[Microbit-Python] Accelerometer orientation

[Loop Space]

Damien,

Thanks for that.  This means that the microbit is the "correct" way up 
when the leds are facing downwards.  As you say, then the axes are 
right-handed.

Thanks,
Andrew

On 18/02/2016 14:10, Damien George wrote:
> Hi Andrew,
>
> Your analysis is correct.  It seems that the (universally accepted?)
> convention for accelerometer chips is that they give a positive
> reading on the axis if that axis is pointing *away* from the centre of
> the Earth.  With this convention the axis is right handed.
>
> Basically their convention is that "up" is positive, for all 3 axis.
> Since gravity is oriented "down" that makes it inverted in your
> analysis.
>
> Note that if you were in free space (free fall, no gravity), then the
> axis will read positive in the direction you accelerate.
>
> Cheers,
> Damien.
>
>
>
>
>
>
> On Thu, Feb 18, 2016 at 12:04 PM, Loop Space <loopspace at mathforge.org> wrote:
>> Hi all,
>>
>> Brief intro: I recently got a microbit as part of the initial roll-out to
>> teachers.  I've no experience with python, but plenty of experience with
>> other scripting languages so using micropython seemed the easiest of the
>> various options available to me.  I'm using the mu editor to write and flash
>> scripts to the microbit via USB serial link.
>>
>> So far, so good.
>>
>> I'm just testing stuff, and was trying to work with the readings from the
>> accelerometer.  When the microbit is just resting, that should basically
>> tell me what direction "down" is in.  I can use the following script to get
>> its values:
>>
>> from microbit import *
>>
>> DELAY=100
>>
>> while True:
>>      sleep(DELAY)
>>      x,y,z =
>> accelerometer.get_x(),accelerometer.get_y(),accelerometer.get_z()
>>      print(x,y,z)
>>
>> Via the REPL interface in mu, I get a stream of values.  As I turn the
>> microbit, obviously those values change.
>>
>> With the microbit essentially flat (leds uppermost), the vector is heavily
>> weighted negatively in its z-component (values of the order of -1000).  This
>> says that the z-axis goes vertically up from the leds.  By turning the
>> microbit, I can find where the other axes point.
>>
>> This gets to my question.  It would appear that the x-axis runs across the
>> microbit with positive x-axis running out towards the B button, whilst the
>> y-axis runs from the leds to the pins.
>>
>> As far as I can tell, that means that the microbit's orientation is
>> left-handed as opposed to the almost universally accepted right-hand axes.
>>
>> Is my analysis right?  Or am I misinterpreting what the numbers from the
>> accelerometer mean?
>>
>> Thanks,
>> Andrew
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