WO2013083192A1 - Optical angle encoder - Google Patents

Optical angle encoder Download PDF

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Publication number
WO2013083192A1
WO2013083192A1 PCT/EP2011/072103 EP2011072103W WO2013083192A1 WO 2013083192 A1 WO2013083192 A1 WO 2013083192A1 EP 2011072103 W EP2011072103 W EP 2011072103W WO 2013083192 A1 WO2013083192 A1 WO 2013083192A1
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WO
WIPO (PCT)
Prior art keywords
angle encoder
magnetic field
fiber bragg
light
fiber
Prior art date
Application number
PCT/EP2011/072103
Other languages
French (fr)
Inventor
Frank De Wit
Hongyu YANG
Original Assignee
Aktiebolaget Skf
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Aktiebolaget Skf filed Critical Aktiebolaget Skf
Priority to PCT/EP2011/072103 priority Critical patent/WO2013083192A1/en
Publication of WO2013083192A1 publication Critical patent/WO2013083192A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
    • G01D5/35338—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using other arrangements than interferometer arrangements
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
    • G01D5/35306—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement
    • G01D5/35309—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer
    • G01D5/35316—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer using a Bragg gratings

Definitions

  • This invention relates to an angle encoder for determining a rotation angle of a rotating part relative to a static part, the angle encoder comprising a magnetic ring, means for measuring a magnetic field and an electronic circuit.
  • the magnetic ring has at least two magnetic poles and is attached to the rotating part.
  • the means for measuring a magnetic field are attached to the static part.
  • the electronic circuit is arranged for determining the rotation angle based on the measured magnetic field.
  • This invention further relates to a bearing unit with such an angle encoder.
  • Angle encoders also called rotary encoders or shaft encoders convert an angular position or motion of a shaft or axle to an analog or digital code.
  • Known angle encoders of the mechanical type may use concentric rings with openings. At each angular position, sliding contacts may detect the openings and an electronic circuit may convert the detected pattern of openings into an angular position.
  • Optical encoders may use a similar principle.
  • the concentric rings with openings can be replaced by a disc with transparent and opaque areas. A light source and a photo detector array make it possible to detect the pattern of opaque and transparent areas at a given angle. However in a bearing environment with grease obscuring the disc, this type of angle encoder cannot be used.
  • a magnetic ring and a number of Hall sensors are used to provide an angle encoder.
  • the magnetic ring is attached to the rotating part of the bearing unit.
  • the Hall sensors capable of detecting magnetic fields, are attached to th a static part of the bearing unit.
  • the magnetic ring provides a magnetic field with a direction depending on its angular orientation.
  • the Hall sensors detect the magnetic field and an electronic circuit coupled to the Hall sensors converts the detected magnetic field to an angular position. This arrangement also works in the greasy bearing environment.
  • the use of such magnetic angular encoders comes with a number of disadvantages. Strong external magnetic fields saturate the sensors and impede the proper operation of the angle encoder.
  • this object is achieved by providing an angle encoder for determining a rotation angle of a rotating part relative to a static part, the angle encoder comprising a magnetic ring, means for measuring a magnetic field and an electronic circuit.
  • the magnetic ring has at least two magnetic poles and is attached to the rotating part.
  • the means for measuring a magnetic field are attached to the static part and comprise an optical fiber, a light source and an optical sensor.
  • the optical fiber comprises a fiber Bragg grating coated with a magnetostrictive material.
  • the light source and the optical sensor are both optically coupled to the optical fiber and electronically coupled to the electronic circuit.
  • the electronic circuit is arranged for determining the rotation angle based on measured signals from the optical sensor.
  • the angle encoder uses the magnetic ring in a similar way as before.
  • the improvements lie in the way of detecting the magnetic field, for which now an optical fiber with one or more fiber Bragg gratings is used.
  • a fiber Bragg grating is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This selective reflectance is obtained by adding a periodic variation to the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror.
  • a fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.
