The New Wave 
in Shock Absorbers

The promise of an old idea is finally fulfilled.

Over the past two model years, General Motors (GM) has introduced one OF THE most interesting and potentially far-reaching new technologies ever developed for automobiles. The Chevrolet Corvette and the Cadillac SRX, STS and XLR models are all available with Magnetic Ride Control. Called MagneRide by its supplier Delphi Automotive Systems, it uses a computer to adjust the shock absorbers' damping rate. While electronic damping adjustment is nothing new, the shock absorbers themselves are different from anything you've seen before. Instead of adjustable valves, these shocks have adjustable oil.

A hydraulic shock absorber dampens suspension movement by forcing a piston to move through oil. Holes in the piston are covered by spring-loaded valves. These valves slow the flow of oil through the holes to control damping rate: the smaller the valve opening, the slower the oil flow and the greater the damping. Adjustable shock absorbers vary the shock's damping rate by varying the size of the valve opening, either by adjusting the spring preload or by selecting a different size oil flow orifice. By using a small motor or a solenoid to operate the valve, damping rate can be adjusted "on-the-fly" by the driver and/or by the computer.

Exactly the same results can be achieved by varying the viscosity of the oil instead of the size of the valve opening. The technology that allows changing oil viscosity on-the-fly presents some exciting possibilities that go far beyond adjustable shock absorbers.

Magneto-rheological fluid
Rheology is a science that studies the deformation and flow of materials. Rheological fluids have flow characteristics that can be changed in a controllable way using electrical current or a magnetic field. Depending on the base fluid and the strength of the electrical current or magnet, the fluid's viscosity can be varied from thinner-than-water to almost-solid and any stage in between. The fluid's response is instantaneous, completely reversible and extremely controllable, but there are some limits.

Inside Story: The 'valve' in the center of the tube is a block with oil passages surrounded by an electromagnet coil. (Photo: Delphi)

Electro-rheological (ER) fluid changes viscosity when an electric current is applied directly to the fluid itself. ER fluid was first invented and patented in the 1940s, and to varying degrees, development has continued ever since. It has been tested in a wide range of applications, from torque converters, clutches and dampers to synthetic muscles and dampers in powered prosthetic arms and legs. It works, but its shear strength -- that is, its resistance to shearing movement -- is limited. Despite huge investments in research and development, ER fluid is still far from ready for any practical applications.

Magneto-rheological (MR) fluid has a shear strength about 10 times stronger than ER fluid. Invented at the same time as ER fluid, the two have many similarities. Both can use oil, silicone, water or glycol as the base fluid, and both contain polarizable particles suspended in the fluid. Polarizable means the particles can be forced to align in a specific way. These suspended polarizable particles are the basic difference between ER and MR fluids. ER fluid uses particles that polarize when directly exposed to an electric current. MR fluid uses somewhat larger particles of iron that polarize when surrounded by a magnetic field.

The typical MR fluid particles are soft iron spheres measuring 3 to 5 microns (3 to 5 thousandths of a millimeter) in diameter. Depending on the application, the fluid will be 20 to 40 percent saturated with the iron particles, and other additives will be used to control particle settling and mixing, fluid friction and fluid viscosity. Specific gravity is generally between 3 and 4; for reference, water's specific gravity is 1. Thus, a 55-gallon drum of MR fluid can weigh almost a full ton. MR fluids are developed specifically for the application. For instance, in addition to automotive uses, MR fluids have been developed for use in dampers that protect buildings and other structures from earthquake damage. These dampers sit still for long periods, so different additives are needed to keep the particles in suspension.

MR fluid can be used for two different functions -- shear control and valve control. Shear control applications control relative movement of adjacent parts, such as in torque converters, clutches and brakes. In valve control mode, it can be used in place of any kind of flow control valve, which brings us back to the most common automotive application, shock absorbers.

Shock absorber valves
It wasn't hard to develop a synthetic oil-MR fluid with viscosity and lubrication qualities similar to normal hydraulic shock absorber oil. The challenge was to develop seals, O-rings and other components that can withstand the fluid's "particle contamination," which is part of the reason it's taken so long for MR fluid to escape the laboratory. According to David Caldwell, communications manager for performance cars at GM, these shocks have been in development for 20 years. He said they were first used on open-wheeled racecars, where cost and durability are not quite as critical as in production cars. Working with Lord Corp., which manufactures the MR fluid, Delphi has finally developed MR shock absorbers that are suitable for real-world applications.

