the incremental sensor 24 includes four groups of interdigitated / staggered sensor elements: A, B, C, and D. As will be appreciated, the present invention is also applicable to non-electrical grating sensor arrays, such as linear CMOS (complementary metal oxide semiconductor) arrays. Figure 5a As shown, there are two groups of incremental sensor elements G1, G2, which are long enough in the direction perpendicular to the measuring dimension so that they are located in the space on both sides of the reference mark sensor element. Accordingly, at least one imaginary line can be identified (see, for example, Figure 5aThe example dashed line 30) in FIG: i) extends parallel to the measuring dimension X of the readhead; ii) passes through the reference mark sensor element; and, iii) passes through at least one incremental sensor element on either side of the reference mark sensor element 26a', 26a", 26b', 26b".

[0084] Extending at least some of the incremental sensor elements so that they are located in the space on either side of the reference mark sensor element can increase the amount of light sensed by the main sensor / incremental sensor 24, which in turn can reduce the amount of noise present on the signal output by the main sensor / incremental sensor 24, which in turn can reduce the amount of jitter error present in the position signal determined based on the output of the main sensor / incremental sensor 24. For example, Figure 6 The shaded area 27 represents (for Figure 2 5 ) compared to incremental sensor designs in which the incremental sensor elements are not located in the space on either side of the reference mark sensor element (e.g., as taught in WO 2017 / 042570). As illustrated, the sensor according to the present invention utilizes more of the footprint 25'' of available light reflected from the scale and falling on the sensor unit 20, thereby improving the reduction of signal noise and the resulting position jitter / jitter errors.

[0085] like Figure 5a As clearly shown in , the lengths of the incremental sensor elements (A, B, C, D, etc.), as measured in a direction perpendicular to the measuring dimension (i.e., along the Y dimension), vary along the measuring dimension X. Accordingly, the contribution I of the individual sensor elements to the position signal output by the incremental sensor 24 varies along the sensor element array of the main sensor / incremental sensor 24 (i.e., varies along the measuring dimension X). Figure 5b is a graph illustrating how the influence I of the individual sensor elements on the position signal output by the incremental sensor 24 varies along the sensor element array of the main sensor / incremental sensor 24 (i.e., how the influence I of the individual sensor elements on the position signal output by the incremental sensor 24 varies along the measuring dimension X). In other words, Figure 5b Shown Figure 5a” of the sensor. As shown, the influence varies according to a multi-peak (in this embodiment, bimodal) distribution that tapers towards its ends. This multi-peak distribution is the result of designing the incremental sensor 24 in a manner that balances the following requirements: i) locating the reference mark sensor element in a position between the ends of the incremental sensor 24, which is optically more advantageous and less sensitive to yaw of the scale and readhead (i.e., rotation about an axis normal to the scale) that might shift the phase of the signals output by the primary and auxiliary sensors; ii) reducing the influence of the incremental sensor element towards the ends of the sensor to reduce measurement errors in the signal derived from its output that appear near the reference mark (as explained in more detail below); and iii) causing as much light as possible to fall on the incremental sensor in order to reduce signal noise and reduce jitter.

[0086] In the described embodiment, the contribution I of the incremental sensor elements to the position signal output by the incremental sensor 24 varies due to differences in the physical lengths of the incremental sensor elements (measured perpendicular to the measurement dimension, i.e., measured along the Y dimension). However, this need not necessarily be the case. For example, a mask may be disposed over at least some of the incremental sensor elements to limit (e.g., block) the amount of light that falls upon them. The mask may be configured so that the effective sensing area of ​​any one individual sensing element is continuous, or the mask may be configured so that each individual sensing element has a plurality of discrete sensing areas separated by blocked / masked areas, such that the sensing areas are spread out along the length of the sensor element. Additionally or alternatively, the contribution I of the incremental sensor elements to the position signal output by the incremental sensor 24 may be controlled by electronic weighting. As will be appreciated, for those incremental sensor elements that are co-located along the measuring dimension (X) as the reference mark sensor element, it is likely that the length of the incremental sensor element must be physically shorter in order to enable the reference mark sensor element to be placed in accordance with the present invention, but for other incremental sensor elements (which are not so co-located along the measuring dimension (X) as the reference mark sensor element), there is more freedom in choosing how to control their effect I on the position signal output by the incremental sensor 24 (for example, the designer can choose whether to do so by electronic weighting, masking and / or giving the incremental sensor elements different lengths).

