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July 4, 2014
Physics, electronics and fixing things…
I’ve been prototyping with this breakout board and have stumbled across something that I’m still struggling to believe… It would appear that the device is photosensitive!
Let me explain, once calibrated the XYZ values appeared to behave themselves in my office, with Z showing approximately 1g and XY at roughly 0g when “horizontal”.
The device is attached to a small UAV project that I’m working on which requires outdoor testing. During this testing I could not get the accelerometer associated part of the project to work as expected. After some analysis I discovered that the XYZ “bias” values were significantly shifted during flight testing.
Initially I decided that the vibration from the UAV’s motor was causing the issue and that I either had to mount it differently or use a part with a better “vibration rectification” specification (this in itself was an illuminating learning experience).
It turns out that after some analysis, the ADXL362 was well within specification with respect to the vibration levels. So the problem was somewhere else… After lot’s of code checking, and test flights designed to log the accelerometer outputs etc. I discovered that the bias levels changed abruptly as soon as I took the UAV outside into direct sunlight. I should also add at this point that the PCB on which the breakout board is mounted is exposed.
So was it temperature related? No… (more testing) However by using a small “hand operated sun shade” to cover only the breakout board I could instantly (within one sample period) modulate the bias values on all axes (by approximately 1g for the Z axis and by similar amounts for X and Y). This was a big surprise! So much so that I that I had to get a colleague up to speed on the experiment and convince him of the effect.
Other than the accelerometer there is only a single ceramic decoupling capacitor on the breakout board, I doubt that this capacitor is causing the effect. I checked on the AD web site and discovered that the black packaging used on the ADXL362 is made from “slica” with 5% added black carbon. i.e. a compound that is not completely opaque to infra-red, of which the Sun produces plenty!!
Has anyone else noticed this behavior?
I’ve lodged a support query with AD, but have not yet heard back.
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