THE substantialfield-induced yield stresses exhibited

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1 Design of a High-efficiency Magnetorheological Valve JIN-HYEONG YOO AND NORMAN M. WERELEY* Alfred Gessow Rotorcraft Center, Department of Aerospace Engineering, University of Maryland, College Park, Maryland 2742 USA ABSTRACT: A high efficiency design was explored for meso-scale magnetorheological (MR) valves (< 25 mm OD with an annular gap < 1 mm). The objective of this paper is to miniaturize the MR valve while maintaining the maximum performance of the MR effect in the valve. The main design issues in the MR valve involve the magnetic circuit and nonlinear fluid mechanics. The performance of the MR valve is limited by saturation phenomenon in the magnetic circuit and by the finite yield stress of the MR fluid. When field is applied to the magnetic circuit in the MR valve, a semisolid plug (as a result of particle chain formation) forms perpendicular to the flow direction through the valve, and a finite yield stress is developed as a function of field. The resulting plug thickness is used to control flow rate through, and pressure drop across, the MR valve. The nondimensional plug thickness is evaluated as a basis for evaluating valve efficiency. Design parameters of the MR valve are studied and an optimal performance was designed using steel (Permalloy) material in the magnetic circuit. A maximum magnetic flux density at the gap was achieved in the optimized valve design based on a constraint on the outer diameter limitation. Valve performance was verified with simulation. A flow mode bypass damper system was fabricated and was used to experimentally validate valve performance. Key Words: Author please supply Keywords??? INTRODUCTION THE substantialfield-induced yield stresses exhibited by magnetorheological (MR) fluids make possible numerous industrial applications (Carlson et al., 1996). Magnetorheological fluids can be implemented in a variety of semiactive smart actuation systems (Stanway et al., 1996) including optical polishing (Kordonski and Golini, 2), fluid clutches (Lee et al., 2), aerospace (Kamath et al., 1999), automotive (Lindler and Wereley, 1999; Gordaninejad and Kelso, 2), and civil damping applications (Dyke et al., 1998; Gavin et al., 21a). Furthermore, severalstudies have focused on the development of active devices utilizing electrorheological (ER) (Lou et al., 1991; Choi et al., 1997, 22) or MR fluids (Yoo et al., 21) in hydraulic actuation systems. Almost all of these applications use relatively large valves. Using MR valves in hydraulic actuation systems accrues many advantages, including: (1) valves have no moving parts, and (2) electronic flow control via an electromagnet. The most important advantages of an MR valve will be weight savings and reduction in complexity and moving parts as compared to a mechanical valve. A Wheatstone bridge-based hydraulic *Author to whom correspondence should be addressed. wereley@eng.umd.edu actuator is being developed at the University of Maryland for compact actuation in such applications as unmanned air vehicles and helicopters. The MR valve is a key component of the actuation system. However, as weight is a key issue in aerospace systems, smaller diameter valves are the focus of this study. Such reductions in size and volume may make actuation systems based on MR valves a feasible means of actuating such devices as trailing-edge flaps in helicopter blades (Milgram and Chopra, 1998). Two limitations of such an actuation scheme are the block force and the cut-off frequency of the actuator. The block force is a function of the yield stress of the MR fluid, and the cutoff frequency is a function of the response time of the MR fluid. The objective of this paper is to design and test a meso-scale MR valve while exploiting the maximum field dependent yield stress of the MR fluid. This entails designing an effective magnetic circuit in the valve: two kinds of steel material are examined a low permeability steel, Permalloy and a high permeability Hiperco 5-A. The MR valves will be analyzed and evaluated experimentally to assess controllability of axial flow rate and pressure drop in the valve. Also, magnetic field analysis will be utilized to optimize electromagnetic performance with given materialproperties. The pressure drop achieved across the MR valve is also measured as a function of applied current to validate JOURNAL OF INTELLIGENT MATERIAL SYSTEMS AND STRUCTURES, Vol. December X/2/ 1 7 $1./ DOI: 1.116/ ß 22 Sage Publications + [ :25am] [1 8] [Page No. 1] FIRST PROOFS i:/sage/jim/jim d (JIM) Paper: JIM Keyword

