Electromagnetics offer smoother ride

An innovative suspension control system is offering automotive designers much greater freedom. Graham Pitcher reports.

Automotive design specialist BWI has introduced the third generation of its MagneRide suspension control system. The system uses dampers whose response can be changed by controlling the properties of the damper fluid electromagnetically.

The fluid is a suspension of magnetically soft particles in a synthetic hydrocarbon base fluid. Mike Zimmerman, manager of controlled suspension systems development for BWI, explained: "The system is based on a magneto rheological fluid in which iron particles are suspended. When a magnetic field is applied, the fluid reacts and the particles line up to resist flow through a small gap in the actuator. Fine control can be applied because the level of resistance varies with the flux applied."

When the coil through which the damper fluid flows is not energised, the particles within the fluid are distributed randomly and the fluid behaves conventionally. When the coil is energised, the particles align and form fibrous structures and this resists flow. By changing the current applied to the magnetising coil, a wide range of damping forces can be applied.

The fluid retains the same viscosity, no matter whether the electromagnetic force is applied or not. When the force is applied and the particles line up, the shear force of the fluid changes dramatically and the alignment needs to be sheared if the fluid is to move.

In automotive applications, MagneRide is available in two sizes; working with dampers of 36 and 46mm diameter. The pistons are 30mm long and the gaps through which the fluid flows can be as small as 0.5mm.
Sensors at the wheels determine wheel position and an electronic control unit (ECU) calculates relative wheel velocities and whether the car body is moving up or down, rolling or pitching.

The ECU then issues commands to each coil to maintain acceptable handling and comfort. Signals are issued at a rate of 1kHz, but the dampers can respond to a 6kHz update rate. According to BWI, it has concentrated on getting current to the damper quickly in the latest version of the system.

A further enhancement has been to allow the fluid to be demagnetised more quickly in order to speed system response. By applying a dual coil approach, the system's range has been broadened to include a 'softer soft' and a 'harder hard'. This allows the suspension to become more or less flexible more quickly.

BWI is now looking to see how vehicle dynamics can be improved by sharing data with other onboard systems in a global chassis control system. This would see a supervisory computer managing an array of subsystem controllers. Potential candidates for this system would include antilock braking, electronic stability control, engine torque, roll control, dampers and electronic steering.

BWI gives the example of a severe emergency turn while braking. In such an instance, the MagneRide system would be able to optimise the vehicle's dynamic behaviour rapidly, helping the driver to maintain control.

Olivier Raynauld, manager of forward engineering controlled suspensions, commented: "The main barrier to this level of integration is the need for a standard high speed communications protocol implemented by the manufacturers of each system. We are participating in various consortia, including FlexRay, to develop the infrastructure that will support safe and reliable collaboration between the many chassis subsystems in today's vehicles."

BWI believes the MagneRide system may have application beyond automotive suspension systems. "We are looking at other markets," said Zimmerman, "but we haven't anything to talk about as yet. But we think the technology is applicable to controlling any vibrating system." However, the company has identified wind turbines as a potential application for the technology.

Graham Pitcher

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