Applications of Magnetoresistive Sensors in
Navigation Systems
Michael J. Caruso
Honeywell Inc.
ABSTRACT
Most navigation systems today u some type of
翘首而望compass to determine heading direction. Using the
earthÕs magnetic field, electronic compass bad on
magnetoresistive (MR) nsors can electrically resolve
better than 0.1 degree rotation. Discussion of a simple 8-
point compass will be described using MR nsors.
Methods for building a one degree compass using MR
nsors will also be discusd. Compensation
techniques are shown to correct for compass tilt angles
and nearby ferrous material disturbances.
INTRODUCTION
The magnetic compass has been ud in
navigation for centuries. The inventor of the compass is
not known, though evidence suggests that the Chine
were using lodestoneÑa magnetic iron oreÑover 2000
years ago to indicate horizontal directions. It appears
that Mediterranean amen of the 12th century were the
first to u a magnetic compass at a [1]. Today, the
balanced needle compass is only a slight variation of this
early discovery. Advances in technology have led to the
莫文蔚的歌曲solid state electronic compass bad on MR magnetic
nsors and acceleration bad tilt nsors. Electronic
compass offer many advantages over conventional
ÒneedleÓ type or gimbaled compass such as: shock
and vibration resistance, electronic compensation for
stray field effects, and direct interface to electronic
navigation systems. Two types of compass will be
discusd in this paperÑa basic eight-point compass
and a one-degree compass.
EARTHÕS MAGNETIC FIELD
The earthÕs magnetic field intensity is about 0.5
to 0.6 gauss and has a component parallel to the earthÕs
surface that always point toward magnetic north. This is
the basis for all magnetic compass. The key words
here are Òparallel to the earthÕs surfaceÓ and Òmagnetic
northÓ.
Hz
Y
¯ = Dip or inclination angle
|Hearth | = Hx 2 + Hy 2 + Hz 2
Figure 2 - EarthÕs Field in X, Y, Z Coordinates
The key to accurately finding a compass
heading, or azimuth, is a two step process: 1) determine the Hx and Hy horizontal components of the earthÕs magnetic field and 2) add or subtract the proper declination angle to correct for true north.
22¡E
20¡E
18¡E 16¡E
14¡E
12¡E
10¡E
8¡E
6¡E
2¡E
0¡
4¡E 2¡W
4¡W
18¡W 16¡W 14¡W 12¡W
10¡W 8¡W 6¡W 20¡W 22¡W Figure 3 - Declination Angle To Correct For True North三年五载的意思
BASICS OF MAGNETIC SENSING
Today, there are veral types of electronic
compass to cho from: fluxgate, magnetoresistive,magnetoinductive, and others. A common type of magnetic compass for navigation systems is the fluxgate nsor. The fluxgate nsor consists of a t of coils around a core with excitation circuitry that is capable of measuring magnetic fields with less than 1 milligauss resolution. The nsors provide a low cost means of magnetic field detection; they also tend to be bulky,somewhat fragile, and have a slow respon time.
Sometimes, fluxgate nsors in motion can have a reading respon time of 2-3 conds. This reading delay may be unacceptable when navigating a high
speed vehicle or an unmanned plane.
Another type of magnetic nsor is the
magnetoresistive (MR) nsor. This nsor is made up of thin strips of permalloy (NiFe magnetic film) who electrical resistance varies with a change in applied magnetic field. The nsors have a well defined axis of nsitivity and are mass produced as an integrated circuit. Recent MR nsors show nsitivities below 0.1milligauss, come in small solid state packages, and have a respon time less than 1 microcond. The MR nsors allow reliable magnetic readings in moving vehicles at rates up to 1,000 times a cond.
A magnetoresistive nsor will be ud in the
remainder of this paper to detect both the sign and magnitude of the earthÕs field as a voltage output. The nsor will also detect any stray field or field anomaly superimpod onto the earthÕs field. The magnetic nsor output will have an X, Y, and Z component referenced to the magnetic nsor, or compass,package. For our navigational reference: the X component will be in the forward looking direction, the Y component to the right, and the Z component will be down as shown in Figure 2.
COMPASS DESIGN
There are many forms of compass ud in
navigation systems. Two forms will be discusd here that u magnetoresistive magnetic nsorsÑthe eight point compass and the one-degree compass.
EIGHT-POINT COMPASSÑA simple eight
point compass depicts the cardinal points (N, S, E, W)and the midway points (NE, NW, SE, SW). This type of compass may be ud for basic automotive u where the driver needs to know the general direction of travel.For this application, the magnetic nsor can be reduced to a two-axis nsor using only the X and Y axis. An automobile usually travels on a level surface, barring any hills or potholes, so that the X and Y nsors will directly measure the earthÕs Hx and Hy magnetic fields. The compass can be mounted on the dashboard with the X axis pointing straight ahead and the Y axis to the left.For now, ignore the magnetic effect of the car on the earthÕs field.
