Encoder device

Abstract

A projection encoder device includes a scale and a read head. The read head includes at least one electromagnetic radiation source for illuminating the scale to generate a projected fringe pattern at a detector configured to detect the projected fringe pattern. The encoder device can be configured to suppress the total harmonic distortion (THD) of the fringe pattern, for example, such that the THD of the fringe pattern is no greater than 6%.

Description

[0001] The present invention relates to an encoder device, and more particularly to a projection position measuring encoder device, the projection position measuring encoder device comprising a scale and a reading head movable relative to each other.

[0002] As is well known, position measurement encoder devices typically include a scale with a series of features (and their derivatives, such as velocity and / or acceleration) that a read head can read to determine and measure relative position. Encoders are generally classified as incremental or absolute. The scale used in incremental encoders includes a series of generally periodic features detected by the read head to determine the relative position and movement of the scale and read head. As will be understood, incremental encoders can be configured to provide two orthogonal (90-degree out of phase with each other) signals, typically labeled as SIN and COS signals (even though they may not actually be sinusoidal or cosine signals). The orthogonal signals can be interpolated to provide accurate measurement of the read head's position to less than one cycle of the repeating scale pattern. Providing such orthogonal signals by the encoder device is well-known to provide an indication of the orientation and relative movement of the read head and scale. One or more reference marks can be provided on the scale to provide reference positions against which the relative position of the scale and read head can be counted. The scale used for absolute encoders includes features that define unique positions along the length of the scale (e.g., a series of unique absolute positions) and enables the reader to determine its absolute position upon startup without any relative motion.

[0003] As will be understood, incremental encoders operate by utilizing the diffraction of light to generate a composite field at the detector that varies with the relative motion of the scale and the read head. For example, light can be diffracted by a diffraction grating in the scale and the read head, thereby forming interference fringes at the detector. It is also known that incremental encoders operate in such a way that scale features selectively prevent (e.g., block) light from reaching the detector, such that a non-imaging representation (e.g., a shadow) of the scale features is projected onto the incremental detector. Such encoders are commonly referred to as projection encoders. Projection encoders can be transmissive (where the electromagnetic radiation source and the detector are located on opposite sides of the scale) or reflective (where the electromagnetic radiation source and the detector are located on the same side of the scale). In prior art systems, common shapes of the light source used to illuminate the scale (for both diffraction encoders and projection encoders) include squares and circles.

[0004] As is well known in the field of position measurement encoders, sub-divisional error (SDE) can occur due to defects in the interpolation of signal readings. This defect may be due to the way readings are processed and / or due to defects in the signal detected by the readhead. It is generally expected that the signal detected by the readhead is substantially sinusoidal in form (e.g., the intensity variation of the pattern falling on the detector varies sinusoidally). Deviation from a sinusoidal signal may mean that the signal includes undesirable frequencies that adversely affect SDE (e.g., the first harmonic / fundamental harmonic of the signal). Consequently, SDE adversely affects the accuracy of the determined position. SDE is also commonly referred to as "interpolation error." In this document, the terms SDE and interpolation error are used interchangeably. In incremental encoders that include orthogonal signals that can be used to generate Lissajous curves, reducing SDE improves the roundness of the Lissajous curve.

[0005] The present invention provides an improved encoder, specifically an improved projection encoder.

[0006] For example, this paper describes an encoder device that includes a scale and a reader head configured to reduce unwanted frequencies (harmonics) in the scale signal in order to reduce the subdivision error of the encoder device.

[0007] This document describes an encoder device comprising a scale and a read head, the read head including at least one electromagnetic radiation source for illuminating the scale to generate a fringe pattern at a detector. The fringe pattern can be a non-imaging representation of the scale. In other words, the fringe pattern can be a projected fringe pattern. The encoder device can be configured to suppress the total harmonic distortion (THD) of the fringe pattern, for example, such that the THD of the fringe pattern is no greater than 6%.

[0008] According to a first aspect of the invention, a projection encoder device is provided, comprising a scale and a read head, the read head including at least one electromagnetic radiation source for illuminating the scale to generate a non-imaging representation / projection fringe pattern of the scale at a detector configured to detect the projection fringe pattern. The encoder device can be configured such that the total harmonic distortion (THD) of the non-imaging representation / projection fringe pattern of the scale is no greater than 6%.

[0009] It has been found that, for a given scale, the configuration of at least one electromagnetic radiation source of the readhead can affect the quality of the non-imaging representation / projected fringe pattern of that scale. It has been found that at least one electromagnetic radiation source of the readhead can be configured to provide an improved non-imaging representation / projected fringe pattern of the scale, for example, by reducing the amplitude of undesirable frequencies such as harmonics, thereby providing, for example, a fringe pattern with a THD of no more than 6%. As described in more detail below, this can be achieved, for example, by appropriately varying the radiated power of the electromagnetic radiation source along an axis substantially parallel to the measurement dimension of the encoder and / or by providing (e.g., along the measurement dimension of the encoder) a plurality of electromagnetic radiation sources that are appropriately offset / spaced.

[0010] The present invention can improve the output of an encoder device, for example, by reducing the SDE of the encoder device (such as SDE caused by, for example, defects in the signal falling on the read head sensor).

[0011] Optionally, at least two electromagnetic radiation sources are provided, and these at least two electromagnetic radiation sources are offset / spaced apart (along the measurement dimension of the encoder) such that the total harmonic distortion of the stripe pattern is no greater than 6%. Optionally, the radiated power curves of the electromagnetic radiation sources can be configured to be non-uniform (along the measurement dimension of the encoder) such that the total harmonic distortion of the stripe pattern is no greater than 6%.

[0012] As will be understood, THD is a well-known and standardized measurement of the harmonic distortion of a signal, and is calculated using the fundamental frequency / first harmonic. Specifically, the THD of a signal / striped pattern can be defined as the ratio of the sum of the power of all (measurable) harmonic components to the power of the first harmonic. In this case, harmonics up to and including the thirteenth harmonic can be considered to determine the THD.

[0013] Optionally, the total harmonic distortion of the stripe pattern is no greater than 3%, optionally no greater than 2%, optionally no greater than 1.5%, optionally no greater than 1%, and optionally no greater than 0.7%.

[0014] Optionally, the amplitude of the third harmonic of the striped pattern is no greater than 3%, optionally no greater than 2%, and optionally no greater than 1% of the amplitude of the first harmonic.

[0015] Optionally, the amplitude of the fifth harmonic of the striped pattern is no greater than 3% of the amplitude of the first harmonic, optionally no greater than 2%, and optionally no greater than 1%.

[0016] A projection encoder may include multiple (e.g., at least a group of multiple) electromagnetic radiation sources for illuminating a scale to produce a stripe pattern.

[0017] Optionally, the electromagnetic radiation sources are spaced apart to be in positions equivalent to offsets / spaced apart by a distance equal to the period of a (specific) harmonic of the first harmonic divided by the number of electromagnetic radiation sources.

[0018] For example, the electromagnetic radiation source can be offset / spaced out by a distance D in the measurement direction of the device, where,

[0019]

[0020] in,

[0021] f is the spacing of the scale;

[0022] h is the order of the (specific) harmonic to be canceled;

[0023] s is the number of sources;

[0024] n i It is an integer (as will be understood, it is an integer including zero);

[0025] M is the encoder's magnification factor.

[0026] This arrangement can essentially reduce / suppress, and even essentially eliminate (specific) harmonics.

[0027] optional

[0028] The (specific) harmonic of the first harmonic can be an odd-numbered harmonic. The (specific) harmonic of the first harmonic can be the third, fifth, and / or seventh harmonic.

