WO2015106732A1

WO2015106732A1 - Magnetostrictive sensor for measuring distance and position

Info

Publication number: WO2015106732A1
Application number: PCT/DE2014/000011
Authority: WO
Inventors: Heiko Mahr; Josef ALBANO; Michael Warber
Original assignee: Balluff Gmbh
Priority date: 2014-01-20
Filing date: 2014-01-20
Publication date: 2015-07-23
Also published as: DE112014006216A5; DE112014006216B4

Abstract

The invention relates to a magnetostrictive sensor for measuring distance and position, comprising at least one optical fibre (300, 305) provided with a magnetostrictive sheath (315), which sensor can be used to detect the position of a magnetic object (307) in the direction of the at least one optical fibre (300, 305), said at least one optical fibre (300, 305) having, in particular, a Bragg grating (310), the lattice constant of which can be altered by the mechanical action of the magnetostrictive effect of the magnetostrictive sheath (315) brought about by the magnetic object (307).

Description

Description title

Magnetostrictive sensor for distance or position measurement

The invention is based on a magnetostrictive sensor according to the preamble of the independent claims.

State of the art

Different approaches are known in the field of distance or position measurement here concerned. Thus, US Pat. No. 3,898,555 A discloses a position measuring device comprising an electrical waveguide (i.e., waveguide) having magnetostrictive properties. The waveguide is acted upon by current pulses which lead to a magnetic field whose field lines run concentrically in the waveguide and surround it concentrically on the outside. Adjacent to the waveguide is a

slidable permanent magnet provided whose poles are arranged such that the magnet is a localized, extending mainly in the axial direction within the waveguide magnetic field

CONFIRMATION COPY caused. The permanent magnet is arranged on the object whose position or movement is to be measured.

The localized in the region of the magnetic field of the permanent magnet superposition with the circular, occurring during the current pulse magnetic field leads to a localized change of the resulting magnetic field, due to the magnetostrictive effect

Changes the length of the waveguide causes, which moves as a sound wave in both directions. At one end of the waveguide is a

Damping element arranged, which absorbs the sound wave impinging there and prevents reflection. At the other end of the waveguide, a wave detection device is arranged, which detects the vibration of the waveguide and converts it into signals. The signals are fed to a signal evaluation arrangement, which determines the position of the permanent magnet and the associated object from the transit time of the shaft.

In the patent application US 3,898,555 A is an apparatus for

Position measurement described, which is also based on the magnetostrictive effect. Instead of the comparatively complex wave detection provided in the device described above, in the measuring device described here, the wave detection is realized as a magnetic coil. The magnetic coil surrounds the waveguide at one end over a short distance, while at the other end the damping element is again arranged. The auxiliary permanent magnet is replaced by an always existing magnetic field of the waveguide, which can be referred to as remanent magnetic field. The short-term change of the (remanence) magnetic field induces a measuring voltage in the magnetic coil. The position of the

Permanent magnet is again determined from the transit time of the wave in the waveguide, which is between the occurrence of the current pulse and the occurrence of the measuring voltage. Applicant's previously published DE 10 2010 008 495 A1 also discloses a method for measuring the position of an object, in which a magnet associated with the object is moved along a magnetostrictive waveguide, whereby the magnet causes a first magnetic field component in a field region in the waveguide. In this case, a current signal is provided with a current pulse which causes a current magnetic field in the waveguide, which has at least one field component in the waveguide, which caused by the magnet

Field component deviates. Due to the magnetostrictive effect, in the field area mentioned, a wave is generated due to the field change during the current pulse, which is detected, the position of the object being determined from the propagation time of this wave in the waveguide.