[0087] Figure 7a 、 Figure 8a 、 Figure 9a and Figure 10a An example of an alternative sensor arrangement according to the invention is shown in . Figure 7b 、 Figure 8b 、 Figure 9b and Figure 10bA graph showing how the influence I of the individual sensor elements varies along the array of sensor elements of the main sensor / incremental sensor 24 (in other words, Figure 7b 、 Figure 8b 、 Figure 9b and Figure 10b They were shown Figure 7a 、 Figure 8a 、 Figure 9a and Figure 10a ). As shown, in each of these embodiments, the main sensor (in these embodiments, the incremental sensor 24) is configured (in particular, shaped) such that: i) the influence on the output position signal of the main / incremental sensor element (in particular, the effective sensing length as measured in a direction perpendicular to the measuring dimension) varies along the measuring dimension according to a multimodal distribution that tapers towards its ends; and ii) at least one imaginary line (e.g., dashed line 30) can be identified that extends parallel to the measuring dimension X and passes through at least one auxiliary (in these embodiments—reference mark) sensor element and passes through at least one main (incremental) sensor element on either side of the at least one auxiliary (e.g., reference mark) sensor element.

[0088] Figure 7a Examples and Figure 5a (as well as Figure 8a 、 Figure 9a and Figure 10a ) differs in that the shape of the incremental sensor 24 ' is shaped with respect to its center line CL extending parallel to the measuring dimension X. X Asymmetric. Moreover, the reference mark sensor element is only located on the incremental sensor center line CL extending parallel to the measuring dimension X X On one side.

[0089] Figure 8a Examples and Figure 5a (as well as Figure 7a 、 Figure 9a and Figure 10a ) differs in that the main sensor (in these embodiments, the incremental sensor 24″) is configured (in particular, shaped) such that the influence on the position signal output of the main sensor element / incremental sensor element (in particular, the effective sensing length as measured in a direction perpendicular to the measuring dimension) varies along the measuring dimension according to a trimodal distribution (tapering towards its ends). Similar to Figure 5a (as well as Figure 7a and Figure 9a), reference mark sensor 26 comprises a so-called "split detector" comprising a first half formed by a first reference mark sensor element 26a' and a second reference mark sensor element 26a", and a second half formed by a third reference mark sensor element 26b' and a fourth reference mark sensor element 26b". However, unlike Figure 5a (as well as Figure 7a and Figure 9a ), first reference mark sensor element 26a′ and second reference mark sensor element 26a″ forming a first half of the “split detector” are separated from third reference mark sensor element 26b′ and fourth reference mark sensor element 26b″ forming the second half of the “split detector” by a group of incremental sensor elements G2. In this embodiment, as illustrated, at least one imaginary line can be identified (e.g., see dashed line 30) that extends parallel to the measurement dimension and passes through at least one reference mark sensor element and passes through at least one main sensor element on either side of the at least one reference mark sensor element (e.g., see incremental sensor element groups G1, G2, and G3). As illustrated in this embodiment, such an imaginary line need not be identifiable for the entire length of the reference mark sensor element. (In this embodiment, the reference mark sensor element extends further in the Y dimension than all of the incremental sensor elements).

[0090] Figure 9a An embodiment is very similar to Figure 5a However, in Figure 5a (as well as Figure 7a and Figure 8a ), the incremental sensor is configured such that (as Figure 5b 、 Figure 7b and Figure 8b 9 (nor FIG. 10 ). While the embodiments of FIG. 9 and FIG. 10 may provide an even greater proportion of light falling on the incremental sensor than the embodiment of FIG. 5 , it has been found that designing the incremental sensor's influence profile so that it tapers between its peak and valley regions helps reduce systematic / positional errors, particularly when the signal falling on the sensor is non-optimal (e.g., distorted).

