Position Encoder Device

Abstract

The position measurement encoder includes a scale and a readhead, the readhead including a sensor for sensing the scale, the sensor including a one-dimensional array of columnar pixels, the one-dimensional array of columnar pixels arranged such that the one-dimensional array of columnar pixels is partitioned into a plurality of rows, each columnar pixel having at least one individual sensing portion in each row arranged to contribute to an output of the columnar pixel, each row being individually activatable such that which one or more of the individual sensing portions in the columnar pixels that contribute to the output of each columnar pixel can be selectively selected and altered on a row-by-row basis.

Description

[Technical field]

[0001] The present invention relates to a position measuring encoder device. [Background technology]

[0002] A position measurement encoder device, hereinafter referred to as an "encoder device" or "position encoder", can be used to determine movement between two relatively moveable parts of a device. A position encoder typically includes a scale and a readhead, one provided on one part of the device and the other on the other part of the device. The scale can include a series of features that the readhead can read to measure position (and / or derivatives, e.g. velocity and / or acceleration) along a measurement direction of the scale. For example, the scale features could be provided on a substrate that is fixedly secured to a part of the device, or could even be integrally formed as part of the device.

[0003] So-called "incremental" position encoders may operate, for example, by "counting" positions along the length of the scale, for example from a starting position and / or from a reference marker(s) defined on the scale. As will be appreciated, the way in which "counting" is done with respect to the position of the readhead may vary from encoder device to encoder device. One way is to create a resultant field, for example a fringe field, modulated spot or image, at a detector in the readhead that changes with relative movement. For example, light from a light source (e.g. in the readhead) may impinge on a scale which diffracts the light into several diffraction orders. Optionally, optical elements in the readhead (e.g. a diffraction grating and / or lenses) may be configured to cause the diffraction orders from the scale to be recombined at the detector to create a resultant electric field. As the scale and readhead move relative to each other, the resultant electric field changes. The readhead can record and / or report movement and position by monitoring changes in the resultant electric field (e.g. movement of the fringe fields). Such a position encoder is described in US5861953. As will be appreciated, reference marks can be provided, for example embedded next to and / or within a diffraction feature of the scale, to provide a defined reference position. Such a position encoder is described in US7659992.

[0004] Furthermore, so-called "absolute" position encoders are also known, which allow the absolute position of the readhead relative to the scale to be determined without the need to count from a predefined position, such as a reference mark or an end position of the scale. Absolute position encoders usually include a scale with unique position data formed along a measuring length relative to the scale. The data can be, for example, in the form of a pseudo-random sequence or discrete code words. By reading the just-mentioned data as the scale reader passes over the scale, the scale reader can determine the absolute position. Examples of absolute position encoders are described in US7499827, US10132657 and US2012 / 0072169. It is known to use an incremental scale together with the absolute scale. Furthermore, it is also known that the absolute scale retains sufficient periodicity such that the scale can be used as a periodic incremental scale (for example as described in US7499827). In any case, said incremental scale can be used, for example, to fine-tune the determined absolute position. For example, after the absolute position has been determined at start-up, the relative position of the readhead and scale can then be measured by "counting" changes in position using the incremental scale. The incremental scale can be read in the same way as described above, for example by analysing the resultant electric field produced (at a sensor in the readhead) by recombination of the diffraction orders produced by the scale.

[0005] It is known to provide a sensor for reading a scale which comprises a one dimensional array of elongated pixels, for example as shown and described in relation to US7659992, US10670431 and WO2010 / 128279. Elongated pixels are particularly useful when detecting elongated one dimensional scale features as increasing the length increases the photometry of the system. Compared to the pixels and photodiodes used in general purpose camera sensors, sensors used in readheads for reading scales are known to have pixels which comprise unusually long photodiodes. For example, the pixels of the one dimensional sensor in the RESOLUTE readhead available from Renishaw plc each comprise one photodiode approximately 1mm long and approximately 7μm wide (ratio of approximately 143:1), with the length of the pixel being measured perpendicular to the measurement direction of the scale / readhead. To improve the charge lead-off time, rather than tapping off the photodiode charge via a single readout line, sensors used in RESOLUTE readheads are arranged to tap off the charge of each photodiode at a number of different points along its length via a number of tap-off points connected to the photodiode along its length (in particular via eight tap-off points) to reduce the distance between the light sensitive points of the photodiodes to the readout line. It is further known to provide more pixels from a RESOLUTE readhead than are required. For example, RESOLUTE includes 320 pixels but only 256 pixels are actually required and used to acquire an image of the scale (for example as described in WO2010 / 128279). Summary of the Invention

[0006] The present invention relates to an improvement in a sensor used in a readhead for reading a scale.

[0007] According to a first aspect of the present invention, there is provided a position measuring encoder including a scale and a readhead, the readhead including a sensor for sensing the scale, the sensor including a one-dimensional array of columnar pixels, the one-dimensional array of sensors / columnar pixels being arranged such that it is partitioned into a number of rows, each columnar pixel having at least one individual sensing section in each row arranged to contribute to the output of the columnar pixel. Preferably, each row of individual sensing sections (of the one-dimensional array of sensors / columnar pixels) is individually activatable such that which one or more (in other words "which one or many") of the individual sensing sections in the columnar pixels that contribute to the output of each columnar pixel can be selectively selected and changed on a row-by-row basis. In other words, preferably all individual sensing sections in the same row are collectively activatable on a row-by-row basis. Thus, in other words, each row would be individually activatable such that a subset of the rows of the individual sensing portions that contribute to the output of each columnar pixel can be selectively selected and altered (as will be appreciated, a subset may include one, some, or even all of the rows, as compared to a proper subset which may not include all of the rows).

