Electronic Compass Sensor Robert Racz, Christian Schott, Samuel Huber Sentron, a Melexis Company, Switzerland, , ,
压缩语段Abstract
In this paper, we describe a new two respective three axis integrated IMC Hall-ASIC that can be ud as a single chip electronic compass and applied in portable low cost elec-tronic equipment like wrist watches and mobile phones. The electronic compass chip consists of ring shaped inte-grated ferromagnetic concentrators (IMC), a CMOS inte-grated circuit and an excitation coil manufactured by di-rect bonding onto the chip surface. The IMC Hall ASIC measures the two in-plan magnetic field components Bx and By and the vertical component Bz of the earth mag-netic field. The IMC Hall nsor exhibits an excellent nsi-tivity of 8000V/T for the X and Y axis which yields to an in-plane heading accuracy of +/-1 °, whereas the field in the Z-axis is measured with a lower nsitivity of 800V/T. The high nsitivity characteristics are obtained by integrating magnetic flux concentrators directly on Hall-ASIC, which amplifies the earth magnetic field by a factor of 10. Further more the compass nsors provides a low energy method to eliminate the perming effect (remnant field effect after sub-jecting to strong magnetic fields)
Keywords
Magnetometer, electronic compass, earth magnetic field, Hall nsors, IMC-Hall ASIC, Triaxis-Technology INTRODUCTION
Magnetic nsors have been in u for direction finding or navigation for centuries. The earth magnetic field is a weak three-dimensional field with an intensity of 50-60µT (0.5-0.6 Gauss) that can be approximated by a dipole model. By definition the X-Y component of the field vector lies paral-lel to the earth surface and points towards the magnetic north pole. For a latitude not clo to the equator the major-ity of the earth magnetic field lies along the Z-axis and points at the northern hemisphere into the ground. How-ever, the horizontal direction of the earth field is always pointing toward magnetic north and is ud to determine the compass direction. Becau of this the compass has to be held parallel to the earth’s surface or has to be gimbaled in order to be accurate. Hence, a two-axis magnetometer held parallel to the earth surface is the minimum configuration for magnetic compass heading.
For an electronic compass applied in portable low cost equipment, the magnetic field nsor has to have the follow-ing features: • to measure at least two orthogonal axes X and Y of the earth magnetic field with an accuracy of better than 1%.
• to be innsitive to thermal and magnetic shock ( per-ming)
• to be small in size and have low power consumption • to be mass-manufacturable at low cost.
Today, there are various types of electronic compass available. The most commonly ud magnetic field nsors for compass applications are bad on the magneto resistive (AMR) effect, on the fluxgate effect or on magneto-inductive effect. Becau of the weakness of the earth mag-netic field Hall-effect nsors were hardly applied due to their poor nsitivity.
It is generally believed that Hall nsors are only applicable at magnetic field higher than 0.5mT, and that the measured field must be perpendicular to the nsors surface. On the other hand magneto resistive (AMR) and fluxgate nsors (FG) are considered to be highly nsitive and well adapted for low fields. However, AMR and FG nsors have some drawbacks compared to Hall-nsors. They are either not fully compatible with mass fabrication at low cost or with low power requirements. For example AMR nsors require an integrated t-ret coil for low field DC measurements and are not fully compatible with conventional CMOS tech-nology. Fluxgate nsors were recently fully integrated on single CMOS chip [4] but due to the principle of chopping the magnetic field, the saturation of an open loop core re-quires a significant current not suitable for low power ap-plication.
An effect that aris for magnetic field nsors containing of ferromagnetic material is that the offt
of the nsor changes after exposure to a high magnetic field. This effect is known as perming and can be explained by the remnant field effect of the material. Perming is a critical issue for low field measurements and has to be considered for com-pass applications.
In this paper we prent a concept of a high nsitivity Hall-nsor consisting of CMOS-Hall ASIC and IMC. Standard CMOS technology allows for embedding amplification and noi reduction circuit, the A/D conversion, calibration and interfacing on the same chip as well as for low cost mass production. The IMC functions as passive magnetic field amplifier and dramatically improves the nsors perform-ance.
