WO2015078860A1

WO2015078860A1 - Metrological scale

Info

Publication number: WO2015078860A1
Application number: PCT/EP2014/075535
Authority: WO
Inventors: Matthew HARRISON
Original assignee: Renishaw Plc
Priority date: 2013-11-26
Filing date: 2014-11-25
Publication date: 2015-06-04

Abstract

A method of configuring a metrological scale. The method comprises taking a metrological scale comprising a series of position features and a plurality of supplemental features spaced apart from each other along the scale at predetermined positions, and processing the scale's material so as to selectively enable or disable at least one supplemental feature as a reference feature.

Description

METROLOGICAL SCALE

The present invention relates to a metrological scale for a position measurement encoder, and in particular to a user configurable metrological scale and associated methods of configuring the metrological scale.

As will be understood, a position measurement encoder typically comprises a metrological scale comprising features and a readhead for reading the features so as to be able to determine relative position and/or movement of the readhead and scale. The scale and readhead are moveable relative to each other. Scale comes in many different forms, including linear, rotary and disc. Various different technologies can be used to detect scale features, including optical, magnetic, inductive and capacitive technologies. Incremental encoders are known wherein the scale comprises a series of incremental features which the readhead can read to determine and measure relative motion. As will be understood, various techniques can be used to read the incremental features, including counting the features as they pass the readhead and (for example in the case of optical encoders (e.g. an encoder in which light in the infra-red to ultraviolet range is used)) simple imaging of the incremental features.

Many optical incremental encoders rely on the use of a combination of gratings arranged to diffract light so as to produce optical patterns at a detector which changes as the readhead and scale move relative to each other. Examples of incremental encoders include those produced by Renishaw pic available under the brand names TONiC™ and SiGNUM™. In the TONiC™ and SiGNUM™ encoders light from a light source in the readhead interacts with the scale to produce diffraction orders, which in turn interact with a diffraction grating in the readhead to produce at a photodetector array in the readhead an interference fringe pattern which moves with relative movement of the scale and readhead. Such an interference fringe can be detected and used to provide signals indicative of relative movement. Often, quadrature signals, i.e. first and second signals which are out of phase with each other (e.g. 90 degrees out of phase, e.g. Sin and Cos signals) are output by a readhead to indicate direction as well as a measure of relative movement. One or more reference features can be provided (e.g. embedded within or next to the series of incremental features, either on the same scale substrate or next to it) such that relative position can be determined with respect to known reference position(s) defined by the reference feature(s). In many cases (but not necessarily) a separate reference detector is provided in the readhead to detect the reference feature(s). In an optical encoder a reference feature can for example provide an increase or decrease in the amount of light received at the reference photodetector. An example of an incremental encoder is disclosed in

US7659992. Absolute encoders are known wherein the scale comprises a series of features which the readhead can read to determine unique positions along the length of the scale. An example of an absolute encoder is disclosed in WO2010/139964. It can be useful in some circumstances to apply reference features to absolute scale. For example, they could be used define the limit of relative movement of the readhead and scale.

WO2009/130449 describes taking a scale with a series of position features, subsequently applying temporary reference features on the scale substrate, checking them, finalising acceptable reference features and removing

unacceptable reference features.

It is known to supply a scale with a plurality of pre-formed reference features and also to supply stickers which a customer can use to cover those reference features which are not to be used as active reference features.

This invention relates to improvements in configuring metrological scale. In particular, it provides an improved technique for enabling someone to select which of a plurality of features are to be identified by a readhead as an

active/valid reference feature, e.g. which of a plurality of reference features are to be readable by the readhead. In particular, this application describes a method of configuring a metrological scale, comprising: taking a metrological scale comprising a series of position features extending in a measurement direction and a plurality of (integral) features at different points along the at least one measurement dimension; and processing the scale's material so as to selectively enable or disable at least one feature as a reference feature.

