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Stable High-Tc rf SQUID NDE system operating in noisy environment
Physica C 436 (2006) 77–80 www.elsevier.com/locate/physc
Stable High-Tc rf SQUID NDE system operating in noisy environment D.F. He a
a,*
, M. Yoshizawa b, H. Itozaki
a
Superconducting Materials Center, National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan b Iwate University, Ueda 4-3-5, Morioka 020-8551, Japan Received 18 October 2005; received in revised form 25 January 2006; accepted 29 January 2006 Available online 10 March 2006
Abstract A high-Tc rf SQUID NDE system was developed for stable, long time operation in a noisy environment. To reduce the influence of high frequency electromagnetic interference (EMI), a carefully designed metal shielding made of 0.1 mm thick metal foil was used. With the metal shielding, the SQUID could still operate well when a cellular phone was used nearby although the cellular phone could produce very strong EMI noises. To reduce the influence of the line interference or low frequency noises, a compensation method was used in the SQUID-based NDE system. With the compensation, the low frequency and the line interference could be reduced well. Using this SQUID-based NDE system, we successfully performed the NDE measurements in a noisy environment. 2006 Elsevier B.V. All rights reserved. PACS: 85.25.Dq; 81.70.Ex Keywords: SQUID; Electromagnetic interference; Eddy-current NDE
1. Introduction For SQUID (superconducting quantum interference device) applications, such as NDE (nondestructive evaluation), the experiment is generally performed in an open environment: airport or factory. A SQUID system with high stability, which can operate well in a noisy environment, is needed. There are normally two kinds of strong noises or interferences in an open environment, which may strongly influence the operation of SQUID. One is the high frequency EMI over 1 MHz, which is produced by AM broadcast, FM broadcast, TV and cellular phone; another is the low frequency noise, which is produced by some devices nearby, such as the line interferences or a car passing in a close proximity.
*
Corresponding author. Tel.: +81 29 859 2356; fax: +81 29 895 2301. E-mail address: [email protected] (D.F. He).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.01.044
For the low frequency noise, if the amplitude exceeds the dynamic range of a SQUID, the SQUID cannot be kept locked, so the low frequency noise should be compensated. High frequency EMI, especially radio frequency EMI (RFI), can seriously affect the operations of SQUIDs. Ishikawa et al. [1] and Koch et al. [2] discussed the effect of EMI on dc SQUIDs and found that EMI of sufficient intensity can reduce the modulation depth of dc SQUID and induce a low frequency noise. For rf SQUID, the situation may be different. The tank circuit of rf SQUID is just like an antenna, which receives the EMI and couples it to rf SQUID sensor and rf SQUID readout electronics. Mu¨ck et al. [3] investigated the effects of EMI in high frequency and ultrahigh-frequency range on various types of rf SQUIDs. EMI of sufficient field strength reduces the voltage versus flux transfer function, and thus increases the flux noise of the rf SQUIDs. Therefore, the high frequency EMI should be shielded. Using high-Tc rf SQUID, we developed a SQUID-based NDE system. For the stable operation of SQUID-based NDE system in noisy environment, the high frequency
78
D.F. He et al. / Physica C 436 (2006) 77–80
EMI should be shielded and the low frequency noise should be compensated. 2. Metal shielding for SQUID Metal shielding is an effective way to reduce the influence of high frequency EMI. However, discontinuities in the shield will cause the problem of EMI leakage. To reduce the leakage of EMI, we carefully designed a metal shielding for the rf SQUID. Fig. 1 shows the design of the metal shielding for the high-Tc rf SQUID magnetometer. We used a copper foil of 0.1 mm thickness to construct the shielding surrounding the cryostat and the SQUID holder. In order to fix the SQUID readout electronics with the cryostat easily, we used aluminum plates of 1 mm thickness to shield the SQUID readout electronics. A second AL plate of 2 mm thickness was placed between the cryostat and the SQUID readout electronics. The shielding frequency was over 100 kHz. The excitation frequency of SQUID-based eddy-current NDE was normally below 10 kHz, so the shielding had little influence on the NDE signals. To reduce the leakage of EMI, the cooper foils were soldered together at the positions of connection; the aluminum shielding for the SQUID readout electronics was fixed tightly with the cryostat using copper screws. Ground was also very important for the shielding. The copper foils and the aluminum plates were connected to the ground for the SQUID readout electronics. We measured the shielding effectiveness (SE), defined as the ratio of the strength of the EMI field outside the shield to that of the field inside the shield. To produce the EMI, a dipole antenna was connected to a rf signal generator and was put 2 m away from the shielding. The power of EMI was 20 dBm and the frequency was varied in a range from 1 MHz to 1 GHz. Another short dipole was put at the position of SQUID as the receiver. It was connected to a spectrum analyzer after an amplifier of 20 dB. After measuring the power of EMI at the position of cryostat with and with-
Fig. 1. The metallic shielding for the high-Tc rf SQUID to reduce the influence of high frequency EMI.
