force vectors

[Nithin Kurup]

Hi,

I am trying to simulate a pedalling, and would like to get the force components produced on the pedal as a result to find torque. The leg is joined to pedal by weld/point constraint. The external load applied to the pedal body is a result of gravitational, inertial and muscular forces of the leg.

I am having some doubts

1.Is it possible to get the force vectors from the com of the pedal geometry eg. calculate the acceleration of the com using custom analysis/ kinematics then finding force from it by multiplying with the pedal segment mass.

2. or do I have to implement a contact geometry [ between leg and pedal but doubt on accuracy as very few idea as how to setup the orientation of the sphere.
- But by using contact geometry will the forces produced by the pedal alone eg. due to gravity (mg) etc be included in the calculation apart from the forces transmitted from the leg?


Looking forward to any guidance,

Nithin

[Jiang Ping]

There are some functions in SimBodyEngine class to get force, kinematics information like:

void OpenSim::SimbodyEngine::getAcceleration
(const SimTK::State & s,
const OpenSim::Body & aBody,
const SimTK::Vec3 & aPoint,
SimTK::Vec3 & rAcc ) const

Please check OpenSim Doxygen.

best regards,
jiang

[Nithin Kurup]

Thanks Jiang as always, I will look into the documentation. Hope this acceleration includes the linear and rotational values.

Jiang, I have a construction of a body A welded to body B, and I want to find the force vectors on body B's com.

So if I multiply the the mass of body B with the acceleration vectors from the com of body B, will it give me the total force vectors which include all the gravitational, inertial forces etc on body B and also include the external force from the body A ?


thanks again,

Nithin

[Jiang Ping]

Thanks Jiang as always, I will look into the documentation. Hope this acceleration includes the linear and rotational values.

Jiang, I have a construction of a body A welded to body B, and I want to find the force vectors on body B's com.

So if I multiply the the mass of body B with the acceleration vectors from the com of body B, will it give me the total force vectors which include all the gravitational, inertial forces etc on body B and also include the external force from the body A ?


thanks again,

Nithin

When two body are welted, they can be regarded as one rigid.
So CoM_body_a and CoM_body_b should have same linear acc. but different angular acc.
The external force will have same effect on linear movement of COM_body_a and COM_body_b but different rotational movement of them.
So I think the vector will include information you mentioned above.

I am not sure if i am correct. I think Sherman is more familiar with it. @Sherman

Jiang

[Michael Sherman]

If you know a body's acceleration and mass properties (which you can get from OpenSim or from Simbody), then you can calculate the net force because f=ma. But as you point out that is composed of a number of different effects including joint and constraint reaction forces, gravity, inertial forces, and applied forces (like muscles). So if all you want is the net that works, but if what you actually want is the individual effects then knowing their sum does you no good.

If you have a weld constraint or weld joint connecting two bodies, you can ask for the reaction forces generated there. Those are likely available through the OpenSim API, but if not they are certainly available by requesting the Simbody System from OpenSim's Model, and then using the Simbody API to obtain reaction forces.

Sherm

[Nithin Kurup]

Dear Sherman and Jiang,

Thank you so much for the explanation. Its really helpful.

thanking you again,

Nithin