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CMOS Three Axis Hall Sensor and Joystick Application
Christian Schott
Sentron AG, Switzerland
Robert Racz
Sentron AG, Switzerland
Samuel Huber
Sentron AG, Switzerland
AbstractWe present for the first time a three-axis CMOS Hall sen-sor based on integrated magnetic concentrator technology
(IMC). The sensor measures the two in-plane magnetic
field components Bx and By and the vertical component Bz
and generates three output voltages proportional to them.
The sensing core consists of four Hall elements arranged at
90 under the edge of a ferromagnetic disk (IMC), which is
attached to the silicon die. By subtracting the Hall voltages
of two opposite Hall elements a voltage proportional to the
in-plane components is generated and by adding them a
voltage proportional to the perpendicular component. In
such a way a planar structure is used to implement a three-
axis sensor device. The sensor further contains current
sources, dynamic offset compensation and signal amplifi-
cation and conditioning.
The sensor aims for applications where two translatory or
rotational movements have to be measured independently
and precisely over a large temperature range. Examples
are joysticks, car mirror sensors and other control devices.
Keywords3-axis Hall sensor, angle sensor, joystick sensor
INTRODUCTIONTraditionally Hall sensors in CMOS technology can only
be used to measure the magnetic field component perpen-
dicular to the chip plane. This is due to the fact that CMOS
is a planar technology providing only for very shallow
structures. Therefore such a sensor can only measure field
strength and not field direction. For direction measurement
several units have to be accurately assembled which makes
the whole sensor bulky and expensive. The advantage of
direction measurement over field strength measurement lies
in the fact that ageing effects, temperature drift effects and
sometimes even some mechanical tolerances of magnet and
sensor are virtually eliminated.
With the integration of ferromagnetic flux concentrators
(IMC) directly on Hall-ASICs [1] also the two in-plane
magnetic field components can be measured. The in-planemagnetic field is locally deflected under the edge of the
metal layer and can so be measured by conventional
CMOS Hall elements (Fig. 1).
Figure 1: IMC Principle: A horizontal magnetic field par-
allel to the chip is locally rotated by the ferromagnetic
flux concentrator so that it can be measured by ordi-nary planar Hall elements.
IMC technology has lead to a series of products for con-
tactless current sensing and angle sensing [2]. The benefits
are additional passive amplification leading to higher sig-
nal-to-noise and signal-to-offset ratios as well as the possi-
bility to measure several components of the magnetic field
in the same chip. A recently presented angle sensor [3]
with a ferromagnetic IMC disk measures two orthogonal
in-plane components.
OPERATION OF 3-AXIS SENSORThe three-axis Hall sensor which we present here combines
the measurement principle of classical Hall sensors with
the one of IMC Hall sensors. Similar to the angle sensor the
IMC is of disk-shape and four Hall elements are arranged
at each 90 under the disk edge (Fig. 2).
Figure 2. Top-View photograph of disk-shape magneticconcentrator and arrangement of the Hall plates
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missing magnetic gain of the IMC in Z direction. The out-
put signal amplitudes are proportional to the applied field
strength and yield a value of 1.8V for X and Y and 0.9V
for Z for a field of 40mT. From these values a sensitivity of
45V/T respectively 22.5V/T is calculated. Depending on
the application this value can be either doubled or cut to
half by setting the corresponding programming parameters.
The noise level of the three outputs is only a few mV so
that an angular resolution of about 0.1 is reached. Addi-tionally to normal operation with a current consumption of
16mA, the sensor can also be operated in a low power
mode with 3mA current consumption.
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500
1000
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0 45 90 135 180 225 270 315 360mechanical angle []
V
x,
Vy,
Vz[mV]
Vx [mV] Vy [mV] Vz [mV]
Figure 6: Measured sensor outputs for a rotation of themagnetic field in the XY-plane
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0 45 90 135 180 225 270 315 360
mechanical angle []
Vx,
Vy,
Vz[mV]
Vx [mV] Vy [mV] Vz [mV]
Figure 7: Measured sensor outputs for a rotation of themagnetic field in the XZ-plane
JOYSTICK APPLICATION
PrincipleDue to measuring the magnetic field components along
three orthogonal axes simultaneously, the device can be
used to measure field direction in two different planes. A
very typical application to illustrate this principle is the
contactless magnetic joystick. Figure 8 shows such a joy-
stick where a permanent magnet in the stick rotates in a
semi-sphere above the sensor. The magnetization axis of
the magnet is always directed towards the sensor.