Fig. 2. The shielding effectiveness of the metal shielding for different EMI frequencies.
out shielding, we could calculate the shielding effectiveness. Fig. 2 shows the shielding effectiveness as the function of the EMI frequency. The shielding effectiveness of EMI was about 22 dB for 1 MHz, 41 dB for 10 MHz, 63 dB for 100 MHz and 80 dB for 1 GHz. 3. Compensation method for SQUID Using a compensation method, we earlier developed the mobile high-Tc SQUID [4]. For the mobile SQUID, the low frequency field was compensated, and the high frequency signal was not influenced, so the mobile SQUID could be kept locked when moving in the earth’s magnetic field. This compensation method could also be used to reduce the influence of the low frequency environmental noises. Fig. 3 shows the setup of the compensation method. A 20-turn compensation coil made of 0.5 mm copper wire was wound outside of the cryostat with the diameter of about 6 cm. The SQUID output signal was sent to the compensation circuit, then to the compensation coil. A low pass filter was included in the compensation circuit. The dc and low frequency signals were feedback to the SQUID, thus canceling the signals of the SQUID output. The cut off
Fig. 3. The compensation method to reduce the influence of the low frequency environmental noise.
D.F. He et al. / Physica C 436 (2006) 77–80
79
frequency of the compensation could be adjusted from 0 Hz to 1 kHz. To supply a higher current to the compensation coil, a power amplifier was also included in the compensation circuit. The maximum output current of the power amplifier was about 200 mA. Low frequency magnetic fields as large as 80 lT could be compensated using this compensation circuit. Choosing the cut off frequency of the compensation circuit above 50 Hz, the strong line interference could also be compensated. 4. SQUID-based NDE system with high stability Using the metal shielding and the compensation method, we developed a high-Tc rf SQUID NDE system with a high stability. Fig. 4 shows the setup. The YBCO high-Tc rf SQUID with step-edge junction was made by Ju¨lich Research Center, Germany. The washer size was 3.5 mm with an inner loop of 100 lm · 100 lm, while the magnetic field sensitivity was about 300 fT/Hz at 10 Hz. The LC tank circuit had a resonant frequency of about 812 MHz [5]. Without rf shielding, the high-Tc rf SQUID could not work in our laboratory. But with the rf shielding, a satisfactory operation was obtained. Using the dipole antenna and the rf generator, we checked the operation of the SQUID for the EMI within the frequency range from 10 MHz to 1 GHz. For an EMI power at the position of SQUID below 40 dBm, the SQUID signal and the noise level were not influenced by the EMI. When the EMI power was about 30 dBm, the white flux noise of the rf SQUID was enhanced about 10%. Normally, the power of EMI radiation is below 40 dBm, so that it has no influence on our SQUID system with metal shielding. Cellular phone used in close proximity can produce strong EMI and may thus seriously influence the operation of the SQUID. The frequency of cellular phones in Japan is about 800 MHz, which is close to the resonant frequency of the tank circuit, thus strongly influencing the operation of SQUID readout electronics.
Fig. 5. (a) The aluminum plate with several holes on it. (b) The 2D graph of the scanning results obtained by the SQUID NDE system.
For the SQUID without metal shielding or with insufficient shielding, the SQUID could not work at all when a cellular phone was used several meters away from SQUID. For our SQUID system, a stable operation was observed when the cellular phone was used 2 m away from the SQUID. However, if the distance was smaller than 50 cm, the rf SQUID could not be kept locked. To check the compensation circuit, we tested our SQUID NDE system in a noisy laboratory, where a lot of experimental devices were working. Some persons used cellular sometimes during the experiments. The noises in this laboratory were very strong. The amplitude of the 50 Hz line interference was about 300 nT. Without the compensation, the SQUID could be kept locked, however, it was difficult to observe the NDE signals. With the compensation, the 50 Hz line interference was reduced to 3 nT, and the defect could be observed by eddy-current NDE. For this eddy-current NDE measurement, we used a planar double-D excitation coil with the diameter of about 5 mm. The distance between the excitation coil and the SQUID was about 6 mm and the distance between the excitation coil and the sample was about 2 mm. The sine output of the lock-in amplifier was sent to the excitation coil to produce the excitation field with the current amplitude of about 10 mA and the frequency of 320 Hz. The sample was an aluminum plate of 10 cm · 30 cm · 1 mm with several holes on it. The sizes of the holes varied from 1 to 9 mm and the holes were covered by another aluminum plate of 2 mm thickness. Fig. 5(a) shows the sample and Fig. 5(b) shows the 2D graph of the scanning results. It can be seen that the hole defects could be successfully detected even in such a noisy environment. 5. Summary
Fig. 4. The setup of the high-Tc rf SQUID eddy-current NDE system with high stability.
