Brake-by-wire

Bosch's first generation brake-by-wire system is used on certain Mercedes vehicles. In this electro-hydraulic system, the brakes are applied by hydraulic pressure; in normal operation the driver demand for braking is interpreted by the control system and translated into requests for braking force for each wheel. This system provides for integration of ABS, traction control and stability control. It also provides the potential for novel functions such as keeping brake discs dry in wet weather by imperceptible brake applications and 'one pedal' control in queues, where the vehicle accelerates and brakes according to driver demand on the accelerator pedal. In the event of a fault the system reverts to a limited direct hydraulic system making it a full-authority system but with a fail-safe mode of operation. Concepts for full-authority brake operation based on electric actuators are already being proposed, eliminating hydraulic systems. Such a system would need to be fault-tolerant, so that the braking functions would remain available in the event of one or more fault.

Steer-by-wire

In a full-authority steer-by-wire system, there would be no direct mechanical link between the steering angle input by the driver and the direction in which the road wheels turn. Such a system would offer a number of advantages in terms of packaging, noise performance and occupant crash protection by eliminating the steering column. It would also open the possibility for an alternative to the traditional steering wheel. First steps towards steer-by-wire have been taken, with manufacturers such as BMW marketing 'active steering' systems. Nevertheless, these systems still retain the traditional steering column mechanical linkage.

Future systems

There is considerable interest in introducing further advanced driving support systems such as lane-keeping assistance, collision avoidance and, ultimately, fully autonomous convoys or 'road trains'. For optimal implementation, these will require vehicles to be equipped with full by-wire control of powertrain, braking, suspension and steering.

Implementing drive-by-wire

The sections above have outlined possible benefits and uses for systems controlled largely by computers, generally removing the constraints imposed by mechanical or hydraulic systems. However, this introduces new issues for the system developer:

• In a by-wire implementation, the relationships between inputs and outputs are not derivable from the physical implementation. This means that a change to any input can change one or more outputs. Furthermore, a very small change in an input may well result in large, multiple output changes - there is no linearity.
• The number of combinations of inputs is too large to be able to prove exhaustively the correctness of the system by testing alone.

In order to implement automotive drive-by-wire systems, a number of specific additional activities are required in the product lifecycle compared to traditional engineering processes.