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Magnetic field sensors based on amorphous ribbons
Sensors and Actuarors
A, 25-27
(1991)
759
159-162
Magnetic Field Sensors Based on Amorphous Ribbons V. E. MAKHOTKIN, Institute
R. P. SHURUKHTN,
ofGeneral Physics,
U.S.S.R.
Academy
V. A. LOPATIN,
ofSciences,
Abstract New sensitive sensors suitable for measuring weak magnetic fields and based on amorphous ribbons are described. Three types of sensors are discussed in the paper. The principle of sensor operation is based on the dependence of the ribbon properties on the magnetic field intensity. The advantage of the sensors is the minimal use of active elements, and consequently their high reliability, thermal stability and small size.
1. Introduction Amorphous ribbons have been intensively studied for nearly two decades. They show interesting scientific properties and also have many applications in actual devices, for example, in magnetic field sensors and magnetometers [l-4]. The principle of their operation is based on the dependence of the ribbon properties on the magnetic field. The new sensors are developed with the aim of getting high sensitivity, minimal size and a broad temperature range down to cryogenic temperatures, particularly because hightemperature superconductivity research is of great interest now. Three types of sensors with amorphous ribbons were made and studied by the authors: (1) resonant, (2) vibrating and (3) resistive. The sensors have a simple construction with minimal use of active elements, which enhances their reliability, thermal stability and dimensions. The sensors mentioned may be used for the development of various devices to measure any physical data: current, 0924-4247/91/$3.50
P. YU. MARCHUKOV
38 Vavilov S&et,
and YU. K. LEVIN
117942 Moscow
(U.S.S.R.)
pressure, linear and angular shifts. In particular the possibility of using the devices in the cryogenic temperature range should be noticed. The amorphous ribbons used have a magnetostriction of about zero and are of the following composition: Fe and Co (75-85)%, B and Si (15-25)%.
2. Resonant Sensor The first type of sensor is a resonant circuit with the amorphous ribbon core inside an inductive coil. When d-c. pulses are passed through the core, ac. pulses are induced in the coil. The form of the induced pulses is the same as the free decayed oscillation of the resonant circuit and its time position corresponds to the initial and the final fronts of the d.c. pulse. This a.c. pulse oscillation frequency depends on the external magnetic field intensity, and its initial phase depends on the sign of the magnetic field projection on the ribbon direction. The frequency measurement of the decayed oscillation is usually fulfilled by registration of one to five initial half periods with a special device, such as a computer’s adapter. The sensor construction is very simple. There is a resonant circuit in the non-magnetic box with a size less than 10 mm x 10 mm x 50 mm, connected to the registration circuit by the connector. The inductive coil has 2000 winds directly wound on the amorphous ribbon. The size of the ribbon is 40 mm x 0.8 mm x 0.03 mm. The capacitor is chosen to provide a resonance frequency in the l-200 kHz range, because the sensor 0 Elsevier Sequoia/Printed in The Netherlands
760
O_‘4
-3
-2
-1 magnetic
6
i field,
8 Oe
j.
4
Fig. I Resonance frequency of the sensor as a function of the magnetic field intensity.
turned out to have the highest magnetic field sensitivity in this range. The sensor has galvanically isolated output and driving circuits because the circuit is excited by the current passing through the inductive coil core. The sensor’s output characteristic is the dependence of the resonance frequency on the magnetic field intensity. The conventional form of such a curve for this sensor is shown in Fig. 1. It is clear that the resonance frequency is changed from 11 kHz to 170 kHz when the magnetic field intensity is changed from 0 to 3 Oe, so the highest sensitivity is in the 0.770.9 Oe field range, when the curve slope achieves a value of 120 kHz/Oe. The repetition frequency of the driving pulse is 200-300 Hz, so the time of one measuring period is 3-5 ms. One advantage of the sensor is its ability to operate in the cryogenic temperature range, because the amorphous ribbon keeps its magnetic field sensitivity down to liquid helium temperature. For example, this sensor may be used for the measurement of an integral coefficient of magnetic field screening in a high-temperature superconducting cylinder.
