Steering: Description and Operation
Electromechanical Power Steering
Electromechanical Power Steering (EPS)
The difference between electromechanical power steering (EPS) and conventional hydraulic steering lies in the type of steering servo assist. EPS assists the driver using an electrical servomotor instead of a hydraulic pump. The servomotor is only activated during steering wheel movements. This means the steering uses no power during straight-ahead driving.
Electromechanical power steering offers the following advantages:
- lower steering forces on parking
- Integrated, vehicle speed dependent steering servo (Servotronic): Ordered as optional equipment or can be enabled after hand over to the customer by the dealer organization using the enabling code.
- lower bumpiness when cornering as well as lower steering wheel vibrations
- active steering wheel return
- Fuel saving of up to 0.3 l/100 km and therefore lower CO2 emissions
- no hydraulic fluid necessary
The electromechanical power steering is also available for the first time as variable sport steering (optional equipment SA2VL) with integrated Servotronic. As opposed to with the basic version of the EPS, the more direct steering gear ratio results in a smaller required steering angle. The vehicle subsequently responds more directly and exhibits increased agility. This is of particular benefit for avoidance maneuvers, for example.
The variable steering gear ratio is realized through a stroke-dependent rack gearing geometry. While directional stability is stable, the steering remains true to the centre position of the steering gear. For steering angles away from the centre position, the ratio becomes ever more direct.
Brief component description
The electromechanical power steering comprises the EPS unit and the mechanical steering gear including steering-torque sensor. The most important electrical components are described below.
EPS unit
The EPS unit consists of the following components:
- EPS control unit
- Servomotor with rotor position sensor
The EPS control unit is a component of the electromechanical power steering. The EPS control unit is connected to the vehicle electrical system with 2 plug connections. The steering-torque sensor is connected via another plug connection to the EPS control unit.
Several characteristic curves for power assistance, active steering wheel return as well as damping characteristics are stored in the EPS control unit. The values calculated from the input variables added to each characteristic curve result in the necessary steering servo.
The EPS unit is powered by the engine compartment power distribution box with terminal 30B. The power distribution box in the junction box supplies the EPS control unit with terminal 15N.
Servomotor with rotor position sensor
The servomotor is a non-collector, synchronous direct current motor (permanent magnet). The servomotor drives the reduction gear. This transfers the power output of the servomotor to the rack. The reduction gear consists of a belt drive and a ball screw. The reduction gear has a reduction ratio of approx.2.4.
There are 2 rotor position sensors (redundancy) on the printed circuit board of the control unit. Both sensors use the principle of the Hall effect (Hall sensor with magnet wheel). The magnet wheel is attached to the shaft of the motor. Rotor position sensor 1 determines the position of the servomotor. The sensor delivers a sine-wave signal and a cosine-wave signal. The signals are used to determine the rotor position. Rotor position sensor 2 is used for monitoring (plausibility check). Both sensors are supplied with voltage by the EPS control unit.
New: The rotor position sensor is at the same time the steering angle sensor.
The EPS does not determine the steering angle using a separate sensor on the steering wheel. Instead, the EPS unit determines the steering angle from the motor position angle of EPS unit motor on the steering wheel. The EPS sends the rack position to the ICM control unit via FlexRay. To do this, the EPS calculates the absolute position of the rack from the current rotor position of the servomotor and from the number of complete rotor revolutions from the neutral position (i.e. straight-ahead driving position).
Based on this position, the ICM control unit uses a saved parameter (rack stroke to wheel-based steering angle) and other values to calculate the wheel-based steering angle and transmits this via FlexRay. This wheel-based steering angle is also used by the DSC as a reference variable for the internal control operation.
The absolute EPS rack value can be lost, i. e. if terminal 30 fails after programming.
The absolute rack position is then determined by the ICM and EPS in conjunction using two different methods:
- Automatically while driving at up to 20 km/h using an integrated adaption function.
