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Demonstration of a new principle for an active electromagnetic pressure sensor
Sensors and Actuators 81 Ž2000. 328–331 www.elsevier.nlrlocatersna
Demonstration of a new principle for an active electromagnetic pressure sensor B.M. Dutoit ) , P.-A. Besse, A.P. Friedrich, R.S. Popovic EPFL— Swiss Federal Institute of Technology, Institute of Microsystems, CH-1015 Lausanne, Switzerland
Abstract A new principle for an active electromagnetic pressure sensor is described and results from a demonstration prototype are presented. Unlike traditional pressure devices, this sensor does not measure the membrane deflection, but the force feed back to the membrane to compensate for its displacement. The membrane is thus always kept close to its rest position. Consequently, the measured signal is independent of the mechanical properties of the membrane, of the fluctuations due to time and temperature, and of the offset due to fabrication tolerances. Moreover, this enables the selection of the membrane material that offers the best resistance for the intended application. This sensor system is hence especially adapted for harsh environments, high temperature applications, and long term measurements. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Pressure sensor; Feedback system; Magnetic force compensation; Harsh environments; Electromagnetic actuation
1. Introduction When measuring small pressure differences, temperature cross-sensitivity offset and long-term stability are challenging problems w1–3x. This is due to the weak measuring signal in comparison with the signal variations produced by the mechanical drift of the measurement membrane due to environment changes Žtemperature, humidity, etc.. or stress relaxation. To avoid these problems, we propose to use an electromagnetic sensor which compensates for the deformation of the membrane ŽFig. 1.. The major benefit lies in the fact that the system becomes completely independent of the mechanical characteristics of the membrane which can, in this case, be made out of any soft material. The fabrication tolerances of the membrane and packaging induced stresses become negligible. This improves the offset and the reproducibility of the characteristics from sensor to sensor. Moreover, it can be favorably exploited by using a material with high resistance to corrosive media to fabricate the membrane. Finally, the number of parts and the assem-
) Corresponding author. Tel.: q41-21-693-6735; Fax: q41-21-6936670; E-mail: [email protected]
bling will hence be considerably simplified in comparison with existing pressure sensors for harsh environments w4x. 2. Demonstration prototype The proposed setup ŽFig. 2. is similar to variable reluctance pressure ŽVRP. sensors w5x. A magnetically soft plate is fixed on a Teflon diaphragm and sandwiched between two half shells. Each half incorporates an E-corerflat coil assembly. The reluctances of the magnetic circuits that surround the coils are determined by the air gaps widths. They vary in opposite directions as a force is applied on the membrane. A 40-kHz current is symmetrically applied on both coils to sense for this inductance variation. The two coils form two branches of a Wheatstone bridge along with two resistances. After signal processing, the current control feedback loop is regulated with a standard PID regulator. This signal is converted to a current Ž Ix . and applied on the same coils to compensate for any deformation of the membrane. A constant bias current Ž Io . of 300 mA is added on both coils as well, to linearize the output characteristic and increase the dynamic response. The novelty in comparison with VRP sensors is that the measurement signal is not deduced from the position detection of the membrane, but from the current that is necessary to feed back into the coils to compensate for a deflection of the membrane.
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 1 0 3 - X
B.M. Dutoit et al.r Sensors and Actuators 81 (2000) 328–331
Fig. 1. Schematic of the current controlled feedback loop. The movement of a membrane due to a pressure difference is detected by a position sensor. The membrane is then put back to its rest position by an actuator. Both sensor and actuator are electromagnetic.
Fig. 2. Simplified cross section of the setup with the currents responsible for the forces: the bias current Ž Io . and the feedback current Ž Ix ..
To demonstrate the principle, a preliminary system has been built ŽFig. 3. with ferrite pots and two wire wound coils. The diameter of the ferromagnetic pots is 28 mm. The system is tested by applying a force on the membrane.
3. Results and discussion This system has been brought into feedback and pressures up to 65 mbar have been compensated with linearity
Fig. 3. Side view of the demonstration prototype.
