Control System Design and Implementation for a Novel Mechanically Overdamped Actuator with Adjustable Stiffness




Wahrmund, Christian

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Traditional rigid robotic actuators have, in recent years, been improved upon by SEAs (Series Elastic Actuators) and VSAs (Variable Stiffness Actuators). SEAs and VSAs decouple the load or output link of an actuator from the driving motor, usually by using a spring or another mechanical device capable of rapidly storing and releasing mechanical energy. This thesis focuses on the mechanical modeling, simulation, and ultimately evaluation of a novel high impedance VSA dubbed MOD-AwAS (mechanically overdamped actuator with adjustable stiffness) under a step input.

After finding equations of motion for the system, a state space model was formed. Then, the response of the open and closed loop system, P, PI, Pole Placement, and an LQR (linear quadratic regulator) controller were simulated and evaluated. The PI controller showed the best response in simulation and was selected for implementation and evaluation on the physical system. Using a Vicon motion capture system, rise time, settling time, percent overshoot, and steady state error were evaluated for the open loop, closed loop, P, and PI controllers at system stiffness values of 35, 50, and 400 Nm under a position step input. It was found that MOD-AwAS exhibited an overdamped open loop response, atypical of most VSA's, which exhibit an underdamped response. The PI controller improved system dynamics, most notably by reducing overshoot from 21.34% to 6.44% and steady state error from 5.44 to .41 degrees.


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Actuator, Control, High Impedance, Variable Stiffness Actuator, VSA



Mechanical Engineering