monolithic silicon magnetic compass

747 Sensors and Actuators, A21 -A23 (1990) 747-750

Monolithic Silicon Magnetic Compass

K MAENAKA, M TSJKAHARA and T NAKAMURA

Department of Electncal and Electrome Engmeermg, Toy+ht Unwerstty of Technology, Toyahashr (Japan)

Abstract

In this report, an integrated magnetic sensor whch offers the direction of the applied magnetic field 1s presented l&s integrated sensor contams a two-dlmenslonal magnetic detector which detects the m-plane magnetic vectors, B, and By, and a signal conversion arcmt which calculates the du-ectlon 8 ( =tan- '(BJBJ) usmg the transhnear carcult technology Tl~s integrated magnetic sensor has not only the tire&on output but the output for the mtenslty (the absolute value) of the applied magnetic field The test device was fabncated using the standard bipolar analog IC technology and its charactenstlcs were examined As a result, the &re&on of the apphed magnetic field was obtained Hrlth manmum error of f 2% full-scale deflection

1. IntToduction

There are some applications where a detection of the dlrectlon (or angle) of the applied magnetic field 1s Important, e g , using a magnet as an intermediary, an angle detector for tension arms of a magnetic tape dnver, a slope detector, a detector for the amount of valve opemng, a JOY- stick, etc In such cases, a bndgecoupled MR element may be used [l] However, MR elements have the followmg serious restntions

(a) the vahd hnear regon of the angle detection is narrow ( < looO),

(b) the output signal 1s a double-valued func- tion of the angle In other words, the four- quadrant detection of the angle cannot be achieved

(c) it 1s not evident when the magnet that dnves the MR element 1s damaged by an accident or a secular change, because the output signal of the MR element does not include mformation about the magnetic field intensity

We present a monohtic silicon magnetic compass that detects the dlrectlon (or angle) of the applied magnetic field wthout the above prob- lems This compass 1s a monohthlc IC cncurt

0924-4247/90/$3 50

conslstmg of a two-&mennonal magnetic detector and the signal-processing arcmtry The two- dnnenslonal magnetic detector IS sensitive to the m-plane magnetic field vectors B, and B,, parallel to the chip surface The detector 1s composed of two novel vertical Hall cells [2] located perpen&c- ular to each other The signal-processmg clrcmtry converts the signals B, and B,, from the detector mto an angle fl by means of the analog operation, 8 = tan-‘(B,/B,) When we actually design the cmzmtry, we use the approximate equation,

e 1 578,

= 0 63B, + (0 88B,* + By*) - I’*

for the calculation [3] and the transhnear tech- mque [4,5] for the cucmt In the above approxl- mate equation, the term, (0 88 Bx2 + B,,*) - II2 indicates the approxnnate mtenslty (absolute value) of the magnetic field Our compass pro- duces not only 0 but also this term m order to momtor the intensity of the apphed magnetic field By momtonng ths term, we know If the dnve magnet IS damaged or no magnetic field 1s applied

2. Magnetic Detector

2 I Vertical Hall Cell Figure 1 shows the structure of the vertical

Hall cell (VHC) whose fabncation process 1s fully compatible Hnth the standard bipolar IC process [2,6] The supply current flows from the center current electrode to the two outer current electrodes through the epltaxlal layers and n+ buned layer In the epitaxIa1 layer, Just under the center current electrode, the current flows perpendlcu- larly to the chp surface When the magnetic field IS parallel to the hne of the current electrodes as shown m Fig 1, the Hall voltage appears at Hall electrodes located near the center current electrode This device IS a mod&d version of the device decnbed m ref 2 Mod&d points are

(1) the p-lsolauon d&sion between the current electrodes wluch avoids the lateral current flow m the epltaxlal layer, and

0 Elsemer Sequola/Pnnted m The Netherlands

748

(b)

Fig 1 Structure of the vertical Hall cell

(2) the enlarged n + buned layer which reduces the offset voltage introduced by the alignment error m the photolithographs