2 2 JIN-HYEONG YOO AND NORMAN M. WERELEY the design methodology. The performance of the MR valve is then evaluated using the nondimensional plug thickness (Wereley and Pang, 1998; Lindler and Wereley, 1999) as a metric of the valve efficiency. MR VALVES The MR valve in this study consists of a core and a flux return forming an annulus through which the MR fluid flows, as shown in Figure 1(a). The bobbin shaft is wound with insulated wire. A current applied through the wire coilaround the bobbin creates a magnetic field in the gap between the bobbin flanges and the flux return. The magnetic field increases the yield stress of the MR fluid between the bobbin flanges and the flux return. This increase in yield stress alters the velocity profile of the fluid in the gap by creating plug flow, which decreases the volume flux, Q, and raises the pressure drop, P, required for a given flow rate. We now consider the approximate rectangular duct analysis of the flow mode valve system containing MR fluid, which is assumed to behave as a Bingham-plastic material. Gavin (21b) has carefully examined the approximation errors when estimating annular duct behavior using the rectangular duct approximation. However, the rectangular duct approximation yields interesting and important insights in many situations, as long as the valve radius, R, is much greater than the annular gap, d. The typical velocity profile is illustrated R8.5 R CW Details in (b) CCW 25.4 in Figure 1(b). The total volume flux (Wereley and Li, 1998) is Q ¼ bd 3 12L a ð1 Þ 2 ð1 þ =2ÞP where the nondimensionalplug thickness is ¼ =d and ¼ for Newtonian flow. Also, L a is the active length in the MR valve, which is the sum of the three bobbin flange thicknesses. Here, b is the mid annulus circumference of the MR valve and is the differentialpostyield viscosity. To verify the performance of the valve, an MR bypass damper was designed and fabricated. A schematic of the flow mode bypass damper is shown in Figure 2. The volume flux displaced by the hydraulic cylinder head is proportional to the cylinder head velocity, v p,orq ¼ A p v p, where A p is the area of the cylinder head minus the area of the shaft. Solving for the force acting at the shaft, the pressure drop becomes P ¼ F A p ¼ 12L a A p bd 3 ð1 Þ 2 ð1 þ =2Þ v p Measuring shaft force, F and velocity, v p, we obtain from (Wereley and Pang, 1998) þ 1 12L aa 2 p v p bd 3 ¼ F In evaluating the valve performance, the nondimensional plug thickness in Equation (1) plays a substantial role. In the case of ¼ 1, there is no flow through the valve as in the case of an ideal valve (fully closed with infinite blocking pressure). Both flow rate, Q, and pressure drop, P, are a function of nondimensional plug thickness,. Thus, the nondimensionalplug ð1þ ð2þ ð3þ φ 42 (a) Valve cross section. All units in mm (b) Velocity Profile Figure 1. Schematic of the valve: (a) valve cross section; (b) velocity profile. Figure 2. Test configuration for the MR valve. + [ :25am] [1 8] [Page No. 2] FIRST PROOFS i:/sage/jim/jim d (JIM) Paper: JIM Keyword