The compass design can be broken into eight
regions to indicate the cardinal directions. To analyze the magnetoresistive nsor respon, plot the
X and Y outputs while the car travels in a circle as indicated in Figure 4. Knowing that the earthÕs magnetic field is always pointing toward the north, start the analysis with the X axis (and the car) directly pointing north. The X output will be at its maximum value while the Y output
will be zeroÑsince no part of the earthÕs field is pointing to the left, or west. As the car turns clockwi toward the east, the X axis will diminish to zero while the Y axis will decrea to its maximum negative value. With the car continuing its clockwi turn to point due south, the X axis will decrea to its most negative value while the Y axis will return to zero. This effect is illustrated in Figure 4 and shows the complete circular cycle for the X and Y axis outputs. The X and Y outputs of the magnetometer can be modeled by the cos(¯) and sin(¯) functions
where ¯ is the azimuth, referenced to magnetic north.
Direction (degree)
O u t p u t (%F S )
Vupper
Vlower
Figure 4 - Magnetic Outputs X And Y For 360¡ Rotation
The X and Y curves in Figure 4 can be split into
eight regions reprenting the four cardinal and four midway points. A combination of the curves can be formed to reprent each region. Two crossover points,Vupper and Vlower, are necessary to distinguish the boundaries of the eight compass direction headings. The crossover points can be determined by knowing the full scale (FS) values for X and Y as:
Vupper = 100*sin(22.5¡) (%FS) = 38 %FS (1)Vlower = -100*sin(22.5¡) (%FS) = -38 %FS
Voltage comparators can be ud to detect
Vupper and Vlower levels to divide the X and Y curves into four regions: A, B, C, and D. The eight points of the compass can be determined by combining the A, B, C,and D using Boolean logic gates, four comparators, and a two axis magnetic nsor as shown in Figure 5. This circuit requires a two axis magnetometer with a signal nsitivity of 1-2 milligauss. Magnetic hysteresis and linearity must be less than 1-2%FS with good repeatability. There are three limitations to consider while using this design: 1) there is no tilt compensation so the compass must be held level, 2) there should be no nearby ferrous material to create magnetic distortions, and 3) the declination angles are difficult to add to this design. The limitations will be addresd in the one-degree compass discussion below.
E NE NW
千禧鸟N
S
SW SE
W
Figure 5 - Eight Point Compass Circuit
阁王腾
ONE-DEGREE COMPASSÑSome navigation
systems require more than just an eight point compass.For instance, the Global Positioning System (GPS) has led to a sophisticated tracking of vehicle position on video maps with accuracy better than 10 meters. The systems rely on telemetry contact from four satellites,sometimes aided by a system radio tower. It is esntial to keep a line of sight with the satellites for position determination. Backup systems are required in cities and tunnels to maintain a cour of direction during short blackouts. This is where a more accurate compass can help in GPS bad navigation systems. During the loss of GPS signals, knowing the vehicleÕs speed and heading direction can maintain proper vehicle tracking.Gyros can be ud to maintain direction but a lower cost MR bad compass is preferred. For the systems, a compass accuracy of one degree is desirable.
To achieve a one degree accurate compass
requires a magnetic nsor that can reliably resolve angular changes to 0.1 degrees. The nsors must also exhibit low hysteresis (<0.05%FS), a high degree of linearity (< 0.5%FS error) and be repeatable. The magnetic fields in the X and Y plane will typically be in the 200 to 300 milligauss rangeÑmore at the equator,less at the poles. Using the relationship:
Azimuth = arcTan (y/x)
(2)
the required magnetometer resolution can be estimated.To resolve a 0.1¡ change in a 200milligauss field would require a magnetic nsitivity of better than 0.35milligauss. Solid state MR nsors are available today that reliably resolve 0.07 milligauss signals giving a five times margin of detection nsitivity.
Using the simple magnetic nsor shown in
Figure 6, the azimuth can be calculated by using the X and Y outputs in a horizontal plane. To account for the tangent function being valid over 180¡ and not allowing the y=0 division calculation, the following equations can beÊud:
Magnetic Sensor
Figure 6 - One Degree Compass System without Tilt Compensation
Azimuth (x=0, y<0) = 90.0(3)Azimuth (x=0, y>0) = 270.0
Azimuth (x<0)
= 180 - [arcTan(y/x)]*180/¹Azimuth (x>0, y<0) = - [arcTan(y/x)]*180/¹Azimuth (x>0, y>0) = 360 - [arcTan(y/x)]*180/¹
The t/ret (S/R) circuit shown in Figure 6 is a
current pul generator ud to eliminate the effects of past magnetic effects and temperature drift [4]. The rial bus output can readily interface to a low cost microprocessor for azimuth computation. Equations (3)provide continuous azimuth angles from 0¡ to 360¡ in the forward direction relative to magnetic north (H North ), e Figure 7. In this example, there is no compensation for tilt and nearby ferrous distortion effects on the azimuth.