For example, this application describes a method of configuring a metrological scale, comprising: taking a metrological scale comprising a series of position features and a plurality of supplemental features (e.g. spaced apart from each other along the scale at predetermined positions); and processing the scale's material so as to selectively enable or disable at least one supplemental feature as a reference feature.

According to a first aspect of the invention there is provided a method of configuring a metrological scale, comprising: taking a metrological scale comprising a series of position features and a plurality of integral reference features; and processing the scale's material so as to selectively enable or disable at least one reference feature. Providing the scale with preformed features which can be enabled or disabled as reference features (as detectable by a readhead for the scale) enables good control over the position, size, quality and/or phasing of the scale's reference features (e.g. with respect to the series of position features). Furthermore, processing the scale's material so as to selectively enable or disable at least one reference feature has been found to be advantageous over known techniques for enabling or disabling features, such as applying stickers over or next to the features that are to be enabled or disabled. Processing the scale's material has been found to be more permanent and also can avoid the significantly raised profile of stickers which can collect dirt and impair cleaning of the scale.

Said processing of the scale's material can comprise chemically processing the scale's material. That is a chemical can be applied to process the scale's material. The chemical could react with the material. Chemically processing can comprise chemically etching the scale's material.

The (enabled) reference features can be optical reference features. Optionally, the series of position features can be optical position features. Accordingly, the series of position features and/or (enabled) reference features can be configured to manipulate light (in the infrared to ultraviolet range).

Said processing can alter the scale's optical behaviour so as to enable or disable at least one reference feature. For example, said processing can alter the scale's optical propagation behaviour, e.g. the direction in which light is propagated (e.g. reflected or transmitted) by the scale, the optical scattering behaviour of the scale, and/or the optical absorption behaviour of the scale.

The method can comprise processing the reference/supplemental feature, e.g. so as to enable or disable it. For example, the method can comprise processing the material of the reference/supplemental feature itself so as to enable or disable the reference feature. For example, the method can comprise processing the reference/supplemental feature so as to change alter its optical behaviour.

Additionally, or alternatively, the method can comprise processing scale material next to the reference/supplemental feature so as to enable or disable it as a reference feature. The scale material next to the reference/supplemental feature could be within the same track in which the plurality of reference/supplemental features are contained or outside (e.g. next to) the track in which the plurality of reference/supplemental features are contained.

Said processing can comprise altering the scale material's form, e.g. its physical form, for instance its profile and/or shape. Said processing can comprise mechanically processing the scale's material, e.g. so as to alter its form, i.e. its physical form, in other words its profile and/or shape. Mechanically processing can comprise at least one of scratching, grinding, abrading the scale's material.

Said processing can comprise displacing scale material. Said processing can comprise removing scale material.

The metrological scale can comprise a substrate in which the series of position features and the plurality of integral reference/supplemental features are formed. Accordingly, the series of position features and the plurality of

reference/supplemental features can be formed in a single piece of material. For example, the series of position features can the plurality of reference/supplemental features can be ingrained within the scale's substrate. The substrate can be metal, for example stainless steel.

The metrological scale can be an optically reflective scale. In other words, the scale can be configured to be used with a readhead that is positioned on only one side of the scale (e.g. its detector is configured to be on the same side of the scale as a light source for illuminating the scale). The metrological scale can be an optically transmissive scale. In other words, the scale can be configured to be used with a readhead in which its detector is on the opposite side of the scale to a light source for illuminating the scale. As will be understood, the series of position features can define a main scale. The series of position features can comprise a series of substantially periodic features. The series of position features can define an incremental scale. The series of position features can form a diffraction grating. Optionally, the series of position features define an absolute scale. Accordingly, the series of position features can comprise a series of unique codes.

At least one reference/supplemental feature (for example some and optionally all reference features) can be embedded within the series of position features. For example at least one reference feature (for example some and optionally all reference features) can be contained within the same track in which the series of position features are provided.

At least one reference feature (for example some and optionally all reference features) can be located beside the series of position features. For example, at least one reference feature (for example some and optionally all reference features) can be located in a separate track to the series of position features.

