Audi's new magnetic semi-active suspension system
June 18, 2006 The Audi TT is a cult car, an icon –from the day it made its debut, in autumn 1998, it took the sports coupe segment by storm, sharpening the brand's profile in the process. We reported on the coming of the second generation of this successful model in April but overlooked one of the most interesting facets of the new machine which uses a completely new form of damping technology developed in conjunction with Delphi that resolves the age-old conflict between comfort and driving dynamics without countenancing any of the otherwise unavoidable compromises. As a continuously adaptive system, it adapts the damping characteristic to the profile of the road and the driver's gear-shifting habits within just a few milliseconds. The shock absorber pistons on the TT do not contain conventional oil, but a magneto-rheological fluid – a synthetic hydrocarbon oil in which microscopically small magnetic particles measuring between three and ten microns are enclosed. When a voltage is applied to a coil – by means of a pulse delivered by a control unit – a magnetic field is created in which the alignment of the particles changes. They position themselves transversely to the direction of flow of the oil, and so inhibit its flow through the piston channels. This alters the characteristic of the damping characteristic much faster than is the case in conventional adaptive dampers.
Audi magnetic ride provides the right degree of damping force at each individual wheel in every situation. The control unit, supplied by complex sensing technology, constantly analyses the situation. Starting from the "Normal" setting, the driver can also activate the "Sport" mode via a button on the centre console. These two programs establish clearly distinctive characteristics.
In the basic mode – when the oil is more viscous and the degree of damping less pronounced – the TT rolls surprisingly smoothly; this is ideal for long-distance driving or uneven road surfaces. In the Sport mode, by contrast – when the oil is less viscous – it reveals an extremely dynamic character that is manifested by a resolutely firm grip of the road surface. Rolling movements are suppressed from the very moment the steering is turned even more effectively than in the basic suspension setting, and the steering response is further improved. The self-steering behaviour is further optimised by the specific stabilisation provided for each individual wheel, resulting in a driving feel reminiscent of a go-kart.
Audi Magnetic Ride is based on a magneto-rheological principle. When in a magnetic field, small iron particles in the suspension fluid align themselves in the direction of the magnetic flux. The electromagnetic coil is integrated into the damper piston in such a way that when it is energized the magnetic flux runs exactly transversely to the admission ports in the damper piston. If the piston moves, the aligned iron particles create flow resistance in the flowing suspension fluid.
The greater the energy applied and the stronger the magnetic field, the greater the resistance and damping power. The energy is controlled in relation to driving dynamics and impulses from the road. This means for every road situation optimal damping power is available. This damping power produces – according to OEM desire – a more comfortable feel or sportier vehicle handling.
Audi uses the magneto-rheological dampers for both – comfort and sport. In its basic mode, the suspension allows for a more comfortable vehicle feel. In sports mode, the vehicle offers a sportier ride thanks to a push of a button. Because of the lower level of damping force in basic mode, the total path of the springs is fully utilized. This assures top comfort over long distances and when driving over uneven surfaces.
The greater damping of the sports mode results in tighter, sportier handling along with better control in the threshold. It also optimally suppresses the rolling tendency of the car’s body in corners.
Since no mechanical parts must be moved, the Delphi technology responds extremely spontaneously. The damping force is only dependent on the power applied to the magneto-rheological fluid and can be adjusted up to 1,000 times a second. Thanks to the variable control of the power, the damping force can also be set infinitely. This means the magneto-rheological dampers respond almost in real time with great sensitivity to impulses from the road. The control system loop with Skyhook algorithms ensures the greatest possible road-to-wheel contact in the case of uneven road surfaces with the least impulses to the car body. This means the Audi TT with Magnetic Ride has the greatest possible driving dynamics and allows for comfort.
Delphi’s magneto-rheological monotube damper requires roughly the same space requirements as conventional dampers or struts. If required, Delphi supplies the complete semi-active system with dampers and struts including the sensors and the control unit.
For Audi, the low energy requirements of the system are another benefit. On average, a damper requires just 5 watts of electrical capacity (25 watts at a maximum).