Hz
Y
a = Azimuth or Heading
Figure 7 - Azimuth Defined In The X-Y Plane
COMPENSATING FOR TILTÑMost often
compass are not confined to a flat and level plane.They are often hand held, attached to an aircraft, or on a vehicle in an uneven terrain. This makes it more difficult to determine the azimuth, or heading direction, since the compass is not always horizontal to the earthÕs surface.Errors introduc
临清流而赋诗ed by tilt angles can be quite large depending on the amount of the Dip angle. A typical method for correcting the compass tilt is to u an inclinometer, or tilt nsor, to determine the roll and pitch angles. The terms roll and pitch are commonly ud in aviation: ROLL refers to the rotation around the X, or forward direction, and PITCH refers to the rotation around the y, or left-right, direction (e Figure 8).
Forward Figure 8 - Compass Tilt Referenced To The
EarthÕs Horizontal Plane
Common liquid filled tilt nsors remble a glass ÒthimbleÓ that us electrodes to monitor the fluid movement as the nsor changes angles. Newer solid state accelerometer tilt nsors are available that measure the earthÕs gravitational field by means of an electromechanical circuit [5]. The output of the devices are an electrical signal equivalent to the angle of tilt. During compass asmbly, the tilt nsor directions must be carefully aligned with the X,Y,Z magnetic axis. Several manufacturers offer the tilt nsors as stand alone circuit boards that provide the roll and pitch angles as outputs.
To compensate a compass for tilt, knowing the roll and pitch is only half the battle. The magnetomete
r must now rely on all three magnetic axes (X, Y, Z) so that the earthÕs field can be fully rotated back to a horizontal orientation. In Figure 8, a compass is shown with roll (q) and pitch (f) tilt angles referenced to the right and forward level directions of the obrver or vehicle. The X, Y, and Z magnetic readings can be transformed back to the horizontal plane (X H, Y H) by applying the rotational equations shown below:
X H = X*cos(f) + Y*sin(q)*sin(f) - Z*cos(q)*sin(f)
赫伯特斯宾塞Y H = Y*cos(q) + Z*sin(q)(4)
Azimuth = arcTan (Y H / X H)
胰岛素的种类
Once the X and Y magnetic readings are in the horizontal plane, equations (3) can be ud to determine the azimuth. For speed in processing the rotational operations, a sine and cosine lookup table can be stored in program memory to minimized computation time.
A block diagram for a tilt compensated compass is shown in Figure 9 with a rial bus interface. After the azimuth is determined, the declination correction can be applied to find true north according to the geographic region of operation.
Figure 9- Tilt Compensated Compass System
COMPENSATING FOR NEARBY FERROUS EFFECTSÑWhen a compass is operating in a open area in the abnce of any ferrous metals there is no distortion effects on the earthÕs magnetic field. In reality, though, compass are mounted in vehicles, aircraft, and platforms that most likely have ferrous materials nearby. The effects of ferrous metals (iron, nickel, steel, cobalt) will distort, or bend, the earthÕs field which will alter the compass heading. The effects can be thought of as a magnetic field that is added to the earthÕs field. If the compass is curely mounted in the vehicle, the ferrous effects can be accounted for and removed from the magnetic readings.
Figure 10 illustrates the X and Y magnetic readings when the compass is turning around in a circle in a horizontal plane. In this example, there is no ferrous interference with the earthÕs field. The readings are taken from HoneywellÕs HMR2300 Smart Digital Magnetometer where each count repr
ents 67 microgauss. The earthÕs field magnitude in the X and Y plane reads 2800 counts which is approximately 190 milligauss. When the X and Y readings are plotted with each other they form a circle centered about the 0,0 point. An azimuth can be determined for each reading using equations (3) as show in Figure 10. This plot shows a sine and cosine output respon for the X and Y directions during rotation.
If the magnetometer is mounted in a car, the effect of the engine and car body would distort the earthÕs magnetic field. Driving the car in a circle would produce the curves shown in Figure 11. Note here that the X,Y plot is not a circle (slightly ellipsoid) and that it is offt from the 0,0 point by -480 and -795 counts. This offt and ellipsoid effect are a result of the fixed distortion of the car on the earthÕs magnetic field. This distortion can be determined systematically and applied to subquent X,Y readings to eliminated the effects of the car.
To compensate for the vehicleÕs distortion, two scale factors Xsf and Ysf can be determined to change the ellipsoid respon to a circle. Offt values Xoff and Yoff can then be calculated to center the circle around the 0,0 origin. The X,Y values ud to compute the azimuth when compensating for the vehicleÕs distortion are:
Xvalue = Xsf * Xreading + Xoff (5)
Yvalue = Ysf * Yreading + Yoff
Here, the scale factors Xsf and Ysf scale each reading to change the ellipsoid to a circle and Xoff and Yoff values shift the center back to the 0,0 origin. The result of this compensation is shown in Figure 12 and should be compared to the Ôno interferenceÕ curves in Figure 10.
-3500
03500
X a n d Y A x i s (c o u n t s )
Figure 10 - No Interference Of Magnetometer Readings For 360¡ Rotation In Level Plane
-3500
03500
X a n d Y A x i s (c o u n t s )
Figure 11 - Car Engine/Body Interference Of Magnetometer Readings For 360¡ Rotation In Level Plane
-3500
3500
X a n d Y A x i s (c o u n t s )
Figure 12 - Car Engine/Body Compensation Of Magnetometer Readings For 360¡ Rotation In Level Plane
