(2)the Research of a Differential Magnetoresistive Linear Displacement Sensor Measurement System

8/10/2019 (2)the Research of a Differential Magnetoresistive Linear Displacement Sensor Measurement System

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Sensors & Transducers, Vol. 161, Issue 12, December 2013, pp. 618-624

SSS eee nnn sss ooo r r r sss &&& TTT r r r aaa nnn sss ddd uuu ccc eee r r r sss 2013 by IFSA

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The Research of a Differential Magnetoresistive LinearDisplacement Sensor Measurement System

Jianhui ZHU, Siqin CHANG, Jianguo DAISchool of Mechanical Engineering, Nanjing University of Science and Technology,

Nanjing 210094, ChinaTel.:/fax: +86-25-84315451

E-mail: [email protected]

Received: 17 December 2013 /Accepted: 29 December 2013 /Published: 30 December 2013

Abstract: For a single magnetic resistance displacement sensor is used for detecting the linear displacement ofelectromagnetic linear actuator (ELA), and the decline of detection precision caused by strong electromagneticinterference, and the problem of low control accuracy etc., a scheme of the dual differential magnetoresistive is

proposed. The sensors with respect to the basis magnet about layout electromagnetic interference signal makesthe output signal have a pair of opposite signal at initial position. The two signals of magnetoresistive sensor areadded so as to improve the signal amplitude, and at the same time interference quantity could decrease after thetwo reverse signal summation of output interference magnetic field on the sensor. The feasibility of scheme istested and verified by simulation with the use of Ansoft software to simulate the change of environmentalmagnetic field. The test results show that the disturb is effectively inhibited with the use of differential schemein this paper, the signal amplitude is increased by two times, the accuracy and resolution is improved.Copyright 2013 IFSA.

Keywords: Displacement sensor, Magnetic resistance, Electromagnetic interference, Linear displacement.

1. Introduction

The perception of location and measurement is anindispensible course for linear actuator motioncontrol process [13]. Displacement feedback to thecontrol system is the key factor to achieve the precisecontrol of movement [4, 5]. Therefore it put forwardhigher requirements on the displacement sensor.While as a non-contact magnetoresistive sensor,

because of its unique advantages, small size, high precision, non-contact, long service life, widemeasuring range, it is widely used in variousindustries [6, 7], such as, gear speed [8], compasstechnology [9, 10], permanent-magnet synchronismmotor (PMSM) [11], pneumatic drive [12] andmicrofabricated spin-valve [13]. Magnetic resistance

displacement sensor transforms the variation of themagnetic field angle into linear displacement. It can

be used for angle displacement measurement anddetection of linear displacement [1416]. But whenapplied to complex industrial environment and

because of the interference by external stray magneticfields, especially the electromagnetic interference. Itresults in precision decrease, and the outputinstability [1719]. In the actual detection ofelectromagnetic linear actuator displacement (Asshown in Fig. 1), and at the initial moment ofactuator loading current. The interference of externalmagnetic field causes a big error to the output of thesensor output. So how to eliminate the errorgenerated at this time has become a problem to besolved in the application of magnetoresistive sensor.
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this small range relative to the A and B is consideredas a parallel magnetic field. Suppose a field as shownin Fig. 4, D is magnetic interfering field, and theangle between the direction of magnetic interferingfield and axis is 0-180. In order to facilitate theanalysis, the marked angle between axes on map is90. In the initial position, the included angle

between axis M and electric current I is 45. Whenthere is the presence of a magnetic interfering field,the magnetic field along the axis M direction andmagnetic interfering field D can be combined.

Fig. 3. The structure of differential magnetoresistivedisplacement sensor.

C u r r e

n t I

C u r r e

n t I

Fig. 4. Distribution of magnetic field interferenceand vector combining.

Relative to the sensor A:

A d A d AsH +H =H +H =H

(5)

Relative to the sensor B:

A d A d BS-H +H =-H +H =H

(6)

In the equation above, A

H and A-H are

internal magnetic field vector of sensor, dH

interfere with the magnetic field vector, AsH and

BSH are synthetic magnetic field respectively A andB. Through the synthesis of magnetic field vector, theoriginal angle between axis and current is changed. Itis + on A, while - on B. Thus an outputvoltage comes. By means of Equation 2, 3, 4, thedisturbances- generated voltage is eliminated throughthe summation method. On a special occasion, whenthe included angle between magnetic interfering fieldD and axis M is 0and 180, The strength of themagnetic field along the axis M direction can only

increase or weaken. It can not cause the angle change between the axis M and current I, therefore it has noeffect on the output voltage. But because A and B aresymmetric arrangement, when the included angle

between magnetic interfering field D and axis M is180-360, it is the same with the above analysis.