In the case of the magnetostrictive sensors mentioned, the transit time of a mechanical shaft is used to determine the position of the position sensor. The wave is created by superposition of a magnetic field from the current pulse with the magnetic field of the position sensor. Signal detection has a decisive influence on the measuring accuracy. In the case of signal converters known from the prior art, a coil is used for this purpose in which the mechanical shaft induces a voltage twice. The mechanical shaft is reflected after passing through the coil and then generates a voltage for the second time. By a suitable geometric arrangement of the components, i. the coil and the

Reflection end, the two signals can be added so that an optimum useful signal, in particular an approximately twice as large useful signal arises. In this case, in the prior art, a characteristic size of the signal, for example a zero crossing, is evaluated. Such a

Zero crossing today can take into account the noise in the

Nanosecond range are evaluated, which corresponds to a measurement accuracy of a few m. Furthermore, from US 2007/0014506 A1 a fiber optic

Position measuring system forth, which comprises a magnetic element and one or more segments of magnetostrictive material. The

Fiber optics include a fiber Bragg grating. This measuring system detects changes in the relative position between a magnetic field source and one of said segments. The local measuring principle is based on

Changes in the segment size, which in turn causes deviations of the wavelength reflected from the Bragg grating, which

be detected interferometrically. In a known spatial arrangement of the magnetic field can be concluded from the wavelength of the reflected wave on said change in the relative position. The fiber optic disclosed therein has more than one lattice constant. In addition, the determination of position data by means of the relatively complex interferometric method described briefly takes place. Since the measuring section is not formed continuously but segmented, the

Measurement resolution fixed by the segmentation. In addition, the measurement length accessible for the measurement is limited by the resolution of said interferometer.

Finally, US Pat. No. 6,433,543 B1 discloses a magnetostrictive magnetic sensor, in which a first glass fiber is attached to a

Bending beam is arranged and a second fiber from the first

Glass fiber is arranged spaced. Light is introduced into the first fiber. The second optical fiber has means for detecting the end of the first optical fiber by optical coupling to the end of the second optical fiber

Glass fiber transmitted light. This optical coupling changes due to the magnetostrictive effect and the concomitant bending of the bending beam, the magnetostrictive effect being proportional to changes in a corresponding component of an external one

Magnetic field is, and therefore allows the determination of the size of the external magnetic field. Disclosure of the invention

The invention has for its object to provide a generic magnetostrictive sensor, which eliminates the disadvantages of the prior art described.

The invention is based on the idea of further developing a sensor based on a fiber optic technology having at least one optical fiber.

In particular, the sensor according to the invention has a Bragg grating known per se, by means of which distance or position measurements are made possible by the interaction of the Bragg grating with the described principle of magnetostriction.

The at least one optical fiber can be formed by one or more individual fibers. The use of such an optical fiber has the advantage that a flexible arrangement or laying of the

Measuring section is made possible. In addition, one over known

Distance measuring sensors higher clocked measurement allows. The measuring method on which the invention is based measures the distance continuously and not only periodically. Magnetostrictive systems known in the prior art can only measure periodically, in particular due to the current impulse excitation.

The better damping compared to sound waves at the end face of an optical fiber allows a higher useful length than ultrasound-based displacement sensors and thus long measuring distances of eg greater than 7 m. Also, with a light guide, ultrasonic noise due to reflections decreases. The measuring method does not generate a mechanical wave but is based on optical reflections taking place in the optical measuring section. Optical reflections can be significantly improve reflective coatings or by chamfering the fiber. In addition, optical reflections can be better damped by corresponding damping elements than mechanical waves. The space to be provided for damping at the end of the measuring section is significantly reduced in optical fiber systems compared to mechanical ones

Systems, and therefore increases advantageously the ratio of measuring distance to length.

In addition, the proposed fiber optics, especially in view of the relatively small number of electrical components, allows the use in an industrial environment, in particular under chemically or physically particularly aggressive or even potentially explosive environments. The elimination of an electrical excitation also offers a higher E MV stability (electromagnetic compatibility) along the entire measuring path.

Preferably, the optical fiber is provided with a continuous lattice constant grating lattice and then coated in a manner known per se with a magnetostrictive envelope or surrounded by such a sheath, e.g. by means of an adhesive bond, Schrumpfbzw. Press connection, or by overmolding, remelting, threading, pressing, coating or doping. On the exact nature of

mechanical connection between the optical fiber and the

magnetostrictive wrapping is not important, but it must be ensured that it is a non-positive connection, which mechanically takes place in the envelope held expansions or compressions on the fiber.

