Granular Media-Based Tunable Passive Vibration Suppressor
- Monday, 01 July 2013
Potential applications include vehicle shock absorbers, earthquake protection systems, and explosion protection systems.
A complete, tested, and tunable shock and vibration suppression device is composed of statically compressed chains of spherical particles. The device superimposes a combination of dissipative damping and dispersive effects. The dissipative damping resulting from the elastic wave attenuation properties of the bulk material selected for the granular media is independent of particle geometry and periodicity, and can be accordingly designed based on the dissipative (or viscoelastic) properties of the material. For instance, a viscoelastic polymer might be selected where broadband damping is desired. In contrast, the dispersive effects result from the periodic arrangement and geometry of particles composing a linear granular chain. A uniform (monatomic) chain of statically compressed spherical particles will have a low-pass filter effect, with a cutoff frequency tunable as a function of particle mass, elastic modulus, Poisson’s ratio, radius, and static compression. Elastic waves with frequency content above this cutoff frequency will exhibit an exponential decay in amplitude as a function of propagation distance.
System design targeting a specific application is conducted using a combination of theoretical, computational, and experimental techniques to appropriately select the particle radii, material (and thus elastic modulus and Poisson’s ratio), and static compression to satisfy estimated requirements derived for shock and/or vibration protection needs under particular operational conditions. The selection of a chain of polymer spheres with an elastic modulus ≈3 provided the appropriate dispersive filtering effect for that exercise; however, different operational scenarios may require the use of other polymers, metals, ceramics, or a combination thereof, configured as an array of spherical particles.
The device is a linear array of spherical particles compressed in a container with a mechanism for attachment to the shock and/or vibration source, and a mechanism for attachment to the article requiring isolation (Figure 1). This configuration is referred to as a single-axis vibration suppressor. This invention also includes further designs for the integration of the single-axis vibration suppressor into a six-degree-of-freedom hexapod “Stewart” mounting configuration (Figure 2). By integrating each single-axis vibration suppressor into a hexapod formation, a payload will be protected in all six degrees of freedom from shock and/or vibration. Additionally, to further enable the application of this device to multiple operational scenarios, particularly in the case of high loads, the vibration suppressor devices can be used in parallel in any array configuration. The parallel application of these devices divides the amplitude of the incident vibrations while preserving the frequency content and thus preserving the designed operation of the invention.
This invention includes the design of a novel, self-contained method for adjustably applying (and simply adjusting or tuning) static compression to the chain of spheres while still transmitting vibration through the dissipative and dispersive media. The dispersive filtering effect for this system only exists as predicted in the presence of static compression, which must be applied in application.
However, the mechanical method for applying this compression must be decoupled enough from the vibration source and payload such that vibrations are not primarily transmitted through the static compression mechanism and around the dissipative and dispersive media. This invention utilizes the solution of a soft spring-loaded casing for the chain of spherical particles, designed so that the first mode of the casing spring mass system is within the pass band of the dispersive filter. Attachment points are coupled directly to the first and last particle of the granular chain for simple attachment in between the payload and vibration source. The soft coupling and low-frequency first mode of the casing ensure the vibrations are transmitted primarily through the filtering media.
Performance of the invention was demonstrated using a prototype single-axis vibration suppressor constructed and tested under both high-amplitude simulated pyroshock and lowamplitude continuous broadband noise perturbations. The results show high attenuation with frequency response characteristics in accordance with the theoretical and numerical predictions. Two orders of magnitude reduction were observed in the shock response spectra at frequencies over 1 kHz, and over two orders of magnitude reduction in the peak accelerations for high-amplitude transient shocklike impacts. Approximately one order of magnitude reduction in the shock response spectra at frequencies below 1 kHz, which was attributed to the dissipative effects of the bulk polyurethane material, was observed.
This work was done by Robert P. Dillon, Gregory L. Davis, Andrew A. Shapiro, John Paul C. Borgonia, Daniel L. Kahn, Nicholas Boechler, and Chiara Daraio of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 321-123
4800 Oak Grove Drive
Pasadena, CA 91109-8099
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