hi,
I have executed opensim analysis with data collected from vicon nexus system.
the methodology used was Scale-IK-CMC.
But when I try to compare the graphs of knee flexion angles of Opensim output and vicon outputs(for validation of analysis), I get a very large time lag (2-3sec)at the end of the opensim output. Even though the time lag seems very less it affects the output. Also the angles are not same.
Why does these errors occur?
CMC output validation
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saylee patkar
- Posts: 35
- Joined: Wed Jun 20, 2018 1:03 am
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Dimitar Stanev
- Posts: 1096
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Re: CMC output validation
Can you post the figures?I get a very large time lag (2-3sec)at the end of the opensim output.
It is possible that the coordinate system in OpenSim (e.g., knee flexion is negative) and Nexus are different. OpenSim follows the ISB conventions.Also the angles are not same.
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saylee patkar
- Posts: 35
- Joined: Wed Jun 20, 2018 1:03 am
Re: CMC output validation
Dear sir,
After some trial and error I noticed that the problem is with Opensim output.
1. Opensim gives me output of knee flexion for both knees as almost equal values but the original trial file clearly shows that it is different for both legs.
2. The output of CMC is not equally distributed(with specific step size). This is the reason the graphs of Opensim and Vicon are having time lag.
Is there any method to solve these issues?
Sorry, I am not allowed to share any data on the general forum post.Can you post the figures?
To solve this I took inverse of all the values to from the Opensim output(multiplied by (-1)).It is possible that the coordinate system in OpenSim (e.g., knee flexion is negative) and Nexus are different. OpenSim follows the ISB conventions.
After some trial and error I noticed that the problem is with Opensim output.
1. Opensim gives me output of knee flexion for both knees as almost equal values but the original trial file clearly shows that it is different for both legs.
2. The output of CMC is not equally distributed(with specific step size). This is the reason the graphs of Opensim and Vicon are having time lag.
Is there any method to solve these issues?
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Dimitar Stanev
- Posts: 1096
- Joined: Fri Jan 31, 2014 5:14 am
Re: CMC output validation
When you open the .sto files containing the results you can read the time (first column) data. Therefore, you can plot the knee flexion angle with respect to time.
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saylee patkar
- Posts: 35
- Joined: Wed Jun 20, 2018 1:03 am
Re: CMC output validation
hi sir,
About your suggestion,
I exported required graph data in excel format and in that there are three columns.
1: Frame/ step
2: Time
3: value/quantity
So, the graph we get from the plot window is with respect to time(second column) or frame/step(first column)?
(Updating with one more doubt:)
Also, is there any step in IK where the tool calibrates the motion and gives same output for both lower limb? because, I am getting output of both knee joints as same whereas ideally it should be different(observed in vicon data).
About your suggestion,
I exported required graph data in excel format and in that there are three columns.
1: Frame/ step
2: Time
3: value/quantity
So, the graph we get from the plot window is with respect to time(second column) or frame/step(first column)?
(Updating with one more doubt:)
Also, is there any step in IK where the tool calibrates the motion and gives same output for both lower limb? because, I am getting output of both knee joints as same whereas ideally it should be different(observed in vicon data).