Biologically inspired elastic transmission for stiffness variability in actuation: Design and implementation

Muscular actions in the biological world are associated with stiffness/impedance variation in a wide range depending upon the need of the task. Again, in artificial world variable-stiffness-actuation finds applications in areas like legged machines, artificial prostheses, vibration control, automotive suspension, and compliant robots etc. Nonlinear elasticity of transmission is indispensable in any passively variable stiffness mechanisms. Many designs exist in the literature; however, majority of the designs attain nonlinearity for the sake of nonlinearity only; in most cases neither is there a preferred choice of the nonlinear force-displacement function, nor any design guideline exists. In this report a design is presented which is inspired by properties of muscles. A principle is derived from passive properties of muscle fibers and an elastic function is obtained from that. The elastic transmission is designed with an objective and a functional specification for some desired stiffness behaviour. Next, a generic method for realizing any continuous monotonic function is illustrated. Finally, as an exemplar implementation, an agonistic-antagonistic simple actuation arrangement similar to musculoskeletal system is described. Initial result of simultaneous control of motion and stiffness/internal-force on this one degree-of-freedom variable-stiffness-actuator is reported.