The magnetic particles in the fluid align with the magnetic field when the magnet is turned on. The fluid resists flow perpendicular to the field lines just as if an orifice plug were suddenly inserted into the passage.

With the fluid and other materials properly matched, developing the valve itself was the simplest part of the system. There are no moving parts, just passages in the piston that the fluid moves through. The oil passages are surrounded by an electro-magnet; it is basically a solenoid coil without the valve core that generates a magnetic field when current is passed through it. When the magnet is turned on, the iron particles in the oil passages align to form fibers in the oil, making the fluid thicker and, therefore, resistant to flow. The thickness or viscosity of the fluid can be infinitely adjusted from that of the base oil to almost plastic in less than two milliseconds simply by adjusting current flow through the coil. When the current is turned off, the fluid reverts to its base viscosity just as quickly. Only the fluid in the oil passage is involved, so the magnet's coil can be small enough to ride on the piston itself. The wires from the coil are routed through the piston rod to a connector on the end of the shock housing. This basic design is currently being used in long-haul truck seats, shocks, struts and air-inflated load-leveling shocks.

Control system
Because adjustable damping has been around for a while, all of the other bits and pieces needed for the MagneRide system are already in place. The control module uses suspension height data supplied by position sensors at each corner. With throttle position sensor (TPS), transmission and wheel speed data supplied by the Powertrain Control Module (PCM), the suspension controller can predict lift and dive at each end of the car and operate the shocks' "valves" to counteract it. With data from a steering wheel position sensor, a two-plane acceleration sensor and a yaw rate sensor, the shocks can be operated as needed to control body roll during any maneuver. The system also checks body movement during antilock brake system (ABS) operation using vehicle speed, wheel speed and other data supplied by the ABS control unit.

The MagneRide controller itself is a stand-alone unit equipped with two parallel processors: one for input signals and one for output. It operates the shocks on 5 volts DC that is pulse-width modulated to adjust current to the magnets. Current draw can spike momentarily at about 5 amps per shock, but normal current draw is about half that much, and there is always some current flowing whenever the key is on.

Like earlier versions, this is a semi-active suspension system. In addition to its main function of keeping the wheels in contact with the road, it can check body motions and, within certain limits, adjust weight bias at each corner by preventing suspension compression. But it is a reactive system, not proactive, and it cannot extend the suspension to make the car lean into a turn. Still, it provides a significant amount of increased control with base settings that are tuned for a more comfortable ride.

Other applications
As noted earlier, MR fluid can become almost solid in the presence of a strong enough magnetic field. Several companies and universities are experimenting with using it in brakes and fluid couplings. The most common design uses two plates facing each other in a sealed housing with a shaft attached to each plate. The housing is filled with MR fluid and a magnet coil is wrapped around the housing. The fluid thickens according to the strength of the applied field current. With one shaft held stationary, it's a brake for the other shaft. With one shaft as input and the other as output, full field current can be applied to use the assembly as a clutch. These applications use the MR fluid in shear mode as opposed to valve mode, so its shear strength is what limits the amount of torque that can be transferred.

So far, the automotive use of MR fluid is limited to shock absorbers and seat dampers for cars and trucks, but active motor mounts are on the way. Other ideas in development include off-road racer Rod Millen's work to develop a damping system to increase high-speed off-road capabilities of the Army's HMMWV. Dana Corp. is developing a center bearing mount for long driveshafts using an MR fluid damper, and also a viscous driveline coupling using an MR fluid clutch. Several companies that make production line equipment are using MR fluid rotary actuators in place of traditional air powered motors or electric stepper motors. Repeatability and accuracy are comparable, but braking is also possible, allowing movement and precise positioning of heavier loads.

Non-automotive applications already in production include the structural dampers mentioned earlier. In addition to earthquake protection, they control wind-induced motion of bridges and buildings. At least one company is using MR fluid as cutting oil for CNC polishing of optical lenses and mirrors. The magnetic field is applied right at the polishing wheel, and the process produces a micro-finish never attained before. There are even non-industrial applications. Nautilus uses MR fluid brakes on some of their home exercise machines.

All new inventions need time to mature enough to be useful, especially in the automotive industry. The scroll compressor was invented in the 1890s, but it took decades to develop the materials and production techniques needed to make it work. Likewise, rheological fluids have been under development for 60 years, and finally the oil and seal materials that can survive in what is basically a contaminated environment are now available, making MR fluid useable in production cars and trucks. With modern electronic controls and a material that can change itself almost as fast as a control unit can "think" about it, we're getting closer to intelligent machines almost every day.