Measuring device

Abstract

An incremental measurement encoder includes a scale and a readhead. The scale includes a periodic series of features forming an incremental track and at least one reference mark. The readhead includes a structured light source and a photodetector array for the reference mark. The at least one reference mark can include at least one imaging element configured to form an image of the structured light source on the photodetector array for the reference mark.

Description

Technical Field

[0001] The invention of the present application relates to a measuring device, in particular, a measuring encoder including a scale and a reading head, wherein the scale is provided with an incremental track and a reference mark.

Background Art

[0002] Generally, a measuring encoder includes a scale and a reading head that are movable relative to each other. One type of known encoder has scale markings provided on the surface of the scale, and the reading head includes a light source that illuminates the scale and a detector that detects the optical pattern generated by the interaction between the light, the scale, and other optical components present, thereby measuring the relative movement of the scale and the reading head. In particular, an incremental measuring encoder generally includes a scale having increment features arranged periodically. Usually, when relative movement occurs between the measuring scale and the reading head, some form of periodic signal is generated. This signal is counted, and the displacement between the scale and the reading head can be measured. Such counting can be performed inside and outside the reading head (for example, the reading head can output a periodic signal and / or the reading head can output the count of the above periodic signal). The periodic signal may be in the form of a sine wave. In many cases, a plurality of periodic signals are generated, and they are offset from each other, for example, by 90°. Usually, these signals are called SIN and COS signals. It is known that these signals can be interpolated to improve the resolution of the encoder.

[0003] Also known are measurement scales having one or more reference marks, i.e., marks that provide a "reference" point for positions on the measurement scale. The reference marks serve to verify the accuracy of the increment count and / or to measure the reference position of the reading head relative to the scale in case, for example, the increment count has been lost (e.g., due to a power failure) or has been performed inaccurately due to factors such as the reading head moving too fast or dirt on the scale. It is known to provide a reference mark adjacent to the increment feature (such that the increment feature and the reference mark are on separate tracks) or to embed the reference mark within the increment feature (such that the increment feature and the reference mark are on the same track). Patent document 1 (WO2005 / 124282) discloses that a reference mark can be embedded within an increment feature. Usually, a reference mark is configured to provide a signal pulse (either positive or negative) in a reference mark detector within the reading head. Such a signal pulse may be in the form of a light pulse (caused by either an increase or a decrease in the light reaching the reference mark detector). The encoder device can analyze this pulse in order to measure the reference position between the scale and the reading head. A reference mark signal may be output in response thereto (e.g., the encoder device can output a reference mark signal to, for example, an interface unit and / or a signal line to an external device). For example, one or more threshold values may be used to identify the presence of a reference mark. In order to make it easier to identify the presence of a reference mark, avoid detecting false reference marks (e.g., caused by dirt), and / or increase the resolution of the reference mark, it is preferred that the pulse be distinct (e.g., having a relatively large amplitude and a relatively narrow width).

[0004] As will be understood, references to "light" in this application refer to electromagnetic radiation (EMR) that falls anywhere within the electromagnetic radiation (EMR) spectrum from ultraviolet to infrared (inclusive). Similarly, the use of the term "optical", such as in "optical encoder", refers to an encoder that operates using EMR that falls anywhere within the EMR spectrum from ultraviolet to infrared (inclusive).

[0005] Patent Document 2 (WO2008 / 053184Al) discloses a reading head for a scale reading device, and FIG. 15 shows the arrangement of components within the reading head. A first LED 15 is shown for illuminating an increment track on the scale, and the light therefrom forms an increment pattern, which is then detected by an increment detector shown on either side of the LED 15. Also shown is an LED 50 for illuminating a reference mark, and the light therefrom is detected by a reference mark detector 51.

[0006] Patent Document 3 (US2006 / 0180748Al) discloses a position measurement system including an incremental rank-order track, and the scanning unit includes a light source 21, an incremental signal detector 230, and a reference mark signal detector 240.

[0007] Patent Document 4 (US7141780) discloses an encoder device, and the reference mark includes a pattern of features that correlates with a mask pattern to provide a signal of the reference mark.

[0008] Patent Document 5 (US2006 / 0267822) discloses an optical encoder including a light source 110 and a scale 120. The scale includes an optical pattern 124 that forms a light and dark pattern on a photodetector array 154, and a reference position pattern 122. The light reflected by the reference position pattern 122 is incident on a group of photodetectors 152.

[0009] Patent Document 6 (WO2015 / 049174) discloses an encoder device including at least one light source configured such that a structure exists in light projected toward a scale. The reading head is configured such that the structure is angled so as to be substantially misaligned with the features of the scale.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Patent Document 9

Summary of the Invention

Means for Solving the Problems

[0011] The present invention relates to an improved measurement encoder, particularly an improved incremental measurement encoder. The encoder includes a scale and a reading head. The scale can include a periodic series of marks forming an incremental scale track. The scale may also include at least one reference mark.

[0012] According to a first aspect of the present invention, there is provided an incremental measurement encoder comprising a scale and a reading head, the scale comprising a periodic series of features forming an incremental track and at least one reference mark comprising at least one imaging element. The reading head can comprise a structured light source (source of light). The reading head can comprise an array of photodetectors for the reference mark. The at least one imaging element may be configured to form / project an image of the structured light source onto the array of photodetectors for the reference mark. The structured light source may comprise an array of light sources spaced non-uniformly. The array of photodetectors may comprise a discontinuous series of photodetector elements arranged to correlate with the structured light source.

[0013] It has been found that the arrangement of the present invention enables a relatively distinct reference mark signal (e.g., pulse) to be generated in the array of photodetectors for the reference mark for a relatively small reference mark. This is particularly advantageous when the reference mark is embedded in the incremental feature. This is because the smaller the reference mark, the smaller the adverse effect on the incremental signal. In particular, it has been found that such an arrangement can assist in enabling repeatable measurements when the reading head is in a position relative to the scale corresponding to the reference mark. For example, such an arrangement can be useful for providing a signal of a reference mark that can be repeated within the width of an incremental scale mark and, if desired, within the measurement unit of the encoder system (e.g., output by the encoder device). This is particularly difficult to do with incremental encoders having a fine pitch (e.g., incremental encoders with a scale period of less than 20 μm (micrometers)).

[0014] As will be appreciated, imaging reverses the copy of the object being imaged spatially. Thus, as will be appreciated, the image formed by the imaging element is a spatially reversed representation / copy of the structured light source. Another optical member can be included to reverse the copy of the structured light source so that it is not spatially reversed when it finally fits onto the light detector array of the reference mark. However, this requires providing additional optical elements. Thus, preferably, the imaging element causes the spatially reversed representation / copy of the structured light source to fit onto the light detector array of the reference mark.

[0015] As will be appreciated, other terms for the imaging element of the reference mark include an imager, or imaging means / device / member. The imaging element may comprise at least one of i) a lens (e.g., a cylindrical or spherical lens, or a Fresnel lens), ii) a diffractive imaging element (such as a Fresnel zone plate (FZP)), or iii) a feature configured to provide an image by means of a pinhole effect. Such a feature (resulting in imaging by means of a pinhole effect) may be an elongated pinhole (e.g., such a feature is elongated orthogonal to the measurement direction of the scale, e.g., such a feature consists of a line on the scale). In that case, as will be explained in detail below, the reference mark / imaging element may provide only a one-dimensional image. Alternatively, such a feature (resulting in imaging by means of a pinhole effect) may consist of a reflective or transmissive feature (e.g., a line). Alternatively, the reference mark / imaging element comprises a single continuous omission of a generally periodic series of features within the incremental track. Such a feature (resulting in imaging by means of a pinhole effect) may consist of a reflective line (e.g., a line taken along the measurement direction and more reflective than the scales on both sides thereof) or, for example, an aperture.

[0016] As an option, the period of the increment feature does not exceed 20 μm (micrometers), for example, it is 10 μm or less, for example, 5 μm or less, for example, 4 μm or less. As can be understood, the period of the increment signal derived from the output of the increment photodetector (for example, the quadrature signals, for example, SIN and COS signals, or the "Lissajous" period) may be equal to the pitch of the scale, but not necessarily so. As an option, the period of the increment signal of the encoder device is 10 μm or less, for example, 5 μm or less, for example, 2 μm or less.

[0017] As an option, the width of the reference mark / image forming element (measured parallel to the measurement direction of the scale) extends to 15% or less, preferably 10% or less of the width of the increment detector array (measured parallel to the measurement dimension). As an option, the width of the reference mark / image forming element (measured parallel to the measurement direction of the scale) does not exceed 150 μm (micrometers), for example, it is 100 μm or less, for example, 75 μm or less.

[0018] As will be appreciated, the image formed by the imaging element extends in at least one dimension. Optionally, the fiducial mark / imaging element can form a one-dimensional image of the structured light source. In other words, the fiducial mark can comprise a one-dimensional imaging means / member / device / element, such as an elongated pinhole, a cylindrical lens, or a one-dimensional FZP. That is, the imaging element can form an image of the first dimension of the structured light source, but not the second dimension. The first dimension can be parallel to the measured dimension of the scale. Such one-dimensional imaging means / devices / members / elements are advantageous because, especially when the fiducial mark is embedded within the incremental feature, they can be performed more simply, less expensively, and more efficiently for the manufacture of the scale (than two-dimensional imaging means). For example, if the imaging element is configured to image via the pinhole effect, the fiducial mark / imaging element can be formed by simply overlooking (or adding) the lines that will later form the incremental feature. Also, in the case of a fiducial mark comprising an FZP, the imaging means can be formed using the same process as the incremental feature.

[0019] As will be understood, the light source will have a structure of at least one dimension. In other words, the intensity of the light emitted from the light source varies in at least one dimension. The light source is captured in at least one dimension and is separated by "dark" regions where the intensity of the light emitted towards the scale is substantially low (e.g., at least 50% less than that of the illumination region, alternatively at least 75% less, e.g., 90% or less), and may be configured to have at least two "bright" or "illuminating" regions where light is emitted towards the scale. Alternatively, no light may be emitted from the dark regions. Preferably, there are at least three bright / illuminating regions (separated by "dark" regions), and optionally at least four bright / illuminating regions (separated by "dark" regions). Alternatively, the width of the region detected parallel to the measurement direction by the light detector array of the reference mark (e.g., the width of the bright / illuminating region or the width of the dark region) is nominally the same. This can serve to provide an alternating set of light detector elements (described in detail below).

[0020] The structured light source can be described as an array of discrete or separated light sources. The array can be regular (e.g., uniform, etc.) or irregular (non-uniform, etc.). In other words, the (bright) elements in the array may be equally spaced such that for all pairs of adjacent (i.e., directly adjacent) elements in the array (e.g., bright elements), the spacing between the elements is the same. Alternatively, the (bright) elements in the array may be non-uniformly arranged such that the spacing between a pair of adjacent (i.e., directly adjacent) (bright) elements is different from the spacing between another pair of adjacent (i.e., directly adjacent) (bright) elements.

[0021] Thus, as can be understood, it can be said that the structured light source projects a pattern (which does not necessarily have to be a repeating pattern) or a code towards the scale. In other words, the structured light source can be said to be a patterned light source or a coded light source (here, as an option, the pattern or the code extends in at least one dimension, and as an option, extends only in one dimension). As an option, the said (at least) one dimension is parallel to the measurement direction of the scale.

[0022] When the structured light source is described as a coded light source, the value of the bits of the code projected by the coded light source can be measured by the intensity of the emitted light. For example, if the light emitted by the structured light source is greater than a predetermined threshold, this can be measured as "1", and if the light emitted by the coded light source is less than a predetermined threshold, this can be measured as "0". As an option, in each set of light detector elements (described in more detail below), at least one corresponding light detector element is provided for each "1" bit or each "0" bit of the code. That is, for each "1" bit (or each "0" bit), there is at least one corresponding light detector element configured (e.g., arranged) to detect that bit. As an option, the code projected by the coded light does not include adjacent "l"s. This can serve to provide an alternating arrangement of sets of light detector elements (described in more detail below).

[0023] The structured light source can comprise a plurality of spatially separated light-emitting elements. For example, the plurality of spatially separated light-emitting elements can consist of a plurality of spatially separated light-emitting diodes (LEDs).

[0024] As described in Patent Document 7 (W02008 / 053184), the light source and the detector element of the reading head (e.g., the photodetector array of the reference mark) may be formed on the same optoelectronic chip / semiconductor compound.

[0025] As an option, the structured light source may be provided by a mask covering a single (extended) light source (e.g., a mask covering a single LED). Thus, each of the above "bright" or "illuminated" regions may be provided by a single light emitting element such as an LED or by a transmission region (e.g., an opening in the mask) within the mask. As an option, the light from the structured light source is not collimated.

[0026] The light detector array for the fiducial marks comprises a set of at least one light detector element for detecting an image provided by fiducial marks arranged to match / be related to the structure (i.e., pattern / code) emitted by the structured light source. As will be appreciated, the light detector array for the fiducial marks can comprise a plurality of sets of such light detector elements (e.g., at least first and second sets of light detector elements each for separately detecting an image provided by the fiducial marks). As will be appreciated, such sets can be arranged such that when the reading head and the fiducial marks pass each other, the image of the structured light source passes sequentially across each set of light detector elements (i.e., not all sets detect the image simultaneously). Thus, the sets can be offset relative to each other in a dimension at least parallel to the measurement direction of the scale. As an option, the sets of light detector elements are arranged alternately.

[0027] The light detector array for the fiducial marks can comprise a continuous series of light detector elements (in other words, a regular array of light detector elements), with selected elements within the continuous series (regular array) being used to form sets (i.e., at least one set of the light detector elements described above). For example, only some of the elements can be used to contribute to the signal for the detection of the fiducial marks.

[0028] As an option, the fiducial mark photodetector array comprises a discontinuous series of photodetector elements (e.g., an array of irregular / interrupted / non-uniform photodetector elements). For example, the elements in the set can be spaced such that when the reading head passes over the fiducial mark, the detector elements are positioned only where it is expected that at least one image of the structured light source's relatively bright (or relatively dark) portion will fall in order to provide a positive (or negative) pulse. Thus, in embodiments where the fiducial mark provides only a single image of the structured light source, the fiducial mark's photodetector array can comprise a set of photodetector elements spaced from each other in an arrangement that correlates (e.g., negatively or positively matches) with the structured light pattern when imaged by the fiducial mark.

[0029] Thus, in any case (i.e., for a continuous or discontinuous series of photodetector elements), the fiducial mark's photodetector array can comprise at least one set of photodetector elements, the elements being spaced from each other within an arrangement (e.g., pattern) that correlates / corresponds to the arrangement (e.g., pattern) of the structured light source when imaged by the fiducial mark. In other words, the detector can comprise an array of elements for detecting the structure of the structured light source that is arranged / located in a configuration (formation) that relates / corresponds to the structure of the structured light source. As will be understood, correlating / corresponding includes matching arrangements that are spatially inverted (or negative versions) of the structured light source (e.g., front-to-back and / or top-to-bottom).

[0030] The structure / pattern of the photodetector elements for detecting the reference mark may be the same as the structure of the structured light source. The structures / patterns of the plurality of photodetector elements for detecting the reference mark may be the same, however, it may be the reverse of the pattern of the structured light source (because, as described above, the image formed by the reference mark is a reverse representation of the structured light source, and this reverse representation may fit into the photodetector array of the reference mark).

[0031] Therefore, the encoder device may be configured such that when the reading head passes through the field of view of the reference mark, at least one image of the structured light source quickly passes through the photodetector array of the reference mark. This may result in a high correlation between at least one image and the plurality of photodetector elements for detecting the reference mark at a specific relative position (e.g., the reference position) of the scale and the reading head, resulting in a signal peak. As will be understood, at least one image of the structured light source may sweep across the photodetector array of the reference mark in a direction opposite to the direction of relative movement of the reading head and the scale.

[0032] As will be appreciated, the reading head can comprise at least one incremental light detector, such as an incremental light detector array. Light from a light source can interact with an incremental scale track to form a field of results detected by the incremental light detector. The resulting field can consist of interference fringes. Alternatively, the reading head can comprise at least one grating (e.g., a diffraction grating) used to form the field of results (e.g., an interference fringe / fringe field). The at least one (e.g., diffraction) grating can be positioned in front of the scale (e.g., to interact (e.g., diffract light) with light from a structured light source before reaching the scale). The at least one (e.g., diffraction) grating can be positioned behind the scale (e.g., to interact (e.g., diffract light) with light reflected / transmitted by the scale). Alternatively, the reading head can comprise at least one (e.g., diffraction) grating in front of the scale and at least one (e.g., diffraction) grating behind the scale. Gratings other than diffraction gratings can be provided in the reading head. For example, a sampling grating can be provided covering the detector, e.g., so as to cover an incremental light detector array.

[0033] Alternatively, the encoder can be configured such that at least a portion of the structured light source provides light that interacts with the incremental scale track to form a field of results. For example, if the structured light source comprises a plurality of individual light sources, then light from, preferably, at least one, and optionally all, of said individual light sources can be configured to form the resulting light field.

[0034] Accordingly, as an option, the structured light source may be configured to illuminate the ink increments and the features of the reference marks on the scale. As an option, the structured light source is configured to generate both the ink increments and the signals of the reference marks detected by the reading head. For example, the structured light source may be configured to illuminate a photodetector of the reference mark and a photodetector for detecting an incremental change in the position of the reading head relative to the scale, to generate a resulting field (e.g., a fringe field).

[0035] By using the same structured light source in this way, a highly efficient measurement encoder becomes possible. Using the same structured light source can reduce the complexity of the encoder and simplify manufacturing, especially the manufacturing of the reading head. Furthermore, by using the same light source to illuminate both the ink increment and the photodetector array of the reference mark, there is no separate light source for illuminating the photodetector array of the reference mark, so the amount of background illumination of the ink photodetector array can be reduced. This can mean a more favorable signal-to-noise (S / N) ratio that enables a higher discrimination of the signal from the signal amplifier in the signal supplied to the signal amplifier.

[0036] A grating (e.g., a diffraction grating) may be provided covering (e.g., directly attached to) the structured light source to diffract the light from the structured light source before it reaches the scale. Thus, for example, the (diffraction) grating may be provided in at least one illumination area of the structured light source, or as an option, in all illumination areas of the structured light source. In such an arrangement, the grating causes the light from the structured light source to form a resulting field (e.g., interference fringes) in the incremental photodetector for detecting the relative movement of the reading head and the scale. As an option, the (diffraction) grating is an amplitude grating.

[0037] The reference mark can comprise a plurality of imaging elements. As an option, the reference mark can comprise two imaging elements that image a structured light source and are provided close to each other on the scale (e.g., measured parallel to the dimension being measured and at a distance of less than 50% of the width of the structured light source). As will be appreciated, in this case, at the reference position, the pattern / code detected by the reading head is not the same as in the case of the structured light source. Rather, the pattern / code detected by the reading head at the reference position is some combination of two or more images formed by each imaging element.

[0038] As an option, the detector elements of the detector array of the reference mark are sized and / or spaced such that they are insensitive to the phase of the fringe field. This can be achieved by the detector elements of the detector array of the reference mark having a lateral extent that is an integer multiple of the pitch of the fringe field in the detector array of the reference mark in the measurement direction, and / or the centers of the detector elements of the detector array of the reference mark being spaced an integer multiple of the pitch of the fringe field in the detector array of the reference mark in the measurement direction. Such an arrangement allows the detector array of the reference mark not to be affected by a particular phase alignment of the fringe field illumination pattern generated by the measurement encoder device.

[0039] As will be understood, the encoder device can be configured to derive at least one signal used to measure the presence of at least one reference mark from the output of an array of reference mark photodetectors. The at least one signal can comprise a main lobe and side lobes that occur when the reading head passes over the reference mark. The encoder device may be configured such that the size of the side lobe does not exceed 75% of the size of the main lobe, alternatively does not exceed 50% of the size of the main lobe, for example does not exceed 35% of the size of the main lobe. Alternatively, the at least one signal (used to measure the presence of at least one reference mark) is derived from at least two sets of photodetector elements. For example, the encoder device can be configured to derive at least one "gate" / "window" signal and / or "zero-point crossing" signal from at least two sets of photodetector elements.

[0040] The at least one signal (e.g., the zero-crossing signal of the reference mark) used to measure the presence of at least one reference mark can be measured based on the outputs from at least two elements of the photodetector array of the reference mark (e.g., from the outputs of at least two sets of the photodetector elements of the reference mark). Alternatively, the at least one signal (e.g., the zero-crossing signal of the reference mark) used to measure the presence of at least one reference mark can be measured based on the outputs from four elements of the photodetector array of the reference mark (e.g., from the outputs of four sets of the photodetector elements of the reference mark). The at least one signal (used to measure the presence of at least one reference mark) (e.g., the zero-crossing signal of the reference mark) can be measured based on the outputs from four or more elements of the photodetector array of the reference mark (e.g., from the outputs of four or more sets of the photodetector elements of the reference mark). In use, based on the outputs of at least two, three, four, or more elements of the photodetector array of the reference mark (e.g., from the outputs of at least two, three, four, or more sets of the photodetector elements of the reference mark), the measurement encoder can monitor the zero-crossing of the reference mark.

[0041] Alternatively, the reference mark is embedded within the incremental track. This enables a more compact scale and may improve efficiency.

[0042] The scale may be a phase scale. The scale may also be an amplitude scale.

[0043] Also disclosed is a measurement encoder comprising a scale and a reading head, the scale comprising a periodic series of marks forming a reference mark and an increment track, and the reading head comprising an increment photodetector array and a photodetector array for the reference mark. Thus, the reading head further comprises a structured light source, and the reference mark photodetector array comprises elements arranged to correspond to the code of the structured light source. As a result, during use, and also when the reading head moves past a predetermined position relative to the scale corresponding to the reference mark, a fringe field is formed to monitor the incremental position such that a change occurs in the illumination of the reference mark photodetector array.

[0044] According to another aspect of the present invention, an incremental encoder comprising a scale and a reading head is provided, the scale comprising a periodic series of features defining an incremental scale and at least one reference mark, the reading head comprising a light source for illuminating the scale and a detector for detecting the reference mark, the light source being configured such that it has an array / arrangement of relatively (alternately) light and dark regions in at least one dimension (e.g., such that there are at least two separated light regions), and the reference mark being configured to project / replicate the structure of the light source towards the reference mark detector (in particular, project a spatially inverted version of the light source towards the reference mark detector) when the reading head and the scale are aligned at the reference mark position, and the detector for detecting the reference mark comprising an array of elements for detecting the structure such that an intensity pulse is provided to the array of elements when the reading head and the reference mark are aligned at the reference mark position (in particular, the detector comprises an array of elements for detecting a structure arranged / positioned in a configuration corresponding to the structure of the structured light source).

[0045] As will be appreciated, the features described above in connection with other aspects of the present invention are equally applicable to this aspect of the present invention, and vice versa.

[0046] As will be appreciated, such light and dark regions can be provided, for example, by an arrangement of light-emitting elements (e.g., LEDs) spaced apart from each other in at least one dimension. Such light and dark regions can be provided by, for example, a mask between at least one light-emitting element and a scale. For example, the mask can have an arrangement of bright and dark portions, where the "bright" portions allow a relatively greater amount of light to propagate towards the scale than the "dark" portions. For example, the "dark" portions may substantially block light from propagating towards the scale.

[0047] The light region can comprise a region where light is emitted towards the scale, and the dark region consists of the absence of light-emitting elements.

[0048] Here, the grating is provided in one or more light regions (and, for example, all light regions).

[0049] According to a further aspect of the invention, an incremental measurement encoder is provided comprising a scale and a reading head, the scale comprising a periodic series of features forming an incremental track and at least one reference mark, the reading head comprising a light source and an array of light detectors for the reference mark, where the reading head can detect when the reading head is in the position of the reference mark within the unit of the resolution of the incremental measurement encoder. Alternatively, the detection when the reading head is in the position of the reference mark relative to the scale consists of forming at least one image of the light source on the light detector of the reference mark. The light source may be a structured light source.

[0050] Embodiments of the invention will be described in more detail herein, by way of example only, and with reference to the following drawings.

Brief Description of the Drawings

[0051]

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[0052] Figure 1 shows a first embodiment of a measurement encoder 1 according to the invention comprising a scale 2 and a readhead 3. In use, the scale 2 and the readhead 3 are fixed to relatively moving parts of a machine or machines and the measurement encoder 1 enables the relative position of the two parts to be measured.

[0053] In the embodiment shown in Figure 1, the measurement encoder 1 is a linear measurement encoder for measuring the position of a readhead 3 relative to a scale 2 in a first direction x as indicated in the figure.

[0054] The scale 2 is an elongated scale having, thereon, a series of periodic marks 200 that define an increment track. In this embodiment, the periodic marks form a phase grating on the scale that, when irradiated, can cause a pattern of a fringe field to be formed by the light reflected therefrom. Also, a reference mark 202 that forms part of the scale is embedded in the increment track in this embodiment. In this example, the increment track consists of a pitch scale of 4 μm (micrometers), and the reference mark 202 extends a distance of 100 μm in the measurement direction (x).

[0055] FIG. 1(b) shows a cross-section passing through a plane orthogonal to the measurement direction x of the measurement encoder of FIG. 1(a).

[0056] FIG. 2 shows the arrangement of the light emitting and light receiving components of the reading head 3 of the embodiment shown in FIG. 1. The components of the reading head 3 face the scale 2. In particular, the reading head 3 includes a structured light source 300 that, in this embodiment, is composed of three individual light sources 302 (i.e., “light” or “illumination” regions) separated by a space 304 (i.e., a “dark” region). As will be described in detail below, due to the non-uniform spacing of the individual light sources 302, the structured light source 300 shown in FIG. 2 can be said to correspond to the code 101001 when read from left to right. In this embodiment, the individual light sources 302 are provided with gratings (diffraction gratings in this particular embodiment). In particular, each of the individual light sources 302 has a grating formed from individual elements 306 formed directly on the individual light sources 302. The grating causes the structured light source to form an interference fringe field.

[0057] FIG. 2 also shows the fiducial mark photodetector array 310. The fiducial mark photodetector array comprises three portions corresponding to the three individual light sources 302 of the structured light source 300. The portions of the fiducial mark photodetector array 310 are arranged in the reading head 3 as a mirror image (i.e., spatially inverted) of the configuration of the individual light sources 302 of the structured light source. As will be described in more detail, it can be said that the portions of the fiducial mark photodetector array and the spaces between them form the code 100101 (the spatial inversion of the above-described code of the structured light source). The embodiment of FIG. 2 shows a fiducial mark photodetector array 310 comprising three portions / photodetectors 3l0a, 3l0b, 3l0c.

[0058] The incremental optical detector array 320 is also shown as part of the reading head 3. An example of a suitable incremental optical detector array is described in Patent Document 2 (WO2008 / 053184A1). The incremental optical detector 320 detects a change in the intensity of a fringe field (e.g., interference fringes) that fits thereon. In this embodiment, the fringe field of light that fits on the incremental optical detector array 320 is created by the wavefront of the light emitted from the structured light source 300 that interacts with the grating formed by the element 306 and then interacts with a second grating, in this case, the incremental track composed of the periodic marks 200. This forms a fringe field (e.g., interference fringes) on the incremental optical detector array. The relative movement of the scale and the reading head causes a change (e.g., movement) of the fringe field across the incremental optical detector, and such a change / movement is detectable by the reading head and is used, for example, to increment / decrement the count of the relative position. As will be understood, it is possible to use regions of results other than the fringe field to measure the increment. For example, the light from the light source can be used to generate a spot (e.g., a change in intensity) that is modulated (e.g., via the scale and, optionally, one or more diffraction gratings in the reading head) by the relative movement of the scale and the reading head. Such an encoder is described, for example, in Patent Document 6 (US6198534).

[0059] As will be appreciated, the signals from the incremental photodetector array can be used to generate one or more incremental measurement signals (e.g., quadrature signals, e.g., SIN and COS signals). Such incremental measurement signals enable an incremental count of the relative position of the scale to be measured and the read head. Such incremental measurement (e.g., quadrature) signals may be interpolated to provide a high resolution of the count / measurement. As will be appreciated, the incremental encoder device will have units of a predetermined / specified resolution. Preferably, the pulses of the reference mark detected by the photodetector array of the reference mark (e.g., by a set of photodetector elements) enable the position of the reference mark to be repeatedly measured within the unit of the resolution of the incremental measurement, in other words, enable the reference position signal measured / output by the encoder device to be repeatable within the unit of the resolution of the incremental measurement. As will be appreciated, this may mean that the reference position signal measured / output by the encoder device is provided at the same position within the unit of the resolution of the incremental measurement, regardless of the relative direction of movement.

[0060] For example, the output from the optical detector array of the reference mark can be used to measure a "gate" or "window" signal while a reference position signal is being measured / output by the encoder device when a predetermined event occurs within the cycle of the incremental signal (e.g., when the quadrature signals are negative and equal). Thus, in order to avoid ambiguity, it is important that the "gate" or "window" signal does not become wider than one cycle of the incremental signal (e.g., one period of the quadrature / SIN signal). The narrower the reference mark pulse detected by the read head (e.g., the narrower the "pulse" of light from the detector), the narrower the "gate" or "window" signal that can be measured from the signal of the optical detector of the reference mark. The use of "gate" or "window" signals to measure the presence of a reference mark is well known and is described in detail, for example, in international applications published under the Patent Cooperation Treaty, Patent Document 8 (WO2017 / 203210) and Patent Document 9 (W02007 / 057645), and will be further described in more detail below. Our inventors have found that providing a structured light source imaged on the optical detector array enables a narrower reference mark pulse to fit within the detector, which in turn can ensure the repeatability of the reference mark signal provided by the encoder and facilitate fitting within the unit resolution of the incremental encoder.

[0061] The embodiments shown in FIGS. 1 and 2 are understood to be reflective measurement encoders in which a light source (here, a structured light source 300) and an optical detector array (here, an optical detector array 310 for the reference mark and an incremental optical detector array 320) are positioned on the same side of the scale 2 during use.

