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Signal processing pipeline - fidelity of 400 Hz signal

In the spec sheet, you may have noticed that the bandwidths of the rate of turn and acceleration signals have a higher value than 50% of the (standard) output data rate of 400 Hz. Theoretically, this would not be possible, but it means that all the information of higher frequencies is integrated into the signal, even if the output data rate is very low. 

The most crucial part of the signal processing chain is when the analog signals are transformed (sampled) to digital domain. At this point, if you have sufficient bandwidth (representing the motion captured) and high enough sampling rate then the information will be stored in the signal. 

The next step in the signal processing pipeline is the SDI algorithm: the 2000Hz signal (refer to this article) stepped down to 400Hz, using the SDI algorithm with coning/sculling compensation, has all the information preserved for the said bandwidths of motions digitized, and it applies only to dq and dv. The frequency components are not there but since we are working on the integrals the delta changes at 400Hz take you through the same motion trajectory as you would with the 2000Hz signal. 

Example:

  • Imagine a gyroscope signal at 2000 Hz, that is rotating at 20 deg/s for 5 ms and 0 deg/s for the next 5 ms. After 10 ms, the integrated signal of the gyroscope (dq) would be 0.1 deg (ignoring all other error sources such as bias and noise). The information in this signal is completely preserved, like in the 2000 Hz signal.
  • When using the Xsens SDI algorithm, all of the signal of the 2000 Hz will be integrated into the 400 Hz signal. The dq values resulting from that will have the high bandwidth, yet the output data rate is lower. The total integrated signal after 10 ms will still be 0.1 deg. This situation is completely different when the signal is discretely sampled at 400 Hz: information will be lost. 

The SDI algorithm in all Xsens' products is extremely important for the orientation performance and when an orientation sensor does not have proper strapdown integration into place, the resulting orientation will have errors. 

See for more information the MTi white paper: https://www.xsens.com/whitepaper-download/

 

 

 

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