Abstract
An incremental measurement encoder including 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 including a structured light source and a reference mark photodetector array. The at least one reference mark can include at least one imaging element configured to form an image of the structured light source onto the reference mark photodetector array.
Claims
1. An incremental measurement encoder comprising a scale and a readhead, the scale comprising a periodic series of features forming an incremental track and at least one reference mark, the readhead comprising a structured light source and a reference mark photodetector array, in which the at least one reference mark comprises at least one imaging element configured to form an image of the structured light source on the reference mark photodetector array, wherein the structured light source comprises an array of unevenly spaced light sources.
2. An incremental measurement encoder according to claim 1, wherein the reference mark comprises at least one of i) a lens, ii) a diffractive imaging element, or iii) a feature configured to image the structured light source by a pinhole effect.
3. An incremental measurement encoder according to claim 1, wherein the image formed comprises a one-dimensional image.
4. An incremental measurement encoder according to claim 1, wherein light from the structured light source is also used to produce a resultant field at a photodetector for detecting an incremental change in position of the readhead relative to the scale.
5. An incremental measurement encoder according to claim 1, wherein a grating is directly mounted on the structured light source.
6. An incremental measurement encoder according to claim 1, in which the reference mark photodetector array comprises at least first and second sets of photodetector elements, each set comprising a plurality of spaced apart photodetector elements, and each set being configured to separately detect the image provided by the at least one imaging element at laterally offset positions.
7. An incremental measurement encoder according to claim 6, in which the at least first and second sets of photodetector elements are interleaved.
8. An incremental measurement encoder according to claim 7, wherein, in use, a reference mark zero-point crossing signal is determined based on output from at least the first and second sets of photodetector elements.
9. An incremental measurement encoder according to claim 8, wherein, in use, based on the output of at least the first and second sets of photodetector elements, the measurement encoder monitors for the reference mark zero-point crossing.
10. An incremental measurement encoder according to claim 1, wherein the elements of the reference mark photodetector array are sized and/or spaced so as to be insensitive to the phase of a fringe field generated by the incremental scale track and falling on the reference mark photodetector array.
11. An incremental measurement encoder according to claim 1, wherein the reference mark is embedded within the incremental track.
12. An incremental measurement encoder as claimed in claim 1, in which at least one signal which is used to determine the presence of the at least one reference mark is derived from the output of the reference mark photodetector array, in which the signal comprises a main lobe and side lobes which occur as the readhead passes over the reference mark, and in which the encoder apparatus is configured such that the magnitude of the side lobes is no greater than 75% of the magnitude of the main lobe.
13. An incremental measurement encoder as claimed in claim 6, in which the signal which is used to determine the presence of the at least one reference mark is derived from at least two sets of photodetector elements.
14. An incremental measurement encoder according to claim 1, configured to provide a reference position signal which is repeatable to within a unit of resolution of the incremental measurement encoder.
Description
[0054] Embodiments of the invention will now be described in greater detail, by way of example only, and with reference to the following drawings, wherein:
[0055] FIG. 1 shows an incremental measurement encoder according to one embodiment of the present invention;
[0056] FIG. 2 shows an arrangement of components for part of the readhead shown in FIG. 1;
[0057] FIG. 3 shows an example of a structured light source which forms part of the readhead of FIG. 1;
[0058] FIG. 4 schematically shows the readhead in a position relative to the scale corresponding to the reference mark;
[0059] FIG. 5 shows the spacing of elements of the reference mark photodetector;
[0060] FIG. 6 shows the processing of the signals output from the reference mark photodetector to determine a reference mark signal;
[0061] FIG. 7 shows arrangements of components for part of the readhead shown in FIG. 1;
[0062] FIG. 8 schematically shows an embodiment in which the reference mark comprises more than one imaging element; and
[0063] FIG. 9(a) is a graph illustrating the side lobes present on the outputs of each of the sets of the reference mark photodetector elements (J, K, L and M); and
[0064] FIG. 9(b) is a graph illustrating the side lobes present on the gate and zero-point crossing signals which are derived from the outputs of the sets of the reference mark photodetector elements (J, K, L and M).
[0065] FIG. 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 readhead 3 are fixed to relatively movable parts of a machine or machines and the measurement encoder 1 allows the relative positions of the two parts to be determined.
