CHAPTER 2 MR FLUIDS AND DEVICES. Magnetorheological fluids (or simply MR fluids) belong to the class of controllable

Transcription

1 CHAPTER 2 MR FLUIDS AND DEVICES Magnetorheological fluids (or simply MR fluids) belong to the class of controllable fluids. The essential characteristic of MR fluids is their ability to reversibly change from free-flowing, linear viscous liquids to semi-solids having a controllable yield strength in milliseconds when exposed to a magnetic field. This feature provides simple, quiet, rapidresponse interfaces between electronic controls and mechanical systems. MR fluid dampers are relatively new semi-active devices that utilize MR fluids to provide controllable damping forces. In this chapter, following the introduction of the essential characteristics of MR fluids, the visco-plasticity models are presented to describe the MR fluid field-dependent characteristics and shear thinning/thickening effects. The advantages of MR fluids and devices in practical applications are also discussed, and comparisons of MR fluids with their companion, ER fluids, are provided. Moreover, various types of MR fluid dampers for civil engineering applications are introduced. Finally, a laboratory-scale structural vibration control experiment (Dyke et al. 1996a) is discussed; the results demonstrate the significant potential of utilizing MR dampers for natural hazard mitigation applications in large-scale structures. 25

2 North South Figure 2.1: MR fluids. H 2.1 Magnetorheological (MR) Fluids The initial discovery and development of MR fluids can be credited to Jacob Rabinow (1948, 1951) at the US National Bureau of Standards in the late 1940s. These fluids are suspensions of micron-sized, magnetizable particles in an appropriate carrier liquid. Normally, MR fluids are free flowing liquids having a consistency similar to that of motor oil. However, in the presence of an applied magnetic field, the iron particles acquire a dipole moment aligned with the external field which causes particles to form linear chains parallel to the field, as shown in Fig This phenomenon can solidify the suspended iron particles and restrict the fluid movement. Consequently, yield strength is developed within the fluid. The degree of change is related to the magnitude of the applied magnetic field, and can occur only in a few milliseconds. A typical MR fluid contains 20 40% by volume of relatively pure, soft iron particles, e.g., carbonyl iron; these particles are suspended in mineral oil, synthetic oil, water or glycol. A variety of proprietary additives similar to those found in commercial lubricants are commonly added to discourage gravitational settling and promote particle suspension, enhance lubricity, modify viscosity, and inhibit wear. The ultimate strength of an MR fluid depends on the square of the saturation magnetization of the suspended particles. The key to a strong MR fluid is to choose a particle with a large saturation magnetization (Carlson and Spencer 1996a). The best available particles are alloys of iron and cobalt that have sat- 26

3 uration magnetization of about 2.4 tesla. Unfortunately, such alloys are prohibitively expensive for most practical applications. The best practical particles are simply pure iron, as they have a saturation magnetization of 2.15 tesla. Virtually all other metals, alloys and oxides have saturation magnetization significantly lower than that of iron, resulting in substantially weaker MR fluids. Typically, the diameter of the magnetizable particles is 3 to 5 microns. Functional MR fluids may be made with larger particles; however, particle suspension becomes increasingly more difficult as the size increases (Carlson and Spencer 1996a). Smaller particles that are easier to suspend could be used, but the manufacture of such particles is difficult. Commercial quantities of relatively inexpensive carbonyl iron are generally limited to sizes greater than 1 or 2 microns. Significantly smaller ferromagnetic particles are generally only available as oxides, such as the pigments commonly found in magnetic recording media. MR fluids made from such pigment particles are quite stable because the particles are typically only 30 nanometers in diameter. However, because of their lower saturation magnetization, fluids made from these particles are generally limited in strength to about 5 kpa and have a large plastic viscosity due to the large particle surface area. Three types of MR fluids manufactured by the LORD Corporation are now commercially available ( Table 2.1 presents the main properties of these three types of MR fluid: MRF-132LD (oil-based), MRF-240BS (water-based), and MRF- 336AG (silicone oil-based). Furthermore, the characteristics of plastic viscosity, magnetic induction, and yield stress of the LORD MRF-132LD oil-based MR fluid are shown in Fig As shown in Fig. 2.2(a), MR fluids exhibit a significant shear thinning effect because of both the addition of suspension agents and changes in the magnetic particle microstruc- 27