Using shielding based on metal foils to reduce the influence of the high frequency EMI and using a compensation method to reduce the influence of low frequency noise, we developed a stable high-Tc rf SQUID NDE system, which
80
D.F. He et al. / Physica C 436 (2006) 77–80
could operate well, even in a noisy environment. As an example, the artificial defects in an aluminum plate could be detected in the noisy environment. Acknowledgement The work was supported by Japan Science and Technology Corporation under CREATE (Collaboration of Regional Entities for the Advancement of Technical Excellence) Iwate on ‘‘Development of practical applications of magnetic field technology for use in the region and in everyday living’’.
References [1] N. Ishikawa, K. Nagata, H. Sato, N. Kasai, S. Kiryu, IEEE Trans. Appl. Supercond. AS-3 (1993) 1910. [2] R.H. Koch, V. Foglietti, J.R. Rozen, K.G. Stawiasz, M.B. Ketchen, D.K. Lathrop, J.Z. Sun, W.J. Gallagher, Appl. Phys. Lett. 65 (1994) 100. [3] M. Mu¨ck, J. Dechert, J. Gail, M. Kreutzbruck, S. Scho¨ne, R. Weidl, Rev. Sci. Instrum. 66 (1995) 4690. [4] D.F. He, M. Daibo, M. Yoshizawa, IEEE Trans. Appl. Supercond. 13 (2003) 200. [5] D.F. He, X.H. Zeng, H.-J. Krause, H. Solter, F. Ruders, Y. Zhang, Appl. Phys. Lett. 72 (1998) 969.
Stable High-Tc rf SQUID NDE system operating in noisy environment D.F. He a
a,*
, M. Yoshizawa b, H. Itozaki
a
Superconducting Materials Center, National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan b Iwate University, Ueda 4-3-5, Morioka 020-8551, Japan Received 18 October 2005; received in revised form 25 January 2006; accepted 29 January 2006 Available online 10 March 2006
Abstract A high-Tc rf SQUID NDE system was developed for stable, long time operation in a noisy environment. To reduce the influence of high frequency electromagnetic interference (EMI), a carefully designed metal shielding made of 0.1 mm thick metal foil was used. With the metal shielding, the SQUID could still operate well when a cellular phone was used nearby although the cellular phone could produce very strong EMI noises. To reduce the influence of the line interference or low frequency noises, a compensation method was used in the SQUID-based NDE system. With the compensation, the low frequency and the line interference could be reduced well. Using this SQUID-based NDE system, we successfully performed the NDE measurements in a noisy environment. 2006 Elsevier B.V. All rights reserved. PACS: 85.25.Dq; 81.70.Ex Keywords: SQUID; Electromagnetic interference; Eddy-current NDE
1. Introduction For SQUID (superconducting quantum interference device) applications, such as NDE (nondestructive evaluation), the experiment is generally performed in an open environment: airport or factory. A SQUID system with high stability, which can operate well in a noisy environment, is needed. There are normally two kinds of strong noises or interferences in an open environment, which may strongly influence the operation of SQUID. One is the high frequency EMI over 1 MHz, which is produced by AM broadcast, FM broadcast, TV and cellular phone; another is the low frequency noise, which is produced by some devices nearby, such as the line interferences or a car passing in a close proximity.
