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Setup and capability of CMOS Hall sensor arrays
Sensors and Actuators A 129 (2006) 100–102
Setup and capability of CMOS Hall sensor arrays Heinrich Gr¨uger ∗ , Uwe Vogel, Steffen Ulbricht Fraunhofer Institut Photonische Mikrosysteme, Grenzstr. 28, 01109 Dresden, Germany Received 23 September 2005
Abstract Hall sensors have been established long ago. Single sensor elements can be purchased from many different suppliers. For array sensors interesting applications like position and rotation detection arise. Arrays setup from single sensors in surface mounted or chip on board technology serve some tasks. Smaller size and higher position accuracy can only be realized by integrated arrays. First chips have been developed some years ago to a level that demonstrates the principal function. Now, new chips have been designed in silicon on insulator (SOI) technology that provides high sensitivity, low noise and a wide temperature range. The chip is 7.8 mm wide and has 32 sensor elements with 250 m pitch. Random access or successive switching of the sensors is realized as well as selectable sensitivity of the sensors. The analogue output is adjusted to 0–5 V suitable for the requirements of most analog to digital converters (ADCs). Applications like highly accurate linear or rotational position detection can be realized. © 2005 Elsevier B.V. All rights reserved. Keywords: Hall array; System on chip; Position detection; Rotation detection
1. Introduction For many applications non-contact measurements of the linear or angular position are desired. Some examples are the position of linear valves or the angular position of a throttle. Magnetic systems using a permanent magnet and suitable detectors are one possibility. Well established is the measurement of the analogue values of one magnet with two sensors in the x and y direction and the evaluation with a sine/cosine algorithm. The main disadvantage is the mounting technology required. A solution working with only one chip would be much more suitable. Using sensor arrays, the detection of the zero field as a marker of the position is used typically. Higher signals can be achieved if the magnet drives the sensors far from the zero field position into saturation. In these applications the field of a permanent magnet has to be measured always. Disturbances may occur from the earth magnetic field with a value in the range of ±45 T. The magnet should deliver a field at the sensor position in a way that one-tenth of the disturbance (10 T) is in the range of the sensor noise or ADC lowest significant bit (LSB). For 10 bit resolution
∗
Corresponding author. Tel.: +49 351 8823 155; fax: +49 351 8823 266. E-mail address: [email protected] (H. Gr¨uger).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.11.024
this means 10 mT field strength, for 12 bit the field should be 40 mT. This way, the sensor target specifications can be set. 2. Integrated Hall sensors The required full scale in the range of 10–50 mT can be measured by different types of sensors. Integrated planar fluxgate sensors [1] offer a sensitivity that results in saturation very close to the zero field position. Thus only one or two sensors may not be saturated and a useful regression is impossible. Magneto resistive (MR) sensors offer the accurate sensitivity but are not CMOS compatible. Thus great problems arise for the system design because the multiplexing, amplification and readout circuitry should be integrated on chip, which in turn is impossible. The best choice are Hall sensors, which reveal sufficient sensitivity and CMOS compatible production capabilities [2,3]. The Hall effect is correlated to the Lorentzian force f = v × B on every moving charges in a magnetic field. For electrons in a conductive tape this leads to a charge transfer induced potential at the sides of the tape. It can be measured and correlated to the field strength if current and materials properties are known. The evaluation of the Hall effect is also possible in integrated silicon devices. Some specific problems occur in silicon Hall sensors. If the silicon crystal is stressed, piezo electric charges
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grated on chip the width varies. The chip design offers the possibility to adjust several chips side by side to enhance the measurement range. Using a driving current of 300 A the signal for 100 mT selected as maximum full scale range is 6.6 mV. To adjust this to an output signal span of 4 V (valid 0.5–4.5 V) an amplification of 333× is required. For more options an adjustable amplification of 333×, 666×, 1666× and 3333× has been realized. The principal chip setup is shown in Fig. 2. 4. Capabilities of the sensor
Fig. 1. Chip picture of a spinning current Hall sensor.