[0091] Figure 10aThe embodiment differs from the above-described embodiments in that reference mark sensor 26 includes only a single sensor element. Moreover, reference mark sensor 26 is completely embedded / surrounded by the substantially continuous array of incremental sensor elements of incremental sensor 24". As shown, there is a small interruption in the array continuity of the sensor array due to electrical connections to sensor 26 (and / or for some other reasons, such as manufacturing reasons), but the size of the gap measured along the measurement dimension of the readhead (i.e., along the X-axis) is less than 10% of the total length of the main sensor measured along the same measurement dimension and therefore has a relatively negligible impact on the metrological performance of incremental sensor 24.

[0092] In all the above embodiments, the influence of the main sensor / incremental sensor on the position signal output by the main sensor / incremental sensor varies with respect to an imaginary center line CL extending perpendicular to the measuring dimension X. Y While this may be beneficial for various reasons (including minimizing positional errors), such a configuration is not required / essential.

[0093] Configuring the main sensor / incremental sensor such that the contribution of the main sensor elements to the output position signal varies along the measurement dimension according to a multimodal distribution (as opposed to the unimodal distribution advocated by WO 2017 / 042570) may have an adverse effect on the presence of segmentation error in the position signal output by the main sensor / incremental sensor (and is therefore contrary to the teachings of WO 2107 / 042570). However, the benefits of reducing noise and jitter that can be achieved by adopting a multimodal distribution may outweigh the reduced segmentation error (SDE) performance. Furthermore, if desired, SDE can be compensated in downstream electronics (e.g., by mapping).

[0094] Figure 11a and Figure 11b An alternative embodiment of a position encoder device 2' is shown, which is Figure 2The position encoder apparatus of the scale 4' is substantially identical, and similar parts share the same reference numerals. In this embodiment, the reference mark 11' does not extend completely across the width of the scale 4' (in the Y dimension), but rather comprises two separate reference mark areas 11a, 11b that are separated / spaced apart along the Y dimension. In the depicted embodiment, the two separate reference mark areas 11a, 11ba are located at the same position along the measuring dimension X. When the read head 6 passes over the reference mark 11', the first reference mark area 11a is imaged onto the first reference mark sensor element 26a' and the third reference mark sensor element 26b', and the second reference mark area 11b is imaged onto the second reference mark sensor element 26a" and the fourth reference mark sensor element 26b". Accordingly, the incremental position feature 10 extends continuously without any interruption between them, but has a reduced length in the region of the reference mark 11'. This can help to increase the strength of the incremental signal generated at the incremental sensor 24 and reduce reference mark induced errors.

[0095] In all of the above-described embodiments, the primary sensor comprises an incremental sensor and the auxiliary sensor comprises a reference mark sensor. Figure 12a and Figure 12b A position encoder device 2' is shown according to an alternative embodiment, wherein the absolute position information is embedded in an incremental scale. Figure 2 The position encoder apparatus of the present invention has many similarities and similar parts share the same reference numerals. In this embodiment, the scale 4" comprises (in accordance with the embodiments described above) a series of incremental features in which absolute coding features are embedded along a first row 15a and a second row 15b extending along the length of the scale, on either side of a row of pure / uninterrupted incremental features 10' (accordingly, the absolute coding features are separated in a dimension perpendicular to the measuring dimension of the scale).