[0008] It has been found to be beneficial to provide a sensor having an array of columnar pixels partitioned into individual (i.e. separate) sensing portion rows. For example, as described in more detail below, such an arrangement can provide greater flexibility in choosing which area (i.e. which row or rows) of the columnar pixel array contributes to reading the scale, and / or can provide faster sensor readout. The above arrangement can be used to provide a number of different advantages, including but not limited to improved ease of manufacture and / or installation, and / or improved encoder performance, e.g. reduced position error.

[0009] Suitable scale signals that can be sensed by the sensor include magnetic, optical, volumetric, or inductive scale signals. As will be appreciated, in the case of an optical position measurement encoder, the readhead can include a light source configured to illuminate the scale. The optical position measurement encoder could be a reflective optical position measurement encoder (where the light source and the sensor are on the same side of the scale) or a transmissive optical position measurement encoder (where the light source and the sensor are on opposite sides of the scale). As will be appreciated, references to "light" and "optical" herein encompass electromagnetic radiation (EMR) from infrared to ultraviolet. For example, the light source could be an infrared light source.

[0010] Optionally, the scale signal could include / be referred to as a composite electric field. The composite electric field could include fringes (e.g. fringe fields). The fringes could include interference fringes. Thus, the scale could include a series of features arranged to diffract light. Optionally, the readhead includes one or more diffraction gratings for generating interference fringes. For example, the one or more diffraction gratings could interact with light directed to / from the scale to generate interference fringes. Optionally, the interference fringes are generated by recombination of light in diffraction orders from the scale and the diffraction grating (optionally in those orders).

[0011] The sensor could also be arranged in a conjugate plane of the scale (e.g. such that an image of the scale is formed on the sensor). Thus, for example, optionally the scale signal / resultant electric field includes an image of the scale. Thus, the readhead could be configured to image the scale onto the sensor. Optionally, the readhead includes one or more optical elements (e.g. one or more lenses) configured to form an image of the scale on the sensor. Suitable optical elements include refractive optical elements (e.g. cylindrical / spherical / Fresnel lenses) or diffractive optical elements (e.g. Fresnel zone plates). Optionally, the sensor is configured to capture an image. As will be appreciated, a one-dimensional array of pixels will capture a one-dimensional image. Optionally, the one or more optical elements could be configured to form only a one-dimensional image on the sensor. However, what has just been stated need not be the case and the one or more optical elements could be configured to form a two-dimensional image on the sensor. In fact, it may be preferred that the one or more optical elements form a two-dimensional image on the sensor.

[0012] The position measuring encoder could include an incremental position measuring encoder. Thus, the scale could include an incremental scale. The scale could include at least one track including a series of (generally) periodically arranged features. As will be appreciated, one or more reference features could be provided adjacent to or embedded within the at least one track.

[0013] The position measuring encoder may be an absolute position measuring encoder. Thus, the scale may include an absolute scale. In other words, the scale may include features that define absolute position information. The scale may include features that define a series of unique absolute positions. The features that define the absolute position information could be included in at least one track. In addition to the features that define the absolute position information, the scale may include features that define incremental positions. The features that define the incremental position information could be the same features that define the absolute position (e.g., absolute position information could be embedded within the incremental position information, such as described in US7499827). Optionally, the scale includes a separate track that includes a series of generally periodically arranged features that can be used to determine the incremental position information.

[0014] In either case (either absolute or incremental), the signal sensed by the sensor can be used to measure the position (or derivative) of the readhead along the measurement direction of the scale, as described above in the background section of this specification.

[0015] Preferably, the scale is a one-dimensional scale.

[0016] As will be appreciated, there are various ways in which the output of the columnar pixels may be read / accessed. For example, the sensor could be configured such that during use / operation, the output of the columnar pixels is continuously streamed to a downstream device / component (e.g., a processing device / component that uses / processes / converts the output). Additionally / alternatively, the encoder / sensor could be configured such that the outputs of multiple columnar pixels are combined. For example, the sensor could include an electrograting, which is a sensor that includes two or more interdigitated / interleaved sets of pixels, each set configured to detect a different phase of the signal falling on the sensor. In this case, the outputs of pixels of the same set could be combined such that their outputs are provided as one output signal to a downstream device / component (e.g., a processing device / component).

[0017] As will be appreciated, references herein to "processing devices" / "processors" / "processing components" and the like are intended to include custom processing devices configured for a particular application (e.g., field programmable gate arrays "FPGAs"), as well as more general purpose processing devices that can be programmed (e.g., via software) according to the needs of the application for which it is used. Suitable processing devices thus include, for example, a Central Processor Unit (CPU), a Field Programmable Gate Array (FPGA), or an Application Specific Integrated Circuit (ASIC), or the like.

