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A new compact 2D planar fluxgate sensor with amorphous metal core
Sensors and Actuators 81 Ž2000. 180–183 www.elsevier.nlrlocatersna
A new compact 2D planar fluxgate sensor with amorphous metal core Pavel Kejık ´ a, Laurent Chiesi b
b,)
, Balazs Janossy b, Rade S. Popovic
b
a Czech Technical UniÕersity of Prague, Department of Measurement, 166 27 Prague, Czech Republic Swiss Federal Institute of Technology Lausanne, Department of Microengineering, CH-1015 Lausanne, Switzerland
Abstract A new compact 2D planar fluxgate sensor using a ferromagnetic amorphous metal core is described. The fluxgate sensor consists of two orthogonal planar metallic coils and a ring shaped amorphous magnetic ribbon mounted on a PCB substrate. Each planar coil is used alternatively as excitation or as pick-up coil. An electronic interface drives alternatively the two coils and performs a feedback on each sensing coil in order to compensate for the two measured magnetic field components. The sensing element has a magnetic sensitivity of 55 VrmT at an excitation frequency of 8.4 kHz with a 160 mA peak driving current. Used as an electronic compass, the reached precision angle is better than 18. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Compass; Planar coils; Fluxgate sensor; Amorphous ferromagnetic ribbons
1. Introduction The known integrated planar w1x or 3D w2x fluxgate sensors are parallel fluxgate sensors w3,4x. Therefore, the excitation and pick-up coils are in the same axis and the combination of two ferromagnetic cores is necessary to cancel the influence of the excitation field on the output signal. Furthermore these integrated fluxgate sensors measure only in one direction. To obtain a 2D fluxgate sensor with such a configuration, it is necessary to use four ferromagnetic cores. The total chip surface and the overall resistance of the excitation coils is then significative. In this paper, we present a more compact 2D planar fluxgate sensor. This sensor works as an orthogonal fluxgate sensor w3,4x with one toroidal ferromagnetic core and only two planar coils. Therefore, the sensor surface and the resistance of the coils are much reduced which is interesting for integration. Furthermore, a smart electronic interface is described which is capable of driving simultaneously the coils and compensate for the external field.
ferromagnetic core, the whole mounted on a PCB substrate. The coil has a density of 50 turnsrcm. The core is obtained from a Vitrovac 6025 w ribbon w5x by photolithography and wet chemical etching. The principle of the detection is based on alternating measurements in two directions. In each case the excitation coil is the one orthogonal to the measurement axis while
2. Sensor and electronics The fluxgate sensor on Fig. 1 consists of two similar orthogonal planar coils and a ring shaped amorphous )
Corresponding author. Tel.: q41-21-693-6614; Fax: q41-21-6936670; E-mail: [email protected] 0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 0 8 3 - 7
Fig. 1. View of the sensor.
P. Kejık ´ et al.r Sensors and Actuators 81 (2000) 180–183
181
Fig. 2. Block diagram of the 2D planar fluxgate sensor.
the sensing coil is the one which is parallel. Therefore, the output voltage on the pick-up coil is free from the excitation field and contains only the useful signal at the frequency twice that of the excitation field. A compensation feedback DC current is applied simultaneously in both axes to avoid the influence of the orthogonal field component on the measurement axis. A block diagram of the 2D magnetometer electronics is shown in Fig. 2. The two switches allow the commutation between the excitation and sensing mode for the two coils. In Fig. 2, the switches close the coil of x channel in sensing mode and the coil of y channel in excitation mode. The generator G, an 8-Bit microcontroller PIC16C84, provides the driving frequency fr10 s 840 Hz. A pulsed current with a slew rate of 20 mArms at a frequency f s 8.4 kHz is supplied to the excitation coil by the current amplifier. This current of 160 mA P generates an AC magnetic field Ž500 mTP . which periodically saturates the amorphous ring core. Fig. 3 shows the waveforms of the excitation current on both axis sensed by resistors R x and R y . This waveform has a pulse time to period ratio of 1:3 which reduces the power consumption. For a 160 mA P current, the RMS value is only 71 mA RMS . During the positive and negative slew rates, induced voltage peaks appear across the pick-up coil. The output voltage of the
Fig. 4. Full-range magnetic field response of the sensor.
pick-up coil contains only even harmonics proportional to the measured magnetic field. The output signal is extracted by the synchronous detector PSD driven by the doubled excitation frequency 2 f from the generator G. The PSD output is connected to the current amplifier through an integrator. The feedback loop compensates the external field component in the measured direction and is updated only during sensing cycles. The sensor output is thus the feedback current sensed by the resistors R x and R y . For DC field precision is achieved by using the integrator in the feedback loop.