  • the fiber Bragg grating is coated with a magnetostrictive material. Magnetostrictive materials change their shape in response to a change in their magnetization. When the magnetic ring is rotated together with the rotating part to which it is attached, the magnetic field at the fiber Bragg grating changes. As a result, the shape of the coating and the fiber core to which it is attached change.
  • the coated part of the fiber core i.e. the part with the fiber Bragg grating, is shortened or elongated. The distortion of the fiber core causes the periodic variation of the refractive index at the fiber Bragg grating to change. The changed shape of the fiber Bragg grating gives it different transmission and reflection properties.
  • each Hall sensor requires its own wiring which may lead to a very complex wiring architecture.
  • one optical fiber comprises multiple fiber Bragg gratings, they should be arranged to reflect light of different wavelengths in order to enable the optical sensor to detect which one has been affected by the magnetic field. This can easily be obtained by providing different fiber Bragg gratings with different patterns of variation of the refractive index.
  • different Bragg gratings are comprised in different optical fibers in order to enable detecting which one of the fiber Bragg gratings has changed shape.
  • Another advantage of using the coated fiber Bragg gratings instead of Hall sensors is that the dynamic range of the coated fiber Bragg sensors is relatively large.
  • the Hall sensors of the prior art quickly become saturated in an external magnetic field, which makes it impossible to detect rotation of the magnetic ring.
  • An external magnetic field that influences all coated fiber Bragg sensors in the same way effectively changes the base wavelength (i.e. the reflection wavelength if the magnetic ring would not be there) of all of them. Due to their large dynamic range, the coated fiber Bragg sensors remain mechanically elastic and will continue to change shape in response to rotation of the magnetic ring.
  • the offset caused by the external magnetic field can easily be filtered out electronically or using software. Similar offsets caused by, e.g., changes in temperature can be filtered out in a similar way.
  • At least one fiber Bragg grating may be provided which is not coated with a magnetostrictive material.
  • the sensor thus obtained can be used as a reference sensor, because it's wavelength of reflection is not affected by the changing magnetic field.
  • the reference sensor can therefore be used to compensate for temperature dependent influences and other non-magnetic phenomena that affect all fiber Bragg gratings to the same extent.
  • the reference sensor may be covered with an inert (and not magnetostrictive) material to prevent unwanted interactions with the bearing environment.
  • FIG. 1 shows an angle encoder according to the invention
  • Figure 2 shows a refractive index of an optical fiber with a coated fiber Bragg grating
  • Figure 3a shows a power distribution of an input light beam
  • Figure 3b shows a spectral response for an optical fiber with a fiber Bragg grating
  • Figure 3c shows a power distribution of light reflected by a coated fiber Bragg sensors at three different orientations relative to the magnetic ring
  • Figures 4a-4c schematically show a coated fiber Bragg grating at three different orientations relative to the magnetic ring.
  • Figure 1 shows an angle encoder 10 according to the invention.
  • the angle encoder 10 is provided for determining a rotation angle of a rotating part relative to a static part 12.
  • a magnetic ring 11 is attached to the rotating part and has two magnetic poles, a north pole N and a south pole S. In principle, the magnetic ring 11 could also have more magnetic poles. For the current invention, it is only important that the magnetic ring 11 provides a different magnetic field for different rotation angles. When the magnetic ring 11 is rotated, the direction of the magnetic field at the static part 12 changes.
  • Means for measuring the magnetic field and a control circuit 17 for processing the measurements detect the changes in the magnetic field and convert the detected changes to a rotation angle of the rotating part relative to the static part 12.
  • the means for measuring the magnetic field comprise an optical fiber 13 with at least one, but preferably more, coated fiber Bragg gratings 14.
  • the fiber Bragg gratings may be created by engraving variations of refractive index into the core of the fiber 13. Fiber Bragg gratings reflect particular wavelengths of light and transmit all others. Which particular wavelengths are reflected depends on the pattern that is engraved in the fiber 13 core.
  • a light source 15 and a light sensor 16 are optically coupled to respective ends of the optical fiber 13.
