Variable stiffness structures are common in nature, including the human body, but are rarely engineered into mechanical designs. HRL Laboratories has developed an isolator mechanism that it says is capable of changing stiffness 100-fold, and can do so in milliseconds, independent of the static load.

HRL Laboratories introduced negative stiffness to achieve positive results in a vibration isolator mechanism.“This performance surpasses existing mechanisms by at least 20 times in either speed or useful stiffness change,” says Christopher Churchill, lead investigator on the project.

One example of a variable stiffness structure is human limbs, which can stiffen to lift a bowling ball, but also can soften to paint with the tip of a brush. Such versatility is rarely seen in engineered systems because of complexity and costs. (Watch a video of the process.)

When variable stiffness is pursued in mechanical structures, the traditional approach has been to start by designing a “soft” system, then add damping and force. HRL Laboratories took the opposite approach, building a stiff system, then softening it. Churchill says that approach resulted in lower cost and higher bandwidth. Moreover, he says the ability to soften stiff systems on demand could solve shock and vibration problems.