3. Magnetic Field Simulation

3.1. The Simulation and Test for MagneticField of Magnetic Steel from DifferentPositions

In order to select the appropriate sensor layoutarea, a 3D simulation model of the magnet is set upwith the help of Ansoft software, to simulate thedirection variation of the magnetic field aroundmagnet steel. The distance of simulation magneticcenter in the X axis direction is respectively 6 mm,8 mm and 10 mm, and 8 mm in the Y axis direction.The simulation results are shown in Fig. 5.

As can be seen from Fig. 5, the closer away fromthe magnetic steel position, the more gentle thevariation of the magnetic field angle. It means thatthe output voltage is lower resolution, and in thenearer distance position, the angle of the magneticfield is failure to maintain consistent, because theangle change is directly reflected in the outputvoltage. Through the test to check the sensor outputunder the three positions, the tested motion range is8 mm as shown in Fig. 6(b). As can be seen from Fig.6(b), it can ensure the whole range of linear line ifsensor output at the 10 mm position. With the

position approach of sensor and magnetic steel, the output is not in its linear range. And compared withthe simulation results, it also verified the further

between sensor and magnetic steel, the higher theoutput resolution. The experimental results show thatthe greater voltage changes, the smaller the slope ofthe curve becomes.


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Fig. 5. The angle of magnetic field in motion region.

Table 1. Bias magnet parameters.

Name Parameter

Magnet types Cylindrical shape and axial

magnetizationDimension 5 mm 6 mmMass 0.2 gMagnetic strength 4000-4500 GSMaterial NdFeB45

3.2. The Strength of the Magnetic Fieldat Different Positions

The bias magnetic field required for sensor haslocation requirements between the above magnetsand sensor, but also consideration should be given tothe saturation problem for the bias magnetic fieldstrength of sensor. In the Ansoft software, itsimulates and calculates magnetic field strength inthe whole range of motion and region. According tothe performance of magnetoresistive sensor,magnetic field intensity is required to reach morethan 80 GS. In the premise of magnetic saturation, tryto use small size magnetic steel as far as possible, onone hand small magnetic steel is helpful for the useof sensor placement space limit, on the other handmagnetic steel with small surface volume meanssmall quality, so the control system dynamic qualityreduces, which is more conducive to improve thedynamic performance of actuators. Therefore, basedon considerations of the above two conditions,simulation is made to the change of magnetic fieldstrength in the motion area. The selected magneticsteel specifications are shown in Table 1. The resultsof simulation and test are shown in Fig. 6. With change of the distance between magnet and sensor,magnetic field intensity can decay continuously, butdoes not meet the requirements of the linear region.And if the distance is too far away, it cannot realizethe saturation of the magnetic field strength.Therefore, after combination for the saturability ofmagnetic field intensity and the consistency ofmagnetic field angle change, the magnetic steel isdetermined to be arranged in 10 mm location area.

Fig. 6. The magnetic field strength in motion region andexperimental test.

4. Scheme of the Dual MagnetoresistiveLayout

4.1. Achievement of Differential Scheme

In order to ensure the accuracy of test results,reluctance type of displacement sensor in the actualuse of the process, must have a better anti-disturbance performance. The resistance value ofmagnetoresistive bridge is easily influenced byexternal environmental temperature changes on theone hand, and on the other hand reluctance is easilyinterfered by external stray magnetic fields,

especially when applied to electromagneticinterference, Such as electromagnetic linear actuator,at the moment of the actuator loading initial current,it causes the change of the magnetic field around,thus bring the fluctuations of sensor output.

In the non-contact displacement sensor, anothersensor using differential method is linear variabledifferential transformer (LVDT) whose working

principle is that transform the change of measureddisplacement into the variation of transformer coilinductance by means of electromagnetic induction.The magnetic field generated by the primary coilthrough the core to the secondary coil, the secondary

coil induces voltage according to Lenz theorem; theinduction voltage by two secondary coils hasdifferential motion, then output voltage signal. In


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conditions of the primary coil voltage fluctuationinterference, and after subtraction of voltage throughthe two secondary coil transformers, it is still able tomaintain a constant output.

Use the principle of differential transformer toeliminate the interference or error of measuringsystem. Compensation method of software orhardware can generally be used to eliminate theinfluence of temperature. Differential method canalso be used to weaken test bias caused bytemperature. And as for electromagnetic interference,in addition to the necessary shielding measures, thedifferential method is proposed to implement in this

paper, the specific scheme is shown in Fig. 7.

Fig. 7. Dual magnetoresistance differential scheme.

The designed differential scheme mainly usesdouble reluctance in both sides of the movingmagnetic steel to generate symmetrical arrangement,installed on the upper end of the electromagneticlinear actuator. The output signal after processconducts conversion of A/D, and then is sent todigital signal processing (DSP) control unit, is givento the control system as the feedback signal, and uselaser displacement sensor to do calibration at thesame time. Finally the signal is transmitted to thehost computer to output display throughEthernet port.

