Dear Matteo, The gravity vector is a vector of magnitude 9.8 m/s^2 which points downwards (i.e. towards the centre of the earth). Using the convention suggested in the manual, when you have the Shimmer orientated such that the sticker/leds are pointing upwards then +z-axis is also pointing upwards. The gravity vector is thus pointing in the opposite direction to the +z-axis and therefore registers a negative value.
Hopefully this helps to resolve the problems you are having. Best Regards, Karol From: [email protected] [mailto:[email protected]] On Behalf Of Matteo Giuberti Sent: 07 September 2012 11:35 To: [email protected] Subject: [Shimmer-users] SHIMMER calibration Dear all, I have some SHIMMERs with the 9DoF module and I have to calibrate the sensors (accelerometer, gyroscope, and magnetometer) in order to use their output in an orientation estimation filter. I'm using the calibration software provided to the member user and, reading the manual, I've noticed that the instructions provided to calibrate the accelerometer lead to a calibration of the accelerometer that is different from all the previous devices that I've experienced in the past. Let me first say that I've used the same coordinate axis convention that you suggested in the manual. Following that convention, I was expected to see, for example when the Shimmer main side (i.e., the one with the sticker and the leds) is pointing toward the ceiling, a z-acceleration almost equal to +9.81 m/sec^2. Instead it is -9.81 m/sec^2. Can you motivate me this choice (that, I repeat, is different from the conventions that I previously experienced in other devices)? Do you think it will be a problem to calibrate ONLY accelerometer using my convention? I hope I clarified my point. Otherwise, I'll try to explain it better. Best regards, Matteo
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