An issue of OpenSim ID analysis need help

[Xiangjie Meng]

Thanks a lot for reading this topic. Recently, I simplified Gait_2354 model into a five link model and try to make inverse dynamical analysis to get driving torques needed for each joints. I try to get angular velocity and angular acceleration by "w=(Theta(i)-Theta(i-1))/(Time(i)-Time(i-1)); a=(w(i)-w(i-1))/(Time(i)-Time(i-1))", but the results generated large noises.

I found that angular velocity and angular acceleration generated by "Analysis Module" both are very smooth. Could you please tell me the OpenSim and Simbody's method of generating corresponding angular velocity and angular acceleration by joint angles? Are there special filter methods used?

Thanks a lot. I am looking forward to your reply.

Best,
Xiangjie

[Jiang Ping]

hi,

There are some method like getAcceleration() or getSpeed() available in OpenSim to access coordinate or body acc. and speed.
w=(Theta(i)-Theta(i-1))/(Time(i)-Time(i-1)) is not precise linear approximation.

best regards,
jiang

[jimmy d]

Hi Xiangiie,

The coordinate data gets splined and defined as a function so it is continuous and differentiable. The souce code can be viewed here

Cheers,
-James

[Xiangjie Meng]

Dear Jiang

Thanks a lot. It is helpful. I want to make a 5 link model in Matlab and use OpenSim to check the results. So, I need to get how to generate angular velocity and angular acceleration.

Best,
Jack Meng

[Xiangjie Meng]

Dear James,

Thanks a lot. This is very helpful. I also found the signal.cpp. Just as what you said, from this file, I found OpenSim uses GCVSpline and 3rd lowpass IIR Butterworth digital filter to get angular velocity and acceleration needed.

Best,
Jack Meng