Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 900 – 903
30th Eurosensors Conference, EUROSENSORS 2016
AMR yokeless current sensor with improved accuracy Pavel Mlejnek*, Pavel Ripka Dept of Measurement, Faculty of Electrical Engineering, Czech Technical University in Prague, Technicka 2, Prague 6, 166 27, Czech Republic
Abstract We analyse the influence of real parameters of KMZ51 magnetic sensors on the accuracy and characteristic of AMR current measurement sensor. The device consists of a circular array of eight magnetic field sensors. The differences in sensitivity of individual AMRs, as well as bridge resistance, the resistance of compensation coil and compensation coil factor are discussed. Finally, we present the method how to compensate the mentioned errors by reading individual sensor outputs. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: AMR; current measurement; error analysis; KMZ51
1. Introduction Yokeless galvanic insulated electric current sensors are needed for applications in which space and weight are strictly limited [1]. In our previous paper [2] we presented AMR AC/DC current measurement device consisting of eight KMZ51 [3] magnetic field sensors arranged around the measurement hole. We have shown serious errors caused by the spread of the sensor parameters. In this paper we describe the new circular version of the device (Fig. 1 (a)) together with the detailed analysis of the influence of real parameters of AMRs on the accuracy of the final device. Estimation of the current from the magnetic field measured by single AMR is problematic due to the strong dependence on the distance a from the conductor (|1/a). In the case of the circular sensor array (Fig. 1), this dependence is theoretically strongly suppressed as shown in [2]. The theoretical residual error strongly depends on the number of sensors in the array: while for 2 sensors the maximum error was 18%, for 16 sensors it drops below 1 ppm.
* Corresponding author. Tel.: +42-022-435-2189; fax: +42-023-333-9929. E-mail address:
[email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.301
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Pavel Mlejnek and Pavel Ripka / Procedia Engineering 168 (2016) 900 – 903
In our case, we chosen to use eight sensors as a compromise between the complexity of the device and the suppression of the off-center error. The sensors are placed around the 10 mm hole on the circle with the radius R 12.7 mm. Numbers of the sensors and their orientation against the coordinate system are illustrated in Fig. 1 (b).
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D 0°
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Fig. 1. (a) AMR current sensor; (b) definition of direction and sensor numbering.
In the real case the spread of the sensor parameters is significant. Main parameters that can affect the error of current estimation are sensitivity S and its field dependence, bridge resistance Rbridge, compensation coil resistance Rcomp and compensation coil field factor Acomp. We observed almost 20 % spread in these parameters even within the sensors from the same batch. In our device, the voltages from all sensors are summed together and the compensation current (that creates compensation magnetic field inside each sensor) is set so that the sum of voltages is zero. The existing version of our AMR yokeless current sensor consists of 8 synchronously flipped AMR sensors. All AMR bridges are connected in series and supplied by constant current so the differences in the bridge resistances directly cause differences in sensitivity. Each floating bridge output is amplified by instrumentation amplifier and the outputs of these amplifiers are summed and demodulated before the AD conversion. On-board microprocessor controls the flipping and data communication. The sensors are compensated in a common feedback loop: the compensation coils are connected in series so in our case their resistance has no effect. Nomenclature AMR a rH r R S Rbridge Rcomp Acomp
anisotropic magnetoresistor distance between the current conductor and AMR sensor radius of the hole of the sensor ( 5 mm) distance between the current conductor and centre of the current sensor radius of the circle of sensors sensitivity of the AMR sensor (mVV1/Am1) bridge resistance (Ω) compensation coil resistance (Ω) compensation coil field factor (Am1/mA)
2. Measurement results 2.1. Measurement of error due to off-centered conductor In the case of measured current is in the centre of the sensor array the basic error of the device is very small, as illustrated in Fig. 2 (a). However, the error caused by off-centre position of the measured conductor can reach 0.5 %
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as shown in Fig. 2 (b), which is almost one order of magnitude larger than theoretical error calculated for ideal sensors. The measured conductor was placed at the edge of the inner hole of the sensor with radius rH 5 mm and moved around the hole from 0° to 360°. The problem is caused by the fact that due to shared compensation current for all sensors, the measured magnetic field (generated by the current) is not fully compensated by the compensation field in a particular sensor. Therefore all sensors work at the non-zero magnetic field and therefore the different sensitivities and compensation coil factors affect the performance of the device.
Fig. 2. (a) Nonlinearity and hysteresis of a new current sensor; (b) measured error due to the non-centered conductor as a function of the conductor angular position. The conductor displacement was 4 mm
2.2. Individual reading of the AMR output Some information about the position of the conductor in the hole can be extracted from the individual reading of the AMR outputs. Let us assume that all the AMR sensors are perfect and there are no differences among them. There is a non-zero voltage at the outputs of particular sensors caused only by the off-center current. It is possible to estimate the position of the conductor from the readings of the AMRs. In the case of real parameters of AMR sensors, there are non-zero outputs even with the centred conductor. Fig. 3 (a) shows the single outputs (“centred offsets”) of the sensors when the current was situated in the centre of the hole. Note, that the sum of the voltages is equal to zero.
Fig. 3. The residual output voltage of the AMR sensors (a) with centred measured current; (b) off-center measured current.
Similar measurements were done for off-center conductor when the conductor moves around the edge of the hole in sixteen positions. The “centred offsets” were subtracted from these measurements. The residual output voltages are shown in Fig. 3 (b). Only three positions of the measured current are shown for better clarity. The conductor was
Pavel Mlejnek and Pavel Ripka / Procedia Engineering 168 (2016) 900 – 903
placed at the edge of the hole (rH 5 mm) in direction 67.5°, 202.5° and 315°. The residual voltages clearly define the position of the conductor. The patterns in the polar chart are similar and symmetrical if the conductor is in the direction of AMR sensor or between two of them. In this case, it is easy to estimate the position of the measured current and provide some correction of the error according to the Fig. 2 (b). Fig. 4 presents measured residual output voltages for all sixteen positions of the current. The results are depicted in polar chart as well as in scatter chart where the symmetry and similarity are evident.
Fig. 4. The residual output voltage of the AMR sensors (a) scatter chart; (b) polar chart.
Above mentioned method for estimation of the position of measured current assumes that the measured current is at the edge of the hole. The voltage outputs of the AMR sensors depend also on the distance of the current from the centre of the current sensor. The estimation of the distance and as well for the conductor diameter can be done by comparison of residual voltages and maximal residual voltages for actual measured current obtained from the calibration process. 4. Summary The goal of this paper was to introduce the possibility of correction of off-centered measurement current of our AMR current sensor. Error due to the off-centered current can be rather high in comparison to the measurement error with the centred conductor. The error can be suppressed by individual measurement of the residual voltage of all AMR sensors. The sum of these voltages have to be zero due to PI regulator that provides common compensation current for all AMR sensors but individual sensors do not work at zero magnetic field. The off-centered current causes different biasing of the sensors and the position of the measured conductor can be estimated from it (see Fig. 3 (b)). According to the position proper error correction can be realised. Next step will be to prepare and test the calibration process for better estimation of current position. This will lead to more precise suppression of the measurement error.
References [1] P. Ripka, Electric current sensors: a review, Meas. Science and Technology 21 (2010) pp.1-23 [2] P. Mlejnek, M. Vopalensky, P. Ripka (2008): AMR current measurement device. Sensors and Actuators A: Physical 141/2, 649-653. [3] Philips (2000): KMZ51 Magnetic field sensor (datasheet). http://pdf.datasheetcatalog.com/datasheet/philips/KMZ51_3.pdf (online)
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