329
Fig. 4. Output voltage of the demonstrator in closed loop. The linearity is better than 2%.
better than 2% ŽFig. 4.. To prove that the deflection of the membrane is compensated, its position is measured with an interferometer in open loop and in closed loop ŽFig. 5.. When the sensor is in feedback mode, the membrane is kept within y0.4rq 1.1 mm around the rest position. In Fig. 6, it is shown that the linearity error of the output voltage is the integral of the positioning error. Hence, the former is the consequence of the latter. If we go one step further, we can see that the positioning error is due to a cross-talk between the low frequency feedback signal and the 40 kHz signal for the position detection. This effect is shown by measuring the output signal while maintaining the inductance of the coils equal and forcing a constant current Io q Ix and Io y Ix , respectively into the coils. The error is directly given by the position voltage Žsee Fig. 1 for the definition.. There are three components responsible for the observed cross-talk ŽFig. 7.. First, the resistance Ž R current source . that controls the voltage–current conversion has a temperature drift Ž200 ppm.. If the voltage–current converter must deliver a large current, the resistance is heated and its value increases. The 40 kHz current signal is consequently reduced. If the
Fig. 5. Measurement of the deflection of the membrane as a function of applied pressure in open loop and in closed loop Žthe measurement is carried out twice to test for the reproducibility.. The measurement is done with an interferometer.
330
B.M. Dutoit et al.r Sensors and Actuators 81 (2000) 328–331
Fig. 6. Linearity error of the output voltage of Fig. 4 and position error of Fig. 5. It is observed that the linearity error is the integral of the position error.
feedback current Ž Ix . is not zero, it induces an error on the position voltage ŽFig. 7.. It is as if the system senses a membrane movement. The actuation force is also influenced by this variation as the feedback currents also depend on these resistances. These errors are partially compensated using low temperature drift resistances Ž20 ppm.. The second source of error is the resistance variation of the coils Žerror R coil on Fig. 7.. The position of the membrane is sensed by the voltage change difference over the coils, which depends on the inductance and on the resistance of the coils. The actuation force is nevertheless not influenced by this variation, as the currents in the coils only depend on the current sources and not on the coils resistances. The third cause of error is the output capacitance variation of the power amplifiers. It changes the cut-off frequency of the voltage to current converter and degrades the 40 kHz signal Žerror OPA on Fig. 7.. It will be smaller
Fig. 7. Graphic of the three errors induced on the position voltage as a function of the applied current Ž Ix . for the demonstration prototype. The error R current source is due to the resistance variation of the current sources using a 200 ppmr8C resistance. The error R coil is due to the resistance variation of the coils. The error OPA is due to the output capacitance variation of the power amplifiers.
Fig. 8. Top view of one part of the flat sensor. A double side flat coil Ža. on a 100 mm PCB is surrounded by an amorphous metal structure Žb..
for the flat coil as the inductance is only 200 mH Ž3 mH for the actual demonstration prototype..
4. Design of the flat sensor A second system has been designed flat ŽFig. 8. to be able to fabricate it using batch processing and to reduce the overall size of the sensor. The ferromagnetic parts are made out of amorphous metal plates ŽVitrovac w , Vacuumschmelze. which are etched using standard photolithographic techniques. The flat coils are made double sided with the printed board technology used to its limits. The track width is 50 mm with a gap width of 50 mm. The height of the copper track is 37 mm with 10 mm tin at the top. The whole system is glued and encapsulated in a stainless steel package. The pressure difference is generated by a hand operated pump. The flat system has also been brought into feedback. But the error on the position voltage is high in comparison with the position voltage. This error is analyzed in open loop ŽFig. 9.. The position voltage is measured when the membrane is kept fixed ŽPosition voltage error. and when the membrane is free ŽPosition voltage.. The position
Fig. 9. Graphic of the position voltage and of the error induced on the position voltage Žerror R coil . as a function of the applied current Ž Ix . for the flat sensor. The other errors mentioned for the demonstration prototype Žerror R current source and error OPA. are negligible.
B.M. Dutoit et al.r Sensors and Actuators 81 (2000) 328–331
voltage error is almost completely due to the variation of the coil resistance. A constant current Ž Io q Ix . of 300 mA in the coil warms it up to 1208C. Consequently, the dc resistance of the coils, which was 28.6 V when the feedback current Ž Ix . was 0, increases to 40 V for the first coil and decreases to 25.8 V for the second coil. As the Q f actor is low Ž Q s 2., this resistance difference induces an error on the position voltage. In the future, it is expected to reduce this problem by working with lower currents and by optimizing the design of the system.
5. Conclusion This article described a new principle for the measurement of differential pressures. The first prototype has a linearity of 2% in the 0 to 65 mbar range. A flat sensor fabricated mainly with batch processing techniques has then been realized. The results presented in this paper show that an active electromagnetic pressure sensor is feasible and worth further investigation in order to improve its performances.
Acknowledgements This work has been partially supported by the Swiss priority program MINAST 3.12.
References w1x C. Berthoud, M. Ansorge, F. Pellandini, Effective static response compensation suitable for low-power ASIC implementation with an application to pressure sensors, Proceedings of the Joint IEEE Instrumentation and Measurement Technology Conference and IMEKO Technical Committee, Vol. 2, Brussels, Belgium, June 4–6, 1996, pp. 1168–1173.
