Latch on, launch off

Spring-powered latch mechanisms have been around for a long time—Leonardo da Vinci’s cam hammer is one early example. Compressed springs store large amounts of potential energy, and latches are used to contain the energy before releasing it all at once. In fact, the fastest organisms in nature are powered by springs, not muscles. Dracula ants use spring-latch systems for some of the fasted recorded strikes in nature, froghoppers use them for some of the highest jumps, and some fungi even use them to eject spores.

“In the past, latches were assumed to be really simple things,” said Sarah Bergbreiter, professor of mechanical engineering. “They were either compressing the spring and holding it in place, or they were just gone.”

Bergbreiter and her team of Carnegie Mellon University researchers teamed up with other universities to publish a paper describing how latches can be far more complex than previously thought.

In general, latch mechanisms can be either instantaneous or delayed. Instantaneous latches release the energy as fast as possible, but delayed latches have more control over the amount of energy that is released and when that happens. Instantaneous latches tend to have straight edges, while delayed latches tend to have round edges. Some latches can even behave like either instantaneous or delayed latches, depending on how fast they are pulled out of the way.

“In this paper, we're looking at the complexity of the unlocking process and how that can ultimately affect the performance of the system,” Bergbreiter said.

When designing, engineers tend to use a straight edge latch in their design to create an instantaneous latch. In biology, however, straight edges are not found. This forced the question: why do these super-fast organisms have round latches that engineers would avoid?

The rounded edges found in biological latches can be thought of as part of a circle with a known radius. If the radius of the latch is big enough, the team found that the latch can display either instantaneous or delayed latch properties.

We’re looking at the complexity of the unlocking process and how that can ultimately affect the performance of the system.

Sarah Bergbreiter, Professor, MechE

To find this result, the team analyzed latch systems using mathematical models, physical models, engineered systems, and Dracula ants in biology. When the latch was quickly released, the projectile was launched at a higher speed, like an instantaneous latch. The opposite was true when the latch was slowly released, and it behaved like a delayed latch. The team was able to find a threshold unlatching speed, above which the latch acts like an instantaneous latch and below which acts like a delayed latch. With these results, unlatching speed can be varied to achieve the desired projectile velocity.

“It comes down to how fast you move it out of the way. If I move it out of the way super-fast, the system hasn't even had time to react, like an instantaneous latch,” Bergbreiter said. “If I move it out of the way really slowly, then I get only a tiny amount of energy out in the end. And then there's the whole swath in between.”

The ability to control launch velocity has many exciting implications. Jumping robots could vary the latch release speed to have more control over the distance of their jumps. Biologists can use these results to look for biological latches. And researchers have a new language to talk about latch mechanisms.

When conducting this research, CMU engineers teamed up with physicists and biologists for a wholistic analysis of latch mechanisms.

“The entire project is very interdisciplinary,” Bergbreiter said. “Working with this group of people has helped me explore some of these more fundamental questions.”

The paper was published in the Journal of the Royal Society Interface in 2020. CMU mechanical engineering Ph.D. students Sathvik Divi and Xiaotian Ma and postdoctoral researcher Ryan St. Pierre were also authors of the study. Additional authors include Mark Ilton from Harvey Mudd College, Babak Eslami from Widener University, and S. N. Patek from Duke University.