[0018] Preferably, each columnar pixel has a shared / common / single signal storage area (e.g., has an associated shared / common / single signal storage area) for storing signals from a plurality (e.g., all) of the individual sensing portions in the columnar pixel. The signal storage area could be referred to as an accumulating signal storage area. The signal storage area could be a charge storage area. Thus, each columnar pixel could include a shared / common / single charge storage area for storing / "accumulating" charges from a plurality (e.g., all) of the individual sensing portions in the columnar pixel. The sensor could be configured such that each of the at least one individual sensing portion in each row can contribute a signal (e.g., charge) stored (e.g., "accumulated") in the signal (e.g., charge) storage area of ​​the columnar pixel. Preferably, the sensor could be configured such that each signal (e.g., charge) storage area can be selectively addressed and read out. As will be appreciated, what has just been described includes configurations in which the storage areas of the columnar pixels are configured to be automatically read in order (e.g., sequentially).

[0019] If the columnar pixel includes a signal (e.g., charge) storage area, preferably each row (of individual / separate sensing units) is individually activatable such that which one or more of the individual sensing units in the columnar pixel contribute to the signal (e.g., charge) stored in the signal (e.g., charge) storage area of ​​each columnar pixel can be selectively selected and changed row by row. What has just been said could be achieved, for example, by each individual sensing unit including its own switch / gate, e.g., a transmission / transfer gate. Thus, the switch / gate can be used to control which of the individual sensing units of the columnar pixel transfers / reads out a signal to the signal storage area of ​​the columnar pixel. As mentioned above, the sensor can be configured such that the switches / gates for the individual sensing units in the same row are collectively activatable row by row.

[0020] Preferably, the individual sensing parts of a columnar pixel can be read (transfer signals) simultaneously. For example, if present, the individual sensing parts of a columnar pixel can transfer signals to the signal storage area of ​​the columnar pixel simultaneously. For example, they could be arranged / connected in parallel to the signal storage area of ​​the columnar pixel. Each individual sensing part could include its own switch / gate, e.g. a transmission / transfer gate. Thus, the switch / gate can be used to control which of the individual sensing parts of the columnar pixel transfers / reads a signal to the signal storage area of ​​the columnar pixel. As in the previous paragraph, the switches / gates for the individual sensing parts in the same row can be connected such that they can be activated collectively, row by row.

[0021] As will be appreciated, columnar pixels will be elongated in configuration, and the invention may be particularly useful for sensors in which the ratio of pixel photosensitive length to photosensitive width is at least 10:1, such as at least 50:1, for example at least 100:1.

[0022] Optionally, the sensor is partitioned into at least four rows, optionally into at least six rows, such as at least eight rows.

[0023] Alternatively, the sensor comprises at least 50 columnar pixels, such as at least 100 columnar pixels, in particular at least 256 columnar pixels, such as at least 500 columnar pixels.

[0024] Optionally, the sensor is configured such that the ratio of columnar pixels to the number of rows is 10:1 or greater (ie, there are at least 10 times as many columnar pixels as there are rows).

[0025] Each sensing portion may include a photodetector (e.g., a photodiode). Preferably, all individual photodetectors in a columnar pixel are configured to detect the same wavelength range.

[0026] Optionally, the columnar pixel has only a single individual sensing portion (e.g., a photodetector) in each row arranged to contribute to the columnar pixel's output (e.g., to the accumulated charge stored in the columnar pixel's charge storage region).

[0027] According to another aspect of the invention, there is provided a method of operating a position measurement encoder as described above, comprising obtaining a scale reading by reading, for each of a plurality of columnar pixels, the output generated by the individual sensing portions (e.g., the charge accumulated in the charge storage region of the columnar pixel) in a first true subset of the row.

[0028] The method could further include obtaining another reading of the scale by reading, for each of the plurality of columnar pixels, the output generated by the individual sensing portions (e.g., the charges stored in the charge storage regions of the columnar pixels) in a second real subset of rows distinct from the first real subset. As would be appreciated, the method could include obtaining further readings of the scale by reading the output generated by the individual sensing portions (e.g., the charges stored in the charge storage regions of the columnar pixels) in further / other real subsets of rows (e.g., a third, fourth or more real subsets of rows). Optionally, but not necessarily, there could be overlapping rows in the real subsets (e.g., the first and second real subsets could include at least one row in common).

[0029] The method may include processing, e.g. comparing, readings obtained from the first and second true subsets to determine at least one characteristic of the position measurement encoder. The at least one characteristic could include at least one of: radius of the scale, relative configuration of the readhead and scale, quality of the scale reading.

[0030] The rows to be included in the first proper subset of rows for a subsequent reading could be selected (e.g. automatically) based on parameters determined from at least one previous reading of the scale. As will be appreciated, what has just been said need not necessarily be the case. For example, the rows to be included in the first proper subset of rows for a subsequent reading could be selected based on the type of scale being used (e.g. if it is known that the readhead will be used with a small face-read disc, the readhead could be configured to use only the middle set of rows).

[0031] The method may include operating the readhead to cause the sensor to repeatedly obtain readings of the scale, simultaneously monitoring at least some of the readings, and based thereon automatically adapting which subset of rows is used to contribute to the output of the columnar pixels (e.g. accumulating charge in a charge storage region) for one or more subsequent readings of the scale.

[0032] According to another aspect of the invention, there is provided a sensor comprising a one-dimensional array of columnar pixels, the one-dimensional array of sensors / columnar pixels being arranged such that the one-dimensional array of sensors / columnar pixels is partitioned into a number of rows, each columnar pixel having at least one individual sensing section in each row arranged to contribute to the output of the columnar pixel. Preferably, each row of individual sensing sections (of the one-dimensional array of sensors / columnar pixels) is individually activatable such that which one or more of the individual sensing sections in the columnar pixels that contribute to the output of each columnar pixel can be selectively selected and modified on a row-by-row basis. In other words, preferably, all individual sensing sections in the same row are collectively activatable on a row-by-row basis. As will be appreciated, the features described above in relation to the other aspects of the invention are also applicable to the aspect just described.

