User's Manual

Document MV0319P.N
© Xsens Technologies B.V.
MVN User Manual
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If a reset is required during measurements, it is also advised that the subject stands still when the reset
is enabled and remains still for 30 seconds thereafter. In these 30 seconds, the gyroscope biases are
estimated and the uncertainties minimized.
If there are issues regarding problematic magnetic field distortions follow the instructions below:
1. Configure the system (see section 7.1).
2. Move all motion trackers in smooth 3D, circular motions. This can be done with hardware
1. In the suitcase or rucksack, for approximately 3 minutes.
2. On the subject for approximately 6 minutes.
3. Perform a basic calibration (N or T pose).
4. Stand still for 30 seconds.
19.10 Update Rate versus Sample Frequency
Since MVN Studio 4.0, Xsens refers to the term “update rate” rather than sample frequency. This is an
intentional change in terminology since the data is sampled in a different way than in the past.
Normal, digital systems using some form of recording relies on sampling of this data. Since data is
sampled, this implies some degree of data loss at those time instances that no sampling took place.
Sampling must take place a measureable time, some amount of times over a given time frame. This is
known as the sampling frequency. Most systems operate at a given sample frequency.
The intentional difference in terminology is needed to highlight that data from new Xsens motion trackers
(MTx and MTw) undergoes a mathematical method called Strap Down Integration. This ensures that no
loss of accuracy is experienced, even though the data is transmitted to the PC at lower update rates.
Update rates as low as 20 or 30Hz still provides very accurate data.
19.11 Strap Down Integration
Strap down integration (SDI) is a method used to compute an orientation/position change given an
angular velocity and linear acceleration of a rigid body. Angular velocity and acceleration data in the
MTx and MTw is sampled at a very high frequency (1000Hz) to maintain accuracy under very dynamic
conditions such as vibrations and impacts. The sampling frequency is too high to be transmitted
wirelessly, and would typically present a computational load that is too high on the receiving host device
(e.g. PC). Therefore the data that has been sampled at a high frequency is digitally filtered and down-
sampled to 600 Hz and calibrated using the individual device calibration
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values on the embedded
microprocessor (MCU) of the MTw. Based on this data the MTx and MTw further calculates velocity
increments and orientation increments using an SDI algorithm. The main advantage of using the SDI
algorithm is that full 3D tracking accuracy can be maintained even if the output update rate from the
MTx and MTw is lower (<100 Hz). An additional advantage is that the MTx and MTw can continue to
track the 3D motion internally when facing transient data loss in the RF transmission and immediately
report the full 3D velocity and orientation increment as soon as the RF link is restored, without the need
to re-transmit all the data that was lost which would a) cause a large delay during real-time tracking b)
use a lot of precious RF bandwidth and consume unnecessary power. The specific use of SDI data in
combination with such a specialized RF protocol, Xsens has named the Awinda protocol
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.
19.12 The Awinda Protocol
The Awinda protocol is the wireless protocol that controls the data stream from the MTws to the Awinda
Station or Awinda Dongle. This wireless protocol provides accurate time synchronization of up to 32
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The devices are calibrated at the factory by Xsens and is a highly sophisticated process compensating for
component errors that are not stochastic (i.e. they can be modeled). Compensation models include bias, gain,
misalignment, g sensitivity, temperature effects, etc.
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Patents pending.