CHAPTER 1 INTRODUCTION
1.2 Motivation and Scope
Prior to discussing about the motivation of this study, it is essential to mention about the concept and the classifications of “structural control”. In the year of 1972, the initial conceptual study of structure control was brought by Prof. Yao [10]. The theory of control originated from the mechanical field, and the theory includes two major control systems, such as open-loop control system and close-loop control system. The difference between these two systems is the feedback function; for the open-loop system, the controller is independent from the system outputs, and the mechanisms of the controller are also remain unchanged; as to the close-loop system, controller will follows designed programs according to the received feedback signals from sensors installed on the control objects to adjust the mechanisms.
As listed by Housner et. al. [11], the applications for control concepts in engineering field could be classified into four types, which are active control, passive control, hybrid control, and semiactive control, respectively. The active control system adopts controllers, such as actuators, to instantly mitigate structure vibrations entirely depending on the input energy. The controllers of the close-loop system will adjust such input energy according to the feedback signals from sensors. However, the huge power demand limits the controlling scale, and engineers will also worry about the negative effects brought from the failure of control systems. By contrast, the passive control system is a simple
open-loop system operating by various passive dampers, such as viscous dampers, viscoelastic dampers, or metallic dampers. Owing to the features of long durability and operating without external power, passive control system is the most popular and widespread system in the world. The only disadvantage of passive control system is that the behaviors of passive dampers are unchangeable. Therefore, the lack of adjusting ability makes passive control system being not as efficient as active control system. On the other hand, the hybrid control system is the combination of passive dampers and active control systems. For example, adopt the active actuators in isolation layer for seismic isolated build, or combine active control systems with tuned mass systems.
However, the hybrid control system is also limited by the disadvantages of both systems.
Therefore, a more efficient system, called semiactive control system, has being invented.
According to Prof. Spencer [12, 13], the versatile semiactive control system combines the merits of both passive and active control system. With less external power, the properties of the passive-like dampers could be changed and acts as other passive dampers. Even if the electronic system is fail, the semiactive dampers could operate as passive dampers.
There are a lot of types of semiactive dampers, including variable-orifice fluid dampers, controllable friction devices, variable-stiffness devices, and controllable fluid dampers.
Among them, magnetorheological (MR) damper, as shown in Fig. 1-4(a), classified to the type of controllable fluid damper, has been widely studied and applied. The MR fluid is dispersed with small magnetic particles, which will automatically self-arrange under magnetic field and causes the MR fluid thickening. Without magnetic field, MR dampers acts as linear viscous dampers, and under various levels of magnetic field, MR dampers become the 1 non-linear viscous dampers with corresponding magnitudes, as shown in Fig. 1-4(b).
However, although the semiactive control system possesses advantages of passive and active control systems, it is still limited by the lack of some techniques, and that hinders the spread of this applications. For example, there are a great deal of studies aim to develop efficient semiactive control and sensing strategies, but when these strategies are implemented into medium or large scale buildings, it is noticed that the insufficient of electronic data delivering technique and computer calculating speed, owing to a large number of monitoring spots and complex structure behaviors, will cause semiactive controllers fail to operate in time. Apart from this, the durability of electronic system will also become another challenge.
On the other hand, although a semiactive damper is very powerful in application to any structure as long as the demand damping force is under the capacity, the damper is still only installed in one target structure. In other words, one already-installed semiactive damper does not actually need the entire control function. For example, normally, we will not drive our cars more than 200 kilometers per hour, even though they are designed much powerful than that; under this circumstance, practically, we usually prefer our cars with better gas mileage rather than higher maximum speed. Similarly, a comprehensive control system is not definitely demanded to mitigate structure vibrations. Therefore, this study aims to design a passive control system with multiple control purposes. The new concept of multiple functional passive control system not only exhibits its efficiency to reduce structural responses under different demand excitations, but also keeps the advantages of passive control system as high durability and free external power.
The previous feasibility study have been start from 2009 [14], leading by Prof. K. C.
Chang, and according to the feasibility study, a simple damper device, with a solid piston head and annular gap around the piston head, is filled with nanofluid, mixed with fumed
silica particles and polypropylene, could exhibits as a 1 non-linear viscous damper.