[0066] In the embodiment shown in FIG. 1, the measurement encoder 1 is a linear measurement encoder for measuring the position of the readhead 3 relative to the scale 2 in a first direction x as indicated in the figure.
[0067] The scale 2 is an elongate scale having a series of periodic marks 200 thereon which define an incremental track. In this embodiment, the periodic marks form a phase grating on the scale which can cause, when illuminated a fringe field pattern to be formed by light reflected therefrom. Also forming part of the scale is a reference mark 202, which in this embodiment is embedded in the incremental track. In this embodiment, the incremental track comprises 4 μm (microns) pitch scale, and the reference mark 202 extends a distance of 100 μm in the measurement direction (x).
[0068] FIG. 1(b) shows a cross section through the measurement encoder of FIG. 1(a) through a plane orthogonal to the measurement direction x.
[0069] FIG. 2 shows the arrangement of light emitting and light receiving components of the readhead 3 for the embodiment shown in FIG. 1. The components of the readhead 3 face the scale 2. In particular, the readhead 3 comprises a structured light source 300, in this embodiment made up of three individual light sources 302 (i.e. “light” or “illuminating” regions) which are spaced apart by spaces 304 (i.e. “dark” regions). As will be described in more detail below, due to the uneven spacing of the individual light sources 302, the structured light source 300 shown in FIG. 2 could 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 a grating (which is this particular embodiment is a diffraction grating). In particular, each individual light source 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.
[0070] FIG. 2 also shows a reference mark photodetector array 310. The reference mark photodetector array comprises three parts which correspond to the three individual light sources 302 of the structured light source 300. The parts of the reference mark photodetector array 310 are arranged on the readhead 3 in a mirror image (in other words, spatially inverted) configuration of the individual light sources 302 of the structured light source. As will be explained in more detail the parts of the reference mark photodetector array and the space therebetween could be said to form the code 100101 (which is the spatially inverse of the above mentioned code of the structured light source). The embodiment of FIG. 2 shows the reference mark photodetector array 310 comprising three parts/photodetectors 310a, 310b, 310c.
[0071] An incremental photodetector array 320 is also shown as part of the readhead 3. An example of a suitable incremental photodetector array is described in WO 2008/053184 A1. The incremental photodetector 320 detects the changes in intensity of the fringe field (for example an interference fringe) falling thereon. In this embodiment, the fringe field of the light falling on the incremental photodetector array 320 is created by the wavefronts of light emitted from the structured light source 300 interacting with the grating formed by elements 306 and subsequently interacting with a second grating, in this case being the incremental track made up of periodic marks 200. This forms a fringe field (e.g. an interference fringe) on the incremental photodetector array. Relative movement of the scale and readhead causes the fringe field across the incremental photodetector to change (e.g. move), such change/movement being detectable by the readhead and, for example, used to increment/decrement a count of the relative position. As will be understood, it is possible to determine incremental measurement using a resultant field other than a fringe field. For example, light from the source can used to generate (e.g. via the scale, and optionally one or more diffraction gratings in the readhead) spots which modulate (e.g. change in intensity) with relative movement of the scale and readhead. Such an encoder is, for example, described in U.S. Pat. No. 6,198,534.
[0072] As will be understood, the signals from the incremental photodetector array can be used to produce 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 and readhead to be measured. Such incremental measurement (e.g. quadrature) signals can be interpolated to provide a higher resolution of count/measurement. As will be understood, an incremental encoder apparatus will have a given/specified unit of resolution. Preferably, the reference mark pulse detected by the reference mark photodetector array (e.g. by a set of photodetector elements) enables the reference mark position to be repeatably determined to within a unit of resolution of the incremental measurement, in other words, such that the reference position signal determined/output by the encoder apparatus is repeatable within a unit of resolution of the incremental measurement. As will be understood, this can mean such that the reference position signal determined/output by the encoder apparatus is provided at the same position, within a unit of resolution of the incremental measurement, regardless of the relative direction of motion.
