WO2010128279A2

WO2010128279A2 - Position encoder apparatus and method of operation

Info

Publication number: WO2010128279A2
Application number: PCT/GB2010/000868
Authority: WO
Inventors: Iain Robert Gordon-Ingram; Andrew Paul Gribble
Original assignee: Renishaw Plc
Priority date: 2009-05-06
Filing date: 2010-04-30
Publication date: 2010-11-11
Also published as: WO2010128279A3; GB0907842D0

Abstract

A method of configuring a position encoder readhead for determining position information in at least one measuring dimension. The readhead comprises an array of sensor elements for sensing position features on a scale which extends along the at least one measuring dimension. The method comprises selecting a subset of the array of sensor elements in the measuring dimension to use in a subsequent reading of the position features on a scale.

Description

POSITION ENCODER APPARATUS AND METHOD OF OPERATION

This invention relates to a position encoder apparatus and a method of operating a position encoder.

Position encoders for measuring the relative position between two moveable objects are well known. Typically, a series of scale markings are provided on one object and a readhead for reading the scale markings is provided on another. The scale markings can be formed integrally with the object or can be provided on a scale which can be secured to the object.

A position encoder is commonly categorised as being either an incremental position encoder or an absolute position encoder, hi an incremental position encoder the scale has a plurality of periodic markings which can be detected by the readhead so as to provide an incremental up/down count. For instance, such a scale is described in European Patent Application no. 0207121. Reference marks can be provided, either next to or embedded in the periodic markings so as to define reference points. For example, such a scale is disclosed in Published International Patent Application WO 2005/124282. An absolute position encoder typically measures relative displacement by a readhead detecting unique series of marks, i.e. codes, and translating those codes into an absolute position. Such a scale is disclosed in International Patent Application no. PCT/GB2002/001629.

A readhead typically comprises an array of sensor elements arranged along the measuring dimension. During readhead manufacture the sensor elements and/or other scale reading components (such as lenses in an optical system) are located in an approximate position before being subjected to a fine tuning operation. The fine tuning operation entails finely adjusting the position of the sensor elements and/or other components to ensure that the sensor elements obtain an optimum signal from a scale during use. The fine tuning operation can be omitted but this typically results in a sub-optimum signal being obtained. The invention provides an improved readhead design which avoids the need for a manual fine tuning process whilst still enabling an optimised signal to be obtained.

In particular the invention provides a method of configuring a position encoder readhead comprising an array of sensor elements for sensing position features on a scale, comprising selecting a subset of the array of sensor elements for reading of the position features on a scale.

In accordance with a first aspect of the invention there is provided a method of configuring a position encoder readhead for determining position information, the readhead comprising an array of sensor elements for sensing position features on a scale, comprising: selecting a subset of the array of sensor elements to use in a subsequent reading of the position features on a scale.

A readhead according to the present invention has more sensor elements than is required to read the scale and uses a select subset of the total available sensor elements in a subsequent reading. This provides for a certain amount of tolerance in the manufacture, installation and/or operation of an encoder readhead and therefore reduces manufacture and/or installation time and costs without impacting on readhead performance. In particular it reduces the assembly time and can simplify tooling used in assembly and/or setting up of the readhead. Rather, the readhead can be manufactured such that the array of sensor elements are only approximately located in the right place, and/or installed such that the readhead is only approximately aligned with the scale. The configuration of the readhead is then determined and the sub-array of the sensor elements used to acquire subsequent readings of position features on a scale can be selected so as to take into account the readhead 's configuration.

In particular, preferably the position encoder readhead is suitable for determining position information in at least one measuring dimension and the array of sensor elements extends along the at least one measuring dimension. Furthermore, the method preferably comprises selecting a subset of the array of sensor elements in the measuring dimension to use in a subsequent reading of the position features on a scale. Accordingly, preferably the readhead according to the present invention has more sensor elements in the measuring dimension than is required to read the scale and uses only a subset of the total available sensor elements in the measuring dimension. Accordingly, taken in the at least one measuring dimension, the subset is smaller than the full extent of the array.

The invention could be used at the manufacturing and/or installation stages, hi a particular embodiment, the method could be used at the manufacturing and/or installation stages so as to set the subset of the array of sensor elements used during the operation of the readhead (e.g. when it is being used by a controller to read and report the relative position of the readhead and a scale). Additionally or alternatively, the invention could be used during operation of the readhead. For instance, as described in more detail, a readhead according to the invention could dynamically adjust the sub-array used during operation.

