Patent Description:
Encoders providing a relative position between two elements are key elements of digital measuring instruments.

Encoders relying on a digital scale with periodic marks separated by a constant pitch for providing a relative position between the reader and the scale, are well known and disclosed, for example, in <CIT> and <CIT>. These encoders comprise readers with sensing elements spaced from each other by a fraction of the marking pitch along the measuring path so as to provide sub-pitch measuring accuracies.

However, main drawback of these encoders stems from the fact that the maximal unambiguous displacement range, i.e. the segment in which an unambiguous unique position can be provided, is limited to the pitch between two adjacent marks. This limitation is typically overcome by combining additional tracks with different pitches and/or by considering the number of (entire) pitch the reader has already overtaken, when moved along the scale from an origin of the scale (e.g. a zero position). The settings of the zero position and of the pitch counter (e.g. at the powering of the encoder) generally requires a displacement of the reader at the origin of the scale.

Absolute position encoders configured to provide an unambiguous position all along the displacement range of the reader, are also well known. For example, <CIT> describes an absolute position encoder relying on a scale with track with markings arranged on adjacent tracks from which a unique digital code can be read, the unique digital code indicating a discrete, absolute position of the reader with respect to the scale. However, the accuracy of this encoder is strictly correlated with dimensions of the markings on the scale as well as on marking alignments on the scale.

Complementary to these approaches, <CIT>, US5'<NUM>'<NUM>, US5'<NUM>'<NUM> and <CIT> describe embodiments of absolute position transducers relying on combining a coarse absolute position and a fine relative position. The transducers are configured to sense a unique code for providing the coarse absolute position of the reader, while the relative fine position is provided by sensing one or more periodic tracks on the scale. In one embodiment of <CIT>, the absolute position transducer relies on reading the absolute code by couples of sensors sensing a first and a second tracks of markings. These tracks contain the same unique digital code. The marking of the second track provides an inverted digit of the digit provided by the first track for increasing the reading robustness. Another example of encoder is disclosed in <CIT>, which describes an encoder configuration comprising an illumination portion, absolute scale pattern comprising absolute tracks, and a detector having a width dimension. An absolute track pattern comprises geometrically congruent sub tracks. The congruent sub tracks are arranged such that if one is translated by the width dimension, then they will nominally coincide. A further example of an absolute encoder is disclosed in <CIT>.

An aim of the present invention is to provide an absolute position encoder for applications requiring high accuracy, such as dimensional metrology measurements.

Another aim is to provide an absolute position encoder more sensitive to inaccurate marks sensing due to an unfavourable positioning of the reader relative to the scale.

According to the invention, these aims are achieved by an absolute position encoder for a measuring instrument or accessory for measuring instrument, comprising a digital scale and a reader movable relative to the digital scale. The digital scale is arranged along a travel direction of the reader. The digital scale comprises at least one absolute position track having a sequence of discrete regions (in this document also referred as marks or markings) sharply separated from each other by separating regions, the discrete regions being detectable by detectors. The reader comprises a first and a second series of detectors configured sense the discrete regions to detect at least one of a first and a second absolute position code, each absolute code representing an absolute position (i.e. unique, non-repetitive all along the digital scale) of the reader with respect to the digital scale. The sequence of discrete regions and the first and second series of detectors are disposed in such a way that none of the detectors of at least one of said first and second series of detectors is aligned with a transition between a discrete region and a separating region at each possible position of the reader relative to the digital scale. The position encoder is configured to select the first or second absolute position code which is read by the series of detectors among which no detector is aligned with said transition. According to the invention, a code can be any information represented in an (electrical) analogue or digital format, e.g. in form of one or more analogue or digital signals, or of a digital flag or value.

In an embodiment, the selection of the first or second absolute code includes testing whether said first and second absolute position codes read by respective first and second series of detectors are coherent with a common assumed position of the reader.

In an embodiment, the digital scale further comprises an additional track adjacent and extending along said at least one absolute position track, the additional track being detectable by the reader. In particular, the additional track is provided with markings being detectable by detectors of the reader. The absolute position encoder is configured to select the absolute position code based on a code derived from the additional track. In such embodiment, the derived code could advantageously represent a spatial or physical information or a relationship between the additional track and said at least one absolute position track.

In an embodiment, the additional track is a periodic coded track (in this document also referred as incremental track) providing an incremental position code, the incremental position code representing a periodic (or cyclic) position of the reader with respect to the digital scale. The absolute position encoder is configured.

In an embodiment, the first and second series of detectors are disposed at equal intervals along the travel direction of the reader.

In an embodiment, the digital scale of the absolute position encoder comprises a first absolute position track having a first sequence of discrete regions and separating regions therebetween, and a second absolute position track having a second sequence of discrete regions and separating regions therebetween. The second sequence is a replica of the first sequence shifted along the travel direction of the reader such that the discrete regions of the first absolute position track are offset with respect to the corresponding discrete regions of the second absolute position track by a constant pitch. Each detector of the first series of detectors is transversally aligned, with respect to the travel direction of the reader, with a corresponding detector of the second series of detectors. The first series of detectors is arranged to move along the first absolute position track while the second series of detectors is arranged to move along the second absolute position track.

In an embodiment, the digital scale of the absolute position encoder comprises a unique absolute position track comprising discrete regions and separating regions therebetween. The first and second series of detectors are disposed along respectively a first and a second row extending along the direction of travel of the reader to detect at least one of said first and second absolute position codes.

In an embodiment, each detector of the first series of detectors is offset with respect to a corresponding detector of the second series of detectors by a constant pitch along the direction of travel of the reader.

In an embodiment, the detectors of the first series of detectors are interleaved with the detectors of the second series of detectors to form pair of adjacent detectors of respective first and second series of detectors.

In an embodiment, the absolute position encoder is configured as a linear position encoder or as an angle position encoder.

In an embodiment, the discrete regions of the at least one absolute position track differ from the separating regions by one of the following properties: optical opacity, optical reflectivity, electric conductivity, magnetization, and magnetic permeability.

Another aspect of the invention relates to a measuring instrument or accessory for measuring instrument comprising the absolute position encoder. The measuring instrument can be a (digital) hand-held measuring instruments, such as a (digital) sliding calliper, a (digital) micrometre, or a (digital) comparator. The measuring instrument can be a (digital) transportable measuring instrument, such as a height gauge, a measuring probe system, an articulated measuring apparatus, or a geodetic system. The measuring system can be a (digital) stationary measuring instrument such as a coordinate measuring machine or system. The accessory for measuring instrument can be, in particular, a rotary table, a rotary axe, an articulated probe head or a survey pole.

In an embodiment, the measurement instrument is configured as a sliding calliper comprising a first jaw fixed relative to the digital scale of the encoder and a second jaw slidable relative to the first jaw and fixed relative to the reader of the encoder. The absolute position encoder provides a value indicative of a distance between the first and second jaws.

In an embodiment, the measurement instrument is configured as (notably it comprises an accessory configured as) a survey pole comprising the absolute position encoder, a first section fixed relative to the digital scale of the encoder and a second section slidable relative to the first section and fixed relative to the reader of the encoder. The absolute position encoder provides a value indicative of a height of the survey pole.

Another aspect of the invention relates to a method of determining a position of a reader relative to a digital scale along a travel direction. The digital scale comprises at least one absolute position track comprising a pattern made of discrete regions and separating regions therebetween detectable by a plurality of detectors in the reader. The detectors are disposed along a first and second rows aligned with the travel direction of the reader. The method comprises determining a first absolute position code from detectors in a first row and a second absolute position code from detectors in the second row. The discrete regions and the plurality of detectors are disposed in such a way that at least one of the first row and second row has no detector aligned with a transition between a discrete region and a separating region at each possible position of the reader relative to the digital scale. The method further comprises selecting the absolute position code provided by the row with no detector aligned with said transition.

In an embodiment, the selection of the absolute position code includes a test whether the first and second absolute codes are coherent with a common assumed position of the reader.

