Since the matrices K {\displaystyle {\boldsymbol {K}}} and D {\displaystyle {\boldsymbol {D}}} have the dimension of stiffness and damping, they are commonly referred to as stiffness and damping matrix, respectively. Clearly, the controlled robot is essentially a multi-dimensional mechanical impedance (mass-spring-damper) to the environment, which is addressed by τ e x t {\displaystyle {\boldsymbol {\tau }}_{\mathrm {ext} }} .