The method could comprise selecting a subset of the array of sensor elements to use in a single subsequent reading of the position features on a scale. The single subsequent reading could be the next, i.e. the immediately subsequent, reading. Optionally, the method can comprise selecting a subset of the array of sensor elements to use in a plurality of subsequent readings of the position features on a scale. This will particularly be the case when the invention is used at a manufacturing and/or installation stage so as to set the subset of the array of sensor elements used during the operation of the readhead. However, this can also be the case when the invention is used during operation of the readhead. For instance, the selected subset could be used for a plurality of subsequent readings before the process of selecting a subset of the array of sensor elements to use in subsequent readings is repeated. In a particular embodiment, the readhead could have a reading mode in which it is configured to read a scale and report position information and a distinct calibration mode in which the method of the invention can be used. Accordingly, the invention could comprise putting the readhead into a calibration mode and selecting the subset of the array of sensor elements for the readhead to use in the reading mode. The subset of the array of sensor elements used by the readhead in the reading mode could be fixed until the next time the calibration mode is used to select the subset of the array of sensor elements to be used.

Preferably the method comprises determining the readhead 's configuration, hi this case, selecting can comprise selecting the subset of the array based at least in part on the readhead's configuration. The readhead's configuration can affect how a reading of position features on a scale will be detected by the array of sensor elements. In particular, the readhead's configuration can affect which group of sensor elements detect the strongest and/or clearest reading of a scale's position features. The readhead's configuration can comprise its internal configuration, for instance the position of the array of sensor elements, the quality of the array of sensor elements or other components which are used in the reading of position features on a scale (e.g. a lens in an optical encoder). The readhead's , configuration can comprise how it is configured relative to an external component, such as a scale. This could be for instance the readhead's installed configuration, which could include its initial installed configuration and/or its configuration relative to the scale during operation.

The subset of the array of sensor elements to use could be selected so as to optimise a subsequent reading of the position features on a scale. As will be understood, the selected subset of the array of sensor elements may not result in the subsequent reading of the position features being the most optimum possible reading that could be obtained. Rather, as explained in more detail below, the subset could be selected to provide an optimisation over the reading a currently selected subset would obtain. The subset of the array of sensor elements to use can be selected so as to optimise the suitability of a subsequent reading of the position features on a scale to provide position information. This can include selecting a subset of the array so as to optimise the signal quality of a subsequent reading. This can comprise selecting a subset of the array so as to optimise the signal strength of a subsequent reading and/or to minimise signal spatial distortion (e.g. positional distortions in the image). The signal strength could include at least one of the signal amplitude, signal intensity and/or the signal modulation transfer function (MTF) of a subsequent reading.

The readhead's configuration could be determined by a visual inspection of the configuration. For instance, a person assembling or testing the readhead prior to sending to a customer could use a microscope or other optical magnification device in order to ascertain the position of the array of sensor elements in the readhead.

Preferably, determining the readhead's configuration comprises acquiring a signal indicative of the readhead's configuration. The signal could be acquired by at least one detector separate to the array of sensor elements. Preferably, the readhead comprises the at least one detector. For instance, the at least one detector could comprise a dedicated rideheight and/or pitch detector, hi embodiments in which the readhead comprises a reference mark detector, the at least one detector could comprise the reference mark detector.

The signal can be acquired by at least one of the array's sensor elements. Preferably, the signal is acquired by at least some of the array's sensor elements. More preferably, the signal is acquired by a subset of at least three consecutive sensor elements. The size of the subset acquiring the signal can be substantially the same as the size of the selected subset. The signal can be acquired by substantially all of the array's sensor elements.

The method can further comprise determining position information from the signal indicative of the readhead's configuration. Accordingly, in this case the signal will be acquired by at least a subset of the array's sensor elements.

Selecting the subset can comprise a manual input of the subset to be used. This method of selection is particularly suitable during manufacture and/or initial installation of the readhead. Accordingly, the method can comprise providing an output indicative of the readhead configuration. The output could comprise the signal indicative of the readhead configuration. For instance, the signal indicative of the readhead configuration could be displayed on a display. The signal displayed could be the raw or processed version of the signal. For instance, the readhead could be connected to an external device, such as a computer or oscilloscope which can display the signal. Optionally, the readhead could comprise a display which could illustrate the readhead's configuration. The user can then analyse the display and input to the readhead the sub-array to be used.

The method can further comprise a processor processing the signal and determining the subset of the array of sensor elements. Accordingly, the subset of the array of sensor elements could be selected automatically, e.g. without user intervention. The readhead could comprise the processor.