[0033] Aspects of the present invention will now be described, by way of example only, with reference to the following drawings, in which: [Brief description of the drawings]

[0034] [Figure 1] 1 shows an encoder device including a scale and a readhead. [Diagram 2] FIG. 2 is a plan view to scale of FIG. 1. [Diagram 3] FIG. 2 is a schematic diagram of the optical components of the readhead of FIG. 1; [Figure 4]FIG. 2 is a schematic diagram of the electronic components of the readhead of FIG. 1; [Diagram 5] FIG. 2 is a schematic diagram of a sensor of a readhead according to the present invention; [Figure 6] FIG. 6 is a detailed view of two of the columnar pixels of the sensor of FIG. 5. [Figure 7] 1 is a top view of a rotary disk encoder apparatus including a face-read disk scale and a readhead. [Figure 8] FIG. 8 is a schematic diagram of an image of two adjacent scale features of the scale of FIG. 7. [Figure 9] FIG. 2 is a schematic diagram of an image of a series of scale features of a face-read disk scale falling on a sensor of a readhead according to the present invention; [Figure 10] 10 is a graph illustrating the distance between adjacent scale features as determined by two images acquired with different true subsets of a row of columnar pixels of the sensor of FIG. 9; [Figure 11] FIG. 9 shows the sensor and image array with rows A, B, G, and H highlighted to illustrate that they can be invalid. [Figure 12] Figure 2 is a schematic diagram of an image of a scale feature falling on a sensor of a readhead according to the present invention as the scale and readhead are yawed relative to each other; [Figure 13] 13 is a graph illustrating positions determined along a scale as determined by two images acquired with different true subsets of a row of columnar pixels of the sensor of FIG. 12; [Figure 14] 11A-11C are plan views of different positions for a readhead along the length of a linear scale mounted in a curved configuration; [Figure 15]Figure 15(a) illustrates generally a rotary encoder including an edge-reading ring scale and readhead, and Figure 15(b) illustrates generally the yaw effect that the readhead of the rotary encoder of Figure 15(a) would experience if the ring scale were mounted so that there was a swash imparted to the rotation. [Figure 16] Figure 11 is a schematic illustration of an image of a scale feature which falls only partially on a sensor of a readhead according to the present invention due to lateral misalignment between the scale and readhead; [Figure 17] 17 is a graph illustrating signal strength acquired by different rows for the sensor and image array shown in FIG. 16. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] 1-3, one embodiment of the present invention is shown in an embodiment comprising an absolute encoder 100 including a readhead 200 and a scale 300. The readhead 200 and scale 300 are provided / mounted respectively to first and second parts of a machine (not shown) that are movable relative to one another along an X axis. In the embodiment described, the scale 300 is a linear scale. However, it will be appreciated that the scale 300 could be other types of scale, such as for example a rotary scale. As will be appreciated, the readhead 200 can be used to measure the relative position (and / or derivatives, e.g. velocity and / or acceleration) of itself and the scale 300 along the X dimension and thus can be used to provide a measurement of the relative position (and / or derivatives, e.g. velocity and / or acceleration) of two moveable parts of the machine along the X dimension.

[0036] The readhead 200 communicates with an external device, such as a controller (not shown), via wired and / or wireless communication channels. The readhead 200 could be configured to report signals from its sensors to the external device, which then processes the signals to determine position information. Additionally or alternatively, the readhead 200 could itself process signals from the detectors and send position information to a controller.

[0037] The signal(s) output by the readhead 200 can take many different forms. For example, as is known in the art of position measurement encoders, a readhead may output digital quadrature (A, B) signals, analogue quadrature (SIN, COS) signals and / or serial data representing position information.

[0038] In another embodiment, an intermediate unit, for example an interface unit, can be located between the readhead 200 and an external device (e.g. a controller) as mentioned above. The interface unit can facilitate communication between the readhead 200 and the external device. For example, the interface unit could be configured to process the readhead signals and provide position information to the external device (e.g. in the form of digital square wave (A, B) signals, analog square wave (SIN, COS) signals, and / or serial data).

[0039] In the embodiment described, the scale 300 is an absolute scale and includes a track 302 having a series of reflective lines 304 and non-reflective lines 306 extending perpendicular to the measurement direction X. The reflective lines 304 and non-reflective lines 306 are typically arranged alternately with a predetermined period (i.e., defining a particular spatial frequency). However, select non-reflective lines 308 are missing from the track 302 to form a discrete codeword by encoding absolute position data in the track 302. Further details of the above absolute scale and how absolute position information is encoded within the tracks are described in International Patent Application No. PCT / GB2002 / 001629 (Publication No. WO2002 / 084223), the contents of which are incorporated herein by reference. Although the scale 300 includes only a single track 302, it could include multiple tracks. For example, separate incremental or separate absolute tracks could be provided in addition to the tracks shown, if desired.

