Semi-active Control System

As we discussed in the previous section, semi-active control systems can have considerable eco-nomic benefits over active control systems due to their low energy requirements and perform a wide excitation bandwidth in comparison with passive control system. Therefore, we will focus on the study of semi-active control devices. Typically, semi-active control devices can be divided into four classes [19]: variable orifice device, variable stiffness device, controllable fluid device and variable friction device. The MFD belongs to the category of variable friction device and will be introduced in next section. Before, these four types of devices are reviewed.

2.2.1 Variable Orifice Device

Figure 2.2: Schematic of a variable orifice device [116]

Variable orifice device is the typical device of semi-active hydraulic devices utilizing variable orifice valves for controlling the flowing fluid force, as show in Fig.2.2. When the valve is closed the device act as a stiffness element and when the valve is open the device become a controllable viscous damper. The valve position will determine the resistant force from the flowing fluid. Sack & Patten [106] conducted an experimental study for energy dissipation of vehicle traffic by controllable orifice dampers in a single-lane model bridge. Kobori et al. [66] investigated a full scale variable orifice damper in a semi-active variable stiffness system at the Kobori Research Complex and showed that the device has great effectiveness of structural response mitigation. Luca & Pastia [81] performed analytical and numerical studies of variable orifice dampers in a single degree of freedom for seismic protection of structures.

In applications, the variable orifice device is first applied in a bridge on interstate highway I-35 for a full scale experiment in 1998 [100], as show in Fig2.3. After two years, the world’s first smart base isolated 9 story building installed with variable orifice devices was constructed in Tokyo, Japan [43].

In this building, variable orifice devices are used for preventing large deformation from base isolators under extreme events.

(a) (b)

Figure 2.3: First full-scale application of variable orifice device in the US [117]: (a) installation; and (b) variable orifice device.

2.2.2 Variable Stiffness Device

As their name suggests, the principle of variable stiffness device is to adapt structural resonant frequency by varying its stiffness element. Although variable orifice device can also generate different stiffness by opening and closing the valve, the stiffness cannot be varied in different levels. Variable stiffness device can modify its stiffness continuously and smoothly to achieve desired state of structural frequency.

Nagarajaiah [92] developed a semi-active variable stiffness (SAIVS) control device for earthquake and wind storm mitigation. Figure2.4shows the SAIVS device; it consists of four spring elements with four pivot joints (joint 1-4) arranged in the vertices of a rhombus configuration. A linear electromechan-ical actuator which has one side fixed in the ground and one side connected to one joint (joint 1) is used for pulling or pushing joint 1 to required position, resulting in varied stiffness from the change of spring element angles. Narasimhan & Nagarajaiah [93] further developed a short time Fourier (STF)

trans-Figure 2.4: Schematic of a SAIVS device [93]

formation control algorithm for the SAIVS to achieve better mitigation performance in base isolated buildings.

Recently, Ghorbani-Tanha et al. [40] designed a semi-active stiffness (SAVS) device and proposed formulations for its application in a semi-active tuned vibration absorber. The 3D and 2D views of SAVS are shown in Fig. 2.5 (a) and (b). The SAVS is a rectangular cross section fixed-end elastic beam and contains free rotational degrees of freedom at two supports, as illustrated in Fig. 2.5(a). It can adapt its stiffness smoothly by rotating the beam to the desired angle using electric motors at the ends and produce resistance force against the loading in the middle of beam, as shown in Fig. 2.5(b).

Ghorbani-Tanha et al further installed SAVS in a semi-active tuned vibration absorber and showed the effectiveness of the SAVS, as shown in Fig.2.6(a) and (b).

Figure 2.5: Semi-active stiffness (SAVS) device [40]: (a) 3D view; and (b) 2D view.

Figure 2.6: Application of SAVS in a semi-active tuned vibration absorber [40]: (a) : schematic repre-sentation; and (b) 3D view.

2.2.3 Controllable Fluid Device

As we discussed before, variable orifice devices employ controllable valves to change the damping force. However, these valves usually are electrically controlled and lack of mechanical reliability [118].

Controllable fluid devices overcome this issue by utilizing special fluids in a fixed orifice damper and contain no moving mechanical parts but only the fluid. These special fluids typically contains silicon or oil mixed with ferric ion particles which can be polarized using an electric or magnetic field [19].

Fluids activated by electric field are termed electrorheological (ER) and ones activated by an magnetic field are termed magnetorheological (MR). Damping force of controllable fluid device are varied by adapting fluid properties of ER or MR. The schematic of controllable fluid device is shown in Fig.2.7.

Figure 2.7: Schematic of controllable fluid device [118].

MR fluids has been shown more reliable than ER fluids for civil engineering applications due to its large yield stress and low power requirement [62]. It gained more popular in the development of damping devices. Spencer et al. [62] first designed a Magnetorheological (MR) damper that use MR fluids to produce damping force, as shown in Fig. 2.8. Subsequently, numerous researches focus on enhancing the MR damper capacity were conducted. In 2002, Yang et al. [134] developed and tested a 200 kN capacity MR damper using maximum 50W power input. After one year, Fujitani et al. [36]

proposed a 400 kN MR damper for a real base-isolated building. Recently, Tu et al. [129] designed and tested a 500 kN capacity MR damping utilizing maximum 200W power input.

Figure 2.8: Cross section of an MR damper [62].

2.2.4 Variable Friction Device

Variable friction devices, which is our primary focus, will be introduced in this subsection. Variable friction devices proposed for control of civil infrastructure are primarily composed of a friction pad and an actuator. The additional controllability is provided by an actuation system that varies the normal force applied on the friction mechanism. These devices can be simply categorized by different types of actuator. Examples of actuators include hydraulic [63], pneumatic [131, 85], electro-magnetic [136, 78], electro-mechanical [94, 64] and piezoelectric [16, 79, 30, 133].

Most of proposed variable friction devices focus on enhancing resisting force capability for the large-scale applicability. Agrawal et al. [2] developed a semi-active electromagnetic friction damper (SAEMFD) of 20 KN maximum force which consists of one friction pad and two steel plate, as shown in Fig.2.9. The normal force is adjusted by changing the electric current at the plate. Narasimhan et al.

[94] introduced a new semi-active variable friction device (SAIVF) which also has 25 KN maximum damping force, as shown in Fig. 2.10. This device is designed based on existing variable stiffness device (SAIVS) mechanism which was introduced in Section2.2.2. The difference is that SAVIF add four friction elements in parallel with four spring elements and damping force is mainly contributed by friction elements instead of spring elements. Lu et al. [79] investigated a piezoelectric friction device (PFD) of 2 KN damping capacity which contains one friction bar and two friction pads. Recently, Dai et al.[24] developed and tested a electromagnetic friction damper (EFD) of 2.84 KN damping capacity

with 12 V power input. Figure2.11show the geometric design and prototype of EFD. The friction force is controlled and varied by the electromagnetic force from the electromagnet plate.

Figure 2.9: Semi-active electromagnetic friction damper (SAEMFD) [2].

Figure 2.10: Analytical model semi-active variable friction damper (SAVIF) [94].

Figure 2.11: Electromagnetic friction damper (EFD) [24]: (a) geometric design; and (b) prototype