System functions: Notes

The Servotronic determines most of the steering torque that driver has to apply. Servotronic is largely defined by the driving speed and steering angle:

• Low driving speed range (urban driving, parking) = low steering torque
• High driving speed range (freeway/highway) = high steering torque

The Servotronic valve is only activated at "terminal 15 On" and with the engine running. The Servotronic control loop controls the current for the connected Servotronic valve. A higher current results in greater steering assistance with the same steering torque. The nominal current is determined from a characteristic map with driving speed and steering angle as the parameters. The characteristic map is stored in the coding data and has 10 data points for the driving speed and 6 data points for the steering angle. Intermediate values of the nominal current are determined by linear interpolation of adjacent data points. A corresponding function limits the value from the characteristic curve to 0 and 860 milliampere.

The Servotronic determines the required steering servo as a function of following factors every 100 milliseconds:

• Current driving speed
• Current steering angle value

The corresponding value in the characteristic map is activated depending on the situation. The specified maximum or minimum values are used if the current value for driving speed or steering angle is outside the defined range.

It is possible to switch over between two characteristic maps depending on the chassis and suspension set-up. The Servotronic evaluates the CAN message in order to switch over between the normal characteristic map and the sport characteristic map. The signal is compared to the coding value sport mode. If the values are equal the Servotronic switches over to the sport characteristic map. If the message fails or in the event of an invalid value, the characteristic map last selected remains active. After a reset Servotronic uses the comfort status and therefore the normal characteristic map. However, the status is not stored in the Servotronic control unit (SVT).

As an example, the following graphic shows the characteristic map for driving speed with steering angle 0 degrees on the E70 with Dynamic Drive and Electronic Damper Control.

Index	Explanation
1	Activation
2	Comfort characteristic curve
3	Sport characteristic curve
4	Driving speed

The following Servotronic system functions are described:

• Steering servo at low driving speeds
• Steering servo at high driving speeds
• Steering servo with Servotronic valve de-energized