[0040] As will be appreciated, absolute position data could be encoded in the track 302 by, as, or in place of missing reflective lines 304 or missing non-reflective lines 306. Moreover, absolute position data could be embedded in the track 302 without adding or removing reflective or non-reflective lines 304 or 306. For example, the width of the lines or the distance between the lines could be varied to embed absolute position data in the scale 300. Still further, rather than providing discrete code words, absolute data could be embedded in the form of a pseudo-random bit sequence (e.g. as described in European Patent No. 0503716). In another aspect, the scale 300 could include an incremental scale such that the encoder device is an incremental encoder. As will be appreciated, one or more reference marks are provided next to or embedded within the incremental scale track.

[0041] As illustrated in Figure 3, the readhead 200 includes a light source 202 (which in this embodiment includes light emitting diodes (LEDs)), a lens 204, a one-dimensional sensor 206, and a window 208. In the described embodiment, the sensor 206 includes 512 columnar / elongated pixels whose lengths extend parallel to the lengths of the reflective and non-reflective lines 304 and 306 of the scale (which are perpendicular to the measurement direction of the scale 300). Further details of the sensor 206 will be described in greater depth in relation to Figures 5 and 6.

[0042] Light emitted from the light source 202 passes through the window 208 and falls on the scale 300. Light reflected by the reflective scale features 304 passes through the window 208 and back through the lens 204, focusing the reflected light onto the sensor 206. In the described embodiment, the sensor 206 is located in a conjugate plane of the scale 200, such that an image of a portion of the scale 300, in particular a portion of the track 302, is formed on the sensor 206. Figure 3 is a schematic representation of an optical arrangement of a readhead configured to image the scale. Further details of suitable optical arrangements can be found, for example, in WO2010 / 116144 and WO2021 / 094456, the contents of which are incorporated herein by reference.

[0043] 4, the read head 200 further includes a processor 224, an analog-to-digital converter (ADC) 230, a memory device 232 in the form of an Electrically Erasable Programmable Read-Only Memory (EEPROM) or flash memory, and an interface 238. As will be appreciated by those skilled in the art, the read head 200 can include other suitable electrical components, such as amplifiers, drivers, etc., which are omitted from FIG. 4 for purposes of clarity.

[0044] The sensor 206 is connected to the processor 224 such that the processor 224 can receive a digitized image of the intensity of light falling across the sensor 206. As will be appreciated, an ADC 230 (which could be separate from the sensor 206 or integrated with the sensor 206) can be used to digitize the sensor output. Additionally, the sensor 206 can be directly connected to the processor 224 such that the sensor 206 can be instructed to take a snapshot of the intensity falling across it upon request by the processor 224. The processor 224 is connected to a memory 232 such that it can store and retrieve data for use in processing the sensor output to determine position information. An interface 238 is connected to the processor 224 such that the processor 224 can receive requests from and output results to an external device (not shown) via line 240. As will be appreciated, FIG. 4 is a schematic example of one readhead configuration and it is not necessary that the identified components are distinct components. For example, all of the components shown in FIG. 4, or some combination, could be provided by, for example, one or several ASICs.

[0045] FIG. 5 illustrates a schematic configuration of a sensor 206 according to one embodiment of the present invention. As shown, the sensor 206 includes 512 columnar pixels 210, labeled 1 through 512 in FIG. 5. Each columnar pixel is individually addressable. In accordance with the present invention, rather than each columnar pixel 210 including a single elongated photodiode 1 mm long and 3.6 μm wide, each columnar pixel is partitioned into eight rows 212, labeled A through H in FIG. 5, and an individual photodiode 214 is provided in each row of each columnar pixel. Thus, in the embodiment described, each columnar pixel 210 is composed of eight separate photodiodes 214, each of the eight photodiodes of a columnar pixel being approximately 125 μm long (and 3.6 μm wide). Each columnar pixel 210 is configured such that each of the photodiodes 214 can contribute to the columnar pixel output. In the described embodiment, each columnar pixel includes a Charge Storage Region (CSR) 216 (which could include, for example, a Floating Diffusion (FD) node) in which charge from each of the columnar pixel's photodiodes can accumulate. Thus, when a columnar pixel 210 is read out, the value provided by the columnar pixel is the accumulated charge stored in the CSR.

[0046] Sectioning the columnar / elongated pixel into multiple shorter photodiodes can make the sensor readout faster compared to sensors where each columnar pixel includes only one long photodiode. This could be for several reasons. For example, longer photodiodes require longer time to read out the charge compared to shorter photodiodes. Also, for example, sectioning the columnar / elongated pixel into multiple shorter photodiodes can provide greater design freedom, allowing design choices that facilitate shorter readout times, such as each photodiode including its own integration / dedicated transfer gate. Thus, a sensor constructed according to the present invention can maintain the good photometric advantages of elongated pixels and can facilitate reduced readout times, especially when the charge from each photodiode of the columnar pixel can be read out and stored simultaneously in the CSR 216.

