Absolute position measurement scale and apparatus therefor

Abstract

An absolute position measurement scale for an optical encoder apparatus, comprising a scale track extending along a measurement dimension, the scale track comprises alternating dark and bright scale features arranged such that there is within at least a section of the scale track, embedded within one another: i) a first series of dark scale features arranged according to a first lattice having a period p; and ii) a second series of dark scale features which are irregularly spaced along the measurement direction but are arranged according to a second lattice having a period P, where P > p.

Description

[0001] ABSOLUTE POSITION MEASUREMENT SCALE AND APPARATUS

[0002] THEREFOR

[0003] The present invention relates to an absolute position measurement scale for an absolute position measurement encoder, and an absolute position measurement encoder comprising a readhead for reading such an absolute position measurement scale.

[0004] Absolute position measurement encoders which enable the absolute position of a readhead relative to a scale to be determined, without requiring any relative motion therebetween (e.g. on start-up) are known. This is to be contrasted with incremental position encoders, having a scale with one or more reference marks, which require relative motion of the readhead to a reference mark in order to establish a known reference position.

[0005] Absolute position measurement encoders typically comprise a scale having at least one track comprising a series of absolute scale features which define a regular series of uniquely identifiable absolute positions along the measurement dimension (such that regardless of where the readhead is along the scale’s measuring dimension, the readhead can read the scale and determine from the pattern of scale features, where it is along the measuring dimension without requiring any relative movement along the scale). As is already known, the scale features of an absolute position measurement scale can be arranged to form discrete codewords (e.g. as described in US Patent no. US7499827 and US11619521). As is also already known, the scale features of an absolute position measurement scale can be arranged to form, for instance, a pseudorandom bit sequence (e.g. as described in European Patent no. EP0503716, wherein as described therein, a pseudo-random sequence is to be understood to mean a sequence wherein a displacement of the readhead over one pitch of the track always produces a specific code for each pitch). Absolute position encoders can provide position information in one dimension, for example as described in US7499827 and EP0503716, or in two dimensions, for instance as described in US Patent no.

[0006] US6603115B1. As is already known, the features in a scale track of an absolute position measurement scale can be arranged in accordance with a nominally regular / periodic “lattice” (or “framework”) structure. This has the advantage that fine-pitch phase information can be extracted from a reading of scale features, i.e. position information which is finer than the period of the nominally regular / periodic lattice structure can be extracted from a reading of absolute scale features. Example embodiments of how this can be achieved are explained in US11619521 and US7499827. As explained in US11619521 and US7499827, this can be achieved even when features are arranged such that they depart from the regular / periodic lattice structure (e.g. so as to define / embed the absolute position information).

[0007] The present invention concerns an improved an absolute position measurement scale.

[0008] According to a first aspect of the invention there is provided an absolute position measurement scale for an optical encoder apparatus, comprising a scale track extending along a measurement dimension, the scale track comprises alternating dark and bright scale features (e.g. lines) arranged such the scale track comprises: i) a first series (e.g. a first set) of dark scale features arranged according to a first lattice (or “framework”) having a period p; and ii) a second series (e.g. a second set) of dark scale features embedded within the first series of dark scale features, wherein the second series of dark scale features are irregularly spaced along the measurement direction but are arranged according to a second lattice having a period , where P > p (i.e. P is greater than p).

[0009] This arrangement has been found the superior over existing known optical absolute position measurement scales, such as those described in US7499827 and EP0503716. In particular, the scale configuration of the invention helps to optimise the optical performance of the scale and / or of a readhead used with the scale by avoiding the over-provision of light which does not contain any position information. In other words, embedding within said first series of dark scale features, a second series of dark scale features means that absolute position information is / can be embedded in the scale track by reducing rather than increasing the light received by the sensor, and this can be advantageous because using bright features of different lengths can result in excessive noise in the signal obtained by a sensor reading the scale. For instance, scale according to the present invention can help to reduce Shot or Poisson noise (compared to embodiments where absolute position information is embedded via bright features) and can also help to reduce the dynamic range of the sensor required to be used with the scale, and thereby can make it easier to set the gain of the sensor to minimise quantization noise. Furthermore, arranging the second series of dark scale features according to a second period P which is different to and greater than the period p increases the ease, speed and / or reliability via which the absolute position information can be extracted from the scale reading.

[0010] It is possible that the dark scale features of the first series and the dark scale features of the second series are substantially the same length as each other, as measured along the measurement dimension (e.g. as per the embodiment of Figure 3 described in more detail below). In such a case, the length of the dark scale features of the first series, and the length of the dark scale features of the second series, can be less than 0.33 / ?, preferably less than 0.3p, more preferably less than 0.25p. It is also possible that the length of the dark scale features of the second series is less than the length of the dark scale features of the first series. However, a beneficial embodiment of the invention is configured such that the dark scale features of the second series, as measured along the measurement dimension, are longer than the length of the dark scale features of the first series (e.g. as per the embodiments of Figures 1 and 4). A particularly advantageous embodiment of the invention is configured such that the dark scale features of the first series all have the same length (Ls) as each other, as measured along the measurement dimension, and that the dark scale features of the second series, as measured along the measurement dimension, are longer than the length of the dark scale features of the first series (e.g. as per the embodiments of Figures 1 and 4).

[0011] It can be beneficial for all of the bright scale features to have substantially the same length (Lh) as each other, as measured in the measurement dimension. This can help to reduce problems caused by noise in a signal generated by / derived from the scale, which in particular can help to reduce errors when deriving position information from the scale at a resolution finer than the period p. The scale can be configured such that the length Lh of said bright features is not more than 0.75 / ? and is not less than 0.25 / ?. In advantageous embodiments, the length Lh of said bright features is substantially 0.5 / ?.

