MiniGRAIL

GravitationalRadiation Antenna In Leiden

Kamerlingh Onnes Laboratory, Leiden University, The Netherlands

Projects for students

2005

MASTER PROJECT (6 months)

Vibration isolation system

Introduction
In order to measure displacements of the order of 10-20 meters, seismic vibrations should be filters at this level around the resonant frequency of the sphere (3.1 kHz). This is being done by suspending the sphere from a vibration isolation system, which consists of a stack of masses and springs. The frequency of a mass-spring system is determined by the mass and the spring constant of the spring. The transfer function of the system can be measured by exciting the base of which the spring is suspended and measure the response of the mass. Below the resonant frequency, there will be no amplification, nor damping. At the resonant frequency, the signal will be amplified and at higher frequency attenuation of the signal will occur (see picture). The graph shows that the attenuation increases with frequency, so it is important to design a spring with a low resonant frequency.

Schematic picture of a system of a mass (m) and a spring (spring constant k).	This graph shows the attenuation A (dB) of the system as a function of frequency. Amplification will occur around the resonant frequency of the system (A>0). At higher frequency there will be attenuation (A<0).

Student project

	The attenuation system of MiniGRAIL consists of a stack of seven masses springs. In order to be able to start cryogenic experiments with MiniGRAIL as soon as possible, the initial attenuation system design was very simple. The springs consist of rods with a ring in the middle to increase the attenuation in the vertical direction. Measurements of the transfer function between two masses are shown in the graph. It is clear that the effect of the higher order modes of the rod decreases the attenuation of the system around 3 kHz. It could also be that the normal modes of the masses (at ~3500 Hz) are too close to the resonant frequency of the sphere.
	This project implies optimizing the vibration isolation system for MiniGRAIL, so that the highest attenuation occurs at the resonant frequency of the sphere (~3 kHz). Designing a new spring using Finite Element Analyses (FEA) with ProMechanica and measure the results in a separate room temperature setup in vacuum. Once the mass-spring system is optimized, it can be installed in the MiniGRAIL setup. Click here to read more about the attenuation system.