How to compute the contact Jacobian?

[xiaosuyang]

Hello pinocchio community. I'm working on Biped robot 's walking control.
In the formula:

How to calculate the contac jacobian using pinocchio?
Can I Establish a coordinate system under the foot surface and then use getFrameJacobian(model,data, footframe_index, LOCAL) or use the getFrameJacobian(model,data,footframe_index,LOCAL_WORLD_ALIGNED) ?

[ManifoldFR]

Hello mate,

You are in the right headspace - one would indeed usually go about this by creating a pin.Frame attached to the foot (adding it to the model object) and grabbing the Jacobian for that frame.

However, pin.getFrameJacobian() actually requires that pin.computeJointJacobians() and pin.updateFramePlacements() were called first - because getFrameJacobian() composes the parent joint's jacobian with an appropriate matrix to account for the frame's relative placement.

As a shortcut (which is more efficient for a single Jacobian because it will skip superfluous computations) you should use pin.computeFrameJacobian() instead. Have fun !

For good measure - you didn't have to close #2200 (which I've deleted) and open this one, editing the previous one was enough (we get a bunch of e-mail notifications when someone opens a discussion/issue 😅)

[xiaosuyang]

Thank you for your guidance. But in above formular, if the external force $f_r$ is expressed in world frame, which frame should we use to map the $f_r$ to the joint space, LOCAL_WORLD_ALIGNED or the WORLD frame?

[xiaosuyang]

I use the MPC to compute the reaction force($f_r\in F^6$) of a point of the stance foot. And I compute the geometric jacobian(Use the biped 's torsor frame as a fixed base) $J (6\times6)$ 6 dof a leg to map the $f_r$ to the joint, and it seems that it work. But the geometric jacobian $J$ is not the contact jacobian including the floating base joint， That was confusing.

[cmastalli]

Hi @xiaosuyang,

If you are interested in computing the "dynamics", then the most efficient option is to map your contact forces into its body frame. ABA and RNEA algorithms recursively account for such a spatial force without involving an inefficient matrix-vector multiplication.

Computing contact Jacobians is needed when you need to impose "contact constraints".