The method can comprise analysing the signal quality. This is particularly suitable when the signal is obtained by at least some of the sensor elements. Accordingly, analysing the quality of the acquired signal can provide an indication of the suitability of the signal to provide position information, and in particular reliable and/or accurate position information. The variation in signal quality can be analysed across at least some of the sensor elements. Analysing the signal quality can comprise analysing the signal strength, for instance at least one of the signal amplitude, signal intensity and/or the modulation transfer function (MTF). Accordingly, for instance, the distribution in signal strength, for instance signal amplitude, can be analysed across at least some of the sensor elements. Analysing the signal quality can comprise analysing the signal spatial distortion of the image. Selecting the subset of sensor elements can comprise selecting the subset that provides the optimum signal quality. Preferably, the subset is an array of consecutively located sensor elements.

Optionally, selecting the subset of the array of sensor elements can comprise shifting a currently selected subset of the array of sensor elements by a predetermined number of sensor elements toward the part of the array that provides the optimum signal quality.

The sensor elements can be individually selectable. Optionally, the sensor elements are arranged into selectable groups. This might be useful for instance in embodiments in which the sensor elements are electrically combined and not available individually, such as photo-diode arrays commonly used in incremental position encoders. Each group could comprise an array of consecutively located sensor elements.

The readhead could be configured to read from all of the sensor elements in the array, but to only process the signals from the selected subset. Preferably, the readhead is configured to read signals from the selected subset of the array only, during a subsequent reading of the position features on a scale. This can significantly reduce the amount of time required to read out the signals from all of the sensor elements and thereby reduce the amount of time to calculate the current position.

Acquiring a signal can comprise the readhead reading an article placed in a reading relationship with the readhead. The article can be a scale comprising a plurality of position features. Acquiring a signal can comprise reading the position features on the scale. As will be understood, articles other than scales can be used. For instance, a calibration article could be used to assess the configuration of the readhead. The calibration article could be an imitation scale or some other non-scale type article such as a mirror or optical test pattern.

The array can be a one-dimensional array of sensor elements. Optionally, the array can be a two-dimensional array of sensor elements.

As will be understood, the position encoder readhead could be for determining position information in two measuring dimensions, for instance two orthogonal measuring dimensions, hi this case the readhead could comprise two separate arrays of sensor elements. One array could extend along one of the measuring dimensions and the other could extend along the other measuring dimension. Optionally, the readhead could comprise at least one two-dimensional array, which extends along the two measuring dimensions. Accordingly, as will be understood, a subset of sensor elements in at least one of the two measuring dimensions could be selected. Optionally, the subset could be a subset in both of the two measuring dimensions, hi other words, in each measuring dimension, the subset is smaller than the full extent of the array of sensor elements.

The sensor elements can be electromagnetic radiation ("EMR") sensitive sensor elements. Optionally the sensor elements are magnetic, capacitive or inductive sensitive sensor elements. Accordingly, the invention is suitable for all types of encoders, including optical, magnetic, capacitive and inductive.

hi particular, the sensor elements can be optical sensor elements. As will be understood, the optical sensor elements could be sensitive to EMR in the infrared to the ultraviolet range.

The readhead can comprise at least one component for providing a representation of the scale onto a plane containing the array of sensor elements. For instance, the readhead could comprise at least one lens for providing an image onto a plane containing the array of sensor elements, hi this case, the array of sensor elements can form an image sensor array.

The position features could define incremental information. The position features can define absolute position information. The position features could be contained in a single track. Optionally, the position features could be spread across multiple tracks. The scale could comprise a first series of position features defining incremental information and a second series of position features defining absolute position information.

Accordingly, this application describes a method of configuring a position encoder readhead comprising an array of sensor elements for sensing position features on a scale, comprising: acquiring a signal indicative of the readhead's configuration; and based at least in part on the signal indicative of the readhead's configuration, selecting a subset of the array of sensor elements to use so as to optimise a subsequent reading of the position features on a scale.

According to a second aspect of the invention there is provided, a position encoder readhead for determining position information in at least one measuring dimension, comprising an array of sensor elements for sensing position features on a scale, the array extending along the at least one measuring dimension, in which a subset of the array of sensor elements in the measuring dimension can be selected for reading position features on a scale.

According to a third aspect of the invention there is provided a position encoder readhead comprising an array of sensor elements for sensing position features on a scale, the readhead being configured to acquire a signal indicative of the readhead's configuration, and in which a subset of the array of sensor elements can be selected, based on the signal indicative of the readhead's configuration, so as to optimise a subsequent reading of the position features on a scale.