The sensltlvlty of the device 1s 41 V/AT wth the noise level of 1 x 10B5 T/,/6 at 40 Hz

2 2 Two-dlmenslonal Magnetic Detector Figure 2 shows the two-dlmenslonal magnetic

detector composed of two vertical Hall cells located at nght angles to each other When the m-plane magnetic vector B IS applied, each Hall cell offers the components of the magnetic field, B, = IBI cos 13 and B,, = IBI sm 8 Since the two Hall cells are closely located to each other, magnetic detection with high spatial resolution 1s achieved In our design, the spatial resolution of 320 pm 1s realized Moreover, since the accuracy of the arrangement of the Hall cells IS determined by the mask pattern, the detection error due to the arrangement error of the Hall cells can be ignored Figure 3 gves expenmental results of the two-

lax= lelcose

Rg 2 Two-dlmenslonal magnetic detector usmg two vertical Hall cells

-6 -

-6 - -10 I I I

0 so 160 270 360

ANGLE (DEGREE )

Rg 3 Charactenstlcs of the two-dlmenslonal rnagnetlc detector

dimensional magnetic detector The magnetic field was rotated around a two-dnnenslonal magnetic detector and the output signals of each Hall cell were measured where the offset voltages were cancelled by the external clrcult and the scnntlvl- ties were equahzed by adjusting the supply cur- rents of the Hall cells The sohd hnes indicate the theoretical value, and the deviation of the measured data from theoretical value 1s less than f 1% In constant current dnve, the temperature- dependence of the sensltlvlty was not detected for the temperature range of - 30” to 70

3. SIgnal-procxdng circuitry

3 1 Operation The reqmred operation of our integrated sen-

sor 1s that 8 = tan - ’ (B,, /B,) To reahzc ths operation, we use the followmg approxlmatc equation

0 = tan- ‘(B,, /B,)

= 1 57B,/(O 63B, + (0 88Bx2 + By’) - “2) (1)

This approximation 1s fairly good and the maxi- mum error is 0 24% for BJB, < 0 and B,, > 0 In eqn (l), the second term of the denommator represents roughly the mtenaty (the absolute value = (B,* + Bv2) ‘12) of the apphed magnetic vector B Using this term, we can momtor whether the magnetic field 1s applied or not so that we can see whether the angle output signal 1s valid or not It 1s clear that this momtonng output 1s important if the situations, B, = B,, = 0 and B, = B,, = 1 T are considered For both situations, the integrated sensor offers 8 = tan - '(B,/B,) = 45”, but the for- mer 1s m an abnormal con&tlon, e g , the dnve magnet 1s damaged

3 2 Reallzatron In order to reahze the operation gven m eqn

( l), the transhnear clrcmt technique (TLC) [4] was

749

Rg 4 Schematic diagram of the l&grated sensor

used The TLC has many advantages for an analog IC design

(1) ability to reahze the hnear and non-linear operation only by transistors,

(2) mput and output signals are at current level, (3) high preclnon, (4) wide dynarmc range, (5) msensltivity to temperature vanations In our integrated sensor, the signal operation

cmrcmt reahzed by TLC technology is a main part of the system In order to connect the TLC (whose input 1s current level) and the Hall cells (whose output signal 1s voltage level), the voltage-to-current converters (V/Z converter) are reqmred The V/Z converters are designed based upon the simple differential-amplifier with current nurrors

The schematic diagram of the integrated sensor is shown m Fig 4 The integrated sensor contams 2 Hall cells, 190 transistors and 40 resistors The external vanable resistors are optionally available for adjusting the operation points of the TLC, the gain of the V/I converters, and the offset voltage and the senslttvltles of the Hall cells The angle output signal is a differential voltage level and the intensity (absolute) output signal 1s a current level The design supply voltage 1s single, 7- 15 V, which is convenient to connect the other analog clrcmtry