331
w2x C.C. Chang, C.T. Lieu, M.K. Hsich, Study of the fabrication of a silicon pressure sensor, Int. J. Electron. 82 Ž1997. 295–302. w3x H. Obermeier, Kapazitive Abgriffsysteme auf keramischer Basis, Sensoren Messaufnehmer 1989, Esslingen, Germany, May 30–June 1, 1989, pp. 3.1–3.26. w4x J. Hermann, C. Bourgeois, F. Porret, B. Kloeck, Capacitive silicon differential pressure sensor, Proceedings of the 8th International Conference on Solid-State Sensors and Actuators and Eurosensors IX, Stockholm, Sweden, 1995, June 25–29, pp. 620–623. w5x R. Proud, VRP transducers for low-pressure measurement, Sensors ŽFebruary 1991. 20–22.
Biographies Bertrand M. Dutoit received his Diploma in Microengineering from the Swiss Federal Institute of Technology, EPF Lausanne, in 1995. He first worked at the ETH in Zurich in the field of Scanning Near field Optical Microscopy. He then joined the Institute of Microsystems at EPFL in 1997, where he has been working towards his PhD in the field of sensor microsystems. Pierre-Andre´ Besse received the diploma in Physics and his PhD from the Swiss Federal Institute of Technology, ETH Zurich, in 1986 and ¨ 1992. In 1986, he joined the group of micro- and optoelectronics of the Institute of Quantum Electronics at ETH Zurich, where he is engaged in research in optical telecommunication science. In August 1994, he joined the Institute of Microsystems at the Swiss Federal Institute of Technology at Lausanne ŽEPFL. as senior assistant. Andreas P. Friedrich received his diploma in microengineering from the Swiss Federal Institute of Technology, EPF Lausanne, in 1993. He joined the Institute of Microsystems at EPFL in 1994, where he is working towards his PhD in the field of sensor microsystems. Rade S. PopoÕic 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 and from 1982 to 1993 he worked for Landis & Gyr, Zug, Switzerland. In 1994 he joined the Swiss Federal Institute of Technology, EPF Lausanne, as professor for microtechnology systems. He teaches Conceptual Product Design and Microelectronics at the Department of Microengineering of EPFL.
Demonstration of a new principle for an active electromagnetic pressure sensor B.M. Dutoit ) , P.-A. Besse, A.P. Friedrich, R.S. Popovic EPFL— Swiss Federal Institute of Technology, Institute of Microsystems, CH-1015 Lausanne, Switzerland
Abstract A new principle for an active electromagnetic pressure sensor is described and results from a demonstration prototype are presented. Unlike traditional pressure devices, this sensor does not measure the membrane deflection, but the force feed back to the membrane to compensate for its displacement. The membrane is thus always kept close to its rest position. Consequently, the measured signal is independent of the mechanical properties of the membrane, of the fluctuations due to time and temperature, and of the offset due to fabrication tolerances. Moreover, this enables the selection of the membrane material that offers the best resistance for the intended application. This sensor system is hence especially adapted for harsh environments, high temperature applications, and long term measurements. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Pressure sensor; Feedback system; Magnetic force compensation; Harsh environments; Electromagnetic actuation
1. Introduction When measuring small pressure differences, temperature cross-sensitivity offset and long-term stability are challenging problems w1–3x. This is due to the weak measuring signal in comparison with the signal variations produced by the mechanical drift of the measurement membrane due to environment changes Žtemperature, humidity, etc.. or stress relaxation. To avoid these problems, we propose to use an electromagnetic sensor which compensates for the deformation of the membrane ŽFig. 1.. The major benefit lies in the fact that the system becomes completely independent of the mechanical characteristics of the membrane which can, in this case, be made out of any soft material. The fabrication tolerances of the membrane and packaging induced stresses become negligible. This improves the offset and the reproducibility of the characteristics from sensor to sensor. Moreover, it can be favorably exploited by using a material with high resistance to corrosive media to fabricate the membrane. Finally, the number of parts and the assem-
) Corresponding author. Tel.: q41-21-693-6735; Fax: q41-21-6936670; E-mail: [email protected]
bling will hence be considerably simplified in comparison with existing pressure sensors for harsh environments w4x. 2. Demonstration prototype The proposed setup ŽFig. 2. is similar to variable reluctance pressure ŽVRP. sensors w5x. A magnetically soft plate is fixed on a Teflon diaphragm and sandwiched between two half shells. Each half incorporates an E-corerflat coil assembly. The reluctances of the magnetic circuits that surround the coils are determined by the air gaps widths. They vary in opposite directions as a force is applied on the membrane. A 40-kHz current is symmetrically applied on both coils to sense for this inductance variation. The two coils form two branches of a Wheatstone bridge along with two resistances. After signal processing, the current control feedback loop is regulated with a standard PID regulator. This signal is converted to a current Ž Ix . and applied on the same coils to compensate for any deformation of the membrane. A constant bias current Ž Io . of 300 mA is added on both coils as well, to linearize the output characteristic and increase the dynamic response. The novelty in comparison with VRP sensors is that the measurement signal is not deduced from the position detection of the membrane, but from the current that is necessary to feed back into the coils to compensate for a deflection of the membrane.
