A magnetorheological damper is a shock-absorbing device that operates by placing a magnetic field on a mixture of liquid oil and iron particles. The iron particles are attracted to the magnetic field and line up along the magnetic field lines that pass through the liquid. This creates a thick liquid that resists movement and can help reduce vibration and shock in a variety of applications. These fluids are sometimes referred to as smart fluids, because they change properties when the magnetic field is added or varied.
Rheology is the study of the effects of liquids and solids when exposed to movement or pressures. In liquids, the primary characteristics that can be useful for vibration control are viscosity and shear stress. Viscosity refers to the thickness of a liquid, and how it can resist movement or flow. Shear stress is a measurement of how a liquid resists being pulled apart or moved suddenly, and also how materials placed in the liquid can move if pulled quickly in any direction.
A damper is a term for a device used to reduce vibration, which is similar to a shock absorber used on vehicles to reduce suspension movement due to uneven road surfaces. Many dampers and shock absorbers use oils of various thicknesses to reduce movement and protect equipment. When small particles of iron are added to oils, magnetic fields can affect the iron particles and change the properties of the liquid.
Adding the iron-oil mixture inside a standard shock absorber, and creating a magnetic field with an electric current, will create a magnetorheological damper. As the magnetic field is increased, the iron particles will increasingly resist movement and create higher levels of vibration dampening. If an electrical controller is added along with software to control the magnetic field, a variable magnetorheological damper can be used to quickly reduce vibration and protect structures or vehicles.
Iron particles in dampers are often coated with a polymer to keep them from sticking together. Keeping the particles very small helps keep them suspended in the oil and prevents them from settling to the bottom of the damper. When the magnetic field is created, the mixture becomes more like a solid than a liquid, and is very resistant to flow or movement. If the oil is pushed with a piston inside a cylinder, the high viscosity can reduce movement of the oil through small holes in the piston.
Shear stress can be utilized by changing the piston to a series of plates submerged in the oil. The plates move back and forth in the liquid, and when the magnetic field is activated, the iron-oil mixture thickens quickly and becomes very resistant to shear. If the plates are connected to a solid shaft extending from the damper, the shaft can be attached to a vehicle or building foundation and provide a damping system.
Earthquake protection became an area of increased research in the late 20th century as human development grew in areas where the potential for building damage was high. One technique was to separate the building from the ground with rubber or other shock-absorbing materials, which allowed some building movement during an earthquake. Without some form of dampening, however, building movement could be extreme and damage or complete failure could occur. Adding a magnetorheological damper system at the base of the building gave the architects a way to reduce the building movement with a controllable system.
Vehicles were another area of interest for magnetic dampening systems in the 20th century. Interest in passenger comfort and an increased level of safety systems led to shock absorbers using magnetorheological damper technology to provide variable suspension. The driver could choose a ride comfort level using a selector, which would tell a controller how much magnetic field to create when vibration was sensed. In addition, safety systems could detect skidding or a possible rollover situation, and change the suspension behavior to counteract it.
Military weapons were another area where magnetorheological dampers could provide a benefit. When fitted to a large cannon, the damper could sense the firing of a projectile and activate to reduce the recoil. This not only reduced wear on the weapon, but in mobile tanks or cannons, the shock reduction could reduce fatigue on the soldiers firing the weapons.