[0029] As will be understood, in the presence of more than two sources, it may be necessary to determine multiple values ​​of D (e.g., D' and D”), and therefore, n i The values ​​for each interval can be the same or different. For example, in the case of three sources (and therefore two offsets / intervals):

[0030]

[0031]

[0032] The at least two sources can also be offset / spaced apart in a direction transverse to the measurement direction of the device. Specifically, when n i This might be useful when the value is 0.

[0033] As explained in more detail below, sources (e.g., a group of sources) can be spaced apart by a specific distance (F) such that these sources effectively behave as if they were a single source (e.g., they actually form / provide a single source). In this case, the form of the fringe pattern can remain unchanged (and therefore the ratio of harmonic amplitudes can remain unchanged). Optionally:

[0034] F = fM

[0035] Where f is the scale spacing and M is the encoder magnification factor.

[0036] optional

[0037]

[0038] in,

[0039] u is the distance from the source to the scale;

[0040] v is the distance from the scale to the detector.

[0041] Optionally, the electromagnetic radiation sources are offset / spaced such that at the detector, the (specific) harmonics of the first harmonic of their respective (projected) fringe patterns substantially cancel each other out. In other words, optionally, the electromagnetic radiation sources are offset / spaced such that the (specific) harmonic content of the fringe patterns cancels out at the detector.

[0042] Optionally, the electromagnetic radiation sources are offset / spaced such that the lateral shift between their respective (projected) fringe patterns is equal to / equivalent to the period of a (specific) harmonic of the first harmonic divided by the number of electromagnetic radiation sources.

[0043] Optionally, the at least one electromagnetic radiation (EMR) source comprises a single EMR source. Optionally, the at least one EMR source comprises more than one EMR source. Optionally, the at least one EMR source comprises two EMR sources. Optionally, the at least one EMR source comprises three EMR sources. As will be understood (and explained in more detail below), any EMR source may be an actual single source comprising multiple sub-sources. In other words, optionally, the at least one EMR source, or each of the at least one EMR source, is formed by more than one sub-source. Optionally, the at least one EMR source, or each of the at least one EMR source, is distributed between two or more equivalent locations. Optionally, the at least one EMR source, or each of the at least one EMR source, is distributed between two or more equivalent locations such that, at the detector, the (specific) harmonics of the first harmonic of their respective (projected) fringe patterns substantially cancel each other out. Optionally, the at least one EMR source, or each of the at least one EMR source, is distributed between two or more equivalent locations such that the lateral shift between their fringe patterns is equal to the period of the first harmonic divided by the number of EMR sources. Alternatively, the at least one EMR source, or each of the at least one EMR source, is distributed between two or more equivalent locations such that their fringe patterns are equivalent to a single EMR source.

[0044] Optionally, at least one group of multiple electromagnetic radiation sources includes two electromagnetic radiation sources. Optionally, at least one group of multiple electromagnetic radiation sources includes three electromagnetic radiation sources. Optionally, at least one group of multiple electromagnetic radiation sources includes one actual electromagnetic radiation source. Optionally, the at least one group of multiple electromagnetic radiation sources includes two groups of multiple electromagnetic radiation sources. Optionally, the at least one group of multiple electromagnetic radiation sources includes three groups of multiple electromagnetic radiation sources.

[0045] Optionally, a first electromagnetic radiation source or a first group of electromagnetic radiation sources and a second electromagnetic radiation source or a second group of electromagnetic radiation sources are provided. Each of the first electromagnetic radiation source or the first group of electromagnetic radiation sources and the second electromagnetic radiation source or the second group of electromagnetic radiation sources may substantially include a portion corresponding to the other of the first electromagnetic radiation source or the first group of electromagnetic radiation sources and the second electromagnetic radiation source or the second group of electromagnetic radiation sources. The corresponding portions of the first electromagnetic radiation source or the first group of electromagnetic radiation sources and the second electromagnetic radiation source or the second group of electromagnetic radiation sources may be spaced apart by a distance D.

[0046] Optionally, a third electromagnetic radiation source or a third group of electromagnetic radiation sources is provided. Optionally, the third electromagnetic radiation source or the third group of electromagnetic radiation sources substantially includes portions of the first electromagnetic radiation source or the first group of electromagnetic radiation sources and / or the second electromagnetic radiation source or the second group of electromagnetic radiation sources. Optionally, the corresponding portions of the first electromagnetic radiation source or the first group of electromagnetic radiation sources, the second electromagnetic radiation source or the second group of electromagnetic radiation sources, and the third electromagnetic radiation source or the third group of electromagnetic radiation sources are spaced apart by a distance D.

[0047] Optionally, the (specific) harmonic of the first harmonic is the third harmonic. Optionally, two electromagnetic radiation sources are separated by a distance equivalent to half the period of the third harmonic. Optionally, three electromagnetic radiation sources are separated by a distance equivalent to one-third the period of the third harmonic. Optionally, the (specific) harmonic of the first harmonic is the fifth harmonic. Optionally, two electromagnetic radiation sources are separated by a distance equivalent to half the period of the fifth harmonic. Optionally, three electromagnetic radiation sources are separated by a distance equivalent to one-third the period of the fifth harmonic.

[0048] Optionally, the at least one EMR source is configured such that its radiated power along its range parallel to the measurement direction of the device is configured to vary such that the total harmonic distortion (THD) achieving the fringe pattern is achieved. Optionally, the at least one EMR source produces a triangular radiated power curve. Optionally, the total radiated power of the middle third of the at least one EMR source is greater than the total radiated power of each of the two outer thirds of the at least one EMR source (e.g., such that the peak of the radiated power is directed toward the middle of the at least one EMR source). Optionally, the at least one EMR source produces a radiated power curve that produces a fringe pattern at the detector with substantially no harmonic content. Optionally, the at least one EMR source produces a radiated power curve for producing a fringe pattern at the detector with substantially zero THD.

[0049] Optionally, the at least one EMR source is shaped such that its width varies along its range to produce this variation in radiated power. Optionally, the at least one EMR source is shaped such that its radiated power curve along its range parallel to the measurement direction of the device is not substantially uniform / constant. Optionally, the EMR source is not square in shape (and is arranged such that its sides are parallel to the measurement dimension of the device). Optionally, the EMR source is not circular in shape. Optionally, the at least one EMR source is shaped such that its radiated power curve along its range parallel to the measurement direction of the device is not a semi-elliptical radiated power curve (e.g., not a semi-elliptical radiated power curve in which the minor axis of an ellipse extends along the measurement dimension of the device). Optionally, the at least one EMR source is shaped such that its radiated power curve along its range parallel to the measurement direction of the device is not a semi-elliptical radiated power curve in which the major axis is twice the minor axis.

[0050] Optionally, the at least one EMR source includes a light source. As will be understood, in this document, light includes EMR at any location in the infrared to ultraviolet range of the EMR spectrum.

[0051] Optionally, the detector includes at least one sensor element, such as an array of sensor elements. Optionally, the array extends along (e.g., parallel to) the measurement dimension of the device. Optionally, the detector includes sensor elements configured to detect different phases of the projected fringe pattern. Optionally, each sensor element includes a selector arrangement (placed above or in front of the sensor element), such as a grating or a mask, for selecting the phase of the fringe pattern detected by the sensor element. Accordingly, the detector may include one or more sensor elements, and optionally other (e.g., optical) components (such as the grating / mask). Optionally, the sensor includes an electro-grating comprising a sensor array comprising two or more sets of interdigitated / interlaced sensor elements, each set configured to detect different phases of the interference fringes. Each set may be referred to as a channel.

