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GMR-Sensor in Use of Stress Measurement Dr.-Ing. Werner Ricken, Prof. Dr.rer.nat. Wolf-Jürgen Becker, Dr.-Ing. Jigou Liu Contact

1 Abstract

The monitoring of the condition of reinforced concrete constructions like bridges requires a great amount of strain measurements. Therefore the measurement of the prestressing of steel bars is important more and more. It does'nt astonish, that some fibre optic measurements are published in the last years [1]. These optic sensors are suitable for stress measurement in a wide area with great distances. With the help of the magnetoresistive sensor a stress measurement of prestressed steel bars in concrete is possible. In the case of the giant magnetoresistive sensors (GMR-Sensor) we prove their facility for a high sensitive stress measurement. The comparison with an eddy current measurement shows the advantages and the limits of magnetoresistive stress measurement.

2 Magnetoelastic Sensor

Many developments in the area of magnetic measurement technology are initialized by the availability of new magnetic materials. So in the begin of the 80th, some new methods of a contactless torque measurement on a rotating shaft were published. At that time, the novel amorphous metal foil, which are prestressed in a former process, covered in entirety or as stripe in a groved structure the shaft. Because of the good softmagnetic properties like small coercitivity H_c and high remanence B_r of the amorphous metal, a magnetoelastic torque sensor with two coils was possible. The principle of these torque measurement systems were the variation of magnetic coupling between a primary and secondary coil arrangement caused by the magnetoelastic effect. Therefore a more complex electronic circuit with the components of an oscillator, ac amplifier and filters was necessary. With the foil coated shaft a sensitivity of s=(Du/u)/(Dl/l)=25%/0.1% of a tensile measurement was achieved [2]. The advantage of this method was the suppression of disturbances like the influence of air gap between sensor and shaft surface, magnetic inhomogenities and temperature by an optimized field condition and a signal processing.

A new approach of a contactless magnetoelastic torque measurement was initialized by the development of a micro system consisting of a hall sensor, planar exciting coils and a softmagnetic yoke on a silicon chip [3, 4]. This micro system was developed in a CMOS technology and based on magnetic field effect transistor MAGFET. The advantages of this micro system is the integration of sensor, field excitation and signal processing on the same chip. Consequently the engineers use semiconductor as sensor material. In this case, the sensitivity and the magnetic field resolution are limited by the CMOS technology. The prototype of a MAGFET attains a sensitivity of s=5 V/T which strongly varies with the sensor geometry. The detectable magnetic field are limited by the flicker noise of the sensor. A smallest amount of is achieved.

The higher sensistivity and the smaller detectable field caused us to investigate GMR-Sensors in use of the magnetoelastic force measurement. In our contribution we will show the sensor characteristic ie the linearity, teh hysteresis and temperature error. Then we will present the results of a stress measurement at the center wire of the prestressed steel braid. Finaly, the measurements with a parallel and orthogonal field alignment follows.

3 Giant Magnetoresistive Sensor

The giant magnetoresistance occurs on very thin magnetic layers so called superlattices. The sensitive element is structured in order of a ferromagnetic and a non-magnetic layer with a thickness less than 10 mm. The electron scattering at the interface of the magnetic and non-magnetic layer is the reason for the electrical resistance. The giant magnetoresistance can be phenomenally explained by the dependence of the electron spin scattering. Electrons whose spins lie antiparallel to the magnetization in the ferromagnetic layers are strongly scattered [5].Against the antiparallel alignment, the parallel magnetization affects a weak scattering of the electrons at the interface. Because of the interaction of an external magnetic field H with the magnetization of the layers, this magnetic field can be measured. The most amount of the giant magnetoresitance

(1)


attain a value up to 82% [6].

Fig 1: Electronicsscattering at Co/Cu-layers[7]

There are three important technological types [8] of a GMR-Sensor. First, the GMR happens in a multilayer structure with alternate magnetic and non-magnetic layers. The thickness of the conductor layer which is precisely controlled to the proper thickness range is the reason of an antiferromagnetic coupling of the adjacent magnetic layers with antiparallel alignment of the magnetization (high resistance state). A large field is necessary to overcome this coupling and make a parallel alignment (low resistance state). This sensor shows the most amount of a GMR up to 70%. The antiparallel alignment in a Co/Fe sandwich layer is obtained by the magnetic anisotropic of the ferrmomagnetic layers. The parallel alignment of the magnetization of the two magnetic layers is attained by a saturated field H_s. Fig. 1 shows the state of the parallel and antiparallel alignment. The third important GMR-Sensor is the spin valve sensor. It has the highest sensitivity of s = 40^(mV/V)/mT and significantly are qualified for technical applications.One of the two magnetic layer of a spin valve sensor is pinned by an antiferromagnetic layer like the FeMn-exchange layer. As figure 2 shows, it is no movement of the magnetization in the pinned layer by an external field. But in contrast to this layer, the external field changes the direction of the magnetization in the free layer.

Fig 2: Spin value layer

Fig 3: Characteristic curves at T=0 and 450C

Before a sensor is used in non destructive testing, it must be investigated under the same environments with defined specimen and wellknown circumstance. Because of this, we investigate the magnetic resolution, linearity, hystersis and the temperature error of a GMR-Sensor [10].Afterwards, the characteristic data of the commercial available GMR-Multilayer sensor AA002 [9] follows. Fig. 4 shows the charactersitic signal curve. This sensor is suitable for the use in the linear range up to 1.1 mT. We calculate the sensitivity of s = 3.75^(mV/V)/mT at a temperature T=20° from the fitted signal curve. The deviation of 2.1% from the mean value of sensitivity depends on the temperature and the mean variation of the item, which is given by the procduction.