[0047] FIG. 6 illustrates the configuration of the columnar pixel 210 of FIG. 5 in more detail. In particular, FIG. 6 illustrates columnar pixels number 1 and 2 of FIG. 5. As shown, each columnar pixel 210 includes eight individual photodiodes 214, each connected to a CSR 216 for the columnar pixel. In the particular embodiment just described, each row (A-H) of photodiodes 214 is individually activatable. As shown, each photodiode 214 of a columnar pixel 210 is connected to the columnar pixel's CSR 216 via a (transmission) switch / gate, in this embodiment a switch transistor 218. When the switch transistor 218 of any given photodiode 214 is closed, light that strikes the photodiode 214 causes a charge to be stored in the columnar pixel's CSR 216, in particular in this embodiment a capacitor 220. According to a particular embodiment of the present invention, any combination of rows (A-H) can be turned on (by activating a line to close the switch transistor 218) such that charge from the photodiodes 214 of the row is stored in the CSR 216 of the columnar pixel, specifically in the present embodiment in the capacitor 220 of the columnar pixel. As illustrated in FIG. 6, the switch transistors 218 of the photodiodes 214 of the same row across all columnar pixels (in other words, all of the photodiodes of the sensor of the same row, e.g., row A) are connected to be turned on or off collectively / "as one". While FIG. 6 explicitly depicts the switch transistors 218 of row A being connected together via a line, for purposes of clarity of depiction, dashed dotted lines are used to generally illustrate that the switch transistors 218 of the other rows are connected together "as one" (i.e., the switch transistors 218 of row B are connected together "as one", the switch transistors 218 of row C are connected together "as one", etc.). As will be appreciated, it may not be practical to have a single line connecting all the switch transistors in a row (e.g., due to capacitance build-up) and it may be necessary to use fan-out circuitry to connect multiple switch transistors together.Nonetheless, all switch transistors 218 in any given row are configured to be activated or deactivated collectively / "as one."

[0048] Each columnar pixel 210 can be addressed and read via an address switch 222. Closing the address switch 222 causes the voltage stored in the CSR 216 to be output via a source follower amplifier and ADC to an external processor via output lines 225A, 225b. Each columnar pixel 210 can be individually / separately addressed and read in the same manner. Once a columnar pixel 210 is read, the charge / voltage stored in the columnar pixel's CSR 216 is reset by a reset signal RST closing the reset switch 226, resetting the capacitor 220 to a known, predetermined voltage V RST The columnar pixels may be reset by setting the pixel count to 0. The above reset could be done for all columnar pixels simultaneously. As will be appreciated, each columnar pixel could have its own output line, as illustrated in FIG. 6, or some or all of the columnar pixels could share one or more output lines (e.g., so that the outputs are read out and presented serially).

[0049] As explained above, in the embodiment of FIG. 6, each row (A-H) of photodiodes is individually activatable such that which one or more of the individual photodiodes 214 in a columnar pixel 210 contribute to the accumulated charge stored in the CSR 216 of each columnar pixel can be selectively selected and changed on a row-by-row basis. In other words, the sensor is configured such that all photodiodes in the same row are collectively activatable on a row-by-row basis. What has just been said does not necessarily have to be the case. For example, the sensor could be configured such that all photodiodes of a columnar pixel 210 are permanently connected to the CSR 216 of the columnar pixel such that the photodiodes cannot be selectively turned on / off. However, providing rows of selectively activatable photodiodes, as in accordance with the embodiment of FIG. 6, can provide many advantages, including, for example, determining information about a scale used with the readhead (such as, for example, radius), improving assembly and / or manufacturing of the readhead, improving position measurement and / or encoder performance, more detailed examples of which will be explained in more detail immediately below.

[0050] Being able to determine information about the scale used with the readhead is useful when the scale is a rotary scale, particularly when the readhead can be used with scales of different diameters. For example, in the case of a rotary encoder, it is often necessary / desirable to convert position information determined by the readhead into angular information. In the case of an absolute encoder, the conversion just mentioned is often performed by the readhead or by an interface unit between the readhead and an external processing device / apparatus (e.g. a computer / controller device / apparatus), although it is possible for an external processing device / apparatus to perform the conversion (e.g. in incremental systems it is more common for an external processing device / apparatus to perform the conversion). In any case, in order to perform the conversion, it is necessary to know some information about the scale with which the readhead is used, such as for example the size (e.g. radius / diameter) of the disc and / or the nominal number of lines of the scale track, or some related information. As will be appreciated, in the case of an absolute encoder, the nominal number of lines may not be the same as the actual number of lines, for example if lines are removed from the scale track to encode absolute data in the scale. Thus, in the above case, the number of lines would be the number of lines that would have been provided on the scale track if the missing lines had not been omitted.

[0051] The present invention allows the encoder device (e.g., readhead) to automatically determine the above information from the scale track features itself. For example, a face-read rotary scale typically has lines extending along a radial direction. FIG. 7 is a plan view of a rotary encoder device 100a including a readhead 200 and a rotary scale disk 300a having a scale track 302a on one of its faces. In the embodiment shown, the scale track 302a includes a series of reflective and non-reflective radially extending lines. The reflective and non-reflective lines are typically alternated with a predetermined period (i.e., defining a particular spatial frequency). However, select non-reflective lines are missing from the track 302a to form a discrete codeword by encoding absolute position data in the track 302a.

[0052] For a given line width, the number of lines in the scale track 302a will depend on the radius of the scale disk 300a. Also, due to the radial nature of the lines in the scale track 302a, the lines in the image of the scale track will spread further apart from each other along the radial direction. The smaller the radius of the scale disk, the greater the spreading just described. The spreading just described can be quantified by calculating the image period / spatial frequency at various points along the radial direction, for example by obtaining multiple images with different rows of the sensor 206 active. For example, FIG. 8 illustrates diagrammatically an image of two adjacent non-reflective lines in the scale track 302a, the center of the image being at the read radius R of the disk scale. θ is equal to the angular spacing between each line with a distance Δx between them. If the radial direction of the read position changes from the initial R read position, Δx also changes. What has just been described is related by the following equation:

[0053]

number

[0054] Encoders usually do not specify the angle between lines, but instead specify the nominal number of lines around the full circumference of a circle (in other words, the nominal number of lines "L"). Therefore,

[0055]

number

[0056] Δx can be calculated by the encoder by taking the Fourier transform of the image and determining the period of the dominant carrier spatial frequency, from which Δx is known.