[0052] As will be understood, a scale may include a generally periodic array of features. A scale may include a periodic arrangement of two (e.g., different) types of features. The periodic pattern of the features can provide alternating degrees of constraint on EMR from the EMR source to the detector (e.g., propagating toward the detector) (in order to form the stripe pattern). Accordingly, when a scale includes a periodic arrangement of two types of features, one type of feature may (e.g., uniformly) provide a first level of constraint, while the other type of feature may (e.g., uniformly) provide a second level of constraint. One of the constraint levels may be substantially zero (e.g., so as to substantially not constrain the EMR from the source toward the detector). For example, the periodic pattern of the features may alternately constrain (e.g., prevent) and facilitate the propagation of electromagnetic radiation toward the detector (in order to form the stripe pattern). For example, the periodic pattern of the features may alternately block and transmit electromagnetic radiation, or, for example, alternately absorb (or scatter) and reflect electromagnetic radiation. The array may extend along (e.g., parallel to) the measurement dimension of the encoder. These features may include substantially elongated features whose length extends substantially perpendicular to the measurement dimension. These features may be discrete / dissimilar features. For example, each of the scale features selected along the measurement dimension of the device (e.g., two types) can provide a degree of uniformity / constancy in the constraint of the EMR propagating from the EMR source toward the detector.

[0053] Optionally, the scale period is at least 40 μm, for example at least 60 μm, for example at least 80 μm.

[0054] Optionally, the encoder is an incremental encoder. Optionally, the encoder is a reflective encoder. Optionally, the encoder is a transmissive encoder. Optionally, the scale includes a reference mark. Optionally, the reference mark is reflective or allows transmission of electromagnetic radiation. Optionally, the portion of the scale containing the reference mark absorbs electromagnetic radiation.

[0055] Optionally, the reference mark absorbs electromagnetic radiation. Optionally, the scale portion containing the reference mark reflects or transmits electromagnetic radiation. Optionally, the scale is a reflective scale. Optionally, the scale is a transmissive scale.

[0056] Optionally, the subdivision error (SDE) of the encoder device is less than 0.15 μm, and optionally less than 0.1 μm.

[0057] Optionally, the stripe pattern is formed by electromagnetic radiation from more than one spaced-apart electromagnetic radiation source. Optionally, the stripe pattern is formed at the detector.

[0058] As will be understood, variations and options of the foregoing aspects of the invention also apply to the following aspects of the invention, and vice versa.

[0059] According to another aspect of the invention, a projection encoder device is provided, the projection encoder device including a scale and a read head, the read head including at least one electromagnetic radiation source for irradiating the scale to generate a projected fringe pattern at a detector configured to detect the projected fringe pattern; wherein the at least one electromagnetic radiation source is configured such that the total harmonic distortion of the projected fringe pattern is not greater than 6%.

[0060] According to a further aspect of the invention, a projection encoder device is provided, the projection encoder device including a scale and a read head, the read head including at least one electromagnetic radiation source for irradiating the scale to generate a projected stripe pattern at a detector, the detector being configured to detect the projected stripe pattern.

[0061] The at least one electromagnetic radiation source is configured such that the amplitude of the third harmonic of the projected fringe pattern is no greater than 3% of the amplitude of the fundamental / first harmonic, and / or is configured such that the amplitude of the fifth harmonic of the projected fringe pattern is no greater than 3% of the amplitude of the fundamental / first harmonic.

[0062] According to a further aspect of the present invention, a projection encoder device is provided, comprising a scale and a read head, the read head comprising: (e.g., at least a group) a plurality of electromagnetic radiation sources for illuminating the scale to generate a projected fringe pattern; and a detector for detecting the projected fringe pattern; wherein the electromagnetic radiation sources are spaced apart by a distance D in the measurement direction of the device, wherein...

[0063]

[0064] in,

[0065] f is the spacing of the scale;

[0066] h is the order of the harmonic to be canceled;

[0067] s is the number of sources;

[0068] n i It is an integer (which may be the same or different for each interval);

[0069] M is the encoder's magnification factor.

[0070] According to a further aspect of the invention, an encoder device is provided comprising a scale and a read head, the read head including at least one electromagnetic radiation (EMR) source for illuminating the scale, the scale comprising a generally periodic array of features that provide alternating degrees of confinement to the EMR from the EMR source to a detector to generate a fringe pattern at the detector, the detector being configured to detect the fringe pattern; wherein the total harmonic distortion (THD) of the fringe pattern is not greater than 6%. Optionally, the scale period is at least 40 μm, for example at least 60 μm, for example at least 80 μm. The scale may comprise a periodic arrangement of two (e.g., different) types of features. Accordingly, when the scale comprises a periodic arrangement of two types of features, one type of feature may (e.g., uniformly) provide a first level of confinement, while the other type of feature may (e.g., uniformly) provide a second level of confinement. One of the confinement levels may be substantially zero (e.g., so as to substantially not confine the EMR from the source toward the detector). For example, the periodic pattern of the features can alternately restrict (e.g., prevent) and facilitate the propagation of electromagnetic radiation toward the detector (in order to form the stripe pattern). For example, the periodic pattern of the features can alternately block and transmit electromagnetic radiation, or, for example, alternately absorb (or scatter) and reflect electromagnetic radiation. The array can extend along (e.g., parallel to) the measurement dimension of the encoder. These features can include substantially elongated features whose length extends substantially perpendicular to the measurement dimension. These features can be discrete / dissimilar features. For example, selected along the measurement dimension of the device, each of (e.g., two types of) scale features can provide a uniform / constant degree of restriction on the EMR propagating toward the detector from the EMR source.

[0071] According to a further aspect of the invention, an encoder device is provided, comprising a scale and a read head, the read head including a plurality of electromagnetic radiation (EMR) sources for illuminating the scale to generate a (projected) fringe pattern at a detector configured to detect the fringe pattern; wherein the EMR sources are offset / spaced such that at the detector, the (specific) harmonics of the first harmonic of their respective (projected) fringe patterns substantially cancel each other out. The (specific) harmonic of the first harmonic can be an odd-numbered harmonic. The (specific) harmonic of the first harmonic can be a third, fifth, and / or seventh harmonic.

[0072] Embodiments of the invention will now be described by way of example only with reference to the following accompanying drawings, in which:

[0073] Figure 1 This is a schematic diagram illustrating an encoder device according to the present invention;

[0074] Figure 2 schematically shown Figure 1 The optical scheme of the encoder device;

[0075] Figure 3 It is a schematic representation of an electro-optic grating;

[0076] Figure 4 This is a schematic ray diagram showing the stripes generated by a point source at an incremental photodetector;

[0077] Figure 5 (a) is a schematic ray diagram showing the generation of stripes produced by an EMR source at an incremental photodetector, the EMR source comprising multiple EMR point sources with infinitesimal spacing between them;

[0078] Figure 5 (b) shows the results of Figure 5 (a) Trapezoidal stripe pattern generated by the EMR source;

[0079] Figure 5 (c) shows Figure 5 (b) Harmonic content of the trapezoidal stripe pattern;

[0080] Figure 6 (a) shows the radiated power curve of an EMR source according to an embodiment of the present invention;

[0081] Figure 6 (b) shows the provision based on Figure 6 (a) An example of a shaped EMR source with radiant power distribution;

[0082] Figure 6 (c) shows the result of Figure 6 (b) Stripe pattern generated by the EMR source;

[0083] Figure 6 (d) shows the result of Figure 6 (b) Harmonic content of the stripe pattern generated by the EMR source;

[0084] Figure 7 (a) shows the radiation power curve of an EMR source according to another embodiment of the present invention;

[0085] Figure 7 (b) shows the provision based on Figure 7 (a) An example of a shaped EMR source with radiant power distribution;

[0086] Figure 7 (c) shows the result of Figure 7 (b) Stripe pattern generated by the EMR source;

[0087] Figure 7 (d) shows the result of Figure 7 (b) Harmonic content of the stripe pattern generated by the EMR source;

[0088] Figure 8 Further alternative embodiments of the forming EMR source are shown;

[0089] Figure 9 Showing the target Figure 6 and Figure 7 The SDE in the embodiment shown varies with the source lateral range error;

[0090] Figure 10 The diagram schematically illustrates two equivalent positions of the point source relative to the scale;

[0091] Figure 11 The equivalent distribution of the forming EMR source is schematically shown;

[0092] Figure 12 An alternative arrangement is schematically shown for altering the radiated power distribution at the detector in order to reduce unwanted harmonics;

[0093] Figure 13 (a) is a schematic ray diagram showing the generation of stripes produced by multiple spaced point sources at the incremental encoder;

[0094] Figure 13 (b) shows the harmonic content of a single triangular EMR source;

[0095] Figure 13 (c) shows the harmonic content of three spaced-apart triangular EMR sources;

[0096] Figure 14The diagram illustrates the variation of SDE with source lateral range error for a single triangular EMR source and three spaced-apart triangular EMR sources; and...