Fig 4: Non-linearity

Fig 5: Hysteresis


Futhermore, the GMR-Sensor shows a low linearity and hystersis error. The non-linearity of less than ±0.6% and the hysteresis of less than ±0.15% in the range of are sufficient for an application as a magnetoelastic stress sensor. The figures 4 and 5 show the calculation of non-linearity and hysteresis. Other important properties of a sensor are the temperature and long run stability. We measured the signal of the GMR-stripes, which are typically wired as a measurement bridge on a chip with a 8-pin SOIC package as housing. The figure 6 shows the curves of the calculated temperature error at three different magnetic flux densities B. The definition of the calculated temperature error f_T is given by

(2)




Fig 6: Temperature error

Fig 7: Long-run stability[10]


The curve surprisingly increase at B =1.8mT with the increasing temperature. The minimum of the temperature error is achieved at B =1.8mT. The reason can phenomenally explained by the different temperature behavior of the electrical resistance, anisotropic and giant magnetoresistance. The figure 7 show the result of a long-run measurement of the GMR-Sensor signal U and the error correction by the compensation of the temperature error. The detectable magnetic field can be calculated from the standard derivation of the temperature error corrected and low pass filtered value . With a standard derivation of 50mV a magnetic resolution of DB=267 × 10^-6Tis achieved. This value of the GMR-Sensor makes the use in non-destructive testing clear.

4 Stress Measurement with GMR-Sensor

According to the magnetoelastic theory [11], the mechanical stress s of a ferromagnetic material can be determined by the measurement of the magnetic permeability. The magnetoelastic coupling factor K₃₃ descibes the ratio of stored elastic stress energy W_s to the total stored energy. The total stored energy is the sum of å_iW_i of the magnetic W_H, mechanical stress W_s, cystralline W_K, uniaxial anisotropy W_u and shape anisotropy energy W_N. In order to achieve a large magnetoelastic coupling factor the magnetic, cystralline, anisotropy and shape energy should be as small as possible.

(3)


A low stored magnetic energy is achieved by a small field excitation coil. We use three different coils of an eddy current sensor with sizes from 8.8x4x2 to 26.7x15x6.4 mm with a parallel and perpendicular magnetization direction to the stress direction.

Fig 8: Measurement equipment

Fig 9: Stray Field Measurement with U-Yoke(15´11´6.4mm)


Manually generating from a stress apparatus the stress is lead to the center wire of a prestressing steel braid with a diameter of d = 5.23mm. Figure 8 shows shematically the picture of themeasurement equipment.With the help of the stress apparatus we run stress measurement up to a value of 930N/mm², which lies in the elastic range of s_g = 974N/mm². To compare the measurement results, we additionally measure the reactance X_L of an eddy current sensor with an impedance analyzer. First we put the GMR-Sensor in the stray field of a softmagnetic yoke on the surface of the steel bar. The arrangement of the field excitation consisting of a softmagnetic U-core ferrite and a coil is presented in figure 9. A small normal force on both the excitation and the sensor should prevent a varying of the air gap. For a cycle with up and down loading the wire we put the measurement values of the GMR-Sensor and the impedance of the excitation coil from a multimeter and impedance analyzer. For a better understanding, the figures 10 and 11 show the standardized measurement values. Figure 10 shows the ratio of the deviation of the reactance to the reactance X_L0 without damping the eddy current sensor at a frequency f= 1kHz . The measurement values U_d of the GMR-Sensor are scaled to the supply voltage U₀=5V of the measurement bridge. Both curves show a hysteresis loop which results from the higher coercitive field strength H_c of the steel. It is considered that a lower H_c decrease the hystersis. A coating with an amorphous foil should improve the measurement results.

Fig 10: Reactance of an Eddy Current Sensor

Fig 11: Signal of a GMR-Sensor

5 Parallel and Orthogonal Field Condition

Afterwards we investigate the influence of a parallel and orthogonal field condition. Therefore two different excitation coils are necessary. We use an E-core ferrite with a size of 8.8x4.2 mm for a parallel magnetic alignment. For an orthogonal field we modified the arrangement by two U-core and one I-core ferrite. The size of this so called cross-sensor is 15x11x6.4 mm.For both the parallel and the orthogonal field direction, we measure the magnetic stray field with the GMR-Sensor and the impedance of the excitation coil. As expected a higher sensitive measurement occurs at the parallel field direction. The results of a parallel and orthogonal field alignment show the figures 12 and 13. The best sensitivity of results from a parallel field alignment and a small excitation coil with the E-core ferrite. It can be improved by coating the wire with a softmagnetic material like amorphous metal. The hysteresis error attained a value up to 10 % for the whole loading cycle. The temperature error which essentially comes from the sensor shows figure 5. These first investigations show that the sensitivity of a GMR-Sensor is sufficient for a magnetoelastic stress measurement.

Fig 12: GMR-Measurement,parallel Direction

Fig 13: GMR-Measurement,orthogonal Direction

Fig 14: Temparature Stability

6 Summary

Our contribution shows the first approach to measure the stress of a ferromagnetic wire with a GMR-Sensor. A strain sensitivity of s=(Du/u)/(Dl/l)=0.033% / % is achieved. This value can be improved by a low-noise amplifier and an optimized field excitation. Computer simulations of a differential field arrangement are the basis for the improvement of the sensitivity. In [12] Dobmann e.a. point to the influence of material properties on the magnetoelastic stress measurement. The softmagnetic coating of the wire could reduce this disturbance. In future the measurement of stray field with high local resolution is planned. For this, the GMR-sensor offers the best properties.

References

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