Active and Semi-active Vibration Isolation

The conflicting nature of vibration and shock isolation requirements, necessitates design compromises in realizing mounting schemes for systems subject to both vibration and shock perturbations. To avoid making such design compromises, one needs to pursue active and semi-active isolation solutions.

One approach to active isolation is combining an active scheme with a traditional passive isolation system. In such an approach a passive, but adjustable mount (such as an air spring) is used in mounting the machine. The control scheme takes advantage of the adjustability of air as the mounting medium and through the use of controls does on-demand adjustment of the parameters (damping and stiffness) of the isolation system. Such a system uses no additional actuators to exercise the control. Such active system can switch its parameters, promptly, from being an ideal vibration isolator to an ideal shock isolator, depending on the nature of the instantaneous perturbation (vibration or shock). More

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Vibration and Shock Isolation

DEICON, Inc. was invited to make a presentation on shock and vibration isolation at the global superyacht forum in Amsterdam. The following is an excerpt of the talk.

There exist a number of different isolation schemes, e.g., single mounting, double mounting, active isolation, semi-active isolation, air mounting, etc., with different pros and cons. Either one of these schemes can use rubber, air, or even metal (steel) as the isolation medium. Each one of these schemes and media have their own pros and cons. Shock and vibration isolation schemes are required to:

  • reduce the propagation of base vibration to the isolated object (machinery),
  • abate the transmission of vibration energy of machinery to the hull, and
  • lower the impact of shock from the hull to isolated object or vice-versa.

By varying the two main attributes of an isolator, i.e., its stiffness and damping, as well as the mass of the isolated machine, one can either emphasize the achievement of one of above mentioned isolation requirements at the expense of others, or optimize the achievement of all the requirements with moderate levels of effectiveness. The latter is the commonly used approach by designers. The following web page provides a good overview of ‘vibration isolation’ http://www.deicon.com/vibration-isolation .

The metric for measuring the performance of an isolation scheme is what is known as ‘transmissibility’ which is a measure of how much of the vibration force is being transmitted from the isolated machine to the base (hull of a yacht) at various frequencies. ‘Transmissibility’ is also the measure of how much of the motion of the base (support structure, e.g., the hull in a watercraft) will be transmitted to the machine. The goal is lowering the transmissibility at the frequencies where the vibration energy lies, as much as possible, without causing the machine to experience excessive motion.

As indicated in the vibration isolation page (http://www.deicon.com/vibration-isolation ), softer isolators with negligible damping will have the lowest transmissibility at off-resonant frequencies. Their excessive transmissibility at resonance is normally addressed by having the resonant frequencies well below the vibration excitation frequencies. Thus, the softer and the more underdamped the mount, the higher is its vibration isolation erformance. Unfortunately, the improved isolation using soft and highly underdamped isolators is achieved at the expense of excessive low-frequency motion of the isolated machine in response to shock disturbances; we all have seen how a Diesel engine experiences excessive undesirable motion during start up and shut down, straining all the plumbing and wiring connections to the engine. On the other hand, stiffer mounts with high damping are good in tightly holding the isolated machine and thus avoid excessive motion, but they transmit most of the vibration to the support structure.

No one passive solution quite satisfies all the requirements of an ideal isolation system. The common practice used by isolation system designer has been centered around a compromise design which satisfies all the requirements to some degree, but not to the highest possible degree. A number of enhancements to the plain passive isolation have been proposed over the years but again none satisfies all the requirements of an ideal isolation. For example double (two-stage) mounting, while effective at high frequencies, has no better low-frequency isolation effectiveness than a single mounting system. Shock isolation of double mounting is also inferior to that of single mounting. In addition, double mounting imposes unfavorable weight and space penalties.

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Air Isolation of Diesel-Generators

DEICON’s Computer Controlled Air Isolation System is an adjustable mounting scheme capable of meeting the conflicting vibration and shock isolation requirements. This is a pneumatic system capable of changing its stiffness and changing its damping, depending on isolation requirements. DEICON has installed a number of these air mounting system on a number of gensets (Diesel generators) onboard superyachts, achieving 35+ dBs reduction in transmitted vibration, especially at low frequencies; traditional elastomeric-based isolation schemes do not even deliver half as many dBs in isolation performance. This technology can be used in any vibration and shock isolation applications including, but not limited to isolation of precision devices such as MRI machines in hospitals. More

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Seminar on Feedback Control

DEICON, Inc. presented a 3 day seminar on feedback control to Caterpillar engineers in Peoria Illinois. Following some introductory concepts, the seminar covered the design techniques for feedback control applications, with lots of examples and hands-on experiences. The commonly used, popular control strategies such as PI, PD, and PID controllers were emphasized. More

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Tonal Vibration Absorption of Diesel-Generators

In a recent project, abating the 1 order (25 Hz) transmitted vibration from three 170 KVA marine diesel generators onboard a motor yacht, to the hull of the boat was successfully accomplished using a number of small tuned vibration absorbers. Four tuned vibration absorbers, targeting the 25 Hz vibration, were designed, fabricated, and installed on each of the three diesel generators, at their mounting feet. This solution reduced the vibration of these 2-ton machines (engine weighs about 900 Kg and generator weighs 1100 Kg) by more than 80%, at their tuning frequency. More

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