BMW 7 Series: Higher-Level Driving Dynamics Control

Observation of the Driving Condition

The Integrated Chassis Management (ICM) control unit calculates the current driving situation from the signals listed below. This essentially means the longitudinal and lateral dynamic driving condition.

• Wheel speed signals from all four wheels
• Lateral acceleration
• Yaw rate

The ICM control unit therefore knows how the vehicle is actually moving at this point.

To be able to optimize the vehicle behavior, the dynamic driving systems require information about how the driver wishes the vehicle to move. The driver's command is determined from the following signals:

• Accelerator pedal angle and current engine torque and gear ratio
• Application of the brake pedal and current brake pressure
• Effective steering angle and steering-angle speed.

The driving condition and driver's command are provided both internally and externally by the ICM control unit. The central driving dynamics control acts as a receiver internally in the ICM control unit. The control units of the dynamic driving systems (e. g. DSC) are the external receivers. They receive the driving condition and the driver's command from the ICM control unit via the FlexRay bus system.

Central Driving Dynamics Control

The aim of the interventions by the dynamic driving system is to improve agility and traction. If required, they can of course also restore the stability of the vehicle. In previous vehicles, separate systems existed that were designed to do this and although they in fact communicated with each other, they tended to have a more restricted range of tasks. The interaction of all systems that ultimately determines the overall driving characteristics was therefore difficult to coordinate.

The Integrated Chassis Management of the F01/F02 incorporates the central driving dynamics control. This compares the command given by the driver with the actual movement of the vehicle at that point and therefore determines whether intervention of the dynamic driving system is required, and also the extent of the intervention.

The yawing force is an output variable of the central driving dynamics control system. This produces a rotation of the vehicle that is superimposed on the existing movement of the vehicle.

This can be used to "readjust" the driving characteristics if the result identified does not match the driver's command. Classic examples of this are understeering or oversteering driving characteristics.

A new feature of the ICM installed in the F01/F02, however, is that the dynamic driving systems are already deliberately activated before a deviation of this nature is identified. The interventions of the dynamic driving systems therefore take place long before the driving characteristics become unstable.

This produces a far more harmonious effect in the vehicle than would be possible from a conventional chassis design. The vehicle reacts neutrally in many more situations and does not even begin to understeer or oversteer.

This new function is possible through the use of extremely precise computing models and new control strategies that can be used to evaluate and influence the driving characteristics.

Fig. 20: View Of Influencing Driving Characteristics Using Driving Dynamics Control System

INDEX REFERENCE CHART

1. Correction of unstable driving characteristics
2. Intervention at an early stage to achieve neutral driving characteristics

1. Braking intervention at individual wheels in order to correct understeering
2. Braking intervention at individual wheels in order to prevent understeering
3. Course of an understeered vehicle
4. ourse of a vehicle with neutral driving characteristics

13. Yawing force that acts on the vehicle due to braking intervention (at individual wheels)

Coordinated Intervention by the Dynamic Driving Systems

The following intervention options for producing the yawing force calculated by the central driving dynamics control system have been available up till now (and will of course remain available) -the corresponding dynamic driving systems are shown in brackets:

• Individual activation of the wheel brake (DSC)
• Adjustment of the current engine torque (ASC+T, DSC, MSR)
• Adjustment of the steering angle of the front wheels, regardless of the driver's input (Active Steering).

Fig. 21: View Of Possible Driving Dynamics Interventions During Understeering

INDEX REFERENCE CHART

1. Prevention of understeering by means of braking at individual wheels
2. Prevention of understeering by means of steering intervention at rear axle

1. Braking intervention at individual wheels
2. Steering intervention at the rear axle
3. Course of an understeered vehicle
4. Course of a vehicle with neutral driving characteristics

13. Yawing force that acts on the vehicle due to braking intervention (at individual wheels)

The option of influencing the lateral dynamics of the vehicle - the rear axle slip angle control (HSR) - was available for the first time in the F01/F02. The customer only receives this innovative dynamic driving system in combination with the established Active Steering feature. This option is referred to as "Integral Active Steering".

A function referred to as "Actuator coordination" follows the central driving dynamics control. This decides which dynamic driving system should be used to produce the yawing force in the specific road situation.

For example, if the vehicle has a tendency to sharply understeer this can be counteracted by means of selective braking intervention at the back wheel on the inside of the curve. If the vehicle is equipped with Integral Active Steering, the same objective can be achieved more harmoniously by applying an appropriate steering angle at the rear axle.

As both actuating options are limited, it may also be beneficial to apply both at once. If understeering is avoided the driver becomes aware of this due to the considerable increase in agility.

The F01/F02 is the first instance where genuine functional networking between the integrated chassis management and Vertical Dynamics Management functions also takes place. This does not simply mean that the ICM records and processes ride-height information and then delivers it to the VDM.

The ICM also actively controls the Active Roll Stabilization as an integral part of central driving dynamics control in order to influence the self-steering characteristics. As the conventional chassis design already demonstrates, a more rigid anti-roll bar on one axle means that the overall achievable cornering stability on the same axle is lower. The effects of more or less rigid anti-roll bars can be emulated with the aid of the hydraulic motors in the anti-roll bars of the Dynamic Drive. This means that the central driving dynamics control of the ICM can selectively influence the degree of available lateral force on one axle via the active anti-roll bars of Dynamic Drive.

