Using Latches for Mediating Energy Output in Ultrafast Systems

 Many small organisms rely on ultrafast movements such as jumping, for a variety of locomotory purposes: from predation to flight. Despite their small size, these organisms achieve accelerations that are far beyond the capabilities of their muscles alone. They circumvent the force-velocity constraints of their muscles by instead utilizing spring-driven actuation to drive the power delivery. Recent research in this interdisciplinary area of ultrafast systems has led to the establishment of the “Latch -Mediated Spring Actuation (LaMSA)” framework for analyzing such systems in both biology and engineering. A key component of this paradigm is the latch that mediates the release of spring energy to power the motion. Latches have traditionally been considered as switches; they maintain spring compression in one state and allow the spring to release energy without constraint in the other. Using a reduced-order template model that incorporates a contact latch, it is shown that latches can mediate the release of energy from the spring resulting in variable energy output. This mediation is influenced by the latch parameters: latch radius and velocity. A critical threshold is identified between instantaneous and delayed release that depends on the latch, spring, and mass of the system. It is shown how spring-actuated systems can attain a wide range of output performance, including instantaneous behavior, by changing either the latch release velocity or radius. These results are then experimentally validated. 

This work is then expanded to test the concept of using a latch for varying the jump performance in a synthetic jumper. The reduced-order model is expanded to accommodate the dynamics of the actuator pulling the latch, and the effect of spring force on the latch. A new metric, ‘Tunability Range’ is defined to capture the range of tunable jump behaviors that a jumper can attain given a fixed range of control inputs (i.e., latch actuation voltage) to choose from. It is shown that, for a fixed set of control inputs, a LaMSA jumper with a larger latch geometry has greater tunability. However, this greater tunability comes with a trade-off in maximum performance. Through modeling and experimental validation with a 4g jumper, it is demonstrated that jump performance in small insect-inspired resource-constrained robots can be tuned to a range of outputs by using latch mediation, despite starting with fixed spring potential energy.
This work is further expanded to explore how these jumpers are tuned to their environments. In particular, the role that latches play in maximizing jump performance is explored. The analytical model is expanded to incorporate the environment. Through this model, the conditions in which a jumper can either lose energy to the substrate or recovery energy back from the substrate is shown. This work illustrates the crucial role that latches play in the ability of a system to adapt its jump performance to a wide range of substrates that vary in their inertia and compliance. These results are then validated experimentally using a 4 g LaMSA jumper for which latch parameters can be changed. Finally, a demonstration of the jumper recovering energy from a tree branch during take-off is shown, thereby extending these mechanistic findings to robots interacting with a more natural environment.
In addition to this, a programmable substrate setup is designed and built to virtually emulate a variety of substrate conditions. This is done through the implementation of a second-order impedance control on a brushless DC motor. This programmable substrate enables rapid testing of a wide variety of environmental conditions by emulating them as a mass-spring-damper system. The viability of this programmable substrate is demonstrated by using a 108 g Salto jumping robot. The robot is made to jump from the programmable substrate for a variety of substrate conditions. The robot’s performance across different substrate conditions is then evaluated to showcase how the trends are similar to that of 4g jumper jumping on physical substrates.
Through the series of works described above, the role of latches in latch-mediated spring actuation systems is studied. This work serves as a foundation to further our understanding of ultrafast systems, both across the engineering and natural world, to help robustly design mechanically intelligent robots at small scales.