3 3 Design of a High-efficiency Magnetorheological Valve thickness, which is a measure of the valve constriction and has a value range from ¼ (open) to ¼ 1 (closed), is an appropriate measure of valve efficiency. MAGNETIC CIRCUIT Bobbin Diameter The bobbin shaft radius is the most sensitive design parameter limiting the magnetic performance. In Figure 4, the averaged magnetic flux density along the bobbin flanges is plotted versus the bobbin shaft radius for a low permeability Permalloy steel, and a high permeability Hiperco 5-A powder metallurgical alloy. It is desired to have as high a magnetic flux as possible. Both materials provide adequate magnetic flux for bobbin shaft radii of at least 4 mm. However, the low permeability Permalloy rolls off below 4 mm much faster than the high Permeability Hiperco 5-A, due to saturation. Therefore, to reduce the bobbin shaft and hence, the valve diameter further, more costly higher permeability magnetic materials must be used. Bobbin Flange Length Figure 5 shows the trend of magnetic flux density at the gap as a function of bobbin flange length, L. In the case of Permalloy, as the bobbin flange length decreases, the magnetic flux density at the gap tends to increase because the magnetic flux density at the bobbin shaft decreases with decreasing the flange length. Decreasing Magnetic flux density (Te) The primary components of the MR valve design are pictured in Figure 3. The yield stress of the MR fluid can be varied as a function of the applied magnetic field. Therefore, the magnetic field applied to the MR fluid must be correlated with prediction or measurement. The main design parameters of the magnetic circuit are gap distance, bobbin shaft diameter, bobbin flanges length, thickness of the flux return and number of windings in the coil, which is related to the length of the bobbin shaft. To achieve an efficient magnetic circuit, the area of the path of magnetic flux should be maintained constant. Theoretically, a smaller gap distance is better because the permeability of the MR fluid in the gap is much less than those of the iron-based bobbin and flux return. Practical gaps typically range from.25 to 2 mm for ease of manufacture and assembly. A constant gap between the core and flux return was maintained for uniformity of the magnetic flux in the gap. Furthermore, the gap must be considered in the flow analysis. The gap determines the flow rate for a given pressure difference as shown in Equation (1). In this study, the gap will be set to.5 mm. The flux return (or hydraulic cylinder) has two functions: one is as an element in the magnetic circuit and the other is as a connector to the hydraulic cylinder cap with a seal as shown in Figure 3. In our small valve design, the minimum wall thickness of the flux return is set by the height of the threads around the hydraulic cylinder used to connect to the caps. A magnetic field finite element analysis for the valve system was conducted using ANSYS/Emag 2D. The purpose of the analysis was to identify saturation phenomenon in the magnetic circuit and to evaluate the effect of the design parameters on the magnetic behavior of the valve. The magnetization data for Permalloy steel (Roters, 1941) and Hiperco 5-A (Harner, 1999) material were used in this analysis. The MR fluid also has a saturation phenomenon in the yield stress as a function of applied magnetic field. Considering the shear stress versus magnetic inductance of MRF-132LD (Lord Corp., 1999),.8 T was the field at which magnetic saturate occurred in the MR fluid and the yield stress of the material was maximized. Throughout this analysis, an optimized MR valve was investigated to achieve smaller outer diameter and longer active length with maintaining magnetic flux density of.8 T at the gap Hiperco 5A Permalloy.2 2 Figure 3. Photograph of the MR valve parts. + [ :25am] [1 8] [Page No. 3] FIRST PROOFS 3 4 Bobbin shaft radius (mm) 5 Figure 4. Magnetic flux density at the gap as a function of the bobbin shaft radius (gap ¼.5 mm, air). i:/sage/jim/jim d (JIM) Paper: JIM Keyword