The plurality of reference/supplemental features can be located at reference positions at different points along the length of the scale/at different points along the extent of the series of position features. The series of position features can define at least one measurement dimension, for example two measurement dimensions. The plurality of reference features can define reference positions at different points along the at least one measurement dimension. Enabling a reference feature can comprise changing a portion of the scale so as to change at least one reference/supplemental feature from a state in which it is not determined by a readhead for the scale to be/represent a valid reference feature (and hence will not cause the readhead to output a reference position signal) into a state in which it is determined by the readhead to be/represent a valid reference feature. Disabling a reference feature can comprise changing a portion of the scale so as to change at least one reference/supplemental feature from a state in which it is determined by the readhead to be/represent a valid reference feature (and hence will cause the readhead to output a reference position signal) into a state in which it is not determined by the readhead to be/represent a valid reference feature. For example, enabling a reference feature can comprise transforming a reference/supplemental feature from a state in which it is not detected by the readhead as a valid reference feature (and hence will not cause the readhead to output a reference position signal) into a state in which it is detected by the readhead as a valid reference feature. Disabling a reference feature can comprise transforming a reference/supplemental feature from a state in which it is detected by the readhead as a valid reference feature (and hence will cause the readhead to output a reference position signal) into a state in which it is not detected by the readhead as a valid reference feature.

As will be understood, the distance by which the plurality of reference features are spaced apart by from each can depend on various factors, but typically they could can be spaced somewhere in the range of 10mm and 400mm for example, optionally in the range of 10mm and 200mm, for example in the range of 30mm to 100mm, for instance in the range of 40mm to 60mm, for example approximately 50mm. The plurality of features could be unevenly spaced. Optionally, the plurality of features are substantially equidistantly spaced.

According to another aspect of the invention there is provided a method of configuring a metro logical scale, comprising: taking a metro logical scale comprising a series of position features extending in at least one measurement dimension and further comprising a plurality of (e.g. integral/pre- formed) reference features (e.g. which are detectable by the readhead) (e.g. which define reference positions at different points along the at least one measurement dimension); and selectively eliminating at least one of the reference features by altering the substrate defining the reference feature so as to change its optical behaviour.

Accordingly, the invention can involve changing the reference feature's detectability. The invention can comprise making at least one reference feature detectable as a valid reference feature by the readhead. The invention can comprise making at least one reference feature undetectable as a valid reference feature by the readhead. For example, the invention can comprise eliminating at least one reference feature so as to make it undetectable by the readhead. Accordingly, the method of the invention can comprise changing the optical behaviour of the scale (e.g. of the reference feature) such that it changes whether or not the readhead will identify a reference feature as a valid reference feature (and hence output a reference signal when it passes the reference feature). E.g. the method of the invention can comprise changing the optical behaviour of a reference feature such that it changes the amount of light it propagates (e.g.

reflects) toward the readhead.

As will be understood, the above described methods could be performed before or after the scale has been supplied to a customer/end user. Accordingly, the method of the invention could be performed before or after the scale is mounted on the machine on which the scale is to be used.

According to another embodiment there is provided a metrological scale comprising a series of scale features and a plurality of optically bright reference features defining reference positions at different points along the length of the scale.

Accordingly to another aspect there is provided a method of configuring a metrological scale for reading by a readhead, comprising: taking a scale comprising a series of position features; and applying chemical etchant to the scale so as to alter the optical behaviour of the scale's material, so as to enable or disable at least one reference feature. According to a further aspect of the invention there is provided an encoder apparatus kit comprising: a metrological scale comprising a series of position features; and a tool comprising chemical etchant for processing the scale's material so as to selectively enable or disable at least one reference feature. According to a further aspect of the invention there is provided an encoder apparatus kit comprising: a metrological scale comprising a series of position features and a plurality of supplemental features; and a tool comprising chemical etchant for processing the scale's material so as to selectively enable or disable at least one supplemental feature as a reference feature.

