Innovative Designs for MR Dampers: Thesis on Fluid Devices and Applications, Papers of Mechanical Engineering

Download Innovative Designs for MR Dampers: Thesis on Fluid Devices and Applications and more Papers Mechanical Engineering in PDF only on Docsity! Innovative Designs for Magneto-Rheological Dampers James Poynor Advanced Vehicle Dynamics Laboratory Virginia Polytechnic Institute and State University Audience of Thesis The primary audience for this thesis will be my graduate advisory committee. A secondary audience may consist of students that are involved in the field of magneto-rheological (MR) damper design. Summary of Thesis This thesis will present the results of evaluating several different MR damper designs and will also make recommendations for the design of new MR dampers. The following excerpt is Chapter 1. 2 Table of Contents Abstract x List of Figures x List of Tables x Chapter 1: Introduction x 1.1 Overview of Magneto-Rheological (MR) Fluid Devices x 1.2 Project Objectives x 1.3 Approach x Chapter 2: Background x 2.1 MR Fluids x 2.2 MR Devices x 2.3 MR Dampers x Chapter 3: Design of MR Dampers for Vehicle Applications x 3.1 Monotube MR Dampers x 3.2 Twin Tube MR Dampers x 3.3 Performance of MR Dampers x Chapter 4: MR Damper Design for Gun Recoil Control x Chapter 5: Hybrid MR Dampers x Chapter 6: Concluding Remarks x 6.1 Summary x 6.2 Recommendations x Appendix A: (Title to Be Determined) x Appendix B: Drawings x References x 5 Force Force MR Fluid Paramagnetic Pole Paramagnetic Pole Magnetic Field Magnetic Field Figure 2. MR fluid used in shear mode. FlowMR Fluid Magnetic Field Magnetic Field Annular Orifice (or Inside Diameter of a Tube) Poles Figure 3. MR fluid used in valve mode. When MR fluid is used in the valve mode, the areas where the MR fluid is exposed to magnetic flux lines are usually referred to as “choking points” (see Figure 4). In the case of the damper depicted in Figure 4, MR fluid restricts the flow of fluid from one side of the piston to the other when the fluid is in the vicinity of the “choking points” shown. Varying the magnetic field strength has the effect of changing the apparent viscosity of the MR fluid. The phrase “apparent viscosity” is used since the carrier fluid exhibits no change in viscosity as the magnetic field strength is varied. Upon exposure to a magnetic field, the MR fluid as (a whole) will appear to have undergone a change in viscosity. As the magnetic field strength increases, the resistance to fluid flow at the choking points increases until the saturation point has been reached. The saturation point 6 is the point where any increase in magnetic field strength fails to yield an increase in damper resistance. This resistance to movement that the iron particles exhibit is what allows us to use MR fluid in electrically controlled viscous dampers. Choking Points Magnetic Coils Fluid Gap Housing Approximate Path of Magnetic Flux Figure 4. Typical MR damper. Monotube and Twin Tube Magneto-Rheological Dampers. A monotube MR damper (see Figure 5) is one that has only one reservoir for the MR fluid and also has some way to allow for the change in volume that results from piston rod movement. In order to accommodate this change in reservoir volume, an accumulator piston is usually used. The accumulator piston provides a barrier between the MR fluid and a compressed gas (usually nitrogen) that is used to accommodate the necessary volume changes. 7 Piston Rod Piston Accumulator Piston Housing Compressed Gas Reservoir Piston Guide MR Fluid Reservoir Figure 5. Monotube MR damper section view. The twin tube MR damper is one that has two fluid reservoirs, one inside of the other. This configuration, which can be seen in Figure 6, has an inner and an outer housing. The inner housing guides the piston/piston rod assembly just as the housing of a monotube damper does. This inner housing is filled with MR fluid so that no air pockets exist. To accommodate changes in volume due to piston rod movement, an outer housing that is partially filled with MR fluid occurs. In practice, a valve assembly called a “foot valve” is attached to the bottom of the inner housing to regulate the flow of fluid between the two reservoirs (see Figure 7). As the piston rod enters the damper, MR fluid flows from the inner housing into the outer housing through the compression valve that is attached to the bottom of the inner housing. The amount of fluid that flows from the inner housing into the outer housing is equal to the volume displaced by the piston rod as it enters the inner housing. As the piston rod is withdrawn from the damper, MR fluid flows into the inner housing through the return valve. In order for a twin-tube MR damper to function properly, the compression valve must be stiff relative to the pressure differential that exists between either side of the piston when it is in operation. The return valve must be very unrestrictive so that as little resistance to fluid flow as possible is provided. The damper should function properly as long as the following conditions are met: (1) the valving is set up properly; (2) MR fluid settling is not a problem; and (3) the damper is used in an upright position. With this type of MR damper, keeping the iron particles (which are an integral part of MR fluid) in suspension is a major concern since these iron particles can settle into the valve area and prevent 10 Outer Housing MR Valve Inner Housing Hydraulic Fluid MR Fluid Hydraulic Valving Figure 9. MR piloted hydraulic damper. 1.2 Project Objectives The primary objectives of this research were as follows: (1) to study different designs that are commonly used for MR dampers; (2) to recommend new designs that were able to improve the performance of existing MR dampers; and (3) to provide recommendations for the effective design and fabrication of MR dampers. To accomplish these goals, several MR dampers were designed, built, and tested to determine their suitability and performance for specific applications. 1.3 Approach The approach used to satisfy the previously stated objectives was to design, build, and test MR dampers for automotive and gun recoil applications. To explore the area of MR dampers for gun recoil use, a gun recoil demonstrator (see Figure 10) was built. A double-ended MR damper was placed behind a single-shot, bolt-action rifle that was mounted on a ball bearing recoil slide. The caliber that was chosen for this test apparatus was the 50-caliber Browning Machine Gun (50BMG) cartridge. This cartridge was chosen because of its high recoil energy, its availability, and the armed force’s familiarity with weapons of this caliber. A force transducer and a LVDT (linear variable differential 11 transformer) were used in conjunction with a Hewlett-Packard dynamic signal analyzer to capture and record data for force and velocity. MR Damper Rifle Action Recoil Slide Figure 10. Gun recoil demonstrator. To explore possibilities in the field of automotive MR dampers, a set of monotube MR dampers was designed and built for use on a Mercedes ML-430 sport-utility vehicle. A drawing of a front damper for this application was previously shown in Figure 5. To determine what force-velocity characteristics the new MR dampers would need, the original hydraulic dampers were tested in an MTS (Material Testing Systems) machine. These tests yielded force-velocity data that was then used to determine what internal geometry the new MR dampers should have. Figure 11 shows the MTS machine with a damper mounted in place. Similarly, a new, easier to manufacture version of the Mercedes ML-430 damper was built and tested. In addition to the monotube MR dampers that were built for the Mercedes ML-430, two different MR piloted hydraulic dampers were built as well. One MR piloted hydraulic damper was built using a valve mode MR damper, and the other was built using a squeeze mode MR damper. Finally, a twin tube MR damper was built and tested. All of these automotive MR dampers were tested on the MTS machine. 12 Figure 11. MTS machine used for automotive damper testing.