A revolution is taking place in the steering system: the classic hydraulic power steering is making way for an electric variant. What initially seemed like a trick to lower CO2 emissions on the dynamometer is now becoming a crucial part for the autonomous driving car. Maarrr … if the electronics fail, there is still a fixed connection between the steering wheel and the wheels. That sounds complicated, but is it?
With the conventional hydraulic power steering, a pump runs continuously. That was no problem for a long time. Only the drive of that pump requires a little energy, even when driving straight ahead, for example on the dynamometer. And in a world where every little bit makes a big difference, such a continuous energy consumer is not ideal for CO2 emissions. So there was a diligent look for an alternative that at least does not require energy on the dynamometer. The solution: electric power steering, which only requires energy when steering (and steering on a dynamometer is out of the question). The fact that in practice there is always (even if it is sometimes minimal) steering does not count for a while, for the dynamometer an energy consumer has been sidelined.
The complete hydraulic system gives way to a sensor, a control unit and an electric motor with a gear transmission. The sensor provides the electronics with information about the steering wheel rotation plus the speed and force (or actually the torque or torque) with which it is steering. This results in an action of the electric motor. That electric motor is initially coupled to the steering column by a gear transmission. The actual steering still takes place via a rack and pinion steering gear.
Those first electrical systems do not excel in sophistication, it seems nothing more than a simple trick to reduce fuel consumption and thus CO2 emissions, and nothing more. There is only limited feed back about what goes on between the tire and the road surface. And that doesn’t seem to matter to the pioneers at all. We see the first systems mainly on compact cars that are even equipped with a button to increase the power during the parking process to increase the fun. Refined steering behavior apparently has less priority.
In these larger and especially heavier cars, the forces in the steering are greater. When even larger cars, say the C and D segments, start electric steering, we see that the electric motor gets a different place: no longer somewhere halfway between the steering wheel and the wheelhouse, but with its own gear wheel directly on the rack.
This allows larger forces to be transferred with stronger electric motors. Furthermore, the calibration is now taken a lot more serious, just like the wheel geometry. This results in more feeling in the steering.
Serious sports cars and even larger (and more luxurious) cars are also getting electric power steering. To cope with the even greater forces that play a role in these cars, the electric motor no longer works via the rack, but with a recirculating ball system. In addition to the information from the steering gear, we also use variables such as vehicle speed, yaw angle and the data from the stability system. It all plays a role in determining the degree of assistance, with the result that in cars such as the Porsche 911, nothing more to be noted on the electric power steering.
The steering system is increasingly part of the large network that is a car. With information from radar, cameras and navigation data, the electric power steering is also capable of independently determining the course. Handy for lane assistance and a prelude to the autonomous driving car.
Of course, safety should not be overlooked. In the unlikely event that the electricity fails or the computer becomes confused, it must still be possible to control. And that is possible, for those unexpected situations, there still appears to be a mechanical connection between the steering wheel and the wheels, alone or without power.