RuuviTag Pro accelerometer alignment

Ruuvi products 🇬🇧 RuuviTag & RuuviTag Pro sensors
1

Questions about the accelerometer in the RuuviTag Pro:

  1. Are the axes deterministically aligned with the case?
  2. If so, is the alignment such that X or Y axis is aligned through the mounting holes?

Perhaps “Ruuvitag Pro - x and y acceleration orientation” answers question #2, but still how much variation is possible from one unit to the next?

2

Hello,

  1. Yes, there is a pin in the bottom part of enclosure which aligns with the slot on PCB which fixes the orientation.

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  2. The alignment is not exactly in line of the mounting holes, @Marko do we have a picture of accelerometer axis available? In any case the orientation can always be calculated with X,Y,Z and trigonometry if tag is not moving, vector sum is 1G pointing to down to Earth.

The error sources in alignment of accelerometer come from:

  • Alignment of accelerometer to PCB
  • Alignment of PCB to enclosure
  • Alignment of bottom part of enclosure to top
  • Sensor noise

Manufacturing tolerances are tight on PCB itself, put there is a bit of intentional free space in PCB to slot to make it easy to fit. The enclosure alignment is probably the biggest source of error, you need to be careful to align the positioning arrows when you close the enclosure. Here they are a bit off:

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We don’t have official tolerance listing for accelerometer alignment, but my guess would be that sub-degree accuracy is not possible and a few degrees is achievable if you’re being careful with mounting.

3

Thanks for this detailed reply, @otso. Okay, so while the alignment is deterministic in the manufacturing process, it’s possible that a user doesn’t re-align the parts of the enclosure properly, although it looks like that would be a few degrees perhaps.

I’m wondering about how necessary a user calibration step would be for an orientation app I’m thinking about. I suppose my main concern might be about future-proofing; the XY alignment might differ in a future PCB revision, if it’s indeed not already aligned with the enclosure by design.