  • the light source 15, e.g. a laser diode sends monochromatic or polychromatic light into the optical fiber 13.
  • the light sensor 16 detects the light that is transmitted through the optical fiber.
  • Multiple light sources 15 and/or light sensors 16 may be used for separately sending out or detecting light of different colors.
  • the fiber Bragg gratings 14 reflect part of the light sent through the fiber 13, the spectrum of the detected light differs from the spectrum of the emitted light. Analysis of the detected light may thus provide information about the presence of fiber Bragg gratings 14 in the fiber 13.
  • a reference fiber 19 may be provided for directly leading part of the emitted light to the light sensor 16 and for enabling direct measurement of the light that enters the optical fibers 13, 19.
  • the coating of the fiber Bragg gratings 14 comprises a magnetostrictive material.
  • the magnetostrictive coating may, e.g., comprise Terfenol-D in the form of particles dispersed in a polymer matrix. Magnetostrictive materials change their shape in response to a change in its magnetization. When the direction of the magnetic field generated by the magnetic ring 11 changes, the coating changes shape. Together with its coating, the fiber Bragg grating 14 itself changes shape. This will change the periodicity of the variations of the refractive index and the range of wavelengths that are reflected by the coated fiber Bragg grating 14. A change of the magnetic field will thus also lead to a change of the light detected by the light sensor 16. Analysis of the detected light makes it possible to get information about the direction of the magnetic field.
  • the control circuit 17 is electrically coupled to the light sensor 16 for receiving signals representing the detected light and converting the received signals into a corresponding rotation angle.
  • the coated fiber Bragg gratings 14 should reflect light of different wavelengths.
  • separate optical fibers 13 are used for multiple fiber Bragg gratings 14 with the same base wavelength.
  • at least one fiber Bragg grating 18 may be provided which is not coated with a magnetostrictive material.
  • the sensor 18 thus obtained can be used as a reference sensor 18, because it's wavelength of reflection is not affected by the changing magnetic field.
  • the reference sensor 18 can therefore be used to compensate for temperature dependent influences and other non-magnetic phenomena that affect all fiber Bragg gratings 14, 18 to the same extent.
  • Figure 2 shows a refractive index of an optical fiber 13 with a fiber Bragg grating 14, 18.
  • Fiber Bragg gratings 14, 18 are obtained by adding a systematic variation to the refractive index, n, of the fiber core.
  • the optical properties of the fiber Bragg grating 14, 18 depend on the difference between the refractive indices (n 2 -n ), the distance between the index transitions and the number of transitions.
  • Figure 3a shows a power distribution of an input light beam.
  • the input light beam used in the angle encoder 10 according to the invention may either be monochromatic or polychromatic. In practice, it is not possible to produce light with only one wavelength and the term monochromatic light is used for light with a narrow frequency bandwidth.
  • Polychromatic light is light with a number of different wavelengths. Polychromatic light may be a combination of monochromatic light of two or more different colors or just some mixture of light in a broad frequency range, such as for example white light.
  • Figure 3a shows a frequency spectrum of a polychromatic input light beam that may be used in an angle encoder according to the invention.
  • Figure 3b shows a spectral response for an optical fiber with a fiber Bragg grating 14, 18. It shows the power distribution of the light transmitted through the optical fiber 13 and the fiber Bragg grating 14, 18 as detected by the light sensor 16. Most frequencies of the light beam of figure 3a are transmitted and detected. Only light in a narrow frequency range around the reflection wavelength, ⁇ ⁇ , is reflected by the fiber Bragg grating 14 and does not reach the light sensor 16.
  • Figure 3c shows a power distribution of light reflected by a coated fiber Bragg sensor 14 at three different orientations relative to the magnetic ring 11.
  • the fiber Bragg grating 14 provides a transmission profile as shown in figure 3b, with a dip at the reflection wavelength, ⁇ ⁇ , of the grating 14.