A suitable material for said enclosure is e.g. a

magnetostrictive polyethylene (PE) layer. The exact material is not relevant to the invention, however, it is advantageous if the magnetostrictive material as similar or coincident mechanical properties such as the material of the fiber optic, in particular with respect to the bending flexibility, on the elongation over the temperature and on the material expansion over the temperature. Therefore, all magnetostrictive acting (MR) materials such as

Plastics, ceramics, magnetostrictive glasses or doped optical fibers suitable for the envelope.

It should be noted that the elongated extent of the magnetostrictive envelope along the optical fiber defines the actual measurement range, i. the area within which the magnetostrictive sensor is sensitive.

According to a preferred embodiment, by means of an optical

Light source a time-varying frequency modulated light signal is introduced or fed into the optical fiber. The same light signal is e.g. fed by means of a splitter in a not provided with such a magnetostrictive envelope optical reference fiber or fed directly to the receiver, wherein the two signals from the reference path and the reflection from the measuring fiber are fed to a receiver. In the receiver, the two exiting signals are superimposed and therefore give a modulated signal. For this purpose, either again an optical component is provided, which combines the two exiting signals, or the two signals are fed directly to the receiver, which is connected to both optical fibers in a light-conducting or light-tight manner. The latter is made possible in particular by the small thickness of a single fiber.

Instead of using only one receiver, the signals emerging from the first fiber and the reference fiber may also be separate

Receivers are supplied and then superimposed (electrically). The light signal fed into the two fibers is preferably infrared (IR) liquid. The use of light this

Wavelength has the advantage that IR light is one in the optical

Fiber measuring technology is common wavelength and many standard components are available in this area. However, the sensor according to the invention can in principle be realized with any wavelength of light

The feeding of said light signals and the detection of the two exiting light signals or the said modulated signal is preferably carried out by means of the known measuring principle of the modulated (FMCW) continuous wave radar. The advantage of the FMCW method in comparison with normal magnetostrictive methods with mechanical waves is in particular that due to the sampling in the MHz to GHz range as well as due to the insensitivity to optical

Reflections the appropriate sampling rate of the measuring length is independent.

In addition to the two mentioned optical fibers, a third optical fiber, which is preferably arranged along the measuring section and which, like the reference fiber, is likewise not provided with a magnetostrictive envelope but is mirrored at its free end, can be provided. By means of this third fiber, e.g. Variations in the thermal expansion of the measuring path caused by changing environmental conditions are detected and compensated by suitable measures, e.g. the length of the reference fiber is determined using the same method, and then the length extension of the measuring fiber is compensated for electronically or mathematically.

The sensor according to the invention has in comparison to the prior art, a Bragg grating, whereby the size of the by magnetostriction

caused measurement effect along the measuring section is essentially independent of the position. Due to the proposed arrangement of the Bragg grating also results in particular a continuous Measuring section. Due to the incoherent superposition of the measurement and reference signal, the evaluation of the modulated

Output signal relatively simple.

In addition, the measurement resolution can be freely adapted to the measurement length, since the chirp frequency can be adapted to the position of the magnet or the application. Said chirp frequency refers to a signal whose frequency changes over time, distinguishing between positive chirps, in which the frequency increases in time, and negative chirps, which have a frequency decrease. Thus, a high-resolution measurement can be ensured even with large measuring lengths. Since the fiber arrangement according to the invention can be extended almost as desired, altogether very different, and in particular very long, measuring lengths are possible.

The sensor according to the invention is preferably suitable for a position or displacement transducer for detecting the position of a magnetic

Measuring body. Also, the sensor can detect internal or external mechanical stresses or deflections of a body. In addition, the sensor is in principle also suitable for detecting changes in temperature or pressure. The very small cross-section and the great mechanical flexibility of the optical fibers used make it possible to use them in curved measuring sections or environments and to lay the measuring path in a mechanically and optically difficult or impossible area, e.g. in the interior of a

opaque cylinder, e.g. for detecting the longitudinal position of a magnet in such a cylinder.

