Magnetorheological Shock Absorber for Crew Seats in the Expeditionary Fighting Vehicle

A magnetorheological shock absorber (MRSA) system is designed and tested to integrate semi-active shock and vibration mitigating technology into the existing EFV (Expeditionary Fighting Vehicle) forward seating positions. Based on the operational requirements of the vehicle, the MRSA is designed so that it can not only isolate occupants from harmful whole body vibration (WBV) during normal operations but also reduce injury risk during extreme events such as a “rogue” wave or ballistic/UNDEX shock event. The MRSA consists of a piston with a circular flow-mode valve, a magnetorheological (MR) fluid cylinder, and a nitrogen accumulator. Piston motion forces MR fluids enclosed in the fluid cylinder to flow through the valve where it is activated by a magnetic field in the valve. Based on the Bingham-plastic constitutive relation and a steady state fluid motion model, the valve parameters are determined using a magnetic circuit analysis tool and are validated by electromagnetic finite element analysis (FEA). The high-speed field-off viscous force of the MRSA is predicted using computational fluid dynamic analysis. To experimentally evaluate the damping performance of the MRSA and validate the design, the MRSA is tested under single frequency sinusoidal displacement excitation on a material dynamic testing machine for low piston velocities (up to 0.9 m/s) performance evaluation. For performance evaluation at high piston velocities (up to 2.2 m/s), the MRSA is tested under impact loading on a rail-guided mass-drop test stand. Equivalent viscous damping is used to characterize the controllable damping behavior of the MRSA. To describe the time response of the MRSA, a dynamic model is developed based on geometrical parameters and MR fluid properties.