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

Figure 1 is a schematic side view an encoder apparatus according to the invention comprising a rotary scale and a readhead;

Figure 2 is a schematic isometric view of the encoder apparatus of Figure 1;

Figure 3a is a schematic block diagram of the various optical and electronic components of the readhead according to a first embodiment;

Figure 3b is a schematic block diagram of the various optical and electronic components of the readhead according to a second embodiment;

Figure 4a illustrates a first example footprint location of an image of the scale at the readhead's imaging plane with respect to the readhead's image sensor and the preferred imaging area;

Figure 4b illustrates a second example footprint location of an image of the scale at the readhead's imaging plane with respect to the readhead's image sensor and the preferred imaging area;

Figures 5 (a) and 5(b) illustrate a first example acquired signal and the preferred image sensor sub-array used by the readhead;

Figures 6(a) and 6(b) illustrate a second example acquired signal and the preferred image sensor sub-array used by the readhead;

Figures 7(a) and 7(b) illustrate a third example acquired signal and the preferred image sensor sub-array used by the readhead;

Figure 8 illustrates a configuration process used to set the image sensor sub-array; Figure 9 illustrates in more detail Figure 8's step of determining the optimum sub- array;

Figure 10 illustrates an example method of operation of a readhead 4 operating in accordance with the present invention;

Figure 11 illustrates in more detail Figure lO's step of sub-array optimisation;

Figures 12(a) and 12(b) illustrate a first example acquired signal and the sub-array used by the readhead; and

Figure 13 (a) and 13(b) illustrate a second example acquired signal and the sub- array used by the readhead.

Referring to Figures 1, 2, 3 and 3b there is shown an encoder apparatus 2 comprising a readhead 4, scale 6 and controller 7. The readhead 4 and scale 6 are mounted to first and second objects respectively (not shown). As will be understood, a readhead has at least one measuring dimension, i.e. the dimension in which it can detect relative movement between it and the scale. In the embodiment described, the readhead 4 has only one measuring dimension, which is illustrated by arrow B in Figures 1 to 4. Furthermore, as illustrated in Figures 3 and 4 and explained in more detail below, the readhead has an array of sensor elements 20 which extends along (and in this case extends substantially parallel to) the readhead' s measuring dimension.

The scale 6 is rotatable about axis A (which extends perpendicular to the page as shown in Figure 1) relative to the readhead. hi the embodiment described, the scale 6 is a rotary scale. Accordingly, the readhead (or controller 7) translates its measurements into rotational position measurements. However, it will be understood that the scale 6 could be a non-rotary scale, such as a linear scale. Furthermore, the readhead 4 and scale 6 are configured to provide measurement in a single dimension only. However, it will be understood that this need not be the case, and for example the scale could enable measurement in two dimensions.

hi the described embodiment, the scale 6 is an absolute scale and comprises a series of reflective 8 and non-reflective 10 lines arranged to encode unique position data along its length. As will be understood, the data can be in the form of, for instance, a pseudorandom sequence or discrete codewords.

The width of the lines depends on the required positional resolution and is typically in the range of 1 μm to 1 OOμm, and more typically in the range of 5μm to 50μm. hi the described embodiment, the width of the lines is approximately 15μm. The reflective 8 and non-reflective 10 lines are generally arranged in an alternate manner at a predetermined period. However, select non-reflective lines 10 are missing from the scale 6 so as to encode absolute position data in the scale 6. For instance, the presence of a non-reflective line can be used to represent a "1" bit and the absence of a non-reflective line can represent a "0" bit. A series of groups of markings can be used to encode a series of unique binary codewords along the scale length defining unique, i.e. absolute, position information. Further details of such a so-called hybrid incremental and absolute scale is described in International Patent Application no. PCT/GB2002/001629 (publication no. WO 2002/084223), the content of which is incorporated in this specification by this reference.

As will be understood, absolute position data could be encoded in the scale 6 by missing reflective lines 8, as well as, or instead of missing non-reflective lines 10. Furthermore, absolute position data could be embedded in the scale 6 without the addition or removal of reflective 8 or non-reflective lines 10. For instance, the width of lines or the distance between them could be varied in order to embed the absolute position data in the scale 6. As will also be understood, the invention could also be used with incremental scales, hi this case, if desired, reference marks could be provided either adjacent or embedded within the incremental scale track.

As illustrated in Figure 3 a the readhead 4 comprises a Light Emitting Diode ("LED") 12, a lens 18, a Complemenary Metal-Oxide-Semiconductor ("CMOS") image sensor 20 and a window 22. Light emitted from the LED 12 passes through the window 22 and falls on the scale 6. The scale 6 reflects the light back through the window 22 which passes through the lens 18 which in turn images the scale onto the CMOS image sensor 20 using the reflected light. Accordingly, the CMOS image sensor 20 detects an image of a part of the scale 6. The figures schematically illustrate that the CMOS image sensor 20 comprises an array of sensor elements, e.g. pixels. In the described embodiment, the CMOS image sensor 20 comprises a single row of 320 elongate pixels, although for illustrative purposes not all of the individual pixels are shown in the figures. As shown, the lengths of the pixels extend parallel to the length of the reflective 8 and non- reflective lines 10 on the scale. As will be understood, the invention is not limited to the use of one dimensional arrays and for instance a two dimensional array of sensor elements could be used instead.

