as the closed-loop system, which is essentially a multi-dimensional mechanical impedance to the environment ( F e x t {\displaystyle {\boldsymbol {\mathcal {F}}}_{\mathrm {ext} }} ) as well. Thus, one can choose desired impedance (mainly stiffness) in the task space. For example, one may want to make the controlled robot act very stiff along one direction while relatively compliant along others by setting