4.2. The Output Signal Processing

Take advantage of magnetoresistive sensor canoutput signal of sine and cosine at the same time,then through signal processing circuit, that is additionand subtraction circuit. If the electromagneticinterference is in the same direction, 180 phase shiftis made on the signal of a sensor output, thensubtracts sine signal from another sensor; if in theopposite direction, then pluses the output sine signal.Results are shown in Fig. 8.

As can be seen from Fig. 8, neither addition norsubtraction will change the original sensor linearzone. And after plus or subtraction, the amplitude ofthe output signal has increased nearly 2 times, thus

increases the output signal of the sensor resolution.The output difference of sensor A and B in theFig. 8(a) is because the error of two sensors in the

initial installation, which causes the amplitude ofoutput voltage is not the same, but throughdifferential method, it can also eliminate theinfluence of error.

(a)

(b)

Fig. 8. The processing of output signal.

5. Experiment Analysis

5.1. The Static Test of Single Reluctance

According to the above analysis, the designeddifferential inductance sensor is used to controlsystem of electromagnetic linear actuator. The test

platform of differential magnetoresistive sensor isestablished as shown in Fig. 9. The bias magnet andelectromagnetic linear actuator coil is connectedtogether to have movement in order to verify thelinear region of designed sensor, and to verify themagnetic field simulation. It can not only satisfy themagnetic saturation region, and consistency ofmagnetic field angle variation, but also ensure thatthe output of the sensor is linear in the whole

movement. The first test is sensors work in staticcondition, and let the electromagnetic linear actuatormove under a very small current, while the magnetic


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field around basically keeps unchanged, the output of sensor at this moment is tested and carries on thecontrast with the laser displacement sensor. Resultsare shown in Fig. 10. As can be seen from Fig. 10,the designed sensor can guarantee its linearity of themotion region in static condition. It is further provedthe feasibility of simulation region around the biasmagnetic field in Ansoft software, which providestheoretical support and guidance for the design ofsensor static condition. It is further proved thefeasibility of simulation region around the biasmagnetic field in Ansoft software, which providestheoretical support and guidance for the designof sensor.

Fig. 9. Sensor experimental test platform

Fig. 10. The results of sensors static test.

5.2. Double Reluctance to EliminateMagnetic Interference under DynamicConditions

In the established test platform of a doublereluctance, let the electromagnetic linear actuator stayin high speed linear motion, and the sensor output

signal feedback to control system. In the condition ofactuator loading large current (bigger than 10 A), alarger interference magnetic field exists around the

sensor, especially at the initial moment of actuatormotion, due to a larger current is need to start, somagnetic field interference at this moment is alsostrong. Also during the actuator has reversemovement, a large reverse current is needed to load.Due to the interference signal on the designed dualmagnetoresistive sensor is reverse, through theaforementioned additive method of signal processing,it can eliminate the interference of externally appliedmagnetic field caused by the actuator motion. Resultsare shown in Fig. 11.

Fig. 11. The results of sensors dynamic test.

As can be seen from Fig. 11, in the actuatorsmotion process, the interference is maximum at the

initial time. The interference is maximum in theinitial startup when reverse motion, and after throughdifferential double reluctance, the interference caneffectively eliminate whether in the actuator rise ordecline stage. In order to further verify the anti-

jamming ability of double reluctance to other externalmagnetic field, magnet steel is applied in aroundsensor to simulate the other magnetic field effect ontest results. Results are shown in Fig. 12.

Fig. 12. The results of sensors dynamic test withsuppression of magnetic fields.


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As seen from Figure, after an interferencemagnetic field is externally added, the sensor outputis capable of ensuring the stable operation. Aftercalculation, the designed differential inductancesensor in contrast with a single reluctance, A/D ofdisturbance variable is: sensor A -138-1269 andsensor B -314-(-1227), the added signal is reduced to0-(-10). It can be seen that the differential scheme bydouble reluctance can effectively suppress theinterference, and can meet the precision control oflinear displacement.

6. Conclusions

In this paper, it presents a design scheme based ondifferential reluctance linear displacement sensormagnetoresistive principle, and sets up a 3Dsimulation model of sensor bias magnetic steel, andtests the variation of magnetic field intensity andangle by simulations, and determines that the sensor

layout area is 10 mm position. The designed sensor isto have static and dynamic test, and the test resultsunder static state and laser displacement sensor arecompared to prove its good linearity; the comparisonof dynamic test results and individualmagnetoresistive sensor verifies the differentialscheme can suppress electromagnetic interferencegood, and can satisfy the high precision of lineardisplacement detection.