  • Figure 3c shows the power distribution 31 of the light reflected by the grating 14, with a peak at the reflection wavelength, ⁇ ⁇ .
  • the reflection wavelength, ⁇ ⁇ of the grating is shifted to, a lower wavelength, ⁇ ⁇ or a higher wavelength, ⁇ ⁇ 3 .
  • This shift in of the reflection wavelength is measured by the light sensor 16 and converted into information about the rotation angle by the electronic circuit 17.
  • Figures 4a-4c schematically show a coated fiber Bragg grating 14 at three different orientations relative to the magnetic ring 11.
  • the magnetic field and the magnetostrictive coating have caused the grating 14 to have different shapes.
  • the elongation and shortening of the grating 14 also changes the distance between two consecutive refractive index transitions, which changes the reflection wavelength of the fiber Bragg grating 14.
  • any reference signs placed between parentheses shall not be construed as limiting the claim.
  • Use of the verb "comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim.
  • the article "a” or “an” preceding an element does not exclude the presence of a plurality of such elements.
  • the invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Abstract

An angle encoder (10) is provided for determining a rotation angle of a rotating part (11) relative to a static part (12). The angle encoder (10) comprises a magnetic ring (11) having at least two magnetic poles and being attached to the rotating part (11) and, attached to the static part (12), means for measuring a magnetic field. The means for measuring the magnetic field comprise an optical fiber (13) with a fiber Bragg grating (14) coated with a magnetostrictive material. The means for measuring the magnetic field further comprise a light source (15) and an optical sensor (16), both being optically coupled to the optical fiber (13) and electronically coupled to an electronic circuit (17). The electronic circuit (17) is arranged to determine the rotation angle based on measured signals from the optical sensor (16).

Description

Optical angle encoder
Field of the invention
This invention relates to an angle encoder for determining a rotation angle of a rotating part relative to a static part, the angle encoder comprising a magnetic ring, means for measuring a magnetic field and an electronic circuit. The magnetic ring has at least two magnetic poles and is attached to the rotating part. The means for measuring a magnetic field are attached to the static part. The electronic circuit is arranged for determining the rotation angle based on the measured magnetic field. This invention further relates to a bearing unit with such an angle encoder.
Background of the invention
Angle encoders, also called rotary encoders or shaft encoders convert an angular position or motion of a shaft or axle to an analog or digital code. Known angle encoders of the mechanical type may use concentric rings with openings. At each angular position, sliding contacts may detect the openings and an electronic circuit may convert the detected pattern of openings into an angular position. Optical encoders may use a similar principle. The concentric rings with openings can be replaced by a disc with transparent and opaque areas. A light source and a photo detector array make it possible to detect the pattern of opaque and transparent areas at a given angle. However in a bearing environment with grease obscuring the disc, this type of angle encoder cannot be used.
In current SKF bearing units, often a magnetic ring and a number of Hall sensors are used to provide an angle encoder. The magnetic ring is attached to the rotating part of the bearing unit. The Hall sensors, capable of detecting magnetic fields, are attached to th a static part of the bearing unit. The magnetic ring provides a magnetic field with a direction depending on its angular orientation. The Hall sensors detect the magnetic field and an electronic circuit coupled to the Hall sensors converts the detected magnetic field to an angular position. This arrangement also works in the greasy bearing environment. The use of such magnetic angular encoders, however, comes with a number of disadvantages. Strong external magnetic fields saturate the sensors and impede the proper operation of the angle encoder. Additional disadvantages are the sensor's sensitivity to electromagnetic interference and its complicated wiring. Possible improvement in sensitivity by using more sensors comes at the cost of wiring complexity. Reduced wire complexity by using fewer sensors is detrimental to the sensitivity. When integrating the electronics inside the bearing housing, the electronics must be able to withstand large temperature gradients and should be small enough to fit in a small space.
Object of the invention
Considering the disadvantages of the known angle encoders, it is an object of the invention to provide a more practical and more reliable angle encoder.
