There are many damper solutions for attenuating undesired vibration. Why choose a magnetorheological damper and not a tuned mass damper, for instance? Here we discuss the advantages and disadvantages of different attenuators: passive damper, tuned mass damper and semi-active magnetorheological damper.
Passive Damper Pros and Cons
A passive damper is the traditional solution to a vibration solution. The idea in short is that you place a dampening pad between the vibrating object (e.g. motor) and the surface the vibration is conducted to (e.g. floor). The pad then absorbs the kinetic energy of the vibration. In many cases a passive damper is still the best solution: the pad is easily installed, the characteristics of various damper materials are well known and have been scientifically researched for decades – even centuries -, there are no moving parts and it is easy to replace.
If your vibration occurs on a fixed frequency band and there is good material available in a decent price for these frequencies, go for the passive damper! There is nothing wrong with it. I would use it whenever possible.
However, it is not always possible. Sometimes the performance of a passive damper is limited on a too limited frequency band. You could try installing passive dampers with different characteristics next to each other attempt killing different frequencies with different dampers, and it might work in some situations. But it does not have to be a very special motor installation, when this approach becomes too complicated. At least that is what I noticed in the case of my thesis.
This is where we come to the other two categories of dampers.
Tuned Mass Damper Pros and Cons
The idea of a tuned mass damper is to somehow intelligently react to the motor vibration. A definition I follow is that a tuned mass damper creates a counteracting force by moving a mass at counter phase with the source vibration.
An advantage of a tuned mass damper is clearly its ability to create a significant difference by imposing a force to the vibrating system through a moving mass. This is also one of its major disadvantages. Should you apply the force systematically at a wrong instant, the vibration system can become instable if the movement of the tuned mass damper occurs in the resonance frequency. And operation at the resonance frequency is quite common, because the resonance frequency is many times the one to be killed. Ok, it is true that with a semi-active damper there is also the danger of messing up with something at the resonance point by controlling the damper poorly, but at least no extra force with a moving mass is applied and therefore the potential damage is not as harmful.
A further disadvantage is the increased number of moving parts. A tuned mass damper is also the one that requires the most manual installation work.
Semi-Active Damper Pros and Cons
The operating principle of a semi-active damper is based on the material characteristics. As for the magnetorheological damper, the dampening material changes its viscosity. The material – magnetorheological fluid – is oil containing metal particles. The idea is that when you apply a magnetic field to the material, the metal particles are aligned according to the field lines and make the fluid stiff. This creates in practice a damper that can be switched on and off in only milliseconds. For more specific information, check out the Wikipedia article, which is quite good.
A disadvantage of the damper is clearly its relatively low amount of use. This is mainly due to the fact that it is quite a new solution in the market and not very widely researched yet. However, this is constantly changing as the scientific research material accumulates.
In my opinion, the semi-active damper combines the best parts of both of the previous damper categories:
- Small size
- Few moving parts
- Reacts dynamically to a number of vibration frequencies
In addition, due to its small size the magnetorheological damper can be installed next to an existing passive damper that is already attenuating some frequencies. The magnetorheological damper can then focus on the vibration that varies over time.
If you want to find out which tools I used in creating and testing a magnetorheological fluid damper from scratch, go see the Damper Equipment section!