[0097] Figure 15 The arrangement of EMR sources spaced apart to cancel out the third harmonic is illustrated schematically.

[0098] refer to Figure 1 and Figure 2 A first example encoder device 2 according to the invention is shown. The encoder device includes a read head 4 and a scale 6'. Although not shown, in use, the read head 4 can be fastened to one part of the machine, and the scale 6' can be fastened to another part of the machine, these parts being movable relative to each other. The read head 4 is used to measure its relative position with respect to the scale 6', and thus can be used to provide a measurement of the relative position of the two movable parts of the machine. The read head 4 communicates with a processor such as a controller 8 via a wired (as shown) and / or wireless communication channel. As will be understood, the processor may include a custom processor configured for a specific application (e.g., a field-programmable gate array "FPGA") and a more general-purpose processor that can be programmed (e.g., via software) as needed by the application using it. The read head 4 may report signals from its detectors to the controller 8, which then processes these signals to determine position information, and / or the read head 4 itself may process signals from its detectors and send position information to the controller 8. In another embodiment, an intermediate unit (e.g., an interface unit) may be located between the read head 4 and the controller 8. This interface unit facilitates communication between the reader 4 and the controller 8. For example, the interface unit can be configured to process reader signals and provide position information to the controller 8.

[0099] Scale 6' includes a plurality of scale marks defining the incremental track 10. In the described embodiment, scale 6' also includes a reference track 12.

[0100] In this embodiment, the encoder device is an optical, projection encoder. Therefore, the incremental track 10 includes a series of periodic scale marks 14, which form a scale grating extending in an elongated scale direction, substantially parallel to the movement direction of the read head 4, such as... Figure 1 As indicated by arrow E in the diagram. In the illustrated embodiment, the periodic scale marks 14 are alternating relative reflection marks and relative absorption marks. When the spacing of the scale gratings is sufficiently large (typically greater than 40 μm), geometric projection is the primary mechanism for forming the fringe pattern on the detector array, and a small amount of diffraction can be ignored.

[0101] Reference track 12 includes a reference position defined by a reflective reference mark 16. The remainder of the track includes a light-absorbing feature 17. It will be understood that in other embodiments, the reference mark 16 may absorb light, while the feature 17 may reflect light. Accordingly, the reference position is defined by a mark that allows relatively more light to reach the reference photodetector 24 than the remainder of the track containing it, and in this case, is relatively more reflective than the remainder of the track containing it. The reference position can be useful for enabling the read head 4 to accurately determine its position relative to the scale 6'. Accordingly, incremental positions can be counted based on the reference position. Furthermore, this reference position can also be a position referred to as a "limit position" because it can be used to define the limits or ends of the scale 6' that the read head 4 is allowed to travel between.

[0102] In this embodiment, the encoder device is a reflective optical encoder because it includes an electromagnetic radiation (EMR) source 18 (e.g., an infrared light source 18) and at least one detector 22, 24 on the same side of the scale 6'.

[0103] However, this is not necessarily the case, and the encoder can be configured such that the EMR source 18 and at least one detector can be located on opposite sides of the scale 6'. Typically, infrared light from the light source 18 is configured to be reflected back towards the read head by the scale 6'. As shown, the light source 18 is divergent, and the illuminated area of ​​the light source falls on both the incremental track 10 and the reference track 12. In the described embodiment, the light source 18 emits EMR in the infrared range; however, it will be understood that this is not necessarily the case and EMR can be emitted in other ranges, for example, anywhere from infrared to ultraviolet. As will be understood, the selection of a suitable wavelength for the light source 18 may depend on several factors, including the availability of a suitable detector operating at electromagnetic radiation (EMR) wavelengths. Also as shown, the read head 4 includes an incremental photodetector 22 and a reference photodetector 24. The incremental photodetector 22 in the illustrated embodiment comprises a detector array in the form of an electro-grating.

[0104] The incremental detector 22 can be in the form of an electro-grating, which is in Figure 3 The details are shown in more detail below. As shown, the incremental detector 22 includes a photosensitive sensor array comprising two or more sets of interdigitated / staggered / interlaced photosensitive sensor elements (also referred to herein as "photodetectors" or "fingerprints"). Each set can, for example, detect a different phase of the projected fringe pattern 28 at detector 22. Figure 3The diagram shows four groups of photodiodes (A, B, C, and D) in which the finger-like / photodiodes are interdigitated / intertwined to form an array of sensor elements extending along the length "L" of the sensor. Multiple groups of photodiodes are arranged in a repeating arrangement with a period "p" (and therefore a frequency "f" of 1 / "p").

[0105] As shown, in the described embodiment, the individual finger / photodiode / sensor element extends generally perpendicular to the length L of the incremental detector 22. Furthermore, the individual finger / photodiode / sensor element is generally rectangular in shape. It will be understood that the invention is also applicable to sensor elements of other shapes and arrangements.

[0106] The outputs from each finger / photodiode in a set are combined to provide a single output, resulting in four channel outputs: A', B', C', and D'. These outputs are then used to obtain quadrature signals SIN and COS. Specifically, A'-C' are used to provide the first signal (SIN), and B'-D' are used to provide a second signal (COS) that is 90 degrees out of phase with the first signal. Although in a particular embodiment the electro-grating includes four sets of photodiodes providing four channels A', B', C', and D', this is not necessarily the case. For example, the electro-grating may include two sets of photodiodes providing only two channels A' and B'.

[0107] exist Figure 3 In the diagram, the projected fringe pattern 28 is represented by lines that schematically illustrate the changing radiated power across the ideal projected fringe pattern of the incremental detector 22 (i.e., caused by the shadow of the incremental scale track 10). As shown, the encoder device is configured such that at any given moment, all photodiodes in any group detect the same phase of the projected fringes (if the projected fringe pattern period p' and the sensor period p are the same).

[0108] like Figure 2 As shown, the light source 18 is positioned between the incremental photodetector 22 and the reference photodetector 24 in a direction substantially transverse to the elongated scale direction. This facilitates uniform illumination of both the incremental track 10 and the reference marker track 12.

[0109] Light from light source 18 is emitted from read head 4 toward scale 6', wherein a portion of the area covered by light source 18 interacts with reference marker track 12 and a portion of the area covered by light source 18 interacts with incremental track 10. In the embodiment described herein, this reference position is defined by feature 16 in reference marker track 12, which modifies the amount of light reflected back toward reference photodetector 24 from light source 18 compared to the remainder of the track containing the reference marker. This can be achieved, for example, by feature 17 in the remainder of reference marker track 12 absorbing, transmitting, and / or scattering more light than reference marker 16. Figure 2 In the position shown, the read head 4 is aligned with the reference position, and therefore the light is shown as being reflected back toward the reference photodetector 24.