According to a further aspect of the invention there is provided an encoder apparatus comprising: a readhead; and a scale comprising a series of position features extending in at least one measurement dimension, at least one readhead readable reference feature defining a reference position, and a plurality of impaired/damaged reference features at different points along the length of the scale that are not determined by the readhead as valid reference features.

According to another aspect of the invention there is provided an encoder apparatus kit comprising: a metrological scale comprising a series of position features extending in at least one measurement dimension and further comprising a plurality of reference features; and at least one (e.g. hand-holdable) tool for selectively eliminating at least one of the reference features by altering the substrate defining the reference feature so as to change its optical behaviour.

The (e.g. hand-holdable) tool can comprise an etchant for etching the scale. Accordingly, this application describes a method comprising taking a scale comprising a substrate in which a series of position features and a plurality of reference features at reference positions are formed, in which the scale (e.g. the scale's substrate) is manipulated so as to alter the optical behaviour of the scale at at least one of the plurality of reference positions.

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

Figure 1 illustrates an optical encoder apparatus comprising a readhead and a scale;

Figure 2 illustrates a scale according to the present invention comprising a plurality of reference features;

Figure 3 illustrates a scale according to the present invention after reference features have been selectively disabled;

Figure 4 illustrates a reference feature eliminator;

Figure 5 illustrates a flow chart illustrating an example method for performing the invention; and

Figure 6 illustrates a scale according to another embodiment of the present invention comprising a plurality of reference features and an additional mark to selectively highlight the enabled reference feature. Referring to Figure 1 there is shown an encoder apparatus 2 comprising a readhead 4 and a scale 6. A cable 10 comprising a plurality of wires is provided for supplying power to the readhead 4 and also for facilitating communication with the readhead (as explained in more detail below). In the embodiment described, the scale 6 is an incremental scale and comprises a first track 12 comprising a series of incremental periodic marks 8 and a second track 14 comprising a reference feature 9. As will be understood, a reference feature can be used to identify a reference (optionally a unique) position along the scale. A reference feature could delineate a boundary of permitted machine motion (e.g. it could be what is commonly referred to as a limit mark). The reference feature 9 shown in Figure 1 does not delineate a boundary, rather it identifies a reference position part way along the scale. Optionally, reference feature(s) can be provided in the same track as series of the incremental period marks 8, e.g. as disclosed in US7659992. As will be understood, the scale's features are shown schematically in order to aid illustration. As will be understood, scale features can be as small as only a few microns wide (e.g. only a few tens of microns wide, or even smaller) as opposed to the size illustrated in the drawing. Reference features can also vary in shape, size and configuration. For instance, reference features can be single-block reference features as shown, or comprise a plurality of distinct marks/attributes. Reference features can be in the form of lines (as shown) dots, or other regular or irregular shapes. In the embodiment described, the encoder apparatus is an optical encoder apparatus 2. The readhead 4 comprises a light emitting diode (LED) (not shown) which is configured to illuminate the scale 6. Light from the LED which falls on the first track 12 comprising the periodic scale marks 8 is diffracted thereby creating diffraction orders. These orders are reflected back toward the readhead 4 and then interact with a readhead diffraction grating (not shown) to generate diffraction orders which then interact to form a resultant field (in this case an interference fringe) on a detector (not shown) in the readhead 4. Position signals, in this case in the form of quadrature signals (i.e. first and second signals which are out of phase with each other (e.g. 90 degrees out of phase, e.g. sine and cosine signals)) are output by the readhead 4 to indicate a measure of relative movement. In this case, the position signals (that is the quadrature signals) indicate direction as well as magnitude of relative movement. Such an arrangement is well known, and for instance is described in US4959542 and US5861953. As will be understood, other arrangements (e.g. other optical arrangements) are possible and well known in the field of position measurement encoders.