3. Vibrating Sensor The principle of operation of the second type of sensor is the measuring of stray magnetic field near one end of the amorphous ribbon by the vibrating Hall sensor. This field
is determined by the magnetic moment of the ribbon, consequently the sensor measures its magnetization. The latter depends on the external magnetic field directed along the ribbon. So the amplitude of the Hall sensor’s alternating signal will be determined by the external magnetic field projection on the longitudinal ribbon axis. The magnetic field near the sensitive ribbon edge gives the main contribution to the output signal. So this sensor may be used to measure the local distribution of the magnetic field. The sensor’s construction consists of two small 2 mm x 2 mm x 60 mm bars. On the remote ends of the bars there is a piece of amorphous ribbon and a Hall sensor. They are fixed in opposite directions to one another. The nearer ends of the bars are connected to the vibrator. The vibrator is made as a cylindrical piezoelectric ceramic ring with a transverse gap. A IO-20 V a.c. voltage with the resonance frequency of the mechanical system is applied to the external and internal sides of the ring. The vibrator is situated in an aluminium box to screen the noise. There are some connectors on the box to supply power to the vibrator and the Hall sensor, and to take out the sensor’s output signal. The latter is processed by a lock-in nanovoltmeter. The amorphous ribbon and Hall sensor are 50 mm from the box due to the bars. This allows the local magnetic field distribution to be measured in a poorly accessible space. The output characteristic of the vibration sensor is the dependence of the a.c. Hall signal on the ribbon projected field intensity. The conventional form of such a dependence is shown in Fig. 2. According to this diagram, the output curve has a considerable linear part in the magnetic field range from -7 to +7 Oe with a slope of about 10 pV/Oe. This allows a sensitivity of about I mOe to be achieved with a non-linearity better than 1% in a wide magnetic field range for the phase detector time constant of about 1 s. An important advantage of this sensor is the possibility to operate in the cryogenic temperature range, because the Hall sensor
761
““““““” -5
magnetic
[ 0
field,
5 Oe
I
10
O-5 4
-4
-3
-2
-1
magnetic
0I
2 field,
3
4
I
5
Oe
Fig. 2. The output of the vibration sensor vs. external magnetic field.
Fig. 3. D.c. field detection characteristic of the resistive sensor at two different frequencies.
can operate at a temperature essentially below liquid nitrogen temperature. In such a way only the bars with the amorphous ribbon and Hall sensor situated on their ends are cooled. For example, the authors used the vibrating sensor to measure the coefficient of magnetic field screening in a high-temperature superconducting cylinder and for a high-temperature superconductor’s surface topography. The measuring temperature was about 78 K.
200 kHz, the curve has its maximum value in zero magnetic field with a monotonic decrease as the absolute value of the field increases. At the same time, when the frequency is 2 MHz the curve has a minimum at zero magnetic field. Then the impedance increases quickly (slope z 40 Q/Oe) with increasing absolute value of the field; it then achieves its maximum with a subsequent monotonic decrease. When the generator frequency is 2 MHz the output characteristic in a zero magnetic field is of great interest. Such sensors may have a broad application as simple sensitive magnetic field zero-indicators. There is also the possibility of developing a compensation magnetometer with a broad dynamic range and high sensitivity if the above-mentioned zero sensor is inserted in the feedback. It should be noted as a conclusion that the sensor output characteristics greatly depend upon the ribbon’s mechanical stress and the d.c. passing through the ribbon. Even slight ribbon stress may cause the failure in zero field to disappear. Besides that, the output characteristic becomes asymmetrical when d.c. passes through the ribbon.
4. Resistive Sensor The principle of operation of the third type of sensor is the measurement of the amorphous ribbon impedance at high frequencies, which is dependent on the applied magnetic field intensity. A constant-amplitude a.c. goes through the ribbon. The high-frequency voltage formed on the ribbon ends is measured with a conventional a.c. millivoltmeter, used to determine the amorphous ribbon impedance. The sensor consists of a piece of amorphous ribbon 50-80 mm long and a load resistance to stabilize the high-frequency current, all situated in the non-magnetic box. A high-frequency connector is fixed on the box to connect the sensor to the millivoltmeter. The dependence of the amorphous ribbon impedance on the measured field intensity is the output sensor characteristic. This relation is presented in Fig. 3. It is clear that the form of the curve is strongly dependent on the current frequency. When the frequency is
5. Conclusions The resonant, vibrating and resistive magnetic field sensors described here are based on amorphous magnetic ribbons. They have sufficiently high sensitivity for a broad temperature range. They may be used to measure
762
integral or local field distribution. They may be the base for developing secondary devices to measure current, pressure, length, etc. All the sensors may be used to study high-temperature superconductors in the liquid nitrogen temperature range.
References I S. Takeuchi and K. Harada, A resonant-type amorphous ribbon magnetometer driven by an operational
amplifier. IEEE Trans. Msg., MAC-20 ( 1984) 17231725. 2 K. Mohri, S. Shigematsu and K. Yoshino, On flux reversal in amorphous-core multivibrator bridge circuits for sensors. IEEE Trans. Msg. Jpn., TJMJ-2 (1987) 556-557. 3 K. Mohri, T. Kondo and J. Yamasaki, Quick re-
sponse magnetometers using amorphous wire cores and mechanocardiographs, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals, Elsevier, Amsterdam, 1985, pp. 1659-1662. 4 K. Reinitz and L. Hart, Magnetometer with a solidstate magnetic-field sensing means, U.S. Patent No. PN 4 517 515
( 1985).