4 4 JIN-HYEONG YOO AND NORMAN M. WERELEY Magnetic flux density (Te) the flange length resulted in higher magnetic flux density at the gap by reducing saturation at the bobbin shaft. To verify this effect, the magnetic flux density at the gap was calculated for cases of 2, 4, and 1 mm flange length per each, as shown in Figure 6. Clearly, a length of 2 mm has the maximum magnetic flux density of the three. Figure 7 shows the magnetic flux density along the air gap with various active lengths. In this narrow range of the flange lengths, as the length increases, the uniformity of the magnetic field across the active length improves, but the level of magnetic flux density also decreases. Based on the results of Figure 7, a bobbin flange length, L ¼ 3 mm per each flange was chosen as our optimal design because we achieve an average magnetic field of.8 T in the bobbin flange, while maximizing the core length and thus the blocking pressure or yield force. Magnetic Material Hiperco 5A Permalloy Bobbin flange length (mm) Figure 5. Magnetic flux density at the gap as a function of the bobbin flange length (gap ¼.5 mm, air). Magnetic flux density (Te) L = 2 mm L = 4 mm L = 1 mm Applied current (A) Figure 6. The magnetic flux density at the gap with various bobbin flange lengths (Permalloy). The magnetic material has a limited achievable magnetic flux density at the gap due to magnetic saturation. This saturation phenomenon must be considered along with the MR fluid yield stress Magnetic Flux Density (Te) Magnetic Flux Density (Te) (b) in detail -2 2 Relative position along the air gap (mm) (a) L=2mm L=1mm L=3mm L=4mm (b) Zoom on circled region in Fig. (a) Figure 7. Magnetic flux density along the gap with various bobbin flange lengths (Permalloy, I ¼ 1 A). Table 1. Valve dimensions. Outer diameter Bobbin diameter Flange length/each Air gap Number of windings Maximum tesla at the gap 25.4 mm 14 mm 3 mm.5 mm 16 turns.8 T saturation, itself to achieve optimized magnetic field in the MR valve. Considering the shear stress versus magnetic inductance of MRF-132LD (Lord Corp., 1999) and magnetization curve for Permalloy,.8 T is the maximum magnetic flux density achievable with these materials. The averaged maximum magnetic flux density of our valve (dimensions are shown in Table 1) is about.8 T with 1.6-A input current, as shown in Figure 8. When high permeability material, Hiperco 5-A, is used as shown in Figures 4 and 5, the bobbin shaft diameter can be reduced, and the core length increased, while still obtaining the same performance as a larger valve made of lower permeability Permalloy steel. From these results, the valve can be reduced in volume on scale, while maintaining the same performance, by using a bobbin/flux return manufactured from a higher permeability material. + [ :25am] [1 8] [Page No. 4] FIRST PROOFS i:/sage/jim/jim d (JIM) Paper: JIM Keyword

5 Design of a High-efficiency Magnetorheological Valve 5 MEASUREMENT OF THE MAGNETIC FLUX DENSITY A thin film (FH-31-6, F.W.BELL) Hall sensor was used to measure the magnetic flux density at the gap and a hand-held Gauss meter (F.W.BELL, model 58) was used to calibrate the Hall sensor. Figure 8 compares the experimentaldata with the analyticalprediction from ANSYS/Emag 2-D for the valve with an air gap. Taking the error of the Hall sensor into consideration, the results in Figure 8 are in good agreement with each other. With these results, we conclude that simulation using ANSYS/Emag 2-D will be sufficiently accurate to predict the steady state performance of the magnetic field at the gap when filled with MR fluid. Thus, at 1.6 A of applied current, we will have induced a magnetic flux of.8 T, which is sufficient for our purposes. Magnetic flux density (Te) Test result, air Simulation, air Simulation, MR Applied current (A) Figure 8. Performance of the magnetic flux density at the gap with MR fluid permeability (simulation) and with air permeability (test and simulation, Permalloy). MAGNETORHEOLOGICAL FLUID CONSTITUTIVE MODEL Due to the nonlinearity of the MR fluid, determination of the performance of the valve requires numerical analysis. We will evaluate the valve performance by calculating the pressure difference and flow rate. We used a commercially available MR fluid, namely MRF- 132LD (Lord Corp., 1999) in our experiments. For the Bingham-plastic model, the MR fluid properties of dynamic yield stress, y, and plastic viscosity,, were required as a function of applied magnetic field. The dynamic yield stress for this fluid was approximated by