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Philips Semiconductors
Electronic Compass Design using
KMZ51 and KMZ52
Practically, the compensation field can be generated by supplying a current through an appropriate coil near the
sensor. As already pointed out, Philips MR sensors for compass applications come with an integrated
compensation coil, allowing to apply the electro-magnetic feedback method without the need for any external
coils. Due to the well defined field factor of these integrated compensation coils, there is a well defined
proportionality between field strength to be measured and compensation current.
Figure 14 shows a block diagram for an SCU channel employing both flipping and electro-magnetic feedback.
The electro-magnetic feedback circuit is a closed-loop controller, in which a current regulator feeds the
compensation coil in order to keep the sensor output voltage at zero. In order to achieve a sensor output voltage
of exactly zero, a current regulator with integral characteristic is required. The measured field strength can be
represented as voltage, proportional to the compensation current.
FLIPPING
SOURCE
Figure 14
5.4 Non-orthogonality compensation
Up to this point it has been assumed, that the two magnetic field sensors are displaced at an angle of exactly
90°. However, in practice the displacement will deviate by an angle β from the desired orthogonality due to
mounting tolerances. This deviation causes an error in compass reading, which is a periodic function of the
azimuth (see Table 2 in section 10). The maximum error is approximately equal to the non-orthogonality β.
Thus, for the KMZ52 with a specified max. non-orthogonality of 2°, the max. azimuth error is also 2°.
If a higher accuracy is desired, β should be compensated. If the compass is rotated with respect to the earth´s
field, then the phase shift between Vx and Vy is 90°±β. Having determined β, the error can be eliminated
mathematically:
Assuming, that the SCU delivers the signals
that a corrected signal
and using the trigonometric relationship
the corrected Vy is:
V
=
V
, y
corrected
cos
LF
LC
CURRENT
REGULATOR
Block diagram of flipping and electro-magnetic compensation circuit
V
=
⋅
α
V
V
sin
, y
corrected
max
α
sin(
y
β
−
⋅
V
tan
β
x
CLOCK
PRE-AMPLIFIER
AND
OFFSET
FILTER
VOLTAGE & CURRENT
OUTPUT
α
β
=
⋅
+
V
sin(
);
Vx
y
max
is desired,
+
β
=
α
β
+
α
)
sin
cos
cos
(8)
20
Application Note
SYNCHRONOUS
RECTIFIER
MBH619_1
α
=
⋅
, where α is the azimuth,
V
cos
max
β
sin
,
AN00022
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