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Philips Semiconductors
Electronic Compass Design using
KMZ51 and KMZ52
5.5.8 SCU without electro-magnetic feedback
If no temperature compensation of sensor sensitivity is required, the electro-magnetic feedback loop can be
interrupted by omitting the compensation coil driver (block 6). Block 5 is still required as low pass filter in order
to suppress flipping spikes on the signal line. However, a resistor R22 then needs to be connected in parallel to
C2 in order to achieve a limited output voltage. The output voltage then is dependent on sensor sensitivity S and
SCU amplification. Assuming that the synchronous rectifier (block 4) has an amplification magnitude of 1, the
output voltage becomes:
=
V
H
out
(
, x
) y
( e
In this case, Vout can be adjusted by varying e.g. R22 or R10.
5.5.9 SCU with Microcontroller
Figure 17 shows a block diagram for an SCU, using a microcontroller. From the analogue circuitry shown in
Figure 15, only the flip coil driver, pre-amps and - optionally - compensation coil drivers are required. The
flipped sensor signals are fed to the µC via an A/D converter. It should be considered, that the resolution at this
stage must be higher than the final compass accuracy. Principally the same considerations are valid for the D/A
converter, driving the compensation coil driver. For a low-end compass, where accuracy is not critical, the
internal 8 bit A/D converter of a low-cost µC could be sufficient. If higher accuracies are demanded, an external
A/D converter with higher resolution can be the solution.
Offset compensation can be implemented as software, according to the equations shown in section 5.2. Further
optional software features could be a control algorithm for electro-magnetic feedback or non-orthogonality
compensation. Besides signal conditioning, the µC software will usually also perform the direction determination
and further optional tasks, such as interference field calibration or true north calibration (see respective
sections).
SCU_UC.AI
R
4
R
22
⋅
⋅
⋅
S
, x
) y
R
R
10
bridge
2-dimensional
field sensor
KMZ52
compensation
coil
flip
coil
Hex-Sensor
compensation
coil
S·Hex
flip
coil
S·Hey
Hey-Sensor
Figure 17
Signal conditioning circuit with microcontroller
(10)
Microcontroller
Flip
coil
I/O
driver
Compens.
coil driver
D/A
D/A
D/A
Conv.
Conv.
Conv.
Compens.
coil driver
Pre-Amps
Vx
A/D
Conv.
Vy
24
Application Note
Software:
- Flipping clock
generation
- Offset-elimination
(see section 5.2)
- T-compens. of
sensor sensitivity
- Interference field
calibration
(see section 7)
- Azimuth calculation
(see section 6.2)
- True north
calibration
(see section 8)
AN00022
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