4 (a) 10 Log Viscosity (pa-s) (b) Shear Rate (1/sec) 1.5 B (Tesla) 1 B 0.5 B-m 0 H H ( x10 5 A/m) (c) Yield Stress (kpas) Magnetic Induction (Tesla) Figure 2.2: Characteristics of the LORD MRF-132LD oil-based MR fluids (courtesy of the LORD Corporation). 28

5 TABLE 2.1: PROPERTIES OF THREE DIFFERENT TYPES OF MR FLUIDS (RHEONETIC MR FLUID SPECIFICATIONS COURTESY OF THE LORD CORPORATION). MR Fluid MRF-132LD MRF-240BS MRF-336AG Fluid Base Synthetic Oil Water Silicone Oil Operating Temperature ( C) Density (g/cc) Weight Percent Solids 80.74% 83.54% 82.02% Coefficient of Thermal Expansion (Volume, 1/ C) Specific 25 C (J/g C) Thermal Conductivity (w/m C) Flash Point ( C) >150 >93 >200 10s -1 /50s -1 (Pa sec) 0.94/ / /- ture during shear (Jolly et al. 1998). Fig. 2.2(b) illustrates the magnetic induction curve, or B H curve, of the MR fluid. It can be seen that MR fluids have an approximately linear magnetic property when the applied magnetic field is small. As the magnetic field increases, a gradual magnetic saturation is observed; consequently, the MR fluid yield stress saturates (Fig. 2.2(c)) due to its direct relationship with the magnetic field. 2.2 MR Fluid Models A simple Bingham visco-plasticity model (Phillips 1969), as shown in Fig. 2.3, is effective at describing the essential field-dependent fluid characteristics. In this model, the total shear stress τ is given by τ = τ 0 ( H)sgn( γ ) + ηγ (2.1) τ 0 where = yield stress caused by the applied field; H = magnitude of the applied magnetic field; γ = shear strain rate; and η = field-independent plastic viscosity, defined as the 29

6 Shear Stress τ 1 η Shear thickening ( m <1) Bingham fluids ( m =1) Shear thinning ( m >1) τ 0 Newtonian Fluid Strain Rate γ τ 0 Figure 2.3: Visco-plasticity models of MR fluids. slope of the measured post-yield shear stress versus shear strain rate. Note that the fluid post-yield viscosity is assumed to be a constant in the Bingham model. Because MR fluids exhibit shearing thinning effect which is shown in Fig. 2.2(a), the Herschel-Bulkley visco-plasticity model (Herschel and Bulkley 1926) can be employed to accommodate this effect. In this model, the constant post-yield plastic viscosity in the Bingham model is replaced with a power law model dependent on shear strain rate. Therefore, τ = τ 0 ( H) + K γ m sgn( γ ) (2.2) where m, K = fluid parameters and m, K> 0. Comparing Eq. (2.2) with Eq. (2.1), the equivalent plastic viscosity of the Herschel-Bulkley model is η e = K γ m (2.3) 30

7 Eq. (2.3) indicates that the equivalent plastic viscosity γ increases when m > 1 η e decreases as the shear strain rate (shear thinning). Furthermore, this model can also be used to describe the fluid shear thickening effect when m < 1. The Herschel-Bulkley model reduces to the Bingham model when m = 1, therefore η = K. 2.3 Advantages of MR Fluids and Devices in Practical Applications There are basically two types of controllable fluids MR fluids and ER fluids. The primary advantage of MR fluids stems from their high dynamic yield strength due to the high magnetic energy density that can be established in the fluid. Energy density in MR fluids is limited by the magnetic saturation of iron particles. For a typical iron-based MR fluid, the maximum energy density is 0.1 Joule/cm 3. ER fluids, on the other hand, are limited by the dielectric breakdown, and the maximum energy density is only about Joule/cm 3. This is the main reason that the yield strength of MR fluids is larger by an order of magnitude than that of ER fluids; however, their viscosity is almost the same. A yield stress close to 100 kpa can be obtained for MR fluids with magnetic suspensions containing powder of carbonyl iron (Weiss and Duclos 1994; Carlson et al 1996), whereas 2-5 kpa appears to be the maximum yield stress for an ER fluid (Havelka 1994). A high dynamic yield stress allows for small device size and high dynamic range. Carlson and Spencer (1996a) indicated that the minimum amount of active fluid in a controllable fluid device is proportional to the plastic viscosity and inversely proportional to the square of the maximum yield stress. For a comparable mechanical performance, the amount of active fluid needed in MR devices will be two orders of magnitude less than that required in ER devices, resulting in much smaller devices. MR fluids can operate at temperatures from -40 to 150 ºC with only slight variations 31