are induced which are well above the Hall signals. This problem can be overcome using spinning current Hall sensors. The layout (Fig. 1) is designed in a way that current and signal directions can be switched. The magnetic signal is accumulated along four or eight directions but the piezo signal in plane is compensated this way. Further improvements can be achieved by the use of silicon on insulator (SOI) technology. Only a thin silicon layer insulated by an oxide layer is used instead of the whole silicon wafer. This leads to a higher current in the active Hall element and to a reduction of the piezo effect. SOI technology is for example well established for the dark current suppression in infrared photo detector. SOI Hall elements are available by different foundry services. A process has been chosen which offers 1 m process resolution, 5–12 V capability and the combination with CMOS circuitry. Here 250 nm device silicon on 1000 m buried oxide and 25 nm gate oxide have been used. The Hall element available in the process library reveals a sensitivity of 205 V/AT. 3. Hall array chip setup
The signal of the sensors is amplified by a three stage amplifier. The amplification of the first and third stage can be adjusted. Over all 2 bit information is used to select four different amplifications. Each sensor can either be addressed through a 5 bit address bus or the 32 sensors are read out in a loop with increasing number. In this case, the address bus serves to indicate the actual sensor number. The signal generation can be started using a “conversion start” pin. As soon as the signal is ready, the “data ready” pin is switched. The data rate is at 1 kHz or more for all 32 sensors. To use several chips in one system a chip enable pin has been designed, so a controller can select one of several chips to operate. 5. Applications One important application is the detection of linear movement, for example in valves. The position is typically detected by the zero field position of a magnet mounted to the valve. In a test system, eight neighboring sensors have been read out. A linear regression across the eight output signals is used to estimate the exact position of the zero field value (Fig. 3). A position accuracy below 1 m is possible using 12 bit sensor values. With the same principle, a rotational detection is possible [4,5]. A special magnet, diametrically magnetized, is used to encode position (Fig. 4). Again the zero field value is detected by a linear regression. The angular position can be measured
The chip design integrates 32 Hall elements with 250 m pitch in a chip 7.8 mm long. Depending on the electronics inte-
Fig. 2. Design of the linear Hall array.
Fig. 3. Linear regression using eight sensor signals.
102
H. Gr¨uger et al. / Sensors and Actuators A 129 (2006) 100–102
whole system. Easier mounting technology and the use of more simple magnets ensure high reliability. The algorithms used to calculate position information from the sensor signals is a very simple linear regression, so compared to the typical sine/cosine algorithms less calculation power is required. In the future 2D arrays may serve applications like pattern recognition of magnetic or magnetized objects like coins. The integration of processor or interface driving circuitry may be realized for high volume products. References
Fig. 4. Diametrically magnetized cylindrical magnet used to encode angular position.
in a 90◦ range with 0.1◦ resolution. An absolute position signal is available at once after power up. A 360◦ capability can be realized with additional sensor elements. Other applications need a fast parallel measurement of high magnetic fields, for example in the quality management of the high field magnet production. 6. Summary and discussion A new setup of an array of Hall sensors has been realized. High end SOI sensors technology results in favorable sensor properties. The setup may serve applications like linear or angular position detection or parallel magnetic field measurements. Driving and readout electronics can be integrated on chip. This system has great advantages concerning the size and effort for the
[1] H. Gr¨uger, R. Gottfried-Gottfried, Performance and applications of a two axes fluxgate magnetic field sensor fabricated in a CMOS process, Sens. Actuators A 2909 (2001). [2] R. Steiner, Ch. Maier, A. Haberli, F. Steiner, H. Baltes, Offset reduction by continuous spinning current method, Sens. Actuators A 66 (1998). [3] R. Popovic, Not-plate-like Hall magnetic sensors and their applications, Sens. Actuators A Phys. 85 (2000) 9–17. [4] H. Gr¨uger, H. Lakner, High resolution absolute angular position detection with single chip capability, in: S. Kr¨uger, W. Gessner (Eds.), Advanced Microsystems for Automotive Applications, Springer Verlag, Berlin, 2002, pp. 222–226. [5] H. Gr¨uger, R. Gottfried-Gottfried, Drehwinkelsensor, EP 1 362 221 B1/WO 2002/068911 (2002).
Biography ¨ Heinrich Gruger was born in W¨urzburg, Germany in 1968. He studied physics at the University of Hannover where he specialized in plasma physics. From 1994 to 1998 he was employed by the Institute of Solid State and Materials Research (IFW) Dresden and the Technical University Dresden. He obtained his PhD in 1999 from the Technical University Dresden with a thesis on the plasma assisted deposition of carbon nitride thin films. In 1999 he was employed by the Fraunhofer-Institute Photonic Microsystems (IPMS) at Dresden, where he is working in the field of sensors. Since 2000 he is group manager and responsible for sensor system developments.