3. Experimental results and discussion The prototype 2D compact planar sensor has been characterized in open and closed-loop configurations. The open-loop characterization indicates a strong dependence of the pick-up output voltage upon a cross magnetic field. The response of the sensor is non-linear with a non-linearity error close to 30% of FS Ž"60 mT..
Fig. 3. Excitation current waveforms of x and y channels sensed by resistors Ž R x s R y s 9 V ..
182
P. Kejık ´ et al.r Sensors and Actuators 81 (2000) 180–183 Table 1 Comparison of sensor errors with 18 error precision for a measured magnetic field of 30 mT and three different projection angles a x Ž8. B x ŽmT. Hysteresis error ŽmT. Non-linearity error ŽmT. Cross sensitivity error ŽmT. Total of sensor error ŽmT. Error ŽmT. corresponds to 18 error
0 30.0 0.072 0.066 0 0.138 0.550
45 21.2 0.072 0.042 0.114 0.228 0.380
90 0 0.072 0 0.240 0.312 0.550
Finally the measured sensor detectivity is 0.1 mT for a dc magnetic field. Fig. 5. Output sensor non-linearity and hysteresis errors over the full-scale.
In the closed-loop configuration, the sensor output shows much better characteristics. Fig. 4 illustrates the full-scale magnetic field response of the sensor. The output sensitivity is 55 VrmT Ž R x s R y s 9 V . in the "60 mT field range. The range is deliberately limited in order to use our sensor as compass device. However, even if the ferromagnetic core is saturated only from 200 mT, the feedback compensation allows an extending of the measurement range up to 500 mT with a well adapted electronic circuit. Fig. 5 gives the total sensor output errors vs. the measured magnetic field. These errors are hysteresis error due to the remanent field and non-linearity error. In order to measure the effect of the remanent field on the sensor output, the ferromagnetic core has been driven into deep saturation by a magnetic field of "4.4 mT Ž22 times higher than the field where the saturation begins.. The measured hysteresis and non-linearity errors are respectively 0.12% and 0.23% of the full-scale. Fig. 6 illustrates the effect of a cross magnetic field on the measurement axis. The cross sensitivity full-scale error is obtained by sweeping a field orthogonal to a measured constant field. The cross sensitivity error varies from 0 to 1% of full-scale. Thus the total output sensor error varies from 0.12% to 1.35% of full-scale between 0 and "60 mT.
4. Compass application Our sensor is well adapted for measuring in plane components of the Earth’s magnetic field. It can also determine the direction of the field, thus it is potentially useful as an electronic compass. Table 1 gives the level of precision of the compass for a 30 mT horizontal field and three different sensor positions. For such a magnetic field, the precision on the angle is better than 18.
5. Conclusion We have presented a new design of a 2D fluxgate sensor in a planar technology based on a ferromagnetic amorphous ring core. The sensor has the following advantageous features: compactness and simplicity due to the reduced number of coils and magnetic cores; high sensitivity due to feedback which reduces hysteresis and non-linearity; and optimized current consumption due to pulsed current excitation and alternative measurement. The excellent performance of the sensor with non-linearity and cross sensitivity errors respectively 0.12% and 1.35% of full scale in a working range of "60 mT makes this device an excellent compass with precision better than 18.
Acknowledgements This work has been supported by the Swiss confederation and MINAST project ‘High-detectivity magnetic sensor microsystem’ 3.11.
References
Fig. 6. Cross sensitivity error.
w1x S. Choi, S. Kawahito, A planar fluxgate magnetic sensor for on-chip integration, Sensors and Materials 9 Ž1997. 241–252. w2x S. Ulbricht, W. Budde, R. Gottfried, A monolithically integrated two-axis magnetic field sensor system, Presented at ESSCIRC’96— Proceedings of the 22nd European Solid-State Circuits Conference, Neuchatel, ˆ 1996.
P. Kejık ´ et al.r Sensors and Actuators 81 (2000) 180–183 w3x F. Primdahl, The fluxgate magnetometer, J. Phys. E: Sci. Instr. 12 Ž1979. 241–253. w4x P. Ripka, Review of fluxgate sensors, Sensors and Actuators A 33 Ž1992. 129–141. w5x Vacuumschmelze, Amorphous metals VITROVAC.