Brief description of the figures

Figure 1 shows a glass fiber with a Bragg grating according to the prior art; Figures 2a-c show in a glass fiber according to Figure 1 resulting signal waveforms;

Figure 3 shows a preferred embodiment of a

magnetostrictive sensor according to the invention;

Figures 4a-e show in a sensor according to the invention according to Figure 3 resulting signal waveforms.

Detailed description of the embodiments

The optical waveguide (in this case a conventional glass fiber or POF) shown in FIG. 1 comprises, as is known, a cladding 100, a core 105 and an outer coating (not shown). By the

Outside coating gives the fiberglass 100, 105 robust properties, making it particularly suitable for use in a harsh environment.

In addition, at regular intervals and in known manner along the core or transversely to the core pattern-like structures 110 with respect to the surrounding material changed refractive index

incorporated. These structures 110 form a "Bragg grating" in which the so-called "Bragg reflection" takes place. The Bragg grating corresponds to a periodic modulation of the refractive index and therefore influences a continuous signal such as an interference filter with a filter bandwidth λ _Β (see FIG. 2).

Said periodic modulation of the refractive index n, with high and low refractive index ranges, causes the light of a certain wavelength to be reflected back according to a band-stop filter. The center wavelength of the filter bandwidth in a monomode fiber is given by the following Bragg condition:

2 (1) In the equation (1), n ₂ and n _{3 are} the effective refractive indices of the core of the optical fiber and Λ the grating period. The spectral width of the band depends on the length of the fiber Bragg grating and the strength of the

Refractive index change between the adjacent refractive index ranges.

Due to the Bragg reflection, only certain wavelengths of the infrared light 115 propagating in the glass fiber are reflected 120 at the Bragg grating 110. The wavelength of the reflected light 120 depends in particular on the elongation of the glass fiber, i. of the

Deformation and mechanical stress on the glass fiber at the location considered. These changes are evaluated in a conventional manner.

The wavelength of the filter bandwidth ΛΒ shifts by \ with the temperature 7 and the relative elongation e of the glass fiber:

with: o «: refractive index change, type 5 ... 8 χ 10 ^{" 6} K ^"1

k constant (typically: 0.78)

The elongation of the fiber is made up of the proportion of the outside

applied strain £ m and the thermal expansion ^e T together: ^e - ^€ va + ^€ τ and one receives for the

Temperature dependence of λβ: With:

-1

ατ: thermal expansion of the glass fiber, type 0.6 χ 10 ^"6 K ^"

and for the strain dependence e:

Therefore, with the temperature and mechanical stress simultaneously changing for the strain e:

A described fiber Bragg grating can resolve compressive forces of several bar and temperature changes of 100 K. A typical Bragg grating tuned to 1500 nm shifts by 0.1 nm with a temperature change of 10 K, as well as a change in length of 10 ^-4 .

FIGS. 2a to 2c show a fiber according to FIG

typically resulting signal waveforms shown, in each case the light output P is plotted against the wavelength λ. The individual values of the light output are the input power P _E corresponding to the input signal 115 in FIG. 1, the output power P _{R of} the reflected signal 120, and the output power P _D of the signal 125 exiting the optical fiber 100, 105.

As can be seen from Figure 2a, the power P _{E of} the input signal has a relatively large bandwidth of wavelengths λ. Due to the above-mentioned filter function of the Bragg grating 110 is a relative

narrow band signal with power P _R and average wavelength AB reflected back at the Bragg grating (Figure 2b). This results in the output of the glass fiber 100, 105, an output signal 125 with the spectral curve shown in Figure 2c, in which in the region of the wavelength λ _{Β of} the back-reflected signal, a corresponding reduction of the power P _D occurs.

FIG. 3 shows an optical fiber according to the invention, in the present case formed from an arrangement of at least two individual fibers 300, 342. The individual fibers 300, 342 may be glass fibers, plastic (polymer) fibers, or the like. Only the single fiber 300 has a Bragg grating 310 arranged in the respective fiber core 305 and is enveloped or coated with a magnetostrictive material 315.