The readhead 4 also comprises a CPU 24, a memory device 32 in the form of Electrically Eraseable Programmable Read-Only Memory (EEPROM) and an interface 38.

The LED 12 is connected to the CPU 24 so that the LED 12 can be operated on demand by the CPU 24. The CMOS image sensor 20 is connected to the CPU 24 such that the CPU 24 can receive an image of the intensity of light falling across the CMOS image sensor 20. The CMOS image sensor 20 is also directly connected to the CPU 24 so that the CMOS image sensor 20 can be operated to take a snapshot of intensity falling across it on demand by the CPU 24. The CPU 24 is connected to the memory 32 so that it can store and retrieve data for use in its processing. For instance, in this embodiment the memory 32 contains a plurality of lookup tables. One or more of the lookup tables will be used in the determination of the relative position of the readhead 4 and scale 6 as explained in more detail below. The interface 38 is connected to the CPU 24 so that the CPU 24 can receive demands from and output results to an external device such as a controller 7 (shown in Figure 1) via line 40. The line 40 also comprises power lines via which the readhead 4 is powered.

The readhead illustrated in Figure 3b is substantially the same as that illustrated in 3 a and like parts share like reference numerals. However, the optical arrangement of the embodiment shown in Figure 3b is slightly different. In this embodiment, the readhead 4 comprises a collimating lens 13, a beam splitter assembly 15 having a reflecting face 17 and a beam splitting face 19, and an imaging lens 21. The collimating lens 13 collimates light emitted from the LED 12 into a beam 23 which is then reflected by the splitter assembly's reflecting face 17 toward the beam splitting face 19. The beam splitting face 19 reflects the beam 23 toward the scale 6 via window 22, which then reflects the light back through the window 22 toward beam splitting face 19 which allows the reflected light to pass straight through it. The reflected light then passes through the imaging lens 21 which forms an image of the scale 6 onto the CMOS image sensor 20.

In use, the readhead 4 waits until a position request is received from a controller 7 via the interface 38. Once received the readhead 4 then operates under the control of the CPU 24 to determine the absolute relative position between the scale 6 and the readhead 4. This involves the CPU 24 causing a snapshot of the scale 6 to be taken by the readhead 6. This is effected by the CPU 24 controlling the LED 12 to temporarily emit light and also controlling the CMOS image sensor 20 to simultaneously sense and register the intensity of the pattern of light falling across it. The CPU 24 then reads from the CMOS image sensor 20 the signal that is representative of the image detected by the CMOS image sensor 20. As will be explained in more detail below, only a sub-array (comprising 256 pixels) of the entire 320 pixels of the CMOS image sensor 20 is read by the CPU 24 and used to determine position information. The CPU 24 then extracts a codeword from the signal that it has read from the sub-array of the CMOS image sensor 20. The relative position corresponding to this codeword can then either be determined by the CPU 24 (for instance by using a look-up table stored in the memory 32) and then sent to the controller 7, or the CPU 24 can simply send the codeword to the controller 7 for further processing. As will be understood, in an alternative embodiment, the raw image data read from the CMOS image sensor 20 could be sent straight to the controller 7 without any processing or analysis by the readhead 4.

As mentioned above, the CMOS sensor 20 is designed such that the number of pixels provided in the array (320 in the described embodiment) is greater than the actual number of pixels needed (256 in the described embodiment) in order to obtain enough information from the scale 6 to determine the relative position between the readhead 4 and the scale 6. This can be advantageous as it enables the sub-array of pixels to be selected manually or automatically so as to ensure that a good quality signal is obtained. This can help to compensate for inaccuracies in the building, and/or setting up, of the readhead 4. Various ways in which this can be implemented are described below.

Referring now to Figure 4a, there is shown a schematic plan view of the CMOS image sensor 20 and the footprint 50 of the image of the scale 6 formed at the CMOS image sensor 20 by the lens 18. As can be seen the size of the footprint 50 in a direction parallel to the extent of the array of sensor elements is larger than the length of the array of sensor elements. However, the image quality varies across the image and this can be due to many factors including illumination levels, variations in MTF, distortion and/or optical aberrations. As will be understood, the poorer the quality of the image the more difficult it is to extract reliable and accurate information and so it is better to process only the higher quality parts of the image. As only a subset of all the pixels are actually needed in order to obtain position information there is some flexibility in which pixels are used in reading the scale.