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 1 0 3 - X
B.M. Dutoit et al.r Sensors and Actuators 81 (2000) 328–331
Fig. 1. Schematic of the current controlled feedback loop. The movement of a membrane due to a pressure difference is detected by a position sensor. The membrane is then put back to its rest position by an actuator. Both sensor and actuator are electromagnetic.
Fig. 2. Simplified cross section of the setup with the currents responsible for the forces: the bias current Ž Io . and the feedback current Ž Ix ..
To demonstrate the principle, a preliminary system has been built ŽFig. 3. with ferrite pots and two wire wound coils. The diameter of the ferromagnetic pots is 28 mm. The system is tested by applying a force on the membrane.
3. Results and discussion This system has been brought into feedback and pressures up to 65 mbar have been compensated with linearity
Fig. 3. Side view of the demonstration prototype.
329
Fig. 4. Output voltage of the demonstrator in closed loop. The linearity is better than 2%.
better than 2% ŽFig. 4.. To prove that the deflection of the membrane is compensated, its position is measured with an interferometer in open loop and in closed loop ŽFig. 5.. When the sensor is in feedback mode, the membrane is kept within y0.4rq 1.1 mm around the rest position. In Fig. 6, it is shown that the linearity error of the output voltage is the integral of the positioning error. Hence, the former is the consequence of the latter. If we go one step further, we can see that the positioning error is due to a cross-talk between the low frequency feedback signal and the 40 kHz signal for the position detection. This effect is shown by measuring the output signal while maintaining the inductance of the coils equal and forcing a constant current Io q Ix and Io y Ix , respectively into the coils. The error is directly given by the position voltage Žsee Fig. 1 for the definition.. There are three components responsible for the observed cross-talk ŽFig. 7.. First, the resistance Ž R current source . that controls the voltage–current conversion has a temperature drift Ž200 ppm.. If the voltage–current converter must deliver a large current, the resistance is heated and its value increases. The 40 kHz current signal is consequently reduced. If the
Fig. 5. Measurement of the deflection of the membrane as a function of applied pressure in open loop and in closed loop Žthe measurement is carried out twice to test for the reproducibility.. The measurement is done with an interferometer.
330
B.M. Dutoit et al.r Sensors and Actuators 81 (2000) 328–331
Fig. 6. Linearity error of the output voltage of Fig. 4 and position error of Fig. 5. It is observed that the linearity error is the integral of the position error.
feedback current Ž Ix . is not zero, it induces an error on the position voltage ŽFig. 7.. It is as if the system senses a membrane movement. The actuation force is also influenced by this variation as the feedback currents also depend on these resistances. These errors are partially compensated using low temperature drift resistances Ž20 ppm.. The second source of error is the resistance variation of the coils Žerror R coil on Fig. 7.. The position of the membrane is sensed by the voltage change difference over the coils, which depends on the inductance and on the resistance of the coils. The actuation force is nevertheless not influenced by this variation, as the currents in the coils only depend on the current sources and not on the coils resistances. The third cause of error is the output capacitance variation of the power amplifiers. It changes the cut-off frequency of the voltage to current converter and degrades the 40 kHz signal Žerror OPA on Fig. 7.. It will be smaller
Fig. 7. Graphic of the three errors induced on the position voltage as a function of the applied current Ž Ix . for the demonstration prototype. The error R current source is due to the resistance variation of the current sources using a 200 ppmr8C resistance. The error R coil is due to the resistance variation of the coils. The error OPA is due to the output capacitance variation of the power amplifiers.
Fig. 8. Top view of one part of the flat sensor. A double side flat coil Ža. on a 100 mm PCB is surrounded by an amorphous metal structure Žb..
for the flat coil as the inductance is only 200 mH Ž3 mH for the actual demonstration prototype..