Figure 1: The IMC locally rotates and amplifies the magnetic field so that it can be measured by stan-
dard Hall elements X1 and X2.
冷漠的意思The focus of this paper is not on low power consumption, but rather on demonstrating that IMC-technology can boost the magnetic nsitivity of CMOS-Hall nsors sufficiently for compass applications. An additional key characteristic of the new compass nsor is that it provides for a low-energy method to virtually eliminate the perming effect. IMC Hall-Sensors
The idea of integrating magnetic flux concentrators directly on Hall-ASICs was first described about 10 years ago [1]. The integrated concentrator converts the external magnetic field parallel with the chip surface locally into a field per-pendicular to the plane, which is nd by conventional planar Hall elements. Figure 1 shows the cross ction of a conventional CMOS-Hall chip combined with a planar magnetic concentrator. The combination of IMC and Hall features higher magnetic nsitivity, lower equivalent offt and offt drift compared to conventional Hall nsors, so that the week earth magnetic field can be measured.
2023春节
The basic structure of the compass-nsor consists of a ring shaped IMC and one pair of Hall elements for each meas-urement axis X and Y. The Hall elements are distributed under the periphery of a ferromagnetic flux concentrator (IMC) where the strongest field concentration appears. Two perpendicular axes X and Y are ud to measure two or-thogonal magnetic field components in the plane (e fig. 2). The two Hall elements along an axis are expod to a field component with opposite orientation, so that subtract-ing the two output voltages yields a output signal propor-tional to the in-plane magnetic field components X and Y. Designating the in-plane direction by an angle α the output voltage of the X-axis is proportional to cos(α), whereas the output for the Y-axis is proportional to sin(α). The vertical component of the field Bz can be measured by adding the output voltages of all Hall elements like for an ordinary Hall nsor without IMC.
If now the ring-shaped IMC is expod to a strong (acciden-tally) magnetic field then the ferromagnetic layer gets mag-netized in the X-Y plane. This diametrical remnant mag-netization prevails even when the accidental field is not prent anymore and leads as conquence to a change of the Hall-output offt voltage.
However, we found an elegant way how to eliminate the remnant diametrical field. The trick is to directly manufac-ture on the ring shaped IMC layer an excitation coil by wire bonding on the chip surf
ace (e fig. 3). The ring shaped IMC behaves as a clod loop magnetic circuit and can be easily circularly magnetized by applying a short current pul I. Circular magnetization corresponds to the lowest energy state and as a conquent no magnetic field lines will leave the magnetic layer at the periphery and no change in offt will be experienced.
Figure 2: SIN-COS compass structure with two Hall elements per each axis (X1-X2 ,Y1-Y2)
α
Figure 3: Elimination of perming effects from external fields by circular magnetization of the IMC ring.
It is an important benefit that the ring shaped IMC features high magnetic amplification and allows for easy magnetiza-tion by an integrated coil with low energy pul.
Applied Technology - IMC Process
The integrated magnetic flux concentrators consist of a high-permeability and very low-coercive-field (very soft) amorphous ferromagnetic layer, which is first glued in form of ribbons onto the wafer containing electronic circuitry and Hall elements and then structured by photolithography and wet-chemical etching. Finally the wafer is cleaned from glue residues. The IMC process ud for the com
pass proto-types (fig. 4) has already been applied on over 300 wafers at Sentron. IMC can be considered as a simple post process applied on completely manufactured CMOS wafers.
龙案
Results
We have realized for the first time a compass nsor en-tirely bad on standard 0.8um CMOS technology, and ad-ditional low-cost IMC post process (e fig. 5). The circuit contains the magnetic nsor front-end and other state of the art electronic circuitry for amplification, offt compensa-tion, filtering etc. The CMOS circuit features an electronic gain of 2500 and a power consumption of 16mA @5V.