[0057] In the sensor of the present invention, it is possible to vary along the radial direction where the readhead reads the scale by enabling and disabling certain rows of the sensor. For the example just given, and with reference to Figure 9 (where the radial spread of the scale features has been exaggerated for convenience of example), the top half rows (rows A to D) could form a first real subset of rows taking a first image of the scale track, and the bottom half rows (rows E to H) could form a second real subset of rows taking a second image of the scale track.

[0058] The centers of the two images are spaced by four rows of photodiodes, which is 4 x 0.125 mm = 0.5 mm for the exemplary sensor shown in Figure 5. Plotting the calculated Δx versus these dimensions provides the graph shown in Figure 10. The slope of the best-fit line is equivalent to:

[0059]

number

[0060] It is therefore possible to use the gradient just described to determine the (e.g. nominal) number of lines (L) to detect what scale disk is given. What has just been described is beneficial as it means that generic disk readheads can be manufactured, stocked and shipped to customers which perform the process just described at installation / start-up to determine the nominal number of lines to read and program themselves accordingly. The above approach is much more convenient and fail-safe compared to programming the readhead for a particular disk at the time of manufacture / shipment and / or requiring the customer / installer to manually program the readhead at the time of installation. What has just been described avoids the need to configure the encoder device (such as the readhead) at the manufacturing site to work with a particular size disk and therefore avoids the need to stock readheads for use with particular size disks.

[0061] Another advantage of the present invention when used with face-read rotary scale disks is explained as follows: The smaller the disk diameter, the greater the amount of radial spread of the lines. It is desirable to have a wide sensing area (e.g. optical footprint) on the scale to increase the reliability and accuracy of position measurements. What has just been mentioned is the amount of scale lines visible in the captured image. However, for a wide radial spread, the image will be blurred at the edges because the imaged lines will fall on multiple adjacent columnar pixels 210. One solution could be to manufacture and stock readheads with different sized sensors so that readheads with smaller sensors can be used with smaller diameter face-read scale disks, but with the present invention it is possible to disable certain rows to shorten the columnar pixel length. For example, with reference to FIG. 11, disabling rows A, B, G and H such that only a true subset of the photodiodes in rows C to F are used to sense the scale would shorten the active / sensing length of the columnar pixels 210, thereby improving the visibility of the sensed image in the example just mentioned.

[0062] Another advantage of the readhead incorporating a sensor according to the invention, and which is beneficial for both linear and rotary scales / encoders, will now be described with reference to Figures 12-15. Figure 12 shows diagrammatically an image of the scale track of Figures 1 and 2 falling on the sensor 206. As shown, the scale track lines 304 / 306 are misaligned by the columnar pixels 210, in this case because the readhead 200 is "yawed" with respect to the scale 300, in other words the misalignment is due to an inadvertent rotation about the z-axis. The angular misalignment can be measured by measuring the position of the readhead from images taken by different rows 212 of the sensor 206. For example a first image can be taken by a first true subset of rows, for example the top four rows A-D, and a second image can be taken by a second true subset of rows, for example the bottom four rows E-H. Plotting the position determined by each of the first and second images relative to the geometric center of the effective sensing portion of each of the images (e.g., +0.25mm and -0.25mm) gives the graph illustrated in Figure 13. The slope of the graph is related to yaw by the following equation: Yaw angle = atan(gradient) (4) The yaw angle, or a related parameter or signal, could for example be used during manufacture and / or installation / assembly of the encoder arrangement. For example, during manufacture, the yaw angle (or a related parameter or signal) could be output and used to determine whether the sensor is sufficiently aligned within the readhead. During installation / assembly, the yaw angle (or a related parameter or signal) could be output and used to determine whether the readhead is correctly aligned with respect to the scale. Furthermore, the yaw angle, or a related parameter or signal, could also be used to determine how to correctly align the sensor / readhead. The above corrections may be made automatically or manually. Thus, as with other aspects of the invention, the yaw angle, or a related parameter or signal, could be used as part of a feedback loop to aid in assembly.

[0063] Furthermore, as with other aspects of the invention, the yaw angle, or a related parameter or signal, could also be used to correct the measurement signal / compensate for misalignment of the scale and / or readhead. For example, referring to FIG. 14, if the linear scale 300 is not mounted on a straight line, the scale will be curved. What has just been said can be measured by measuring the yaw angle of the readhead 200 with respect to the scale 300 as the readhead moves along the length of the linear scale. This curvature of the scale 300 can not only be measured, but can also be compensated for. For example, by assuming that the readhead 200 moves on a straight line as shown by the straight line 201 in FIG. 14, by measuring the relative yaw of the scale and readhead at multiple points along the length of the scale (e.g. each of the points indicated by the dots on the arrow 201 - five of these points are illustrated diagrammatically by the readhead 200 in dashed outline), it is possible to quantify how long the scale is (measured along its centre line) compared to the straight linear distance the readhead has actually moved, and compensate for the resulting error.

