BMW 7 Series: Electronic Ride-Height Control

Function of Air Springs

The various control modes on the F01/F02 are similar to those on other model series such as the E70:

Fig. 131: EHC Control Mode Flowchart, F01/F02

Control Modes with Single-axle Air Suspension

Ongoing control operations are not affected by transitions from one mode to another. However, in the case of load cutout OFF (VA_AUS), control operations are always concluded in order to safeguard system deactivation.

The EHC control unit then sets Sleep mode.

IMPORTANT: All technical data quoted are code-specified as at SOP.

Sleep Mode

The vehicle is in Sleep mode at the latest by the time it has been parked for longer than a few minutes without a door or hood/trunk lid being operated or the terminal status changing. This is the initial state of the control system. No control function is performed in Sleep mode. The control system goes into Pre-mode when a wakeup signal is received by the EHC control unit.

Post-mode

Post-mode is activated in order to compensate for any inclination or to adjust the ride height after driving and between the Pre-mode and Sleep mode.

Duration of Post-mode is limited to approximately 1 minute. This mode is only executed if the engine has been running before the system switches into this mode. If the engine has not been running, the system switches directly from Pre-mode to Sleep mode.

Control adjustments are made within a tight tolerance band using the following coding parameters:

POST MODE CODING PARAMETER

The fast signal filter is used.

In the event of an inclination (Kerb mode), the control operation takes place for the nominal heights applicable in this situation.

Pre-mode

Pre-mode is activated by the "Load cutout OFF" signal (e. g. in response to opening of the door or unlocking with the radio remote control). The Pre-mode then stays set for 16 minutes and is restarted with a change in status.

The ride height of the vehicle is monitored and evaluated with a wide tolerance band.

In Pre-mode, the vehicle is only controlled up to the nominal height if the level is significantly below the nominal height. This control tolerance ensures that the vehicle is only controlled up in the case of large loads in order to increase the ground clearance prior to departure. Small loads give rise to small compression travel and this is compensated only when the engine is started. This control setting helps to reduce the battery load.

PRE MODE CODING PARAMETERS

(1) Average figure

With single-axle air suspension, the vehicle is controlled down when the mean value of both ride height signals is > 0 mm and one side is in excess of + 10 mm.

In this mode, only the mean value of the two ride height signals (fast filter) is considered when deciding whether there is a need for a control operation.

There is no inclination detection in Pre-mode.

Normal Mode

Normal mode is the starting point for the vehicle's normal operating state. It is obtained by way of the "Engine running" signal.

Levelling is possible. The compressor starts up as required.

NORMAL MODE CODING PARAMETER

A narrower tolerance band than that in Pre-mode can be used because the battery capacity does not have to be protected. The fast filter is used with a narrow tolerance band of +- 10 mm. In this way, ride level compensation takes place outside a narrow tolerance band of +- 10 mm. The faster filter allows the system to respond immediately to changes in ride level. Evaluation and control are performed separately for each wheel.

When a speed signal is detected, the EHC control unit switches into Drive mode. When the vehicle is stopped, the EHC control unit remains in Drive mode. The system switches back into Normal mode only when a door or the luggage compartment lid is also opened. If none of the doors or the luggage compartment lid is opened, the vehicle logically cannot be loaded or unloaded.

This prevents a control operation happening when the vehicle is, for example, stopped at traffic lights and the ride height is above the mean value due to a possible pitching motion at the rear axle.

Drive Mode

Drive mode for the single-axle air suspension is activated when a speed of > 1 km/h is detected.

DRIVE MODE CODING PARAMETERS

Low-pass filters are used. In this way, only changes in ride height over a prolonged period of time (1000 seconds) are corrected. These are merely the changes in ride height, caused by vehicle compression and a reduction in vehicle mass due to fuel consumption. The high-pass filter (fast filter) is used during the control operation. The slow filters are reinitialized at the end of the control operation. The markedly dynamic height signals caused by uneven road surfaces are filtered out.

Kerb (curb)

KERB MODE CODING PARAMETERS

The Kerb mode prevents the inclination caused by the vehicle mounting an obstacle with only one wheel from being compensated. Compensation would cause a renewed inclination of the vehicle and result in a renewed control operation after the wheel came off the obstacle.

Kerb mode is activated if the difference in height between the left and right-hand side of the vehicle is > 24 mm and is present for longer than 0.9 seconds. No speed signal may be present for this mode to be set. The system switches from single-wheel control to axle control.

Kerb mode is quit if the difference between the left and right-hand side of the vehicle is < 28 mm and this difference remains for longer than 0.9 seconds or if the speed is > 1 km/h.

If the system switches from Kerb mode to Sleep mode, this status is stored in the EEP-ROM.

If the vehicle is being loaded or unloaded in Kerb mode, the EHC control unit calculates the mean value for the axle from the changes in ride height determined from the spring travel on the right and left-hand side.

A change in ride level is initiated if the mean value of compression or rebound at the axle is outside the tolerance band of +- 10 mm. The left and right sides of the vehicle are raised or lowered in parallel. The height difference between the two sides is maintained.