Summary of the invention
According to a first aspect of the invention, this object is achieved by providing an angle encoder for determining a rotation angle of a rotating part relative to a static part, the angle encoder comprising a magnetic ring, means for measuring a magnetic field and an electronic circuit. The magnetic ring has at least two magnetic poles and is attached to the rotating part. The means for measuring a magnetic field are attached to the static part and comprise an optical fiber, a light source and an optical sensor. The optical fiber comprises a fiber Bragg grating coated with a magnetostrictive material. The light source and the optical sensor are both optically coupled to the optical fiber and electronically coupled to the electronic circuit. The electronic circuit is arranged for determining the rotation angle based on measured signals from the optical sensor.
The angle encoder according to the invention uses the magnetic ring in a similar way as before. The improvements lie in the way of detecting the magnetic field, for which now an optical fiber with one or more fiber Bragg gratings is used. A fiber Bragg grating is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This selective reflectance is obtained by adding a periodic variation to the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.
The fiber Bragg grating is coated with a magnetostrictive material. Magnetostrictive materials change their shape in response to a change in their magnetization. When the magnetic ring is rotated together with the rotating part to which it is attached, the magnetic field at the fiber Bragg grating changes. As a result, the shape of the coating and the fiber core to which it is attached change. Typically, the coated part of the fiber core, i.e. the part with the fiber Bragg grating, is shortened or elongated. The distortion of the fiber core causes the periodic variation of the refractive index at the fiber Bragg grating to change. The changed shape of the fiber Bragg grating gives it different transmission and reflection properties. With the light source and the optical sensor coupled to the optical fiber, this change of transmission and reflection properties can easily be detected. It is a major advantage of the angular encoder according to the invention, that multiple fiber Bragg gratings may be provided in one optical fiber. In the prior art angular encoders, each Hall sensor requires its own wiring which may lead to a very complex wiring architecture. When one optical fiber comprises multiple fiber Bragg gratings, they should be arranged to reflect light of different wavelengths in order to enable the optical sensor to detect which one has been affected by the magnetic field. This can easily be obtained by providing different fiber Bragg gratings with different patterns of variation of the refractive index. Alternatively, different Bragg gratings are comprised in different optical fibers in order to enable detecting which one of the fiber Bragg gratings has changed shape.
Another advantage of using the coated fiber Bragg gratings instead of Hall sensors is that the dynamic range of the coated fiber Bragg sensors is relatively large. The Hall sensors of the prior art, quickly become saturated in an external magnetic field, which makes it impossible to detect rotation of the magnetic ring. An external magnetic field that influences all coated fiber Bragg sensors in the same way effectively changes the base wavelength (i.e. the reflection wavelength if the magnetic ring would not be there) of all of them. Due to their large dynamic range, the coated fiber Bragg sensors remain mechanically elastic and will continue to change shape in response to rotation of the magnetic ring. The offset caused by the external magnetic field can easily be filtered out electronically or using software. Similar offsets caused by, e.g., changes in temperature can be filtered out in a similar way.
In a special embodiment, at least one fiber Bragg grating may be provided which is not coated with a magnetostrictive material. The sensor thus obtained can be used as a reference sensor, because it's wavelength of reflection is not affected by the changing magnetic field. The reference sensor can therefore be used to compensate for temperature dependent influences and other non-magnetic phenomena that affect all fiber Bragg gratings to the same extent. The reference sensor may be covered with an inert (and not magnetostrictive) material to prevent unwanted interactions with the bearing environment.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
Brief description of the drawings
In the drawings:
Figure 1 shows an angle encoder according to the invention,
Figure 2 shows a refractive index of an optical fiber with a coated fiber Bragg grating, Figure 3a shows a power distribution of an input light beam,
Figure 3b shows a spectral response for an optical fiber with a fiber Bragg grating, Figure 3c shows a power distribution of light reflected by a coated fiber Bragg sensors at three different orientations relative to the magnetic ring, and
Figures 4a-4c schematically show a coated fiber Bragg grating at three different orientations relative to the magnetic ring.