In an embodiment, the test comprises taking as assumed position a position given by the first absolute position code, using the assumed position to establish which detectors of the second row are not facing a transition of the digital scale, determining the expected values of bits of the second absolute position code given the assumed position, and comparing the expected values with values of corresponding bits of the second absolute position code derivable from the detectors not facing said transition.

In an embodiment, the method comprises determining an incremental position code from a periodic coded track on the digital scale, interpolating a phase value of the incremental position code, using the interpolated phase to select the absolute position code, and providing a precise absolute position code comprising a most significant part derived from the selected absolute position code and a least significant part derived from the interpolated phase.

<FIG> show an absolute position encoder <NUM> for a measuring instrument or for an accessory thereof, notably for a dimensional metrological measuring instrument, with a linear accuracy of more than (i.e. inaccuracy less than) <NUM>, preferably more than <NUM>. The absolute position encoder <NUM> comprises a digital scale <NUM> and a reader <NUM> movable relative to the digital scale <NUM> along a measuring path <NUM>. The reader <NUM> is configured to sense a first and a second absolute position track 30a, 30b for providing an absolute position of the reader <NUM> with respect to the digital scale <NUM> along the measuring path <NUM>, by reading a first and a second absolute position code.

The digital scale <NUM> further comprises an incremental track <NUM> for providing a position of the reader <NUM> within a division of scale divisions <NUM>, <NUM>, <NUM> along the first absolute position track 30a extending along the measuring path. Typically, each scale division <NUM>, <NUM>, <NUM> is dimensioned along the measuring path so as to correspond to the maximal displacement range (e.g. Pitch P) in which the incremental track <NUM> can provide an unambiguous position <NUM>, as illustrated in <FIG>.

The scale divisions <NUM>, <NUM>, <NUM> can be located along the measuring path, such that one edge 37a of each division corresponds to one edge 37b of an adjacent division as shown in <FIG>. In a non-illustrated variant, each scale divisions can be spaced apart from each other along the measuring path with free space between the adjacent divisions. Preferably, each scale division <NUM>, <NUM>, <NUM> extends along the measuring path by a same spatial extension T. The spatial extension T can be notably a linear as illustrated in the embodiment of <FIG>, or a curved or an angular spatial extension according to a variant.

The incremental track <NUM> comprise a plurality of periodic markings <NUM>, <NUM>, <NUM> spaced apart from each other along the measuring path for providing such precise position <NUM> (in this document also referred as periodic position) within each of these scale divisions <NUM>, <NUM>, <NUM>. In particular, the periodic markings <NUM>, <NUM>, <NUM> can be regularly spaced apart from each other along the measuring path by a pitch P. The pitch P essentially correspond to the spatial extension T of the scale divisions of the first absolute position track 30a, to a multiple of the spatial extension T or to a fraction of the spatial extension T. The pitch P can be notably a linear, round, or angular pitch. The periodic markings may however be spaced part from each other along the measuring path by an unregular pitch, according to a variant. The unregular pitch may depend, for example, on the position of the periodic marks along the measuring path, e.g. an incremental, decremental, logarithmic, exponential, or a polynomial pitch, or a combination thereof.

The scale divisions can also extend along the measuring path by an unregular spatial extension T that can be, for example, function of the pitch and/or of the position of the scale divisions along the measuring path, e.g. an incremental, decremental, logarithmic, exponential, or a polynomial spatial extension, or a combination thereof. The pitch P and the spatial extension T are advantageously shaped according to the (local) shape of the measuring path.

The reader <NUM> comprises an incremental sensing unit <NUM> with a plurality of detectors <NUM>-<NUM>. Each detector is configured to sense the periodic markings <NUM>, <NUM>, <NUM> of the incremental track <NUM>, so as to derive an incremental position code. In the illustrated embodiment of <FIG>, the detector of the incremental sensing unit <NUM> are phase shifted by a fraction of the pitch P along the measuring path so as to generate sinusoidal signals (ucos, usin ) with a period corresponding to the pitch P as shown in <FIG>. A relationship between these sinusoidal signals (typically based on an tan-<NUM> or arctan-<NUM>) can provide said unique relative position <NUM> within the pitch P (and then within the corresponding scale division <NUM>, <NUM>, <NUM>), as well known in the field.

Advantageously, the reader <NUM> and the digital scale <NUM> can be configured so as the edges 37a, 37b of each scale division <NUM>, <NUM>, <NUM> are aligned with a zero-cross of one of such phase-shifted sinusoidal signals (cf.

As illustrated, the detectors <NUM>-<NUM> are advantageously grouped in four couples of sensing elements for reducing uncorrelated sensing errors while compensating the common ones. These groups are phased-shifted from each other by P/<NUM>. This leads to four sinusoidal signals (ucos(+), ucos(-),usin(+),usin(-)) phase-shifted from each other by P/<NUM>, P/<NUM> and 3P/<NUM> (typically indicated as <NUM>°-, <NUM>°- and <NUM>°- phase-shifted signal). The relationship between the sensed values of these sinusoidal signals (i.e. the incremental position code) determines the relative position <NUM> within the pitch P and then within the corresponding scale division <NUM>, <NUM>, <NUM>. The reader can be configured to correct the relative position, e.g. by applying linear and polynomial corrections of the (incremental) tracks.

Each of the first and second absolute position tracks 30a, 30b comprises discrete regions 32a, 32b and separating regions 33a, 33b which define regions extending between two discrete regions. These combinations of discrete and separating regions which may vary according to different embodiments described subsequently, are referred hereafter as absolute marking. These regions are associated with the incremental track <NUM> for providing a coarse absolute position of the current position of the encoder, i.e. to uniquely identify or determine on which of scale divisions <NUM>, <NUM>, <NUM> of the first absolute position track 30a the reader <NUM> is (currently) positioned. The first and second absolute position tracks 30a, 30b of the digital scale <NUM> can be sensed by a first and a second absolute sensing unit 60a, 60b of the reader <NUM> provided with sensing elements 70a-77a, 70b-77b. The encoder <NUM> can thus provide the absolute position <NUM> (<FIG>) of the reader with respect to the scale as function of the precise relative position <NUM> (<FIG>) provided by sensing the incremental track <NUM> and of (an absolute reference associated with) the sensed scale division <NUM>, <NUM>, <NUM>, i.e. the first absolute position. Advantageously, the absolute position <NUM> can be provided by adding (or subtracting) the precise relative position <NUM> to (or from) and an absolute reference associated with the sensed scale division <NUM>, <NUM>, <NUM> of the first absolute position track 30a, i.e. the first absolute position, preferably corresponding to one of opposite edges 37a, 37b of the scale division <NUM>-<NUM>.

The identification of each scale division <NUM>-<NUM> efficacy relies on arranging the absolute marking of the first absolute track 30a so as the reader <NUM> can sense a first plurality of unique identifiers (A<NUM>,A<NUM>) along the measuring path, each unique identifier being distinct and dependent on the positional relationship between the reader and the digital scale along the measuring path.

The unique identifier can be provided within the first plurality of unique identifiers along the measuring path, i.e. each unique identifier occurs exactly once as a portion of the sequence, the portion of each code partially overlapping the portion of the adjacent code. Alternatively, the first plurality of unique identifiers can be provided within a series of juxtaposed identifiers (i.e. placed side by side without overlap) along the measuring path.

As illustrated, the absolute marking of the first absolute track 30a can be arranged along the measuring path <NUM>. Alternatively or complementary, sets of absolute markings can be grouped (e.g. delimited or aligned) into tracks being oriented perpendicular or inclined with respect to the measuring path <NUM>.

