Accelerometer calibration

Note: most of this is actually been superseded by my next post, but having written it, I begrudge just deleting it.

Test framework / beer fridge:

Accelerometer calibration rig

Accelerometer calibration rig

Although neither had been part of my original specification for the beer fridge, I realized a couple of its features were mandatory for it to be used as a test rig in addition to a beer fridge:

  • the Peltier effect heat pump has no moving parts (unlike the normal compressor cooling mechanism), so the fridge generates no noise to disturb the accelerometer
  • the glass door (rather than the normal metal) is necessary for WiFi contact with the outside world – a metal door would make the fridge into a Faraday cage.

When the photos was taken, Phoebe’s just done the ‘cold’ shots, and she and the fridge are warming up for the warm shots.

The LiPo wrapped in foam to keep itself warm, and to stop it warming up my beer and the test environment during the cold-shot phase.

The sloping platform is some left over carbon fibre sheet tilted at 28.5°.  Any strong material will do – I just had the CF lying around from a previous build of Phoebe’s frame.

At a given temperature, measuring gravity G on the X, Y and Z axes (θ = 0 for horizontal tests)

G * sin θ = gain * (sensor1 + offset)
- G * sin θ = gain * (sensor + offset)

Add them together and dividing by gain makes

0 = sensor1 + sensor2 + 2 * offset

or

offset = - (sensor1 + sensor2) / 2

Leading to

gain = sin θ / (sensor1 + sensor2)

Do that for 3 axes and 2 temperature ranges, and you can put together a formula for gains / offsets at any temperature.

I’d bother to describe that in more detail, but as I say at the top, this is now redundant info.  Still the beer fridge does its secondary role beautifully!

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