8 in yield stress (Carlson and Weiss 1994). This arises from the fact that magnetic polarization is not strongly influenced by temperature. Additionally, MR fluids are not sensitive to impurities commonly encountered during manufacturing and usage. Furthermore, because the magnetic particle polarization mechanism is not affected by surfactants and additives, it is easier to stabilize MR fluids against particle/carrier separation, even though the particles and carrier liquid have a large density mismatch. Antiwear and lubricity additives can generally be included in MR fluids to enhance stability, seal life and bearing life since electro-chemistry does not affect the magneto-polarization mechanism (Carlson and Spencer 1996a). From a practical implementation perspective, although the total energy requirements for the ER and MR devices are almost equal, only MR devices can be easily driven by common low-voltage power sources (Carlson and Spencer 1996a). MR devices can be controlled with a low-voltage, current-driven power supply outputting only ~1-2 amps. ER devices, on the other hand, require a high-voltage power source (~ volts) which may not be readily available, especially during strong earthquake events. Moreover, such a high voltage may pose a safety hazard. Table 2.2 provides a summary of the key properties of both ER and MR fluids. 2.4 MR Devices and MR Fluid Dampers The maximum force that an MR damper can deliver depends on the properties of MR fluids, their flow pattern, and the size of the damper. Virtually all devices that use MR fluids can be classified as operating in: (a) a valve mode, (b) a direct shear mode, (c) a squeeze mode, or a combination of these modes (Carlson and Spencer 1996a). Diagrams of these basic modes of operation are shown in Fig Examples of valve mode devices 32

9 TABLE 2.2: SUMMARY OF THE PROPERTIES OF ER AND MR FLUIDS. Property MR Fluids ER Fluids Max. Yield Stress kpa 2 5 kpa Maximum Field ~250 ka/m ~4 kv/mm Apparent Plastic Viscosity η Pa-s Pa-s Operable Temp. Range ºC ºC Stability τ 0 Unaffected by most impurities Cannot tolerate impurities Density 3 4 g/cm g/cm 3 η τ s/pa s/pa Maximum Energy Density 0.1 Joules/cm Joules/cm 3 Power Supply (typical) 2 50 V, 1 2 A V, 1 10 ma include servo-valves, dampers, shock absorbers and actuators. Shear mode devices include clutches, brakes, chucking and locking devices, dampers and structural composites. While less well-understood than the other modes, the squeeze mode has been used in some small-amplitude vibration dampers. To date, several MR fluid devices have been developed for commercial use by the LORD Corporation (Carlson et al. 1996; Jolly et al. 1998). Linear MR fluid dampers have been designed for use as secondary suspension elements in vehicles. MR fluid rotary speed displacement force pressure flow Q force H H H a. Valve Mode b. Direct Shear Mode c. Squeeze Mode Figure 2.4: Basic operating modes for controllable fluid devices. 33