It goes without saying that in order to increase the signal security, it is also possible to bundle a plurality of fibers to form a single fiber 300, 342, since then the breakage of a single fiber has no effect on the measurement or evaluation. However, this requires that the individual fibers of these bundles are non-positively connected, so that any changes in length also occur bundled.

An incoming) frequency modulated as described herein and provided by an IR laser (e.g., an LED based laser) 302

Light signal 320 is fed to a splitter 325. The splitter 325 is used to feed the input signal 320 into both the first measuring fiber 300 and the second reference fiber 342. The signal fed into the measuring fiber 300 is fed to the signal current position of a magnetic object or measuring body 307 due to

magnetostrictive cladding 315, and in combination with the Bragg grating, and deflected by means of the beam splitter 325 so as to strike an optical receiver 370.

The identical input signal 320 fed into the reference fiber 342, which is deflected by means of the splitter 325 and into the reference fiber 342 is initiated 340, is accordingly reflected at the end of the reference fiber to a mirror 344 and in the present case via the beam splitter 325 also supplied to the receiver 370.

It should be emphasized that the arrangement shown here of splitter and deflecting mirrors is only an example, and it is based on the exact

Signaling for the present invention does not matter. Thus, the optical elements shown here can also be dispensed with altogether if the measuring fiber and the reference fiber are arranged relatively close, e.g. are bundled so that the signals due to the small lateral offset are instantaneous, i. can be supplied to a receiver without the 90 ° deflection shown, or to be supplied to separate receivers and then to be electrically superimposed.

In addition, a third fiber can be provided, which is also fed via the identical input signal. In contrast to the other two fibers, the third fiber also has a mirror finish at the end. Due to this mirroring the input signal is reflected back, whereby a change in length of the third fiber due to a

Temperature change in turn, known per se via the FMCW method can be determined. Based on the resulting value of the change in length, the sensor can be calibrated to a

Tem perature compensation.

At the current position of a magnetic object or measuring body 307, the magnetostrictive envelope 315 is stretched or compressed by the magnetic field shown. The magnetic measuring body 307 is formed in the present embodiment by a permanent magnet, but can also be realized by an electromagnet or the like, since it does not depend on the type of generating the magnetic field. The position of the measuring body 307 in the longitudinal direction of the measuring fiber 300 and the reference fiber 342 corresponds to the position to be detected of the present sensor. Alternatively, by means of the present sensor, the path traveled by the magnetic measuring body along the length of the fibers can be detected. Also, by means of the sensor based on the above equation (5), a temperature change, a mechanical stress (or

mechanical strain according to a strain gauge), a

Bending or a pressure change are detected.

Due to the mentioned frictional connection, the sheath of the measuring fiber 300 is correspondingly stretched or compressed by the magnetostrictive effect. This deformation causes due to the Bragg grating 310 an intrinsic influence on the coupled-in light spectrum, which in turn a

Causes reflection or phase change. This intrinsic influence

i

Therefore, it is possible to correlate the respective change mentioned with the position of the magnetic object 307 in the manner described below.

The measuring principle on which the invention is based is illustrated below with reference to the signal curves shown in FIGS. 4a-4e. In Figure 4a, a typical frequency response, i. shown in the embodiment shown a sawtooth with constant pitch. In FIG. 4b, as a further exemplary embodiment of the aforementioned frequency characteristic, a triangular function with a constant gradient is shown. FIG. 4c shows a typical transmission signal, in the present case an amplitude characteristic with the named sawtooth frequency response. FIG. 4d shows a typical receiver signal 400, which results from the incoherent superimposition of the measurement signal 405 and the reference signal 410. To determine the position, the frequency of the beat, i. the envelope shown, are determined. Finally, FIG. 4e illustrates how twice as large a distance as compared to FIG. 4a is twice as large

Beating frequency of the receiver signal 415. The elongation or compression of the measuring fiber 300 caused by the magnetic measuring body 307 caused by the magnetic measuring body 307 at the location of the magnetic influencing, ie in the area of the mentioned measuring section, causes an intensified reflection of the coupled-in light signal due to the corresponding distortion of the Bragg grating. This reflected light is supplied to the aforementioned optical receiver 370. In addition, the light reflected from the reference fiber 342 is supplied to the receiver 370. At the receiver 370, the light components of the measuring fiber 300 and the reference fiber 342 are superimposed incoherently or coherently, resulting in a correspondingly modulated signal. In addition, the light signals from the measuring fiber and the reference fiber, as already described, can be supplied to separate receivers and then subsequently be electrically superimposed.

