Sensors 2012, 12, 15520-15541; doi:10.3390/s121115520 OPEN ACCESS sensors ISSN 1424-8220 www.mdpi.com/journal/sensors Article A Current Sensor Based on the Giant Magnetoresistance Effect: Design and Potential Smart Grid Applications Yong Ouyang 1, Jinliang He 1,*, Jun Hu 1 and Shan X. Wang 1,2 1 State Key Lab of Power Systems, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China; E-Mails: [email protected] (Y.O.Y.); [email protected] (J.H.) 2 Center for Magnetic Nanotechnology, Stanford University, 450 Serra Mall, Stanford, CA 94305, USA; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +86-10-6278-8811; Fax: +86-10-6278-4709. Received: 27 September 2012; in revised form: 29 October 2012 / Accepted: 30 October 2012 / Published: 9 November 2012 Abstract: Advanced sensing and measurement techniques are key technologies to realize a smart grid. The giant magnetoresistance (GMR) effect has revolutionized the fields of data storage and magnetic measurement. In this work, a design of a GMR current sensor based on a commercial analog GMR chip for applications in a smart grid is presented and discussed. Static, dynamic and thermal properties of the sensor were characterized. The characterizations showed that in the operation range from 0 to ±5 A, the sensor had a sensitivity of 28 mV·A−1, linearity of 99.97%, maximum deviation of 2.717%, frequency response of −1.5 dB at 10 kHz current measurement, and maximum change of the amplitude response of 0.0335%·°C−1 with thermal compensation. In the distributed real-time measurement and monitoring of a smart grid system, the GMR current sensor shows excellent performance and is cost effective, making it suitable for applications such as steady-state and transient-state monitoring. With the advantages of having a high sensitivity, high linearity, small volume, low cost, and simple structure, the GMR current sensor is promising for the measurement and monitoring of smart grids. Keywords: current sensing; giant magnetoresistance; sensor design; smart grid Sensors 2012, 12 15521 1. Introduction Smart grids are the latest trend in the World’s power system. They represent an evolution towards a more optimized and sustainable energy system using the implementation of information technology [1–3]. Advanced sensing and measurement techniques are key technologies in a smart grid to successfully provide accurate information for real-time monitoring and control of the entire distributed power system. The giant magnetoresistance (GMR) effect was discovered in some ferromagnetic metallic materials. The resistance of these materials decreases greatly under the influence of external magnetic field. Grünberg and Fert separately discovered this phenomenon in 1988 [4–6], which revolutionized the field of data storage and magnetic measurement. For their significant contributions, they were awarded the 2007 Nobel Prize for Physics [7]. Since IBM Corporation developed the first commercial read head based on the GMR effect in 1997, the area density of the devices has grown hundreds of times [8,9]. In the field of magnetic measurement, GMR sensors are widely utilized for position and angular sensing, current sensing and biological sensing. A number of integrated detectors containing GMR elements with wide bandwidth have been developed for motor drives, power electronic modules, and regulator modules [9–13]. A GMR sensor for highly-sensitive stress measurement has been introduced [14]; additionally, an eddy current testing probe based on the spin-valve GMR effect has been developed for inspecting printed circuit boards [15]. In the field of biological sensing, GMR sensors have been applied in crop production for sensing growth conditions, fertilization, and irrigation; GMR sensors are also useful for plant protection, harvesting, and fleet controls [16]. Sensors based on the GMR effect have also been used in protein and DNA assays, to perform molecular diagnostics of diseases [17–20]. In order to realize linear sensing, the sensed field range, linearity, and sensitivity are three important parameters. For example, the sensitivity is 25 mV·A−1, and the sensed field range is from 0 to ±10 mA in certain linear spin-valve GMR sensors reported in the literature [21–24]. In this work, we designed the structure of a GMR sensor for current sensing that can be implemented in a smart grid. In addition, the characteristics of the GMR sensor, such as its static characteristics, dynamic characteristics and thermal characteristics were tested. 