*
Corresponding author. Tel.: +81 29 859 2356; fax: +81 29 895 2301. E-mail address: [email protected] (D.F. He).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.01.044
For the low frequency noise, if the amplitude exceeds the dynamic range of a SQUID, the SQUID cannot be kept locked, so the low frequency noise should be compensated. High frequency EMI, especially radio frequency EMI (RFI), can seriously affect the operations of SQUIDs. Ishikawa et al. [1] and Koch et al. [2] discussed the effect of EMI on dc SQUIDs and found that EMI of sufficient intensity can reduce the modulation depth of dc SQUID and induce a low frequency noise. For rf SQUID, the situation may be different. The tank circuit of rf SQUID is just like an antenna, which receives the EMI and couples it to rf SQUID sensor and rf SQUID readout electronics. Mu¨ck et al. [3] investigated the effects of EMI in high frequency and ultrahigh-frequency range on various types of rf SQUIDs. EMI of sufficient field strength reduces the voltage versus flux transfer function, and thus increases the flux noise of the rf SQUIDs. Therefore, the high frequency EMI should be shielded. Using high-Tc rf SQUID, we developed a SQUID-based NDE system. For the stable operation of SQUID-based NDE system in noisy environment, the high frequency
78
D.F. He et al. / Physica C 436 (2006) 77–80
EMI should be shielded and the low frequency noise should be compensated. 2. Metal shielding for SQUID Metal shielding is an effective way to reduce the influence of high frequency EMI. However, discontinuities in the shield will cause the problem of EMI leakage. To reduce the leakage of EMI, we carefully designed a metal shielding for the rf SQUID. Fig. 1 shows the design of the metal shielding for the high-Tc rf SQUID magnetometer. We used a copper foil of 0.1 mm thickness to construct the shielding surrounding the cryostat and the SQUID holder. In order to fix the SQUID readout electronics with the cryostat easily, we used aluminum plates of 1 mm thickness to shield the SQUID readout electronics. A second AL plate of 2 mm thickness was placed between the cryostat and the SQUID readout electronics. The shielding frequency was over 100 kHz. The excitation frequency of SQUID-based eddy-current NDE was normally below 10 kHz, so the shielding had little influence on the NDE signals. To reduce the leakage of EMI, the cooper foils were soldered together at the positions of connection; the aluminum shielding for the SQUID readout electronics was fixed tightly with the cryostat using copper screws. Ground was also very important for the shielding. The copper foils and the aluminum plates were connected to the ground for the SQUID readout electronics. We measured the shielding effectiveness (SE), defined as the ratio of the strength of the EMI field outside the shield to that of the field inside the shield. To produce the EMI, a dipole antenna was connected to a rf signal generator and was put 2 m away from the shielding. The power of EMI was 20 dBm and the frequency was varied in a range from 1 MHz to 1 GHz. Another short dipole was put at the position of SQUID as the receiver. It was connected to a spectrum analyzer after an amplifier of 20 dB. After measuring the power of EMI at the position of cryostat with and with-
Fig. 1. The metallic shielding for the high-Tc rf SQUID to reduce the influence of high frequency EMI.
Fig. 2. The shielding effectiveness of the metal shielding for different EMI frequencies.
out shielding, we could calculate the shielding effectiveness. Fig. 2 shows the shielding effectiveness as the function of the EMI frequency. The shielding effectiveness of EMI was about 22 dB for 1 MHz, 41 dB for 10 MHz, 63 dB for 100 MHz and 80 dB for 1 GHz. 3. Compensation method for SQUID Using a compensation method, we earlier developed the mobile high-Tc SQUID [4]. For the mobile SQUID, the low frequency field was compensated, and the high frequency signal was not influenced, so the mobile SQUID could be kept locked when moving in the earth’s magnetic field. This compensation method could also be used to reduce the influence of the low frequency environmental noises. Fig. 3 shows the setup of the compensation method. A 20-turn compensation coil made of 0.5 mm copper wire was wound outside of the cryostat with the diameter of about 6 cm. The SQUID output signal was sent to the compensation circuit, then to the compensation coil. A low pass filter was included in the compensation circuit. The dc and low frequency signals were feedback to the SQUID, thus canceling the signals of the SQUID output. The cut off
Fig. 3. The compensation method to reduce the influence of the low frequency environmental noise.