Figure 8: Gimbaled Mount Type Joystick
The situation of the magnetic field components for inclin-
ing the stick along the two directions is illustrated in Figs
9-11. Fig. 9 shows the situation where the stick is vertical,
so that only the Bz component is present.
Figure 9: In center position all the field is perpendicular
to the chip
Why three axes ?First reason: Inclining the stick along the angles and
(Fig. 10) results in a sine function field components for Xand Y and in a cosine field decrease for Z.
Figure 10: The horizontal field components depend onthe total field strenght B, whereas the ratios Bx/Bz and
By/Bz are indpendent of B.
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We can see, that as long as B is constant, Bx and By are
sufficient to yield the inclination angles. However, when B
changes (age, temperature etc) an error occurs. In this case
the field independent angle information is obtained by
building the ratios Bx/Bz and By/Bz.
Second reason: If the stick is inclined along both axes at
the same time (Fig. 11), then all three measured compo-
nents depend upon both inclination angles and . How-
ever, building the ratios Bx/Bz and By/Bz two angles can
be decorrelated.
Figure 11: All three magnetic field components depend
on both inclination angles and .
Third reason: Due to mechanical tolerances or caused by
wear-out the distance between magnet and sensor may
change (Fig. 12). In this case the total field strenght B also
changes and with it the one of the three field components
Bx, By and Bz.
Figure 12: By changing the distance magnet to sensor
the total field strength and the one of the componentschanges, whereas the ratios Bx/Bz and By/Bz remain
constant.
If here again we only rely upon measurement of the com-
ponents, this changes leads to errors. However, the ratios
Bx/Bz and By/Bz cancel out the total field change and adistance change has no effect.
Temperature DriftTo illustrate the influence of temperature variation on the
position drift when only the field strenght is used, we have
performed a temperature drift measurement. Fig. 13 shows
how much the output signal at a fixed inclination position
changes in degree angle for a temperature increase of 60C.
Signal drift with 60 temperature change
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inclination angle []
drift[]
Vx
Vx / Vz
TC = -1250ppm/C
TC 0ppm/C
Figure 13: Measured drift of the output position signal
with temperature. When only one channel is used the
drift is about -1100ppm/C whereas with the ratio Vx/Vzvirtually no drift is present.
The voltage Vx changes for an equivalent of about 2 for
an inclination angle of 25. This corresponds to a tempera-
ture coefficient of -1250ppm/C which is the sum of the
TC of sensitivity of the sensor and the TC of the magnet.
On the other hand, the difference of the ratio of the volt-
ages Vx/Vz at both temperatures remains nearly perfectly
zero.
CONCLUSION AND OUTLOOKBy the integration of a ferromagnetic layer on a CMOS
Hall circuit three orthogonal magnetic measurement axes
can be realized in one single chip. Such a sensor allows to
measure the magnetic field direction in two orthogonal
planes, representing the two spherical angles of a two-axis
joystick. The principle of building the ratio of the in-plane
magnetic field components Bx and By with the vertical
component Bz not only makes the inclination angle signalindependent of magnet strength and distance magnet to
sensor, it also virtually eliminates all effects of temperature
drift and ageing. In such a way the three-axis Hall sensor is
the ideal component to build accurate, robust and low-cost
magnetic joysticks. For further convenience the CMOS can
be equipped with a divider logic so that the outputs are
already the desired angle signals. This can be either done in
an analog way by using a PI-regulator feedback [4], or in a
digital way by using an on-chip microcontroller.
REFERENCES[1] US Patent 6545462
[2] For more info, datasheets etc, see www.sentron.ch
[3] R. S. Popovic et al., A new CMOS Hall angular Posi-
tion sensor, tm Technisches Messen, 68, June 2001,
pp. 286-291.
[4] US Patent 6731108, Fig. 11
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