The 991 electrically-assisted steering is an engineering marvel. It uses a closed-loop controlled electric motor attached to the rack via a belt actuating a ring with recirculating ball bearings.

 

1 Housing with steering rack
2 Steering spindle connection
3 Torque sensor
4 Steering pinion
5 Sensor cable
6 Electronic control unit
7 Electric motor
8 Power steering gear

 

1 Small toothed disc/electric motor
2 Toothed belt
3 Steering rack/ball thread
4 Large toothed disc/ball recirculating nut
5 Ball recirculating nut
6 Balls/ball chain
7 Return channel

The feedback loop which incorporates inputs from a myriad of sensors, actually drives the steering on its own. For example it senses different left and right grip surfaces and turns the steering to compensate (independently of the driver input).

Amazing steering evolution. Next step: the car drives itself. :)


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