Questions about qrb5165-imu-server and specifically ICM42688 sample rates

peterkrull

I saved a log using voxl-logger --preset_odometry with the intent of creating some data sets to try other VIO solutions. I was curious about the sample rate (before I thought about looking in /etc/modalai/voxl-imu-server.conf, where it is configured to 1000 Hz) and decided to calculate it based on the actual data. The image shows a sequence of 1e9/data_imu['timestamp(ns)'].diff()[1:200] which corresponds to the instantaneous frequency between two samples.

First I noticed that there is an offset from the 1000 Hz configured, which according to icm42688.c on line 117, the IMU should be able to reach. I find that the mean of this frequency, which is around 1023.89, is suspiciously related to the inverse of the #define STARTING_CLOCK_RATIO 0.9765625 (which is excactly 1/1.024) parameter in the ICM42688 driver. Is this supposed to mean that the IMU runs at a deterministic 1.024x too fast, or am I missing something?

The second thing I notice is that the frequency (and in turn the delta-time) varies a lot. I see that the timestamp from the sensor itself it not used, but that they are instead generated manually using the rc_ts_filter_... functions. I imagine that the sensor itself is a lot more consistent than this, even if the actual frequency might have some bias. So is this timestamp misleading? Too rough of an approximation?

Moderator

@peterkrull These IMU have inaccuracies in their clocks. The code you are referring to is trying to calculate and adjust for those inaccuracies to provide a more consistent measurement. Otherwise, there would be gaps. The frequency offset could be different for each power cycle or each IMU, so it is generated online.

peterkrull

@Moderator I would just have expected the frequency graph to be smoother. I guess I could adjust the phase_constant and scale_constant parameters if it even turns out to be an issue. I am still a bit confused by the STARTING_CLOCK_RATIO 0.9765625 == 1/1.024 thing. Do these IMUs come from the factory with a clock that is biased like that, and is it just chance that a good initialization for the clock ratio is 1e3/(2^10)?

James Strawson

0.9765625 = 32/32.768

The ICM42688 will run at roughly 32khz +- a few percent when running off its own internal oscillator. Optionally, you can drive it from a standard 32.768khz crystal which is what we opted to do on VOXL2 so the sample rate is consistent between boards.

peterkrull

@James-Strawson That makes a ton of sense, thanks for the explanation! Also just noticed that it mentions this right above the definition. I should just learn to use my eyes