Setup and capability of CMOS Hall sensor arrays Heinrich Gr¨uger ∗ , Uwe Vogel, Steffen Ulbricht Fraunhofer Institut Photonische Mikrosysteme, Grenzstr. 28, 01109 Dresden, Germany Received 23 September 2005
Abstract Hall sensors have been established long ago. Single sensor elements can be purchased from many different suppliers. For array sensors interesting applications like position and rotation detection arise. Arrays setup from single sensors in surface mounted or chip on board technology serve some tasks. Smaller size and higher position accuracy can only be realized by integrated arrays. First chips have been developed some years ago to a level that demonstrates the principal function. Now, new chips have been designed in silicon on insulator (SOI) technology that provides high sensitivity, low noise and a wide temperature range. The chip is 7.8 mm wide and has 32 sensor elements with 250 m pitch. Random access or successive switching of the sensors is realized as well as selectable sensitivity of the sensors. The analogue output is adjusted to 0–5 V suitable for the requirements of most analog to digital converters (ADCs). Applications like highly accurate linear or rotational position detection can be realized. © 2005 Elsevier B.V. All rights reserved. Keywords: Hall array; System on chip; Position detection; Rotation detection
1. Introduction For many applications non-contact measurements of the linear or angular position are desired. Some examples are the position of linear valves or the angular position of a throttle. Magnetic systems using a permanent magnet and suitable detectors are one possibility. Well established is the measurement of the analogue values of one magnet with two sensors in the x and y direction and the evaluation with a sine/cosine algorithm. The main disadvantage is the mounting technology required. A solution working with only one chip would be much more suitable. Using sensor arrays, the detection of the zero field as a marker of the position is used typically. Higher signals can be achieved if the magnet drives the sensors far from the zero field position into saturation. In these applications the field of a permanent magnet has to be measured always. Disturbances may occur from the earth magnetic field with a value in the range of ±45 T. The magnet should deliver a field at the sensor position in a way that one-tenth of the disturbance (10 T) is in the range of the sensor noise or ADC lowest significant bit (LSB). For 10 bit resolution
∗
Corresponding author. Tel.: +49 351 8823 155; fax: +49 351 8823 266. E-mail address: [email protected] (H. Gr¨uger).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.11.024
this means 10 mT field strength, for 12 bit the field should be 40 mT. This way, the sensor target specifications can be set. 2. Integrated Hall sensors The required full scale in the range of 10–50 mT can be measured by different types of sensors. Integrated planar fluxgate sensors [1] offer a sensitivity that results in saturation very close to the zero field position. Thus only one or two sensors may not be saturated and a useful regression is impossible. Magneto resistive (MR) sensors offer the accurate sensitivity but are not CMOS compatible. Thus great problems arise for the system design because the multiplexing, amplification and readout circuitry should be integrated on chip, which in turn is impossible. The best choice are Hall sensors, which reveal sufficient sensitivity and CMOS compatible production capabilities [2,3]. The Hall effect is correlated to the Lorentzian force f = v × B on every moving charges in a magnetic field. For electrons in a conductive tape this leads to a charge transfer induced potential at the sides of the tape. It can be measured and correlated to the field strength if current and materials properties are known. The evaluation of the Hall effect is also possible in integrated silicon devices. Some specific problems occur in silicon Hall sensors. If the silicon crystal is stressed, piezo electric charges
H. Gr¨uger et al. / Sensors and Actuators A 129 (2006) 100–102
101
grated on chip the width varies. The chip design offers the possibility to adjust several chips side by side to enhance the measurement range. Using a driving current of 300 A the signal for 100 mT selected as maximum full scale range is 6.6 mV. To adjust this to an output signal span of 4 V (valid 0.5–4.5 V) an amplification of 333× is required. For more options an adjustable amplification of 333×, 666×, 1666× and 3333× has been realized. The principal chip setup is shown in Fig. 2. 4. Capabilities of the sensor
Fig. 1. Chip picture of a spinning current Hall sensor.
