movesense - Android app dev: Finding the best way to synchronize the timestamps of two sensors

Question

There's already a good answer on the technical details and constraints of timing the gyro measurement: Movesense, timestamp source of imu data, and timing issues in general

However, I would like to ask more practical question from the Android app developer perspective working with two sensors and requirement for high accuracy with Gyro measurement timing.

What would be the most accurate way to synchronize/consolidate the timestamps from two sensors and put the measurements on the same time axis?

The sensor SW version 1.7 introduced Time/Detailed API to check the internal time stamp and the UTC time set on the sensor device. This is how I imagined it would play out with two sensors:

1. Before subscribing anything, set the UTC time (microseconds) on the sensor1 and sensor2 based on Android device time (PUT /Time)
2. Get the difference of the "Time since sensor turned on" (in milliseconds) and "UTC time set on sensor" (in microseconds) (on sensor1 and sensor2) (GET /Time/Detailed).
3. Calculate the difference of these two timestamps (in milliseconds)(for both sensors).
4. Get the gyro values from the sensor with the internal timestamp. Add the calculated value from step 3 to the internal timestamp to get the correct/global UTC time value.

Is this procedure correct? Is there a more efficient or accurate way to do this? E.g. the GATT service to set the time was mentioned in the linked post as the fastest way. Anything else?

How about the possible drift in the sensor time for gyro? Are there any tricks to limit the impact of the drift afterwards? Would it make sense to get the /Time/Detailed info during longer measurements and check if the internal clock has drifted/changed compared to the UTC time?

Thanks!

Answer 1

Very good guestion!

Looking at the accuracy of the crystals (+- 20 ppm) it means that typical drift between sensors should be no more than 40 ppm. That translates to about 0.14 seconds over an hour. for longer measurements and or better accuracy, a better synchronization is needed.

Luckily the clock drift should stay relatively constant unless the temperature of the sensor is changing rapidly. Therefore it should be enough to compare the mobile phone clock and each sensor UTC at the beginning and end of the measurement. Any drift of each of sensors should be visible and the timestamps easily compensated.

If there is need to even more accurate timestamps, taking regular samples of /Time/Detailed from each sensor and comparing it to the phone clock should provide a way to estimate possible sensor clock drift.

Full Disclosure: I work for the Movesense team