[0096] The sensor unit 20' includes a main (incremental) sensor 24 and an absolute sensor 40, which includes a first photodiode array 40a and a second photodiode array 40b (in this embodiment, they are one-dimensional photodiode arrays, but they can be two-dimensional photodiode arrays). The first optical focusing (e.g., imaging) element 22a forms an image of the first row of absolute scale features 15a on the first photodiode array 40a, and the second optical focusing (e.g., imaging) element 22b forms an image of the second row of absolute scale features 15b on the second photodiode array 40b. In the embodiment described, the absolute encoding of the first and second rows of absolute scale features 15a, 15b is the same. Accordingly, the outputs of the first and second photodiode arrays 40a, 40b can be combined and viewed as one. As is known, and as described, for example, in US Pat. Nos. 7,499,827, 5,279,044, and 1,098,9567, images of the first and second rows of absolute scale features 15a, 15b can be processed to extract the absolute / unique code and thereby determine the absolute position. The absolute position can be combined with the incremental position determined from the incremental detector 24 to provide a fine-pitch absolute position. Alternatively, once the absolute position has been determined, subsequent positions can be determined solely by monitoring the output from the incremental detector 24.

[0097] In the depicted embodiment, the first and second rows of absolute scale features 15a, 15b also include finer pitch / period incremental features in the spaces between the coarser pitch / period absolute features (although this need not necessarily be the case). As an example, the coarse features encoding absolute position information may have a nominal period of approximately 32 μm, while the finer pitch / period incremental features may have a period of approximately 8 μm. Accordingly, the first and second rows of absolute scale features 15a, 15b also contribute to forming an incremental position signal within the light footprint 25'' falling on the sensor unit 20'.

[0098] In all of the above embodiments, the primary sensor is configured (e.g., shaped) such that the effect on the output position signal of the primary sensor element (e.g., effective sensing length as measured in a dimension perpendicular to the measurement dimension) varies along the measurement dimension according to a multimodal distribution that tapers toward its ends. A benefit of configuring the effect of the sensor such that it tapers toward its ends is that it can help reduce errors in the position signal near the reference mark. This will refer to Figure 3 and Figure 13. As mentioned above, Figure 3The ideal scenario is shown where the interference fringes, represented by line 27, have a purely sinusoidally varying intensity and the fringe period p' matches the electrical grating period "p" (e.g., the period of the centers of two photodiodes (such as two "A" photodiodes)), thereby providing an integer number of repetitions of the periodic pattern of interference fringes for each channel. In other words, the spatial frequency of the interference fringes (e.g., 1 / p') is an integer multiple of the spatial frequency of repetition of the multiple groups of photodiodes (e.g., 1 / p) (and specifically, in this case, the spatial frequency of the interference fringes is equal to the spatial frequency of repetition of the multiple groups of photodiodes). Various circumstances can cause the fringe period p' to not match the sensor period p. For example, variations in the spacing between the readhead 6 and the scale 4 can cause variations in the fringe period p' that falls on the sensor (e.g., due to a magnification effect that causes the interference fringes to stretch or shrink depending on whether the spacing increases or decreases). Distortions in the fringe field (e.g., caused by poor optical quality) can also cause a mismatch between the fringe period and the sensor period. This variation in the period of the interference fringes is illustrated by dashed line 27', which schematically illustrates the varying intensity of the interference fringes across the incremental detector 24. As can be seen, there is now a mismatch between the fringe period (of line 27 ′) and the electrical grating period, the effects of which will now be described with reference to FIG. 13 .

[0099] FIG13 schematically illustrates a small, central portion of a sensor array (such as incremental sensor 24, described above) represented by a blank rectangular shape. The optical signal (e.g., interference fringes) impinging on the sensor is represented by a dot-filled rectangular shape. The optical signal is shown as only partially overlapping the sensor elements of the incremental sensor in the Y dimension, making it easier to see their relative positions. However, as will be appreciated, in normal operation, the optical signal will completely overlap the sensor elements of the incremental sensor in the Y dimension.

[0100] Figure 13a represents the ideal scenario where the fringe period matches the sensor period (i.e., according to Figure 3 line 27). Figure 13b represents a scenario where the fringe period no longer matches the sensor period (e.g. due to the aforementioned amplification effects and variations in the separation of the read head and scale) (i.e., according to Figure 3 As can be seen in FIG13 , the phase difference θ between the interference fringes and the individual sensor elements of the incremental sensor 24 increases with the distance from the center line CL of the sensor. Y (i.e., the centerline extending perpendicular to the measurement dimension X). However, because the phase difference changes about the centerline CL of the sensor Y symmetric, so the phase difference is averaged over the sensor, and therefore, according to Figure 13a, the average phase position of the signal is relative to the center line CL of the sensor Y overlap, thus not causing position errors.