[0073] For example, the output from the reference mark photodetector array can be used to determine a “gate” or “window” signal during which a reference position signal is determined/output by the encoder apparatus when a predetermined instance in the cycle of the incremental signal occurs (e.g. when the quadrature signals are both negative and equal). Accordingly, it can be important for the “gate” or “window” signal to be no wider than one cycle of the incremental signal (e.g. one period of the quadrature/SIN signal) to avoid ambiguity. The narrower the reference mark pulse detected by the readhead (e.g. the narrower the “pulse” of light on the detector), the narrower the “gate” or “window” signal which can be determined from the reference mark photodetector's signals. The use of “gate” or “window” signals to determine the presence of a reference mark are well known, and for example described in more detail in International Applications published under the Patent Cooperation Treaty WO2017/203210 and WO2007/057645, as well as described in more detail below. Our inventors have found that providing a structured light source, which is imaged onto a photodetector array, can enable a narrower reference mark pulse to fall on the detector, which in turn can help to ensure the repeatability of the reference mark signal provided by the encoder, to within a unit resolution of the incremental encoder.
[0074] The embodiment shown in FIGS. 1 and 2 will be understood to be a reflective measurement encoder as the light source (here the structured light source 300) and the photodetector arrays (here reference mark photodetector array 310 and incremental photodetector array 320) are located on the same side of the scale 2 in use.
[0075] FIG. 3 shows a structured light source 300 used in the embodiments of FIG. 1. FIG. 3 shows the structured light source having the code 101001 as is used in the embodiment shown in FIG. 2. Here an individual light source 302 of a given width represents “1” while a space of a given width represents “0”. Here it can be seen that a first space 304a (when reading from left to right) has a width equal to a first individual light source 302a while a second space 304b has a width equal to twice the width of the first space 304a. In this embodiment space 304b represents “00”. Other example possible codes for the structured light source include the code 10000001000100001000001 with the individual light sources representing “1” while spaces represent “0”s, with the width of the spaces compared to the individual light sources determining the number of “0”s. 110010000010100001 is an example of another code that could be used. It will be appreciated that in this structure the left most individual light source will double the width of the other individual light sources, because here one individual light source (the width of which is twice the given width) represents “11” within the code. Such an individual light source could be a single continuous light source or be made up of adjacent separate adjacent light sources. The code 111001100011011011 could also be used in embodiments. In other embodiments the structured light source may be a Barker code. It will be appreciated, that as was described above in relation to the embodiment shown in FIG. 2 having an individual light source having 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 the mirror image (e.g. spatially inverted version) of the structured light source 300.
[0076] The reference mark photodetector array 310 arranged as the mirror images of the above described structured light source 300 codes are 100101 (for 101001 shown in FIG. 3), 10000010000100010000001 (for the code 10000001000100001000001), 100001010000010011 (for the code 110010000010100001), and 110110110001100111 (for the code 111001100011011011).
[0077] While FIG. 1 and FIG. 2 describe a reflective embodiment, it will be understood that other arrangements may be possible, for example the reference mark photodetector array may be located on the opposite side of scale 2 to the structured light source 300, such a system may be referred to as a transmissive system.
[0078] FIG. 4(a) schematically shows the light path from the structured source 300 to the reference mark photodetector array 310 for a transmissive system. It will be understood that with the exception of reflection/transmission of the light through the scale 2 at the reference mark 202, the underlying principles are the same for the reflective embodiment of FIGS. 1 and 2 and the transmissive embodiment of FIG. 4(a). For ease of explanation, the transmissive embodiment of FIG. 4(a) will now be described.
[0079] In FIG. 4(a) the readhead 3 is shown in a position relative to the scale 2 corresponding to a reference mark position. 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 form part of the readhead 3 with the reference mark photodetector array 310 being located on a surface of the readhead 3. The structured light source 300 and the reference mark photodetector array 310 maintain a fixed relationship to each other and can be moved in a movement direction x relative to the scale 2. As the readhead 3 moves relative to the scale 2, the readhead will also move relative to the reference mark 202. In this embodiment, the reference mark 202 in the scale 2 allows light to be transmitted therethrough and acts as an imaging element; in particular the reference mark comprises a feature which images the structured light source 300 onto the photodetector array 310 by a pinhole effect. The transmitted light forms an image of the structured light source 300 on a plane containing the surface on which the reference mark photodetector array 310 is located. When the readhead 3 is in a position corresponding to the reference mark 202 relative to the scale 2 then the image formed is a spatially inverted representation of the code of the structured light source 300. As the reference mark photodetector array 310 is configured to be a spatially inverted representation of the structured light source in this embodiment, when the readhead 3 is in the position corresponding to the reference mark 202 relative to the scale 2, the reference mark photodetector array 310 is illuminated to its fullest extent by the structured light source 300. The reference mark photodetector array 310 is configured to output a signal based on the amount of light falling thereon.