a cubic equation of the magnetic field, B, so that y ¼ a 3 B 3 þ a 2 B 2 þ a 1 B þ a : The polynomial coefficients were determined by least-squares fit of the dynamic yield stress data as a function of magnetic field from data supplied by Lord Corporation (1999), and are: a ¼ :877 kpa, a 1 ¼ 17:42 kpa=t, a 2 ¼ 122:56 kpa=t 2 and a 3 ¼ 86:51 kpa=t 3 : To simplify the analysis, the MR fluid is assumed to have a nominalplastic viscosity of.3 Pa s. EXPERIMENTALRESULTS To validate our nondimensional analysis using the nonlinear Bingham-plastic shear flow and the magnetic circuit design with ANSYS/Emag 2-D, a high stroke ( 2 cm) MR damper was constructed. The damper consists of four main parts: a hydraulic cylinder, industrialtube fittings, an accumulator and an MR bypass valve as pictured in Figure 9. The damper was charged with MR fluid, MRF-132LD (Lord Corporation). The accumulator connected to the cylinder was used to DC Power supply Hydraulic Cylinder MR Valve Load cell Accumulator Signal Conditioning Amplifier MTS Controller MTS Hydraulic Actuator Digital Oscilloscope Figure 9. Experimental setup for performance measurement of the MR valve. + [ :25am] [1 8] [Page No. 5] FIRST PROOFS i:/sage/jim/jim d (JIM) Paper: JIM Keyword

6 6 JIN-HYEONG YOO AND NORMAN M. WERELEY pressurize the MR fluid inside the damper for purposes of compressing adsorbed air bubbles in the fluid, and preventing cavitation. For the experimental validation of the flow mode bypass valve equations, force measurement from a constant velocity amplitude square wave on an MTS servo-hydraulic testing machine were recorded on a computer. The MR bypass damper was mounted in the clevises, and the shaft of the damper was oscillated. The shaft displacement was measured using an LVDT and the applied load was measured using a 25 lb load cell. The shear stress of the MR fluid in the case of the valve is shown in Figure 1. The maximum shear stress is about 47 kpa with 1.6-A input current. There are two saturation effects accounted for simultaneously in this diagram, the magnetic flux density and the yield stress of the MR fluid. The pressure drop versus flow rate diagram is shown in Figure 11 with experimentalresults. The analysis, based on the quoted MR fluid data, tended to overpredict the experimentalpressure difference data. At low currents this overprediction was substantial, almost 3% error. We will be studying the reasons for such overpredictions in future studies. With this valve configuration, nominally 25 psi of pressure drop can be achieved. Figure 12 compares the test results for the nondimensional plug thickness to analysis in the valve, for a Shear Stress (kpa) Applied current (Amp.) Figure 1. Shear stress of the MR fluid at the gap (Permalloy). Pressure Difference (kpa) Flow Rate (cc/sec) 1.6 A 1.2 A.8 A.6 A.4 A Figure 11. Flow characteristics (symbols: test and lines: simulation). constant current input. This data demonstrates that a plug thickness of from 91% to just over 72% can be achieved over the pressure range of kpa. From these data, we deduce that kpa of block pressure with 1.6-A input current can be achieved. Figure 13 shows the time response of the valve with input step current. With this result, we can expect that the maximum drive frequency will be about 1 Hz. CONCLUSION Considering the shear stress versus magnetic inductance of the MR fluid (MRF-132LD, Lord Corp., 1999) and magnetization curve for Permalloy, a maximum magnetic flux density at the gap was achieved with an optimized design and was verified with simulation and experiment. Based on these results, we conclude the following: 1. Using low permeability Permalloy steel material, kpa block pressure can be achieved with our MR valve design with a 25.4 mm outer diameter. 2. Using high permeability material, the size of the valve can be reduced and the active core length can be Pressure Response (kpa) Plug Thickness, δ[1] Pressure Difference (kpa) Test Simulation Figure 12. The nondimensional plug thickness, (I ¼ 1.6 A)..1.2 Time (sec) Figure 13. The time response of normalized pressure difference of the valve (test results). + [ :25am] [1 8] [Page No. 6] FIRST PROOFS i:/sage/jim/jim d (JIM) Paper: JIM Keyword

