Mechatronic design of an actuated biomimetic length and velocity sensor

Biological designs offer roboticists a rich source of mechanisms for the challenge of controlling movement. Our device draws upon this resource, modeling the muscle spindle, a biological sensor which transduces muscle length and velocity for kinesthetic awareness and movement control. The three core neural and mechanical elements of the muscle spindle are identified and implemented in precision engineering hardware using performance specifications derived from biological literature. Intrafusal muscle is modeled by a linear actuator fast enough to replicate muscle dynamics. Its step response exhibits 27 ms rise time and 9.2% overshoot. Sensory region transduction of strain into voltage is modeled by a strain-gauged cantilever 51 μm thick. A voltage-controlled oscillator, encoding voltage as a frequency-modulated square wave, models action potential frequency encoding. The transducer exhibits the desired linear response with a 34-nm/Hz sensitivity. The three subsystems were combined to perform integrated systems testing. Driving the actuator with simple position control, the device detects trajectory-tracking errors introduced by phase lag and perturbation. Driving the actuator with physiologically based force control, the device successfully replicates the major features of muscle spindle response under ramp and sinusoidal position inputs. Applications include motor control research and novel sensor design for prosthetics and engineering.