Sina S.

https://www.iitk.ac.in/nicee/wcee/article/2834.pdf

According to my understanding, active systems are systems where the actuator is driven by the frequency to be controlled while semiactive systems are systems where the actuator is driven by another frequency to modify a passive system response for control purposes.

For example, automatically tuning a tuned vibration absorber would be a semiactive system.

This definition is not related to energy at all - active systems and energy is an interesting topic anyway as many (most?) active systems are mainly working with virtual power to change impedances and therefore do not "consume" any energy (besides internal losses of the components) and some might even be tuned for "energy harvesting". Whenever an active system requires an energy input (besides internal component losses), it will radiate / excite and as a result will not reduce vibration / noise globally but only locally at some specific location.

Greetings,

Thanks for bringing the light such an important point if you are talking about active and semi-active control strategies in civil engineering structures. I can give you a few points to help you clear your thoughts.

- Semi-active devices manufactured in series are mostly Magneto-Rheological (MR) dampers or Electro-Rheological (ER) dampers. Both are based on piston-like devices with a fluid sensitive to the magnetic or electric field (basically, when submitted to a magnetic or electric field, the fluid slightly changes its characteristics to produce more resistance to the external force pushing on the piston).

These devices require small energy input to create the required fields (electric or magnetic) properly. This energy does not exceed 12 Volts, according to bibliography and literature. Even if higher voltages are input, the devices tend to saturate (i.e., they don't produce more response reduction or energy dissipation).

On the other side, active dampers are usually hydraulic pistons driven by larger and continuous energy input. Without this energy, they fail and don't produce any response reduction.

I hope a cleared some thoughts herein.

Best regards.

Mahdi Abdeddaim , thanks for your practical example in civil engineering. I think they fit exactly into my definition above:

• The semiactive ER / MR dampers work by changing the material properties of the passive damper, however, the damping (or, in case of tuning vibration absorbers this way, the "vibration control") itself happens in/through the passive system. Changing the material properties does not happen with the vibration frequency itself, but the properties are adapted independently (typically with a much lower frequency, at least for periodic excitation).
• The active hydraulic systems on the other side are actuated with the excitation frequency. The power required to pressurize them could be recouped in the expansion phase and thus is virtual power only. The constant energy is required to compensate for "internal component losses", i.e. flow losses, but also additional machinery like hydraulic pumps, valves, etc. These are required to operate / manage the hydraulic system). Whether or not recuperation of the virtual power makes sense or not will for sure depend on the actual application. For constant operation, e.g. wind excitation of tall buildings, it might make sense while it is not worth the effort for rare seismic excitation / operation.

Rolf Schirmacher thank you for your detailed response and good interpretation.

I want to add one more point I missed in my previous answer.

The semi-active devices depend strongly on the relative input at their ends (nodes), such as displacement, velocity, and acceleration. Hence, placing a semi-active damper between two points with small relative displacement won't produce a good performance even if you input a large voltage. Contrary, active devices are independent of relative response at their ends.

I hope this contributed more to enrich the discussion going on here.

Please see Vibration Control for Optomechanical Systems, World Scientific, 2021, ISBN 9789811237331, page 233 for the distinction between active and semi-active systems; Journal of Sound and Vibration 186, no. 1, 1995, p.1-21 for general estimates of work performed by active isolation systems.

Mahdi Abdeddaim

Hello,

Thanks a lot for reporting the 12 v voltage from literature.

Rolf Schirmacher

There exists a particular kind of semi-active system which relies on an on/off switching mechanism with the purpose to switch the system off when it is not necessary (to save/harvest energy).

Article Optimal switch timing for piezoelectric-based semi-active vi...

Dear Sina S. ,

Greetings, I hope you are doing well,

Regarding the on/off system you mentioned, this is related to the controller itself, not the device. Hence, a MR/ER damper may have an on/off switching controller, a fuzzy logic-based controller, or even a clipping law controller.

Regards.

Mahdi Abdeddaim

Yes, that's right,

But for instance in semi-active systems, the "on/off/semi-on" modes of controller determine the global stiffness and damping.

Sina S. ,

In a Semi-active device, you can apply as much voltage as you want, and use any controller you want, the device won't produce any force if there is not a relative response between its nodes. This is a very important point to start with.

Contrary to an active device that produces the control force, the semi-active device, just dissipates energy by varying the density of the fluid inside the damper.

Sina S. For a better understanding of the difference between these control devices and their utilization with control algorithms for civil structures, please see the following article.

Article Research Developments in Adaptive Intelligent Vibration Cont...

Mahdi Abdeddaim , I would like to add some comment regarding "large displacement is required for semi-active systems".

I think, from a physics point of view, this is not necessary. The distinction here is more about damping / dissipation / losses / energy absorption vs. impedance change / reflection.

To maximize dissipation with a damping term relative to displacement / velocity, you need to have such displacement / velocity. And this is how semi-active dampers are working: controlling the viscosity of some ER/MR fluid to maximize damping / dissipation within the fluid.

If you would think about a semi-active impedance change / reflection device, you would end up with a tuned mass "damper" where you could semi-actively control the tuned eigenfreqency to match some excitation. This would generate a force at the mounting point like the "active" devices you described while being fully semi-active.

Such semi-active, automatically tuned systems are of course more useful for periodic excitation, so their applications to civil engineering structures are probably quite limited.

Article Further steps towards the tuning of inertial controllers for...

Mahdi Abdeddaim

Hello,

May I ask if there is any reference for the 12 v voltage, you mentioned?

Thanks for attention

Sina S. , depending on the actuator principle, going with 12 V is not necessarily an issue.

For example, if you think about magnetorheological dampers, then you would need a certain magnetic field and therefore some "high" current, but not necessary high voltages. Have a look at Wikipedia for example applications of MR dampers. I would assume that all of the automotive applications will run on 12 V to avoid expensive voltage amplification to higher levels above standard 12V supply voltage (battery voltage) levels.

Of course, for electrorheological systems or piezos, this is going to be a completely different story as you would require a certain electrical field strength for the mechanical effect and therefore higher voltages (or very tiny distances of your "capacitor" / electrodes).