BMW 7 Series: Higher-level Dynamic Handling Control

Centralized Dynamic Handling Control

The interventions by the dynamic handling control systems are performed with the aim of improving agility and traction. Quite obviously, they also re-stabilize the vehicle when required. On previous vehicles the various functions were performed by a number of discrete systems which, although they communicated with one another, nevertheless had strictly defined limits to their spheres of operation. Accordingly, the interaction of all systems which ultimately determines the overall handling response, was difficult to harmonize.

Fig. 86: View Of Influence On Handling Characteristics By Dynamic Handling Control System

INDEX REFERENCE CHART

1. Correction of unstable handling
2. Early intervention to bring about neutral handling

1. Individual modulation of brakes to correct understeer
2. Individual modulation of brakes to prevent understeer
3. Course of an understeering vehicle
4. Course of a vehicle with neutral handling

13. Yaw force acting on the vehicle as a result of individual modulation of brakes

The Integrated Chassis Management system on the F01/F02 employs centralized dynamic handling control. It compares the vehicle response desired by the driver with the actual motion of the vehicle at that moment. By so doing, it is able to determine whether and in what way intervention in the dynamic handling systems is required.

The output variable of the centralized dynamic handling control system is a yaw force. It brings about a yawing motion on the part of the vehicle that is superimposed over the existing movement of the vehicle. In that way, the behavior of the vehicle can be "corrected" if a difference from what is desired by the driver is detected. The classic examples of that are vehicle understeer or oversteer.

A new feature of the ICM on the F01/F02, however, is that the dynamic handling systems are brought into action even before such a discrepancy is detected. Thus, the interventions by the dynamic handling systems take place long before the vehicle becomes unstable. As a result, the vehicle feels much better balanced than would be achievable with a conventional suspension and steering set-up. The vehicle displays neutral handling characteristics in many more situations and does not even begin to under or oversteer. This new function is made possible by very precise computation models and new control strategies by which the handling characteristics can be assessed and influenced.

Co-ordinated Intervention by the Dynamic Handling Systems

The possibilities for intervention available in the past (and, of course, still available now) in order to generate the required yaw force calculated by the central dynamic handling controller are listed below. In brackets in each case are the dynamic handling systems concerned.

• Modulation of individual brakes (DSC)
• Adjustment of engine torque (ASC+T, DSC, MSR)
• Adjustment of front-wheel steering angle independently of driver input (Active Steering).

Subordinate to the centralized dynamic handling control system is an "actuator coordination" function. It decides which dynamic handling system can be used to produce the yaw force in the particular situation concerned.

For example, if the vehicle is exhibiting significant understeer, it can be counteracted by controlled braking of the rear wheel on the inside of the bend. If the vehicle has Integrated Active Steering, the same effect can be brought about even more harmoniously by steering the rear wheels to an appropriate degree.

As both means of intervention are limited in their degree, it can also be useful to use them both simultaneously.

Avoidance of understeer is noticeable to the driver in the shape of a significant gain in agility.

The F01/F02 is also the first model on which there is true functional networking between the Integrated Chassis Management and vertical dynamics management functions. But that doesn't simply mean that the ICM registers the ride height data, processes it and passes to the VDM.

An integral component of the dynamic handling control system is that the ICM also actively initiates the Dynamic Drive function in order to affect the self-steering characteristics. As is familiar from conventional suspension and steering designs, a stiffer anti-roll bar results in a lower achievable overall lateral friction force at the pair of wheels concerned. The actuator motors in the Dynamic Drive anti-roll bars can be used to simulate the effect of stiffer and more flexible anti-roll bars.

Thus the ICM centralized dynamic handling control system can use the Dynamic Drive's active anti-roll bars to selectively control the available lateral friction force at a pair of wheels. If the vehicle is oversteering, that means there is too little lateral friction force on the rear wheels. In that case, it is better to reduce the roll limiting force on the rear suspension. In return, there is a gain in lateral friction force on the rear wheels which helps to stabilize the vehicle.

The input/output diagram on the following page summarizes the effect of the centralized dynamic handling control functions on the ICM control unit.

Fig. 87: Input/Output Signal Diagram - Centralized Dynamic Handling Control On ICM

INDEX REFERENCE CHART

1. Input signals from external sensors
2. Integrated Chassis Management (ICM)
3. Dynamic stability control
4. Active Steering control unit
5. AS actuating unit
6. HSR control unit
7. HSR actuator unit
8. VDM control unit
9. Active stabilizer bar
10. "Actuator co-ordination" function on ICM
11. "Centralized dynamic handling control" function
12. "Sensor signal processing" function
13. Integrated DSC sensor (combined linear acceleration, lateral acceleration and yaw rate sensor)
14. Integrated DSC sensor (additional combined lateral acceleration and yaw rate sensor)

Distributed Functions: ICM and Actuator Control Units

The distribution of functions between the ICM and the other dynamic handling control units in the case of Integrated Active Steering is described below.

Fig. 88: Input/Output Signal Diagram - ICM And Actuator Control Units AL And HSR

INDEX REFERENCE CHART

1. Wheel speed sensors
2. Dynamic stability control
3. Steering column switch cluster with steering-angle sensor
4. Active Steering control unit
5. AS actuating unit
6. HSR control unit
7. HSR actuator unit
8. Integrated Chassis Management (ICM)
9. Other input and output signals(1)
10. Integrated DSC sensor (combined linear acceleration, lateral acceleration and yaw rate sensor)
11. Integrated DSC sensor (additional combined lateral acceleration and yaw rate sensor)

(1) Instrument cluster failure BLS-CAS braking DME engine torque increase.

The Integrated Chassis Management is the control unit which computes the higher-level dynamic handling control functions for the Integrated Active Steering.

From the current vehicle handling status and the desired course indicated by the driver, the Integrated Chassis Management calculates individual settings for the variable steering- gear ratio and the superimposed yaw rate.

Once they have been prioritized, the ICM provides a required setting in each case for the AL and HSR control units. The setting specified is a required steering angle to be applied to the front and rear wheels respectively.

The AL control unit receives the required setting and has the main job of controlling the actuators so as to correctly apply the specified setting. Thus the AL Active Steering control unit is purely an actuator control unit.

The same applies to the HSR control unit. It too is an actuator control unit. Like the AL control unit, it is responsible only for implementing the required steering angle specified by the ICM.

With the introduction of the ICM on the E71, this type of function distribution was used for the first time. On the F01/F02, it has been expanded to the extent that

• the ICM now controls all linear and lateral dynamics systems (AL, HSR and also DSC) and
• the ICM is the master control unit both for linear dynamics and unstable handling situations.

The interface between the Integrated Chassis Management and the Dynamic Stability Control (DSC) represents a special case.

READ NEXT: