  This work is devoted to an impedance grasping strategy for a class of
standard mechanical systems in the port-Hamiltonian framework. We embed a
variable rest-length of the springs of the existing impedance grasping strategy
in order to achieve a stable non-contact to contact transition, and a desired
grasping force. We utilize the port-Hamiltonian structure of standard
mechanical systems. First, we utilize a change of variables that transforms the
port-Hamiltonian system into one with a constant mass-inertia matrix. We then
achieve impedance grasping control via a \emph{virtual spring} with a variable
rest-length. The force that is exerted by the virtual spring leads to a
dissipation term in the impedance grasping controller, which is needed to
obtain a smoother non-contact to contact transition. Simulations and
experimental results are given in order to motivate our results.