[0012] The length of the dark scale features in the first series of dark scale features can have a length Ls, as measured in the measurement dimension where Ls is not more than 0.75 / ? and not less than 0.25 / ?. As will be understood, the length of the dark scale features in the first series of dark scale features can have a length Ls, as measured in the measurement dimension, of p-Lh.

[0013] The length of the dark scale features in the second series of dark scale features can have a length LI, where LI is greater than Ls, and in particular is greater than p. This can be particularly preferred because it can reduce the noise in the signal generated by the scale. Accordingly, the dark scale features in the first series of dark scale features could be referred to as “short” dark scale features, and the dark scale features in the second series of dark scale features could be referred to as “long” dark scale features. In other words, “short” dark scale features can be those dark scale features having a length less than p (e.g. 0.75 or less) and “long” dark scale features can be those dark scale features have a length greater than p. Therefore, in accordance with the invention, it could be described that the scale comprises: i) a first series (e.g. first set) of “short” dark scale features arranged according to a first lattice (or “framework”) having a period p; and ii) a second series (e.g. set) of “long” dark scale features which are irregularly spaced along the measurement direction but are arranged according to a second lattice (or “framework”) having a period P, where P > / ?.

[0014] Accordingly, it can be that all dark scale features having a length greater than p (e.g. all “long” dark scale features) are considered to be part of the second series of dark scale features, whereas all dark scale features having a length less than p (e.g. all “short” dark scale features) are considered to be part of the first series of dark scale features.

[0015] The length LI of the dark scale features in the second series of dark scale features can be substantially equal to N. / ?+Zs, wherein N is a positive integer. Whilst not essential, it can be preferred that N is a constant positive integer, such that the lengths of all of dark scale features in the second series of dark scale features, as measured along the measurement dimension, are substantially the same length as each other.

[0016] The first series of dark scale features could be arranged such that their centres lie on the first lattice having a period p. The second series of dark scale features could be arranged such that their centres lie on the second lattice having a period P.

[0017] Whilst P can take any value which is greater than p, it can been preferred that P = (C / 2). / ? where C is a positive integer greater than 2. (In the embodiments of Figures 1, 3, 5 and 6 described in more detail below, P = (C / 2). / ?, where C = 6; and in the embodiment of Figure 4 described in more detail below, P = (C / 2). / ?, where C = 7).

[0018] In particular, it can be most preferred that P = Cp and where C is a positive integer greater than 1. Such a configuration can be beneficial when it is desired that the lengths of all the dark scale features in the second series of dark scale features, as measured along the measurement dimension, are substantially the same length as each other. (In the embodiments of Figure 1, 3, 5 and 6 described below, P is equal to Cp, where C = 3. In the embodiment of Figure 4 described below, P is not equal to C.p).

[0019] The second lattice could be offset along the measurement dimension relative to the first lattice, e.g. by 0.5 / ? (e.g. as per the embodiments of Figures 1, 3, 5 and 6).

[0020] However, as will be understood, this need not necessarily be the case. Indeed, when P = (C / 2). / ? where C is an odd positive integer greater than 2, then it could be that every even numbered pitch of the second lattice is directly in phase with the first lattice and every odd numbered pitch of the second lattice period is offset along the measurement dimension relative to the first lattice by 0.5 / ? (e.g. as per the embodiment of Figure 4). Whilst C can be any positive integer (greater than 1 or 2), it can be beneficial (e.g. for reasons of code compactness / density) that C is not too large, for example not more than 50, optionally not more than 20, optionally not more than 10, for example not more than 6. Optionally, C is not more than 128 / / ?.

[0021] The dark scale features in the second series of dark scale features can be spaced relative to each other along the measurement dimension such that for all of the dark scale features in the second series of dark scale features (e.g. for all of the “long” dark scale features), the spacing between its centre and the centres of all of the other dark scale features in the second series of dark scale features (e.g. all of the other “long” dark scale features) is substantially equal to M.(D / 2). / ?, where M is a variable positive integer and where D is a constant positive integer greater than 2. This is the case for the embodiments of Figures 1, 3 and 4 as described in more detail below, wherein in the embodiments of Figures 1, 3, 5 and 6, D = 3, and in the embodiment of Figure 4, D = 7.

[0022] The dark scale features in the second series of dark scale features can be spaced relative to each other along the measurement dimension such that for all of the dark scale features in the second series of dark scale features (e.g. for all of the “long” dark scale features), the spacing between its centre and the centres of all of the other dark scale features in the second series of dark scale features (e.g. all of the other “long” dark scale features) is substantially equal to M.D. / ?, where M is a variable positive integer and where D is a constant positive integer greater than 1. This is the case for the embodiments of Figures 1, 3, 5 and 6 as described in more detail below, wherein in the embodiments of Figures 1, 3, 5 and 6, D = 3. This is not the case for the embodiment of Figure 4 described in more detail below.

[0023] As will be understood, M can be different for different pairs of dark scale features in the second series of dark scale features. For example, in the embodiment of Figure 1 (described in more detail below), D = 3 (which is the same as C) and M = 1 for the pair of features 10a and 10b, M = 2 for the pair of features 10b and 10c, and M = 3 for the pair of features 10a and 10c.

[0024] As will be understood, in connection with the above description of the size of the period P of the second lattice, D can be equal to C.

[0025] Optionally, M is greater than N. Optionally, M is less than 11. For example, M can be less than 7.

[0026] The configuration of the invention can be particularly useful for arrangements where p is not greater than 48 pm, optionally not greater than 32 pm, in particular not greater than 16 pm, for instance not greater than 8 pm. The configuration of the invention can be particularly useful for arrangements wherein P is not greater than 128 pm, optionally not greater than 64 pm, optionally not greater than 32 pm, optionally substantially 32 pm.

[0027] The absolute position measurement scale could be an optical reflective scale for a reflective-type optical encoder apparatus. In such a case, a dark scale feature can be a feature / region of (relatively) low optical reflectivity, and a bright scale feature can be a feature / region of (relatively) high optical reflectivity (a feature / region of (relatively) low optical reflectivity having lower optical reflectivity than that of said feature / region of (relatively) high optical reflectivity). The absolute position measurement scale could be an optical transmissive scale for a transmissive-type optical encoder apparatus. In such a case, a dark scale feature can be a feature of (relatively) low optical transmissivity, and a bright scale feature can be a feature / region of (relatively) high optical transmissivity (a feature / region of (relatively) low optical transmissivity having lower optical transmissivity than that of said feature / region of (relatively) high optical transmissivity).

[0028] In the embodiments in which the absolute position measurement scale is an optical reflective scale, the dark scale features can have a higher surface roughness compared to that of the bright scale features. The relatively dark scale features can comprise laser-ablated regions. Optionally, dark scale features comprise a material (e.g. printed on a substrate) having have a higher optical absorbency compared to that of the bright scale features.

[0029] The absolute position measurement scale can comprise a metallic substrate, or could be for example a glass substrate.

[0030] The width of the scale track, as measured in the dimension perpendicular to the measurement dimension, can be at least 1 mm, optionally at least 2 mm, for example at least 5 mm.

[0031] Whilst not essential, it can be advantageous that the widths of the dark scale features and the widths of the bright scale features, as measured in the dimension perpendicular to the measurement dimension, are substantially the same.

[0032] Optionally, the greatest separation between any two consecutive dark features of the second series of dark scale is not more than 50. / ?.

[0033] Optionally, the proportion of: a) the cumulative length of all of the bright scale features, as measured along the measurement dimension; to b) the cumulative length of all of the dark scale features, as measured along the measurement dimension, is not more than 1 :3 (0.33), optionally not more than 1 :4 (0.25).

[0034] Optionally, proportion of: a) the cumulative length of all of the bright scale features, as measured along the measurement dimension; to b) the cumulative length of all of the dark scale features, as measured along the measurement dimension, is not less than 1 :25 (0.04), optionally not less than 1 : 10 (0.1).

[0035] Optionally, the alternating dark and bright scale features within said at least a section of the track are arranged such that such that a Fourier analysis of said section of the track (e.g. of at least 25% of the length of the scale track, optionally at least 50% of the length of the scale track, optionally of at least 75% of the length of the scale track, optionally the entire length of the scale track), would show that for non-zero spatial frequencies, the spatial frequency having the most significant amplitude is a positive integer multiple “M” of the spatial frequency having the second most significant amplitude, where M is greater than 1.

[0036] According to a second aspect of the invention there is provided: an absolute position measurement scale for an optical encoder apparatus, comprising a scale track extending along a measurement dimension, the scale track comprising alternating relatively bright regions and relatively dark regions (e.g. lines). Within at least a section of the scale track: the scale track can have a relatively dark region consistently at regularly spaced locations along its measurement dimension at a nominal period p. Within said at least a section of the scale track, all of the relatively bright regions can have substantially the same length Lh as each other, as measured in the measurement dimension, and said length being not more than 0.75 / ? and not less than 0.25 / ?. Within said at least a section of the scale track the lengths of the relatively dark regions, as measured along the measurement dimension, can be different at different locations along the measurement dimension. E.g. this can be such that there is: a) a plurality of short relatively dark regions of substantially the same length Ls, said short relatively dark regions having a length Ls, as measured in the measurement dimension, of p-Lh, and b) a plurality of long relatively dark regions, the length of said long relatively dark regions, as measured in the measurement dimension, being substantially p+Ls, wherein N is a positive integer. Within said at least a section of the scale track, the long relatively dark regions can be spaced relative to each other along the measurement dimension in an irregular pattern. It can be that for all of said long relatively dark regions, the spacing between its centre and the centres of all of the other long relatively dark regions is substantially equal to M.D. / ?, where M is a variable positive integer and where D is a constant positive integer greater than 1.

[0037] As will be understood, the scale can be either: a) a reflective scale for a reflective-type optical encoder apparatus and a relatively dark region is a region of relatively low optical reflectivity, and a relatively bright region is a region of relatively high optical reflectivity; or b) a transmissive scale for a transmissive-type optical encoder apparatus and a relatively dark region is a feature of relatively low optical transmissivity and a relatively bright region is a feature of relatively high optical transmissivity.

[0038] A scale configured in accordance with the second aspect of the invention has been found to be superior over existing known optical absolute position scales, such as those described in US7499827 and EP0503716. In particular, it has been found that ensuring that all of the relatively bright regions have substantially the same length Lh as each other (as measured in the measurement dimension) wherein said length is not more than 0.75 / ? and not less than 0.25 / ?, can help to ensure that sufficient light is received by the readhead’s sensor without oversaturating sensor with light that doesn’t contribute to the scale signal. Furthermore, encoding the absolute position information via “short” and “long” relatively dark regions, via a scheme which requires that the long relatively dark regions are spaced relative to each other along the measurement dimension such that for all of said long relatively dark regions, the spacing between its centre and the centres of all of the other long relatively dark regions is substantially equal to M.C. / ?, where M is a variable positive integer and where C is a constant positive integer greater than 1, increases the ease, speed and / or reliability via which the absolute position information can be extracted from the scale reading.

[0039] Features described in connection with the first aspect of the invention are equally applicable to the second aspect of the invention, and vice versa.

[0040] According to a third aspect of the invention there is provided an absolute position measurement scale comprising a scale track extending along a measurement dimension, the scale track comprising alternating relatively bright regions and relatively dark regions (e.g. lines), the lengths of the relatively dark regions, as measured along the measurement dimension, being different at different locations along the measurement dimension so as to encode absolute position data within the scale track, wherein the alternating relatively bright regions and relatively dark regions are configured such that a Fourier analysis of at least 25% of the length of the scale track, optionally at least 50% of the length of the scale track, optionally of at least 75% of the length of the scale track, optionally the entire length of the scale track, would show that for non-zero spatial frequencies, the spatial frequency having the most significant amplitude is a positive integer multiple “M” of the spatial frequency having the second most significant amplitude, where M is greater than 1.

[0041] Features described in connection with the first aspect of the invention are equally applicable to the third aspect of the invention, and vice versa.

[0042] According to a fourth aspect of the invention there is provided an optical absolute encoder apparatus comprising an absolute position measurement scale as claimed in any preceding claim, and a readhead configured to read the scale track and determine therefrom an absolute relative position of it and the absolute position measurement scale along the measurement dimension.

[0043] Accordingly, in other words, the optical absolute encoder apparatus can comprise: a) an absolute position measurement scale comprising a scale track extending along a measurement dimension; and b) a readhead configured to read the scale track and determine therefrom information concerning the position of the readhead relative to the absolute position measurement scale along the measuring dimension; wherein the scale track comprises alternating dark and bright scale features (as sensed by the readhead), wherein the alternating dark and bright scale features of the scale track are arranged such that the scale track comprises: i) a first series (e.g. a first set) of dark scale features arranged according to a first lattice having a period p; and ii) a second series (e.g. a second set) of dark scale features embedded within the first series of dark scale features, wherein the second series of dark scale features are irregularly spaced along the measurement direction but are arranged according to a second lattice having a period P, where P > p.

[0044] The optical absolute encoder apparatus could be a reflective-type optical encoder apparatus, or could be a transmissive-type optical encoder apparatus. The readhead can comprise at least one light source for illuminating the scale and at least one sensor for the scale (e.g. for sensing light manipulated by the scale). In those embodiments in which the optical absolute encoder apparatus is a reflective-type optical encoder apparatus, the at least one light source and at least one sensor can be arranged on the same side of the scale, such that the readhead (i.e. its one or more sensors) are configured to detect light reflected by the scale. In those embodiments in which the optical absolute encoder apparatus is a transmissive-type optical encoder apparatus, the at least one light source and at least one sensor can be arranged on the opposite sides of the scale, such that the readhead (i.e. its one or more sensors) are configured to detect light transmitted / passing through by the scale.

[0045] The apparatus can be configured to determine information concerning the position of the readhead relative to the absolute position measurement scale along the measuring dimension by analysing the particular arrangement of alternating bright and dark scale features of a reading of the scale track as obtained / sensed the at least one sensor; e.g. in accordance with the above description, by analysing the particular arrangement of alternating features / regions of relatively low light intensity / amplitude and relatively high light intensity / amplitude as sensed the at least one sensor - wherein the features / regions of relatively low light intensity / amplitude have a relatively low intensity compared to the features / regions of relatively high light intensity / amplitude.

[0046] In those embodiments in which the optical absolute encoder apparatus is a reflective- type optical encoder apparatus, a dark scale feature can be a feature / region of (relatively) low optical reflectivity, and a bright scale feature can be a feature / region of (relatively) high optical reflectivity (a feature / region of (relatively) low optical reflectivity having lower optical reflectivity than that of said feature / region of (relatively) high optical reflectivity). In those embodiments in which the optical absolute encoder apparatus is a transmissive-type optical encoder apparatus, a dark scale feature can be a feature / region of (relatively) low optical transmissivity, and a bright scale feature can be a feature / region of (relatively) high optical transmissivity (a feature / region of (relatively) low optical transmissivity having lower optical transmissivity than that of said feature / region of (relatively) high optical transmissivity).

[0047] The apparatus can be configured to extract absolute position information (e.g. a coarse absolute position, e.g. an absolute position codeword) from a reading of the scale track obtained by the readhead’s sensor(s), by analysing the reading of the scale track. The apparatus can be configured to determine the absolute position information at the resolution of the period of the first lattice (i.e. at a resolution of “ / ?”). Such position information could be referred to as a “coarse” absolute position.

[0048] The apparatus can be configured to extract absolute position information (e.g. a coarse absolute position, e.g. an absolute position codeword) from a reading of the scale track obtained by the readhead’s sensor(s), by analysing the reading of the scale track (e.g. the signal amplitude thereof) solely at spaced apart locations consistent with P = (C / 2). , where C is a positive integer greater than 2 (and optionally solely at spaced apart locations consistent with P = C.p, where C is a positive integer greater than 1).

[0049] The apparatus can be configured to determine from a scale signal obtained by the readhead’s sensor(s) position information concerning the position of the readhead relative to the scale along the measuring dimension at a resolution finer than p (e.g. via interpolation and / or Fourier Analysis / Transform scale signal).

[0050] The apparatus can be configured to determine such position information having a resolution finer than p in two parts, e.g. a first part being a determined “coarse” absolute position determined to a resolution of p and the second part being a “phase reading”, which is a measure of where the readhead is relative to the absolute position measurement scale as a subdivision of / within the period p. The apparatus can be configured to determine the “phase reading” from a scale reading obtained by the same sensor that was used to obtain the signal from which the above mentioned (“coarse”) absolute position was determined. Optionally, the apparatus is configured to determine the “phase reading” from a scale reading obtained by a different sensor to that used to obtain the signal from which the above mentioned (“coarse”) absolute position was determined.

[0051] Optionally, the apparatus is configured to combine the above mentioned “coarse” absolute position information and “phase reading” so as to obtain a “fine resolution” absolute position, e.g. as described in US7499827 and US11619521. As described in US11619521, the “coarse” absolute position part of the reading could be determined from a current reading of the scale, or could be extrapolated from previous readings.

[0052] The light source can be configured to emit light anywhere in the electromagnetic spectrum from (and including) the infrared range to (and including) the ultraviolet range. Similarly, the sensor(s) could be configured to sense light anywhere in the electromagnetic spectrum from (and including) the infrared range to (and including) the ultraviolet range. The optical absolute encoder apparatus could be a reflective optical absolute encoder apparatus or a transmissive optical absolute encoder apparatus.

[0053] Features described in connection with the first, second and third aspect of the invention are equally applicable to the fourth aspect of the invention, and vice versa.

[0054] Embodiments of the invention will now be described, by way of example only, with reference to the following drawings, in which:

[0055] Figure 1 schematically illustrates an absolute position encoder apparatus and shows a section of an absolute position scale according to a first embodiment of the present invention;

[0056] Figure 2 shows a graph representing a Fourier Transform of the scale track of an absolute position scale having absolute position features configured in accordance with the present invention; Figure 3 schematically illustrates an absolute position encoder apparatus and shows a section of an absolute position scale according to a second embodiment of the present invention;

[0057] Figure 4 schematically illustrates an absolute position encoder apparatus and shows a section of an absolute position scale according to a third embodiment of the present invention;

[0058] Figure 5 schematically illustrates an absolute position encoder apparatus and shows a section of an absolute position scale according to a fourth embodiment of the present invention; and

[0059] Figure 6 schematically illustrates an absolute position encoder apparatus and shows a section of an absolute position scale according to a fourth embodiment of the present invention.

[0060] Referring to Figure 1, there is shown a reflective optical absolute encoder apparatus 2 comprising a readhead 4 and a scale 6. In Figure 1, the readhead 4 is illustrated as being transparent such that the scale behind it can be seen, but as will be understood this need not necessarily be the case. The readhead 4 and scale 6 are respectively provided on first and second parts of a machine (not shown) which are moveable relative to each other along the X axis (in the direction indicated by arrow M, which is also herein referred to as the “measurement dimension”). In the embodiment described, the scale 6 is a linear scale. However, it will be understood that the scale 6 could be another type of scale, such as a rotary scale (e.g. ring or disc scale). The readhead 4 has a light source (not shown) for illuminating the scale and a sensor (not shown) for sensing light reflected by the scale track 7 so as to take a reading of a section of the scale track.

[0061] Accordingly, in the embodiment described, the scale 6 is an optical absolute scale for a reflective optical encoder apparatus. Accordingly, the scale 6 is a reflective optical absolute scale and comprises a substrate 12 having a track 7 comprising alternating regions of relatively high optical reflectivity 8 and regions of relatively low optical reflectivity 10. For the sake of brevity and clarity, regions of relatively low optical reflective can referred to simply as “non-reflective” regions / features 10, and similarly the regions of relatively high optical reflectivity can be referred to simply as “reflective” regions / features 8. Because the " reflective” regions will appear bright to the readhead 4 (i.e. its sensor), then they could be referred to as relatively bright regions, or just “bright” scale regions or “bright” scale features (which can be and are used interchangeably herein). Similarly, because the “non-reflective” regions will appear relatively dark to the readhead 4 (i.e. its sensor), then they could be referred to as relatively dark regions, or just “dark” regions or “dark” scale features (which can be and are used interchangeably herein).

[0062] As will be understood, in other embodiments, the encoder apparatus can be a transmissive optical absolute encoder apparatus, in which case the readhead 4 can have a light source on one side of the scale and a sensor on the other side of the scale so as to take a reading of a section of the scale track. In such a case, the scale can be an optical absolute scale for a transmissive optical encoder apparatus. Accordingly, in such a case, the scale can be a transmissive optical absolute scale comprising a substrate having a track comprising alternating regions of relatively high optical transmissivity and regions of relatively low optical transmissivity. In such a case, because the regions of relatively high optical transmissivity will appear relatively bright to the readhead 4 (i.e. its sensor), then they could be referred to as relatively bright regions, or simply as “bright” regions or “bright” scale features (which can be and are used interchangeably herein). Similarly, because the regions of relatively low optical transmissivity will appear relatively dark to the readhead 4 (i.e. its sensor), then they could be referred to as relatively dark regions, or simply as “dark” regions or “dark” scale features (which can be and are used interchangeably herein).

[0063] As illustrated in Figure 1, the scale track 7 comprises relatively dark features 10 of various different lengths (as measured along the measurement dimension M), which is how the absolute position information is embedded therein. In particular, as shown in Figure 1, the scale track comprises a first series of dark scale features 10’ having the same length as each other, as measured along the measurement dimension, the first series of dark scale features being arranged so as to lie on a first lattice having a period p. As a result, and as illustrated in Figure 1, the scale track is configured to have a relatively dark region consistently at regularly spaced locations along its measurement dimension at a nominal period p i.e. as illustrated in Figure 1, there is an identifiable period p at which (at each and every period p) the scale track appears relatively dark, or in other words, there is an identifiable period p at which (at each and every period p) the scale has a relatively dark region. In the embodiment described, the period p of the nominally regular / periodic lattice is 8 pm.

[0064] As explained earlier in this document, arranging the scale features in a scale track in accordance with a nominally regular / periodic lattice structure has the advantage that fine-pitch phase information can be extracted from a reading of absolute scale features, i.e. position information which is finer / smaller than the period p of the nominally regular / periodic lattice structure can be extracted from a reading of absolute scale features. Example embodiments of how this can be achieved are explained in US 11619521 and US7499827. As explained in US 11619521 and US7499827, this can be achieved even when features are arranged such that they depart from the regular / periodic lattice structure (e.g. so as to define / embed the absolute position information), so long as sufficient periodic structure remains in the absolute scale features.

[0065] As also shown in Figure 1, embedded within said first series of dark scale features 10’, there is a second series of dark scale features 10”. The lengths (as measured in the measurement dimension) of the dark scale features within the second series are longer than the length of the dark scale features of the first series. In particular, in the embodiment described, the length Ls of the dark scale features within the first series are equal to substantially 0.5 / ? and the lengths LI of the dark scale features within the second series are equal to substantially N. ?+Zs where N is a positive integer. In the embodiment described and shown in Figure 1, all of the “long” relatively dark features 10 have the same length, and therefore N is a constant positive integer (i.e. it is the same for all of the “long” relatively dark features), but this need not necessarily be the case (in which N will be a variable positive integer).

[0066] Furthermore, and in accordance with the present invention, the dark scale features of the second series of dark scale features are arranged so as to lie on a second lattice having a period , where P = C.p and where in this embodiment C = 3 (but as will be understood, in accordance with the present invention can be any positive integer greater than 1, but preferably is not more than 10). Accordingly, all of the dark scale features of the scale track 7 that aren’t arranged according to the first lattice having period p (e.g. aren’t arranged such that their centres lie on the first lattice having period p) are arranged according to a second lattice having the period P, where P is greater than p.

[0067] As illustrated in Figure 1, all of the relatively bright regions / features 8 have substantially the same length (Lh) as each other (as measured in the measurement dimension). In the embodiment described, the length Lh is substantially 0.5 / ?, and whilst this can be preferred for optimal optical performance, it need not necessarily be the case, and Lh could for instance reasonably be not more than 0.75 / ? and not less than 0.25 / ?.

[0068] Accordingly, it could be said that the scale track comprises “short” relatively dark features 10 having a length Ls, which will be equal to p-Lh. As also shown, there are “long” relatively dark features 10 having a length LI, which is equal to N. / ?+ / .. s' where N is a positive integer. All of the “short” relatively dark features 10 have the same length Ls. In the embodiment described and shown in Figure 1, all of the “long” relatively dark features 10 have the same length, and therefore N is a constant positive integer (i.e. it is the same for all of the “long” relatively dark features), but this need not necessarily be the case (in which N will be a variable positive integer). As it could be said that the “long” relatively dark regions (i.e. those relatively dark regions having a length of N. / ?+ / .. s) are spaced relative to each other along the measurement dimension such that for all of said long relatively dark regions, the spacing between its centre and the centres of all of the other long relatively dark regions is substantially equal to M.D. / ?, where M is a variable positive integer and where D is a constant positive integer greater than 1. In the embodiment described and shown in Figure 1, the constant “D” is 3. Accordingly, D is equal to C.

[0069] Accordingly, the scale described above essentially has relatively dark scale features (e.g. relatively non-refl ective scale features) arranged at two identifiable nominal scale periods. That is the “short” relatively dark features / regions are arranged such that their centres lie on a first lattice having a nominal period p, and the “long” relatively dark features / regions are irregularly spaced along the measurement dimension but arranged such that their centres lie on a second lattice having a different nominal period , in particular where P = Cp (where C is a positive integer). That is, in other words, the scale track comprises (“dark”) position features arranged according to two different periods / two different spatial frequencies. This means that, as illustrated in Figure 2, a Fourier Transform of a substantial length of the scale track (e.g. of any continuous region of at least 20 periods p) would show significant amplitude at the spatial frequencies \ / p and 1 / (K. / ?) along the entire length of the scale, where K is an integer greater than 1. Accordingly, a Fourier analysis of the scale track, would show that for non-zero spatial frequencies, the spatial frequency having the most significant amplitude is a positive integer multiple (greater than 1) of the spatial frequency having the second most significant amplitude. This is also true for the scales arranged according to the other embodiment described herein; that is a Fourier analysis of a substantial length of a scale track arranged according to the embodiments of any of Figures 3 to 6, would show that for non-zero spatial frequencies, the spatial frequency having the most significant amplitude is a positive integer multiple (greater than 1) of the spatial frequency having the second most significant amplitude.

[0070] In the embodiment described, the readhead 4 operates in the infra-red region of light, but this need not necessarily be the case (e.g. an optical encoder / readhead can operate anywhere in electromagnetic spectrum from the infra-red region to the ultraviolet region). The reading could be, for instance, an image of a section of the scale track formed on the sensor by one of more optical devices (e.g. such as a lens). The reading could be a shadow-cast reading of a section of the scale track formed. In any case, the absolute position of the readhead 4 and scale 6 along the measurement dimension M can be determined at any position along the measurement dimension M by the particular combination of the reflective 8 and non-reflective features 10 in a scale reading obtained by the readhead’s 4 sensor.

[0071] In the embodiment described, the absolute data is encoded in the form of discrete codewords which are each a predetermined number of bits or periods long (e.g. typically will be anywhere between 15 periods p long and 100 periods p long, but as will be understood, could be a greater or smaller number of periods p long, depending on the requirements of the scale / encoder). Accordingly, in the embodiment described, the readhead’s 4 reading window (e.g. it’s sensor’s sensing length) is sufficiently long such that the readhead / sensor will always be able to detect at least one whole codeword. As will be understood, other embodiments are possible, such as each codeword being unique (not reused) and therefore reading just one discrete codeword by itself is sufficient to determine the absolute position of the readhead and scale. In other embodiments, the absolute data can be encoded as a pseudorandom bit sequence (e.g. as described in European Patent no. EP0503716, wherein as described therein, a pseudo-random sequence is to be understood to mean a sequence wherein a displacement of the readhead over one pitch of the track always produces a specific code for each pitch. As shown in Figure 1, in the embodiment described, a “0” bit is encoded by way of a longer dark scale feature, but this need not necessarily be the case.

[0072] In any case, the absolute position of the readhead 4 and scale 6 can be determined by extracting the codeword(s) (defined by the particular combination of the reflective 8 and non-reflective features 10) contained in the reading obtained by the readhead, and (for instance) looking up in a look-up table what position along the measurement dimension M that(those) codeword(s) represents. In the embodiment of Figures 1, 3, 5 and 6 the apparatus can be configured to extract the codeword from a scale signal obtained by its sensor, by (e.g. following a Fourier Transform of the scale signal at the spatial frequency 1 / to identify phase of the 2ndlattice) analysing the scale signal (e.g. the signal amplitude thereof) solely at spaced apart locations consistent with P (where in these embodiments P = C.p and C = 3). In the embodiment of Figure 4 the apparatus can be configured to extract absolute position information (e.g. a coarse absolute position, e.g. an absolute position codeword) from a scale signal obtained by its sensor, by (e.g. following a Fourier Transform of the scale signal at the spatial frequency 1 / to identify phase of the 2ndlattice) analysing the scale signal (e.g. the signal amplitude thereof) solely at spaced apart locations consistent with P (where in this embodiment P = (C / 2). and where C = 7). That is, for example, the apparatus can be configured to analyse the amplitude of the signal at those locations according to above to determine whether a 0 or a 1 is encoded (e.g. an amplitude below a predetermined threshold can be interpreted as a 1 and an amplitude above a predetermined threshold can be interpreted as a 0 (or vice versa)). As will be understood, in these embodiments, the resolution of such absolute position information will be equal to p. As mentioned above (and as explained in more detail in US11619521 and US7499827), fine-pitch / phase information (i.e. position information which is finer than the period p of the nominally regular / periodic lattice structure; and therefore position information which finer than the period / ?) can also be extracted from a reading of the absolute scale features (e.g. via interpolation and / or via a Fourier Analysis of the reading of the absolute scale features), and for example combined with the coarser absolute position determined from the absolute codeword.

[0073] In the embodiment the scale substrate comprises a metallic substrate, for example steel.

[0074] In the embodiment described, the surface roughness of the “dark scale features” or “non-reflective regions” 10 is greater than that of the “bright scale features” or “reflective regions” 8. As will be understood, the surface roughness can be measured by, for example, an atomic force microscope.

[0075] In the embodiment described, the “dark scale features” or “non-reflective regions” 10 are formed via laser ablation. An alternative embodiment is shown in Figure 3. This embodiment is substantially the same as that of Figure 1, and like parts share like reference numbers. However, in this embodiment, the dark features of the first and second series of dark features are all substantially the same length and have a length of approximately 0.25 / ?.

[0076] Another alternative embodiment is shown in Figure 4. This embodiment is substantially the same as that of Figure 1, and like parts share like reference numbers. However, in the embodiment of Figure 1 (and in the embodiment of Figures 3 and 5), the dark scale features within the second series are arranged according to a lattice having a period P where P = C.p and where C is a positive integer greater than 1 (in the particular embodiments of Figures 1, 3, 5 and 6, C = 3), but in the embodiment of Figure 4, the dark scale features within the second series are arranged according to a lattice having a period P where P = (C / 2). / ? and where C is a positive integer greater than 2 (in the particular embodiment shown in Figure 4, C = 7 / 2). As shown in Figure 4, in this embodiment, the lengths of the dark features in the second series are not all the same length (as measured along the measurement dimension).

[0077] Another alternative embodiment is shown in Figure 5. This embodiment is substantially the same as that of Figure 1, and like parts share like reference numbers. However, in the embodiment of Figure 1 (and in the embodiment of Figures 3 and 5), the widths (as measured in a dimension perpendicular to the measurement dimension) of the dark scale features 10” of the second series of dark scale features are the same as the widths (as measured in a dimension perpendicular to the measurement dimension) of the dark scale features 10’ of the first series of dark scale features, whereas in the embodiment of Figure 5, the widths (as measured in a dimension perpendicular to the measurement dimension) of the dark scale features 10” of the second series of dark scale features are not the same as the widths (as measured in a dimension perpendicular to the measurement dimension) of the dark scale features 10’ of the first series of dark scale features. Other configurations are possible. For instance, the dark scale features 10” of the second series of dark scale features could comprise laterally offset (in the dimension perpendicular to the measurement dimension) bands, e.g. as illustrated in Figure 6. As will be understood, other configurations and arrangements are possible. For example, referring to Figure 5, the lengths of the dark scale features 10” of the second series of dark features could be longer than that shown (in the direction down the page), such that they also extend outside the boundaries of the dark scale features 10’ of the first series of dark features (i.e. whilst still being embedded within the first series of dark scale features, they can also have a part which sticks out from the first series of dark scale features).

[0078] Herein, the first lattice and could be described as being a first “notional” lattice. Similarly, the second lattice could be described as being a second “notional” lattice. Furthermore, instead of saying that the first series of dark scale features are arranged according to a first (notional) lattice having a period p, it could be said that the first series of dark scale features are arranged according to a first (notional) “grid” or “framework” having a period p. Similarly, instead of saying that the second series of dark scale features are arranged according to a second (notional) lattice having a period , it could be said that the second series of dark scale features are arranged according to a second (notional) “grid” or “framework” having a period P.