- While motionless by steering from full lock to full lock (e. g. straight-ahead position, left limit position, right limit position, straight-ahead position).
Steering-torque sensor
The steering-torque sensor records the steering torque applied by the driver. The working area is approx. 3 steering wheel turns from limit position to limit position.
The steering torque twists the torsion bar spring and transmits the steering torque to the pinion. The function of the steering-torque sensor is based on the magneto-resistive principle. This utilizes the effect that arises in response to changes in the magnetic field and changes in resistance. Different voltage signals are generated by these magneto-resistive elements and these are then forwarded to the EPS control unit. The EPS control unit uses this information to calculate the assistance torque required.
System network
ICM: Integrated chassis management
The ICM control unit serves as a central signal provisioning point and distributes the signals created in the vehicle. In addition to the dynamic handling characteristic elements such as longitudinal acceleration, lateral acceleration and yaw rate, a driving speed is created as reference. Relevant wheel speeds supplied by the DSC are used to do this. Taking into account the different kinematic relationships, different steering angles are calculated from the absolute rack position calculated by the EPS. These include:
- Steering angle of steering wheel
- Steering angle on front axle
- Absolute steering angle
All deviations are ironed out before output signals are sent. Suitable monitoring systems are in place to check that all output signals are correct. For example, the steering angles are checked for plausibility against a steering angle calculated from the yaw rate.
Additional functions are integrated in the ICM, whereby the control unit is responsible for data preparation and calculations. The EPS serves merely as the actuator for executing the control variable.
Two examples of this are outlined in brief below:
Limit range feedback:
This ICM function dramatically improves steering within the limit range. The objective is to make the positive characteristics of hydraulically assisted steering movements available for EPS as well. The reason for this is that the steering feedback during oversteer and understeer is suppressed by the increased friction and inertia in the EPS. In addition, the increased friction makes active feedback a necessity, which leads to even poorer feedback within the limit range.
During untersteer, the steering servo assist, feedback and damping action are reduced in accordance with the reduced self-aligning torque of an oversteered front axle. This is perceptible to the driver as a "softening" of the steering in the same manner as with conventional steering. During oversteer, the front axle is subjected to an additional torque in the stabilizing direction applied by the EPS unit. This action is supported by an additional reduction in feedback and damping action. An oversteer intervention makes steering into the oversteer lighter and the vehicle stabilizes itself in a driving situation that opposes the oversteer forces.
Predicting steering power:
This function uses physical driving variables to analyze the current driving situation. Driving situations using intensive steering power are thereby identified in advance. Energy management is informed of the expected steering power demand.
DME or DDE: Digital Motor Electronics or Digital Diesel Electronics
The engine control emits the "Engine running" signal, which is required by the steering servo as a central switch-on condition. In this way the steering behaves in the same manner as hydraulic steering.
Additional safety in comparison to a purely hydraulic steering servo:
If the engine stalls while driving or switches off because the START-STOP button is pressed, the steering servo remains active until the vehicle comes to a standstill. Only then is it switched off.
Instrument panel: Instrument panel
In the event of failure of the electromechanical power steering (EPS), a yellow Check Control symbol appears in the LC display. Simultaneously, the fixed indicator lamp lights up in the instrument cluster. The Check Control symbol has the following meaning: Steering behavior. Drive with care.
Possible causes for the Check Control message lighting up can be:
- Fault in the EPS control unit, in an integrated sensor or in the servomotor
- EPS overheat protection
- Undervoltage or overvoltage
- Failure of external signals with influence on steering servo assistance
- Faulty or incomplete initialization of the steering (e.g. learning end stops)
System functions
The following system functions are described for the electromechanical power steering (EPS):
- Servotronic
- Active steering return
- Active road feedback
- Overvoltage detection and undervoltage detection
- Overheat protection
- Identification of overload current
- End stop as software function
Servotronic
The Servotronic, the electronic control of the speed-dependent steering servo is integrated in the system (encoding data record, as optional equipment). The EPS control unit determines the required steering servo assistance from various input variables.