Later, during 2012 and 2014 [15-19], the studying group verified the theoretical properties of nanofluid primed damper through damper performance tests and simple finite element analysis, as shown in Fig. 1-5. Until the early in the 2017, the double curves property of nanofluid primed damper have been discovered and verified by experiments [20]. Summarizing from the above studying and development results, the features of the nanomaterial based multi-parameters damper, or called nanofluid damper in brief hereafter, are listed as following:
Double-exponent force curve
Conventional viscous dampers with exponent smaller than one are popular with their damping force properties of reaching the demand magnitude quickly at low speed and not continually increasing at high speed, shown as red dashed line in Fig. 1-6. In recent years, engineers discovered that it is inappropriate for large damping force happening at low velocity, because that will somehow restrain other structural control function, such as isolation system. On the contrary, the force behavior of one idealized nanofluid damper is sketched as blue solid line in Fig. 1-6, which comprising two continual power law curves;
the first curve possesses an exponent that larger than one at low speed and the second curve possesses exponent smaller than one at general speed, respectively. The property allows nanofluid dampers can generate low damping force at low speed, steeply increase damping force at specific velocity range, and behave as conventional viscous dampers under general velocity range.
Force curve meets any engineering demand
Passive control devices should be designed to meet engineering demands. For example, viscoelastic dampers could adjust the behaviors via adjusting the thickness, area,
and number of viscoelastic material layers; lead rubber bearings or high damping rubber bearings could also be designed with different lead core diameter, shape modulus, and number of layers to fit the isolation system requirements. However, it is a challenge to implement the same concept on conventional viscous dampers, since the complex mechanical designs of piston head, as mentioned in the last section and shown in Fig 1-2, are difficult to slightly and accurately adjust damper behaviors. Further, the silicon fluid family, as the fluids filled in the dampers, merely contains few types of fluid with different viscosities, such as Silicon Oil 100, Silicon Oil 350, and Silicon Oil 1000, where the number stands for the fluid viscosity in the unit of cps (centipoises). Under this circumstance, manufacturers could merely try to offer several damper designs for engineers to choose for the most appropriate ones, and such strategy usually causes engineers have to modify the structure designs again in order to install dampers which are different from the original designs. This work requires much more additional time and has been bothering engineers for a long time. In contrast to conventional viscous dampers, nanofluid dampers only possess simple solid piston head and could adjust damper properties by slightly changing the combinations and concentrations of nanofluids; hence, with the database of nanofluid rheology properties, one damper mechanism design could reach a large number of engineering requirements.
Free internal pressure
According to the same situation that mentioned in the last part, the complex piston head design and lack of fluid viscosity choice cause conventional viscous dampers difficult to meet engineering requirements. However, manufacturers still have few methods to adjust the force behaviors. One of them is to increase the initial pressure when fluid is pumping into the cylinder. Additional initial pressure could identically increase
the damping force for any damper velocity, but the existing of initial pressure will speed up the abrasion of damper seal and decrease durability. Hence, since the nanofluid dampers do not acquire internal pressure to adjust damper behaviors, the durability performances are better than conventional dampers.
Easily vary the damper behaviors during the R&D stage
Since the damper behaviors are controlled by the properties of nanofluid, it is easy to change the damper behaviors by replacing the nanofluids. This advantage brings convince during the R&D stage and practical practices. During the R&D stage, researchers do not need to produce a large number of dampers with various mechanical designs. Further, when dampers are installed in the structures, once the behaviors of structural changed, engineers merely need to replace the nanofluid with the appropriate ones from the nanofluid dampers on site.
Advantage for application on bridges
Bridge structures always suffer from small vibrations due to the thermo effective and moving vehicles. The viscous dampers that installed between bridge girders and piers will continually operating at small velocity. Since conventional viscous dampers possess high damping forces at low velocity and internal pressure, the durability of seals in bridge dampers will be significantly decreased by the regular vibrations, which will lead to leaking on the seals. This is a serious engineering problem that occurs all over the world;
to improve this problem, nanofluid dampers with small damping force under low velocity zone could be adopted.
Continue from the previous study, this study aims to perform a methodology study and design method of nanofluid damper. The development of nanofluid damper comprises three parts, such as material properties, damper behaviors, and applications, in
brief. The first part is studied in Chapter 3, where 90 nanofluid samples are fabricated with various combinations of different types of fumed silica particles, PPGs, and fluid concentrations. The viscous curves and fluid curves of nanofluids are obtained from rheology tests and also simulated by a triple-Cross model with eleven parameters. The second part, discussed in Chapter 4, aims to apply the fluid properties into the damper devices by means of theoretical derivations and full-scale damper performance tests.
Chapter 5 discussed the last part, where several properties of nanofluid dampers are
discovered by transforming the simplified solution into the combination of two continuous curves with typical forms of viscous dampers, F CV, and the same nanofluid dampers, used in the full-scale damper performance tests, are adopted to verify that the nanofluid dampers meet the requirements listed in the European Standard (EN15129) for the shock-transmission units (STU). Finally, by means of inputting the recorded data from a practical bridge experiment, it is verified that the energy dissipated by nanofluid dampers is much less than such results analyzed from conventional dampers, which implies that the seal systems will receive smaller pressure and wear, as well as comprise longer durability.