Development of a nonlinear viscoelastic actuator for mastication robot

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In this research, we have proposed and confirmed the effectiveness of a nonlinear viscoelastic actuator for a mastication robot. Human muscle can be approximately modeled as two elements, that is, a contraction element and a serial elastic element. The contraction element includes an elastic and a viscous element. In case of chewing a food, the muscle's stiffness becomes high We focused on these characteristics of the muscle, and designed and made a mechanical nonlinear viscoelastic actuator. Three research steps, development of mathematical models, development of a mechanical model, and basic experiment of the mechanical model were performed.Development of mathematical models : Three mathematical models, type A,B,and C have been proposed. Type A is 'X' shaped model, type B in '<' shaped model, and type C is '<' shaped model. We confirmed the elastic nonlinearity on all mathematical models as results of computer simulations. Furthermore, we confirmed the nonlinear viscoelasticity on type C by computer simulations, that is, the force increases in nonlinear to the elongation and the contraction velocity of the muscle.Development of a mechanical model : A '<' shaped mechanical model was made. A rotary damper (viscous element) was connected in parallel with a helical torsion spring (elastic element). These two elements' characteristics that they have rotary characteristics are transferred into nonlinear characteristics by the attached tendon that pulls robot's mandible.Basic experiment was performed to measure the force-displacement-velocity characteristic of the developed mechanical model. Both of the force-displacement and the force-velocity characteristics of the developed mechanical model showed the nonlinearity in force-displacement-velocity relationships.As conclusions, a nonlinear viscoelastic actuator that has a rotary damper and a helical torsion spring was developed.