Session: 02-04-01: Experimental Studies in Structural Dynamics

Paper Number: 121625

121625 - Zero Stiffness in Bellow-Type Soft Pneumatic Actuators for Modal Analysis of Flexible Aircraft Wings 

Modern aircraft designs lean towards wings with higher aspect ratios to decrease induced drag. While this design change offers aerodynamic benefits, it also leads to more flexible wings. More flexible wings result in pronounced in-flight deformations and significantly lower eigenfrequencies, with the lowest potentially lying below 1 Hz. This flexibility can increase susceptibility to flutter, an aeroelastic phenomenon that poses significant risks. It is crucial to accurately determine a wing's vibrational modes, especially in its deformed, in-flight state. To replicate these conditions in a ground-based experimental test stand, simulating the in-flight air loads becomes necessary. This implies using actuators that can introduce forces without adding stiffness. In the field of vibration isolation, such load-bearing elements are commonly referred to as quasi-zero-stiffness (QZS) vibration isolators.

Most QZS isolators are constructed by combining a positive and a negative stiffness element to achieve the desired zero-stiffness. While they serve well for vibration isolation, their fixed load-bearing capability makes them unsuitable for varying conditions like simulating different wing load cases. This leads to the search for mechanisms with QZS properties under varying loads. While active systems, particularly those that are closed-loop controlled, would have all the desired properties, they are often bypassed due to their associated cost and complexity.

Pneumatic cylinders, while not classified as QZS systems, can inherently exhibit near-zero-stiffness properties across different loading conditions due to their ability to decouple force and stiffness through large external volumes. However, a significant drawback is their susceptibility to the stick-slip effect, especially evident for low-frequency vibrations. In response to this challenge, bellow-type soft pneumatic actuators present a compelling alternative. These actuators not only bypass the stick-slip issues due to their frictionless design but can also exhibit a zero or even overall negative stiffness at certain points.

This research revolves around a 3D printed air bellow, devised for iterative design and parameter studies. Initial experiments quantify the actuator stiffness for different deformations and internal pressures, although no zero-stiffness region is observed. A simulation model is setup, which sheds light on the critical parameter combinations influencing the occurrence of a zero-stiffness region. Notably, factors like the ratio of radius to height of the bellow and the internal pressure are identified as significant, while the wall thickness has lesser influence. Follow-up experiments are designed to validate these findings from the simulations.

In conclusion, this study indicates the potential of bellow-type soft pneumatic actuators as versatile zero-stiffness actuators. Preliminary results suggest their capability to achieve zero stiffness under specific conditions. A promising avenue for future work is to expand the zero-stiffness region of these actuators for various combinations of pressure and displacement by introducing an additional external volume, enhancing their adaptability for modal analysis of flexible aircraft wings.

Presenting Author: Moritz Sprengholz Technische Universität Braunschweig

Presenting Author Biography: Moritz Sprengholz started his mechanical engineering studies at Braunschweig University of Technology in 2015 and wrote his Bachelor's thesis on "Runway Detection in a Simulation Environment via Neural Networks" in 2018. He then shifted to aerospace engineering from 2019 to 2021, wrapping up with a master's thesis on "Structural and Aerodynamic Tailoring of Triply Periodic Minimal Surface Structures" at the Institute of Mechanics and Adaptronics. Since then, he's been a research assistant and PhD student at the same institute. His work focuses on the challenges of flexible aircraft wings for next-generation planes. Collaborating with various industries and universities, Moritz is looking at problems like flutter and is developing a ground-based test stand to safely investigate new aircraft technologies.