Audi recently unveiled its all-new A8 flagship which now features a fully active, electromechanical suspension system. A front camera detects road irregularities and signals a predictive adjustment of the active suspension to suit the road conditions. Each wheel is fitted with an electric motor powered by the 48-V main electrical system. Additional components include gears, a rotary tube together with internal titanium torsion bar and a lever which exerts up to 1100 N·m (811.3 lb·ft) on the suspension via a coupling rod. In an SAE exclusive, U.K.-based contributor Ian Adcock interviewed Thomas Muller, Audi's head of suspension development on the A8 active suspension.
Q: What are the benefits that customers will experience with this new suspension technology?
With the introduction of 48V it has enabled us to introduce new technologies like the e-Booster and, now, electro-mechanical active suspension with no hydraulics integrated into the system that complements the air-suspension which gives the freedom to raise and lower each wheel independently.
It’s about improving comfort and dynamics and safety. We centralize the chassis ‘brain’ into one module; the signal computing and calculations for the air suspension, damper control, quattro sport rear differential and Audi active integrated steering system are all fed into one control module which is essential if you want to have a smooth interaction with the car.
The Audi active steering is very innovative, integrating the steering rack, the dynamic steering column and the rear axle steering into one system. Together with the active suspension we believe this will deliver a lot of driving pleasure.
Q: The dynamic all-wheel steering system is a new development as well?
Yes. It reduces the A8’s turning circle to less than that of an A4, 38 ft (11.6 m) compared to 37.4 ft (11.4 m), making it easy to maneuver in tight urban confines. We achieve that by turning the rear steering by 5° which is twice that of any rival. Combine that with the active suspension and you have a car that is both very comfortable and maneuverable during lane changing, for example, reducing pitch and roll.
The all-wheel steering shifts the point of rotation further back reducing the rear seat passengers lateral head movement, resulting in a more composed ride when combined with the active suspension.
Q: How is the active suspension system controlled?
We use a Mobileye mono camera from the driver assistance system that generates the vertical information from the road with a resolution of 1.2 to 6 in (3 to 16 cm), signaling to the chassis controller to raise or lower the suspension accordingly when countering upcoming imperfections such as sleeping policemen or pot holes.
Once you get into higher G-forces under cornering, body roll is halved; otherwise it feels very artificial. It’s the combination of the active dynamic systems that result in a level of agility and balance you wouldn’t expect from a luxury sedan like the A8. It is also linked into pre-sense side sensors for side impacts instantly raising the body height, so the sill takes the collision and not the door, reducing the impact by up to 50%.
Q: How fast is the system’s reaction time and what sort of energy and forces are involved?
The forces that we have at wheel level are 3.7 kN (832 lbf) front and 3.3 kN (742 lbf) rear. But the really important figures is that we have a gradient at each wheel of 16 kN (3597 lbf) per second. This is a huge force we can apply at each wheel and, combined with the bandwidth of 0 to 6 Hz, means the damper is being altered every 15 ms.
It’s also an efficient system. Its default setting is on the air suspension system when no energy is consumed; force is only generated when active movement is required. Studies show that in a city environment it consumes 40-60 W, on highways it is even less, 10-20 W, rising to 250 W on rough roads. And even lapping the Nürburgring it’s only 400 W. It’s difficult to quantify but that’s two-to-three times more efficient than hydraulic systems with a higher gradient and bandwidth as well.
Q: How is it set up?
There are 2-kW electric motors at each corner, connected by belt to an Ovalo Strain Wave Gear, that delivers a very high transmission ratio of 1:80 transmitted through a torsion shaft and bar to a link into the steering knuckle. It’s different at the rear because it couldn’t go through the Cardan shaft connected to the differential. Additionally there are 48-V power ECUs front and back networked to the central chassis controller.
Information is generated every five milliseconds to create a picture of what is happening and what needs to be done in controlling the chassis dynamic actuators. Data from the road surface, restricted to 16–65 ft (5-20 m) ahead of the car, is read by the forward-facing ADAS camera and analyzed by the chassis controller ECU to preset the suspension ahead of encountering damaged road surfaces, although it works very well without the camera.
Q: What are the prospects of this technology appearing in other Audi products?
Currently we’re only talking about the new A8. But I think that active suspension will have a bright future, especially the combination of manual and what you might call ‘piloted’ driving. Providing a car that eliminates lateral and longitudinal forces so you can work or relax, like a high-speed train, is a huge benefit.
Driving pleasure is something our customers want: comfort or sporty, it’s not either-or. Allowing you to decouple yourself from the road is a big differentiator for us.
Q: How many driver settings are there?
We have three settings with Audi Drive Select, comfort, auto and dynamic that includes the active suspension, all-wheel dynamic settings and quattro sport.
Q: Who are your technology partners?
We don’t have one technology supplier; pre-development and concept phase was done internally and we keep the IP for that. Then we broke it down and generated a network of suppliers working with us. Ovalo is a main supplier for the corner module hardware and Continental for electronic hardware.