Abstract
Devices and systems using magnetorheological (MR) fluid technology have been commercialized across a wide range of industries, the most prominent and promising being automotive applications in recent years. Commercial MR fluid devices and systems have been developed for primary suspension in passenger automobiles; driver's seat suspension in off-highway, construction and agricultural equipment; and steer-by-wire operator control in industrial, off-highway and marine vehicles.
MR fluids are materials that respond to a magnetic field with a dramatic change in rheological behavior. These fluids can reversibly change instantaneously from a free-flowing liquid to a semi-solid with controllable yield strength when exposed to a magnetic field. In the absence of an applied field, MR fluids are reasonably well approximated as Newtonian liquids. For many engineering applications a simple Bingham plastic model is effective in describing the essential, field-dependent fluid characteristics.
A typical MR fluid consists of 20-40 percent by volume of relatively pure, 3-10 micron diameter iron particles, suspended in a carrier liquid such as synthetic oil, water or glycol. A variety of proprietary additives to discourage gravitational settling and promote particle suspension are commonly added to Lord's state-of-the-art MR fluids to enhance lubricity, modify viscosity, and inhibit wear. The rheological properties of MR fluids depend on the concentration, size, distribution and shape of the iron particles, the carrier properties, additives, applied field, temperature and other variables. The interdependency of all of these factors is complex and important to understand in optimizing the performance of these fluids within an MR device and control system for a particular application, as trade-offs must constantly be made in the design process. MR fluids exhibit maximum yield strengths of 50- 100 kPa for applied magnetic fields of 150-250 kA/m. MR fluids are not highly sensitive to moisture or other contaminants that might be encountered during manufacture and usage. MR fluid-based devices is relatively insensitive to temperature over a broad temperature range.
MR fluid formulations are tailored for specific applications, providing high strength and systems that are durable and robust. Two areas of performance that received particular attention during Lord's decade-long MR development program in the 1990s were settling stability and durability (wear and seal life) as MR fluids, when first invented, were not stable to long-term settling and could be quite abrasive in use.
The company also has compiled scientific data through aggressive life cycle testing and installed commercial applications that clearly demonstrate the effectiveness, durability and performance. MR fluid-based automotive shock absorbers were tested after actual consumer use in both southern and northern climates in the USA. These shocks were found to be within performance specifications even after 150% of lifetime durability. Today thousands of vehicles are in operation and more than 100,000 MR dampers, shock absorbers and brakes are in use. For automotive platform applications alone, anticipated annual growth in demand for MR devices is more than 1 million.
Keywords:MR, Magnetorheological Fluid, Suspension, Vehicle, Vibration Control