are induced which are well above the Hall signals. This problem can be overcome using spinning current Hall sensors. The layout (Fig. 1) is designed in a way that current and signal directions can be switched. The magnetic signal is accumulated along four or eight directions but the piezo signal in plane is compensated this way. Further improvements can be achieved by the use of silicon on insulator (SOI) technology. Only a thin silicon layer insulated by an oxide layer is used instead of the whole silicon wafer. This leads to a higher current in the active Hall element and to a reduction of the piezo effect. SOI technology is for example well established for the dark current suppression in infrared photo detector. SOI Hall elements are available by different foundry services. A process has been chosen which offers 1 m process resolution, 5–12 V capability and the combination with CMOS circuitry. Here 250 nm device silicon on 1000 m buried oxide and 25 nm gate oxide have been used. The Hall element available in the process library reveals a sensitivity of 205 V/AT. 3. Hall array chip setup
The signal of the sensors is amplified by a three stage amplifier. The amplification of the first and third stage can be adjusted. Over all 2 bit information is used to select four different amplifications. Each sensor can either be addressed through a 5 bit address bus or the 32 sensors are read out in a loop with increasing number. In this case, the address bus serves to indicate the actual sensor number. The signal generation can be started using a “conversion start” pin. As soon as the signal is ready, the “data ready” pin is switched. The data rate is at 1 kHz or more for all 32 sensors. To use several chips in one system a chip enable pin has been designed, so a controller can select one of several chips to operate. 5. Applications One important application is the detection of linear movement, for example in valves. The position is typically detected by the zero field position of a magnet mounted to the valve. In a test system, eight neighboring sensors have been read out. A linear regression across the eight output signals is used to estimate the exact position of the zero field value (Fig. 3). A position accuracy below 1 m is possible using 12 bit sensor values. With the same principle, a rotational detection is possible [4,5]. A special magnet, diametrically magnetized, is used to encode position (Fig. 4). Again the zero field value is detected by a linear regression. The angular position can be measured
The chip design integrates 32 Hall elements with 250 m pitch in a chip 7.8 mm long. Depending on the electronics inte-
Fig. 2. Design of the linear Hall array.
Fig. 3. Linear regression using eight sensor signals.
102
H. Gr¨uger et al. / Sensors and Actuators A 129 (2006) 100–102
whole system. Easier mounting technology and the use of more simple magnets ensure high reliability. The algorithms used to calculate position information from the sensor signals is a very simple linear regression, so compared to the typical sine/cosine algorithms less calculation power is required. In the future 2D arrays may serve applications like pattern recognition of magnetic or magnetized objects like coins. The integration of processor or interface driving circuitry may be realized for high volume products. References
Fig. 4. Diametrically magnetized cylindrical magnet used to encode angular position.
in a 90◦ range with 0.1◦ resolution. An absolute position signal is available at once after power up. A 360◦ capability can be realized with additional sensor elements. Other applications need a fast parallel measurement of high magnetic fields, for example in the quality management of the high field magnet production. 6. Summary and discussion A new setup of an array of Hall sensors has been realized. High end SOI sensors technology results in favorable sensor properties. The setup may serve applications like linear or angular position detection or parallel magnetic field measurements. Driving and readout electronics can be integrated on chip. This system has great advantages concerning the size and effort for the
[1] H. Gr¨uger, R. Gottfried-Gottfried, Performance and applications of a two axes fluxgate magnetic field sensor fabricated in a CMOS process, Sens. Actuators A 2909 (2001). [2] R. Steiner, Ch. Maier, A. Haberli, F. Steiner, H. Baltes, Offset reduction by continuous spinning current method, Sens. Actuators A 66 (1998). [3] R. Popovic, Not-plate-like Hall magnetic sensors and their applications, Sens. Actuators A Phys. 85 (2000) 9–17. [4] H. Gr¨uger, H. Lakner, High resolution absolute angular position detection with single chip capability, in: S. Kr¨uger, W. Gessner (Eds.), Advanced Microsystems for Automotive Applications, Springer Verlag, Berlin, 2002, pp. 222–226. [5] H. Gr¨uger, R. Gottfried-Gottfried, Drehwinkelsensor, EP 1 362 221 B1/WO 2002/068911 (2002).
Biography ¨ Heinrich Gruger was born in W¨urzburg, Germany in 1968. He studied physics at the University of Hannover where he specialized in plasma physics. From 1994 to 1998 he was employed by the Institute of Solid State and Materials Research (IFW) Dresden and the Technical University Dresden. He obtained his PhD in 1999 from the Technical University Dresden with a thesis on the plasma assisted deposition of carbon nitride thin films. In 1999 he was employed by the Fraunhofer-Institute Photonic Microsystems (IPMS) at Dresden, where he is working in the field of sensors. Since 2000 he is group manager and responsible for sensor system developments.