4. Experiment

4 1 Fabncatton The integrated magnetic sensor shown m Fig 4

was fabncated using the standard bipolar analog

Fig 5 Photograph of the .&con magnehc compass

IC technology From the vlewpomt of the yield m our laboratory, the system was separated into two IC chips (clup 1 vertical Hall cells and V/I converter part, and ctnp 2 signal-processing part) and these chps were interconnected on the glass epoxy pnnted cucmt board Figure 5 is a photograph of this prmted arcmt board mcludmg two IC chps and additional vanable resistors The IC clup located at the end of the prmted circuit board includes the two-dimensional magnetic detector The sizes of both IC chips are. 4 7 x 4 7 mm* and the size of the prmted ctrcult board mcludmg the vanable resistors is 20 x 113 x 10 mm3 From now on, this pnnted arcmt board system 1s re- ferred to as a magnetic compass

4 2 Characterrstuzs of the Magnetu Compass The charactenstics of the magnetic compass

shown m Fig 5 were exammed

750

..I 0 MAGNETIC

OlRECTlON OF THE APPLIED MAGNETIC FIELD Ldogl

Fig 6 Angle output of the magnetic compass vs dwectlon of the apphed magnetic field

Figure 6 shows the angle output of the compass when the magnetic field of 0 1 T turns around on the compass The hnear output ngnal for the rotation (angle) of the apphed magnetic field was obtained with the maxunum error of less than f 2%/FSD The amphtude (or full scale) of the output signal can be changed by adjusting the vanable resistors

For the lower intensity of the applied magnetic field, the error m the angle detection 1s increased as shown m Fig 7, due to the lower current level m the TLC When it 1s used under the low magnetic field, the gam of the V/Z converter must be increased by adJustmg the vanable resistors For the higher intensity of the magnetic field ( -G 1 T), the circuit and the detector can operate accurately wth an error of less than f 2%

The magnetic intensity output of this compass shows the roughly absolute value of the applied magnetic field Figure 8 shows the magnetic mtenslty output versus the apphed direction of the magnetic field wth the apphed magnetic intensity

8 4-

& 2- z 0 E -2

p -4-

? -6-

5 z -a-

-10 I . *..I . I.._

ld3 1ti2 16’ loo MAGNETIC FLUX DENSITY ( T 1

Rg 7 Error m angle output vs mtenslty of the apphed magnetic field

5T

I A’ --_A_---

180

Fig 8 Intensity output of the magnetic. compass vs dwcuon of the appbed magnetic field wth the mtenslty of the appbed magnetic field as parameters

as parameters m the polar coordinate system As indicated m eqn (l), the accuracy IS rather low and dependent on the dlrectlon of the magnetic field (because there 1s a coefficient 0 88 for only B,) However, it can play an important role for the momtor or the check of the apphed magnetic field

5. Conelu!3ionB

The magnetic compass was reahzed using the standard bipolar IC technology The device con- tam 190 transistors, 40 resistors and 2 vertical Hall cells The maNmum error m detection of the dnectlon of the magnetic field was measured to be +Z%/FSDatB=OlTforti=O”to360

References

1 Catalog, Cat No SGOI4, Murata (Japan) Inc 2 K Maenaka, T Ohgusu, M IsLda and T Nakamura,

Novel vertxal Hall cells m standard b@ar technology, Elecrron Lerr , 23 (1987) 1104-l 105

3 E Seevmck, Srmple, wde-range approxunahons to tngone metnc and inverse tngonometnc functmns useful m real- tune signal processmg, Proc IEEE, 128 (1) (1981)

4 B Gdhert, Translmear cmxut A proposed classification, Elecrron L&r, II (1975) 14-16

5 E Seevmck, Analysts and synthew of translmear integrated circmts, D SC Dwertatlon, Umveraty of FVetona, South Africa, 1981

6 K Macnaka, T Ohgusu, M I&da and T Nakamura, Integrated three&menslonal magnetic sensors, Tram IEE Jpn , 109-C (1989) to be published (m Japan%)

monolithic silicon magnetic compass

Documents