Biographies PaÕel Kejık in the Czech Republic in 1971. He ´ was born in Prostejov, ˇ received an Ing. degree in 1994 at the Czech Technical University of Prague. There he has been a PhD student of Department of Measurement since 1995. He was with the Technical University Munich in 1996 and with EPFL Lausanne in 1997–1998 as a visiting researcher. His research field is about fluxgate magnetometry and contactless current measurement. Laurent Chiesi was born in Cluses, France, in 1967. He received his degree in microelectronics engineering from the University of Lyon, France, in 1991. In 1994, he joined the Institute for Microsystems at the Swiss Federal Institute of Technology, EPF Lausanne, to prepare a doctoral thesis in microsensor systems. His current research interests are in magnetic sensor microsystems. Balazs Janossy was born in Budapest, Hungary, in 1967. He received his diploma in Physics at Joseph Fourier University in Grenoble and his PhD on magnetic properties of high temperature superconductors at the University Paris XI, Orsay. He continued his research in superconductors as a visiting scientist at the Kammerlingh Onnes Laboratory then at the High Magnetic Field facilities in Grenoble and Toulouse. He joined the Institute of Microsystems in 1996 to work on high sensitive magnetic field sensors. His current field of interest is in very low magnetic field detection.
183
Rade S. PopoÕic was born in Yugoslavia ŽSerbia. in 1945. He obtained the Dipl. Ing. degree in applied physics from the University of Beograd, Yugoslavia, in 1969, and the MSc and Dr. Sc degrees in electronics from the University of Nis, Yugoslavia, in 1974 and 1978, respectively. From 1969 to 1981 he was with Elektronska Industrija, Nis, Yugoslavia, where he worked on research and development of semiconductor devices and later became head of the company’s CMOS department. From 1982 to 1993 he worked for Landis & Gyr, Central R&D, Zug, Switzerland, in the field of semiconductor sensors, interface electronic, and microsystems. There he was responsible for research in semiconductor device physics Ž1983–1985., for microtechnology R&D Ž1986–1990. and was appointed vice president ŽCentral R&D. in 1991. In 1994 he joined the Swiss Federal Institute of Technology at Lausanne ŽEPFL. as professor for microtechnology systems. He teaches Conceptual Product and System Design and Microelectronics at the Department of Microengineering of the EPFL. His current research interests include sensors for magnetic, optical, and mechanical signals, the corresponding microsystems, physics of submicron devices, and noise phenomena.
A new compact 2D planar fluxgate sensor with amorphous metal core Pavel Kejık ´ a, Laurent Chiesi b
b,)
, Balazs Janossy b, Rade S. Popovic
b
a Czech Technical UniÕersity of Prague, Department of Measurement, 166 27 Prague, Czech Republic Swiss Federal Institute of Technology Lausanne, Department of Microengineering, CH-1015 Lausanne, Switzerland
Abstract A new compact 2D planar fluxgate sensor using a ferromagnetic amorphous metal core is described. The fluxgate sensor consists of two orthogonal planar metallic coils and a ring shaped amorphous magnetic ribbon mounted on a PCB substrate. Each planar coil is used alternatively as excitation or as pick-up coil. An electronic interface drives alternatively the two coils and performs a feedback on each sensing coil in order to compensate for the two measured magnetic field components. The sensing element has a magnetic sensitivity of 55 VrmT at an excitation frequency of 8.4 kHz with a 160 mA peak driving current. Used as an electronic compass, the reached precision angle is better than 18. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Compass; Planar coils; Fluxgate sensor; Amorphous ferromagnetic ribbons
1. Introduction The known integrated planar w1x or 3D w2x fluxgate sensors are parallel fluxgate sensors w3,4x. Therefore, the excitation and pick-up coils are in the same axis and the combination of two ferromagnetic cores is necessary to cancel the influence of the excitation field on the output signal. Furthermore these integrated fluxgate sensors measure only in one direction. To obtain a 2D fluxgate sensor with such a configuration, it is necessary to use four ferromagnetic cores. The total chip surface and the overall resistance of the excitation coils is then significative. In this paper, we present a more compact 2D planar fluxgate sensor. This sensor works as an orthogonal fluxgate sensor w3,4x with one toroidal ferromagnetic core and only two planar coils. Therefore, the sensor surface and the resistance of the coils are much reduced which is interesting for integration. Furthermore, a smart electronic interface is described which is capable of driving simultaneously the coils and compensate for the external field.
ferromagnetic core, the whole mounted on a PCB substrate. The coil has a density of 50 turnsrcm. The core is obtained from a Vitrovac 6025 w ribbon w5x by photolithography and wet chemical etching. The principle of the detection is based on alternating measurements in two directions. In each case the excitation coil is the one orthogonal to the measurement axis while
2. Sensor and electronics The fluxgate sensor on Fig. 1 consists of two similar orthogonal planar coils and a ring shaped amorphous )
Corresponding author. Tel.: q41-21-693-6614; Fax: q41-21-6936670; E-mail: [email protected] 0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 0 8 3 - 7
Fig. 1. View of the sensor.