Curve

Since rolling motions have a direct impact on the measured ride levels, an unwanted control operation would be initiated during longer instances of cornering with an appropriate roll angle in spite of the slow filtering of the Drive mode. The control operations during cornering would cause displacement of the air volume from the outer side to the inner side of the curve. Once the curve is completed, this would produce an inclination which would result in a further control operation. Curve mode prevents that adjustment by stopping slow filtering when cornering is detected and cancelling any adjustment that may have been started.

Curve mode is activated upwards a lateral acceleration of > 2 m/s2 and deactivated at < 1.0 m/s2. The lateral acceleration is recorded by the DSC sensor.

Substitute conditions:

CURVE MODE CODING PARAMETERS

Lift

The Lift mode is used to prevent control operations when a wheel is changed or during work on the vehicle while it is on a lifting platform.

LIFT MODE CODING PARAMETERS

This mode is detected when the permitted rebound travel at one or more wheels is exceeded > 55 mm. A jacking situation is also detected and the ride height stored if the lowering speed drops below the level of 2 mm/s for 1 second.

If the vehicle is raised only slightly and the permitted rebound travel has not yet been reached, the control operation attempts to readjust the ride height. If the vehicle is not lowered, a car jack situation is recognized after a specific period of time and this ride height is stored.

A reset is performed if the vehicle is again 10 mm below this stored ride height.

Special Modes (Transport, Belt)

Transport mode is set and cancelled by means of diagnostics control. It serves to increase the ground clearance in order to ensure safe transportation of vehicles on transporter trucks. The nominal height of the vehicle is raised in this mode by 30 mm.

When Transport mode is activated, the air suspension symbol is lit on the variable indicator lamp on the instrument cluster and a message is shown on the Check Control display to alert the driver to this special mode.

Control operations do not take place in this mode because the vehicle mass does not change during transportation.

Belt mode is set during assembly in the works to prevent control operations.

When Belt mode is activated, the air suspension symbol is lit in the variable indicator lamp in the instrument cluster and a message is output in the Check Control display to alert the driver to this special mode.

Production-line mode is cleared by means of diagnostics control only. The Belt mode can no longer be set.

New EHC control units (spare part) are supplied with Belt mode set.

Control operations are not performed, the safety concept only operates with limited effect.

CONTROL MODES REFERENCE CHART

Method of Operation

Initialization/reset behavior

Different checks and initialization are carried out when the EHC control unit is powered up after a reset (triggered by an undervoltage or also by load cutout off VA_AUS).

The system is only enabled after the tests have been successfully completed and starts to execute the control programs on a cyclical basis. Occurring faults are stored and displayed.

Control sequence

In an ongoing control operation, the high-pass filter (fast filter) is always used to prevent the controlled height from overshooting the specified setting.

If a low-pass filter (slow filter) were used to calculate the ride height, brief changes of ride height would be "absorbed".

The low-pass filter is used when the vehicle is in motion (see NORMAL MODE) to filter out vibrations induced by prevailing road conditions on this basis of this method of filtering.

The high-pass filter is used to respond quickly to ride level deviations from setpoint. These take place while the vehicle is stationary in the event of large load changes (see PRE-MODE).

Both sides of the vehicle are controlled individually, i. e. even the setpoint/actual-value comparison for both sides is carried out individually.

Exception: check for falling below the minimum height in Pre-mode and Kerb mode. The left/right mean values are taken into consideration here. The following stipulations are applicable here:

• Raising or lowering
• All valves controlled with control in the same direction
• Individual wheel deactivation

To ensure reliable closing of the non-return valve in the air drier, the exhaust valve is operated briefly for 200 ms by the EHC control unit after each control sequence.

The permissible ON period is monitored while performing upward adjustment operations.

Safety concept

The safety concept is intended to inhibit any system malfunction, particularly unintentional control operations, through the monitoring of signals and function-relevant parameters.

If faults are detected, the system is switched over or shut down depending on the components concerned. The driver is informed of any faults by the display, and detected faults are stored for diagnostic purposes.

In order to ensure high system availability, existing faults, as far as possible, are cleared with terminal 15 ON.

This is done by resetting the logistics counter to zero. However, the fault memory content in the EEPROM is retained and can be read out for diagnostic purposes. The system is then operational again. Fast fault detection means that existing faults are detected before control operations can take place.

Only lowering is permitted if:

• the power supply voltage drops below the permissible minimum of 9 volts.
• the permissible compressor duty periods, and therefore compressor temperatures, based on a temperature duty period algorithm* are exceeded.

A reset is carried out if the voltage is within the required range of 9 to 16 volts or after expiry of the compressor pause period of approximately 100 seconds.< Only raising is permitted if:

• The permissible control down period of 40 seconds is exceeded.
• The reset takes place the next time the vehicle is driven or after the next control-up procedure.

No control if:

• The permissible supply voltage of 16 volts is exceeded.

The reset takes place as soon as the voltage is within the required range.

* Temperature duty period algorithm:

In order to prevent overheating problems when the compressor is running or when outside temperatures are very high, a temperature duty period algorithm has been implemented.

T-Start (starting temperature) is assumed from the coil resistance of the exhaust valve and the theoretical temperature change is calculated on the basis of the change in coil resistance.

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