[0080] As will be understood, in other embodiments, the imaging element of the reference mark could comprise a lens (e.g. cylindrical lens) and/or a diffractive optical element (such as a Fresnel Zone Plate). As will also be understood, the term imaging can mean the convolution of the structure/pattern of light emitted by the structured light source with the optical impulse response, or point spread function, of the reference mark. Likewise, the term image can refer to the pattern of light formed at the reference mark detectors by this process.
[0081] FIG. 4(b) shows how the output of the reference mark photodetector array 310 may vary as the readhead 3 and scale 2 are moved relative to each other in the measurement direction x. As can be seen in FIG. 4(b) a pulse 500 is formed in the output of the reference mark photo detector array 310 when the readhead 3 is in and around the reference mark position relative to the scale 2. It is possible to vary the relative proportions of the pulse 500 by adjustments to the reference mark 202. For example, widening the reference mark allows more light to be transmitted through the scale in a transmissive system and so increases the intensity of the peak, conversely narrowing the reference mark 202 allows less light to be transmitted though the scale and so diminishes the intensity of the pulse 500. However, widening the reference mark 202 will also cause the pulse 500 to become broader and also will disrupt the incremental signal more. Other factors such as the perpendicular distance of the structured light source 300 from the scale (i.e. along the z-axis shown in FIG. 1) can also affect the intensity of the light reaching the reference mark 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 in the structured light source; thereby helping to provide a more distinct pulse. Accordingly, using a structured light source which is imaged by the reference mark can be particularly useful when trying to provide a small reference mark, whilst still obtaining a distinct pulse at the detector.
[0082] It will be appreciated that in the described transmissive arrangement the light wavefronts emitted from the structured light source 300 interact with the grating comprising elements 306 directly on the structured light source 300 and subsequently interact with the scale 2 (as is described above in relation to the incremental track formed from the periodic marks 200). As the light passes through reference mark 202, the fringe field formed due to the elements 306 is formed on the parts 310a, 310b, 310c of the reference mark photodetector array 310.
[0083] In some embodiments, the parts 310a, 310b, 310c may comprise more than one section/element. For example, each of the parts 310a, 310b, 310c may be formed of four sections/elements J, K, L, M. In such an embodiment reference mark photodetector array 310 comprises three quad cells 310a, 310b, 310c. For example, each of parts 310a, 310b and 310c can be seen to be made up of four sections J, K, L, M. As will be understood, the same section in the three parts form a set of photodetector elements, which are spaced in a manner to match/correlate with the structured light source. For example, all of the J sections are spaced a manner to match/correlate with the structured light source, and so are all of the 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 a third set, and the M sections a fourth set. The outputs from these sets can be combined to aid determination of the reference mark position, as described in more detail below in connection with FIG. 6.
[0084] FIG. 5(a) shows a first way in which a part 310a of the reference mark photodetector array 310 comprises a quad cell having four sections J, K, L, M may be configured in order to be insensitive to the fringe field formed thereon. FIG. 5(a) shows the intensity of the fringe field 600 formed at the reference mark photodetector. As can be seen, each section J, K, L, M of the illustrated quad cell has a width which is an integer multiple of the period of the fringe field. The figure also shows each finger J, K, L, M as being the same width.
[0085] FIG. 5(b) shows a first way in which a part 310a of the reference mark photodetector array 310 comprises a quad cell having four sections J, K, L, M may be configured in order to be insensitive to the fringe field formed thereon. FIG. 5(b) shows the intensity of the fringe field 600 formed at the reference mark photodetector. As can be seen, each section J, K, L, M of the quad cell has the same width and the distance between the centres of adjacent fingers J, K, L, M is an integer multiple of the fringe field.