Advantageously, the first plurality of unique identifiers can correspond to a first plurality of digital codes, each digital code being unique and assigned to one of the scale divisions <NUM>-<NUM> of the first absolute position track 30a. A digital code can be any code having a discrete, discontinuous representation, notably being processable by a computer (e.g. by means of a processor or controller) or by an equivalent thereof (e.g. a microprocessor, a FPGA, or an ASIC). Preferably the digital code is a binary code with a plurality of digits, preferably having N digits with N=<NUM>,<NUM>,<NUM> or greater than <NUM> so as to provide a unique identification of N<NUM> distinct scale divisions of the first absolute position track 30a.

The first plurality of digital codes can rely on a Gray coding for limiting the number of different bits between two adjacent digital codes. This coding is particular advantageously in case of individual codes are sensed along a perpendicular or inclined (virtual) axis with respect to the measuring path (e.g. in case of a juxtaposition of codes oriented perpendicular or inclined with respect to the measuring path).

Alternatively, the first plurality of digital codes can rely on or correspond (entirely or partially) to a DeBruijn sequence in which every possible code occurs exactly once as portion of the sequence. This coding optimally enables a sensing of a (continuous) sequence of codes along the measuring path, the measuring path being notably a linear, a circular or even a circular (ring/disc-shaped) path.

In the illustrated embodiment of <FIG>, the first plurality of unique identifiers take the form of a continuous sequence of digital codes, each digital code AN being unique and assigned to one of the scale divisions <NUM>-<NUM> of the first absolute position track 30a, as above mentioned.

As illustrated in <FIG>a to <NUM>d, the first plurality of digital codes A<NUM>,A<NUM> can be binary codes with N digits, N=<NUM> providing a unique identification up to <NUM> distinct scale divisions. Each digit of each digital code (A<NUM>,A<NUM>) is advantageously provided by one detector 70a-77a of the first absolute sensing unit 60a of the reader.

In the illustrated embodiment of <FIG>, the first plurality of digital codes relies on (a portion of) a DeBruijn sequence so as to enable a continuous sequence of codes along the measuring path. Each digital code is extracted (sensed) from such sequence depending on the relative position of the reader <NUM> with respect to the digital scale <NUM> along the measuring path. The first plurality of digital codes can be equally used for linear as well for angular measuring instrument (cf. <FIG>), i.e. along linear measuring path up to a circular (ring/disc-shaped) measuring path of an angular measuring instrument.

<FIG>a to <NUM>d schematically shows sensing of the first absolute position code, in form of a digital code that is part of a first plurality of digital codes, when the reader <NUM> is relatively moved from a first to a second relative position relative to the digital scale <NUM>. The first position of the reader is within a central portion of (i.e. centred in) a first scale division <NUM> of the first absolute position track 30a and the second position of the reader is within a central portion of (i.e. centred in) a second scale division <NUM> of the first absolute position track. In this embodiment, the second scale division <NUM> is adjacent to the first scale division <NUM>.

<FIG> schematically shows a sensing of a digital code of the first plurality of digital codes when the reader is relatively centred in the first scale division <NUM>, the position of the reader being schematically indicated by the arrow <NUM>. In this spatial relationship, the detectors 70a-77a of the first absolute sensing unit 60a sense a subset of first absolute marking 32a, 33a being located in the sensing volume of each detector so as to provide (individual) signals being discretized in the code A<NUM> =<NUM>. This code A<NUM> provides the unique identification of this first scale division <NUM>.

The absolute position <NUM> of the reader <NUM> can thus be determined by adding or mathematically combining the precise relative position provided by sensing the incremental track <NUM> with the absolute reference associated with the first scale division <NUM> of the first absolute position track 30a. The absolute reference corresponds in this embodiment to the (lower value) edge 37a of said first scale division <NUM>.

<FIG> schematically shows a sensing of a digital code of the first plurality of digital codes when the reader <NUM> is slightly displaced with respect to the centre of this first scale division <NUM>. Even if not the entire subset of discrete region 32a of the first absolute marking of the first absolute position track 30a are completely located in the sensing volume of each detector, the provided individual signals still provide a discretisation of the code A<NUM> assigned to the first scale division <NUM>. The absolute position <NUM> of the reader can still be determined by adding the current precise relative position provided by sensing the incremental track <NUM> with the absolute reference 37a associated with the first scale division <NUM> (i.e. the first absolute position).

<FIG> schematically shows a sensing of a digital code of the first plurality of digital codes when the reader <NUM> approaches the intermediate position between two adjacent scale divisions <NUM>, <NUM>, i.e. the reader <NUM> is relatively positioned within a (right) peripheral portion of the (first) scale division. In other words, some of the first series of detectors 70a-77a of the reader <NUM> are aligned with transitions between separating and discrete regions 32a, 33a. In this peripheral portion, some discrete regions 32a of the absolute marking are (essentially) halfway located in the sensing volume of some detectors (i.e. detector 70a, 76a) of the first absolute sensing unit, as the digital code is changing from a first to a second code. The discretization of the provided signals can lead to an unreliable discriminated digital code Ax51 whose transient digits could be undetermined up to erroneously discriminated. Thus, the absolute position <NUM> of the reader can cannot be determined reliably as the absolute reference associated with the sensed scale division AX51 (i.e. the first absolute position) can be instable up to wrong. This position of the reader are parts of said unfavourable positionings.

<FIG> schematically shows a sensing of a digital code of the first plurality of digital codes when the reader is relatively positioned within the central portion of another scale division <NUM>. None of the detectors 70a-77a is aligned with a transition between separating and discrete regions 32a, 33a. In this spatial relationship, a new subset of absolute marking is fully located in each of the sensing volume of the detectors of the first absolute sensing unit so as to provide a (correct) discretization of each transient digits. This discretization leads to a new code A<NUM> =<NUM> undoubtedly identifying the second scale decision <NUM>. The absolute position <NUM> of the reader can still be determined by adding the current precise relative position with the absolute reference 37b associated with second scale division <NUM>.

In order to provide robustness in intermediate positions between two adjacent scale divisions <NUM>, <NUM>, sensing elements of the reader and the digital scale are further configured to provide, for the same positioning of the reader <NUM>, a second absolute position code, notably representing a second absolute position that is different from the first absolute position, so as to provide a reliable position in this unfavourable positioning of the reader. In this embodiment, the digital scale is further configured to allow the reader to sense a second position code, notably in form of one of a plurality of unique identifiers B<NUM>,B<NUM>. Each unique identifier is assigned to a peripherical portion T2a, T2b of each of the scale divisions (<FIG>), notably to the left and right peripherical portion of two adjacent scale divisions. In these intermediate positions of the encoder, the absolute position <NUM> can be provided by mathematically combining (e.g. adding or subtracting) the precise relative position <NUM> with an absolute reference associated with the sensed overlapping scale division <NUM>', <NUM>', <NUM>' (i.e. the second absolute position) of the second absolute position track 30b. The second absolute position preferably correspond to one of the (lower or upper value) edges 37a', 37b', 37c' of the sensed overlapping scale division <NUM>'-<NUM>', or to a central position of the sensed overlapping scale division.

In particular, each unique identifier of the second plurality of identifiers can be assigned to each of a plurality of overlapping scale divisions <NUM>'-<NUM>', the overlapping scale divisions being assigned to the incremental track <NUM> and/or to the scale divisions <NUM>-<NUM> of the first absolute position track 30b.

The plurality of overlapping scale divisions is thus provided on the second absolute position track 30b of the digital scale along the measuring path, each overlapping scale division <NUM>' spatially overlapping (adjacent) parts 31a, 31b of (both) two adjacent scale divisions <NUM>, <NUM> (cf. <FIG>) of the first absolute position track 30a. Advantageously, each overlapping scale division comprises the (two) adjacent peripherical portions of two adjacent scale divisions, notably the (right-positioned) peripherical portion of a scale division and the (left) peripherical portion of the (right-sided) adjacent scale division. The second absolute position track 30b of the digital scale is configured to enable the reader to sense the unique identifier B<NUM>,B<NUM> of the second plurality of identifiers, notably assigned to each of these overlapping scale divisions <NUM>'-<NUM>'.

