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Vibration isolators can vary stiffness 100-fold in less than 10ms

23 February, 2016

A Californian research-and-development organisation owned by Boeing and General Motors has developed an active variable-stiffness vibration isolator capable of 100-fold changes in stiffness and actuation times of less than 10ms, independent of the load.

HRL Laboratories says that the development, by researchers in its Sensors and Materials Laboratory, has many potential applications, including variable-stiffness joints for robots that could lead to improvements in safety and efficiency, and to more natural movements.

According to the principal investigator on the project, Christopher Churchill, the performance of the isolator technology “surpasses existing mechanisms by at least 20 times in either speed or useful stiffness change”.

The development has been inspired by the human body which contains various variable-stiffness structures that enable efficient load-bearing and nimble activity. “The most ubiquitous tunable stiffness structures are our own joints, which use antagonistic muscle contractions to vary joint stiffness continuously,” Churchill explains. “For example, limbs will stiffen to lift a bowling ball, but soften to paint with the tip of a brush.”

Yet these characteristics are rarely replicated in engineered systems due to the complexity, power, and cost of doing so. Churchill says that the traditional approach – building a soft system and then adding damping and force – is expensive and low-bandwidth.

“We developed a new paradigm, and instead built a stiff system and then softened it,” he says. The result is a low-cost and high-bandwidth answer to long-standing challenges.

A demonstration rig for HRL's variable-stiffness vibration isolating technology
Photo: Dan Little /HRL Laboratories

The HRL researchers say that their technology could be applied in robotic surgical systems that can vary stiffness dynamically to manipulate a scalpel and lift a patient from their bed with the same joint.

Another possible application is in semi-passive mechanical isolators that avoid multiple input frequencies encountered in transport systems. “Advanced lightweight materials are increasingly finding their way into transportation platforms to achieve low mass and high stiffness,” says Churchill. “Utilising adaptive negative stiffness to soften stiff systems on demand has the potential to solve shock and vibration problems that only get more difficult with these next-generation platforms.”

The HRL researchers are continuing their work, focusing currently on multi-axis loading, optimising actuators and improving their efficiency, and handling low-frequency load variations.

A paper describing the HRL research team’s findings, called Dynamically Variable Negative Stiffness Structures, is published in the February 2016 edition of Science Advances.

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