10 brakes are smooth-acting, proportional brakes which are more compact and require substantially less power than competing systems. MR fluid vibration dampers for real-time, active-control of damping have been used in numerous industrial applications. In civil engineering applications, the expected damping forces and displacements are rather large in magnitude. Therefore, MR dampers primarily operating under direct shear mode or squeeze mode might be impractical. Usually valve mode or its combination with direct shear mode are employed. Some examples of recently-developed MR dampers are given below. These dampers are capable of meeting real-world requirements and are presently either in commercial production or in production prototype trials. The MR fluid damper investigated in this dissertation is a 20-ton prototype largescale seismic MR fluid damper developed under cooperation between the LORD Corporation and the Structural Dynamics and Control/Earthquake Engineering Laboratory (SDC/ EEL) at the University of Notre Dame (Carlson and Spencer 1996a; Spencer et al. 1997b, 1998; Yang et al. 2000a,b, 2001a,b,c). The MR fluid damper schematic is given in Fig For the nominal design, a maximum damping force of 200,000 N (20 tons) and a dynamic range equal to ten were chosen. The damper uses a particularly simple geometry in which the outer cylindrical housing is part of the magnetic circuit. The effective fluid orifice is the entire annular space between the piston outside diameter and the inside of the damper cylinder housing. Movement of the piston causes fluids to flow through this entire annular region. The damper is double-ended. The advantage to this arrangement is that a rod-volume compensator does not need to be incorporated into the damper, although a small pressurized accumulator is provided to accommodate thermal expansion of the fluid. The damper has an inside diameter of 20.3 cm and a stroke of ±8 cm. The electromagnetic 34

11 Magnetic Flux Flow Thermal Expansion Accumulator 3-Stage Piston Piston Motion MR Fluid LORD Rheonetic Seismic Damper MRD-9000 Figure 2.5: Schematic of the prototype 20-ton large-scale MR fluid damper. coils are wound in three sections on the piston. This results in four effective valve regions as the fluid flows past the annular gap. The coils contain a total of about 1.5 km of magnetic wire. The completed damper is approximately 1 m long, has a mass of 250 kg, and contains approximately 6 liters of MR fluid. However, the amount of fluid energized by the magnetic field at any given instant is approximately 90 cm 3. The design parameters of the 20-ton MR fluid damper are summarized in Table 2.3. Fig. 2.6 shows a small-scale SD-1000 MR fluid damper manufactured by the LORD Corporation (Carlson and Spencer 1996a; Dyke 1996a,b; Jolly et al. 1998; Spencer 1997a). In this damper, MR fluids flow from a high pressure chamber to a low pressure chamber through an orifice in the piston head. The damper is 21.5 cm long in its extended position, and the main cylinder is 3.8 cm in diameter. The main cylinder houses the piston, 35

12 TABLE 2.3: DESIGN PARAMETERS OF THE 20-TON LARGE-SCALE MR FLUID DAMPER. Stroke Maximum Velocity Nominal Cylinder Bore (ID) Maximum Input Power Nominal Maximum Force Effective Axial Pole Length Coils Fluid Maximum Yield Stress τ 0 Apparent Fluid Plastic Viscosity η ±8 cm 10 cm/s cm < 50 watts 200,000 N ~ cm ~3 x 1000 turns ~70 kpa 1.5 Pa-s Fluid 2 η τ 0 2 x s/pa Gap ~1.5 2 mm Active Fluid Volume ~90 cm 3 Wire 16 gauge Inductance (L) ~6 henries Coil Resistance (R) ~3 7 ohms the magnetic circuit, an accumulator, and 50 ml of MR fluid. The damper has a ±2.5 cm stroke. The magnetic field, which is perpendicular to the fluid flow, is generated by a small electromagnet in the piston head. Forces of up to 3,000 N can be generated with this device. Fig. 2.7 shows a bypass-type 20-ton MR fluid damper designed by the Sanwa Tekki Corporation (Fujitani et al. 2000; Sunakoda et al. 2000). Unlike dampers mentioned previously, MR fluids in this damper flow from a high pressure chamber to a low pressure chamber in valve mode through a bypass outside the main cylinder. The bypass has an annular gap between the outside of the magnetic pole and the inside of the bypass cylinder. 36

13 Wires to Electromagnet Bearing & Seal MR Fluid Coil Diaphragm Accumulator Annular Orifice Piston Cross Section Figure 2.6: Small-scale SD-1000 MR fluid damper. Reservoir Reservoir Cylinder Check Valve Piston Piston Rod Piston Rod Flow Outer Cylinder Outer Cylinder Bypass Portion Bypass Portion Magnetic Flux Flow Flow Magnetic Pole Magnetic Pole Coil Figure 2.7: Bypass type 20-ton MR fluid damper developed by the Sanwa Teki Corporation. 37

