Drive-by-wire concept car steers hi-tech course

Filippo Zingariello, SKF UK Limited

Sophisticated by-wire systems are coming out of the laboratory and onto the road, promising to revolutionise the driving experience.

The Bertone-SKF Filo, probably the most technically advanced independent concept car yet created, epitomised this year's Geneva Motor Show. The car incorporated a suite of state-of-the-art by-wire systems and exploited the interior design and man-machine interface flexibility that such systems offer.

'Our [Bertone] prototypes used to be very different from the cars produced for the market. Just think of the Stratos from the 1970s,' says Mr Piatti, managing director of Stile Bertone. 'Compared to cars on the road at that time, it was an object of science fiction. Today, this gap doesn't exit. In this case we decided to utilise an innovative technology as an opportunity for design. This was the challenge for the Filo.'

It is quite feasible to integrate by-wire technology into a conventional car interior, with all the controls arranged as they have been for decades, but this would seem to miss so many opportunities. Drive-by-wire systems give designers new freedom to explore radical reassessments of what have been seen until now as rigid, immovable boundaries. And it is in the Filo concept car's driver's control - the Guida - that the most immediate and striking aspects of the possibilities of drive-by-wire are seen. The Guida removes the need for pedals and any vestige of a conventional steering column. Mechatronic controls - the parts that we humans use as the interface with the vehicle - have a very subtle and important role to play. Far from being mere switches, they incorporate 'feel', giving the driver tactile feedback which reflects the vehicle's behaviour as it interacts with the road.

Electromechanical solution
SKF has concentrated on electromechanical actuation, the so-called 'dry' systems, which involve just servomotors and geared actuators. This approach draws heavily on the company's existing expertise, and is the purest in engineering terms, eliminating the environmentally unfriendly hydraulic fluids of conventional systems. As the specific automotive developments gathered speed, SKF was in the enviable position of having substantial expertise on which to draw. Ball and roller screws, linear actuators and positioning slides have long been products in the company's range. Sensor integration for both industrial and automotive applications has been increasing in use and sophistication year on year in many of SKF product families. SARMA, the group's aerospace arm has been providing fly-by-wire cockpit controls and actuators to the civil aviation market for over 15 years. Its expertise, particularly in the ergonomic aspects of mechatronic controls, was used to good effect when it came to the design and development of the Filo's driver control, the Guida.

Drive-by-wire is a much-heralded vision for the future of the automotive industry. Building on the foundations set in aerospace and even high-end industrial automation systems, the use of by-wire control in cars and trucks is the next step. Extending computer control from the engine to all major systems in a vehicle offers interesting possibilities for the vehicle manufacturer and user alike.

For any major change in the automotive industry the timing must be right. By-wire's recent move from the research lab for road-capable prototyping was no exception.

New systems
The new systems required a power-supply and control environment. A car's electrical power consumption has been increasing steadily at about 4 per cent per year since the early 1980s. High-output 14V alternators have a practical limit of about 2kW, for all but the smallest cars.. The answer is the 42V supply system. Both alternator and battery manufacturers are preparing suitable equipment. Here too, the old order is being challenged with integrated starter alternator (ISA) packages being announced. The 42V system was not initiated for by-wire systems, but it is certainly a must for higher-power applications such as braking and steering.

Another key element is a fault-tolerant network architecture. Networking protocols are already employed in certain cars. Today these look after non-safety critical devices such as electric windows, although on a few high-end cars they are already very sophisticated. These protocols are event driven. Such protocols are generally considered to lack the necessary level of reliability for true safety-related systems such as steering or braking. The emergence of suitable, robust, fault-tolerant, time-triggered architectures and, perhaps more importantly, the emergence of loose consortia supporting specific flavours of fault-tolerant network protocols have been a prerequisite for the step into mainstream systems development. The actuators and the controls interact in a sophisticated network of logic. The network with its fault tolerant protocol acts like a very reliable postal service, ensuring that control messages are collected and delivered to the correct addresses. Individual power stages deliver precisely controlled current to the smart electromechanical actuating unit's prime mover: an electric motor.

The steering and braking is on the 42V emerging standard. The steering smart actuation system, packaged to fit within the existing confines of the front sub-frame and bulkhead of the donor platform, features full redundancy. Even the power rails used to feed the paired actuators are separate. The driver's steering control and the steering actuation systems are linked via an active feedback arrangement. With maximum movement being only an arc of 40? the steering sensitivity is very different to a conventional, mechanical system. 'We have noticed that the more experienced a driver is, the more difficulty they have adapting to this new arrangement,' comments Gianluca Oberto, project manager of the SKF drive-by-wire unit. 'But, once you have caught on to the improvement, it really is difficult to go back to old habits.'

The braking system was developed in close collaboration with Brembo. Both front and rear wheels rely on a development based around a proven single-piston calliper design from Brembo. The design evolved through several stages, reducing in size and weight with each. The system already out-performs the conventional hydraulic arrangement it replaced in terms of response, clamping force and overall weight.

For the clutch and gearshift by-wire the conventional 14V system was used. The throttle-by-wire arrangement also runs on 14V. The clutch and gearshift actuators are more of an exercise in precision control than they are in pure power. The adoption of 14V power was an exercise to evaluate the immediate possibilities of implementing full electromechanical by-wire control in existing vehicles. Surprisingly perhaps, clutch control provided some of the biggest challenges. Mimicking the subtle interaction between throttle and clutch pedal that drivers of manual gearbox cars carry out instinctively is a complex business.

Safety and integrity
Total assurance of safety and integrity is indispensable for any by-wire system, whether on the road or in the air. In line with established civil aerospace practice for by-wire control systems, the mechatronic systems were subjected to a rigorous failure mode, effects and criticality analysis (FMECA) review. FMECA is a proven route to establishing appropriate levels of redundancy and back-up. The architecture of the Filo concept by-wire systems fulfil this using a rational approach, in a technology application area where there is little previous experience on which to build. In such a pioneering position, where no conventional standards exist, choices have to be built on the knowledge of conventional systems' behaviour.

The Filo incorporates some of the vehicle stability and driver-aid aspects within the various systems' control software. The steering features a speed-mapping algorithm. By relating the movement of the driver's steering control and the actuator's control of the turn of the front wheels to the vehicle's speed, vehicle movement remains within safe operating parameters. Similarly, up and down shifts and reverse gear engagement are protected from inappropriate selection.

Integrating the more sophisticated vehicle systems within an over-mantling computer control arrangement offers significant possibilities to enhance active safety systems. Even existing stability systems such as ABS, TCS and skid-control arrangements can be transformed. With electromechanical operation, these systems, which today rely on complex hydraulic valve arrangements, will be taken care of by software.

By-wire will open up the possibility of more advanced driver aids, such as active collision avoidance. In emergency situations, on-board computers employing position telemetry and proximity sensors will take charge of steering and brakes, bringing the vehicle safely to a halt. These developments may be a decade away, but the first steps will be with us much sooner.

Exciting future
This is an exciting time for all involved in the automotive world. Announcements on by-wire are coming thick and fast in all areas: in trucks and off-highway vehicles, not just cars. Whatever your involvement with the industry, over the next decade we will all witness dramatic changes. By the end of this decade, the automotive industry will have gone through a revolution. SKF is on the leading edge of this wave of change - a great vantage point.

Author

Filippo Zingariello is director of the drive-by-wire business unit at SKF Automotive Division. Mr Zingariello has a degree in engineering from the University of Turin, Italy.
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