Electronic Power Steering
The optional equipment "active steering" is not offered for the E72. The E72 is only available with electromechanical power steering. This is also referred to as "Electric Power Steering EPS".
Since power steering must also be available while the vehicle is driven in pure electric mode it was decided in development that the E72 would use electromechanical power steering instead of hydraulic steering.
The electric motor which generates the steering servo is arranged axially parallel to the rack and pinion power steering gear. The design principle and the functions of the electromechanical power steering in the E72 are largely identical to the steering implemented in the E89.
The most important changes are:
- Design adaptation to the axle geometry of the E72
- Increase in power of the electric motor based on greater track-rod forces
- Integration into the on-board communication system and the onboard energy system of the E72.
Although a power increase was necessary for the electromechanical power steering for the E72, it is not a high voltage component, but rather a 14 V component. This is the primary purpose for the second 12 Volt battery.
The general characteristics of electromechanical power steering systems are as follows:
- Damping disturbances which originate from the road surface (increased ride comfort)
- Active steering wheel return (increased ride comfort)
- Speed-dependent steering servo "Servotronic" (increased ride comfort and improved driving safety)
- No hydraulic circuit (easier vehicle integration and improved environmental compatibility)
- Requirements-based activation of the electric motor (contribution to lower fuel consumption)
- Steering servo independent of the speed of the combustion engine.
The EPS steering system consists of the following main components:
- Steering-torque sensor
- EPS control unit
- Electric motor with motor position sensor
- Speed reducer
- Rack-and-pinion steering box
These components form a unit (the "EPS rack-and-pinion steering box") which can only be replaced as a complete assembly during service.
After a new EPS rack-and-pinion steering box is installed, a wheel alignment with toe adjustment must be carried out. In the course of the start-up, a service function must be run to teach in the end stops.
Please compare a Hydraulic Power Steering (HPS) with an Electric Power Steering (EPS) system.
| Hydraulic power steering | Feature | Electric power steering |
|---|---|---|
| Driven by | ||
| Energy source | ||
| Power consumption (peak) | ||
| Power consumption (combustion engine at idle speed, but no steering) | ||
| Power consumption (combustion engine at max. speed, but no steering) | ||
Which parts of the EPS can you replace separately?