A lot of vehicle models are nowadays equipped with electrically-assisted power steering systems, which have lately replaced hydraulic-assisted steering systems. Such a system consists of a torque sensor in the steering column and an electric actuator supplying adequate steering support. The existence of such components on vehicles has already opened the field for new, more advanced systems in the area.
Such systems are those that adjust the steering ratio, which is the turn of the vehicle wheels per turn of the steering wheel according to vehicle speed, which has started to enter the market. In such systems, there is no direct mechanical linkage between the steering wheel and the vehicle wheels, this is the so-called by-wire approach. Although it is technically possible to design by-wire systems as safe as mechanical systems, the by-wire systems would require significant modifications in the automotive area, this being the reason why such systems have not yet widely entered the market.
In a steer-by-wire system, the steering axle actuator can either follow driver commands, under normal driving or follow commands from the automated system. In such systems, there should be steering feedback to the driver via the steering wheel. The system should be able to override the driver’s command if required, and for safety reasons, there should be several fail-safe positions in case of system failure.
VW has presented a system that can fully undertake to steer while parking, using the electric power steering as an actuator. The BMW active steering system, shown in the above figure, involves a planetary gear set integrated into the steering column.
When the sensors detect a turning of the steering wheel, the system analyzes the data and sends the adequate command to an electric motor and linkage, which turns the front wheels appropriately. The actuators used in this system are a DC motor controlled by the control unit of the system and a planetary gear set between the steering rack and the
steering column, which is driven by the DC motor.
At low speeds, the system makes the front wheels turn more with small angles of the steering wheel, thus facilitating parking and other maneuvers. At high speeds of around 120–140 km/h, the system reduces the change in the steering angle for every turn of the steering wheel, thus the vehicle responsiveness at high speeds is reduced and vehicle stability is improved. If skidding or sliding is detected by the yaw rate sensors, the system can adjust the steering axle of the front wheels in order to stabilize the vehicle.