[0062] FIG. 3 shows a structured light source 300 used in the embodiment of FIG. 1. FIG. 3 shows a structured light source 300 having the code 101001 used in the embodiment shown in FIG. 2. Here, each individual light source 302 of a given width represents "1", while a space of a predetermined width represents "0". Here, it can be seen that the first space 304a (when read from left to right) has the same width as the first individual light source 302a, while the second space 304b has a width twice that of the first space 304a. In this embodiment, the space 304b represents "00". Examples of other possible codes for the structured light source are the code 100000100010001000001 in a state where each individual light source represents "1", while the space represents "0" and the width of the space compared to the individual light source measures the number of "0"s. 110010000010100001 is another example of a code that may be used. In this structure, it will be understood that the leftmost of the individual light sources doubles the width of the other individual light sources. This is because here, one individual light source (whose width is twice the given width) represents "11" within the code. Such an individual light source can be a single continuous light source or composed of separate adjacent light sources adjacent to each other. The code 111001100011011011 can also be used in an embodiment. In other embodiments, the structured light source may be a Barker code. As described above in connection with the embodiment shown in FIG. 2 having individual light sources with the code shown in FIG. 3, the reference mark photodetector array 310 is configured to correspond to the structured light source 300 and may be a mirror image of the structured light source 300 (e.g., a spatially inverted version).

[0063] The code of the photodetector array 310 of the reference mark arranged as a mirror image of the code of the structured light source 300 described above is 100101 for 101001 (shown in FIG. 3), 10000010001000100010000100001 for the code 100000010001000010000010000001, 100001000001001 for the code 1100100000101000001, and 110110110001100111 for the code 111001100011011011.

[0064] FIGS. 1 and 2 illustrate a reflective embodiment, but other arrangements are possible. For example, the photodetector array of the reference mark may be located on the opposite side of scale 2 with respect to the structured light source 300, and it will be understood that such a system may be referred to as a transmissive system.

[0065] FIG. 4(a) schematically shows the optical path from the structured light source 300 to the photodetector array 310 of the reference mark for a transmissive system. It will be understood that the basic principle is the same for the reflective embodiments of FIGS. 1 and 2 and the transmissive embodiment of FIG. 4(a), except for the reflection / transmission of light through scale 2 in the reference mark 202. For ease of explanation, the transmissive embodiment of FIG. 4(a) will now be described.

[0066] In FIG. 4(a), the reading head 3 is shown at a relative position with respect to the scale 2 corresponding to the position of the reference mark. The structured light source 300 is shown on one side of the reference mark 202, and the reference mark photodetector 310 is shown on the opposite side of the reference mark 202. Both the structured light source 300 and the reference mark photodetector array 310 are such that the reference mark photodetector array 310 is positioned on the surface of the reading head 3 and forms part of the reading head 3. The structured light source 300 and the reference mark photodetector array 310 maintain a fixed relationship with each other and are moved in the moving direction x with respect to the scale 2. As the reading head 3 moves relative to the scale 2, the reading head also moves with respect to the reference mark 202. In this embodiment, the reference mark 202 on the scale 2 allows light to pass through and functions as an imaging element. In particular, the reference mark has the feature of imaging the structured light source 300 onto the photodetector array 310 by the pinhole effect. The transmitted light forms an image of the structured light source 300 on a plane including the surface where the reference mark photodetector array 310 is located. When the reading head 3 is at a position corresponding to the reference mark 202 with respect to the scale 2, the formed image is a spatially inverted representation of the code of the structured light source 300. Since the reference mark photodetector array 310 is configured as a spatially inverted representation of the structured light source in this embodiment, when the reading head 3 is at a position corresponding to the reference mark 202 with respect to the reference mark 2, the reference mark photodetector array 310 is maximally illuminated by the structured light source 300. The reference mark photodetector array 310 is configured to output a signal based on the amount of light received thereon.

[0067] As will be appreciated, in other embodiments, the imaging element of the fiducial mark may consist of a lens (e.g., a cylindrical lens) and / or a diffractive optical element (such as a Fresnel zone plate (FZP)). Also, as will be appreciated, the term "imaging" may mean the convolution of the optical impulse response of the fiducial mark, or the structure / pattern of light emitted by a structured light source having a point spread function. Similarly, the term "image" may mean the pattern of light formed in the fiducial mark detector by this process.

[0068] Figure 4(b) shows how the output of the fiducial mark's photodetector array 310 varies in accordance with the reading head 3 and the scale 2 that are moved relative to each other in the measurement direction x. As can be seen in Figure 4(b), when the reading head 3 is at the position of the fiducial mark and its vicinity with respect to the scale 2, a pulse 500 is formed in the output of the fiducial mark's photodetector array 310. By adjusting the fiducial mark 202, the relative ratio of the pulse 500 can be changed. For example, widening the fiducial mark allows more light to pass through the scale in a transmissive system, thus increasing the peak intensity. Conversely, narrowing the fiducial mark 202 allows less light to pass through the scale, thus weakening the intensity of the pulse 500. However, widening the fiducial mark 202 also causes the pulse 500 to become wider and also more disrupts the incremental signal. Other factors such as the vertical distance from the scale to the structured light source 300 (i.e., along the z-axis shown in FIG. 1) can also affect the intensity of the light reaching the fiducial mark's photodetector array 310. Advantageously, the intensity / amplitude of the pulse 500 can be increased without increasing the width of the pulse 500 by increasing the number of "bright" regions of the structured light source, thereby helping to provide a clearer pulse. Thus, using a structured light source imaged by the fiducial mark is particularly useful when trying to provide a small fiducial mark while obtaining a clear pulse at the detector.

[0069] In the described transmissive arrangement, the light wavefront emitted from the structured light source 300 directly interacts with a grating having elements 306 on the structured light source 300 and then interacts with the scale 2 (in relation to the incremental track formed from the periodic marks 200 as described above). As the light passes through the fiducial mark 202, a fringe field formed due to the elements 306 is formed in the portions 3l0a, 3l0b, 3l0c of the fiducial mark's photodetector array 310.

[0070] In some embodiments, portions 3l0a, 3l0b, 3l0c may include multiple sections / elements. For example, each of portions 3l0a, 3l0b, 3l0c may be formed from four sections / elements J, K, L, M. In such embodiments, the fiducial mark photodetector array 310 comprises three square cells 3l0a, 3l0b, 3l0c. For example, it can be seen that each of portions 3l0a, 3l0b, and 3l0c is composed of four sections J, K, L, M. As will be understood, the same sections of the three portions form a set of photodetector elements, which are spaced in a manner that coincides / correlates with the structured light source. For example, all J sections are spaced in a manner that coincides / correlates with the structured light source, and the same is true for all K sections, L sections, and M sections. The J sections form a first set of photodetector elements, the K sections form a second set, the L sections form a third set, and the M sections form a fourth set. The outputs from these sets may be combined to assist in measuring the position of the fiducial mark, as will be described in detail below in connection with FIG. 6.