14 The magnetic field is generated by a 10-stage electromagnet and is perpendicular to the fluid flow. 2.5 Scale-Model Studies on Semi-Active Structural Vibration Control Using MR Dampers Because MR dampers are intrinsically nonlinear, the development of appropriate control algorithms that take advantage of the unique characteristics of the device is a design challenge. Various approaches have been proposed for the control of semi-active systems, such as control based on the Lyapunov stability theory (Leitmann 1994), decentralized bang-bang control (McClamroch and Garvin 1995), clipped-optimal control (Dyke et al. 1996a,b), and modulated homogeneous friction (Inaudi 1997). These control strategies have been evaluated through numerical simulations using model structures with MR dampers (Dyke and Spencer 1997; Jansen and Dyke 2000; Yi and Dyke 2000). To experimentally evaluate the effectiveness of semi-active control strategies using an MR damper, a structural vibration control experiment of a three-story model building subjected to seismic excitation was conducted in the SDC/EEL at the University of Notre Dame (Dyke et al., 1996a,b). Fig. 2.8 shows a schematic of the experimental setup. The test structure used in these experiments is designed as a scale model of the prototype building discussed in Chung et al. (1989), and it is subjected to one-dimensional ground excitations. The MR damper employed herein is the aforementioned LORD SD-1000 linear MR fluid damper. Clipped-optimal control strategies based on acceleration feedback were implemented on the model structure. The three-story model structure was subjected to a scaled version of the N-S component of the 1940 El-Centro earthquake. Fig. 2.9 shows the uncontrolled responses and semi-actively-controlled responses for the tested 38

15 3-Story Scale-Model Building x a3 x a2 Current Driver x a1 f, x d x g Control Computer Rheonetic SD-1000 MR Damper Height: 158 cm Mass: 304 kg Figure 2.8: Experimental setup of model building structural vibration control using MR dampers (Dyke et al. 1996a). structure. The effectiveness of the proposed semi-active control strategy is clearly seen through a 74.5% peak third-floor displacement reduction and a 47.6% peak third-floor acceleration reduction. Moreover, the semi-active control system significantly outperformed passive control strategies. A 24.3% improvement in the reduction of the peak third-floor displacement, along with a 29.1% improvement in the reduction of the maximum interstory drift, were achieved over the best passive case; the maximum accelerations were kept almost the same. Overall, semi-active systems significantly surpass passive systems, and can achieve performance similar to that of fully active systems. These results demonstrate significant potential for the use of MR technology in structural vibration control. 39

16 1.3 Uncontrolled Controlled x 3 (cm) x a3 (cm/sec 2 ) Time (sec) Figure 2.9: Controlled and uncontrolled structural responses due to 120% El-Centro earthquake excitation (Dyke et al. 1996a). 2.6 Summary Magnetorheological (MR) fluids are smart materials with rheological properties that can be substantially, but reversibly, altered in milliseconds when exposed to a magnetic field. The Herschel-Bulkley visco-plasticity model has been shown to be effective in describing the MR fluid field-dependent characteristics and shear thinning/thickening effects. Note that the Herschel-Bulkley model reduces to the well-known Bingham model when the fluid parameter m = 1. Compared with electrorheological (ER) fluids, MR fluids have a higher yield stress and are insensitive to impurities and temperature. Moreover, MR devices are readily driven by common low-voltage power supplies. These advantages make MR fluids and devices more attractive in practical applications. 40

17 To date, several types of MR fluid dampers have been developed for commercial use or are under various stages of development. Previously reported laboratory studies using small-scale commercial MR dampers have indicated the promise of applying MR technology for vibration mitigation. However, there are still many challenges to be met before MR dampers may be implemented in large-scale structural vibration control applications. In the following chapters, topics on large-scale MR damper modeling, testing and control are addressed, intending to provide fundamental insight into the behavior of MR fluid dampers. 41