Claims

CLAIMS:

1. An absolute position measurement scale for an optical encoder apparatus, comprising a scale track extending along a measurement dimension, the scale track comprising alternating dark and bright scale features arranged such that the scale track comprises: i) a first series of dark scale features arranged according to a first lattice having a period p; and ii) a second series of dark scale features embedded within the first series of dark scale features, wherein the second series of dark scale features are irregularly spaced along the measurement direction but are arranged according to a second lattice having a period , where P >p.

2. An absolute position measurement scale as claimed in claim 1, wherein P = (C / 2). where C is a positive integer greater than 2.

3. An absolute position measurement scale as claimed in claim 1, wherein P = Cp where C is a positive integer greater than 1.

4. An absolute position measurement scale as claimed in any preceding claim, wherein: the dark scale features of the first series all have the same length as each other, as measured along the measurement dimension, and the dark scale features of the second series, as measured along the measurement dimension, are longer than the length of the dark scale features of the first series.

5. An absolute position measurement scale as claimed in claim 2 or 3, wherein C is not more than 50, optionally not more than 20, optionally not more than 10, for example 4 or less.

6. An absolute position measurement scale as claimed in any preceding claim, wherein all of the bright scale features have substantially the same length Lh as eachother, as measured in the measurement dimension, wherein said length is not more than 0.75 / ? and not less than 0.25 / ?.

7. An absolute position measurement scale as claimed in any preceding, wherein: the length of the dark scale features in the first series of dark scale features have a length Ls, as measured in the measurement dimension, where Ls is not more than 0.75 / ? and not less than 0.25 / ?; and the length of the dark scale features in the second series of dark scale features have a length LI, as measured in the measurement dimension, where LI is substantially equal to N. / ?+Zs, wherein N is a positive integer.

8. An absolute position measurement scale as claimed in claim 7, wherein N is a constant positive integer.

9. An absolute position measurement scale as claimed in any preceding claim, wherein p is not greater than 32 pm, optionally not greater than 16 pm, optionally not greater than 10 pm, optionally substantially 8 pm.

10. An absolute position measurement scale as claimed in any preceding claim, wherein P is not greater than 128 pm, optionally not greater than 64 pm, optionally not greater than 32 pm, optionally substantially 32 pm.

11. An absolute position measurement scale as claimed in any preceding claim, wherein the absolute position measurement scale is an optical reflective scale, and wherein the dark scale features have a higher surface roughness than the bright scale features.

12. An absolute position measurement scale as claimed in any preceding claim, comprising a metallic substrate.

13. An absolute position measurement scale as claimed in any preceding claim, wherein the width of the scale track, as measured in the dimension perpendicular to the measurement dimension, is at least 1 mm, optionally at least 5mm.

14. An absolute position measurement scale as claimed in any preceding claim, wherein the widths of the dark scale features and the widths of the bright scale features, as measured in the dimension perpendicular to the measurement dimension, are substantially the same.

15. An absolute position measurement scale as claimed in any preceding claim, wherein the relatively dark scale features comprise laser-ablated regions.

16. An absolute position measurement scale as claimed in any preceding claim, wherein the greatest separation between any two consecutive dark features of the second series of dark scale is not more than 50. / ?.

17. An absolute position measurement scale as claimed in any preceding claim, wherein the absolute position measurement scale is either: an optical reflective scale for a reflective-type optical encoder apparatus and a dark scale feature is a feature of relatively low optical reflectivity, and a bright scale feature is a feature of relatively high optical reflectivity; or an optical transmissive scale for a transmissive-type optical encoder apparatus and a dark scale feature is a feature of relatively low optical transmissivity, and a bright scale feature is a feature of relatively high optical transmissivity.

18. An absolute position measurement scale for an optical encoder apparatus, comprising a scale track extending along a measurement dimension, the scale track comprising alternating relatively bright regions and relatively dark regions, wherein within at least a section of the scale track: i) the scale track has a relatively dark region consistently at regularly spaced locations along its measurement dimension at a nominal period / ?;ii) all of the relatively bright regions have substantially the same length Lh as each other, as measured in the measurement dimension, and said length being not more than Q.TSp and not less than 0.25 / ?; iii) the lengths of the relatively dark regions, as measured along the measurement dimension, is different at different locations along the measurement dimension, such that there is: a. a plurality of short relatively dark regions of substantially the same length Ls, said short relatively dark regions having a length Ls, as measured in the measurement dimension, of p-Lh, and b. a plurality of long relatively dark regions, the length of said long relatively dark regions, as measured in the measurement dimension, being substantially N. ?+Zs, wherein N is a positive integer; wherein the long relatively dark regions are spaced relative to each other along the measurement dimension in an irregular pattern, but such that for all of said long relatively dark regions, the spacing between its centre and the centres of all of the other long relatively dark regions is substantially equal to M.D. / ?, where M is a variable positive integer and where D is a constant positive integer greater than 1.

19. An absolute position measurement scale comprising a scale track extending along a measurement dimension, the scale track comprising alternating relatively bright regions and relatively dark regions, the lengths of the relatively dark regions, as measured along the measurement dimension, being different at different locations along the measurement dimension so as to encode absolute position data within the scale track, wherein the alternating relatively bright regions and relatively dark regions are configured such that a Fourier analysis of at least 25% of the length of the scale track would show that for non-zero spatial frequencies, the spatial frequency having the most significant amplitude is a positive integer multiple “M” of the spatial frequency having the second most significant amplitude, where M is greater than 1.

20. An optical absolute encoder apparatus comprising an absolute position measurement scale as claimed in any preceding claim, and a readhead configured toread the scale track and determine therefrom an absolute relative position of it and the absolute position measurement scale along the measurement dimension.

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