A smooth ride under virtually all conditions

Conventional vehicle suspension systems typically consist of shock absorbers connected in parallel with springs between the frame and axle, minimizing the up-and-down motion of the vehicle. Since the force output typically depends on the relative movement of the frame and axle components, a shortcoming of these so-called dashpot assemblies is that they can on occasion amplify the motion, rather than attenuate it.

Active hydraulic actuators, which produce forces that are completely independent of the relative motion of components, are one way to solve the problem. One such example is the Continuously Variable Road Suspension System (CVRSS) that Delphi Automotive designed for General Motors. However, its weight and bulk present a packaging challenge for engineers.

Seeking to streamline the design, Delphi engineers have developed a magneto-rheological (MR) fluid-based actuator. In the absence of a magnetic field, MR fluid from Lord Corp. (Cary, NC) exhibits nearly Newtonian behavior. But when magnetized by an applied magnetic field, particles suspended in the fluid align with magnetic flux lines and the fluid exhibits the characteristics of a Bingham Plastic. There is direct mapping of the fluid rheology into the force vs. velocity characteristics of the damper, producing the correct degree of motion attenuation under virtually all conditions.

The new MR fluid-based actuators are part of a new semi-active system called MagneRide, which is a standalone vehicle suspension control system also consisting of four wheel-to-body displacement sensors and a computer that provides real-time control of suspension damping. It will be introduced on 2002 production Cadillacs.

The design is inherently simpler: The actuator consists of a single tube that is gas-pressurized, with no valves or associated wiring. The MR fluid is separated from the nitrogen gas by a floating piston. Control takes place in the damper's main piston. "The MagneRide has 60% fewer parts," says Dave Hoptry, a Delphi design engineer.

A primary advantage of the design is that the MR effect is fast-engineers estimate that the response time is less than 1 msec. The quicker response time (three times faster than conventional hydraulic actuators) is particularly beneficial under sudden impact conditions. In tests, Delphi engineers say that the system has performed favorably, compared to both passive and active hydropneumatic suspensions.

The new damping system is just the starting point for Delphi. Engineers are also investigating the applicability of MR fluids to braking and steering systems, possibly tying together multiple systems in cars.