Specifications

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Oops, something appears to have gone completely wrong here. The accelerations are noisy and several
times larger than before. What could be the problem? The problem is actually, that the tiny amount of
random noise we introduced by truncating some of the decimals from the marker trajectory gets amplified
by an order of magnitude for each differentiation. The noise is indistinguishable on the position, hardly
detectable on the velocities, but very large on the accelerations. This is a significant problem because the
inertia forces in a mechanical system depend on the accelerations, so the noise will completely dominate the
computation of forces in the system.
AnyBody interpolates the data by a smooth spline interpolation, and it is actually possible to dampen the
noise somewhat by increasing the order of the spline interpolation and/or downsampling the data. Especially
the latter is a good choice because most MOCAP systems sample at a much higher frequency than necessary
for most movements. However, the best way to solve the problem is to low pass filter the data before
sending them to AnyBody and making sure that the data is stored in the text file with as many decimals as
possible. This will get rid of most of the high frequency noise in the MOCAP data. Furthermore, whenever a
model is driven by MCOAP data it is advisable to check that accelerations are not oscillating more than
expected and that accelerations of body parts are not higher than expected. Distal body parts are likely to
sustain more acceleration than proximal parts.
The last lesson in the tutorial on mechanical elements is Lesson 5: Forces
.
Lesson 5: Forces
There are several types of forces in an AnyBody model; forces in joints, forces in muscles, and gravity forces
working on the segments. This section, however, deals only with the application of external forces. The
following two examples illustrate many of the feature.
1. Demo.Forces.any
2. Demo.AnyForce.any