In the embodiment described, the reference feature 9 is an optically bright reference feature. Accordingly, the reference feature 9 is configured to reflect relatively more light back to the readhead 4 compared to the rest of the second track 14. The readhead 4 comprises a sensor for detecting light reflected by the second track 14. The readhead 4 can be configured to determine the presence of a reference feature 9, and output a reference position signal, when it detects an increase in the amount of light detected above a threshold value (or in the case of a dark reference feature can be configured to determine the presence of a reference feature when it detects a dip in the amount of light detected below a threshold value). Such an arrangement is well known, and for instance is described in US7659992, US7624513, US7289042 and US6198534. As will be understood, other optical arrangements are possible and well known in the field of position measurement encoders.

As shown in Figure 2, the scale 6 can initially be configured with a plurality of reference features 9, positioned at different points along the length of the scale 6, thereby defining different reference positions at different points along the length of the scale. Accordingly, the scale 6 can be manufactured with a plurality of reference features 9. The reference features 9 could all be active/finalised/fully- formed reference features 9 in that in their current state (e.g. as supplied to the customer/end user) they would each cause the readhead 4 to detect and identify them as reference features 9 and cause the readhead 4 to issue a reference feature signal/pulse. In the embodiment shown, the plurality of reference features 9 are spaced substantially equidistantly (e.g. their spacing varies by not more than ΙΟΟμιη (microns), and preferably by not more than 50μιη (microns)). Providing the plurality of reference features 9 such that they are spaced substantially equidistantly can be advantageous because it can simplify manufacture, storage, supply and provide predictability to the customer. However, as will be understood, this need not necessarily be the case. For instance, their spacing could vary predictably or randomly. Furthermore, the reference features are finalised reference features in that they need not further processing to make them permanent, active, readable reference features.

In the described embodiment the reference features 9 are integral reference features. In the present example, the reference features 9 comprise the scale's 6 substrate. For instance, in this example, the scale's 6 substrate comprises a metal, e.g. stainless steel. In particular, the periodic incremental features 8 the reference features 9 are formed in a single piece of material by a common process. For example, the process can comprise a laser marking process whereby a laser is used so as to affect the optical characteristic of the scale substrate, (in this case so as to reduce the intensity of light reflected back toward the readhead 4 (in other words they are what are commonly referred to in the field as amplitude features - they affect the amplitude of light being returned back toward the readhead 4)). Accordingly, in this embodiment, the scale's 6 substrate at the reference positions is left unprocessed (i.e. not laser marked) so that the optical characteristic of the scale is unaffected (in this case so that they reflect relatively more light back toward the readhead 4 than the regions 7 between the reference features 9). In an alternative manufacturing process, an etching process can be used to etch the periodic markings 8 and the regions 7 between the reference features 9. For instance, a laser can be used to selectively remove (or selectively cure) etch resist applied on top of the scale 6, and then an etchant is used to etch those parts of the scale on which etch resist is not present. Use of laser in scale manufacture is known and for example described in US8466943 and US7723639.

It is likely that in use only one or some of the reference features 9 is/are needed. For example, it might be that three reference features are needed, two defining limit boundaries (one at each end) and one reference defining a unique position part way between the limit boundaries.

Accordingly, at some point after manufacture (and this could be before or after supply to the customer) one or more of the reference features 9 need to be disabled/eliminated so that when the readhead passes over it, the

disabled/eliminated reference marks do not cause the readhead 4 to output a reference feature signal/pulse (e.g. it is disabled such that the readhead does not identify it as a (e.g. valid) reference feature). This could be done via an etching process. For example, as illustrated in Figure 4, a handheld etching tool 20 comprising a housing 22 comprising an etchant reservoir 24 containing etchant (in this embodiment an acid) and an application nib 26 can be used by a scale installer

(for example), so as to apply etchant locally onto those reference features 9 that are to be disabled. In the embodiment described, the etchant reacts with the substrate defining the reference feature 9 so as to change its optical behaviour in a way such that it propagates less light back toward the readhead 4 (in this case such that it reflects less light back toward the readhead 4). Accordingly, in the present embodiment, etching the scale's 6 substrate that defines the reference mark 9 causes the reference feature 9 to reflect less light back toward the scale such that the amount of light detected by the readhead 4 as it passes over the reference mark is less than the threshold value which would cause the readhead 4 to issue a reference signal/pulse. In the present example, the scale's 6 substrate (and hence the substrate defining the reference feature 9 comprises martensitic stainless steel. Suitable etchant for reacting with the substrate defining the reference feature 9 includes acids, such as hydrochloric acid or ferric chloride acid. Other compounds could be included, such as selenious acid for discolouring the scale. A suitable etchant tool as described is widely available, for example the Metal Etching Pen, from Linear Tools Limited or RS Components Limited.