[0086] Depending on the configuration of the structured source elements and/or reference mark photodetector element size the JKLM detectors may or may not be able to be partially or even fully interleaved (e.g. because the gaps between the sources are large enough to place the four detectors needed for each element where the signal is detected). If the gaps between source elements are not large enough (and/or the desired size of reference mark photodetector elements are too large) such that it is not possible to fit all four elements where the signals are detected, then the detectors can be placed in two or more adjacent tracks as required. Fully interleaved detectors are advantageous because they are an efficient use of expensive semiconductor area. Non-interleaved detectors in adjacent tracks are advantageous in that they offer more freedom to choose any desirable code pattern, and can allow larger detector areas giving bigger, lower noise signals, at the cost of requiring more semiconductor area. FIG. 5(c) (and also FIGS. 7(b) and 7(c)) shows a schematic embodiment in which the detectors are non-interleaved, and FIG. 5(d) (and also FIG. 7(a)) shows a schematic embodiment in which the detectors are interleaved.
[0087] FIG. 6(a) shows the signals from each section J, K, L, M of the quad cell 310a of the reference mark photodetector array 310 as the readhead 3 moves relative to the scale 2 along the measurement direction x. In order to identify when the readhead 3 is a position relative to the scale 2 which corresponds to the reference mark 202, the signals from each section of the quad cell are processed. In particular, as is already known in the field of encoders, so-called “gate” and “zero-point crossing” signals (also known as “sum” and “diff” signals) are derived, which are analysed to determine the presence of a reference mark. FIG. 6(b) shows the signals from the sections J, K, L, M of the quad cell of FIG. 6(a) combined according to equation (1), so as to provide the “gate” or “sum” signal.
(K+L)−(J+M) (1)
[0088] FIG. 6(b) also shows a threshold 700. The output from equation (1) is used as a gate signal to enable a zero-crossing detector. When the gate signal is above the threshold 700, here between the points x.sub.1 and x.sub.2, the zero-crossing detector is activated. The zero-crossing detector processes the signals of the sections of the quad cell according to equation (2a), to obtain a “zero-point crossing” signal, the output of which is shown in FIG. 6(c).
K−L (2a)
[0089] In other embodiments the zero-crossing detector could process the signals of the quad cell according to equation (2b).
(J+K)−(L+M) (2b)
[0090] FIG. 6(c) also shows the positions x.sub.1 and x.sub.2 between which the zero-crossing detector is activated. In one embodiment, the encoder apparatus could be configured such that when the output of the of the zero-crossing detector is 0 (i.e. it crosses “0”), it is determined that the readhead 3 is in the position relative to the scale 2 corresponding to the reference mark 202, and therefore a reference mark signal is output by the readhead. However, alternative embodiments are possible. For example, an additional pair of thresholds (e.g. t.sub.1, t.sub.2) could be used to analyse the zero-point crossing (or ““diff”) signal, such that when the zero-point crossing (or “diff”) signal is between those thresholds, a window W signal is generated which in turn is used to analyse the incremental SIN and COS signals (W, SIN and COS are schematically shown superimposed in FIG. 6(d). In particular, this window W can be used such that a reference mark signal is output by the encoder apparatus when, within such window W, it is determined that the SIN and/or COS signals meet a particular predetermined condition (e.g. when it is determined that the SIN and COS signals are equal and negative). As will be understood, such window signal W needs to be sufficiently narrow so that such predetermined condition of the SIN and COS signals only occurs once within the window W, thereby ensuring that the reference mark is repeatable to within one unit resolution of the encoder apparatus.
[0091] FIG. 9(a) is similar to FIG. 6(a) in that it shows the signals from each section J, K, L, M of the quad cell 310a of the reference mark photodetector array 310 as the readhead 3 moves relative to the scale 2 along the measurement direction x. However, it illustrates that in reality the signals are likely to comprise sidelobes as well as the main lobe. FIG. 9(b) also illustrates how that these sidelobes are still present on the so-called gate (or “sum”) and zero-point crossing (or “diff”) signals (e.g. the signal derived from the outputs of the sets of reference mark photodetector elements). Such sidelobes can adversely affect the processing of these signals, especially if their magnitude is too large. For a given optical system, the “code” or pattern of the structured light source can have an effect on the magnitude of the side lobes. Accordingly, it has been found that by choosing an appropriate code, it is possible to configure the encoder apparatus such that in the signals derived from the outputs of the sets of reference mark photodetector elements (e.g. in the so-called “gate” and/or “zero-point crossing” signals) the magnitude of the most significant sidelobe is not more than 75% of the magnitude of the main lobe. In the embodiment simulated in FIG. 9 (where the light source has the code 110010000010100001), the magnitude of the most significant sidelobe is less than 30% of the magnitude of the main lobe.