[0064] 15(a) and (b), in a similar manner, any swash of an edge-read ring scale 300b (i.e. a rotary scale in which the scale features are provided on the outward circumferential edge / side of a ring or disc shaped body) can be measured as it appears as a yaw error in the image read by the readhead, as illustrated diagrammatically by FIG. 15(b). Thus, determining the yaw measurement as the ring scale 300b rotates relative to the readhead 200 can be used to compensate for any measurement error caused by swash of the ring scale. As will be appreciated, swash can be seen as a periodically reciprocating axial movement of the edge of the ring scale as it rotates about its axis, and can be caused, for example, by any non-planarity of the ring scale and / or by a non-perpendicular mount with respect to the axis of rotation.

[0065] Another advantage of a readhead incorporating a sensor according to the invention will now be explained with reference to Figures 16 and 17. The scale lines are written to a finite length and ideally the image of the scale lines should have the scale lines extending across the entire range / width of the sensor (i.e. across all the rows). If the readhead is offset too far laterally (perpendicular to the measurement direction) with respect to the scale, as depicted diagrammatically in Figure 16, the scale lines will only extend part way across the range / width of the sensor. A readhead with a sensor according to the invention can detect what has just been said by taking images of a scale with different rows and determining where in the sensor width the lines are no longer imaged and therefore how far the readhead is offset in the direction perpendicular to the measurement direction with respect to the scale. For example, separate images of the scale could be taken for each row A-H and the signal strength of each image (e.g. the amplitude of the dominant spatial frequency in the Fourier transform of each image). If what has just been said was done for the example of Figure 16 and the results plotted against the y position of the centre of the image of each row, a graph similar to that shown in Figure 17 would be obtained. Figure 17 shows that it can be concluded that the sensor 206 has no signal in rows A and B, approximately half the normal signal in row C, and full signal in rows D through H, with the scale line ending half the width of the photodiode in row C of the sensor. As with other aspects, the information and / or related parameters just described could be output and / or used during manufacture, assembly / installation and / or operation of the encoder. For example, the above information / output could be used to facilitate adjustment of the sensor, readhead and / or scale to improve alignment.

[0066] Furthermore, a readhead incorporating a sensor according to the invention can be configured so that some rows of photodiodes are shut off / unused during operation. For example, it is possible to select the row that will provide the optimum signal in the image capture process. For example, with reference to Figure 16, if it is not possible or desirable to change the relative position of the readhead and scale in the dimension perpendicular to the measurement direction, image visibility can be improved by deactivating rows A to C, so that only rows D to H contribute to the image acquired by the sensor 206 during normal operation of the readhead 200 / encoder arrangement.

[0067] Another advantage of a readhead incorporating a sensor according to the invention is that the readhead (either in calibration mode or during operation) would be able to learn the location of areas of the scale that are damaged or dirty, by comparing images from individual rows, and the readhead would then be able to selectively turn off appropriate rows in the vicinity of areas identified as damaged or dirty to minimise their effect.

Claims

1. 1. A position measuring encoder comprising a scale and a readhead, the readhead including a sensor for sensing the scale, the sensor including a one-dimensional array of columnar pixels, the one-dimensional array of columnar pixels being configured such that the array is partitioned into a plurality of rows, each columnar pixel having at least one individual sensing portion in each row arranged to contribute to an output of the columnar pixel, and each row being individually activatable such that which one or more of the individual sensing portions in the columnar pixels contribute to the output of each columnar pixel can be selectively selected and changed on a row-by-row basis.

2. 2. A position measurement encoder according to claim 1, wherein the individual sensing portions in a columnar pixel can be read simultaneously.

3. 2. The position measurement encoder of claim 1, wherein each columnar pixel has a shared signal storage area for storing signals from a plurality of individual sensing portions in the columnar pixel.

4. 2. A position measuring encoder as claimed in claim 1, wherein the ratio of photosensitive length to photosensitive width of each columnar pixel is at least 10:1, and optionally at least 50:

1.

5. 2. A position measuring encoder according to claim 1, wherein the sensor is partitioned into at least four rows, selectably at least six rows, for example at least eight rows.

6. 2. A position measurement encoder as in claim 1, comprising at least 256 columnar pixels.

7. 2. The position measuring encoder of claim 1, wherein each sensing portion includes a photodetector.

8. 8. A position measuring encoder as claimed in claim 7, wherein all the individual photodetectors in a columnar pixel are configured to detect the same range of wavelengths.

9. 2. A position measurement encoder according to claim 1, wherein the sensor is arranged in a conjugate plane of the scale.

10. 2. A position measuring encoder according to claim 1, wherein the scale is a one-dimensional scale.

11. 11. A position measuring encoder according to any preceding claim, wherein each columnar pixel has only a single individual sensing portion in each row arranged to contribute to the columnar pixel's output.

12. obtaining a reading of the scale by reading the outputs generated by the individual sensing portions in a first proper subset of the rows for each of a plurality of the columnar pixels; 2. The method of claim 1, comprising:

13. obtaining another reading of the scale by reading the outputs generated by the individual sensing portions in a second real subset of the row, different from the first real subset, for each of a plurality of the columnar pixels; The method of claim 12 further comprising:

14. 13. The method of claim 12, wherein the rows to be included in the first proper subset of rows for a subsequent reading are selected based on parameters determined from at least one previous reading of the scale.

15. causing the sensor to repeatedly take readings of the scale; simultaneously monitoring at least some of said readings; and automatically adapting, based thereon, which subset of rows is used to contribute to the output of the columnar pixels for one or more subsequent readings of the scale.

13. The method of claim 12, further comprising operating the read head so that

Images