Take a brake

Car makers are taking ABS several steps further. In under a decade we should be driving vehicles with the electronics equivalent of the F1 driver’s skills.

What separates F1 drivers from the rest of us?

Put average drivers on a test track where they can explore their car’s limits in safety and most will quickly work out how hard they can brake without skidding. Similarly they’ll soon discover the limit of their vehicle’s ability to take a corner at speed. But ask most people to do both at once – perform an emergency lane change while braking hard, say – and they’re in trouble.

When you brake in a straight line you’re using the tyre’s capacity to grip longitudinally. In cornering it’s lateral capacity. The more capacity you use in one direction, the less is available in the other. Most people can sense the limits in one direction at a time, but if you’re using 50 per cent of the tyre’s capacity longitudinally, it’s hard to tell how much you’ve got left laterally. F1 drivers excel at optimally exploiting the capacity of the tyre in both directions simultaneously.

Suppose you could devise an electronic box of tricks to do this job for you. Not surprisingly, car makers are very interested in this.

Electronics systems to improve vehicle stability are now commonplace. Most people are familiar with anti-lock braking: it’s becoming almost a standard fitting, and European legislation has been proposed to make it exactly that.

Traction control takes ABS a stage further, allowing the driven wheels to be braked individually if they start to lose adhesion. Stability control goes further still, and adds sensors to detect if the car is losing grip while cornering (technically it senses if the car’s yaw rate is greater than you would expect for the amount of steering input) and applies the brakes to apply counteracting forces.

Some manufacturers have marketed cars with elements of an active suspension system. Citroën produced a Xantia with active roll control. The Land Rover Discovery uses electronic roll control to reconcile two conflicting demands: allowing large amounts of suspension travel to deal with uneven ground off-road, while limiting the amount of roll when cornering on normal roads. Jaguar’s CATS adaptive damping system modifies the ride softness according to speed and road conditions.

Mercedes-Benz’s Active Body control uses hydraulic pistons acting on the vehicle springs to change the spring characteristics according to road conditions, and is said to eliminate body movements almost completely when moving off, cornering and braking.

In general, though, these are systems to manage the car’s stability better, said Lotus Engineering’s chief control engineer Steve Kenchington. The aim, he said, should be a system that allows the car’s behaviour to be controlled rather than just managed, under all conditions up to the limit of adhesion, optimising the forces between the tyre and road – the electronic equivalent of the F1 driver.

Lotus calls this Controlled Vehicle Dynamics or CVD. ‘CVD manages the whole vehicle to follow the optimum path from one position to the next,’ said Kenchington. CVD would simultaneously control both lateral and longitudinal slip, and would incorporate active suspension to control the distribution of loads to each tyre, giving the maximum possible control over forces between the tyre and the road.

Sufficient electronic tools have been developed to allow CVD to be achieved in a variety of ways. Lotus has been experimenting with active control systems since 1981, when it first developed active suspension for F1 cars. Potentially at the disposal of the car designer, though at different stages of development, is a menu of controls affecting braking, suspension, steering and transmission systems.

‘There are two approaches to this,’ said Kenchington. ‘Car makers can adopt an incremental approach and end up with many discrete systems rather than an integrated one; or we could choose a subset of the available systems capable of doing everything that’s needed, integrate them, and do the whole thing at lower cost.’

Lotus believes that adding rear wheel steer and active suspension to today’s ABS/traction control and stability control will do the job of handling the five types of inputs that have an effect on the vehicle chassis: longitudinal (acceleration and braking), lateral (cornering), vertical ride inputs (bumps on the road), aerodynamic (wind) and transient (anything that is not steady-state such as a sudden steering movement to avoid an obstacle).

Rear wheel steering might sound like a slightly worrying concept to manydrivers, but Honda used such a system in its Integra coupé in the 1990s. ‘This was a mechanical system with no feedback, so it was limited, but it improved transient response for manoeuvres such as lane changes,’ said Kenchington. ‘Rear wheel steer is a good way of introducing stability.’

Lotus is currently re-evaluating how to apply rear wheel steer. An active system could compensate for bump steer (unintended direction changes caused by hitting bumps) and cross winds; it could correct understeer or oversteer (when the front or rear starts to lose grip when cornering), and counteract changes in the friction between tyre and road (for example if one wheel hits a patch of oil or ice while the other remains on dry tarmac).

All these stability improvements can be achieved using rear steering angles of only one to two degrees, for which electric actuators are sufficient. Higher angles of up to 10 degrees can help with low-speed manoeuvring but would need more expensive, bulkier and heavier hydraulic actuators.

Active suspension has a great deal of potential because it can have an impact on all five types of input. A hydraulic or electronic actuator at each wheel would counteract the tendency of the car’s weight to be thrown forward when braking or to one side when cornering, both of which allow the other less heavily loaded tyres to lose grip.

‘It could control pitching when braking, complementing ABS,’ said Kenchington. ‘It can help aerodynamics by allowing the ride height to be lowered at high speed.’ It can also assist with transient inputs, while additional traction control comes free, he added.

If a wheel starts to lose grip the actuator can push it down to maintain traction. This would obviate the need for an active differential, another electronic stability aid under development. A powerful factor in an active suspension set-up would be twin-channel roll control, as distinct from the Discovery’s single-channel version. Twin channels allow the front and rear of the car to be treated separately, allowing the balance between understeer and oversteer to be optimised.

Lotus envisages two levels of signal processing for CVD, with a central master processor to interpret inputs from sensors around the car and decide which system or systems to deploy – for example, rear steer would be preferable to ABS if the system detected lack of grip due to a slippery surface – while intelligent units at each wheel would then handle the response, communicating with the central processor via a synchronous messaging bus.

According to Kenchington, integrated rear steer could be on the market by around 2007, with active suspension following two years later and full CVD by 2010. ‘Research people at the major car makers have been evaluating the technology for some time,’ he said. ‘Now we’re beginning to talk to them with a view to addressing production issues.’

One problem that could hold up development of a car in which electronics is so integral is the question of how to deal with battery failure in a failsafe way – unless a battery with much longer guaranteed life than the current three years or so becomes available.

Another issue, which at first glance does not appear especially problematic, is the question of the failure of an actuator. A CVD system would be so sophisticated that to a large extent the functioning actuators could automatically compensate for a faulty one. This could leave the driver unaware of the problem or at least with no incentive to get it fixed. So it might be necessary to degrade the performance artificially in a way that was safe but also impossible to ignore.

Kenchington is convinced of the potential of CVD. ‘It will homogenise vehicle behaviour across all conditions. It will make the car more responsive and fun to drive. You will be able to numerically engineer the feel.’ Customers will benefit from better steering feel, car body control and refinement. Car makers will be better able to meet growing customer demand for cars that are fun to drive but safe.

In perhaps as little as 10 years a vehicle without CVD could be seen as unacceptable and could be impossible to market, just as vehicles without airbags or ABS are becoming unacceptable to customers today.