[0092] FIG. 7(a) shows an alternative component geometry to that shown in FIG. 2. In this embodiment each part of the reference mark photodetector array 310 is shown comprising four sections of a quad cell. In FIG. 7(a) the structured light source represents the code 10000001000100001000001. This code has the advantage that the sets of reference mark detector elements can be interleaved which increases the detector area for a given chip size. As will be understood, other codes could be provided which also enable the sets of reference mark detector elements to be interleaved.
[0093] FIG. 7(b) shows another alternative geometry of components of the read head 3. In this embodiment the structured light source has the code 110010000010100001. As can be seen in the figure, this code is not implemented with interleaved reference mark photodetectors, instead the reference mark photodetector array has sections J, K, L, M which overlap in the measurement direction x.
[0094] FIG. 7(c) shows an arrangement of components for the code 111001100011011011.
[0095] In alternative embodiments, each part 310a, 310b, 310c of the reference mark photodetector array 310 may be made up of only two sections, i.e. be a bi-cell which may have a first part K and a second part L. In such an embodiment, a gate signal may be obtained by combining the outputs of sections K and L as shown in equation 3 and applying a threshold, while a zero-crossing may be detected using by combining the outputs of the sections K, L as shown in equation 4 and monitoring when equation 4 has a value of 0.
K+L (3)
K−L (4)
[0096] In further embodiments each section of the parts of the reference mark photodetector 310 may have a width equal to an integer multiple of the period of the fringe pattern formed thereon and be space apart such that their centres are spaced apart by an integer multiple of the fringe field formed thereon.
[0097] While the invention has been described above in connection with at least one embodiment, it will be appreciated that a number of other embodiments are possible without departing from the scope of the claims. For example, the incremental track of the scale 2 may comprise an amplitude (or “Ronchi”) scale, instead of the above described phase scale. Further, FIGS. 7(a) to (c) show the components of the readhead 3 in various configurations, it will be understood that the location of a component in a direction perpendicular to the measurement x can be varied. While the embodiments above describe a linear measurement encoder, the encoder could be a rotary measurement encoder. In other embodiments the structured light source could be provided by a mask located over a light source, the light source may be an extended light source. As will be understood, rather than the grating being formed directly on the structured light source, the grating may be provided on a separate element spaced apart from the structured light source. Alternatively, there could be provided a grating subsequent to the scale for interacting with light from the scale to form the resultant field (e.g. fringe field, for example interference fringe, at the incremental photodetector). Furthermore, a separate light source could be provided for illuminating the incremental scale track and for forming the resultant field at the incremental photodetector. However, our inventors found that the same light source could be used, and that by using the structured light source for illuminating the incremental scale track and for forming the resultant field at the incremental photodetector is a particularly advantageous arrangement. In particular, it can reduce the number of components needed, the size of the readhead. Also, it reduces background illumination levels, which reduce signal noise, and reduces the required amplifier dynamic range needed to process the signals.
[0098] In still other embodiments, the reference mark photo detector array may be configured to detect a decrease in brightness when the readhead is in the reference mark position relative to the scale. An example embodiment of such an incremental measurement encoder may comprise a structured light source having the code 101001 (as shown in FIG. 3), the reference mark photodetector array may have parts corresponding to the code 011010 (where a “1” is used to indicate the presence of a photodetector and a “0” is used to indicate the absence of a photodetector), i.e. have parts arranged to correspond to areas of the image of the structured light source where a drop in illumination from a background level of illumination to a lower level of illumination is expected when the readhead is in the reference mark position in relation to the scale. As will be understood, in this case, the graph of FIG. 6(a) would look be the same, but be upside down, such that a negative pulse, or “dip”, in the signal from each set of photodetector elements would occur as the readhead passes the reference mark.
[0099] So far, all the examples have used a single mark (single imaging element) on the scale to form an image of the coded source. However, it is possible for a reference mark to comprise a plurality imaging elements. Each of the imaging elements forms an image of the coded source, but each image is laterally shifted by an amount determined by the position of each mark on the scale. This produces a much more complex pattern on the reference mark detectors. An example embodiment of this is schematically shown in FIG. 8.
