Comments (3)
Hi @Youras,
If your MTi device is configured to output SampleTimeFine (which it should be, as otherwise the imu/time_ref topic would not be published) then the MTi will output all of its Acceleration data samples together with its corresponding SampleTimeFine value, as a single data packet.
Upon arrival of this data packet, the imu/time_ref and imu/acceleration topics are created and the header.seq and header.stamp values are assigned by ROS. Only the time_ref field of the imu/time_ref topic is filled with the SampleTimeFine value.
I am not sure what causes the difference in header.seq value here, but considering the above, if the two topics have the same ROS timestamp then I would expect them also to correspond to each other.
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Thanks. for your fast answer @StevenXsens
Okay if time_ref and acc come with the same data packet, then the delay of the low pass filter shouldn't be measurable this way. I can also confirm that /imu/angular_velocity share the same header.seq value with /imu/time_ref at approx. same ROS time stamp.
I forgot to mention, that above I was comparing with /filter/free_acceleration, not with the raw acc data.
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Do time_ref and free_acceleration still come with the same data packet?
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Are the digital low pass filters for the accelerometer and the gyroscope the same (and therefore share the same time delay)?
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Apart from that, when does the SampleTimeFine function actually stamp the time? In the Output Message Generator? considering the following Block diagram:
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Hi @Youras,
Apologies for the delayed response; I was unavailable for the past two weeks.
- The time_ref and free_acceleration messages are indeed grouped together as a single data packet. In fact, the time_ref (SampleTimeFine) timestamp is added to every data packet that is sent out by the MTi.
- The block diagram you copied here actually does not apply to the MTi-620, but to the MTi 100-series. One of the differences is that the MTi 100-series uses analog accelerometers/gyroscopes whereas the MTi-620 uses digital sensors. Unfortunately I do not have details regarding the low-pass filtering but I can say that we take care of properly synchronizing the accelerometer and gyroscope data.
- The SampleTimeFine value represents the time at which each 400 Hz sampling interval starts. The SDI (Strap-Down Integration) block is responsible for this, integrating sensor data sampled at high rates (several kHz depending on your MTi model) down to 400 Hz.
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