[0110] Relative to the incremental track 10, light from source 18 falls on periodic scale marks 14. As the read head 4 moves relative to scale 6' (in the direction indicated by arrow E), some light is absorbed, transmitted, and / or scattered by scale marks 14, and some light is reflected back toward the read head 4 and reaches the incremental photodetector 22. Ideally, the output from the incremental photodetector 22 (which includes the electro-grating as described above) varies sinusoidally with relative motion (i.e., sinusoidally in space).

[0111] It will be understood that while the current embodiment has been described in conjunction with an electro-grating, other embodiments may use different devices and / or methods to monitor the position of the read head 4. For example, the incremental photodetector 22 may include one or more photodiodes with corresponding (e.g., chromium) masks to allow the detection of phase information.

[0112] Figure 4 The diagram schematically illustrates the interaction of light from point source 26 with scale 6, producing a projected fringe pattern 28 at incremental photodetector 22. For clarity, Figure 4 A transmission arrangement is shown, in which light from point source 26 passes through scale 6' instead of being reflected by scale 6'. It will be understood that both reflection and transmission arrangements are possible.

[0113] As in Figure 4 As can be seen, the EMR (infrared light in this embodiment; in other embodiments, the EMR can be any wavelength in the ultraviolet to infrared spectrum) from point source 26 is directed toward scale 6. Scale 6 allows light to pass through / propagate along the scale in some areas, but prevents such passage / propagate in other areas. Figure 4In the diagram, the shaded areas represent regions where light passage / propagation has been blocked by scale 6. Due to the spacing of scale 6 (approximately 80 μm in this embodiment, but typically any length greater than 40 μm), geometric projection is the primary mechanism for forming the fringe pattern on the incremental photodetector 22, and square wave fringe pattern 28 is detected. The minimum scale spacing is determined by the extent to which diffraction reduces the visibility of the fringes below an acceptable level. Using the Rayleigh criterion for visibility, the lower limit of the scale spacing (f) is given by equation (1):

[0114]

[0115] in,

[0116] λ is the EMR wavelength;

[0117] u is the distance from the EMR source to the scale.

[0118] Incremental photodetector 22 ( Figure 4 (Not shown in the text) Detect stripe pattern 28 to generate a signal (as described above) Figures 1 to 3 As described, the signal is output from the read head 4 to an external device (such as the controller 8). It will be understood that the output from the incremental photodetector 22 for the square wave projected fringe pattern 28 generated by a single point source deviates substantially from an ideal, purely sinusoidal signal. This deviation of the detected projected fringe pattern 28 from the ideal sinusoidal projected fringe pattern causes the output of the incremental photodetector 22 to deviate from a purely sinusoidal signal, resulting in SDE (Signal Defect).

[0119] In practical applications, EMR sources are not such as Figure 4 The point source 26 shown is a point source, but it will have a finite range. The range involved here is the range of the EMR source that is substantially parallel to the direction of the elongated scale. An EMR source with a finite range can be considered as an infinite number of point sources 26 with infinitesimally small intervals. Figure 5 (a) illustrates such an EMR source with a limited range, and for clarity only three point sources 26 are shown. Light from each point source 26 is directed toward a scale 6, which allows light to pass along the scale in some areas but prevents such passage in others. Each point source will produce a separate square wave fringe pattern 28i, which are offset from each other and produce a combined fringe pattern 28d, which is detected by an incremental photodetector 22. As can be seen, the combined fringe pattern 28d is trapezoidal. It will be understood that the shape of the combined fringe pattern 28d will vary depending on the range of the EMR source and may, for example, be triangular. This is in contrast to an ideal sinusoidal variation (e.g., as...). Figure 3Compared to the schematically shown in the diagram, these patterns have a considerably higher harmonic content (i.e., multiples of the fundamental frequency).

[0120] Figure 5 (b) shows an example trapezoidal combined stripe pattern 28d. Figure 5 (c) shows Figure 5 The associated harmonic content of the stripe pattern shown in (b). Figure 5 (c) It has a logarithmic scale and shows the fundamental frequency 31 (also known as the first harmonic), the third harmonic 33, the fifth harmonic 35, the seventh harmonic 37, and the ninth harmonic 39. These higher harmonics 33, 35, 37, and 39 cause errors during interpolation; that is, harmonics 33, 35, 37, and 39 cause SDE, and thus cause errors in the measured position of the readhead 4 relative to the scale 6. The amplitude is shown on a logarithmic scale and has been normalized, where the fundamental frequency / first harmonic is set to a value of 1.

[0121] It has been recognized that by changing the radiation power of the EMR source along its range (which is substantially parallel to the direction of the elongated scale), the projected fringe pattern 28d detected by the incremental photodetector 22 can be adjusted and the SDE can be reduced by doing so, which in turn improves the accuracy of the measured position of the read head 4 relative to the scale 6.

[0122] The radiated power of an EMR source along its extent can be altered by configuring the shape of the EMR source such that there is a larger emission area at the center of the EMR source and a smaller emission area at the edges, as described in more detail below. However, it will be understood that the invention is not limited to altering the radiated power of an EMR source along its extent by shaping the EMR source, and other methods of altering the radiated power of an EMR source along its extent are possible, such as applying a mask to a larger source (or otherwise positioning the mask between the EMR source and the scale), which may be an opaque mask with a clear window, or a mask with a graded neutral density filter whose density varies according to a desired distribution. The radiated power of an EMR source along its extent, which is substantially parallel to the elongated scale, can also be altered by controlling the distribution of current density within the source. Other methods of altering the radiated power of an EMR source along its extent, which is substantially parallel to the elongated scale, are possible, as will be apparent to those skilled in the art.

[0123] Figure 6 (a) shows a possible radiated power curve for reducing the harmonic content in the striped pattern (compared to an equivalent system using a light source with a uniform / square radiated power curve) (which shows how the radiated power of the EMR source varies along the range of the EMR source). Figure 6 (b) shows that Figure 6The shape of one possible embodiment of the EMR source shown in (a) is illustrated by the radiated power curve. In use, it has... Figure 6 The EMR source of the shape shown in (b) will be aligned such that the x-axis extends substantially parallel to the direction of the elongated scale, and as Figure 6 (b) shows that the y-axis extends substantially orthogonally to the direction of the slender scale. Figure 6 The radiant power curve of (a) can be described as being configured such that the radiant power of the light source is maximized towards the center of the EMR source and smaller at the edges (where the edges are along the curve). Figure 6 (a) The extreme values ​​of the x-axis, with the center located between these edges). If the EMR source emits uniformly on its surface, the desired radiated power distribution can be achieved by physically shaping the light source so that the lateral extent of the light source is proportional to I(x). Figure 6 The shape of the radiation power curve in (a) (and therefore) Figure 6 The width of the shape of the EMR source uniformly emitted on its surface as shown in (b) can be described by equation (2).

[0124]

[0125] in,

[0126] I(x) is the radiated power of the source in a direction that is substantially transverse to the direction of the elongated scale (i.e., the radiated power emitted by the EMR source at a given point along the range that is substantially parallel to the direction of the elongated scale in use).

[0127] f is the spacing of the scale;

[0128] x is the position in a direction that is substantially parallel to the direction of the slender scale;

[0129] k is a proportionality constant.

[0130] If there is any non-uniformity in the emission of the EMR source on its surface (e.g., due to opaque electrodes on the surface of the source, non-uniform current injection, or for any other reason), the physical shape of the EMR source can be modified to provide the same characteristics as the emission from the source. Figure 6 The radiated power curves of (a) are essentially the same as those of (a).