[0101] However, if Figure 13c and Figure 13d As shown, this is no longer the case when the read head passes over a reference mark or contamination on the scale. Figure 13c and Figure 13d The striped filled box in the figure indicates that when the sensor 24 is scanned, the phase difference of the incremental position signal is no longer about the center line CL of the sensor. Y This in turn causes the position signal determined from the output of the incremental sensor 24 to be initially shifted in one direction along the measuring dimension X, and then shifted in the opposite direction along the measuring dimension as the readhead passes over the portion of the scale containing the reference mark, resulting in an erroneous change in the position signal as the readhead passes over the reference mark / contaminant on the scale, such as Figure 15 shown.

[0102] As can be seen, the phase difference increases as the distance from the center line CL of the incremental sensor increases. Y Accordingly, the incremental sensor is configured such that the influence of the incremental sensor element on the output position signal tapers gradually towards its ends, thereby reducing the amplitude of the above-mentioned variation in the position signal error.

[0103] Figure 14 An embodiment of the present invention is presented, which is Figure 5a The embodiment is very similar except that to accommodate the non-circular shape of the light footprint "' reflected from the scale (not shown in this embodiment), the main sensor 24 (represented by the grey shaded area) is oriented about the main sensor centerline CL extending parallel to the measurement dimension. X As can be seen, the center line CL of the sensor element at the end of the main sensor 24 X 1 and CL X 2 and the center line of the sensor element at the middle of the main sensor 24 (which coincides with the center line CL of the main sensor X Accordingly, in the dimension perpendicular to the measurement dimension, the center line CL of the sensor element at the end of the main sensor 24 is X 1 and CL X 2 is laterally offset relative to the centerline of the sensor element at the end of the main sensor 24.

[0104] Figure 14 Examples and Figure 5a The embodiment of is also different in that the central valley section is flat. Figure 14Examples and Figure 5a The embodiment of the invention is also different in that, Figure 14 The incremental sensor is configured so that the effective sensing length of the sensor elements decreases / increases within each group of sensor elements (in this embodiment, within groups of four sensor elements). Accordingly, the aforementioned taper of the sensor's influence distribution between its peak and valley regions will also be stepped. If the sensor is an electric grating type sensor (as described above), the sensor can be configured so that within a group of adjacent sensor elements forming the electric grating (e.g., a group comprising one "A" sensor element, one "B" sensor element, one "C" sensor element, and one "D" sensor element), the sensor elements have the same length.

[0105] The above-described embodiments include diffraction-based incremental encoders, whereby diffraction orders from a scale interact with a diffraction grating in a readhead to produce interference fringes at an incremental sensor. As will be appreciated, other types of incremental encoders exist that generate signals at the sensor in different ways depending on the diffraction orders. Furthermore, other types of incremental encoders exist that do not rely on diffraction orders to generate signals at the incremental sensor, but instead rely on other optical phenomena / properties, such as a shadow cast or image of the scale on the sensor. Accordingly, the present invention is not limited to the types of encoders depicted in the above-described embodiments, but may also be used with these other types of encoders.

[0106] In the above embodiment, the primary sensor comprises an incremental sensor. However, this need not necessarily be the case. For example, the primary sensor may comprise an absolute sensor, such as an image sensor, for sensing an absolute position code on the scale. In this case, the auxiliary sensor may comprise one or more supplementary sensors, for example, for sensing another aspect of the scale, such as the relative pitch between the readhead and the scale, or for sensing functional safety limit markings on the scale.