P. Kejık ´ et al.r Sensors and Actuators 81 (2000) 180–183
181
Fig. 2. Block diagram of the 2D planar fluxgate sensor.
the sensing coil is the one which is parallel. Therefore, the output voltage on the pick-up coil is free from the excitation field and contains only the useful signal at the frequency twice that of the excitation field. A compensation feedback DC current is applied simultaneously in both axes to avoid the influence of the orthogonal field component on the measurement axis. A block diagram of the 2D magnetometer electronics is shown in Fig. 2. The two switches allow the commutation between the excitation and sensing mode for the two coils. In Fig. 2, the switches close the coil of x channel in sensing mode and the coil of y channel in excitation mode. The generator G, an 8-Bit microcontroller PIC16C84, provides the driving frequency fr10 s 840 Hz. A pulsed current with a slew rate of 20 mArms at a frequency f s 8.4 kHz is supplied to the excitation coil by the current amplifier. This current of 160 mA P generates an AC magnetic field Ž500 mTP . which periodically saturates the amorphous ring core. Fig. 3 shows the waveforms of the excitation current on both axis sensed by resistors R x and R y . This waveform has a pulse time to period ratio of 1:3 which reduces the power consumption. For a 160 mA P current, the RMS value is only 71 mA RMS . During the positive and negative slew rates, induced voltage peaks appear across the pick-up coil. The output voltage of the
Fig. 4. Full-range magnetic field response of the sensor.
pick-up coil contains only even harmonics proportional to the measured magnetic field. The output signal is extracted by the synchronous detector PSD driven by the doubled excitation frequency 2 f from the generator G. The PSD output is connected to the current amplifier through an integrator. The feedback loop compensates the external field component in the measured direction and is updated only during sensing cycles. The sensor output is thus the feedback current sensed by the resistors R x and R y . For DC field precision is achieved by using the integrator in the feedback loop.
3. Experimental results and discussion The prototype 2D compact planar sensor has been characterized in open and closed-loop configurations. The open-loop characterization indicates a strong dependence of the pick-up output voltage upon a cross magnetic field. The response of the sensor is non-linear with a non-linearity error close to 30% of FS Ž"60 mT..
Fig. 3. Excitation current waveforms of x and y channels sensed by resistors Ž R x s R y s 9 V ..
182
P. Kejık ´ et al.r Sensors and Actuators 81 (2000) 180–183 Table 1 Comparison of sensor errors with 18 error precision for a measured magnetic field of 30 mT and three different projection angles a x Ž8. B x ŽmT. Hysteresis error ŽmT. Non-linearity error ŽmT. Cross sensitivity error ŽmT. Total of sensor error ŽmT. Error ŽmT. corresponds to 18 error
0 30.0 0.072 0.066 0 0.138 0.550
45 21.2 0.072 0.042 0.114 0.228 0.380
90 0 0.072 0 0.240 0.312 0.550
Finally the measured sensor detectivity is 0.1 mT for a dc magnetic field. Fig. 5. Output sensor non-linearity and hysteresis errors over the full-scale.
In the closed-loop configuration, the sensor output shows much better characteristics. Fig. 4 illustrates the full-scale magnetic field response of the sensor. The output sensitivity is 55 VrmT Ž R x s R y s 9 V . in the "60 mT field range. The range is deliberately limited in order to use our sensor as compass device. However, even if the ferromagnetic core is saturated only from 200 mT, the feedback compensation allows an extending of the measurement range up to 500 mT with a well adapted electronic circuit. Fig. 5 gives the total sensor output errors vs. the measured magnetic field. These errors are hysteresis error due to the remanent field and non-linearity error. In order to measure the effect of the remanent field on the sensor output, the ferromagnetic core has been driven into deep saturation by a magnetic field of "4.4 mT Ž22 times higher than the field where the saturation begins.. The measured hysteresis and non-linearity errors are respectively 0.12% and 0.23% of the full-scale. Fig. 6 illustrates the effect of a cross magnetic field on the measurement axis. The cross sensitivity full-scale error is obtained by sweeping a field orthogonal to a measured constant field. The cross sensitivity error varies from 0 to 1% of full-scale. Thus the total output sensor error varies from 0.12% to 1.35% of full-scale between 0 and "60 mT.