[0071] FIG. 5(a) shows a first way in which a portion 3l0a of the fiducial mark photodetector array 310 can be configured to have a square cell with four sections J, K, L, M and be insensitive to the fringe field formed thereon. FIG. 5(a) shows the intensity of the fringe field 600 formed on the fiducial mark photodetector. As can be seen, each section J, K, L, M of the illustrated square cell has a width that is an integer multiple of the period of the fringe field. The figure also shows that each finger J, K, L, M has the same width.

[0072] Figure 5(b) shows a first method that can be configured such that the portion 3l0a of the photodetector array 310 of the reference mark comprises a square cell having four sections J, K, L, M and is insensitive to the fringe field formed thereon. Figure 5(b) shows the intensity of the fringe field 600 formed on the photodetector of the reference mark. As can be seen, each section J, K, L, M of the square cell has the same width, and the distance between the centers of adjacent fingers J, K, L, M is an integer multiple of the fringe field.

[0073] Depending on the size of the structured source element and / or the photodetector element of the reference mark, the detectors of JKLM may be arranged partially or fully interleaved (for example, because the gap between the sources is large enough to place the four detectors required for each element where the signal is detected). If the gap between the source elements is not large enough to accommodate all four elements where the signal is detected (and / or the desired size of the photodetector element of the reference mark is too large), the detectors may be arranged in two or more adjacent tracks as needed. Fully interleaved detectors are advantageous because they are an efficient use of the expensive semiconductor area. Non-interleaved detectors in adjacent tracks are advantageous in that they allow for a larger detector area that can provide a larger and lower noise signal, at the expense of providing more degrees of freedom to select the desired code pattern and requiring more semiconductor area. Figures 5(c) (and also Figures 7(b) and 7(c)) show schematic embodiments where the detectors are non-interleaved, and Figures 5(d) (and also Figure 7(a)) show schematic embodiments where the detectors are interleaved.

[0074] FIG. 6(a) shows signals from each section J, K, L, M of the square cells 310a of the fiducial mark photodetector array 310 when the reading head 3 moves relative to the scale 2 along the measurement direction x. Signals from each section of the square cells are processed to identify when the reading head 3 is at a position corresponding to the fiducial mark 202 relative to the scale 2. In particular, so-called "gate" and "zero-point crossing" signals (also known as "sum" and "diff" signals), which are already known in the field of encoders, are derived and these are analyzed to measure the presence of the fiducial mark. FIG. 6(b) shows signals from sections J, K, L, M of the square cells of FIG. 6(a) combined according to equation (1) to provide a "gate" or "sum" signal. (K + L) - (J + M) (1)

[0075] FIG. 6(b) also shows a threshold 700. The output from equation (1) is used as a gate signal enabling a zero-crossing detector. When the gate signal exceeds the threshold 700, here between the points x1 and x2, the zero-crossing detector is activated. The zero-crossing detector processes the signals of the sections of the square cells according to equation (2a) to obtain the "zero-point crossing" signal which is the output shown in FIG. 6(c). K - L (2a)

[0076] In other embodiments, the zero-crossing detector could process the signals of the square cells according to equation (2b). (J + K) - (L + M) (2b)

[0077] FIG. 6(c) also shows the positions x1 and x2 where the zero-crossing detector is activated in between. In one embodiment, the encoder device may be configured such that when the output of the zero-crossing detector is 0 (i.e., crosses “0”), the reading head 3 is at a position corresponding to the reference mark 202 relative to the scale 2, and thus the signal of the reference mark is output and measured by the reading head. However, alternative embodiments are also possible. For example, an additional pair of thresholds (e.g., t1, t2) may be used to analyze the zero-crossing (i.e., “diff”) signal such that a window signal W is generated when the zero-crossing signal is between these thresholds, which is then used to analyze the incremental SIN and COS signals (the W, SIN, and COS signals are schematically shown superimposed in FIG. 6(d)). In particular, this window signal W may be used such that within such a window signal W, when the SIN and / or COS signals are determined to meet certain predetermined conditions (e.g., when the SIN and COS signals are equal and negative), the signal of the reference mark is output by the encoder device. As will be understood, such a window signal W needs to be narrow enough such that such a predetermined condition of the SIN and COS signals occurs only once within the window signal W, thereby ensuring that the reference mark is repeatable within one unit of resolution of the encoder device.

[0078] FIG. 9(a) is similar to FIG. 6(a) in that it shows the signals from each of the sections J, K, L, M of the four-corner cells 3l0a of the optical detector array 310 of the reference mark when the reading head 3 moves along the measurement direction x relative to the scale 2. However, in reality, it shows that there is a high probability that the signal has not only the main lobe but also side lobes. FIG. 9(b) also shows how these side lobes are present in the so-called gate (or, "sum") and zero-crossing (or, "diff") signals (e.g., signals derived from the outputs of a set of optical detector elements of the reference mark). Such side lobes can have an adverse effect on the processing of these signals, especially if their size is too large. For a given optical system, the "code" or pattern of the structured light source can affect the size of the side lobes. Therefore, by selecting an appropriate code, it is possible to configure the encoder device such that in the signals derived from the outputs of a set of optical detector elements of the reference mark (e.g., the so-called "gate" and / or "zero-crossing" signals), the size of the most significant side lobe does not exceed 75% of the size of the main lobe. In the embodiment simulated in FIG. 9 (when the light source has the code 110010000010000100001), the size of the most significant side lobe is less than 30% of the size of the main lobe.

[0079] FIG. 7(a) shows the geometry of the alternative components shown in FIG. 2. In this embodiment, each part of the optical detector array 310 of the reference mark is shown with four sections of square cells. In FIG. 7(a), the structured light source represents the code 10000001000100001000001. This code has the advantage that it allows an alternating arrangement of a set of detector elements of the reference mark that increases the detector area for a given chip size. As will be understood, other codes may be provided that allow the set of detector elements of the reference mark to be arranged alternately.