[0107] This document also describes a novel scale arrangement. The scale comprises an incremental scale track comprising a series of generally periodically arranged features extending along a measurement dimension. The scale further comprises auxiliary position features comprising features at least partially embedded within the series of main position features and laterally separated in a dimension perpendicular to the measurement dimension. An embodiment of such a scale is shown and described above in conjunction with Figures 11 and 12. Of course, for this aspect of the invention, the sensor of the read head need not be formed in accordance with the first aspect of the invention. Accordingly, as Figure 17As shown in FIG18 (which is identical to the embodiment of FIG11 and FIG12 except for the shape of the incremental sensor), a standard / known shape incremental sensor 24a (in this case, having a rectangular shape) can be used to replace the sensor 24 of the other above-mentioned embodiments. Accordingly, the embedded reference mark and absolute coding features of these embodiments are divided into at least two parts in a dimension perpendicular to the measuring dimension, wherein a pure / uninterrupted periodic arrangement of incremental scale features is provided in the space between the at least two parts.

[0108] In the above-described embodiments, the reference mark comprises a single continuous block feature in a dimension parallel to the measurement dimension. However, this need not necessarily be the case, and for example, the reference mark may comprise a patterned reference mark, for example, strips (optionally of varying widths) that are each separated along the measurement direction (e.g., by at least one incremental feature therebetween). The term "patterned" in the expression "patterned reference mark" is not intended to imply that the reference mark has a repetitive design / form, but rather to connote that the reference mark has a non-uniform design / form along the measurement dimension. Of course, a "patterned reference mark" may or may not have a repetitive form / design along the measurement dimension.

Claims

1. A position encoder readhead for reading a scale member, the readhead comprising: a primary sensor comprising a substantially continuous array of primary sensor elements for detecting a primary scale signal falling upon the primary sensor, the array extending along a measurement dimension, the primary sensor being configured to output a signal that is dependent on the relative position of the readhead and scale along the measurement dimension; an auxiliary sensor for detecting an auxiliary scale signal falling on the auxiliary sensor, the auxiliary sensor comprising at least a first auxiliary sensor element positioned along the measurement dimension at a position between the ends of the main sensor; The auxiliary sensor is configured such that: i) the contribution of the primary sensor element to the position signal output by the primary sensor varies along the measurement dimension according to a multimodal distribution that tapers towards its ends; as well as ii) A first imaginary line can be identified, extending parallel to the measurement dimension and passing through the at least first auxiliary sensor element and passing through at least one main sensor element on either side of the at least first auxiliary sensor element.

2. The position encoder read head of claim 1 is configured such that: for at least 15%, for example at least 25%, optionally at least 50%, preferably at least 75% of the length of the at least first auxiliary sensor element measured perpendicular to the measuring dimension, it is possible to identify the first imaginary line, which extends parallel to the measuring dimension and passes through the at least first auxiliary sensor element and passes through at least one main sensor element on either side of the at least first auxiliary sensor element.

3. The position encoder read head according to claim 1 or 2, wherein: The first imaginary line passes through the at least first auxiliary sensor element and through a group of at least X primary sensor elements on either side of the at least first auxiliary sensor element, where X is at least 5% of a total number of sensing elements in an array of sensing elements of the primary sensor.

4. A position encoder readhead as claimed in any preceding claim, wherein: The multimodal distribution tapers between its peak and valley regions.

5. A position encoder readhead as claimed in any preceding claim, wherein: The at least first auxiliary sensor element is positioned substantially centrally relative to the main sensor along the measuring dimension.

6. A position encoder readhead as claimed in any preceding claim, wherein: The at least first auxiliary sensor element is arranged beside the main sensor, at least predominantly on a first side of an imaginary center line of the main sensor extending parallel to the measuring dimension.

7. The position encoder read head of claim 6, wherein: The auxiliary sensor comprises at least a second auxiliary sensor element positioned along the measuring dimension at a position between the ends of the main sensor and arranged beside the main sensor, on a second side of the imaginary center line of the main sensor extending parallel to the measuring dimension, and wherein the main sensor is configured such that: i) A second imaginary line can be identified, extending parallel to the measurement dimension and passing through the at least second auxiliary sensor element and passing through at least one main sensor element on either side of the at least second auxiliary sensor element.

