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By: Alex Kovnat
Army researchers are always searching for new and innovative ideas to apply to ground vehicle platforms in order to make them more mobile, durable and survivable. A “smart” substance called Magnetorheological (MR) fluid, which can be applied to a vehicle’s shock absorbers, may be one of the emerging technologies Army scientists and engineers can use to improve vehicle performance and durability.
MR fluids are referred to as “smart” because their consistency can be varied rapidly in response to changing conditions. They consist of magnetic particles such as iron, suspended in a carrier fluid with the particles occupying 20 to 40 percent of the total volume. When a magnetic field passes through the fluid, the particles align themselves with the applied field, causing the fluid to become thicker. The viscosity change is nearly instantaneous and fully reversible.
This technology has important applications in ground vehicles, which the U.S. Army Tank Automotive Research, Development and Engineering Center (TARDEC) and its partners in the Program Executive Office for Ground Combat Systems (PEO GCS) and PEO Combat Support and Combat Services Support (CS&CSS) have investigated.
MR Fluids in Experimental Suspensions
Shock absorbers in most vehicles rely on a hydraulic fluid, a sliding piston and other parts that suffer wear and tear. In addition, the characteristics of a conventional shock absorber are either fixed, or else cannot be rapidly varied on demand. MR fluid is the enabling technology to design entirely new suspension systems that avoid these limitations, leading to improvements such as reduced wear and tear and more reliability through the life cycle.
LORD Corp. is the leading supplier of MR fluids in the United States. “The automotive original equipment manufacturers recognized the value of an adaptive suspension system utilizing [MR fluid in] dampers designed without moving parts,” noted LORD National Accounts Manager Stephen Webb. “We began developing the adaptive suspension system for military vehicles after first realizing success in the automotive markets.”
TARDEC’s Ground Vehicle Power and Mobility Group’s Track and Suspension Team (GVPM T&S), along with government and industry partners, have created semi-active suspension systems using MR fluids that have been successfully tested and demonstrated on a variety of Army platforms. These MR fluid-based suspension systems are an upgrade to modern suspension systems that minimize the impact of vehicle motions on its components and occupants.
“The first area examined is ride quality, which translates to decreased warfighter fatigue, fewer injuries and less vehicle wear. Increasing vehicle safety is another, if not more important, aspect,” Webb explained.
LORD Corp.’s MR technology has been proven through the licensing and broad intellectual property portfolio used in developing BWI Group’s MagneRideTM Suspension System.
The MR fluid TARDEC uses is oil-based with micron-sized iron particles and has a consistency similar to house paint. When a magnetic field is applied, the iron particles line up and thicken the fluid. It is then in a semi-solid state with a consistency like putty.
MR-based suspension systems use the fluid in specially designed dampers, which take the place of standard dampers (shock absorbers). The MR damper contains the fluid and an electric coil, which generates a magnetic field that controls fluid viscosity at any given moment. The overall MR suspension system also consists of a controller that manages the strength of the magnetic field applied to the fluid.
“Think of this as a ‘smart shock,’ one which can be turned on only when needed,” Webb explained. When the suspension needs to be looser, the controller reduces the current to the coil, which relaxes the magnetic field and makes the fluid less viscous. If the suspension needs to be firmer, the controller increases the current, which makes the fluid thicker. Additionally, each shock is controlled independently, to provide the best overall control for the whole vehicle.
MillenWorks is a supplier for MR damper design, development and integration. “This controlled suspension fits into the same envelope as previous suspensions and can interface with existing vehicle technologies,” explained MillenWorks Director of Suspension systems Systems Peter LeNoach. MR suspensions can be retrofitted into existing vehicle designs with little to or no suspension modifications. Various options are available for installing these upgrades, including a vehicle’s normal RESET periods and field-retrofits.
Testing Lays Groundwork
Testing has been conducted on a number of vehicle types over the past 13 years. It started with a Small Business Innovation Research (SBIR) grant awarded to create a proof-of-concept High-Mobility Multipurpose Wheeled Vehicle (HMMWV) modified with an MR-based suspension system in 1999. Since then, the Family of Medium Tactical Vehicles (FMTV), Caiman Mine-Resistant Ambush-Protected (MRAP) vehicle and Stryker have all been outfitted with MR suspension systems. MR fluid-based suspension systems have also been considered for the Bradley Fighting Vehicle.
An increased focus on warfighter protection systems, particularly armor, has added to overall vehicle weight. This has presented significant challenges for engineers designing suspension systems for durability and performance. For example, the shocks in an HMMWV are about twice as large as those in a passenger vehicle. “Adapting the system to work in severe off-road environments required big jumps in technologies. The peak forces are an order of magnitude larger,” LeNoach observed.
TARDEC’s GVPM T&S team and its industry partners have discovered that the new suspension provides various efficiencies. The smoother ride, resulting from less energy being transmitted to the vehicle hull and occupants, improves vehicle component durability and crew responsiveness during target acquisition and engagement. The gains in chassis stability improve overall mobility, allowing the vehicle to traverse rougher terrain at higher speeds and perform more aggressive maneuvers without losing control or rolling over. Better handling and less travel time reduce risk to warfighters, and increase the effective life span of vehicles and components.
Results of MR suspension testing included up to 72 percent increase in cross-country speed, up to 60 percent reduction in peak vehicle shock/vibration levels, and up to 30 percent improvement in handling and stability by reducing vehicle roll rate, specifically during the NATO Lane Change Maneuver Test.
Future Technology Transitions
Researchers are conducting durability tests to show the technology works and will handle military environments. MR suspensions have been researched and developed for use on platforms such as the HMMWV, Joint Light Tactical Vehicle (JLTV) variants and the Caiman MRAP vehicle. The MR suspension has also been demonstrated on the Stryker platform.
This diagram shows how MR fluid works. Under a mag- netic field, the iron particles (red dots) in the fluid align with the field, thus increasing the viscosity of the fluid and changing the damping characteristics of the suspen- sion system. (U.S. Army image.)
Reinforcing the Army’s efforts, the Office of the Secretary of Defense provided a Technology Transition Initiative in 2009 to accelerate MR fluid technology to align with the Stryker ECP Improvement Program, formerly referred to as the Stryker Modernization Program.
Testing has already been completed to validate MR suspension systems on the Stryker. Teams completed 4,000 miles of durability testing on an upweighted Medical Evacuation Vehicle (MEV) variant at APG in April 2011, with zero failures. They also ested a Stryker Infantry Carrier Variant (ICV) with an MR suspension system at Yuma Proving Ground, AZ. It was also tested Mobile Gun System (MGS) Stryker variant. Testing showed the system is capable of stabilizing the MGS chassis and increasing performance. The MR suspension system tripled the Stryker MGS’s maximum target tracking speed over a bump course. Mr. David Dopp, the project manager for the Stryker Brigade Combat Team agrees that, “the MR System provided greater platform stability and significantly reduced the shock and vibration transmitted to the Stryker crew and vehicle occupants.”
The technology has achieved Technology Readiness Level 7 (system prototype demonstration in an operational environment). “This is a good example of a fundamental change of technology through a concerted effort between industry and government to keep going and to realize benefits over the long haul thanks to patience,” LeNoach remarked.
Alex Kovnat is a TARDEC GVPM T&S Team mechanical engineer. He holds a baccalaureate degree in mechanical and aerospace engineering from Illinois Institute of Technology, and master of science degrees in nuclear engineering from Purdue University and automotive engineering from Lawrence Technological University. He is Level III-certified in Systems Planning, Research, Development and Engineering, and is currently serving as Contracting Officer’s Technical Representative on two Phase II SBIR programs.
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