A Tuned Mass Damper (TMD) is device that is added to the top portion of the stack that increases the damping of the stack, which will suppress wind induced vibration. The device consists of some addition of mass and springs, that is tuned to the natural frequency of the structure. The TMD can take different forms, but the most common style is to have a cylinder that acts as a mass, which is larger than the diameter of the stack. The mass is supported by some pendulum rods, which suspend the mass. Between the mass and stack are springs. The entire assembly is designed to a specific natural frequency of the structure, so that it will provide damping to suppress any vibrations.
Tuned mass damper (also called vibration absorbers or vibration dampers) is a device mounted to a specific location in a structure, so as to reduce the amplitude of vibration to an acceptable level whenever a strong lateral force such as an earthquake or high winds hit. A tuned mass damper (TMD) is a vibrating mass that moves out of phase with the motion of the structure it is suspended to. With its out of phase motion, the inertial force of the TMD mass abates the resonant vibration of the structure by dissipating its energy.
Advantages
Provides the greatest damping
All stainless steel components
Relatively low weight and wind drag
Can visually see if the TMD is working
Disadvantages
Higher cost
Needs to be near top of stack
Sometimes complicates design if there are attachments (ladders, platforms, piping, etc.)
One TMD can only address one mode of vibration
FREQUENTLY ASKED QUESTIONS:
Does the damper have to be at the top of the stack? The damper is most effective where the deflection of the stack due to vibration is maximum. For the first mode of vibration, that would typically be at the top of a stack. The TMD can be placed lower than the top, but the trade-off is that the mass of the TMD must increase to compensate for being at a less desirable elevation.
Is the damper mass always 360° in circumference? Ideally the mass does wrap 360° around the stack; however, there are many instances where the mass cannot be wrapped 360° around the stack due to Ladders, Piping or other attachments on the stack. In some instances the mass ring is broken, to allow the obstruction to pass through (i.e. a ladder). In other instances where there are multiple obstructions, a special TMD can be designed to utilize independent masses which do not require a continuous ring.
What inspection is required on a TMD? A TMD can usually be observed from grade using a pair of binoculars. An obvious failure, such as a spring or Pendula failing, would probably be visible from grade with binoculars. If it is feasible, we would recommend performing a visual inspection of the TMD once every 1 to 2 years from a close proximity, but if that is not realistic then using binoculars will be adequate.
MECA ENTERPRISES INC.
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…
TMD performances
The main advantage of the TMDs are their simplicity but
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
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
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.
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