MEAM Ph.D. Thesis Defense: “Bistable Structures Enable Passive Transitions in Mobile Robots”

Making robots more capable, agile, and efficient requires careful design of the robot’s mechanical body to match task requirements. Passive components allow a robot to perform a task without a dedicated actuator, often improving both power consumption and overall performance. In this thesis, we investigate robotic applications of bistable mechanisms, mechanical structures that exhibit two stable equilibria, to enable passive actuation and locking for systems with discrete task modes.

The main underlying idea of this thesis is that “force reversal” provides a practical means of causing bidirectionally passive snap-through of bistable structures, meaning that in the frame of the bistable mechanism, force must be applied toward each target equilibrium state. Force reversal can be produced through a variety of means that do not necessarily require direction change in the actuators. In particular, the design of the mechanism surrounding the bistable structure serves as transmission to redirect actuation forces. We demonstrate this idea in the context of three unique systems: a gripper that uses contact, a gripper that uses a twisting Kresling origami pattern, and a morphing aerial vehicle that uses inertial forces.

More specifically, the main theoretical contribution of this thesis is a method for determining the actuation force requirements for dynamically-actuated bistable mechanisms, where inertial forces are responsible for producing snap-through. In this case, there is a direct relationship between the inertial forces and the output force of the actuators that produce the associated motions. We find that the minimum actuating force required for snap-through depends on the ratio between the mass on the bistable structure and the robot’s total mass, and that it also depends on friction but not on viscous damping. The main experimental contribution includes demonstrations of the impact of bistable mechanisms on grasping and flying systems. For perching, we show that attaching a linkage to a passive bistable structure augments a gripper’s locking strength, leading to passive grasping with a high strength-to-weight ratio. For aerial reconfiguration, we demonstrate that the energy cost of passive dynamic transformation can be offset by the efficiency gains of transforming from a quadrotor to a fixed wing mode. Overall, this thesis shows that passive bistable mechanisms can eliminate the need for task-specific actuators by repurposing existing locomotion actuators.