[0080] FIG. 7(b) shows another alternative geometry of the components of the reading head 3. In this embodiment, the structured light source has the code 11001000001010001. As can be seen in the figure, this code is not implemented with photodetectors of reference marks arranged alternately. Instead, the photodetector array of the reference marks has sections J, K, L, M that overlap in the measurement direction x.

[0081] FIG. 7(c) shows the arrangement of the components of the code 111001100011011011.

[0082] In an alternative embodiment, each part 3l0a, 3l0b, 3l0c of the photodetector array 310 of the reference marks may be composed of a dual cell that can have only two sections, namely, a first section K and a second section L. In such an embodiment, the gate signal is obtained by combining the outputs of sections K and L and applying a threshold as shown in Equation (3), while the zero crossing may be detected by combining the outputs of sections K and L as shown in Equation (4) and monitoring when Equation (4) has a value of 0. K + L (3) K - L (4)

[0083] In a further embodiment, each section of the part of the photodetector 310 of the reference marks is spaced such that it is equal to an integer multiple of the period of the fringe pattern formed thereon and their centers are spaced by an integer multiple of the fringe field formed thereon.

[0084] Although the present invention has been described above in connection with at least one embodiment, it will be understood that many other embodiments are possible without departing from the scope of the claims. For example, the increment track of scale 2 may comprise an amplitude (i.e., "Ronchi") scale instead of the phase scale described above. Further, FIGS. 7(a)-(c) show the components of the reading head 3 in various configurations, and it will be understood that the position of the components in a direction orthogonal to the measurement direction x can be varied. The above embodiments describe a linear measurement encoder, but the encoder can be a rotary measurement encoder. In other embodiments, the structured light source can be provided by a mask positioned over the light source, and the light source can be an extended light source. As will be understood, the grating may be provided on another element spaced from the structured light source rather than being formed directly on the structured light source. Alternatively, a grating following the scale may be provided to interact with the light from the scale to form a resulting field (e.g., a fringe field, e.g., interference fringes at an incremental photodetector). Further, another light source may be provided to illuminate the incremental scale track and form the resulting field at the incremental photodetector. However, our inventors have found that it is a particularly advantageous arrangement to use the same light source and a structured light source that illuminates the incremental scale track and forms the resulting field at the incremental photodetector. In particular, it can reduce the number of components required and the size of the reading head. Also, it reduces the background illumination level, reduces signal noise, and reduces the dynamic range of the amplifier required for signal processing.

[0085] In yet other embodiments, the fiducial mark photodetector array may be configured to detect a decrease in brightness when the reading head is at the position of the fiducial mark relative to the scale. An example of such an incremental measurement encoder may include a structured light source having code 101001 (as shown in FIG. 3), but the fiducial mark photodetector array may have a portion corresponding to code 011010 (where "1" is used to indicate the presence of a photodetector and "0" is used to indicate the absence of a photodetector), i.e., a portion arranged to correspond to the area of the structured light source image where a decrease in illumination from the background level of illumination to a lower level is expected when the reading head is at the position of the fiducial mark with respect to the scale. As will be appreciated, in this case, the graph of FIG. 6(a) will look the same, but will be inverted such that a negative pulse, i.e., a "dip", occurs in the signal from each set of photodetector elements as the reading head passes over the fiducial mark.

[0086] So far, all examples have used a single mark (a single imaging element) on the scale to form an image of the coded source. However, it is possible for the fiducial mark to comprise a plurality of imaging elements. Each of the imaging elements forms an image of the coded source, but each image is shifted laterally by an amount measured by the position of each mark on the scale. This produces a much more complex pattern for the fiducial mark detector. This embodiment is schematically illustrated in FIG. 8.

Claims

1. An incremental measurement encoder comprising a scale and a reading head, wherein the scale comprises a periodic series of features forming an incremental track and at least one reference mark, the reading head comprises a structured light source and an array of light detectors for the reference mark, the at least one reference mark comprises at least one imaging element configured to form an image of the structured light source on the array of light detectors for the reference mark, the structured light source comprises an array of non-uniformly spaced light sources, and the non-uniformly spaced light sources are spatially separated light emitting diodes (LEDs), the reference mark comprises features configured to image the structured light source by means of a pinhole effect, characterized incremental measurement encoder.

2. The incremental measurement encoder according to claim 1, characterized in that the image formed comprises a one-dimensional image.

3. The light from the structured light source is also used in a light detector to generate a result field so as to detect an incremental change in the position of the reading head relative to the scale, characterized in that the incremental measurement encoder according to claim 1 or 2.

4. The incremental measurement encoder according to any one of claims 1 to 3, characterized in that the grating is directly attached to the structured light source.

5. The array of light detectors for the reference mark comprises at least a first and a second set of light detector elements, each set comprising a plurality of spaced light detector elements, and each set is configured to individually detect the image provided by the at least one imaging element at a laterally offset position, characterized in that the incremental measurement encoder according to any one of claims 1 to 4.

6. The incremental measurement encoder according to claim 5, characterized in that the at least first and second sets of light detector elements are arranged alternately.

7. The incremental measurement encoder according to claim 6, characterized in that, in use, the signal of the zero point crossing of the reference mark is measured based on the outputs from the at least first and second sets of light detector elements.

8. In use, based on the outputs of the set of at least the first and second light detector elements, the measurement encoder monitors a zero crossing of the reference mark, the incremental measurement encoder according to claim 7.

9. The elements of the light detector array of the reference mark are generated by an incremental scale track and are sized and / or spaced so as to be insensitive to the phase of the fringe field that fits within the light detector array of the reference mark, the incremental measurement encoder according to any one of claims 1 to 8.

10. The reference mark is embedded within the incremental track, the incremental measurement encoder according to any one of claims 1 to 9.

11. At least one signal used to measure the presence of the at least one reference mark is derived from the output of the light detector array of the reference mark, the signal comprising a main lobe and side lobes that occur when the reading head passes over the reference mark, and the encoder device is configured such that the size of the side lobe does not exceed 75% of the size of the main lobe, the incremental measurement encoder according to any one of claims 1 to 10.

12. The signal used to measure the presence of the at least one reference mark is derived from at least two sets of light detector elements, the incremental measurement encoder according to claim 11.

13. Configured to provide a reference position signal that is repeatable within the unit of resolution of the incremental measurement encoder, the incremental measurement encoder according to any one of claims 1 to 12.

Images