Semi-active suspension systems using magneto-rheological fluids

For a given vehicle, the suspension system isolates the passengers and payload from shock and vibration. In addition, the system also enables the wheels to maintain contact with the surface, thereby affecting stability and control of the vehicle. The challenge in designing a (traditional) passive suspension system is the conflicting criteria of road holding, payload and passenger comfort. This means that a passive suspension system either has harsh ride characteristics, but good control characteristics, or vice versa. Typically, the type of system used is largely determined by the intended usage of the designed vehicle. Semi-active suspension systems, and in particular active suspension systems (both characterised as controllable suspension systems), are considered to be a way of achieving both excellent ride and handling characteristics, and both systems are capable of varying its parameters actively whilst the vehicle is in motion. A semi-active suspension system is a good compromise between a passive and an active suspension system, and such a system has in principle no additional energy demand. Regarding semi-active suspension systems, a couple of different technical solutions (i.e. principles) exist. Semi-active suspension systems using magneto-rheological (MR) fluids are perhaps the most promising technology among this group of systems. Such fluids are characterised as controllable fluids, and are manufactured by suspending ferromagnetic particles in a carrier fluid. MR fluids (MRF) exhibit a change in rheological properties (including the viscosity) when being exposed to a magnetic field. The viscosity changes can be controlled accurately by varying the magnetic field strength, giving MR fluids the ability to reversibly change from viscous liquids to semi-solids in milliseconds. This gives a rapid response interface between electronic controls and mechanical systems, making MRF technologies attractive for many applications (e.g. dampers). Semi-active suspension systems using MR fluids have among other successfully been installed in luxury automobiles, sports cars, light trucks and sport utility vehicles. In addition, some research projects exist where such systems have been integrated in military vehicles (e.g. Stryker and HMMWV). Controllable suspension systems, including semi-active systems, can greatly improve the performance of a military vehicle. Such systems can clearly have a positive impact on health and comfort factors for occupants in such a vehicle. This is due to reduced vibrations in critical frequency ranges. A controllable suspension system can also improve the tactical mobility of a vehicle, since a reduction in absorbed power is experienced for a given velocity. Increased mobility can also increase the survivability. Controllable suspension systems can also give additional benefits, e.g. increased hit probability (i.e. the enemy), increased payload, increased durability of the vehicle, increased operational range and more consistent driving characteristics. Unfortunately, a controllable suspension system integrated into a military vehicle will add complexity to the vehicle, that is, compared to a conventional (passive) suspension system. This could have a significant impact on maintenance needs. Another current disadvantage with controllable suspension systems is that, up to now, only research projects exist, leading to a lack of field experience.