Another approach to controlling large degree of freedom movements is to better exploit the mechanical properties of the actuators and physical structures. I have developed several types of actuators, based on a `Real-time Tunable Spring.’ One actuator (used in the amoeba-like soft robot) can change its resting length and therefore its stiffness by winding/unwinding the coiled spring. Another can change its stiffness actively without changing its resting length. I developed an elastic ring whose thickness distribution was carefully designed such that the dynamic stiffness can be altered by rotating the elastic ring (the Youtube Video below). These passive actuators allow the robot’s body parts to interact through body dynamics (e.g., continuum deformation and body structures) without restraining the overall softness of the robots. This also means that a robot designer can focus on the interaction manner, not designing precise trajectory of the each body part.