SparkFun Forums

[m]

I've been working on a magnetometer based nav unit for a model aeroplane.

I'm using the NVE giant magnetoresitive AA002-02 single axis magnetic sensors plus a 2 axis accelerometer based around the MSP430F149 microprocessor.

Schematic is at


The current board layout is at


(click on the pictures for zoomed in images)

I'm using the MSP because it's 16-bit, low -power and runs at 3.3v. The SAM-LS GPS bolts onto the back of the board with a flat flex cable running around to the main board, and the TRF-24G transceiver solders onto the right hand side. I'm using the MSP eval board as a daughter-board type arrangement both because I doubt my soldering is up to QFP, and because the eval board already has all the programming interface, caps, crystals etc etc.

The MSP also has the advantage that is has 12-bit ADC's rather than the 8-bit on the ATMEL.

The 3rd GMR sensor is on a tiny daughterboard to get the 3rd axis.

I'd looked at using the rate gyro's but decided against it because
a) The drift error is terrible.
b) They suck amazing amounts of power.
c) They need a 5V supply.
d) The integration error is terrible.

The magnetometers are nice because you're not integrating to get position. The accelerometers are mostly just there to calibrate the GMR sensors for zero.

There are 8 servo interfaces (8 servo inputs, and 8 outputs). The idea here is that the board can take normal servo output from the RC receiver and modify it on the way through.

The 74xx541s are used to do level conversion between the 5V servos and the 3.3v MSP.

The board is a terrible pain to layout. The fix of 0.05" spacings, 1.25mm spacings, and 0.5mm spacings causes the autorouter a lot of problems.

This is the first design I've done in my than 15 years so I'm very very rusty.

Any comments?

[Oznog]

I've never figured how you really use these things.
Did I read correctly the AA002-02 only has like 3 mV across the entire sweep of the earth's gravitational field. How do you keep the offset error reasonable between the device and the ADC to maintain any sense of accuracy over a temperature chance?

[m]

The sweep is a bit higher than that, but yes, it's a very small signal. I'm most using a high precision op-amp to keep the offset error small enough to be trimmed in software (i.e. it won't normally use the full scale of the ADC).

The temp drift won't be a significant issue as long as all the magnetometers maintain the same temperate, because it's only the ratio between them that's interesting, not the absolute magnitude.