Thank you Karol for your feedback! Best regards, Matteo
2012/9/11 Karol O'Donovan <[email protected]> > Hi Matteo,**** > > I agree with your explanation in that the direction of the gravity vector > as measured by the accelerometer is infact upwards. It is important to be > aware that under quasi-static conditions the assumed direction does not > effect the calculation of inclination as long as the user is consistent in > their calculations. In the next release of the 9DOF Calibration application > we will reverse the direction of the reference vector. Thankyou for your > assistance on this topic.**** > > Best Regards,**** > > Karol**** > > ** ** > > *From:* [email protected] [mailto:[email protected]] *On Behalf Of *Matteo > Giuberti > *Sent:* 07 September 2012 16:47 > *To:* Karol O'Donovan; [email protected] > > *Subject:* Re: [Shimmer-users] SHIMMER calibration**** > > ** ** > > Dear Karol,**** > > thank you for the quick reply.**** > > ** ** > > Actually, I think the idea behind an accelerometer is much more involved.* > *** > > Just to go step-by-step, I want you to reason upon this: when an > accelerometer is in free-fall (imagine you let it fall from your hand to > the ground) what will it measure? **** > > Maybe I'm wrong, but I always thinked that it will measure 0g. If this is > right, then it is not actually measuring the gravity force but the > accelerations that act on the device apart from the gravity force. **** > > Indeed, my previous idea (that of the + and - signs in the previous mail) > was based on the fact that if a device was leaned still on a table it > should measure an acceleration that counteracts that of the gravity force > (even if physicians will tell me that it is actually a force and not an > acceleration) in order to let the device still!**** > > So, I think that a SHIMMER whose z-axis is pointing vertically up should > measure a +1g (or +9.8m/sec^2) AND NOT -1g (that is obtained instead > through your calibration).**** > > ** ** > > If there is something wrong in my reasoning please try to let me > understand!**** > > ** ** > > P.S. Just to provide you more evidence I just found on wikipedia this:**** > > http://en.wikipedia.org/wiki/Accelerometer**** > > If you read in the introduction and in the "physical principles" sections > you will find more or less the explanation I just provided to you.**** > > I know Wikipedia is not exactly a trustworthy source, but it actually said > what I knew about that topic!**** > > ** ** > > Regards,**** > > Matteo**** > > ** ** > > 2012/9/7 Karol O'Donovan <[email protected]>**** > > 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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