Hello there BlueRobotics team,
I have compiled a lot of questions to ask. You may see a few more questions asked by me in a few hours. Firstly, the Bar30 pressure sensor:
We are experiencing two different/related problems when reading the pressure sensor.
When reading the pressure logs recorded by our vehicle, I noticed that the pressure sensor sent a reading of -4580mm, -4500mm and then -60mm. The reading then stabilized around 60-70mm of depth. The problem is, the vehicle wasn’t in the pool yet, so a reading of more than 4.5meters in depth is concerning. I presume the -60 was because the pressure wasn’t calibrated before the flight. Is there a known explanation for this? the abnormal reading lasted for less than 100ms, but our vehicle relies on the pressure sensor for its depth movement PID. Such a rapid change would change the Proportional and Dericative terms drastically, leading to (perhaps) dangerous movement.
My next question is about that lingering -60mm term. When testing our vehicle above water, we apply pressure to the bar30 with our thumbs. But, we have noticed that after releasing our thumb, the pressure reading does not return back to 0, rather it stabilizes at a value in-between 0 and the maximum pressure reading. For example, the depth reading goes up to -330mm, then stabilizes at -130 or something.
I have instructed our team to stop testing the pressure sensor with this method to stop this from happening, but in the mean time I wanted to know why the reading isn’t stabilized around 0 again. Is is because the physical space the sensor occupies get squished or something?
Is this happening regularly? If not I would guess something hit/poked the sensor, or perhaps the vehicle bumped something quite hard and shook the sensor. Large step changes go via a low pass filter, so a single very large measurement could have a largish jump that then takes a few readings to die out. That’s also less likely in the water because vibrations are damped much faster.
Direct physical sensors generally exhibit at least some degree of hysteresis, whereby lost energy on the way back from a measurement peak causes the sensor’s position to stabilise between the peak position and the desired (accurate) position. I expect that’s exacerbated for a sensor like the Bar30 when it’s used in air, because the gel on the sensor isn’t saturated and pressurised with water, so is less capable of ‘springing back’ from its deformed state when the pressure is reduced. That could be confirmed by checking sensor readings at known depths in the water across multiple cycles.
A brief thumb test is probably ok for confirming the sensor value is changing, but if you do that it’s likely worth dunking the vehicle a bit before doing a barometer calibration, to avoid the initial offset being incorrect.
Thanks a lot!
For now, it seems like the drastic pressure changes was a one-off situation.
We will keep in mind the saturation and wetness of the bar30 when calibrating from now on. Knowing about the low pass filter also helps understand things better.