Edit:
I found the cause of the spikes.
The BMI055 chip is internally flawed, I am sure of it now.
If you read out the FIFO in Bypass mode (that means only one frame is kept) then you have to expect data errors.
If you read it faster than the sampling rate you receive 10% 0x8000 (smallest possible number) in the mix. Not zero, not the last value .. a MAXIMUM value!
If you read the data a bit slower than the sampling rate, you receive valid data with 1-8 spikes per second.
This happens if you use the FIFO register in 6 or 8 byte burst mode.
Now I got curious, I read the 0x02 register in 6 byte burst and this gives the same data (x,y,z) and I changed absolutely nothing else.
The spikes are reliable gone.
previous text:
It seems that the gyroscope itself is flawed and it is not just that one.
I first expected an error in I2C communication, turned down the speed and exchanged the voltage shifting circuit without any change in results.
Now as I identified the problem to come from random high spikes I could also find many other people with similar issues or reports.
Surprising fact is that there are rarely any answers.
My first test was a moving exponential average filter, but as I expected that ruins many subsequent readings and only dampens the spikes.
The best solution I can think about is to use two gyroscopes, that is what I will do longterm.
They will likely both have spikes and by comparing both frames with each other it should be possible to get a very good result and no spikes.
The simple solution is to filter spikes based on a hardcoded max-change value.
I first considered capturing data at twice the current rate and then always lagging behind one frame.
That way I would know the 'future' and the past, if the 'current' data is way off from future and past it is a spike (or someone hit a hammer on the sensor).
But for gyros with high degree/s resolution and pratical use outside of 2000deg/sec the filtering can be easier.
I played around and was shaking the gyro heavily, the raw values rarely exceeded +10k and no matter what I did they did not have any such sudden changes, even during a drop on the table.
I use this now:
if (abs(old-current)>0x3000) use_value();
So if the result is off by more than 12,000 raw values in comparison to the previous value than the previous value will stay in the variable. Otherwise the variable is updated with the new value.
Now I have a quite low drift which even works for many minutes.
Sidenotes about the used gyro:
a) The BMI055 gyroscope is specified to return ZERO if you sample it too fast, in reality it returns random values.
b) The BMI055 gyroscope crashes the I2C bus if you send a soft_reset to it (the onchip ACC doesn't do that)
c) Sudden spikes are not described in the datasheets of the gyros and still many people notice them. That's quite strange.
I would appreciate a better answer, maybe I am wrong.
However, the spike filtration made my program work quite reliable compared to the unusable results earlier.
Tentative answer:
There may be some relaxation mechanism, the way a dent will appear in a carpet if you leave a chair leg sitting on it too long. This specification places an upper limit on the effect that will have on the reading.
So if you read 100N force after 1 second, the reading may have drifted by 5% or 5N (or less) after 10 seconds, 10N (or less) after 100 seconds, 15N after 1000 seconds and so on. (I am assuming they mean log(base 10) but the datasheet is not explicit on that point)
It's not obvious if there is a lower limit to this behaviour (i.e. 15% difference between 1ms and 1 second) but I would assume so, down to the 5 us specified settling time.
Another question is : does the same drift apply when you then remove the weight? (the chair leg depression disappears eventually after you move the chair). If I were contemplating this sensor the first thing I would do is get my hands on one and characterise it with actual measurements. It may be better than the spec, bearing in mind that future production may vary within these limits.
Whether this is good or bad depends on whether it meets your needs, and how well it compares with other sensor technologies within your budget.
Best Answer
Source: https://ae-bst.resource.bosch.com/media/_tech/media/product_flyer/BST-BMI088-FL000.pdf
I don't think I would believe that either unless listed in the datasheet. Only the temperature drift is listed in the datasheet. You would have to know the internal workings of their Kalman filter and how the errors of the gyroscope are being integrated to be able to calculate this yourself. The best course of action would be to contact Bosch and ask them about the drift rate and to put that figure in the datasheet.