MR Fluids consist of magnetic (typically iron) particles in a carrier fluid. In the presence of a magnetic field, the micron-sized particles link (see Figure 1) and change the fluid to a semi-solid in milliseconds. When the magnetic field is removed, the fluid just as quickly reverts back to its natural free-flowing state (see Figure 2). Furthermore, the degree to which the fluid changes to a semi-solid is proportional to the strength of the magnetic field, giving the fluid infinite controllability and precision.
MR fluids can be used in shear mode, with the fluid flowing between two surfaces (as in our SbW Tactile Feedback Device - see Figure 3) or in a valve mode with fluid flowing through an orifice (as in a damper - see Figure 4 below), which move relative to each other. In the absence of a magnetic field applied across the gap the fluid occupies, the fluid flows freely or allows free movement. Upon application of a magnetic field, the particles align like chains with the direction of the field.
The formation of these particle chains restricts the movement of the fluid within the gap since the fluid's yield strength is increased. Altering the inter-particle attraction by increasing or decreasing the strength of the field permits continuous control of the fluid's rheological properties and hence the damping or clutch or braking force.
LORD's patented MR fluids exhibit fast response time, high dynamic yield stress, low plastic viscosity, broad operational temperature range, resistance to settling, easy remixing, and excellent wear and abrasion resistance.
Similar to passive hydraulic dampers, an MR damper consists of a fluid that moves between different chambers via small orifices in the piston, converting "shock" energy into heat (see Figure 4). However in an MR damper, an electrical circuit is introduced in the piston assembly. As electrical current is supplied to the damper, a coil inside the piston creates a magnetic field and instantaneously changes the properties of the MR Fluid in the piston (see Figure 5). Consequently, the resistance of the damper can be continuously changed in real time by modulating electrical current to the damper.
Adaptive suspension systems rely on quick detection of a disturbance and precise control of the damper for optimal suspension performance. Our control systems leverage a network of sensors that continuously monitor the driving situation in a vehicle and send data to the control unit via the CAN bus. The control unit interprets these signals and regulates electrical current to the damper using sophisticated proprietary control algorithms. This process occurs continually thousands of times per second during vehicle operation to ensure the ideal suspension characteristics for the specific driving condition. LORD has spent decades developing proprietary control algorithms that optimize the unique capabilities of MR Fluid technology.
Typical performance characteristics of various suspension technologies are displayed in Figure 7 using a force-velocity curve. The speed at which MR technology changes damper forces enables more use of the damper stroke to control motion, improving the ride and handling for the operator.