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Shock Isolation Seminar Report

This Report is the study on an experimental switching stiffness device for shock isolation. Shock isolation of a lightly damped system is achieved by switching the stiffness of isolator between a high-state and a low-state. The system uses magnetic forces to exert a restoring force, which results in an effective stiffness that is used to isolate a payload. Switchable stiffness is obtained by turning the magnet on and off. Characterization of the physical properties of the device is presented. They are estimated in terms of the percentage stiffness change and effective damping ratio when switched between two constant stiffness states. Additionally, the setup is used to implement a control strategy to reduce the shock response and minimize residual vibration.

The conclusions drawn from the seminar are that the system was found to be very effective for shock isolation. The response is reduced by around 50 percent compared with passive isolation showing good correlation with theoretical predictions, and the effective damping ratio in the system following the shock was increased from about 4.5 percent to 13 percent.

Shock is a very short or transient excitation characterized by high displacements and accelerations that can potentially lead to large mechanical stresses and can cause human discomfort. The process of shock isolation is based on energy storage by an elastic element normally requiring large deflections and the subsequent dissipation of the stored energy by some damping mechanism.

A shock isolator is a passive mount comprising some form of mechanical spring and/or viscoelastic elements. The analysis of shock isolation systems is often performed by considering single degree-of-freedom models with low damping under the effect of pulse input functions.

The concept of switchable stiffness can be used as a means of energy dissipation in lightly damped systems, where it is difficult to implement another form of external damping. This is effectively a semi-active control strategy. The model presented in comprises a mass supported by two springs, one of which can be disconnected. Switching in and out of the spring involves a two-stage control strategy; stiffness control during the shock to reduce the maximum response of the payload, and reduction of the residual vibration after the shock has occurred. The theoretical simulations presented demonstrate that it is possible to obtain better shock isolation by switching the stiffness in lightly damped systems. The main motivation in developing such strategies is to improve methods of shock isolation. Potential engineering applications include the protection of sensitive electronic devices in harsh environments, for example in ships and military vehicles, low frequency and semi-active vibration isolators, aerospace structures and earthquake engineering.

The methods of vibration isolation can be classified as: passive, active, and semi- active. A simple passive vibration isolator consists of a spring and damper. The design of the isolator spring and the isolator damper requires a trade-off. The dynamic stiffness of an isolator spring should be as low as possible in order to increase the region of vibration isolation. However, if a linear spring is used, low stiffness isolator causes unacceptably large static deflection. A desirable vibration isolator should possess high-static–low-dynamic stiffness (HSLDS) so that it can support a large load statically while having low dynamic stiffness.

In general passive isolators are incapable of coping with varying conditions such as changes in the exciting frequency or in system parameters. Active vibration isolators can achieve superior performance through a feedback system. However, they are not cost- effective. Semi-active vibration isolators, in which the system properties can be adjusted in real time, combine the advantages of both passive and active isolators. Semi-active vibration isolators preserve the reliability of passive isolators even in the event of power loss; they also possess the versatility and adaptability of active isolators. In a semi-active isolator, the isolation is achieved by passive elements whose properties can be changed on-line to maintain an optimal performance

Compared to mechanical or hydraulic devices, electromagnetic devices possess some unique advantages, such as cleanliness, faster response, no mechanical contact, easy integration with control system, and compactness. Therefore, the use of electromagnetic techniques in semi-active vibration control continues to receive considerable attention

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