Characteristic maps for the support and damping characteristics are stored in the control unit. The values calculated from the input parameters combine with the characteristic curve to produce the required level of steering servo assistance. The servomotor and reduction gear provide the steering servo assistance.
Important input variables for steering servo assistance are:
- Driving speed
- Steering torque on the driver side via steering-torque sensor
- Current vehicle voltage
- Operating temperature
Active steering return
After cornering, the active steering return brings the steering back to the straight-ahead position as soon as the driver no longer applies steering torque. The active steering return ensures a harmonious return of the steering across the entire operating temperature. Active steering return from the end positions is particularly important at low temperatures. This increases operating convenience.
A gyrostabilizer is integrated in the active steering return.
Active road feedback
Information on the roadway is e.g. changes in the coefficient of friction or properties. The EPS provides this information by changing the steering torque.
The following variables are used to calculate the acceleration at the front axle:
- Driving speed
- Yaw rate
- Lateral acceleration
In turn, the EPS calculates a proportion of the steering torque for the active road feedback.
Overvoltage detection and undervoltage detection
In the case of overvoltage exceeding 16 V, the engine performance is reduced (linear) to 17 V. Steering servo assistance is no longer provided above 17 volts. The Check Control message is issued at 40 % of steering servo assistance (with fault entry). When the voltage drops below 16 Volt, the steering power assistance returns to the currently requested value. The Check Control symbol goes out when 100 % of the requested steering servo assistance is reached.
In the case of undervoltage less than 10 V, the engine performance is reduced (linear) to 9 V. Below 9 Volts, steering servo assistance is no longer provided. The Check Control message is issued at 40 % of steering servo assistance (with fault entry). When the voltage returns to above 10 Volts, the steering servo assistance returns to the currently requested value. The Check Control symbol goes out when 100 % of the requested steering servo assistance is reached.
Overheat protection
The EPS provides a monitoring function to protect against excess temperature. The temperature of the following components is monitored:
- Activation electronics
- Power electronics
- Motor winding
If one or several measuring points reach the temperature threshold value, the EPS reduces the steering servo assist (linear) from the threshold value (1 %) to the limit value (100 %). If in doing so the steering servo assist drops below the value of 40 %, a Check Control symbol lights up in the instrument cluster. The Check Control symbol goes out if the steering servo assist returns to the nominal value of 100 %.
Identification of overload current
The EPS identifies overload current situations using an internal monitoring routine. The steering angle, steering wheel angular velocity, driving speed and steering torque applied by the driver are all used to calculate the required steering power. If a permitted load limit is exceeded, the steering servo is reduced or switched off completely in order to protect the power electronics.
Overload current situations can be caused by the following traffic situations:
- The vehicle is very close to the kerb and is pushed away by a steering action.
- Quick countersteer while skidding.
End stop as software function
The software is used to replicate the left and right mechanical steering full lock positions as software functions.
These software-based end stops are determined during the start-up routine and stored in the control unit.
This function protects the mechanics from premature wear by applying counter-torque before an end stop is reached.
Notes for Service department
Replacing a steering system
After a steering system is replaced, the new steering gear must be "electrically" mated to the existing steering mechanism using the "Start-up" service function.
Steering start-up
The following service function is available in the diagnosis system:
- Initial operation
Using this service function, the electromechanical power steering parameters are reset to the factory settings and then determined again.
Information on encoding and programming
The EPS contains a large number of data records. The data record is selected by encoding depending on the engine version and other marginal conditions. For this reason it is essential that the EPS control unit is encoded correctly if it has been replaced (including control unit swap).
Operating conditions
The steering servo assistance is activated under the following conditions:
- No internal control unit fault
- Ignition is switched on
- Engine running
The steering servo is deactivated under the following conditions:
- Internal control unit fault is present
- Vehicle is stationary and engine is off
- Overvoltage exceeding 17 V
- Undervoltage below 9 V.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.