Giant magnetoresistance based galvanically isolated voltage and current measurement Fei Xie, Roland Weiss, and Robert Weigel, Fellow, IEEE Abstract—Nowadays, direct current (DC) is much more widely used than a decade ago. Not only in the well-known high voltage direct current (HVDC) transmission but also as DC powered marine system, DC data center and DC fast charging system for electric vehicles. However, galvanically isolated DC voltage or current sensors are, in terms of accuracy, noise level or dimensions, still far behind the AC measurement devices in low and medium voltage area. In this paper a giant magnetoresistive (GMR) based sensor system for galvanic isolated DC current and voltage measurements is introduced. The system consists mainly of a special coil arrangement with low inductance for voltage measurement, a special U-turn for current measurement and AD converters with a low-cost FPGA for signal conditioning. The sensor system showed an outstanding measurement accuracy of 0.3% for voltage values up to ±550V and 0.2% for current values up to ±100A in a temperature range from -30°C to 90°C. This completely new approach for the galvanically isolated measurement of electric voltage and current avoids the use of bulky magnetic components and expensive power consuming analog electronics, which allows to integrate it in the current AC measure unit for power monitoring. Index Terms—Giant magnetoresistance (GMR); current sensor; voltage sensor; power measurement; signal conditioning; open loop.
I. INTRODUCTION
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HIS paper is based on [1], presented at the 2014 IEEE Workshop on Applied Measurements for Power Systems. It extends the work by using novel GMR sensors and further improved mathematical methods for galvanically isolated current measurement. The new designed current sensors show an improved accuracy, and together with the in [1] introduced voltage sensor, an important step toward a galvanically isolated DC power sensor system has been made. The main methods used so far for a galvanically isolated current measurement are: current transformers, Rogowski coils, Hall effect transducers, fluxgate transducers and This work is partly supported by ENIAC project No. 296108-2. F. Xie is currently pursuing the Ph.D. at the Friedrich-AlexanderUniversity Erlangen Nuremberg in cooperation with Siemens Corporate Technology, 91058 Erlangen, Germany (e-mail:
[email protected]). R. Weiss is with Sensor System Integration, Corporate Technology, Siemens AG, 91058 Erlangen Germany (e-mail:
[email protected]). R. Weigel is with the Insititute for Technical Electronic, FriedrichAlexander-University Erlangen Nuremberg, 91058 Erlangen, Germany (email:
[email protected]).
magnetoresistance-effect transducers [2]-[6]. Nevertheless, current transformers and Rogowski coils are not suitable for DC measurement; Hall effect transducers cannot keep their accuracy in a wide temperature range [6], and good fluxgate transducers are always very expensive due to the complex analog techniques. Galvanically isolated voltage measurement is usually based on current measurement. With a convenient resistive pre-resistor, the voltage signal is converted to a small current signal. The weak magnetism of the small current is handled by using a high magnetic flux concentrator with a high permeability and a huge number of windings, leading to a high inductance and a weak frequency response. Nevertheless, due to the modest sensitivity of commonly used Hall effect sensors, a voltage measurement using this technology can hardly reach an accuracy level below 1% at room temperature, and the accuracy would be much worse over a wide temperature range [7]. If, instead of the Hall effect sensor, a fluxgate sensor or isolation amplifier based sensor is used, a better accuracy over temperature from -40°C to 85°C (0.6% or 1%) can be achieved [8] [9]. However, similar to current transducers, due to the more complex analog techniques needed, these alternatives are very expensive. We present here a development of a low cost, high accuracy, GMR based openloop voltage and current sensor system combining accuracy with a compact and cost efficient system design. A compact U-shaped conductor with better performance is designed for current measurement. For voltage measurement a magnet intensifier based on a flat coil system is developed so that currents in the mA range can be detected. The nonlinearity and temperature influence of the GMR sensors can be corrected through B-Spline modeling introduced by Bluemm et al [10]. For the experiments shown here, sophisticated GMR spin valve sensors were used. Here the sensors are demonstrated for voltage and current measurements within a range of -550V to 550V and -100A to 100A, which is suitable for isolated power monitoring, for example, in commercial buildings with direct current electric distribution system [11][12] or in solar power systems. The proposed sensor has the potential to be applied in higher voltage and current sensing applications up to 1500V and 150A. The measure principle is described in Section II. Section III describes the experimental setup. The absolute error and relative error are measured and shown in Section IV. Section V concludes the paper and gives an
outlook. II. MEASURE PRINCIPLE A. Current sensor structure To exploit the advantages of the high frequency (up to several GHz) and high sensitivity (20mV/(V*mT)) capabilities of GMR sensors, an open loop configuration without flux concentrator is used for current measurement normally (Fig. 1). According to Ampère’s circuital law, the primary current causes a magnetic field H around the conductor, which it passes through. This conductor is attached to the GMR element with an isolation layer made of PCB material inbetween. As shown in Fig. 1 the GMR sensors are arranged in a dedicated Wheatstone configuration on a U-shaped conductor to enable differential measurement. Basically a change of the current in the U-shaped conductor alters the magnetic field, which in turn results in a rotating magnetization of the free layer of the spin valve GMR element. The resulting change in the resistance value will alter the output voltage of the Wheatstone bridge UBR. So the output voltage of the Wheatstone bridge UBR represents the current strength.
RGMR 1 = RGMR + R∆
U VCC
Fig. 2. Manufactured busbar device with “U-turn” outline, Cu 400 µm thick, with arrows indicating the direction of current flux.
RGMR 2 = RGMR - R∆
UBR
RGMR 4 = RGMR+ R∆
RGMR 3 = RGMR - R∆
current direction Fig. 1. Current measurment based on U-shaped conductor and Wheatstone bridge sensor structure.