In the example of Figure 4a, the readhead's 4 configuration is such that the footprint 50 is positioned approximately centrally with respect to the CMOS image sensor 20 and the lens is such that the image quality is better towards the middle of the image than towards the edge. As schematically illustrated by region 52, analysis of the quality of the signal falling across the CMOS image sensor 20 is likely to show that the 256 pixels receiving the best quality signal are the middle 256 pixels. In this case, the middle consecutively located 256 pixels will be chosen to read the scale 6, leaving 32 unused pixels at either end of the CMOS image sensor 20. Box 21 illustrates the selected sub-array of pixels. As will be understood, the quality of the signal within region 52 will vary and will likely increase in quality toward the middle of the region 52.

hi the example of Figure 4b, the readhead's 4 configuration is such that the footprint 50 falls more to the right-hand side of the CMOS image sensor 20. Again, the lens is such that the image quality is better towards the middle of the image than towards the edge and so as schematically illustrated by region 52, analysis of the quality of the signal falling across the CMOS image sensor 20 is likely to show that the 256 pixels receiving the best quality signal are the 256 pixels toward the right-hand end of the CMOS image sensor and as illustrated by box 21 these, consecutively located 256 pixels will be selected to read the scale 6, leaving 64 unused pixels at the left-hand end of the CMOS image sensor 20.

Figures 5, 6 and 7 illustrate various signals that might be output by the CMOS image sensor 20 depending on the configuration and set up of the readhead 4. The Figures also illustrate the likely selected sub-array to be used in such scenarios. Figure 5(a) illustrates the signal 25 likely output from the CMOS image sensor 20 when a good quality lens is used and in which the footprint 50 of the image of the scale 6 falls substantially centrally on the CMOS image sensor 20. In this case, there is a substantially symmetric illumination distribution, with the signal being stronger toward the middle than toward the ends of the CMOS image sensor 20. Such a signal will likely be obtained by the situation illustrated, and described above, in connection with Figure 4a. Box 21 in Figure 5(b) illustrates that in this situation the middle 256 pixels of the CMOS image sensor 20 are selected to read the scale 6.

Figure 6(a) illustrates an offset illumination distribution signal 27. Such a signal could be caused by, for example, the lens 18 and CMOS image sensor 20 being offset relative to each other, a poor quality sensor (e.g. being less sensitive at one end than the other) and/or by the readhead 4 being pitched relative to the scale 6 (e.g. angled about an axis parallel to the plane of the scale and perpendicular to the measuring dimension). As shown the signal is stronger toward the right hand side of the CMOS image sensor 20 and so as illustrated by box 21 in Figure 6(b) the right-hand 256 pixels of the CMOS image sensor 20 are selected to read the scale 6 as they provide the best quality signal for processing.

Figure 7(a) illustrates a signal 29 having a substantially symmetric illumination distribution, but with better signal contrast toward the right hand side of the CMOS image sensor 2. Such a signal could be caused by, for example, imperfections in the lens 18 and/or CMOS image sensor 20. Depending on how the signal is processed, better contrast may be more important than the average signal intensity and so as illustrated by box 21 in Figure 7(b) the right-hand 256 pixels of the CMOS image sensor 20 are selected to read the scale 6. However, it may be the case that better contrast is less important than the average signal intensity in which case the middle 256 pixels of the CMOS image sensor 20 could be selected to read the scale 6.

The invention can be utilised at various times during the lifetime of a readhead 4. For instance, the invention can be used during the manufacture of the readhead 4 and before it is shipped to customers so as to identify and set the pixels that are to be used during reading operation once installed by the customer. This therefore helps to compensate for any manufacturing inaccuracies and avoids the need for the fine tuning of the position of the CMOS image sensor 20 and the lens 18. The invention could also be used during installation by the customer so as to avoid fine tuning of the relative position of the readhead 4 and the scale 6. Pitching of the readhead 4 relative to the scale 6 can cause the illumination footprint 50 to shift relative to the CMOS image sensor 20 and so without use of the invention it is necessary for the user to ensure that the pitching is minimised because otherwise the readhead 4 would be working with a sub-optimal signal. Such fine-tuning during installation can be avoided as a readhead 4 has the flexibility to cope with such pitching and still work with optimum signals. Furthermore, the encoder apparatus 2 could also be configured to monitor the signal quality during reading operations and adjust the used sub-array of pixels accordingly so as to try to maintain an optimum signal being used for processing. This could be advantageous, for example, in situations in which the relative pitch of the readhead 4 and scale 6 changes along the scale length or over time.

Figure 8 illustrates an example process 100 of how the readhead could be configured during manufacture and prior to shipping to a customer in accordance with the present invention. In this case, the method begins at step 102 with the readhead 4 being assembled. As will be understood, this could involve a complete or only a partial assembly of the readhead 4. hi either case the readhead 4 is arranged relative to a testing article and at step 104 is made to obtain a signal across the entire CMOS image sensor 20.