A, 25-27
(1991)
759
159-162
Magnetic Field Sensors Based on Amorphous Ribbons V. E. MAKHOTKIN, Institute
R. P. SHURUKHTN,
ofGeneral Physics,
U.S.S.R.
Academy
V. A. LOPATIN,
ofSciences,
Abstract New sensitive sensors suitable for measuring weak magnetic fields and based on amorphous ribbons are described. Three types of sensors are discussed in the paper. The principle of sensor operation is based on the dependence of the ribbon properties on the magnetic field intensity. The advantage of the sensors is the minimal use of active elements, and consequently their high reliability, thermal stability and small size.
1. Introduction Amorphous ribbons have been intensively studied for nearly two decades. They show interesting scientific properties and also have many applications in actual devices, for example, in magnetic field sensors and magnetometers [l-4]. The principle of their operation is based on the dependence of the ribbon properties on the magnetic field. The new sensors are developed with the aim of getting high sensitivity, minimal size and a broad temperature range down to cryogenic temperatures, particularly because hightemperature superconductivity research is of great interest now. Three types of sensors with amorphous ribbons were made and studied by the authors: (1) resonant, (2) vibrating and (3) resistive. The sensors have a simple construction with minimal use of active elements, which enhances their reliability, thermal stability and dimensions. The sensors mentioned may be used for the development of various devices to measure any physical data: current, 0924-4247/91/$3.50
P. YU. MARCHUKOV
38 Vavilov S&et,
and YU. K. LEVIN
117942 Moscow
(U.S.S.R.)
pressure, linear and angular shifts. In particular the possibility of using the devices in the cryogenic temperature range should be noticed. The amorphous ribbons used have a magnetostriction of about zero and are of the following composition: Fe and Co (75-85)%, B and Si (15-25)%.
2. Resonant Sensor The first type of sensor is a resonant circuit with the amorphous ribbon core inside an inductive coil. When d-c. pulses are passed through the core, ac. pulses are induced in the coil. The form of the induced pulses is the same as the free decayed oscillation of the resonant circuit and its time position corresponds to the initial and the final fronts of the d.c. pulse. This a.c. pulse oscillation frequency depends on the external magnetic field intensity, and its initial phase depends on the sign of the magnetic field projection on the ribbon direction. The frequency measurement of the decayed oscillation is usually fulfilled by registration of one to five initial half periods with a special device, such as a computer’s adapter. The sensor construction is very simple. There is a resonant circuit in the non-magnetic box with a size less than 10 mm x 10 mm x 50 mm, connected to the registration circuit by the connector. The inductive coil has 2000 winds directly wound on the amorphous ribbon. The size of the ribbon is 40 mm x 0.8 mm x 0.03 mm. The capacitor is chosen to provide a resonance frequency in the l-200 kHz range, because the sensor 0 Elsevier Sequoia/Printed in The Netherlands
760
O_‘4
-3
-2
-1 magnetic
6
i field,
8 Oe
j.
4
Fig. I Resonance frequency of the sensor as a function of the magnetic field intensity.
turned out to have the highest magnetic field sensitivity in this range. The sensor has galvanically isolated output and driving circuits because the circuit is excited by the current passing through the inductive coil core. The sensor’s output characteristic is the dependence of the resonance frequency on the magnetic field intensity. The conventional form of such a curve for this sensor is shown in Fig. 1. It is clear that the resonance frequency is changed from 11 kHz to 170 kHz when the magnetic field intensity is changed from 0 to 3 Oe, so the highest sensitivity is in the 0.770.9 Oe field range, when the curve slope achieves a value of 120 kHz/Oe. The repetition frequency of the driving pulse is 200-300 Hz, so the time of one measuring period is 3-5 ms. One advantage of the sensor is its ability to operate in the cryogenic temperature range, because the amorphous ribbon keeps its magnetic field sensitivity down to liquid helium temperature. For example, this sensor may be used for the measurement of an integral coefficient of magnetic field screening in a high-temperature superconducting cylinder.
