How It Works
A zero-play planetary gearbox with override function is the core element of the Active Steering system. The gearbox is of the planetary type with two input shafts and one output shaft.
The first input shaft is the lower steering spindle of the steering column. The second input shaft is a worm gear on the planetary gearbox. The electric servomotor (actuated by the AL control unit) drives the worm gear of the planetary gearbox. The ratio for transmission of movement of the worm to the worm gearwheel is 20.5:1. The worm gear is self-inhibiting in the event of a malfunction. The worm gear superimposes additional movement on the steering lock that results from direct input from the steering spindle. The cumulative steering angle at the steering pinion comprises:
- Steering angle, applied at the steering wheel
- Steering angle applied by the electric servomotor
The amount of override over and above the steering lock applied by the driver depends on dynamic variables, the most important of which is the vehicle's speed on the road. At low speeds (such as when the vehicle is being manoeuvred into a parking slot, for example), the Active Steering applies considerable override over the steering angle imposed directly by the angle of rotation of the steering spindle. Steering becomes very direct.
At high speeds the electric servomotor works in the opposite direction to the driver's steering angle. The overall steering linkage becomes more indirect.
The electric servomotor on the planetary gearbox with override function receives its supply from three phases (= wires) (U, V, W). In the event of a short circuit, the electric servomotor can turn a maximum of 120°. This prevents the electric servomotor making unintentional starting movements in the event of a short circuit. There is a temperature sensor in the electric servomotor. This temperature sensor monitors heating in the electric servomotor. The electric servomotor lock blocks the electric servomotor:
- If A Fault Develops
- When The Vehicle Is At A Standstill
The electric servomotor lock inhibits movement by engaging the teeth of the worm gear. The electromagnetic lock is spring-loaded, with the force of the spring countered by the power supply. Any interruption of the power supply consequently causes the lock to engage. The driver can still control the vehicle by means of the steering wheel, even if the planetary gearbox with override function is locked in this way. Under these circumstances, the steering performs in much the same way as conventional steering systems. The purely mechanical transmission link between steering wheel and front wheels is always sustained.
The electric servomotor lock releases when the supply voltage is approximately 3.2 volts.