The standard T-motor Air Gear 200 props are plastic and very flexible. During takeoff they bend upwards as they spin up until the bend is strong enough to ‘lift’ the quad off the ground – this makes the lift very gentle.
In contrast, the CF props have no flex, any lift is directly and immediately applied to the frame, and therefore to the sensors. The sensors feel everything; there’s no smoothing or caressing; sharp spikes in motor speed make it to the sensors, to the extent that acceleration may exceed the range of the sensors; I believe this is the cause of my negative G problem.
With the IMU configured to suppport a range of ±2g (the highest resolution), then an additional 1g spike a take-off will overflow. With the hard light higher pitch CF props, it seems entirely possible this could happen.
There are several ways to fix this each with various pros and cons. I could
- increase the low pass filter, but this does still allow the overshoot but then filters it out, along with other valid data – this works though it’s a hack / workaround / sticky plaster, not a fix.
- extend the range of the sensor to ±4g but there’s a corresponding reduction in sensor resolution which means larger undetectable drift
- add physical buffering like the props do – I did this previously with the HoG floating on 8 very solt silicone gromits. I don’t want to use this again (expensive and breakable) but something similar with thicker, softer foam tape sticking HoG to the frame could work
- soften the flight plan transition in software.
What I’d going to do it a combination of safety precautions and diagnostics
- I’ll add diagnostics to flag these 0g events, and to skip any data including such an event for safety reasons.
- I’ll up the range of the sensors to ±4g, and add further diagnotics to flag overshoots over ±2g just to make sure I’m right!
I’ll decide the final solution based on what I find out.