By Dr. Ir. Nicolas Loix
Some vibrations problems are directly related to structural resonances, which are specific frequencies at which dynamic perturbations are, sometimes hugely, amplified. The level of this amplification is inversely proportional to the internal damping of the structure. Hence, in order to reduce vibration problems related to resonances, one will want to increase the structural damping and this can be done by passive (friction joints, viscous material, Tuned Mass Dampers…) or by active means. When the target structure is very stiff, i.e. buildings, bridges, machine tools…, the choice comes down to either a passive Tuned Mass Damper (TMD) or an Active Damping Devices (ADD). The article below compares the advantages and drawback of both technologies.
Tuned Mass Dampers are passive vibration reduction devices consisting of a mass, a spring, and a damper that is attached to a structure in order to reduce its dynamic response. The frequency of the damper is tuned to a particular structural frequency so that when that frequency is excited, the damper will resonate out of phase with the structural motion. Energy is dissipated by the damper inertia force acting on the structure. The TMD tuning parameters (mass, frequency & damping) are mainly dependent on the mass ratio between the TMD mass and the target structure modal mass (see Den Hartog method). Adequately tuned, TMDs will replace the high peak at the structure resonance frequency by two 2 much smaller peaks. Micromega has successfully implemented TMDs in pipings, wind turbines, semicon machines…
The main advantage of the TMDs are their simplicity but
- One TMD can only damp one structural mode
- TMD performances are sensitive to resonance frequency changes
- The maximum damping that can be reached by a TMD is proportional to the mass ratio between the target structure and the TMD, ie high damping ration can only be achieved with quite heavy TMD…
- It is sometimes difficult to achieve the high internal damping ratio (e.g. 40%) required by the equations
- TMD implementation require a lot of engineering which can be inefficient for a single device production case
Alternatively, Active Damping Devices based on inertial actuator will perform the same job than TMD (e.g. increased structural damping, reduced dynamic amplification,…) but with better performances! An ADD is also based on a spring-mass system but in this case the system is equipped with an electromagnetic force generator (i.e. voice Coil, electromagnet, motor…) between the static and moving part of the actuator. While, with a TMD, the spring mass frequency is tuned on the targeted resonance, in an ADD, the spring mass frequency can be chosen arbitrarily as long as it is below the first resonance frequency. In fact, an inertial actuator can be considered as a pure force generator for frequencies above its internal suspension frequency.
Contrary to TMD, ADDs are active systems, based on a closed-loop control relying on sensors, actuators and control/power electronics. Being active system, they doe not need to be tuned to a specific resonance but damp all the resonances they can see. As the selected closed-loop damping strategy with DVF (Direct Velocity Feedback) is quite universal, the ADD can be applied with no or no deep previous knowledge of the structural behavior of the structure or machine it is attached to. No in-depth preliminary modeling or structural analysis is required before being able to generate damping with ADDs. In addition, being closed-loop feedback system based on very simple controllers (ie DVF), the ADD response time is “nearly” immediate making them suitable for the reduction of vibrations due to shocks.
The figure below shows the results obtained with one ADD attached to a steel plate (1500x840x10mm) clamped at its base. The first modal frequency is around 40Hz. One can clearly see that that significant damping can be introduced in the structure and that all the modes up to 400Hz have been damped. Micromega has successfully implemented ADDs in pipings, machine tools, semicon machines, textile machines, printing machines, pick & place robots…
The main advantages of the ADDs over TMDs are
- ADDs can damp all the structural modes they can see
- ADD performances are insensitive to resonance frequency changes
- The mass of the ADD does not depend on the mass of the target structure but on the amount of vibration energy to be removed (ea an optical bench of 400kg can be damped with a 20gr ADD while it is excited by nano-vibrations)
- While based on existing products, test and implementation can be very quick
Both TMDs and ADDs can be fixed on existing machine or device, with little design changes or structural modifications needed, which makes them very useful for machine retrofitting or unexpected “last minute” vibration problems.
The table below compares the advantages and drawbacks of TMDs and ADDs
– Number of damped resonances
– Sensitivitity to resonance frequency changes
– Manufacturing cost
– Implementation cost
|– Simple mechanical device|
– Damps only one resonance
– Doe not require power
– Proportional to target mass
– Complex (tuning) / engineering
|– Complex mechatronic device|
– Damps all resonances it can see
– Not Sensitive
– Requires electrical power
– Proportional to vibration energy
In summary, TMDs are very effective for situations where unwanted vibration is due to one single resonance, whose frequency is rather constant, and where additional weight is not a constraint. On the other side, ADDs are very effective for situations where several resonances are involved, where these are varying significantly and where weight considerations are critical.
Finally, ADDs can also be used in the frame of R&D processes where it can be useful to assess the effect of the increased structural damping on the process performance before spending time, and money, on the design of a cheaper (in volume) TMD-based solution.