[0131] Figure 6 (c) shows the radiation power curve of the projected stripe pattern 28 when it falls on the detector 22, as formed by the present embodiment.

[0132] Figure 6 (d) shows the use of, as Figure 6 The radiant power curve shown in (a) is generated by the light source. Figure 6The harmonic content of the stripe pattern shown in (c) is as follows. The fundamental frequency / first harmonic 31 has been normalized to a value of 1. (As in...) Figure 6 As can be seen in (d), in relation to Figure 5 Compared to the harmonics shown in (c), higher harmonics 33, 35, 37, and 39 are eliminated. The EMR rays from the EMR source, after interacting with scale 6, reaching detector 22 have various angles and radiant powers, resulting in a substantially pure sinusoidal fringe pattern at photodetector 22, with a fringe period p' substantially matching the period p of the detection element of photodetector 22. This leads to a lower SDE and thus an improved encoder. Figure 6 Total harmonic distortion (THD) of the stripe pattern shown in (c) (using Figure 6 The information in (d) is calculated as the ratio of the root mean square (RMS) of the amplitude of the higher harmonics to the RMS of the amplitude of the fundamental frequency / first harmonic 31. Total harmonic distortion can be calculated using all harmonics up to the thirteenth harmonic of the fundamental frequency / first harmonic 31. In other embodiments, total harmonic distortion can be calculated using more or fewer harmonics of the fundamental frequency / first harmonic 31. Figure 6 The total harmonic distortion of the stripe pattern shown in (c) is zero.

[0133] Figure 7 (a) shows a further possible radiated power curve for reducing the harmonic content in the striped pattern (compared to an equivalent system using a light source with a uniform / square radiated power curve) (which shows how the radiated power of the EMR source varies along the range of the EMR source). Figure 7 The radiated power curve shown in (a) is triangular. Figure 7 (b) shows that Figure 7 The shape of one possible embodiment of the EMR source is shown in (a) for the radiated power curve. Figure 7 In the example shown in (b), the EMR source is triangular. In use, it has... Figure 7 The EMR source of the shape shown in (b) will be aligned such that the x-axis extends substantially parallel to the direction of the elongated scale, and as Figure 7 (b) shows that the y-axis extends substantially orthogonally to the direction of the slender scale. Figure 7 The radiant power curve of (a) can be described as being configured such that the radiant power of the light source is maximized towards the center of the EMR source and smaller at the edges (where the edges are along the curve). Figure 7 (a) The extreme values ​​of the x-axis, with the center located between these edges). If the EMR source emits uniformly on its surface, the desired radiated power distribution can be achieved by physically shaping the light source so that the lateral extent of the light source is proportional to I(x). Figure 7 The shape of the radiation power curve in (a) (and therefore) Figure 7 The width of the shape of the EMR source uniformly emitted on its surface as shown in (b) can be described by equation (3).

[0134]

[0135] in,

[0136] I(x) is the radiated power of the source in a direction that is substantially transverse to the direction of the elongated scale (i.e., the radiated power emitted by the EMR source at a given point along the range that is substantially parallel to the direction of the elongated scale in use).

[0137] f is the spacing of the scale;

[0138] x is the position in a direction that is substantially parallel to the direction of the slender scale;

[0139] k is a proportionality constant.

[0140] and Figure 6 As illustrated in the embodiment, if there is any inhomogeneity in the emission of the EMR source on its surface, the physical shape of the EMR source can be modified to detach from the surface. Figure 7 (b) The triangular shape, to provide Figure 7 (a) Triangular radiated power curve.

[0141] Figure 7 (c) shows the radiant power of the projected stripe pattern 28 when it falls on the detector 22, as formed by the present embodiment.

[0142] Figure 7 (d) illustrates the use of, as Figure 7 The radiated power curve shown in (a) represents the harmonic content generated at the EMR source, with amplitudes normalized (where the fundamental frequency / first harmonic 31 is set to a value of 1) and plotted on a logarithmic scale. (See also...) Figure 7 As can be seen in (d), in relation to Figure 5 Compared to the harmonics shown in (c), the higher harmonics 33, 35, 37, and 39 are significantly reduced compared to the fundamental frequency 31. Figure 7 The total harmonic distortion of the stripe pattern shown in (c) is 1.8%. This results in a lower SDE and thus an improved encoder.

[0143] Will understand, Figure 6 The shapes of the EMR sources shown in (b) and 7(b) are merely examples, and as will be apparent to those skilled in the art, the desired radiated power profiles (such as...) can be provided. Figure 6 Other EMR shapes (radiation power curves shown in (a) and 7(a)). Figure 8Examples of further alternative EMR source shapes are shown. Figure 8 (a) and 8(b) show that they can provide Figure 6 The shape of the radiated power curve is shown in (a). It should be noted that... Figure 8 (a) shows the relationship with the Figure 6 The shape of the radiated power curve shown in (a) (as defined in equation (2)) and the shape formed by the x-axis correspond to the shape of the curve. Figure 8 (b) shows a further shape that could potentially achieve the desired radiated power curve. Figure 8 (c) shows the implementation Figure 7 The shape of the desired radiated power curve (as defined in equation (3)) is shown in (a). Figure 8 (d) A further shape is shown that may achieve the desired radiated power profile. It will be understood that the invention is not limited to the disclosed shape, and the shape of the EMR source can be configured such that the radiated power profile of the EMR source varies in a direction substantially parallel to the elongated scale direction, so as to substantially reduce or eliminate the radiated power profile compared to an equivalent system using a light source with a uniform / square radiated power profile. Figure 5 At least some of the harmonic signals shown in (c).

[0144] In embodiments where a varying radiated power profile is achieved by physically shaping the light source, the shaped EMR source can be, for example, a light-emitting diode (“LED”), and the LED emission distribution can be achieved by etching an epitaxial semiconductor structure (and / or by applying a shaping mask to the emission surface). However, any shaping method can be used, for example, the source (e.g., an LED, which can be an organic LED “OLED”) can be printed into the desired shape.

[0145] Although the EMR source described above has been described as a single source, this is not necessarily the case, and such as Figure 6 , Figure 8 or Figure 10 The EMR sources shown can be formed from multiple different sources arranged sequentially to form a single composite EMR source.

[0146] As described above, having as Figure 6 The EMR source with the radiated power curve shown in (a) provides radiation at detector 22 with Figure 6 The projected fringe pattern 28d of the harmonic content shown in (d) indicates the presence of the fundamental wave 31 without higher harmonics 33, 35, 37, and 39. If there is a deviation in the emitted radiated power curve, the higher harmonics 33, 35, 37, and 39 will not be completely eliminated from the fringe pattern 28d. Figure 9This illustrates how the SDE varies with source lateral range error (potentially due to manufacturing tolerances, etc.) at an 80 μm scale, where curve 40 is for a shape as shown. Figure 6 (b) the EMR source of the shape, and curve 42 is for forming as shown Figure 7 The EMR source of the shape in (b). For example, from... Figure 9 (It shows the source lateral range error against SDE) as can be seen, although the EMR source is shaped as Figure 6 A lower minimum SDE can be achieved with the shape shown in (b), but as indicated by curve 40, this shape is very sensitive to the lateral range error of the EMR source. If the EMR source is shaped as shown in... Figure 7 The shape shown in (b) is such that, as indicated by curve 42, the variation of SDE is relatively stable for the lateral range error of the EMR source compared to curve 40.

[0147] Now refer to Figures 10 to 14 Other embodiments and methods for implementing the present invention will be described first. Refer to [reference needed] first. Figure 10 (a) illustrates the first case, in which point source 26 is located at a first position relative to scale 6. Point source 26 generates a projected stripe pattern 28 at the detector (not shown). Figure 10 (b) illustrates the second case, in which the point source 26 has been moved a distance F along the elongated scale direction for the same scale and detector arrangement. As can be seen, Figure 10 (b) Projected stripe pattern 28 and Figure 10 The projected fringe patterns in (a) are identical and in phase, meaning that the two positions are equivalent.