Detailed description of the invention
Figure 1 shows an angle encoder 10 according to the invention. The angle encoder 10 is provided for determining a rotation angle of a rotating part relative to a static part 12. A magnetic ring 11 is attached to the rotating part and has two magnetic poles, a north pole N and a south pole S. In principle, the magnetic ring 11 could also have more magnetic poles. For the current invention, it is only important that the magnetic ring 11 provides a different magnetic field for different rotation angles. When the magnetic ring 11 is rotated, the direction of the magnetic field at the static part 12 changes. Means for measuring the magnetic field and a control circuit 17 for processing the measurements detect the changes in the magnetic field and convert the detected changes to a rotation angle of the rotating part relative to the static part 12. The means for measuring the magnetic field comprise an optical fiber 13 with at least one, but preferably more, coated fiber Bragg gratings 14. The fiber Bragg gratings may be created by engraving variations of refractive index into the core of the fiber 13. Fiber Bragg gratings reflect particular wavelengths of light and transmit all others. Which particular wavelengths are reflected depends on the pattern that is engraved in the fiber 13 core. A light source 15 and a light sensor 16 are optically coupled to respective ends of the optical fiber 13. The light source 15, e.g. a laser diode, sends monochromatic or polychromatic light into the optical fiber 13. The light sensor 16 detects the light that is transmitted through the optical fiber. Multiple light sources 15 and/or light sensors 16 may be used for separately sending out or detecting light of different colors. If the fiber Bragg gratings 14 reflect part of the light sent through the fiber 13, the spectrum of the detected light differs from the spectrum of the emitted light. Analysis of the detected light may thus provide information about the presence of fiber Bragg gratings 14 in the fiber 13. Optionally, a reference fiber 19 may be provided for directly leading part of the emitted light to the light sensor 16 and for enabling direct measurement of the light that enters the optical fibers 13, 19.
The coating of the fiber Bragg gratings 14 comprises a magnetostrictive material. The magnetostrictive coating may, e.g., comprise Terfenol-D in the form of particles dispersed in a polymer matrix. Magnetostrictive materials change their shape in response to a change in its magnetization. When the direction of the magnetic field generated by the magnetic ring 11 changes, the coating changes shape. Together with its coating, the fiber Bragg grating 14 itself changes shape. This will change the periodicity of the variations of the refractive index and the range of wavelengths that are reflected by the coated fiber Bragg grating 14. A change of the magnetic field will thus also lead to a change of the light detected by the light sensor 16. Analysis of the detected light makes it possible to get information about the direction of the magnetic field. The control circuit 17 is electrically coupled to the light sensor 16 for receiving signals representing the detected light and converting the received signals into a corresponding rotation angle.
When using only one coated fiber Bragg grating 14 it is possible to determine a change in the rotation angle, but not to determine an absolute value of it. The same detected reflection wavelength is found at two different rotation angles. In order to accurately determine an absolute value of the rotation angle, it is needed to use multiple coated fiber Bragg gratings. In principle, two fiber Bragg gratings is enough, but the optimum number of gratings depends on the overtones of the magnetic ring, the expected external magnetic field influences, etc. If multiple fiber Bragg coatings 14 with the same base wavelength (i.e. the reflection wavelength if the magnetic ring would not be there) were comprised in the same optical fiber 13, it would not have been possible to find out which grating is reflecting (part of) the light. Therefore the coated fiber Bragg gratings 14 should reflect light of different wavelengths. Alternatively, separate optical fibers 13 are used for multiple fiber Bragg gratings 14 with the same base wavelength. Optionally, at least one fiber Bragg grating 18 may be provided which is not coated with a magnetostrictive material. The sensor 18 thus obtained can be used as a reference sensor 18, because it's wavelength of reflection is not affected by the changing magnetic field. The reference sensor 18 can therefore be used to compensate for temperature dependent influences and other non-magnetic phenomena that affect all fiber Bragg gratings 14, 18 to the same extent.