D.F. He et al. / Physica C 436 (2006) 77–80
79
frequency of the compensation could be adjusted from 0 Hz to 1 kHz. To supply a higher current to the compensation coil, a power amplifier was also included in the compensation circuit. The maximum output current of the power amplifier was about 200 mA. Low frequency magnetic fields as large as 80 lT could be compensated using this compensation circuit. Choosing the cut off frequency of the compensation circuit above 50 Hz, the strong line interference could also be compensated. 4. SQUID-based NDE system with high stability Using the metal shielding and the compensation method, we developed a high-Tc rf SQUID NDE system with a high stability. Fig. 4 shows the setup. The YBCO high-Tc rf SQUID with step-edge junction was made by Ju¨lich Research Center, Germany. The washer size was 3.5 mm with an inner loop of 100 lm · 100 lm, while the magnetic field sensitivity was about 300 fT/Hz at 10 Hz. The LC tank circuit had a resonant frequency of about 812 MHz [5]. Without rf shielding, the high-Tc rf SQUID could not work in our laboratory. But with the rf shielding, a satisfactory operation was obtained. Using the dipole antenna and the rf generator, we checked the operation of the SQUID for the EMI within the frequency range from 10 MHz to 1 GHz. For an EMI power at the position of SQUID below 40 dBm, the SQUID signal and the noise level were not influenced by the EMI. When the EMI power was about 30 dBm, the white flux noise of the rf SQUID was enhanced about 10%. Normally, the power of EMI radiation is below 40 dBm, so that it has no influence on our SQUID system with metal shielding. Cellular phone used in close proximity can produce strong EMI and may thus seriously influence the operation of the SQUID. The frequency of cellular phones in Japan is about 800 MHz, which is close to the resonant frequency of the tank circuit, thus strongly influencing the operation of SQUID readout electronics.
Fig. 5. (a) The aluminum plate with several holes on it. (b) The 2D graph of the scanning results obtained by the SQUID NDE system.
For the SQUID without metal shielding or with insufficient shielding, the SQUID could not work at all when a cellular phone was used several meters away from SQUID. For our SQUID system, a stable operation was observed when the cellular phone was used 2 m away from the SQUID. However, if the distance was smaller than 50 cm, the rf SQUID could not be kept locked. To check the compensation circuit, we tested our SQUID NDE system in a noisy laboratory, where a lot of experimental devices were working. Some persons used cellular sometimes during the experiments. The noises in this laboratory were very strong. The amplitude of the 50 Hz line interference was about 300 nT. Without the compensation, the SQUID could be kept locked, however, it was difficult to observe the NDE signals. With the compensation, the 50 Hz line interference was reduced to 3 nT, and the defect could be observed by eddy-current NDE. For this eddy-current NDE measurement, we used a planar double-D excitation coil with the diameter of about 5 mm. The distance between the excitation coil and the SQUID was about 6 mm and the distance between the excitation coil and the sample was about 2 mm. The sine output of the lock-in amplifier was sent to the excitation coil to produce the excitation field with the current amplitude of about 10 mA and the frequency of 320 Hz. The sample was an aluminum plate of 10 cm · 30 cm · 1 mm with several holes on it. The sizes of the holes varied from 1 to 9 mm and the holes were covered by another aluminum plate of 2 mm thickness. Fig. 5(a) shows the sample and Fig. 5(b) shows the 2D graph of the scanning results. It can be seen that the hole defects could be successfully detected even in such a noisy environment. 5. Summary
Fig. 4. The setup of the high-Tc rf SQUID eddy-current NDE system with high stability.
Using shielding based on metal foils to reduce the influence of the high frequency EMI and using a compensation method to reduce the influence of low frequency noise, we developed a stable high-Tc rf SQUID NDE system, which
80
D.F. He et al. / Physica C 436 (2006) 77–80
could operate well, even in a noisy environment. As an example, the artificial defects in an aluminum plate could be detected in the noisy environment. Acknowledgement The work was supported by Japan Science and Technology Corporation under CREATE (Collaboration of Regional Entities for the Advancement of Technical Excellence) Iwate on ‘‘Development of practical applications of magnetic field technology for use in the region and in everyday living’’.
References [1] N. Ishikawa, K. Nagata, H. Sato, N. Kasai, S. Kiryu, IEEE Trans. Appl. Supercond. AS-3 (1993) 1910. [2] R.H. Koch, V. Foglietti, J.R. Rozen, K.G. Stawiasz, M.B. Ketchen, D.K. Lathrop, J.Z. Sun, W.J. Gallagher, Appl. Phys. Lett. 65 (1994) 100. [3] M. Mu¨ck, J. Dechert, J. Gail, M. Kreutzbruck, S. Scho¨ne, R. Weidl, Rev. Sci. Instrum. 66 (1995) 4690. [4] D.F. He, M. Daibo, M. Yoshizawa, IEEE Trans. Appl. Supercond. 13 (2003) 200. [5] D.F. He, X.H. Zeng, H.-J. Krause, H. Solter, F. Ruders, Y. Zhang, Appl. Phys. Lett. 72 (1998) 969.