[0148] This occurs according to equation (4).

[0149]

[0150] in,

[0151] f is the scale spacing;

[0152] u is the distance from the EMR source to the scale;

[0153] v is the distance from the scale to the detector.

[0154] Now go to Figure 11 Various EMR source configurations are shown, with a triangular source shown here (in (a) and (b)), although the principles shown also apply to other shaped EMR sources. Figure 11 (a) shows two possible locations for the forming source, which are equivalent (i.e., as described above in conjunction with...). Figure 10 (As explained). In Figure 11 In the diagram, shaded / black areas represent the formed EMR source (or a portion thereof) (unshaded areas indicate the absence of an EMR source). Figure 11 (a) shows a single triangular source that, when the scale 6 is positioned in the path between the shaped EMR source and the photodetector 22, will produce a projected stripe pattern at the photodetector 22. Figure 11 (b) shows the relative Figure 11 (a) The triangular source is shifted by a distance F from the equivalent placement of a single triangular EMR source. Figure 11 (b) The shaped EMR source will generate at photodetector 22 with Figure 11 (a) produces essentially the same projected stripe pattern as the forming EMR source.

[0155] Figure 11 (c) to Figure 11 (e) illustrates the arrangement of the EMR sources, which will also generate signals at photodetector 22 similar to those at photodetector 22. Figure 11 The forming EMR sources in (a) produce substantially the same projected stripe pattern (i.e., these EMR sources are equivalent to a single forming source as shown in (a) or (b)). In these examples, the projected stripe pattern is produced by two forming EMR sources arranged to be equivalent to a single triangular EMR source. Figure 11 (f) shows that this is not limited to dividing the shaped EMR source into two parts, but that an equivalent EMR distribution can still be achieved at the photodetector 22 even when a single EMR source is divided into three or more parts. Figure 11 (g) shows that this is not limited to splitting a single forming EMR source between two equivalent locations, but can be split between three (or more) equivalent locations.

[0156] exist Figure 11 In each of the cases shown in (a) through (g), the total surface area of ​​the EMR sources is the same. Furthermore, the distribution among the equivalent sites ensures that, in the case where EMR sources from equivalent sites are placed at a single equivalent site (i.e., in the case where equivalent sites are superimposed), the EMR sources essentially do not overlap. However, as will be understood, this is not necessarily required. Figure 11 (h) shows a first triangular source and a second triangular source located at equivalent positions, if the radiated power of each source is Figure 11 Half of the source shown in (a) (for example) will achieve the same radiated power distribution. Figure 11 Each of the first and second sources shown in (h) can be derived from the above regarding... Figure 11 Multiple spaced-apart sources are formed as described in (c) to (g). Figure 11(i) illustrates the following situation: the theoretical second source of the triangle (such as...) has been... Figure 11 A portion of the second source in (h) has moved a distance F in the direction toward the theoretically triangular first source. Figure 11 The source shown in (i) is in a state of producing equivalent to Figure 11 The configuration of the radiation power distribution of (h). Figure 11 (j) shows the following situation: a portion of the first source of the theoretical triangle has been moved by a distance F in the direction toward the second source of the theoretical triangle, and a portion of the second source of the theoretical triangle has been moved by a distance F in the direction toward the first source of the theoretical triangle. Figure 11 The source shown in (j) is in a state of producing equivalent to Figure 11 The configuration of the radiation power distribution of (h).

[0157] Now about Figure 12 A further example is described where a radiated power curve is formed by multiple sources. Figure 12 (a) shows three equivalent positions (as described above) Figure 10 The described embodiments. (and) Figure 11 Similarly, shaded / black areas indicate the presence of an EMR source (unshaded areas indicate the absence of an EMR source). For Figure 12 The arrangement shown in (a) will cause the EMR sources to overlap when the EMR sources from equivalent locations are placed in a single equivalent location (i.e., when the equivalent locations are superimposed). It will be understood that Figure 12 The arrangement of the EMR sources shown in (a) provides for generating and providing, as in the detector. Figure 12 The same stripe pattern produced by a single shaped EMR source is shown in (b) of the radiation power curve.

[0158] Many other distributions will be apparent to those skilled in the art; for example, the forming EMR source can be distributed between equivalent locations spaced apart by nF, where n is an integer and can be greater than 1.

[0159] Figure 13 (a) illustrates another way to significantly reduce the SDE of a projection encoder device. In this embodiment, three substantially identical discrete EMR point sources 26a, 26b, 26c are spaced apart by finite intervals D' and D''. (It will be understood that two, three, four, five, or more point sources spaced apart by finite intervals will also produce the effect shown). For clarity, Figure 13 (a) shows a transmission arrangement in which light from EMR point sources 26a, 26b, 26c passes through scale 6 instead of being reflected by scale 6. It will be understood that both reflection and transmission arrangements are possible.

[0160] As can be seen, the point sources 26a, 26b, 26c can be arranged such that even though each point source itself produces substantially the same square wave fringe pattern 28a, 28b, 28c at the detector, the spacing D' and D" of the point sources 26a, 26b, and 26c can be chosen (as explained in more detail below) such that the square wave fringe patterns 28a, 28b, 28c at the detector are slightly shifted relative to each other, so that the detected fringe pattern 28d (which is the sum of the individual fringe patterns 28a, 28b, 28c) generated by the spaced EMR sources 26a, 26b, 26c shows a movement away from the square wave fringe pattern toward an ideal sinusoidal pattern.

[0161] exist Figure 13 In (a), source 26b is separated from source 26a by a distance D' and from source 26c by a distance D.

[0162] Distances D' and D” are defined by equation (5). Figure 13 In the example shown in (a), for interval D', n i =1, while for interval D”, n i =2. It will be understood that in other embodiments, both D' and D” can have the same value n. i .

[0163] or

[0164] in,

[0165] f is the spacing of the scale;

[0166] h is the order of the harmonic to be canceled;

[0167] s is the number of sources;

[0168] n i It is an integer (which may be the same or different for each interval);

[0169] M is the magnification factor.

[0170] The magnification M is given by the following formula:

[0171]

[0172] in,

[0173] u is the distance from the source to the scale;

[0174] v is the distance from the scale to the electro-optic grating.

[0175] It should be noted that Mf = F (as defined in equation (4) above).

[0176] The spacing of sources 26a, 26b, and 26c causes a lateral shift between the fringe patterns 28a, 28b, and 28c of these sources equal to one-third of the period of the third harmonic 33, which results in the amplitude of the third harmonic 33 of the combined fringe pattern 28d being reduced to zero.

[0177] This effect is not limited to using three spaced sources. For example, a similar effect can be achieved with two spaced sources. If the spacing D between the two sources causes the lateral shift between their fringe patterns to be equal to half the period of the third harmonic 33, then the amplitude of the third harmonic 33 of the combined fringe pattern can be reduced to zero because the third harmonic component is in perfect out of phase.

[0178] Similarly, this effect can be achieved for other higher harmonics. For example, by separating three separate sources such that the stripe pattern they individually form is laterally shifted by one-third of the period of the fifth harmonic 35, the cancellation of the fifth harmonic 35 will then occur.

[0179] Figure 13 (c) shows the use of Figure 7 Examples of three triangular sources of the type shown in (b).

[0180] Figure 13 (b) shows Figure 7 The harmonic content of the stripe pattern of a single triangular source of the type shown in (b) (and shown in the description above) Figure 7 (d) Same information).