The use of overlapping scale divisions provides thus an absolute position detection in the peripherical portions of the scale divisions <NUM>-<NUM> of the first absolute position track 30a, where a unique identifier of the first plurality of identifiers has to change from a first to a second value, constituting unfavourable positioning of the reader where the first absolute position is likely/probably unreliable.

The overlapping scale divisions are shaped according to the (shape of) the pitch P and/or to the (shape of) of the scale divisions of the first absolute position track and/or to the (local) shape of the measuring path. Each overlapping scale division can notably be linear-, curved- or angularshaped along the measuring path.

Similar to the scale divisions of the first absolute position track, the unique identification of overlapping scale divisions of the second absolute position track are arranged so as the reader can sense a plurality of unique identifiers B<NUM>,B<NUM> along the measuring path, each intermediate unique identifier being distinct (i.e. unique) and dependent on the positional relationship between the reader and the scale along the measuring path, according to the scale divisions <NUM>'-<NUM>' of the second absolute position track 30b.

This embodiment enables that, thanks to a particular relative disposal between the sequence of discrete regions <NUM>; 32a, 32b and the first and second series of detectors 70a-77a, 70b-77b, none of the detectors of at least one of the first and the second series of detectors is aligned with a transition between a discrete region <NUM>; 32a, 32b and a separating region <NUM>; 33a, 33b at each possible position of the reader <NUM> relative to the digital scale.

The sensing of a unique identifier of the first and second plurality of identifiers can rely on a unique absolute position track <NUM> comprising discrete region 32c and separating region 33c therebetween as shown in the embodiment of <FIG>, on a first and second absolute marking provided on a first and a second absolute position track 30a, 30b as per the embodiment of <FIG> or on a combination thereof (non-illustrated).

The overlapping scale divisions <NUM>'-<NUM>' can be spaced apart along the measuring path. Alternatively, the overlapping scale divisions can be located (essentially) one borders another (i.e. without overlapping and without free space between the divisions), along the measuring path, so as an edge of one overlapping scale division essentially corresponds to a edge of the adjacent one.

Advantageously, each overlapping scale division <NUM>'-<NUM>' can be (essentially) centred between two adjacent main scale divisions (i.e. in correspondence with an intermediate position and/or a common edge 37a, 37b between two adjacent main scale divisions) along the measuring path so as to provide a more robust absolute position detection in each transition between a discrete region and a separating region.

Alternatively or complementarily, each overlapping scale division <NUM>'-<NUM>' extends along the measuring path by the same spatial extension T of the main scale divisions <NUM>-<NUM>.

The reader <NUM> of the encoder <NUM> can comprise a second absolute sensing unit 60b with a plurality if detectors 70b-77b as shown for example in <FIG> for sensing discrete regions 32b and separating regions 33b therebetween.

The unique identifiers of the second plurality of identifiers can be technically enabled similarly to the unique identifiers of the first plurality of identifiers, as above described.

The unique identifiers of the second plurality of identifiers can be provided along the measuring path, i.e. each unique identifier of said second plurality of identifiers occurring exactly once as a portion of the sequence. Alternatively, the unique identifiers are in the form of a series of juxtaposed identifiers (i.e. placed side by side without overlap) along the measuring path.

The second plurality of unique identifiers can correspond to a second plurality of digital codes, each of those being unique and assigned to one of said overlapping scale divisions. Similarly, a digital code of the second plurality of digital codes can be any code having a discrete, discontinuous representation, notably being processable by a computer (e.g. by means of a processor or controller) or by an equivalent thereof (e.g. a microprocessor, a FPGA, or an ASIC). Preferably the second plurality of digital codes are binary codes with a plurality of digits, preferably having N digits with N=<NUM>,<NUM>,<NUM> or greater than <NUM> so as to provide a unique identification of N<NUM> distinct overlapping scale divisions. More preferably, the second plurality of digital codes have the same representation as the first plurality of digital codes.

Similarly to the first plurality of digital codes, the second plurality of digital codes can rely on a Gray coding for limiting the number of different bits between two adjacent digital codes or can rely or correspond (entirely or partially) to a DeBrujin sequence.

In the illustrated embodiment of <FIG>, the second plurality of unique identifiers take the form of a (continuous) sequence of digital codes (B<NUM>,B<NUM>), each of the second plurality of digital codes BN being unique and assigned to one of said overlapping scale divisions <NUM>-<NUM>, as above mentioned. The second of plurality of digital codes are binary code with N digits, N=<NUM> providing a unique identification up to <NUM> distinct scale divisions. The second plurality of digital codes relies on (a portion of) a DeBruijn sequence so as to enable a continuous sequence of codes along the measuring path, each digital code being extracted (sensed) from such sequence depending on the relative position of the reader with respect to the scale. In the illustrated embodiment of <FIG>, the second plurality of digital codes have the same digital representation as the first plurality digital codes and relies on the same code generation, i.e. the codes (and the absolute markings thereof) are correlated as shown in particular in <FIG>.

Alternatively, identification of overlapping scale regions can rely on the second plurality of unique identifiers having other digital representations, being or not correlated with the digital representation of the first plurality of unique identifiers. In an embodiment shown in <FIG>, the digital scale <NUM> comprises a second absolute position track 30b provided with discrete and separating regions 32b, 33c arranged to correspond to binary inverse codes of the first plurality of digital codes.

In <FIG>, the overlapping scale divisions <NUM>'-<NUM>' of the second absolute position track 30b are spatially shifted by a spatial period T/<NUM> (corresponding to half of the spatial extension T of the first scale division) along the measuring path with respect to the scale divisions <NUM>-<NUM> of the first absolute position track 30a. In other words, each scale division of the second absolute position track 30b is essentially centred with respect to two adjacent scale divisions of the first absolute position track 30a along the measuring path. The marking of the second absolute position track 30b is shifted towards the left with respect to the marking of the first absolute position track 30a but may be shifted to the right according to another embodiment.

Alternatively, as illustrated in <FIG>, the digital scale <NUM> may comprise a first and a second absolute position track 30a, 30b. The absolute marking 32a, 33a of the first absolute position track 30a is the same (or inverted according to a variant not shown) as the absolute marking 32b, 33b of the second absolute position. The first and second absolute position tracks 30a, 30b are aligned along the measuring path. In this embodiment, the reader <NUM> of the <FIG> comprises a first absolute sensing unit 60a with a first series of detectors 70a-77a, and a second absolute sensing unit 60b with a second series of detectors 70b-70b. Each detector of the first absolute sensing unit 60a is offset from a corresponding detector of the second absolute sensing unit 60b by a spatial period T/<NUM>.

Alternatively to the embodiment of <FIG>, the digital scale <NUM> can comprises a single absolute position track <NUM> with the same and aligned absolute markings <NUM>, <NUM> as illustrated in <FIG>. In order to provide both the first and second unique identifiers, the reader <NUM> of <FIG> comprises two adjacent absolute sensing units 60a, 60b extending along the travel direction of the reader. As for the embodiment of <FIG>, each detector of the first absolute sensing unit 60a is offset from a corresponding detector of the second absolute sensing unit 60b by a spatial period T/<NUM>.

The above-described embodiments enable a robust detection of the absolute position <NUM> of the reader <NUM> at each of its possible positions relative to the digital scale <NUM>. These embodiments rely on a digital scale <NUM> comprising: an incremental track <NUM> for providing a relative position <NUM> of the reader <NUM> within a scale division of a series of scale divisions <NUM>-<NUM> on a first absolute position track 30a. The reader can individually read each of the first and second plurality of unique identifier A<NUM>, A<NUM>, B<NUM>, B<NUM>. Each of the first plurality of identifiers A<NUM>, A<NUM>, is assigned to a (central) portion of a scale division of the first absolute position track 30a, while each of the second plurality of identifiers B<NUM>, B<NUM>, is assigned to another portion of the scale division, notably the (right or left) peripherical portion of the scale division. Preferably, each of the second plurality of identifiers is assigned to an overlapping scale division (continuously) comprising the adjacent peripherical portions of two adjacent scale divisions of the first absolute position track 30a.

