HCR - Chains of Joints that Refers to the Same End-Effector

The selection of the contact velocities depends on the model of the contact that we used.

There are 3 main contact models:

1. Hard finger with friction
2. Hard finger without friction
3. Soft finger

We will use the 1-st one (Hard finger with friction)

To understand the difference between the first and the second model let’s consider us holding a pen and wanting to write on a piece of paper:

• If the paper is normal (it has friction) I can easily write on it (1-st case)
• If the paper is oily (it has no friction) I cannot write on it, I can only impose a vertical force, I slip away when I apply tangential forces (2-nd case)
• In case of Soft fingers we have reduced control on it, instead of having for its wrench 3 forces and 3 moments we only have 3 forces and 1 moment:$W_{\text{soft-finger}}={\left[\begin{array}{l}\overrightarrow{F}{}_{[3\times 1]}\\ {\overrightarrow{M}}_{[1\times 1]}\end{array}\right]}_{[4\times 1]}$