cmos three axis hall sensor and joystick application schott3axisjoystick

7/31/2019 CMOS Three Axis Hall Sensor and Joystick Application Schott3axisJoystick

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CMOS Three Axis Hall Sensor and Joystick Application

Christian Schott

Sentron AG, Switzerland

[email protected]

Robert Racz


[email protected]

Samuel Huber


[email protected]

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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0

500

1000

1500

2000

2500

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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cmos three axis hall sensor and joystick application schott3axisjoystick

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