With respect to the embodiment of <FIG>, <FIG> schematically show the robust effect of the scale divisions <NUM>'-<NUM>' of the second absolute position track 30b on sensing the absolute coarse position of the reader <NUM>.

When the detector 70a of the first absolute sensing unit of the reader <NUM> is relatively positioned in a transition region between two adjacent scale divisions <NUM>, <NUM> of the first absolute position track 30a (i.e. in the peripherical portion) the detector 70b of the second absolute sensing unit of the reader <NUM> is then relatively located (notably centred) within the edges 37b', 37c' of an overlapping scale division <NUM>' (e.g. in the central portion thereof) of the second absolute position track 30b, as illustrated in <FIG>. In contrast to the unreliable discriminated main digital code Ax51, as none of the detectors 70b-77c is aligned with a transition between discrete and separating regions 32a, 33a, the intermediate digital code B<NUM> can be reliably discriminated and allowing an undoubtably identification of the related overlapping scale division <NUM>'.

According to the invention, the central portion of a scale division of the first absolute position track 30a or of a unique absolute position track <NUM> is a portion of the scale division where the unique identifier is undoubtedly discriminable by the reader, e.g. the absolute marking providing the unique identifier are relatively located (along the travel direction of the reader) in the sensing volume of the detectors of the reader, completely down to <NUM>% (i.e. equal or more than <NUM>%), preferably down to <NUM>% (i.e. equal or more than <NUM>%).

According to the invention, the (left and the right) peripherical portion of a scale division is a portion of the scale division where the first unique identifier is doubtfully discriminable by the reader, e.g. the absolute marking providing a first unique identifier are relatively located (along the travel direction of the reader) in the sensing volume of the detector of the reader less than <NUM>%, preferably less than <NUM>%.

The encoder <NUM> can thus provide the absolute position <NUM> of the reader <NUM> with respect to the scale as function of the precise relative position <NUM> provided by sensing the incremental track <NUM> and (an absolute reference associated with) the scale division <NUM>'-<NUM>'. The absolute reference associated with the scale division of the second absolute position track 30b can correspond to the (lower or upper value) edges 37a', 37b', 37c' of the sensed scale division <NUM>'-<NUM>' of the second absolute position track 30b. Alternatively, depending on the precise relative position <NUM> provided by sensing the incremental track <NUM>, the absolute reference associated with scale division can selectively correspond to the (lower or upper value) edges 37a, 37b, 37c of one of the overlapped scale divisions <NUM>-<NUM>, notably to the overlapped edges.

Similarly to the first plurality of digital codes, the second plurality of digital codes are also affected by edge effects in correspondence to code transitions (e.g. peripherical portions of the overlapping scale divisions). These transitions are however unaligned with the transitions of the first plurality of digital codes, as illustrated in <FIG> and <FIG> (e.g. Bx61, Bx62).

The use of overlapping scale divisions <NUM>'-<NUM>' of the second absolute position track 30b, each overlapping a part 31a, 31b of each of (i.e. both) two adjacent scale divisions <NUM>-<NUM> (notably the peripherical portions thereof) of the first absolute position track 30a, provides thus an absolute position detection in critical spatial (i.e. peripherical) regions of the scale divisions, this critical regions corresponding to transition regions of the (sensed) first plurality of unique identifiers.

As the intermediate zones of the overlapping scale divisions are not aligned with the (edges of the) intermediate zones of the scale divisions of the first absolute position track 30a, i.e. the peripherical portions of the scale divisions of the first absolute position track 30a are not aligned (preferably not overlapping) with the peripherical portions of the overlapping scale divisions, the encoder <NUM> can provide a more robust detection all along the travel direction of the reader by the use of the first and second plurality of unique identifiers obtained from sensing the marking of both the first and second absolute position tracks 30a, 30b by respective first and second absolute sensing unit 60a, 60b of the reader <NUM>.

The reader <NUM> is configured to select one of the first and second unique identifiers to determine the absolute position <NUM> of the reader <NUM>, according to (as the function of) the relative position provided by sensing the incremental track <NUM>, as illustrated in <FIG>, so as to use a reliable (first or second) absolute position.

Knowing the relative position of the (transitional) regions of the scale divisions of the first absolute position track <NUM> with respect to incremental track <NUM>, the encoder <NUM> rely on the position of the first or the second absolute position (via the first and/or the second unique identifiers), depending on the sensed incremental track, notably by determining relative position within the scale division of the first absolute position track 30a. If the relative position is near or within a portion of a transition that may impact the reliability of the unique identifier obtained by sensing the first absolute position track (i.e. the encoder estimates that some of the detectors 70a-77a are probably aligned with a transition between separating and discrete regions), the reader can decide to rely on the second unique identifier rather than first unique identifier.

In the exemplary illustration of <FIG>, each scale division (of spatial extension T equal to the pitch P) can be split in four distinct quadrants. Each quadrant can be uniquely identified by the relative position of the reader within the scale division of the first absolute position track 30a. Alternatively or complementarily, each quadrant can be uniquely identified by the signs of the (unbiased) four sinusoidal signals of <FIG> provided by the series of detectors <NUM>-<NUM> of the incremental sensing unit <NUM> of the reader by sensing the incremental track <NUM>. A first unique identifier can thus be assigned to a subset of these quadrants (e.g. the second and the third quadrant of <FIG>), while a second unique identifier can be assigned to the remaining subset (e.g. the first and the fourth quadrant of <FIG>). In particular, the central portion T<NUM> of the scale division of the first absolute position track 30a can be defined to correspond to the <NUM>nd and <NUM>rd quadrant, while the (left) peripherical portion T2a to the <NUM>st quadrant and the (right) peripherical portion T2b to the <NUM>th quadrant.

By sensing the incremental track <NUM> by the incremental sensing unit <NUM> (notably via the sensed relative position) and by knowing the spatial relationship between the incremental track and the first and the second absolute position track (notably the spatial relationship between the transitions thereof), the encoder can estimate, for the current position of the reader, if none of the detectors of the first series of detectors 70a-77a is aligned with a transition between a discrete region and a separating region and/or if none of the detectors of the second series of detectors 70b-77b is aligned with a transition between a discrete region and a separating region. Depending on the estimations, the encoder can select the first or the second absolute position code provided by the first and the second absolute position track respectively.

Advantageously, the reader and the marking arrangement can be configured to permit a separate sensing of one or more second unique identifiers from the sensing of one or more first unique identifiers (and vice versa). This allows the reader to select the first unique identifier or the second unique identifier for determining the identifier, notably as function of the determined (or forecasted) relative position provided by sensing the incremental track. Advantageously, the first and the second absolute position can be simultaneously (i.e. at the same time, i.e. within a time span of less than <NUM>, preferably less than <NUM>) provided.

The absolute position encoder <NUM> of <FIG> is used for providing an absolute position of the reader <NUM> relatively to a circular or round track. The track comprises an incremental track <NUM>, a first absolute position track 30a and a second absolute position track 30b. The second absolute position track can be located on a (disc-shaped) scale <NUM> along a circular (disc- or ringshaped) path for providing an angular absolute position of the reader. The angle can be concaved or convex. The reader comprises a first and a second absolutes sensing unit 60a, 60b to sense respective first and second absolute position tracks 30a, 30b and an incremental sensing unit <NUM> for sensing the incremental track <NUM>.

The absolute position encoder <NUM> can thus enable a method for providing an absolute position of the reader <NUM> relatively to a digital scale <NUM>.

The absolute position encoder <NUM> can be configured to inductively sense at least a subset of the marking arrangement. The absolute markings and/or the periodic markings can be thus conducting and/or permeable elements, while the reader <NUM> can comprise inductive or eddy current sensing units 60a, 60b <NUM> for inductively sense the absolute markings and/or the periodic markings.