If the vehicle is currently oversteering, the cornering force at the rear axle is insufficient. The roll stabilizing torque at the rear axle tends to reduce in this case. This loss of torque is compensated for by additional cornering stability at the rear axle which helps stabilize the vehicle.

The activity of the central driving dynamics control in the ICM control unit is summarized in the input/output graphic on the following page.

Fig. 22: Input/Output Signal Diagram - Central Driving Dynamics Control In ICM

CENTRAL DRIVING DYNAMICS CONTROL LEGENDS

1. Input signals from external sensors
2. Integrated Chassis Management
3. Dynamic Stability Control
4. AS control unit
5. AS actuating unit
6. HSR control unit
7. HSR actuating unit
8. VDM control unit
9. Active anti-roll bar
10. "Actuator coordination" function
11. "Central driving dynamics control" function
12. "Sensor signal preparation" function
13. DSC sensor in the ICM control unit (longitudinal acceleration, lateral acceleration, yaw rate)
14. Redundant DSC sensor in the ICM control unit (lateral acceleration, yaw rate)

Distributed functions: ICM and actuator control units

A description of the distribution of tasks between the ICM and the other driving dynamics control units follows using Integral Active Steering as an example.

Fig. 23: Input/Output Signal Diagram - ICM And Actuator Control Units AS And HSR

ICM AND ACTUATOR CONTROL UNIT LEGENDS

1. Wheel-speed sensors
2. Dynamic Stability Control
3. Steering column switch cluster with steering angle sensor
4. AS control unit
5. AS actuating unit
6. HSR control unit
7. HSR actuating unit
8. Integrated Chassis Management
9. DSC sensor in the ICM control unit (longitudinal acceleration, lateral acceleration, yaw rate)
10. Redundant DSC sensor in the ICM control unit (lateral acceleration, yaw rate)

The Integrated Chassis Management (ICM) is the control unit that performs the calculations for higher-level driving dynamics functions of the Integral Active Steering.

The Integrated Chassis Management uses the current driving situation and the driver's directional input to calculate the individual setpoint values for the variable steering-transmission ratio and the Yaw-Rate Control Plus. Once these have been prioritized, the ICM produces a reference value for the AS and HSR control unit respectively. This is a reference angle that should be set at the front or rear wheels.

The AS control unit receives this reference value and has the principal task of controlling the actuating elements in order to achieve the reference value. The AS control unit is therefore purely an actuator control unit. The same applies for the HSR control unit: this is also an actuator control unit. As with the AS control unit, this control unit is purely responsible for implementing the reference steering angle requested by the ICM.

This type of task distribution was implemented for the first time with the introduction of the ICM in the E71.

The expansions in the F01/F02 mean that

• the ICM now controls all longitudinal and lateral dynamics systems centrally (AS, HSR and also DSC) and that
• ICM is the master control unit both in the linear range and also in unstable driving conditions.

However, the interface between the Integrated Chassis Management and Dynamic Stability Control is a special case.

Fig. 24: Input/Output Signal Diagram - ICM And DSC

INDEX REFERENCE CHART

1. Steering column switch cluster with steering angle sensor
2. Wheel-speed sensors
3. Integrated Chassis Management
4. DSC sensor in the ICM control unit (longitudinal acceleration, lateral acceleration, yaw rate)
5. "Driving dynamics control" function in the ICM control unit
6. "Actuator coordination" function
7. Dynamic Stability Control
8. "Driving dynamics control" function in DSC control unit
9. "Actuator control" function
10. Wheel brake
11. Drive

The Dynamic Stability Control also has its own internal driving dynamics controller that normally implements the reference value (reference yawing force) sent from the ICM control unit in the F01/F02. This is achieved through braking intervention at individual wheels and also by influencing the input torque.

The DSC driving dynamics controller is also able to detect an unstable road situation itself using corresponding signals on the driving condition provided by the ICM in which case the stabilizing braking or engine interventions are implemented automatically by DSC. Corresponding feedback is of course also sent to the ICM. In this case, the interventions of the driving dynamics control in the ICM are cancelled.

Control and Adjustment of Steering

The Servotronic function in the F01/F02 is included in the basic steering and Integral Active Steering (option).

This speed-dependent power steering assistance function is implemented by the Servotronic valve at the steering gear.

The Servotronic valve is always activated from the ICM control unit irrespective of the equipment specification.

It follows that the ICM control unit also incorporates the logic of the Servotronic function.

Again, regardless of the options fitted, the steering system also contains a proportional valve that is also controlled by the ICM control unit. The volumetric flow in the steering hydraulic circuit can be adjusted electronically assisted by this valve which is why it is also referred to as an "electronic volumetric flow adjustment" valve, or EVV valve for short.

This valve is also controlled by the ICM control unit. The volumetric flow generated by the power steering pump is distributed between a circuit to the steering valve and a bypass circuit according to the level of power steering assistance required. This distribution is infinitely variable. The less power steering assistance is required, the more hydraulic oil is diverted to the bypass circuit. Because the hydraulic oil in the bypass circuit has no task to perform, this means that the power steering pump consumes less power. In this way, the EVV valve contributes to reducing fuel consumption and CO2 emissions.

Fig. 25: Input/Output Signal Diagram - Control Of Steering By ICM

INDEX REFERENCE CHART

1. Steering column switch cluster
2. Wheel speed sensor
3. Dynamic Stability Control
4. EVV valve
5. Servotronic valve
6. Integrated Chassis Management
7. "Steering control" function

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