7 Design of a High-efficiency Magnetorheological Valve 7 increased for a higher blocking pressure. The Hiperco 5-A steelcore has same magnetic flux density at the gap with only 6% of the shaft radius for the Permalloy steel case. 3. The nondimensionalplug thickness is a useful measure of valve efficiency, and the valve configuration of this paper achieves up to 9% efficiency. 4. The valve design based on the magnetic analysis in this paper achieves 1 Hz of dynamic range including the response time of MR fluid. ACKNOWLEDGMENT The authors thank Monique Gabrieland Dr. Mark Jolly of Lord Corporation (Cary, NC) for providing the MR fluid (MRF-132LD, 1999) used in this study. REFERENCES Carlson, J. D., Catanzarite, D. M. and Clair, K. A. S Commercial Magnetorheological Fluid Devices Technology, International Journal of Modern Physics Part B, 1(22 23):2857. Choi, S.-B., Cheong, C.-C., Jung, J.-M. and Choi, Y.-T Position Control of an ER Valve-Cylinder System via Neural Network Controller, Mechatronics, 7(1): Choi, S.-B., Sung, K. -G. and Lee, J. -W. 22. The NeuralNetwork Position Control of a Moving Platform Using Electrorheological Valves, ASME Journal of Dynamic Systems, Measurement and Control, 124(3): Dyke, S. J., Spencer, B. F., Sain, M. K. and Carlson, J. D ExperimentalStudy of MR Dampers for Seismic Protection, Smart Materials and Structures, 7(5): Gavin, H., Hoagg, J. and Dobossy, M. 21a. OptimalDesign of MR Dampers, In: Proceedings U.S.-Japan Workshop on Smart Structures for Improved Seismic Performance in Urban Regions, 14 August 21, Seattle WA, pp Gavin, H. P. 21b. Annular Poiseuille Flow of Electrorheological and Magnetorheological Materials, Journal of Rheology, 45(4): Gordaninejad, F. and Kelso, S. P. 2. Fail-safe Magneto- Rheological Fluid Dampers for Off-Highway, High-Payload Vehicles, Journal of Intelligent Material Systems and Structures, 11(5): Harner, L. L A Simplified Method of Selecting Soft Magnetic Alloys, Carpenter Technology Corporations Technical Articles. ( Kamath, G. M., Wereley, N. M. and Jolly, M. R Characterization of Magnetorheological Helicopter Lag Damper, Journal of the American Helicopter Society, 44(3): Kordonski, W. I. and Golini, D. 2. Fundamentals of Magnetorheological Fluid Utilization in High Precision Finishing, Journal of Intelligent Material Systems and Structures, 1(9): Lee, U., Kim, D., Hur, N. and Jeon, D. 2. Design Analysis and Experimental Evaluation of an MR Fluid Clutch, Journal of Intelligent Material Systems and Structures, 1(9): Lindler, J. and Wereley, N. M Analysis and Testing of Electrorheological Bypass Dampers, Journal of Intelligent Material Systems and Structures, 1(5): Lou, Z., Ervin, R. D. and Filisko, F. E Behaviors of Electrorheological Valves and Bridges, In: Proceedings of the International Conference on Electrorheological Fluids: Mechanics, Properties, Structure, Technology and Applications, October, 1991, World Scientific Publishing Co., Carbondale, Illinois, pp Milgram, J. H. and Chopra, I., Parametric Design Study for Actively Controlled Trailing Edge Flaps, Journal of the American Helicopter Society, 43(2): Roters, H. C Electromagnetic Devices, John Wiley & Sons, Inc. Stanway, R., Sproston, J. L. and El-Wahed, A. K Applications of Electro-Rheological Fluids in Vibration Control: A Survey, Smart Materials and Structures, 5(4): Yoo, J.-H., Sirohi, J. and Wereley, N. M. 21. Design of an MR Hydraulic Power Actuation System, In: Proceedings of SPIE The International Society for Optical Engineering, Vol. 4327, 21, Smart Structures and Materials 21 Smart Structures and Integrated Systems 5 8 March, 21, Newport Beach, CA, pp Wereley, N. M. and Pang, L Nondimensional Analysis of Semi-active Electrorheological and Magnetorheological Dampers using Approximate Parallel Plate Models, Smart Materials and Structures, 7(5): [ :25am] [1 8] [Page No. 7] FIRST PROOFS i:/sage/jim/jim d (JIM) Paper: JIM Keyword

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