Principle Of Operation

The steering column adjustment is electrically controlled by a button located on the left side of the steering column.

The electric steering column is adjusted by two motors in the up/down and in/out directions. The various positions are stored in the driver's seat module when one of the seat memory buttons are pressed.

The steering column is automatically moved to the uppermost forward position to provide easy entry and exit from the driver's seat.

There are no Check Control messages for the steering column adjustment because the driver receives immediate feedback in the event of a fault.

The CIM module will deactivate the steering column adjustment motors when overvoltage is detected (>16 V). The motors will be activated again when the voltage drops below 16 V for more than 2 seconds.

The motors are also protected from overheating by the CIM module. A temperature calculation is performed in the CIM based on activation time (in case of jamming) and the motors will be deactivated to allow a "cool down" time.

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. If 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. If the acceleration values are greater than 1.3 g, the speed is interpreted as being implausible and the CIM formulates a "fail-safe" 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. he 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.

Servotronic Safety Requirements 

The CIM will output the Check Control message "Servotronic failed" to make the driver aware of a fault. In addition, a fault code will be stored in the CIM module. If the fault is still present upon the next engine start, the Check Control message will re-appear.

Overvoltage:  The CIM module will not apply power to the solenoid valve if a voltage value greater than 17 V is detected (fail-safe).

Undervoltage:  A momentary dip in voltage will self correct, the CIM functions will resume again after a delay time. The undervoltage fault code will not set during engine starting (crank signal).

• < 7.5 V CIM module operates normally
• V...7.5 V Reset after 200 ms; CIM will assume sleep mode, woken by CAN Bus
• < 6.5 V Reset after 20 ms; CIM will assume sleep mode, woken by CAN Bus

Speed Signal Monitoring:  The CIM module evaluates the vehicle speed signal for three different reasons and will produce different fault codes when faults are detected.

Timeout Vehicle Speed:  The vehicle speed (V_DSC) is read cyclically (every 100 ms) by the CIM module via the K-CAN Bus. If an update is not received within 500 ms, the solenoid valve is deactivated. The sport characteristic (power assist reduced) will be in operation.

A CAN timeout fault code will be stored in the CIM after 10 failed messages. A Check Control message will be sent if an update is not received in 5 seconds. The timeout fault code is reset with the next engine start. The fault code counter will be cleared by a correctly received message within the 5 second monitoring period. When a general CAN fault is detected, the solenoid valve is deactivated and the speed monitoring is ignored.

Fault Value of Speed Message:  The CIM is informed of speed faults detected by the DSC by a fault value. A Check Control message is sent and the solenoid valve is deactivated when the fault value is received ten times in succession.

Plausibility Check by Calculating the Acceleration:  The acceleration is calculated based on the vehicle speed. The Servotronic is regulated to the speed up to an acceleration of 1.3 g. If the acceleration exceeds the value of 1.3 g:

• The speed is calculated internally during braking by deducting 4 km/h per 100 ms from the last speed and storing this value as the current speed.
• The speed is calculated internally during acceleration by adding 6 km/h per 100 ms to the last speed and storing this value as the current speed. As soon as an acceleration value less than 1.3 g is observed between the stored speed and the speed supplied on the Bus, the Servotronic regulation is resumed.

Solenoid Valve Circuit Monitoring:  The circuit is monitored for shorts (B+ and ground) and breaks. If a fault is detected in the control circuit for the solenoid valve, the circuit and solenoid valve will be deactivated. Only minimum power assisted steering will be provided. This check is conducted within the initialization phase of 1 second. This function will repeat each time the ignition is cycled. The Servotronic will not operate during the engine start procedure if a fault is present for longer than 1 second.

Current Plausibility Check:  The solenoid valve current is monitored by the CIM module and must be within a tolerance range. This takes place within the initialization phase and the output stage for the solenoid valve is deactivated if a fault is detected.

Damper Program Signal Status Monitoring:  The EDC-K status is observed by the CIM module cyclically (every 200 ms) over the K-CAN Bus. The EDC-K provides the comfort or sport request. If a message is not received within 2 seconds, the comfort characteristic will be set and the fault code "timeout EDC-K status" will be stored. A Check Control message will not be sent.

Diagnosis Information 

External Faults:  All inputs, actuators and mechanical functions are monitored. Open or short circuits in the wires and actuators to the CIM module are monitored.

Internal Faults:  The electronic function of the CIM module and the output stages are also monitored. When a fault occurs, the entire system is shut down and a fault code is stored. The cyclic CAN signal is also monitored.

Fault Generation:  When a fault is detected in a function or component, that output function is deactivated. The fault information is stored in the fault code memory. If correct function of the component cannot be determined after a fault has occurred, the function will remain deactivated. After repairs, the fault can be deleted by the DISplus.