electromechanical power steering

electromechanical power steering

Until now, the MINI had electrohydraulic power steering.
In the R56, purely electromechanical power steering (EPS: Electronic Power Steering) is used.

The electromechanical power steering (EPS) differs from conventional hydraulic steering in that it has steering power assistance. The EPS supports the driver with an electric motor instead of a hydraulic drive.
The actuator motor is only active when the steering is used. This means that the actuator motor consumes no power during straight-ahead driving.

Electromechanical power steering offers the following advantages:

• Amount of fuel saved up to 0.2 l/100 km and this reduction in the CO₂ emissions
  
• Integrated, driving-speed-dependent power steering (Servotronic)
  
• Switchable steering force (SPORT button)
  
• Active steering wheel return
  

On the R56, the EPS is arranged down at the steering shaft. This means that the transfer channel for the force initiated is very short.

Brief description of components

The following components are described for the electromechanical power steering:

Servo unit

The servo unit consists of the following components:

• EPS control unit
  
• Electric motor with motor position sensor
  
• Reduction gear with steering momentum sensor
  

The EPS control unit is part of the electromechanical power steering. The EPS control unit is connected with 2 plug connections to the vehicle power system.
The steering momentum sensor is connected via another plug-in connection at the EPS control unit.

A number of characteristic curves for servo 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 power assistance.

The fuse box in the engine compartment supplies the servo unit with terminal 30.

Electric motor with motor position sensor

The electric motor is a synchronous DC motor without collectors. The electric motor drives the reduction gear, thus applying the steering momentum.

The motor position sensor determines the position of the electric motor.
The motor position sensor has 3 coils. The electronics in the EPS control unit excite a coil with oscillations. The two other coils use this to form a signal (sinus and cosinus). An analog/digital converter forms the enveloping curve of the signals. The EPS control unit uses this to calculate the engine position.

Reduction gear and steering momentum sensor

The reduction gear driven by the electric motor transfers the steering momentum to the steering gear.

The measurement procedure of the steering momentum sensor consists of 2 resonant circuits.

An oscillating voltage is applied both coils. When steering, the steering column initiates steering momentum. The steering column twists via the torsion bar, thus changing the position of the measurement rings. The position of the measurement rings in turn influences the magnetic field around the coils. The steering momentum sensor uses this change to form its signal regarding the applied steering momentum.

Sport button

When the Sport button is pressed, a switch is made to a more sports-oriented configuration of the EPS.

The Sport button is in the switch block on the centre console.
The Sport button is connected to the junction box electronics (JBE). The JBE is connected to the EPS via the PT-CAN (Powertrain CAN).

The following additional control units are involved in the electromechanical power steering:

DSC, ASC+T, ABS: dynamic stability control, automatic stability plus traction control, anti-lock braking system

The driving stability control system delivers the driving speed as a bus signal. To this end, the corresponding control unit is connected via the PT-CAN (Powertrain CAN) with the EPS.

Note: DSC and ASC+T are optional extras!

ABS is fitted as series standard.

SZL: Steering column switch cluster

With High equipment, the steering-angle sensor is integrated in the steering column switching centre (SZL). The steering-angle sensor delivers the steering angle as a signal. The steering-angle speed is calculated by the EPS from the engine position.

The steering column switching centre is connected via the PT-CAN.

Note: No Chassis CAN on vehicles without DSC!

On vehicles without dynamic stability control (DSC), the steering column switching centre has no steering-angle sensor. This means there is no F-CAN, either. The steering column switching centre is then only connected at the PT-CAN.

DME and DDE: Digital Engine Electronics and Digital Diesel Electronics

The engine management system delivers the signal ”Engine running” on the PT-CAN.

KOMBI: Instrument cluster

In the event of failure of the electromechanical power steering (EPS), a Check Control symbol appears in the LC display. The Check Control symbol has the following meaning:
”Power steering failure. Significantly changed steering characteristics.
Continued driving is possible in moderation. Have this checked by the nearest MINI service centre.

On vehicles with Car Communication Computer (CCC): If there is a Check Control message, the relevant instructions can be displayed on the Central Information Display (CID).

System functions

The following system functions are described for the electromechanical power steering (EPS):

• Steering power assistance
  
• Adaptation of the steering forces (Sport button)
  
• Active steering return
  
• Overvoltage detection and undervoltage detection
  
• Check function
  
  

Steering power assistance

Integrated in the system is the Servotronic, the electronic control of the speed-dependent steering power assistance. The EPS control unit determines the required steering power 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 variables added to each characteristic curve result in the necessary steering power assistance. The electric motor and reduction gear provide the steering momentum.

Important input variables for steering power assistance are:

• Road speed
  
• Steering momentum on the driver side via steering momentum sensor
  
• Steering-angle speed
  
• Steering angle
  
• Operating temperature
  
  

On vehicles without steering-angle sensor, the EPS control unit calculates the steering angle. The calculation involves an algorithm on the basis of the wheel speed. The EPS learns the centre position of the steering via the motor position sensor after a short driving time.

Adaptation of the steering forces

The Sport button selects a more sports-oriented driving style. From the junction box electronics, the signal from the Sport button is forwarded across the PT-CAN to the EPS control unit.

The EPS control unit switches to a characteristic map for sports-style road behaviour. The deflection and holding forces are geared to a more sporting style (higher steering force characteristics).

The sports configuration uses other characteristic curves that also affect the accelerator pedal module and automatic gearbox.

Active steering return

The active steering return is a centering of the steering. To achieve this, the EPS control unit requires the steering angle from the steering-angle sensor. The active steering return ensures a harmonious return of the steering across the entire operating temperature.

For front-wheel-drive vehicles, active steering return from the end positions is particularly important at low temperatures. This increases operating convenience.

Overvoltage detection and undervoltage detection

In the case of overvoltage greater than 17 Volts, the EPS switches off. The EPS control unit stores a fault. A Check Control symbol lights up in the instrument cluster. When the voltage returns to below 16 Volts, the steering power assistance returns to the currently requested value. The Check Control symbol goes out when 100 % of the requested steering power assistance is reached.

In the case of undervoltage less than 9 Volts, the EPS switches off. The EPS control unit stores a fault. A Check Control symbol lights up in the instrument cluster. In the case of undervoltage less than 10 Volts, the engine output is reduced. When the voltage returns to above 10 Volts, the steering power assistance returns to the currently requested value. The Check Control symbol goes out when 100 % of the requested steering power assistance is reached.

Check function

There is a Check Control message for the electromechanical power steering (EPS) in the instrument cluster.

Possible causes for deactivation of the system can be:

• Fault in the EPS control unit, in an integrated sensor or in the engine control unit
  
• Over-temperature protection of the EPS
  
• Undervoltage or overvoltage
  
  

Notes for Service department

General information

In the first two years, the steering gear is always replaced completely. After this time has elapsed, it is likely that the unit consisting of electric motor and EPS control unit can be replaced.

Diagnosis instructions

The following service function is available in the BMW diagnosis system:

• Calibration of steering momentum sensor
  

This calibration must be control unit after replacement of the unit consisting of electric motor and EPS control unit. The prescribed sequence must be adhered to.

If the complete steering gear is replaced, this calibration is not required.

Notes on coding / programming

After replacement, the EPS control unit must be coded.

Switch-on conditions

The EPS is activated under the following conditions:

• Ignition on (terminal 15 and wake-up line)
  
• Engine running
  
  

No liability can be accepted for printing or other errors. Subject to changes of a technical nature