Compliant actuators are more advantageous than stiff actuators in some circumstances, for example, unstructured environment\nrobots and rehabilitation robots. Compliant actuators aremore adaptive and safe. Constant stiffness compliant actuators have some\nlimitations in impedance and bandwidth. Variable stiffness actuators improve their performance owing to introducing an extra\nmotor to tune the stiffness of the actuators. However, they also have some limitations such as the bulky structure and heavy weight.\nIt was also found that there are some waste functions existing in the current variable stiffness actuators and that the fully decoupled\nposition control and stiffness tune are not necessary, because there exist some regular phenomena during most circumstances of\nhuman interaction with the robots which are ââ?¬Å?low load, low stiffness and high load, high stiffnessââ?¬Â. In this paper, a design method\nfor nonlinear stiffness compliant actuator was proposed which performed the predefined deflection-torque trajectory of the regular\nphenomenon. A roller and a cantilever which has special curve profile constitute the basic mechanical structure of the nonlinear\nstiffness compliant actuators. An error compensation method was also proposed to analyze the stiffness of elastic structure. The\nsimulation results proved that the proposed method was effective in designing a predefined nonlinear stiffness compliant actuator.
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