Alternatively or complementarily, the reader can be configured to capacitively and/or magnetically and/or optically sense at least a (or another) subset the marking arrangement and/or the periodic markings, said subset of marking arrangement and/or the periodic markings enabling a capacitively and/or resistively and/or magnetically and/or optically sensing.

With reference to <FIG>, the method comprises a first step S1 involving a sensing of the incremental track <NUM> of the digital scale <NUM> by the reader <NUM>.

The method comprises a second Step S2 involving a determination or a forecasting of a relative position <NUM> of the reader <NUM> within one of the scale division <NUM>-<NUM> of at least one absolute position track <NUM>; 30a, 30b, for example within one of the scale division <NUM>-<NUM> of a first absolute position track, by means of the sensed incremental track. In other words, the second Step S2 involves an estimation based on the sensed incremental track <NUM> (notably on the determined/forecasted relative position) if, for the current position of the reader <NUM> relative to the digital scale <NUM>, none of the detectors of the first and/or of second series of detectors is aligned with a transition between a discrete region <NUM>; 32a, 32b and a separating region <NUM>; 33a, 33b.

A third step S3 consist in selecting a first digital code AN (S4) or a second digital codes BN (S5) provided by sensing the markings on both first and second absolute position tracks 30a, 30b as function of the determinate or forecasted relative position <NUM> of the reader <NUM>. In other words, the third Step S3 involves a selection of the first or the second absolute position code based on the estimation that none of the first or second series of detectors is aligned with a transition.

The step of determining the relative position <NUM> of the reader can comprise a detection of the determinate or forecasted relative position <NUM> within the central portion and/or within the (left and/or right) peripherical portion of a scale division of the first absolute position track 30a. The selection of first or a second digital code AN, BN can be function of the detected central and/or peripherical portion of the scale division of the first absolute position track 30a.

Step S6 comprises providing the absolute position <NUM> of a reader as function of the sensed relative position <NUM> and of the determined digital code AN, BN. The step of providing the absolute position <NUM> of the reader can also comprise correcting the relative position, e.g. by applying linear and polynomial corrections.

The absolute position encoder <NUM> and the method can be advantageously implemented and enabled in a measuring instrument and/or an accessory thereof, notably for dimensional metrology (i.e. quantification of one or more physical sizes of or distance from a given object). More particularly, the absolute position encoder <NUM> and the method can be advantageously implemented and enabled in a measuring instrument and/or an accessory thereof notably providing one or more of the following metrological features: a linear dimension, a thickness, a radius, an inner diameter, or an outer diameter of an object; or a coordinate, roughness, or surface finish of a surface of an object. The measuring instrument can be a hand-held measuring instrument (e.g. digital sliding calliper, digital micrometre) or a transportable measuring instrument (e.g. height gauge, measuring probes). The measuring instrument can also be a stationary measuring instrument, such as a coordinate measuring machine (CMM) or measuring robot provided with a granite (or reference) table on which object have to be placed for measurement (e.g. bridge CMM). The accessory can be a rotary table, a rotary axis or an articulated probe head for orienting a (contactless or contact) probe (notably with respect to a mobile or fixed portion, or component, of the measuring instrument). The measuring instrument can be connected to, or part of, a measuring apparatus, an inspection system or a machine tool.

The encoder <NUM> can also include more than one reader, i.e. a plurality of readers each individually movably relatively to the (same) scale, notably along the travel direction of the reader, e.g. for providing position of a plurality of mobiles components of the measuring instrument.

The surface of scale on which the encoder(s) is moveable can be not only (substantially) planar but also non-planar, e.g. curved or round.

Advantageously, the absolute position encoder can comprise an electronic circuit <NUM> (cf. <FIG>), the electronic circuit being configured to provide the position <NUM> of the reader with respect to the scale along the measuring path, notably to provide a digital representation thereof. The electronic circuit <NUM> can be part of the reader.

The electronic circuit can comprise a programmable electronic circuit (such as a microcomputer, a microcontroller, or a FPGA), or a dedicated electronic circuit (e.g. an ASIC, or discrete-element circuit).

<FIG> shows another embodiment of the absolute position encoder <NUM> for less accurate applications. The absolute position encoder <NUM> comprises a reader <NUM> with a first and a second sensing unit 60a, 60b configured to sense respective discrete regions 32a, 32b and separating regions 33a, 33b of respective first and second absolute position tracks 30a 30b of a digital scale <NUM>. The first and second absolute position tracks 30a 30b extend next to each other along the travel direction of the reader <NUM> so as to provide the first and the second absolute positions. This absolute position encoder <NUM> is devoid of any incremental track as it is designed for less accurate applications.

The absolute position encoder <NUM> of <FIG> is thus able to provide the position <NUM> of the reader <NUM> (<FIG>) even in an unfavourable positioning of the reader as it has always at disposal at least one robust absolute position provided by the first and/or the second absolute position tacks 30a, 30b.

In another embodiment schematically shown in <FIG>, the absolution position encoder comprises a digital scale <NUM> having a single marking made of discrete regions <NUM> and separating regions <NUM> therebetween. The reader comprises a first series of detectors 70a-77a and a second series of detectors 70b-77b. The first and second series of the detectors extend along the travel direction of the reader and next to each other. The position of each detector of the first series of detectors 70a-77a is offset with respect to the corresponding detector of the second series of detectors 70a-77a along the travel direction of the reader. The offset may for example correspond to half the sensing area along the travel direction. The position encoder is configured to select the first or second absolute position code which is read by the series of detectors among which no detector is aligned with a transition between a discrete region <NUM> and a separating region <NUM> of the digital scale <NUM>.

In another embodiment schematically shown in <FIG>, the absolution position encoder comprises a digital scale <NUM> having a single marking made of discrete regions <NUM> and transition regions <NUM> therebetween. The reader comprises a first series of detectors 70a-77a and a second series of detectors 70b-77b. The detectors of the first series of detectors 70a-77a are interleaved with the detectors of the second series of detectors 70b-77b to form pair of adjacent detectors 70a-70b, 71a-71b, 72a-72b, 73a-73b, 74a-74b, 75a-75b, 76a-76b, 77a-77b. The position encoder is configured to select the first or second absolute position code which is read by the series of detectors among which no detector is aligned with a transition between a discrete region <NUM> and a separating region <NUM> of the digital scale <NUM>.

Advantageously, the absolute position encoder <NUM> can be configured to estimate or determine a reliability and/or an unreliability of the first absolute position, and/or of the second absolute position respectively, in order to assure a more reliable position of the reader. The reliability and/or unreliability estimation or determination can be based on (e.g. a verification of): a given spatial relationship between the first and the second absolute position. In fact, if the first and the second absolute position are related to adjacent absolute positions, the first absolute position (and even the second absolute position) can be estimated as reliable. The absolute position encoder <NUM> can thus estimate/determine that the reader is probably positioned in an intermediate zone between the centres of the two divisions). The reader position <NUM> can thus relies on the first absolute position. Alternatively or complementarily, the reader position <NUM> can relies on the second absolute position.

Alternatively or complementarily, the absolute position encoder <NUM> can be configured to estimate or determine a reliability and/or an unreliability based on (e.g. a verification of) a given digital pattern of a digital representation, or relationship between digital representations related to first and/or the second absolute position, e.g. the first and the second unique digital identifier.

As illustrated in <FIG>, in case of a failure of the above-described estimation/determination, the absolute position encoder <NUM> can be configured to estimate or determine if the reader is an unfavourable position with respect to the second absolute position, i.e. the (sensed) second absolute position is incorrect (as one or more detectors thereof is/are aligned with transitions). This situation is similar to these represented by <FIG>. This spatial situation can be verified by checking if the digital representation on which the second position relies presents a given pattern. The pattern can correspond to the common digits (or invariable pattern, i.e. non-transient digits) of the two adjacent unique digital identifiers that are spatially related to the first absolute position (i.e. whose overlapping scale divisions overlap the main scale division indicated by/linked by the first absolute position). Common digits correspond thus to bits of the second absolute position code derivable from the detectors of the second series of detectors 70b-77b that are not facing a transition. Using the exemplary situation illustrated by <FIG>, considering the first absolute position 37a, the encoder can verify the reliability of this first absolute position 37a by verifying if the digital representation (B), on which the second position relies, comprises the common digits of the division <NUM>' and <NUM>' (i.e. BX61).