According to a further embodiment, the feeding of said light signals and the corresponding detection of the light signals emerging from the fibers 300, 342 and the said modulated signal by means of the known measuring principle of the modulated (FMCW) continuous wave radar takes place. A radar signal is emitted with a constantly changing frequency. The frequency increases either linearly to drop abruptly at a certain value back to the initial value (sawtooth pattern), according to the waveform shown in Figure 4a, or it rises and falls alternately at a constant

Rate of change, corresponding to that shown in Figure 4b

Waveform. By linear change of the frequency and by continuous transmission, it is possible, in addition to the differential speed between transmitter and object in particular to determine their absolute distance from each other. The exact position of the generated reflection can be over the

Determination of the beat frequency (envelope) can be determined, as can be seen from the waveform in Figure 4d. At twice as big

Distance also doubles the beat frequency accordingly, as seen in Figure 4e. It should be noted that a network-like arrangement of such sensors formed from the above-described individual Bragg grating sensors also enables the detection of information about existing deformations and loads of an entire component.

Claims

claims

1. Magnetostrictive sensor for distance or position measurement,

which comprises at least one optical fiber (300, 305) provided with a magnetostrictive envelope (315), by means of which the position of a magnetic object (307) in the direction of the at least one optical fiber (300, 305) can be detected, characterized in that at least one optical fiber (300, 305) has a Bragg grating (310) whose grating constant is at least locally changeable by mechanical action of the magnetostrictive effect of the magnetostrictive cladding (315) caused by the magnetic object (307).

2. Sensor according to claim 1, characterized in that the at least one optical fiber (300, 305) has a continuous Bragg grating (310) with a uniform lattice constant.

3. Sensor according to claim 1 or 2, characterized in that the

Position of the magnetic object (307) by means of the changed by the changed lattice constant of the Bragg grating reflectivity of the at least one optical fiber (300, 305) can be determined.

4. Sensor according to claim 3, characterized in that the at least one optical fiber (300, 305) by a said

magnetostrictive cladding (315) having measuring fiber (300) and a said magnetostrictive cladding (315) not having reference fiber (342) is formed, wherein the modified

Reflectance of the measuring fiber by a comparison of the measuring fiber (300) and the reference fiber (342) can be determined.

5. Sensor according to claim 4, characterized in that in the measuring fiber (300) and the reference fiber (342) a time-variable, in particular frequency-modulated light signal is introduced and the at the measuring fiber (300) and the reference fiber (342) exiting

Light signals (350, 360) are superimposed for the purpose of evaluation.

6. Sensor according to claim 5, characterized in that in the measuring fiber (300) and the reference fiber (342), a light signal, in particular an infrared light signal is introduced.

7. Sensor according to one of claims 4 to 6, characterized in that on the measuring fiber (300) and the reference fiber (342)

exiting light signals (350, 360) by means of a modulated

Continuous wave radar are evaluated.

8. Sensor according to one of the preceding claims, characterized

in that the magnetostrictive envelope (315) is non-positively connected to the at least one optical fiber (300, 305).

9. Sensor according to one of the preceding claims, characterized

in that the magnetostrictive sheath (315) is formed from a magnetostrictive material that matches as closely as possible with the mechanical properties of the material of the optical fiber (300, 305).

10. Sensor according to claim 9, characterized in that the

magnetostrictive sheath (315) is formed from a plastic, in particular a magnetostrictive acting polyethylene (PE) layer, or from a ceramic or a magnetostrictive glass or a doped optical fiber.

11. Sensor according to one of the preceding claims, characterized

in that an optical calibration fiber (347) not provided with a magnetostrictive envelope (315) is provided, which is mirrored at its free end and by means of which a change in the thermal expansion of the at least one optical fiber (300, 305) can be detected.