Figure 2 shows a refractive index of an optical fiber 13 with a fiber Bragg grating 14, 18. Fiber Bragg gratings 14, 18 are obtained by adding a systematic variation to the refractive index, n, of the fiber core. The optical properties of the fiber Bragg grating 14, 18 depend on the difference between the refractive indices (n2-n ), the distance between the index transitions and the number of transitions.
Figure 3a shows a power distribution of an input light beam. The input light beam used in the angle encoder 10 according to the invention may either be monochromatic or polychromatic. In practice, it is not possible to produce light with only one wavelength and the term monochromatic light is used for light with a narrow frequency bandwidth. Polychromatic light is light with a number of different wavelengths. Polychromatic light may be a combination of monochromatic light of two or more different colors or just some mixture of light in a broad frequency range, such as for example white light. Figure 3a shows a frequency spectrum of a polychromatic input light beam that may be used in an angle encoder according to the invention.
Figure 3b shows a spectral response for an optical fiber with a fiber Bragg grating 14, 18. It shows the power distribution of the light transmitted through the optical fiber 13 and the fiber Bragg grating 14, 18 as detected by the light sensor 16. Most frequencies of the light beam of figure 3a are transmitted and detected. Only light in a narrow frequency range around the reflection wavelength, λΒ, is reflected by the fiber Bragg grating 14 and does not reach the light sensor 16.
Figure 3c shows a power distribution of light reflected by a coated fiber Bragg sensor 14 at three different orientations relative to the magnetic ring 11. In a first position, the fiber Bragg grating 14 provides a transmission profile as shown in figure 3b, with a dip at the reflection wavelength, λΒ, of the grating 14. Figure 3c shows the power distribution 31 of the light reflected by the grating 14, with a peak at the reflection wavelength, λΒ. When the magnetic ring 11 is rotated, the orientation of the magnetic field relative to the coated fiber Bragg grating 14 changes. The changing magnetic field causes the magnetostrictive coating to change shape and thereby alters the pattern of variations in the refractive index of the grating 14. As a result, the reflection wavelength, λΒ, of the grating is shifted to, a lower wavelength, λΒΤ or a higher wavelength, λΒ3. This shift in of the reflection wavelength is measured by the light sensor 16 and converted into information about the rotation angle by the electronic circuit 17.
Figures 4a-4c schematically show a coated fiber Bragg grating 14 at three different orientations relative to the magnetic ring 11. In the three different figures 4a, 4b and 4c, the magnetic field and the magnetostrictive coating have caused the grating 14 to have different shapes. The elongation and shortening of the grating 14 also changes the distance between two consecutive refractive index transitions, which changes the reflection wavelength of the fiber Bragg grating 14.
In practice there will be a trade-off when selecting either mono-chromatic or polychromatic light. Using mono-chromatic light, the light has to be modulated to reach the sensitivity band of all sensors. As a consequence, the sensors are addressed sequentially which is a disadvantage for precisely determining angle in a rotating system, but is expected to have an advantage in circuit complexity and therefore in cost. The polychromatic variant does allow parallel read-out, but also requires parallel circuits for each of the Bragg gratings. As a consequence higher speeds can be obtained, but the cost may go up. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

An angle encoder (10) for determining a rotation angle of a rotating part (11) relative to a static part (12), the angle encoder (10) comprising:
• a magnetic ring (11) having at least two magnetic poles and being attached to the rotating part (11),
• attached to the static part (12), means for measuring a magnetic field, and
• an electronic circuit (17) for determining the rotation angle based on the measured magnetic field,
characterized in that
the means for measuring the magnetic field comprise an optical fiber (13), the optical fiber comprising a fiber Bragg grating (14) coated with a magnetostrictive material, and wherein the means for measuring the magnetic field further comprise a light source (15) and an optical sensor (16), both being optically coupled to the optical fiber (13) and electronically coupled to the electronic circuit (17), the electronic circuit (17) being arranged to determine the rotation angle based on measured signals from the optical sensor (16). An angle encoder (10) as claimed in claim 1, wherein the means for measuring the magnetic field comprise at least two fiber Bragg gratings (14). An angle encoder (10) as claimed in claim 2, wherein the at least two fiber Bragg gratings (14) are arranged to reflect light of at least two distinct wavelengths, the light source (15) being arranged to emit light comprising the at least two distinct wavelengths. An angle encoder (10) as claimed in claim 2, wherein the means for measuring the magnetic field comprise at least two optical fibers (13) and the at least two fiber Bragg gratings (14) are comprised in respective ones of the at least two optical fibers (13).