As will be understood, in an alternative embodiment in which the readhead 4 is looking for a dip in the amount of light it receives, the scale could be provided with a plurality of bright reference features, and the user could use the etchant tool to alter (e.g. reduce) the amount of light reflected by one or more of such bright reference features toward the readhead in a way which causes the readhead to identify it as a valid reference feature when it passes the altered reference feature. In another alternative embodiment, the scale could be provided with a plurality of reference features having a relatively rough surface which causes light to be scattered away from the readhead, and a tool could be used to smoothen one or more reference features so as to cause more light to be reflected back toward the readhead.

As will be understood, an encoder readhead could (as well as or instead of looking for a threshold level of light) look for a particular light pattern, signature or other characteristic to determine the presence of a reference feature. Accordingly, the method of the invention might be used to affect the particular light pattern, signature or other characteristic as detected by the readhead instead of or as well as affecting the amount of light detected by the readhead. Figure 5, illustrates an example process 100 for configuring a scale in accordance with the invention. The process 100 shown summarises what is essentially described above, in that the process begins at step 102 in which a scale 6 having a plurality of ready-to-use reference marks on it is mounted on a machine 16.

Unwanted reference features are identified at step 104 by the scale installer and then at step 106 the installer uses the etching tool 20 to etch the unwanted reference features 9 so that they are no longer detectable by the readhead and therefore are disabled.

Figure 3 illustrates an example scale 6 in which a plurality of reference features 9a have been etched so as to disable them. As schematically illustrated, the etchant reduces the amount of light that a reference feature reflects back toward the readhead 's reference mark detector, making them (as far as the readhead is concerned) less distinct from the other regions 7 of the second track 14 and therefore undetectable by the readhead. As schematically illustrated, as the etchant is being applied by a hand-held tool 20, the form and extent of etchant applied can vary between different reference features 9, and in some cases the etchant might be inadvertently applied to the first track 12 containing the incremental position features 8. However, it has been found that the inconsistency in the form and extent to which etchant is applied to the reference features is largely inconsequential to eliminating the reference features, and also the incremental position features can take some damage caused by the etchant without the encoder's incremental detection system failing because it essentially looks like dirt (and most encoders can cope with some level of dirt; but as will be understood, the extent by which the encoder can cope with damage to the incremental features caused by etchant will depend on the encoder type). In the embodiments described above, the scale initially comprises a plurality of valid/active reference features which are selectively deactivated. However, this need not necessarily be the case. For instance, the scale could be provided with a plurality of inactive reference features which are selectively enabled (e.g. by using a chemical etchant to process (e.g. mark) the scale).

As will be understood, the invention can be implemented in ways other than that described above. For example, rather than altering the reference feature itself, regions adjacent to the reference feature can be altered. For instance, in the case where the reference feature is configured to propagate (e.g. reflect or transmit) more light toward the readhead than the rest of the track containing the reference feature, then the reference feature could be disabled (so as to make it undetectable by the readhead as a reference feature) by increasing the amount of light propagated toward the readhead by a/the region/s next to it within the track that the reference mark is contained. In this case, the width of the reference feature is effectively increased, causing the readhead to effectively dismiss/ignore the reference feature. Optionally, a part of the scale's material adjacent the reference mark, but not within the track that contains the reference mark can be processed. For instance, as shown in Figure 6, the etchant pen 20 could be used to add a mark 9b next to the reference mark 9 that is to be enabled as a valid reference mark. In this case, the readhead can include an additional detector for detecting the extra mark 9b.