3. Vibrating Sensor The principle of operation of the second type of sensor is the measuring of stray magnetic field near one end of the amorphous ribbon by the vibrating Hall sensor. This field
is determined by the magnetic moment of the ribbon, consequently the sensor measures its magnetization. The latter depends on the external magnetic field directed along the ribbon. So the amplitude of the Hall sensor’s alternating signal will be determined by the external magnetic field projection on the longitudinal ribbon axis. The magnetic field near the sensitive ribbon edge gives the main contribution to the output signal. So this sensor may be used to measure the local distribution of the magnetic field. The sensor’s construction consists of two small 2 mm x 2 mm x 60 mm bars. On the remote ends of the bars there is a piece of amorphous ribbon and a Hall sensor. They are fixed in opposite directions to one another. The nearer ends of the bars are connected to the vibrator. The vibrator is made as a cylindrical piezoelectric ceramic ring with a transverse gap. A IO-20 V a.c. voltage with the resonance frequency of the mechanical system is applied to the external and internal sides of the ring. The vibrator is situated in an aluminium box to screen the noise. There are some connectors on the box to supply power to the vibrator and the Hall sensor, and to take out the sensor’s output signal. The latter is processed by a lock-in nanovoltmeter. The amorphous ribbon and Hall sensor are 50 mm from the box due to the bars. This allows the local magnetic field distribution to be measured in a poorly accessible space. The output characteristic of the vibration sensor is the dependence of the a.c. Hall signal on the ribbon projected field intensity. The conventional form of such a dependence is shown in Fig. 2. According to this diagram, the output curve has a considerable linear part in the magnetic field range from -7 to +7 Oe with a slope of about 10 pV/Oe. This allows a sensitivity of about I mOe to be achieved with a non-linearity better than 1% in a wide magnetic field range for the phase detector time constant of about 1 s. An important advantage of this sensor is the possibility to operate in the cryogenic temperature range, because the Hall sensor
761
““““““” -5
magnetic
[ 0
field,
5 Oe
I
10
O-5 4
-4
-3
-2
-1
magnetic
0I
2 field,
3
4
I
5
Oe
Fig. 2. The output of the vibration sensor vs. external magnetic field.
Fig. 3. D.c. field detection characteristic of the resistive sensor at two different frequencies.
can operate at a temperature essentially below liquid nitrogen temperature. In such a way only the bars with the amorphous ribbon and Hall sensor situated on their ends are cooled. For example, the authors used the vibrating sensor to measure the coefficient of magnetic field screening in a high-temperature superconducting cylinder and for a high-temperature superconductor’s surface topography. The measuring temperature was about 78 K.
200 kHz, the curve has its maximum value in zero magnetic field with a monotonic decrease as the absolute value of the field increases. At the same time, when the frequency is 2 MHz the curve has a minimum at zero magnetic field. Then the impedance increases quickly (slope z 40 Q/Oe) with increasing absolute value of the field; it then achieves its maximum with a subsequent monotonic decrease. When the generator frequency is 2 MHz the output characteristic in a zero magnetic field is of great interest. Such sensors may have a broad application as simple sensitive magnetic field zero-indicators. There is also the possibility of developing a compensation magnetometer with a broad dynamic range and high sensitivity if the above-mentioned zero sensor is inserted in the feedback. It should be noted as a conclusion that the sensor output characteristics greatly depend upon the ribbon’s mechanical stress and the d.c. passing through the ribbon. Even slight ribbon stress may cause the failure in zero field to disappear. Besides that, the output characteristic becomes asymmetrical when d.c. passes through the ribbon.
4. Resistive Sensor The principle of operation of the third type of sensor is the measurement of the amorphous ribbon impedance at high frequencies, which is dependent on the applied magnetic field intensity. A constant-amplitude a.c. goes through the ribbon. The high-frequency voltage formed on the ribbon ends is measured with a conventional a.c. millivoltmeter, used to determine the amorphous ribbon impedance. The sensor consists of a piece of amorphous ribbon 50-80 mm long and a load resistance to stabilize the high-frequency current, all situated in the non-magnetic box. A high-frequency connector is fixed on the box to connect the sensor to the millivoltmeter. The dependence of the amorphous ribbon impedance on the measured field intensity is the output sensor characteristic. This relation is presented in Fig. 3. It is clear that the form of the curve is strongly dependent on the current frequency. When the frequency is
5. Conclusions The resonant, vibrating and resistive magnetic field sensors described here are based on amorphous magnetic ribbons. They have sufficiently high sensitivity for a broad temperature range. They may be used to measure
762
integral or local field distribution. They may be the base for developing secondary devices to measure current, pressure, length, etc. All the sensors may be used to study high-temperature superconductors in the liquid nitrogen temperature range.
References I S. Takeuchi and K. Harada, A resonant-type amorphous ribbon magnetometer driven by an operational
amplifier. IEEE Trans. Msg., MAC-20 ( 1984) 17231725. 2 K. Mohri, S. Shigematsu and K. Yoshino, On flux reversal in amorphous-core multivibrator bridge circuits for sensors. IEEE Trans. Msg. Jpn., TJMJ-2 (1987) 556-557. 3 K. Mohri, T. Kondo and J. Yamasaki, Quick re-
sponse magnetometers using amorphous wire cores and mechanocardiographs, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals, Elsevier, Amsterdam, 1985, pp. 1659-1662. 4 K. Reinitz and L. Hart, Magnetometer with a solidstate magnetic-field sensing means, U.S. Patent No. PN 4 517 515
( 1985).