4. Compass application Our sensor is well adapted for measuring in plane components of the Earth’s magnetic field. It can also determine the direction of the field, thus it is potentially useful as an electronic compass. Table 1 gives the level of precision of the compass for a 30 mT horizontal field and three different sensor positions. For such a magnetic field, the precision on the angle is better than 18.
5. Conclusion We have presented a new design of a 2D fluxgate sensor in a planar technology based on a ferromagnetic amorphous ring core. The sensor has the following advantageous features: compactness and simplicity due to the reduced number of coils and magnetic cores; high sensitivity due to feedback which reduces hysteresis and non-linearity; and optimized current consumption due to pulsed current excitation and alternative measurement. The excellent performance of the sensor with non-linearity and cross sensitivity errors respectively 0.12% and 1.35% of full scale in a working range of "60 mT makes this device an excellent compass with precision better than 18.
Acknowledgements This work has been supported by the Swiss confederation and MINAST project ‘High-detectivity magnetic sensor microsystem’ 3.11.
References
Fig. 6. Cross sensitivity error.
w1x S. Choi, S. Kawahito, A planar fluxgate magnetic sensor for on-chip integration, Sensors and Materials 9 Ž1997. 241–252. w2x S. Ulbricht, W. Budde, R. Gottfried, A monolithically integrated two-axis magnetic field sensor system, Presented at ESSCIRC’96— Proceedings of the 22nd European Solid-State Circuits Conference, Neuchatel, ˆ 1996.
P. Kejık ´ et al.r Sensors and Actuators 81 (2000) 180–183 w3x F. Primdahl, The fluxgate magnetometer, J. Phys. E: Sci. Instr. 12 Ž1979. 241–253. w4x P. Ripka, Review of fluxgate sensors, Sensors and Actuators A 33 Ž1992. 129–141. w5x Vacuumschmelze, Amorphous metals VITROVAC.
Biographies PaÕel Kejık in the Czech Republic in 1971. He ´ was born in Prostejov, ˇ received an Ing. degree in 1994 at the Czech Technical University of Prague. There he has been a PhD student of Department of Measurement since 1995. He was with the Technical University Munich in 1996 and with EPFL Lausanne in 1997–1998 as a visiting researcher. His research field is about fluxgate magnetometry and contactless current measurement. Laurent Chiesi was born in Cluses, France, in 1967. He received his degree in microelectronics engineering from the University of Lyon, France, in 1991. In 1994, he joined the Institute for Microsystems at the Swiss Federal Institute of Technology, EPF Lausanne, to prepare a doctoral thesis in microsensor systems. His current research interests are in magnetic sensor microsystems. Balazs Janossy was born in Budapest, Hungary, in 1967. He received his diploma in Physics at Joseph Fourier University in Grenoble and his PhD on magnetic properties of high temperature superconductors at the University Paris XI, Orsay. He continued his research in superconductors as a visiting scientist at the Kammerlingh Onnes Laboratory then at the High Magnetic Field facilities in Grenoble and Toulouse. He joined the Institute of Microsystems in 1996 to work on high sensitive magnetic field sensors. His current field of interest is in very low magnetic field detection.
183
Rade S. PopoÕic was born in Yugoslavia ŽSerbia. in 1945. He obtained the Dipl. Ing. degree in applied physics from the University of Beograd, Yugoslavia, in 1969, and the MSc and Dr. Sc degrees in electronics from the University of Nis, Yugoslavia, in 1974 and 1978, respectively. From 1969 to 1981 he was with Elektronska Industrija, Nis, Yugoslavia, where he worked on research and development of semiconductor devices and later became head of the company’s CMOS department. From 1982 to 1993 he worked for Landis & Gyr, Central R&D, Zug, Switzerland, in the field of semiconductor sensors, interface electronic, and microsystems. There he was responsible for research in semiconductor device physics Ž1983–1985., for microtechnology R&D Ž1986–1990. and was appointed vice president ŽCentral R&D. in 1991. In 1994 he joined the Swiss Federal Institute of Technology at Lausanne ŽEPFL. as professor for microtechnology systems. He teaches Conceptual Product and System Design and Microelectronics at the Department of Microengineering of the EPFL. His current research interests include sensors for magnetic, optical, and mechanical signals, the corresponding microsystems, physics of submicron devices, and noise phenomena.