The testing article could be a scale, and in particular could be a scale similar to that on which it is intended to ultimately be used. For instance, if the readhead 4 is configured to be used with an absolute scale such as that illustrated in Figure 2, then the testing article could be an absolute scale. However, this need not necessarily be the case. For instance, and as is the case with the presently described embodiment, the testing article could be an incremental scale. This can be advantageous because an absolute scale can make analysing the quality of the signal more difficult due to irregularity of the features which are needed to encode position information. Further still, the testing article need not be a scale at all. For instance, it could be a mirror or optical test pattern.

Once the signal has been acquired, then at step 106 the optimum sub-array is determined and set (the process for which is exampled in more detail in connection with Figure 9).

Figure 9 illustrates an example method for determining the optimum sub-array 106. In this embodiment the signal output by the CMOS image sensor 20 is displayed at step 202 on an output display, such as an oscilloscope or on a computer monitor. Examples of the sort of signal 25, 27, 29 that will be displayed are shown in Figures 5(a), 6(a) and 7(a). The user then looks at the signal and gauges which pixels on the CMOS image sensor 20 have provided the best quality signal. For instance, with respect to Figure 5(a) a user could see that the intensity of the output is best over the middle pixel range. The user then at step 204 inputs the pixel range having the best quality signal. This can be done in various ways, and for instance could comprise the user entering via a keyboard the pixel numbers which delineate the preferred pixel range. For example, with an output signal like that shown in Figure 5(a) the user could enter pixel numbers 32 and 287 to select the middle range of pixels. Pixel numbers 60 could be provided on the output display to aid the user in identifying the pixel numbers to enter. The pixel range is received by the readhead's 24 CPU at step 204 and stored in memory 32 at step 206 for subsequent use.

The readhead 4 could be configured such that the pixel range stored in memory 32 cannot be changed. Accordingly, after the above set up method is performed the readhead will only use pixels 32 to 287 in order to read the scale. However, it will be understood that this need not necessarily be the case and the readhead could be configured such that the range can be altered subsequently, for instance manually during another set up routine, or dynamically during operation of the readhead as explained in more detail below in connection with Figures 10 and 11.

Instead of or in addition to performing the method of Figures 8 and 9 during manufacture, the same method could be used during installation of a readhead 4 with respect to a scale 6, e.g. by the customer.

As mentioned above, the readhead 4 could be configured such that pixel range it uses to read the scale can dynamically change during operation of the readhead. Figure 10 illustrates the process 300 the readhead 4 can be configured to follow in order to achieve this. First of all, at step 302 the readhead 4 takes a reading of the scale 6, for example in response to a position demand request from a controller 7. Taking a reading involves acquiring a signal across the sub-array identified by the pixel range stored in memory 32. The pixel range can have been set either as a default value (e.g. pixels 1 to 256) or could have been set as part of the manufacture or installation routine as described above in connection with Figures 8 and 9.

At step 304, position information is extracted from the signal and passed to the controller 7.

At step 306, the sub-array to be used for acquiring the next signal is optimised. The sub-array optimisation process 306 is explained in more detail in connection with Figure 11. The process begins at step 402 in which a straight line is best- fit to the signal. For example, Figure 12(a) shows a first example signal 80 acquired by the selected sub-array 21 of the CMOS image sensor 20 (illustrated in Figure 12(b)). Figure 13(a) shows a second example signal 82 acquired by the selected sub-array 21 of the CMOS image sensor 20 (illustrated in Figure 13(b)). Also shown in Figures 12(a) and 13 (a), there is shown a straight line 70, 72 best-fit to their signal 80, 82. At step 404 the gradient of the straight line is determined. The gradient of the straight line 70 in Figure 12(a) will be 0, whereas the gradient of the straight line 72 in Figure 13 (a) will have a positive value. At step 406 it is then determined if the gradient is 0, positive or negative in value. If the gradient is 0, then this is an indication that the sub-array is capturing the best quality signal it can and so no action need be taken and the sub-array remains unchanged (step 410). However, if the gradient is positive or negative then this is an indication that the sub-array is not capturing the best quality signal that it can. For instance, it is clear from Figure 13 (a) that there is an offset illumination distribution and that a better quality image would be obtained by shifting the sub-array used to obtain the next signal. The gradient of the best-fit straight line reflects this by indicating that the intensity of the signal increases toward the right of the signal. Accordingly, in the case of Figure 13 (a) the gradient of the best- fit straight line 72 is positive, at step 412 the readhead's 4 CPU 24 shifts the boundary of the sub- array to the right. Accordingly, the next time a signal is obtained the sub-array used should obtain a more symmetric illumination distribution.

