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HRL laboratories team achieves breakthrough in dynamically variable negative stiffness structures

Researchers in the HRL Laboratories Sensors and Materials Laboratory have developed an active variable stiffness vibration isolator capable of 100x stiffness changes and millisecond actuation times, independent of the static load. According to Principal Investigator Christopher Churchill, “This performance surpasses existing mechanisms by at least 20 times in either speed or useful stiffness change.”

This isolator has broad applications for makers of automobiles, aircraft, watercraft, rotorcraft, and robotics. An open-access paper on their work is published in the journal Science Advances.

Dynamic stiffness control could be useful to several engineering disciplines that must adapt to large variations in loading rate and frequency, as might be found in automotive aerospace, robotic, and assistive robotic systems. These platforms may use active isolators when vibrations are large, mixed with shocks, or vary in their frequency spectrum. Commercial active systems control dynamics through a counteracting force from an actuator, for example, hydraulics and voice coil. Cost, parasitic weight, and narrow-band effect on vibration and shock have limited their widespread adoption.

—Churchill et al.

Churchill notes that the human body is home to a range of variable stiffness structures that enable efficient load-bearing and nimble activity. Joints, for example, use antagonistic muscle contractions to vary joint stiffness continuously.

Such features in the human body 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.

To develop their system, the HRL team combined active vibration control and nonlinear negative stiffness (NS). They started with a classical quasi-zero–stiffness (QZS) system consisting of positive stiffness (PS) and NS springs in parallel, which together add up to zero stiffness. They then added a solid-state actuator to control the amount of compression in the nonlinear mechanism, which provides NS. This let them control stiffness rapidly and continuously between the stiffness of the positive spring and nearly zero while still retaining excellent isolation performance.

Our system builds on the capabilities of state-of-the-art variable stiffness systems in two ways: speed and amount of stiffness change. Mechanistic methods have inherently low performance at low stiffness, that is, zero force/torque at zero stiffness. This limits the practical stiffness change to about 5× when supporting a load. Pneumatic air springs are easily tunable but trade off friction for volumetric efficiency, whereas smart material–based methods are fast but are limited to about 30% stiffness change. Some NS systems have the ability to change stiffness but only for initial assembly and zero-stiffness matching. To our knowledge, dynamic stiffness switching has not been attempted at this scale and speed.

—Churchill et al.
F1.large
Design and performance of a dynamically variable stiffness structure. (A to D) Mechanical assembly (A and C), equivalent model (B), and measured force-displacement responses (D) for individual components. The vertical k3 (PS) spring is a simple steel plate. The horizontal k2 spring is principally the compliance of the piezoelectric stack. The snap-through mechanism transforms the compression of the k2 spring into a vertical NS spring with a controllable stiffness between +10 and −100 N/mm.

The assembled system stiffness is the sum of the NS and PS springs, resulting in a system with highly variable stiffness. Unlike other variable stiffness technologies, load capacity and stiffness are completely decoupled; in this example, the supported load is 130 N. Source: Churchill et al. Click to enlarge.

 

Advanced lightweight materials are increasingly finding their way into transportation platforms to achieve low mass and high stiffness. Utilizing 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.Christopher Churchill

Resources

  • Christopher B. Churchill, David W. Shahan, Sloan P. Smith, Andrew C. Keefe, Geoffrey P. Mcknight (2016) “Dynamically variable negative stiffness structures” Science Advances doi: 10.1126/sciadv.1500778

Comments

Davemart

Handy for in-wheel motors?

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