Servotronic

Systems in the CIM Module - Servotronic




The Servotronic system consist of the following components:

- CIM module
- Servotronic solenoid valve (1)

Principle of Operation

Servotronic provides speed dependent control of the power assisted steering. The flow of hydraulic oil is reduced (variable) by an electro-hydraulic pressure converter (solenoid valve) located on the steering rack (1). The degree of oil flow reduction is varied by the current supplied to the solenoid valve from the CIM control circuit.

In connection with EDC-K, the driver can influence the power assisted steering by choosing between two characteristic curves, comfort or sport. The comfort or sport request dictates the current supplied to the solenoid valve dependent on the vehicle speed.

Start Preconditions: The ignition "ON" signal is the only requirement and the solenoid valve is activated only when the engine is running. It a speed signal or steering angle input (movement) is not present within 5 seconds after starting the engine, the Servotronic switches over to the fast drive characteristic (power assist reduced).

This is the substitute value (without power) when there is a fault present. When a speed signal is received or steering angle input, the solenoid valve is activated by current from the CIM module control circuit to increase the power steering assist (speed variable).

Servotronic Initialization: A short initialization phase (approximately 1 second) is necessary to achieve the characteristic curve as fast as possible during the starting procedure. During this phase, vehicle standstill is detected. A current flow plausibility test is conducted and concluded within the initialization phase (test for short to B+ or ground).

Speed Acquisition: The speed signal is generated by the DSC module and is transferred over the PT-CAN and K-CAN Busses to the CIM module to calculate the acceleration. It the acceleration values are greater than 1.3 g, the speed is interpreted as being implausible and the CIM formulates a "failsafe" speed.

Determining Setpoint: Depending on the measured vehicle speed value, the Servotronic is adjusted every 100 ms in the comfort or sport characteristic curve. This provides a smooth transition in power assisted steering regulation.

Characteristic Curve Changeover: The transition between the comfort and sport characteristic curves is gradual in order to avoid jolts in the power steering during changeover. The time required for the changeover is dependent on the cyclic CAN message "comfort or sport" sent by the EDC-K. The comfort characteristic curve is used if the vehicle is not equipped with EDC-K.

Actual Value Acquisition: The voltage drop generated by the solenoid valve current during operation is monitored by the CIM for plausibility, comparing the actual value in relation to the Servotronic required.

Solenoid Valve Control: The solenoid valve is activated by a pulse width modulated signal (PWM) that has a period duration of 2.5 ms at 400 Hz. The pulse duty factor can be set in 2000 steps from 0 - 99.95%, at a time of 1.25 ms per step change.

Operation at the Controller: The Controller and Control Display form the interfaces to the driver. If the vehicle is equipped with EDC-K, the driver can set the chassis to sports tuning. The Servotronic will also switch to the sport steering characteristic curve.

The comfort characteristic provides greater power assist to the steering over the entire vehicle speed range.




The sport characteristic provides less assist over the speed range, giving the driver more "road feel" and feedback required for this driving style.