prescribed dof in forward dynamics

[Anne Schmitz]

I have a knee model with contact. I would like to run a forward dynamic simulation by prescribed knee flexion and have the simulation predict the other degrees of freedom. How do I prescribe the motion of 1 dof in a forward dynamics simulation?

Thanks,

Anne Schmitz

[Ayman Habib]

Hi Anne,

You can modify the model file (osim) by specifying that a coordinate is Prescribed and provide a function that describes its trajectory.

Good Luck,
-Ayman

[Shiuan-Lin Huang]

Hi Ayman

Would you please show me how to specify the <prescribed_function> in .osim file?

I tried to input something like:
<prescribed_function>q= 15.526*t^2 + 2.8708*t - 0.2104</prescribed_function>

But it didn't work. Besides this, is there any tag that should be added to prescribe the motion of a doF?

Thanks!
Shiuan-Lin

[Ayman Habib]

Hi Shiuan-Lin,

The functions available are those that can be represented in XML (typically: Constant, Linear, Spline(s)). We plan to offer Symbolic functions in the future but this is not supported yet. These should be functions of time (x is time).

Hope this helps,
-Ayman

[Shiuan-Lin Huang]

Thank you, Ayman. It works.
I copied the cubic spline function used in .osim,
and pasted experimental values as <y> values.
It works!
I really appreciate your help.

[Sina Porsa]

Hi Ayman
Can I use a spline to define a custom nonlinear passive torque actuator? e.g. the amount of torque which is applied to the joint is a non-linear function of joints angle.
Ragards,
Sina

[Ayman Habib]

Hi Sina,

Which kind of Actuator were you planning to use for this purpose? It's definitely possible to write your own (if that's what you mean by custom).

Please let me know if you need help proceeding in this direction.

Best regards,
-Ayman

[Sina Porsa]

Hi Ayman,
I want to use a non-linear function for "<CoordinateLimitForce>" and also "<BushingForce>".
It would be great if you could help me.
Regards,
Sina