Acknowledgements

This project is supported by the InnovationProgram of Postgraduates in Jiangsu Province(CXLX11-0234). We would like to thank thesponsors.

References

[1]. P. Dimitrova, S. Andreev, L. Popova, Thin filmintegrated AMR sensor for linear positionmeasurements, Sensors and Actuators A: Physical ,Vol. 147, Issue 2, 2008, pp. 387390.

[2]. H. Kwun, K. Bartels, Magnetostrictive sensortechnology and its applications, Ultrasonics , Vol. 36,

Issue 1, 1998, pp. 171178.[3]. M. Caruso, T. Bratland, C. Smith, R. Schneider, Anew perspective on magnetic field sensing, Sensors ,Vol. 15, Issue 12, 1998, pp. 3447.

[4]. D. J. Mapps, Magnetoresistive sensors, Sensors and Actuators A: Physical , Vol. 59, Issue 13, 1997, pp. 919.

[5]. P. Freitas, R. Ferreira, S. Cardoso, F. Cardoso,Magnetoresistive sensors, Journal of Physics:Condensed Matter , Vol. 19, Issue 16, 2007, pp. 121.

[6]. E. Hristoforou, Magnetostrictive delay lines:engineering theory and sensing applications,

Measurement Science and Technology , Vol. 14,Issue 2, 2003, pp. 1547.

[7]. A. Lopez-Martin, J. Osa, M. Zuza, A. Carlosena,Analysis of a negative impedance converter as atemperature compensator for bridge sensors, IEEETransactions on Instrumentation and Measurement ,Vol. 52, Issue 4, 2003, pp. 10681072.

[8]. S. Butzmann, R. Buchhold, A new differentialmagnetoresistive gear wheel sensor with highsuppression of external magnetic fields [automotiveapplications], in Proceedings of the IEEE Conferenceon Sensors , 24-27 October 2004 , pp. 1619.

[9]. A. Platif, J. Kubik, M. Vopalensky, P. Ripka, PreciseAMR magnetometer for compass, in Proceedings ofthe IEEE Conference on Sensors , 22-24 October2003 , pp. 472476.

[10]. Y. C. Lai, S. S. Jan, F. B. Hsiao, Development of alow-cost attitude and heading reference system usinga three-axis rotating platform, Sensors , Vol. 10,Issue 4, 2010, pp. 24722491.

[11]. R. Ramakrishnan, A. Gebregergis, M. Islam,T. Sebastian, Effect of position sensor error on the

performance of PMSM drives for low torque ripple

applications, in Proceedings of the InternationalConference on Electric Machines Drives Conference(IEMDC) , Chicago, IL, 12-15 May 2013,

pp. 11661173.[12]. T. Reininger, F. Welker, M. von Zeppelin, Sensors in

position control applications for industrialautomation, Sensors and Actuators A: Physical ,Vol. 129, Issue 12, 2006, pp. 270274.

[13]. J. Sanchez Moreno, D. Ramrez Muoz, S. Cardoso,S. Casans Berga, A. E. Navarro Antn, P. J. Peixeirode Freitas, A non-invasive thermal driftcompensation technique applied to a spinvalvemagnetoresistive current sensor, Sensors , Vol. 11,Issue 3, 2011, pp. 24472458.

[14]. F. Seco, J. M. Martn, A. R. Jimnez, L. Caldern, Ahigh accuracy magnetostrictive linear position sensor.Sensors and Actuators A: Physical , Vol. 123, 2005,

pp. 216223.[15]. Y. Zhang, W. Liu, J. Yang, C. Lv, H. Zhao, Design

of compensation coils for EMI suppression inmagnetostrictive linear position sensors, Sensors ,Vol. 12, Issue 5, 2012, pp. 63956403.

[16]. C. Connolly, Technological improvements in positionsensing, Sensor Review , Vol. 27, Issue 1, 2007,

pp. 1723.[17]. S. Arana, N. Arana, F. Gracia, E. Castano, High

sensitivity linear position sensor developed usinggranular AgCCo giant magnetoresistances, Sensorsand Actuators A: Physical , Vol. 123, 2005,

pp. 116121.[18]. S. M. Quintero, A. Braga, H. I. Weber, A. C. Bruno,

J. F. Arajo, A magnetostrictive composite fiber Bragggrating sensor, Sensors , Vol. 10, Issue 9, 2010,

pp. 81198128.[19]. P. Prochazka, F. Vanek, New methods of non-contact

sensing of blade vibrations and deflections inturbomachinery, in Proceedings of the Conference on

Instrumentation and Measurement TechnologyConference (I2MTC) , Minneapolis, MN 6-9 May2013, pp. 339344.

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(2)the Research of a Differential Magnetoresistive Linear Displacement Sensor Measurement System