Electric power steering (ES) driving mode, function

P46.35-2020-05

A1 Instrument cluster

L6/1 Left front rpm sensor

L6/2 Right front rpm sensor

L6/3 Left rear rpm sensor

L6/4 right rear rpm sensor

N47-5 ESP and BAS control unit

N68 Electric power steering (ES) control unit

Function requirements:

 Circuit 15 ON

 Engine speed > 300 rpm

A high power steering assistance is desired at low vehicle speeds and when the vehicle is stationary. By contrast, power steering assistance is not desired at high vehicle speeds. For rapid steering maneuvers specific power steering assistance can improve vehicle dynamics. The electric power steering (ES) control unit (N68) requires the following input signals to calculate the power steering assistance required:

 The currently used steering torque, from the electrical power steering (ES) torque sensor (A91b1)

 The current wheel speeds, over the Controller Area Network bus class C (engine compartment) (CAN-C) from the ESP and BAS control unit (N47-5)

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 The steering angle and steering speed from the steering angle sensor (N49) in the steering column module (N80) via the CAN C

 The current engine speed from the ME-SFI [ME] control unit (N3/10) or CDI control unit (N3/9) via the CAN C

 The current rotor position of the electric power steering (ES) actuator motor (A91m1) from the motor position sensor in the electromechanical controller unit (ES) (A91)

When the driver makes a steering movement, the electric power steering (ES) control unit (N68) calculates the current and rotation direction to be applied to the electric power steering (ES) actuator motor (A91m1) based on the performance map using the input signals. The electric power steering (ES) actuator motor (A91m1) is energized accordingly. The electric power steering (ES) actuator motor (A91m1) actively intervenes in the steering operation via a worm gear. A current-recording resistance in the electric power steering (ES) control unit (N68) checks the actual current.

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 The maximum power steering assistance is required when the vehicle is stationary. As the vehicle speed increases the power steering force decreases continuously. During rapid steering maneuvers and at high speeds power steering assistance can take place depending on all input signals.

Steering assist function

The steering assist function is intended to help the driver achieve the ideal steering ability when in a critical situations. The steering assist function is realized by means of the electrical power steering (ES) control unit (N68) and additional software in the ESP and BAS control unit (N47-5). The following functions are performed here:

 Driver support for a vehicle with oversteer taking the following input parameters into account:

- yaw velocity

- steering wheel angle

- Vehicle speed

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 Driver support, e.g. through staying on course when braking on single-side road surfaces taking the following input parameters into consideration:

- wheel braking forces

- yaw velocity

- steering wheel angle

- Vehicle speed

Using the Electronic Stability Program (ESP) sensor data the Steer Assist is able to detect critical driving situations and determine the required steering ability. If the driver's current steering ability does not match the determined values, the basic steering wheel torque has an additional steering wheel torque superimposed onto it which then supports the driver to steer in a manner appropriate to the situation at hand. The ESP and BAS control unit (N47-5) forwards the "Steering assist torque request" message over CAN-C to the electrical power steering (ES) control unit (N68).

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Emergency mode function

For safety reasons, a fault in the electromechanical steering must not lead to a loss of steering capability. This operating condition is achieved by switching off the steering force assist. The vehicle can then still be steered with increased manual effort. When the electromechanical steering is switched on as of "circuit 15 ON" the electrical power steering (ES) control unit (N68) performs a self-test. If a fault is detected the servo assist is not switched on. During vehicle operation the electrical power steering (ES) control unit (N68) checks the signals of the Electrical power steering (ES) torque sensor (A91b1) and the motor position sensor in the electromechanical controller unit (ES) (A91) as well as the current sensors of the electrical power steering (ES) actuator motor (A91m1). If these signals are outside the defined upper and lower limits, the electromechanical steering is immediately switched off by the electrical power steering (ES) control unit (N68). The servo assist is slowly run down at "circuit 15 OFF".

Display in instrument cluster (A1)

If the electrical power steering (ES) control unit (N68) detects a fault condition the driver is notified about it with a warning message ("Power steering defective! Service Required!") in the multifunction display (A1p13) in the instrument cluster (A1). The warning message comes from the electrical power steering (ES) control unit (N68) over CAN-C and is intended to draw the driver's attention in the event of a breakdown in the steering power assist to the vehicle's altered steering ability.