4. Design of the flat sensor A second system has been designed flat ŽFig. 8. to be able to fabricate it using batch processing and to reduce the overall size of the sensor. The ferromagnetic parts are made out of amorphous metal plates ŽVitrovac w , Vacuumschmelze. which are etched using standard photolithographic techniques. The flat coils are made double sided with the printed board technology used to its limits. The track width is 50 mm with a gap width of 50 mm. The height of the copper track is 37 mm with 10 mm tin at the top. The whole system is glued and encapsulated in a stainless steel package. The pressure difference is generated by a hand operated pump. The flat system has also been brought into feedback. But the error on the position voltage is high in comparison with the position voltage. This error is analyzed in open loop ŽFig. 9.. The position voltage is measured when the membrane is kept fixed ŽPosition voltage error. and when the membrane is free ŽPosition voltage.. The position
Fig. 9. Graphic of the position voltage and of the error induced on the position voltage Žerror R coil . as a function of the applied current Ž Ix . for the flat sensor. The other errors mentioned for the demonstration prototype Žerror R current source and error OPA. are negligible.
B.M. Dutoit et al.r Sensors and Actuators 81 (2000) 328–331
voltage error is almost completely due to the variation of the coil resistance. A constant current Ž Io q Ix . of 300 mA in the coil warms it up to 1208C. Consequently, the dc resistance of the coils, which was 28.6 V when the feedback current Ž Ix . was 0, increases to 40 V for the first coil and decreases to 25.8 V for the second coil. As the Q f actor is low Ž Q s 2., this resistance difference induces an error on the position voltage. In the future, it is expected to reduce this problem by working with lower currents and by optimizing the design of the system.
5. Conclusion This article described a new principle for the measurement of differential pressures. The first prototype has a linearity of 2% in the 0 to 65 mbar range. A flat sensor fabricated mainly with batch processing techniques has then been realized. The results presented in this paper show that an active electromagnetic pressure sensor is feasible and worth further investigation in order to improve its performances.
Acknowledgements This work has been partially supported by the Swiss priority program MINAST 3.12.
References w1x C. Berthoud, M. Ansorge, F. Pellandini, Effective static response compensation suitable for low-power ASIC implementation with an application to pressure sensors, Proceedings of the Joint IEEE Instrumentation and Measurement Technology Conference and IMEKO Technical Committee, Vol. 2, Brussels, Belgium, June 4–6, 1996, pp. 1168–1173.
331
w2x C.C. Chang, C.T. Lieu, M.K. Hsich, Study of the fabrication of a silicon pressure sensor, Int. J. Electron. 82 Ž1997. 295–302. w3x H. Obermeier, Kapazitive Abgriffsysteme auf keramischer Basis, Sensoren Messaufnehmer 1989, Esslingen, Germany, May 30–June 1, 1989, pp. 3.1–3.26. w4x J. Hermann, C. Bourgeois, F. Porret, B. Kloeck, Capacitive silicon differential pressure sensor, Proceedings of the 8th International Conference on Solid-State Sensors and Actuators and Eurosensors IX, Stockholm, Sweden, 1995, June 25–29, pp. 620–623. w5x R. Proud, VRP transducers for low-pressure measurement, Sensors ŽFebruary 1991. 20–22.
Biographies Bertrand M. Dutoit received his Diploma in Microengineering from the Swiss Federal Institute of Technology, EPF Lausanne, in 1995. He first worked at the ETH in Zurich in the field of Scanning Near field Optical Microscopy. He then joined the Institute of Microsystems at EPFL in 1997, where he has been working towards his PhD in the field of sensor microsystems. Pierre-Andre´ Besse received the diploma in Physics and his PhD from the Swiss Federal Institute of Technology, ETH Zurich, in 1986 and ¨ 1992. In 1986, he joined the group of micro- and optoelectronics of the Institute of Quantum Electronics at ETH Zurich, where he is engaged in research in optical telecommunication science. In August 1994, he joined the Institute of Microsystems at the Swiss Federal Institute of Technology at Lausanne ŽEPFL. as senior assistant. Andreas P. Friedrich received his diploma in microengineering from the Swiss Federal Institute of Technology, EPF Lausanne, in 1993. He joined the Institute of Microsystems at EPFL in 1994, where he is working towards his PhD in the field of sensor microsystems. Rade S. PopoÕic 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 and from 1982 to 1993 he worked for Landis & Gyr, Zug, Switzerland. In 1994 he joined the Swiss Federal Institute of Technology, EPF Lausanne, as professor for microtechnology systems. He teaches Conceptual Product Design and Microelectronics at the Department of Microengineering of EPFL.