2. Principle of Operation Current flowing through a metal wire creates a magnetic field around the wire. The relationship between the current and magnetic field is described by the Biot-Savart Law. The principle of operation of the GMR current sensor is shown in Figure 1. The current flowing through the metal wire can be measured from the output of the GMR current sensor, when the GMR current sensor is placed close to the wire. The output of the GMR current sensor can be generally described by: VF (, If ,TV , , ,, r , B ) GMR S D (1) where V is the output voltage of the GMR current sensor, I and f are the magnitude and frequency of the current passing through the wire, respectively, T is the environmental temperature, VS is the power supply voltage of the sensor, µ is the relative permeability around the sensor, r is the distance from the Sensors 2012, 12 15522 wire to the sensor, θ is the angle between the magnetic field direction and the axis of sensitivity of the GMR chip, and BD is the surrounding parasitic magnetic field. Figure 1. Principle of operation of the GMR current sensor. I is the current passing through the wire, and the direction is from left to right. B is the magnetic field generated by the current I, and the direction is rotating around the wire. r is the distance from the wire to the sensor, and θ is the angle between the magnetic field direction and the axis of sensitivity of the GMR chip. 3. Sensor Design The sensor system contains three main subsystems: the sensing head, the signal processing subsystem and the power supply subsystem. Each subsystem affects the overall performance of the sensor. The block diagram of the sensor system is shown in Figure 2. Figure 2. Block diagram of sensor system. The whole sensor system contains three main subsystems: The function of the sensing head is to convert the current signal measured to the voltage signal. The function of the signal processing subsystem is to process the voltage signal from the sensing head and calculate the current measured. The function of the power supply subsystem is to provide the power source for the whole sensor system. 3.1. Sensing Head The sensing head, which is shown in Figure 3, is the kernel of the whole measuring system, and its function is to convert the current signal measured into the voltage signal that can be measured easily. Sensors 2012, 12 15523 A wire with the current line to be measured passes through a magnetic ring. A DC coil is uniformly wrapped around the magnetic ring. A GMR chip is placed in the gap of the magnetic ring, and a thermal compensation unit is used to compensate the temperature drift of the GMR chip. The output of the GMR chip is subsequently processed by the signal processing subsystem. Figure 3. Schematic diagram of sensing head. In order to convert the magnetic signal to a voltage signal that can be easily measured, a Wheatstone bridge is used in the design of the GMR chip (Figure 4). In the GMR chip, four resistive elements are arranged as follows: two are active sensing elements and two are shielded elements. Due to limitations in material processing and manufacturing technology, the four resistive elements cannot be manufactured identically, so their magnitudes are slightly different when the magnetic field is not present. Figure 4. Wheatstone bridge of GMR chip. The two shield resistors are shielded elements, and the two GMR resistors are active elements. V+ and V− are two terminals of the power supply of the GMR chip. OUT+ and OUT− are the positive and negative output of the GMR chip. Sensors 2012, 12 15524 The output of the GMR chip is differential, and can be described as follows: R Vout (V V ) V 2RR RR Vout (V V ) V (2) 2RR RR Vout Vout (V V ) (V V ) 22R RR where Vout+ and Vout− are the voltage outputs of the GMR chip, V+ and V− are the power supply voltages of the GMR chip, and R is the intrinsic resistance of the four elements when no magnetic field is applied. ∆R is the change of the resistance of the two active elements under a magnetic field, and ∆R is linearly proportional to the magnetic field to be measured. The MR ratio (∆R/R) is about 7%. Because ∆R is much smaller than R, the output of the GMR chip can be treated as linearly proportional to ∆R. The magnetic ring is made of a soft ferromagnetic material with a large relative permeability, and its structure is shown in Figure 5. It can make the magnetic field to remain stable and nearly unaffected by position fluctuation of the current line. The magnetic field in the gap of the magnetic ring can be described as: I I B 00 2 rd d d (3) r where B is the flux density, μ0 is the permeability of vacuum, μr is the relative permeability of the magnetic ring, r is the average radius of the magnetic ring, d is the length of the gap, and I is the measured current. Figure 5. The structure of the magnetic ring.