Optionally, the reference feature is a structured reference feature (e.g. rather than being a single block as illustrated in the above embodiment, it could comprise a plurality of components/part, e.g. like a barcode). In this case eliminating the reference mark could comprise altering part or all of the structured reference feature. For example, in the case of a structured reference feature comprising a plurality of distinct lines, the reference feature could be disabled by, for example removing or adding one or more lines, joining lines, or generally defacing the reference feature. In the embodiments described above, the reference features 9 and series of position features 8 are formed in one piece of material/substrate. However, this need not necessarily be the case. For instance, the scale 6 can comprise a piece of glass on which chrome is deposited via sputter deposition, and subsequently selectively removed to form the series of position features 8 and a plurality of integral reference features 9. The chrome on glass scale could then be installed, and unwanted reference features 9 disabled by at least partial destruction, deformation and/or removal of the chrome reference features 9, e.g. by using a chemical etchant.

In the embodiments described above, the reference feature is disabled using chemical etchant which reacts instantly with the substrate defining the reference mark. This need not necessarily be the case. For instance, a selectively activateable chemical etchant could be used that only reacts with the substrate defining the reference mark so as to change its optical behaviour when activated. For example, an electrochemical etchant could be applied which only acts on the substrate defining the reference mark when an electrical current is applied.

Rather than chemical etching as described, other techniques can be used to alter the substrate defining the reference feature. For example, mechanical force (e.g. scratching, abrading, grinding) can be used to alter the substrate defining the reference feature. Accordingly, a tool for applying such mechanical force can be supplied. Other possible techniques for altering the substrate defining the reference feature so as to change its optical behaviour comprise spark erosion, laser manipulation (e.g. laser ablation), and/or heat treatment. In the above described embodiments, the reference marks 9 are provided beside the series of position features 8. However, as will be understood, one, some or all of the reference marks 9 could be provided in the same track as the series of position features 8 in the first track 12 (i.e. they could be embedded within the series of position features 8 in the first track 12, e.g. as described in US7659992).

Claims

CLAIMS:

1. A method of configuring a metrological scale, comprising:

taking a metrological scale comprising a series of position features and a plurality of supplemental features spaced apart from each other along the scale at predetermined positions; and

processing the scale's material so as to selectively enable or disable at least one supplemental feature as a reference feature.

2. A method as claimed in claim 1 , in which processing comprises chemically processing the scale's material.

3. A method as claimed in claim 2, in which chemically processing comprises chemically etching the scale's material.

4. A method as claimed in claim 1, in which said processing alters the scale's optical behaviour so as to activate or deactivate the at least one reference feature.

5. A method as claimed in claim 1, in which processing comprises altering the supplemental feature so as to change its optical behaviour.

6. A method as claimed in claim 5, in which said processing reduces the amount of light propagated toward a readhead for the scale.

7. A method as claimed in claim 1 , in which processing comprises mechanically processing the scale's material so as to alter its physical form.

8. A method as claimed in claim 7, in which mechanically processing comprises at least one of scratching, grinding, abrading the scale's material.

9. A method as claimed in claim 1, in which processing comprises removing scale material.

10. A method as claimed in claim 1 , in which the metrological scale a substrate in which the series of position features and the plurality of

supplemental features are formed.

1 1. A method as claimed in claim 1 , in which the metrological scale is an optically reflective scale.

12. A method as claimed in claim 1 , in which the series of position features comprise a series of periodic features defining an incremental scale.

13. A method as claimed in claim 1 , in which at least one supplemental feature is located beside the series of position features.

14. A method of configuring a metrological scale, comprising:

taking a scale comprising a series of position features and a plurality reference features; and

applying chemical etchant to the scale so as to disable at least one reference feature.

15. An encoder apparatus kit comprising:

a metrological scale comprising a series of position features and a plurality of supplemental features; and

a tool comprising chemical etchant for processing the scale's material so as to selectively enable or disable at least one supplemental feature as a reference feature.