Experiment A1: Tuned Vibration Absorber
Background
The natural frequency of a system is the frequency at which it will vibrate freely in simple harmonic motion, when set in motion. An n degree-of-freedom system will possess n natural frequencies, and n modes of vibration, which can be determined by solving the equations of motion for the system in free vibration. It is often the case that only the first few modes will be significant.
If a lightly-damped system is excited at or near one of its natural frequencies, large amplitude oscillations will occur. This phenomenon is known as resonance. Such large displacements are likely to cause severe user discomfort in the case of a building, and may generate stresses large enough to cause ultimate failure. Over a long period, the likelihood of damage due to fatigue will also be increased. Thus it is important in design to know both the natural frequencies of the structure and the frequencies at which excitation is likely to occur, and to keep them separate. In general, the excitation frequency cannot be controlled, but the natural frequency of the structure (which depends on its mass and stiffness) can be altered to avoid resonance. Another method of controlling vibrations is to attach a vibration absorber to the system which will extract energy at the resonant frequency.
Topics Covered
- One- and two-degree-of-freedom systems with the addition of damping through simple constant-damping-factor dashpots
- Looking at the impact of a tuned mass damper on the vibration of the model structure (in software)
- Harmonic response of one- and two-degree of freedom systems with the addition of damping
- Step response of damped one- and two-degree of freedom systems
Experiment
In this experiment, students look at the parameters for a tuned mass damper for their structure. This
builds on the theory learnt in the Part 1A Mechanical Vibrations course, and extends this to look at
two-degree-of-freedom, mass-spring-dashpot models.
© Cambridge University, Engineering Department
Last updated 12/10/2012 by hrs@eng