The integrated IMC structure consists of five rings. A cen-tral ring with a diameter of 1mm is surrounded by 4 sym-metrically arranged smaller rings with the diameter of 0.5mm (e fig. 6). Such an arrangement allows achieving a high magnetic gain through a long metallic structure, which at the same time features minimum distance to the bonding wires. In this 5-ring IMC structure again each two Hall elements per axis are placed under the center ring concen-trator extremities shown as yellow dots. But in addition we add for each axis two Hall-elements below the surrounding IMC just beneath the air gap shown as white dots. Bonding pads are placed in the center of the IMC rings in order to build an integrated coil for the circular magnetization.
The nsors output characteristic was measured in a Helm-holtz coil by performing a sweep of the m
agnetic field in the range of +/- 400µT. The voltage output is plotted in fig. 7. The magnetic field nsitivity in X and Y is 8000 V/T which includes a passive magnetic gain contribution from the IMC of a factor 10. The Z-axis doesn’t feature magnetic gain from the IMC. The linearity error is 1% for the field range of +/- 150µT.
Figure 4: SEM Photograph of a ring shaped Inte-
grated Magnetic Concentrator (IMC)
Figure 7: Respon of the compass-nsor to
external magnetic field
Figure 5: Layout of the CMOS IMC-Hall compass
nsor
Figure 6: Clod up photograph of the 5-ring
IMC structure.
The horizontal component of the earth magnetic field can be considered to be 20µT. To be able to estimate the achievable resolution we have measured the field equivalent noi versus the integration time. Fig. 8 shows that an inte-gration time of 10ms is required in order to achieve a signal resolution of about 1% of the earth magnetic field.
In order to quantify the effect of magnetic perming, the compass nsor was subjected to a strong magnetic field of > 20mT. The corresponding field error from perming is about 5-6 uT (e fig. 9). After applying a short current pul (25µs) with the amplitude of 20mA, the perming was reduced to 0.2 µT, which corresponds to 1% of the earth magnetic field. The wire-bonded prototype of this coil con-sists of 4 loops. By applying flip chip technology the num-ber of loops can be incread and so the current amplitude will proportionally decrea.
五鹿
Conclusions and outlook
We have prented the first time that a single chip CMOS-Hall ASIC in combination with ring shaped integrated mag-netic concentrator (IMC) can be ud to measure the in plane components of the earth magnetic field with an accu-racy of 1%.
The compass nsor consists of ring shaped integrated fer-romagnetic concentrators; a CMOS integrated circuit and an excitation coil manufactured by direct bonding onto the chip surface and feature a low cost, small size, single chip solution.
The IMC structure amplifies the in plane magnetic field components Bx and By by a factor of 10 and yields a output nsitivity Sx and Sy of 8000V/T. Although the compass nsor measures the 3 axes of the earth magnetic field, the field component Bz doesn’t benefit from the IMC and ex-hibits a nsitivity of 800 V/T.
Perming is virtually eliminated by applying a low energy pul on excitation coil so that the ring shaped IMC is cir-cularly magnetized.
旷工Next steps will be to increa the signal quality of the z-axis in order to make the performances similar to X and Y, and to adapt the circuit for low power consumption. REFERENCES
[1] Patent application EP772046W
[2] R.S. Popovic, P.M. Drljaca, C. Schott, R. Racz, "Inte-
grated Hall Sensor / Flux Concentrator Microsystems", Invited Lecture, 37th International Conference On Mi-
croelectronics, Devices And Materials, MIDEM 01,
Bohinj, Slovenia, October 2001
[3] R. S. Popovic, R. Racz, C. Schott, “A new CMOS Hall
angular Position nsor”, tm – Technisches Mesn, 68, June 2001, pp. 286-291.
[4] P.M. Drljaca, P, Kejik, B. Janossy and R.S.Popovic, “
Low noi CMOS Micro-fluxgate Magnetometer”,
Transducers 2003, 12th international conference on
胖头鱼炖豆腐
solid state nsors, actuators and microsystems, Bos-
ton, June 8-12, 2003
Figure 8: Field equivalent noi vs. integration
time
告知
Figure 9: Field equivalent perming effect vs. cir-cular magnetization current.