In case this digital representation comprises the pattern, the absolute position encoder can thus estimate/determine that the reader is probably positioned in a central zone of the scale division of the first absolute position track that correspond to a transition zone (i.e. unfavourable positioning) of the overlapping scale division. The reader position <NUM> can thus relies on the first absolute position as considered reliable, at the contrary of the second absolute position.

Complementarily or alternatively, notably in case of a failure of the above-described estimation/determination, the absolute position encoder can be configured to estimate or determine if the reader is an unfavourable position with respect to the first absolute position, i.e. the (sensed) first absolute position is incorrect (as one or more detectors thereof is/are aligned with transitions).

This situation is similar to those represented by the <FIG>. Similarly to the above-described proceeding, this spatial situation can be verified by verifying if the digital representation on which the first position relies (e.g. identifier of the first plurality of identifiers) presents a given pattern. The pattern can correspond to the common digits (i,e, non-transient digits) of the two adjacent unique digital identifiers that are spatially related to the second absolute position (i.e. whose scale divisions overlap the overlapping scale division indicated by/linked by the second absolute position). In this case, the common digits corresponds to bits of the first absolute position code derivable from the detectors of the first series of detectors 70a-77a that are not facing a transition Using the exemplary situation illustrated by <FIG>, considering the first absolute position 37a, the encoder can verify the reliability of this second absolute position by verifying if the digital representation (A) on which the first position relies comprises the common digits of the scale divisions <NUM> and <NUM> (i.e. AX51) of the first absolute position track.

In case this digital representation comprises the pattern, the absolute position encoder can thus estimate/determine that the reader is probably positioned in the junction between a discrete region and a transition region (i.e. unfavourable positioning) related to the first absolute position. The reader position <NUM> can thus relies on the second absolute position as considered reliable, at the contrary of the first absolute position.

Alternatively, the absolute position encoder can estimate/ determine a reliability of the first and/or of the second absolute position by verifying a given digital property of the digital representations thereof (e.g. the unique digital identifiers thereof). The digital property can be: a digital signature, a redundance code, a cyclic or block redundancy check (e.g. CRC).

The absolute position encoder can thus provide the position <NUM> (<FIG>) relying on:.

<FIG> shows an exemplary usage of the absolute position encoder for dimensional metrology, notably for a sliding calliper.

The sliding calliper <NUM> comprises one of the above-described absolute position encoders, preferably an absolute position encoder relying on period position as illustrated in the exemplary embodiment of <FIG>. The sliding calliper <NUM> comprises a first jaw <NUM> fixed to a calliper casing <NUM> and a second jaw <NUM> connected to a digital scale <NUM> and slidable relative to the first jaw <NUM>. The sliding calliper <NUM> further comprises a display <NUM> for displaying the distance between the first and second jaws <NUM>, <NUM> provided by the absolute position encoder, preferably by means of the electronic circuit <NUM>.

<FIG> and <FIG> show of the numerous possible usages or applications of the invention an exemplary usage for geodetic measuring or surveying. As known in the art of geodetic surveying, target points are surveyed by placing specifically embodied transportable target objects, for example surveying poles <NUM>, <NUM>', <NUM>", at the geodetic target point <NUM>. In order to measure and/or stake-out terrain points, generic surveying pole systems can be used as equipment cooperating with Terrestrial Position Systems (TPS) <NUM> such as a total station or as a cooperative or stand-alone Global Navigation Satellite Systems (GNSS) device. The term surveying pole system refers at least to a surveying pole and a surveying pole with an outsourced (but connected) processing unit.

That is, said target objects comprise for instance a plumb pole <NUM> with a retroreflector <NUM> for defining the measurement section or the measurement point. Another pole <NUM>' depicted has a target plate <NUM>' for instance measurable using a camera of a surveying instrument. Alternatively, a pole <NUM>" may have a GNSS antenna <NUM>". Combinations thereof are known, too.

The position measurement of a measurement or stake-out target point <NUM> is an indirect one: with a TPS <NUM> as shown in <FIG>, a reference point on the surveying pole <NUM> is measured for example by using a reflector and measuring distance and direction from the TPS <NUM> to the reflector. Such a reference point is for example the central point of the retroreflector <NUM>, or in case of a GNSS-pole, a GPS-antenna mounted on the pole <NUM>". Since a tip <NUM>, <NUM>', <NUM>" of the surveying pole <NUM>, <NUM>', <NUM>" is placed on the actual target point <NUM> in the terrain, the position of this target point <NUM> can be derived due to a determinable spatial relationship between the reference point and the tip <NUM>, <NUM>', <NUM>" of the pole <NUM>. This approach particularly allows to measure or stake-out a point <NUM> that could not be measured or staked-out directly due to an obstacle such as the wall shown in <FIG> between the TPS <NUM> or GNSS and the point <NUM>.

Such an indirect measurement may require a free line of sight between the primary sensor, e.g. a laser based optical distance meter of TPS <NUM>, and reflector/antenna. Additionally, the spatial relationship between a measured center of the reflector or target plate or antenna <NUM>, <NUM>', <NUM>" and the pole tip <NUM>, <NUM>', <NUM>" needs to be known.

To provide further flexibility for such obstacle workarounds, surveying poles <NUM>, <NUM>', <NUM>" as shown provide a length adjustability for the pole <NUM>, <NUM>', <NUM>", which enables a continuously variable pole length or several distinct pole length (the latter for example by multiple lock positions). For this purpose, a telescopic structure of the pole <NUM>, <NUM>', <NUM>" with at least two elements <NUM>, <NUM>', <NUM>" and <NUM>, <NUM>', <NUM>" moveable relative to each other provides not only the length adjustability, but also involves an encoder <NUM> according to the invention for absolute length measurement and determination of the actual pole length and optionally for conversion between measurement units or standards, too.

As is shown in <FIG>, the length adjustment of the surveying pole can be realised in several ways. For instance, the surveying pole <NUM>, <NUM>', <NUM>" has two telescopic sections, whereby as is shown on the exemplary pole <NUM> with the reflector <NUM>, the upper telescopic pole section <NUM> of the surveying rod <NUM> can be screwed as indicated by the swung arrow onto the lower rod section <NUM> (or vice-versa). Alternatively, and as is shown on the pole <NUM>" with the GNSS antenna <NUM>", the upper tube (section) <NUM>" may be thinner in order to be slidable into the lower tube <NUM>" or the other way round as shown with pole <NUM>' where the lower part <NUM>' is slidable into the upper rod part <NUM>'.

A pole <NUM>, <NUM>', <NUM>" with continuously variable pole height h has a built-in measuring unit which allows to determine the pole height h at in each position of the two telescopic sections. Namely, the length-adjustable surveying pole <NUM>, <NUM>', <NUM>" provides its current length, that is the distance between the length reference point (e.g. center of reflective prism <NUM> or target plate <NUM>') and the pointing tip <NUM>, <NUM>', <NUM>" using the absolute position encoder <NUM> according to the invention. Thereby, the scale <NUM> is read by reader <NUM> as in principle described above, allowing to determine the actual rod length and actual pole height h at any position. The surveying pole <NUM>, <NUM>', <NUM>" may integrate more than one encoder <NUM>, e.g. if there are more than two telescopic sections present.