A realization of the U-shaped conductor used for the experimental results is shown in Fig.2. Fig.3 gives a result of a finite element modeling (FEM) for current density inside the U-shaped conductor as well as the magnetic field vector in the preferred plane of the sensors, above the conductor plane. It is important to note that for a high quality current sensor the magnetic field should be perpendicular to the GMR-Sensor elements, which are represented by the yellow strips in Fig. 3.
Fig. 3. Simulation of optimized “U-turn” busbar (inner vertical length 12.5mm, width 2.35mm for current lines in antiparallel state); part a) showing the current density (max. J=2e+06 A/m²) in the bar; part b) with magnetic field vectors (max. H=120 A/m) in a plane 0.75mm above the Cu plate; the yellow rectangles represent the position of the GMR sensor elements, both using ANSYS “Maxwell” software for electromagnetic field simulation.
B. Voltage sensor structure As mentioned the voltage measurement is in fact a current measurement with the help of a pre-resistor, in this paper the 20kΩ pre-resistor is used (Fig. 4a). The current flows through a pre-resistor and then goes in the multi-layer flat coil system. The coil structure configuration leads the current to flow in opposite directions on each side of the sensor and therefore generates opposite fields there (Fig. 4b). The four GMR elements of the sensor are composed in a full-Wheatstone bridge configuration (Fig. 4c).
system indicating the enhanced magnetic field at the position of the GMR-Sensor. Each coil consists of 16 layers with 30 windings. Magnetic fields up to 1860A/m are available in the central sensor region with 40 mA current flow.
Fig. 4. The galvanic isolated voltage measurement over a pre-resistor. a) Schematic with a pre-resistor. b) Coil in rectangle shaped section. c) MR sensor in a full Wheatstone bridge configuration.
The two flat coils are designed with 16 layers and are connected in a series-opposing connection (Fig. 5). This configuration is enhancing the magnet field along the sensitive axis (x axis) of the GMR sensor, which allows a very small current to generate strong gradient field at the sensor position. Similar to the GMR current sensor, when current flows in the coil system, the resistors at one side decrease their resistance value, whereas the resistors at the other side increase it. Thereby an electric voltage UBR (output voltage of the Wheatstone bridge) gives a measure for the electrical voltage by remaining galvanically isolated.
C. Sensor electronic Printed circuit boards (PCB) are designed for current (Fig. 7) and voltage (Fig. 8) sensors to convert the analog sensor signal and the temperature signal into a digital form. The converted digital signals can be transferred via the connection ports to a FPGA for further processing. For current measurement the U-shaped conductor is fixed by soldering to the current input and output connectors. The current sensor is placed right over the U-turn, behind the cables (Fig. 7). The digital signal and the supply voltage for the PCB are combined in the connector in the middle. The voltage sensor is placed between the middle of two flat coils. The coils are stacked on both sides of the sensor board and are fixed by four plastic gearheads. For installation in the rack mounted supply and control unit, the sensor board has been implemented in a housing for “top-hat” mounting as shown in Fig. 8.
Fig. 5. Measuring principle with two coils.
Fig. 7. Views of a functional prototyp of a GMR current sensor in a housing, (top), and complete housing with digital output(down).
Fig. 6. FEM 2D simulation of a two-sided coil model with 16 layers/30 windings each; and magnetic field, arising from a 0.04A current per winding flowing through both legs of the coil system.
The FEM simulation in Fig. 6 represents a double-sided coil
Fig. 8. Views of a funtional prototyp of a GMR voltage sensor in a “top-hat” housing, (left), and complete housing with digital output adapted at a “tophat” rail (right).
The output amplitude of the GMR full bridge UBR is slightly dependent on the temperature. However, with a temperature change over 140°C (from -40°C to 100°C) during the
calibration, this slight dependence cannot be neglected. Furthermore the resistance of the coil system for voltage measurement is due to its fine structure and huge number of windings, nearly 800Ω at room temperature. In the aforementioned temperature range the resistance value of the coil system can vary up to 50%, which equate to 2% of the total resistance. Since the current is inversely proportional to the resistance, the measuring error of this small current and later the voltage can be strongly affected. In conclusion, it is necessary to measure the temperature of the GMR sensor system as well as the output voltage of the Wheatstone bridge (UBR) in order to correct the influence of the ambient temperature on the obtained voltage value. This can be achieved without using an extra temperature sensor. The changes of the GMR resistors are used to sense the temperature (Fig. 9). Due to the dependence of the GMR resistor value on temperature, a suitable SMD shunt is used to detect the current change, which is inversely proportional to the RGMR. The shunt value is very small by comparison to the resistance of the Wheatstone bridge; therefore it doesn’t affect the whole system. The temperature signal and the output signal UBR are sampled by two analog digital converters using a synchronized clock signal from the FPGA.
board provide 72 I/O pins, 5V power pins, two 3.3V power pins and four ground pins [13], which allow a multi channel measurement of voltage and current. Oven Vötsch U-turn VT7004 Sensor
LEM IT600
Labview DAQ NI PCI-6052E
Sensor
Pre-resistor
Labview
RGMR 2
~5V UGMR ~
UBR
A/D
FPGA board
Oven Vötsch Coil VT7004 System
5V RGMR 1
Stabizet D2425
Agilent N5772A
Keithley 2000 DAQ NI PCI-6052E
FPGA board
U0 = 5 V
RGMR 3
RGMR 4
Fig. 10. Block diagram of the measurement station for the GMR current sensor(top) and voltage sensor (down). U S