5. An angle encoder (10) as claimed in any preceding claim, wherein the optical fiber (13) comprises a reference fiber Bragg grating (18), the reference fiber Bragg grating (18) not being coated with a magnetostrictive material.
6. An angle encoder (10) as claimed in any preceding claim, wherein the light source (15) is arranged to provide monochromatic light to the optical fiber (13).
7. An angle encoder (10) as claimed in any preceding claim, wherein the light source (15) is arranged to provide polychromatic light to the optical fiber (13).
8. An angle encoder (10) as claimed in any preceding claim, wherein the magnetostricitve material comprises Terfenol-D.
9. An angle encoder (10) as claimed in claim 8, wherein the magnetostricitve material comprises particles of Terfenol-D, dispersed in a polymeric matrix.
10. A bearing unit comprising a static part (12), a rotating part (11), and an angle encoder (10) according to one of the claims 1 to 9 for determining a rotation angle of the rotating part (11) relative to the static part (12).
PCT/EP2011/072103 2011-12-07 2011-12-07 Optical angle encoder WO2013083192A1 (en)

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WO2015106732A1 (en) * 2014-01-20 2015-07-23 Balluff Gmbh Magnetostrictive sensor for measuring distance and position
CN106001827A (en) * 2016-06-14 2016-10-12 华中科技大学 Preparation method for fiber bragg grating magnetic sensor based on reflow soldering
CN109084824A (en) * 2018-07-28 2018-12-25 张健 Angular encoder structure
CN113253169A (en) * 2021-04-30 2021-08-13 上海交通大学 Magnetic resonance safe rotary encoder and rotation angle detection method
CN114273143A (en) * 2021-12-03 2022-04-05 中科芯(苏州)微电子科技有限公司 Glue spreading device for wafer mounting

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US7154081B1 (en) * 2002-11-26 2006-12-26 Luna Innovations Incorporated Composite structures, such as coated wiring assemblies, having integral fiber optic-based condition detectors and systems which employ the same
EP1553386A2 (en) * 2004-01-08 2005-07-13 Delphi Technologies, Inc. System and method for sensing angular position using ring magnet with sensor in flux return path
WO2006099143A1 (en) * 2005-03-12 2006-09-21 Baker Hughes Incorporated Optical position sensor

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2015106732A1 (en) * 2014-01-20 2015-07-23 Balluff Gmbh Magnetostrictive sensor for measuring distance and position
CN106001827A (en) * 2016-06-14 2016-10-12 华中科技大学 Preparation method for fiber bragg grating magnetic sensor based on reflow soldering
CN106001827B (en) * 2016-06-14 2018-03-09 华中科技大学 A kind of preparation method of the fiber grating Magnetic Sensor based on Reflow Soldering
CN109084824A (en) * 2018-07-28 2018-12-25 张健 Angular encoder structure
CN113253169A (en) * 2021-04-30 2021-08-13 上海交通大学 Magnetic resonance safe rotary encoder and rotation angle detection method
CN113253169B (en) * 2021-04-30 2022-03-01 上海交通大学 Magnetic resonance safe rotary encoder and rotation angle detection method
CN114273143A (en) * 2021-12-03 2022-04-05 中科芯(苏州)微电子科技有限公司 Glue spreading device for wafer mounting
CN114273143B (en) * 2021-12-03 2023-02-10 中科芯(苏州)微电子科技有限公司 Glue spreading device for wafer mounting

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