Likewise, if the gradient is negative, then at step 408 the readhead's 4 CPU 24 shifts the boundary of the sub-array to the left.

In the embodiment described, the sub-array is shifted by 1 pixel but of course it could shift it by a different number of pixels. In particular, the number of pixels it shifts the sub-array by could be based on the magnitude of the gradient.

After step 306 the process 300 of Figure 10 continually repeats until the power to the readhead is turned off. The process 300 could be repeated with each reading of the scale the readhead takes, or at some other predetermined or random interval. For example, the process 300 of Figure 10 could be performed every second, tenth or one hundredth reading. The process 300 of Figure 10 will continue to repeat indefinitely until the power to the readhead is turned off.

The embodiment shown is of the reflective type, but as will be understood, the invention can be used with transmissive type encoder apparatus (in which the light > is transmitted through the scale rather than being reflected from it). Furthermore, as will be understood, the scale's pattern can be formed via mechanisms other than features having different optical properties. For instance, as is well known, features having different magnetic, capacitive or inductive properties can be used to encode position information onto a scale. In these cases an appropriate magnetic, capacitive or inductive sensor arrangement will be provided in place of the lens 18, CMOS image sensor and LED in the readhead.

The invention is also not limited to scales having position information contained in a single track only. Rather, the invention is also suitable for use with multiple- tracked scales, for instance scales having a first track containing absolute position information and a second track containing incremental position information.

Claims

CLAIMS:

1. A method of configuring a position encoder readhead for determining position information in at least one measuring dimension, the readhead comprising an array of sensor elements for sensing position features on a scale, the array extending along the at least one measuring dimension, the method comprising: selecting a subset of the array of sensor elements in the measuring dimension to use in a subsequent reading of the position features on a scale.

2. A method as claimed in claim 1, further comprising determining the readhead' s configuration; and in which the selected subset is based at least in part on the readhead' s configuration

3. A method as claimed in claim 2, in which determining the readhead' s configuration comprises acquiring a signal indicative of the readhead 's configuration.

4. A method as claimed in claim 3, in which the signal is acquired by at least some of the array's sensor elements.

5. A method as claimed in claim 4, in which the signal is acquired by a subset of the array of sensor elements.

6. A method as claimed in claim 5, further comprising determining position information from the signal indicative of the readhead' s configuration.

7. A method as claimed in any of claims 3 to 6, comprising a processor processing the signal and determining the subset of the array of sensor elements.

8. A method as claimed in any of claims 4 to 7, comprising determining the variation in signal quality across the array of sensor elements.

9. A method as claimed in claim 8, comprising selecting the subset of sensor elements that provide the optimum signal quality.

10. A method as claimed in claim 8, in which determining the subset of the array of sensor elements comprises shifting a currently selected subset of array of sensor elements by a predetermined number of sensor elements toward the part of the array that provides the optimum signal quality.

11. A method as claimed in any preceding claim, in which the sensor elements are individually selectable.

12. A method as claimed in any preceding claim, in which the readhead is configured to read signals from the selected subset of the array only, during a subsequent reading of the position features on a scale.

13. A method as claimed in any preceding claim, in which acquiring a signal comprises the readhead reading an article placed in a reading relationship with the readhead.

14. A method as claimed in claim 13, in which the article is a scale comprising a plurality of position features.

15. A method as claimed in claim 14, in which the acquiring a signal comprises reading the position features on the scale.

16. A method as claimed in any preceding claim, in which the sensor elements are optical sensor elements.

17. A method as claimed in any preceding claim, in which the subset of the array of sensor elements comprises an array of consecutively located sensor elements.

18. A method as claimed in any preceding claim, in which the subset of the array of sensor elements in the measuring dimension are selected for use in a plurality of subsequent readings of position features on a scale.

19. A method according to claim 18 of configuring a readhead during at least one of the manufacture and installation of the readhead, comprising selecting the subset of the array of sensor elements to be used during the operation of the readhead.

20. A position encoder readhead for determining position information in at least one measuring dimension, comprising an array of sensor elements for sensing position features on a scale, the array extending along the at least one measuring dimension, in which a subset of the array of sensor elements in the measuring dimension can be selected for reading position features on a scale.

21. A position encoder readhead as claimed in claim 20, in which the subset of the array of sensor elements in the measuring dimension can be selected for use in a subsequent reading of position features on a scale.

22. A position encoder readhead as claimed in claim 21 , in which the subset of the array of sensor elements in the measuring dimension can be selected for use in a plurality of subsequent readings of position features on a scale.

23. A position encoder readhead as claimed in any of claims 20 to 22, in which the subset of the array of sensor elements comprises an array of consecutively located sensor elements.