Planetary gearbox with override function with servomotor and servomotor lock

The overriding gear of the active steering is located on the steering box. The overriding gear is installed in the split steering shaft. The overriding gear is a planetary gear train with 2 inputs and one output. The first input is formed by the lower steering shaft of the steering column. The second input shaft is a worm gear on the planetary gear train. The servomotor (actuated by the AL control unit) drives the worm gear of the planetary gear train. The gear ratio from the worm to the worm gear is 20.5:1. For possible failures, the worm gear pair is designed as self-inhibiting. The worm gear pair overrides the steering angle of the direct drive-through from the steering shaft. The total steering angle at the steering pinion comprises:

• Steering angle, applied at the steering wheel
• Steering angle applied by the electric servomotor
  Fig 1: Identifying Planetary Gearbox With Override Function Servomotor

  Courtesy of BMW OF NORTH AMERICA, INC.

The electric servomotor on the planetary gearbox with override function receives its voltage 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 inadvertent start-up movements of the servomotor in the event of a short circuit. A temperature sensor in the control unit for active steering (AL) monitors the output stage for the servomotor.

The electric servomotor lock blocks the electric servomotor:

• in the event of faults to prevent it from rotating due to external forces
• when the vehicle is at a standstill and the is engine off (not when the vehicle is at a standstill and the engine is running) If the engine is switched off by the automatic engine start-stop function, the electric servomotor lock remains applied.

In the event of blocking, the electric servomotor lock engages in the corresponding gearing on the worm. The lock is spring-loaded, with the force of the spring countered by the preload. Any interruption of the voltage 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 transfer between the steering wheel and front wheels is always maintained.