[0181] exist Figure 13 (a) and Figure 13 In (b), the values ​​in the figure have been normalized, where the fundamental frequency / first harmonic 31 has a value defined as 1.

[0182] By generating stripe patterns from a single triangular source ( Figure 13 The amplitude of the higher harmonics in (b)) and the combined fringe pattern produced by three triangular sources (these three triangular sources are spaced apart such that each triangular source produces a fringe pattern with a phase shift of one-third of the period of the third harmonic 33) Figure 13 (c) By comparing the two sources, it can be seen that by combining the EMRs from three spaced-apart triangular EMR sources, the amplitude of the third harmonic 33 can be reduced to zero and the amplitude of the fifth harmonic can be reduced. Figure 13 The total harmonic distortion of the stripe pattern shown in (c) is 0.7%.

[0183] Since some higher harmonic components 35 and 39 are retained in the fringe pattern formed on the photodetector 22, the optimization of the lateral range of the source needs to take into account the harmonic filtering effect of the electro-grating structure. This harmonic filtering by the electro-grating structure is a well-known result from sampling theory. Since different electro-grating configurations can suppress different harmonics, this needs to be taken into account when optimizing the source range.

[0184] Figure 14 The SDE as a function of source lateral range error was compared for a single triangular EMR source (curve 42) and for three spaced triangular EMR sources (curve 44). As can be seen, manufacturing tolerances have a significant impact on the performance of a single source, while the performance of the three spaced sources 44 is almost insensitive to reasonable possible errors in source size (due to manufacturing tolerances, etc.).

[0185] As will be understood, this aspect of the invention is not limited to using three spaced-apart EMR sources, and for example, two, three, four, five, ten or more sources may be used. Similarly, it will be understood that this aspect of the invention is not limited to triangular EMR sources and / or sources shaped as shown in the diagram. Figure 6 (b) or Figure 8 The source of the shape shown in the image.

[0186] also, Figure 11 or Figure 12 The concept of the embodiments can be related to Figure 13 (a) combines the concepts of the embodiments. For example, Figure 15 (a) shows two triangular sources 26a and 26b spaced apart by a distance D' according to equation (5). In the example shown, n i =1, h=3 and s=2. In this example, the third harmonic component will be perfectly out of phase, and the projected fringe pattern will not have a third harmonic component.

[0187] exist Figure 15 (b) with Figure 11 and Figure 12 As in the case of the image, the shaded / black area represents the formed EMR source (or a portion thereof) (the unshaded area indicates that there is no EMR source).

[0188] Figure 15 (b) shows an example in which, as in the example, Figure 15 (a) The third harmonic component is perfectly out of phase, and the projected fringe pattern does not have a third harmonic component. Figure 15 In (b), the triangular source 26b has been formed by two sub-sources 26b(i) and 26b(ii). The sub-sources 26b(i) and 26b(ii) are spaced apart by a distance F as defined by equation (4). (The last sentence appears to be incomplete and requires further context.) Figure 11As described, the two sub-sources 26b(i) and 26b(ii) together are equivalent to a single source and provide a valid source (which is equivalent to...). Figure 15 (a) Arrangement). For example Figure 15 As shown in (b), source 26a and source 26b are a single source (e.g. Figure 15 In case (a), the positions are spaced apart by a distance D' according to equation (5). Since sub-sources 26b(i) and 26b(ii) are equivalent to a single source ( Figure 15 (as shown in (b)) therefore, the third harmonic component of the shadow stripe pattern will be in perfect out-of-phase, and the projected stripe pattern will have no third harmonic component.

[0189] Although Figure 15 (b) shows two sources: a single source 26a and a source formed by sub-sources 26b(i) and 26b(ii), but it will be understood that more than one source can be formed by multiple sub-sources. It will be further understood that it is not necessary to use a single source, and all sources can be formed by multiple sub-sources.

[0190] To understand, as about Figure 12 The description involves forming one or more single sources from multiple sub-sources in order to achieve the cancellation of harmonics (such as third harmonic 33 or fifth harmonic 35) of the first harmonic / fundamental frequency 31 by spacing more than one source apart by a distance D as defined by equation (5).

Claims

1. A projection encoder device, the projection encoder device comprising a scale and a read head, the read head comprising a plurality of electromagnetic radiation sources for irradiating the scale to generate a projected stripe pattern at a detector, the detector being configured to detect the projected stripe pattern; The projection encoder device is configured such that the total harmonic distortion of the projected fringe pattern falling on the detector is no greater than 6%. in, The plurality of electromagnetic radiation sources are configured such that their radiation power along a range parallel to the measurement direction of the device is configured to vary, thereby causing the total harmonic distortion of the projected fringes, wherein the period of the scale is at least 40 μm. Wherein, the plurality of electromagnetic radiation sources are spaced apart by a distance D in the measurement direction of the device, wherein: in, f is the spacing of the scale; h is the order of the harmonic to be canceled; s is the number of sources; n i It is an integer, and if there are more than two sources, it can be the same or different for each interval; M is the magnification factor.

2. The projection encoder device according to claim 1, wherein, The total harmonic distortion of the projected stripe pattern is no greater than 3%.

3. The projection encoder device according to claim 1, wherein, The amplitude of the third harmonic of the projected stripe pattern is no greater than 3% of the amplitude of the first harmonic of the stripe pattern, and / or the amplitude of the fifth harmonic of the projected stripe pattern is no greater than 3% of the amplitude of the first harmonic.

4. The projection encoder device according to claim 1, wherein, The plurality of electromagnetic radiation sources includes two electromagnetic radiation sources or three electromagnetic radiation sources.

5. The projection encoder device according to claim 1 or 4, wherein, The harmonics to be canceled are the third or fifth harmonics.

6. The projection encoder device according to claim 1, wherein, The plurality of electromagnetic radiation sources are shaped such that their width varies along their range in order to produce such variation in radiated power.

7. The projection encoder device according to claim 1 or claim 6, wherein, The multiple electromagnetic radiation sources generate a triangular radiation power curve.

8. The projection encoder device according to claim 1, wherein, The encoder is an incremental encoder.

9. The projection encoder device according to claim 1, wherein, The subdivision error (SDE) is less than 0.15 μm.

10. The projection encoder device according to claim 1, wherein, The plurality of electromagnetic radiation sources are configured such that the total harmonic distortion of the projected fringe pattern is no greater than 6%.

11. A projection encoder device, the projection encoder device comprising a scale and a read head, the read head comprising a plurality of electromagnetic radiation sources for irradiating the scale to generate a projected stripe pattern at a detector, the detector being configured to detect the projected stripe pattern. in, The plurality of electromagnetic radiation sources are configured such that the amplitude of the third harmonic of the projected fringe pattern is no greater than 3% of the amplitude of the first harmonic of the projected fringe pattern, and / or the amplitude of the fifth harmonic of the projected fringe pattern is no greater than 3% of the amplitude of the first harmonic. Wherein, the plurality of electromagnetic radiation sources are spaced apart by a distance D in the measurement direction of the device, wherein: in, f is the spacing of the scale; h is the order of the harmonic to be canceled; s is the number of sources; n i It is an integer, and if there are more than two sources, it can be the same or different for each interval; M is the magnification factor.

12. A projection encoder device, the projection encoder device comprising a scale and a reader head, the reader head comprising: At least one set of multiple electromagnetic radiation sources are used to illuminate the scale in order to produce a projected stripe pattern. and a detector for detecting the projected stripe pattern; Wherein, the electromagnetic radiation sources are spaced apart by a distance D in the measurement direction of the device, wherein, in, f is the spacing of the scale; h is the order of the harmonic to be canceled; s is the number of sources; n i It is an integer, and if there are more than two sources, it can be the same or different for each interval; M is the magnification factor.

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