8. The position encoder read head of claim 7 is configured such that: for at least 50%, optionally at least 75%, and preferably at least 95% of the length of the at least second auxiliary sensor element measured perpendicular to the measuring dimension, a second imaginary line can be identified, the second imaginary line extending parallel to the measuring dimension and passing through the at least second auxiliary sensor element and passing through at least one main sensor element on either side of the at least second auxiliary sensor element.

9. A position encoder readhead as claimed in claim 7 or 8, wherein: The at least second auxiliary sensor element is positioned substantially centrally relative to the main sensor along the measuring dimension such that the at least second auxiliary sensor element is at least partially located within a centrally located valley-shaped section of the main sensor.

10. A position encoder readhead as claimed in any preceding claim, wherein: The main sensor is configured such that an effect of the main sensor element on a position signal output by the main sensor varies along the measurement dimension according to a bimodal distribution that tapers towards its ends.

11. A position encoder readhead as claimed in any preceding claim, wherein: The variation in the effect of the primary sensor on the position signal output by the primary sensor is substantially symmetrical about an imaginary centerline of the primary sensor extending perpendicular to the measurement dimension.

12. The position encoder read head of claim 1, wherein: The sensing area of ​​the entire main sensor is shaped such that an effective sensing length of the main sensor element measured in a direction perpendicular to the measuring dimension varies along the measuring dimension according to a multimodal distribution tapering towards its ends.

13. A position encoder readhead as claimed in any preceding claim, wherein: The main sensor comprises an incremental position sensor, the continuous array of main sensor elements comprises a continuous array of incremental position sensor elements, and the main scale signal comprises an incremental position scale signal.

14. A position encoder readhead as claimed in any preceding claim, wherein: The auxiliary sensor comprises a reference mark sensor, and wherein the at least first auxiliary sensor element comprises at least a first auxiliary reference mark sensor element.

15. A position encoder readhead as claimed in any preceding claim, wherein: The main sensor element and the auxiliary sensor element include light-sensitive elements.

16. A position encoder readhead as claimed in any preceding claim, wherein: The main scale signal comprises a fringe field, optionally interference fringes.

17. The position encoder readhead of claim 16, wherein: The read head comprises one or more diffraction gratings for generating the interference fringes.

18. A position encoder device comprising: A readhead according to any preceding claim; as well as A scale comprising at least a series of position features that produce the primary scale signal.

19. A scale for a position encoder device, the scale comprising: an incremental scale track comprising a series of periodically arranged incremental scale features extending along a measurement dimension; as well as One or more auxiliary position features, the one or more auxiliary position features are at least partially embedded in the incremental scale graduation track, the one or more auxiliary position features are divided into at least two parts in a dimension perpendicular to the measurement dimension, wherein an uninterrupted periodically arranged incremental scale feature is provided in the space between the at least two parts.

20. The ruler of claim 19, comprising: a series of auxiliary position features extending along the same measurement dimension as the series of periodically arranged incremental scale features, wherein the features in the series of auxiliary position features are divided into at least two parts in a dimension perpendicular to the measurement dimension, so that the scale includes two rows of scale features including both the incremental scale features and the auxiliary position features, and an uninterrupted row of periodically arranged incremental scale features located between the two rows of scale features.

21. A scale according to claim 19 or 20, wherein: The one or more auxiliary position features define a reference marker.

22. A scale according to claim 19 or 20, wherein: The one or more auxiliary position features define a series of unique absolute positions along the measurement dimension.

23. A scale according to any one of claims 19 to 22, wherein The one or more auxiliary position features are contained in at least two rows extending along the measurement dimension, the at least two rows being laterally separated in a dimension perpendicular to the measurement dimension.

24. A position encoder device comprising: A scale as claimed in any one of claims 19 to 23; as well as a readhead comprising at least one incremental sensor for sensing features of the incremental scale, and at least first and second auxiliary position sensors located on opposite sides of the at least one incremental sensor in a dimension perpendicular to the measurement dimension.

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