In the poles <NUM>, <NUM>' with the retroreflector <NUM> or the target plate <NUM>', the absolute scale <NUM> is extending at least over a part of the lower part <NUM>, <NUM>' of rod <NUM>. The scale <NUM> extends at least along the complete possible length variation and is read by reader <NUM> situated in the upper part <NUM>, <NUM>'. In exemplary GNSS pole <NUM>", the configuration is different in that the scale <NUM> of encoder <NUM> is on the inner surface of the hollow lower part <NUM>" (wherefore it is not shown in <FIG>). Of course, it is possible, too, to place the reader <NUM> in or at the lower part <NUM>, <NUM>', <NUM>" and the scale <NUM> in or on the upper part <NUM>, <NUM>', <NUM>".

The extension of the scale <NUM> in another direction (scale width, e.g. perpendicular to the length variation/measuring path) can for example be chosen dependent on the possible degree of freedom of movement in that direction. If for example the upper part <NUM>, <NUM>', <NUM>" and the lower part <NUM>, <NUM>', <NUM>" are fully rotatable to each other (<NUM>° rotation possible), the scale <NUM> can be a complete ring structure, encoding the rod length and thus being readable in any position on the whole <NUM>°circumference. Additionally or alternatively, the reader <NUM> is designed in such a way, e.g. being ring-like, that the scale <NUM> can be read in any rotational position of the two segments <NUM>, <NUM>', <NUM>‴ and <NUM>, <NUM>', <NUM>" to each other.

In some embodiments, the scale <NUM> is embedded in the pole rod <NUM>. That is, the scale <NUM> can be integrated or incorporated into the structure of one of segments <NUM>, <NUM>', <NUM>‴ and <NUM>, <NUM>', <NUM>", e.g. mechanically or already during the build-up process of the rod <NUM>.

As a further option, some or all parts are embodied as separate modules. For example, the complete encoder <NUM> can be supplied as an add-on module to upgrade conventional poles. Therefore, the scale <NUM> may e.g. be stuck onto a segment <NUM>, <NUM>', <NUM>" or <NUM>, <NUM>', <NUM>" and the reader <NUM> clamped or fastened onto the other segment <NUM>, <NUM>', <NUM>" or <NUM>, <NUM>', <NUM>", thereby the encoder <NUM> providing preferably a self-calibration functionality. As another example, preferably if the scale <NUM> is part of the lower part <NUM>, <NUM>', <NUM>", in some embodiments, the lower part <NUM>, <NUM>', <NUM>" may be exchangeable such that pole <NUM>, <NUM>', <NUM>" may be equipped with different lower parts <NUM>, <NUM>', <NUM>" comprising different scales. Similarly, the reader <NUM> or the whole encoder <NUM> may be exchangeable, e.g. by the user respectively in the field. As other options, units such as a communication device, power unit or tilt sensors are embodied as (add-on) modules, which may be exchangeable.

The encoder <NUM> allows for continuously determining the pole height h. In some embodiments, the rod <NUM>, <NUM>', <NUM>" is designed in such a way that several fixed lock positions are provided for secure fixation of distinct pole lengths. In this case, the encoder <NUM> can be used to control or verify these lock positions. For example, based on the encoder measurement it is checked if a lock position is indeed installed and a warning can be given out to a user if not, e.g. if the installed pole length is only close to a lock position but actually not a lock position. This principle can be used, too, to verify if a lock position itself is position true i.e. if the actual lock position corresponds to its nominal position. For instance due to environmental or aging effects, which is particularly critical in case of telescopic structures, the lock position may change which however can be noticed using the encoder <NUM>. Above that, the encoder measurements can be used to calibrate such distinct lock positions wherefore e.g. rod aging effects can be compensated. Spoken generally, measurement errors caused by the false assumption of using a pole length according to a lock position (when in fact the pole length is not) can advantageously be prevented.

As indicated in <FIG>, the actual pole height h measured using the integrated absolute position encoder <NUM> can advantageously be automatically transmitted to the measuring unit <NUM>. For instance, as depicted, a wireless communication <NUM> such as a wireless or mobile network between pole <NUM> and TPS <NUM> is used to transfer the actual height value instantaneously and/or simultaneously to the measurement of the reference point to the instrument <NUM>. Thereby, the value may be signed with a unique identifier in order to securely and unambiguously link it with the according GNSS or distance measurement. As another option, the measured height h or any other measurement or processing data can be transmitted to other external instruments or control devices such as a field controller or smartphone or to a communication network. Additionally or alternatively, as shown in <FIG> on the right for exemplary pole <NUM>", a pole may comprise a display <NUM> and operational data or measurement data such as the pole height h is shown on the display to a user.

Further, the according communication device (not shown) may be configured for receiving a request/demand which triggers (a) the communication device to transmit a signal, and/or (b) the reader <NUM> to read a code. As another option, a trigger is automatically executed if a significant change of position is detected. In addition or as an alternative, an operation or action may be triggered by the operator of the pole <NUM>, <NUM>', <NUM>", for example by pushing a button provided at rod <NUM>, <NUM>', <NUM>". Such an action is for instance the command "store point" or "start target search" which is transmitted to the TPS <NUM> by the communication device such that the TPS <NUM> is remote controlled that way.

The signal transmitted to the surveying instrument <NUM> or any other external controlling device may optionally further be based on the according type of equipment arranged at the reference point. For example, the signal may further comprise information on the target type being a prism, certain parameters of the target such as presence of an additionally attached pole length extension, and/or certain assignable parameters of specific targets.

For optionally further considering a tilt and/or an orientation of the surveying pole <NUM>, <NUM>', <NUM>", a tilt sensor and/or an orientation sensor (not shown) may be comprised by the surveying pole <NUM>, <NUM>', <NUM>". The signal transmitted to the total station <NUM> may further be based on the determined tilt and/or orientation. Either of the optional tilt and orientation sensor may exemplarily comprise at least one of an acceleration sensor, a gyroscope, and a geomagnetic sensor. The extra information on the six degree of freedom state of the pole is for example useful in situations where the pole is tipped against a ceiling or a side wall. Accordingly, the signal transmitted from the pole <NUM>, <NUM>', <NUM>"may not only comprise information on the length or height h, but also on upside/down orientation relative to the gravity field, tilt information, delta position and delta orientation information during movement of the pole, and/or absolute positions of the pole <NUM>, <NUM>', <NUM>".

The pole <NUM>, <NUM>', <NUM>" may have a power unit for supplying the reader <NUM> and other units such as above-mentioned optional communication device with power. In the particular case the surveying pole <NUM>, <NUM>', <NUM>" also having a processing unit, this might as well be supplied by the power unit. The power unit may comprise at least one of a rechargeable or nonrechargeable battery and optionally a power cable for obtaining electric power from an external source.

Claim 1:
Absolute position encoder (<NUM>) for a measuring instrument or accessory for measuring instrument, comprising a digital scale (<NUM>) and a reader (<NUM>) movable relative to the digital scale, the digital scale (<NUM>) being arranged along a travel direction (<NUM>) of the reader (<NUM>), said digital scale comprising at least one absolute position track (<NUM>; 30a, 30b) having a sequence of discrete regions (<NUM>; 32a, 32b) sharply separated from each other by separating regions (<NUM>; 33a, 33b), and wherein the reader (<NUM>) comprises a first and a second series of detectors (70a-77a, 70b-77b) configured to sense said discrete regions (<NUM>; 32a, 32b) and separating regions (<NUM>; 33a, 33b) to detect at least one of a first and a second absolute position code, characterized in that said sequence of discrete regions (<NUM>; 32a, 32b) and the first and second series of detectors (70a-77a, 70b-77b) are disposed in such a way that none of the detectors of at least one of said first and second series of detectors is aligned with a transition between a discrete region (<NUM>; 32a, 32b) and a separating region (<NUM>; 33a, 33b) at each possible position of the reader (<NUM>) relative to the digital scale (<NUM>), and in that the position encoder (<NUM>) is configured to select the first or second absolute position code which is read by the series of detectors among which no detector is aligned with said transition.