Abstract
Keywords - vehicle dynamics, innovative variable damper system, semiactive system, ride comfort, handling
Excellent ride comfort and handling performance are key issues in the premium segment of passenger cars. With a conventional spring / damping system it is not possible to fulfil these requirements, namely minimization of vertical body acceleration and wheel-load fluctuation, at the same time. Additionally the possibility of individualising the driving behaviour, for example with a switch, becomes more and more important. Therefore more and more vertical dynamic chassis control systems are being used in the premium segment.
This paper proposes a semiactive chassis control system with variable dampers, because it is more efficient in terms of energy consumption and cost compared to fully active systems. Known variable damper systems either have switchable damper adjustment acting in conjunction with or independently of the compression and rebound directions, or fully variable adjustment acting only in conjunction with the compression and rebound directions. BMW will be the first OEM to use a fully variable damper system, acting separately in the compression and rebound directions. The paper describes the components, characteristics and functional benefits of this system.
The system consists of four variable damper actuators with integrated control units including acceleration sensors, additional sensors and a master control unit.
The fully variable damper actuator has electric adjustment of the damping force even at low damper velocities. This is very important for good primary ride comfort and handling. Since the difference between the soft and the hard damping characteristics is wide, it is possible to offer the customer several different selectable settings of driving behaviour. The characteristic of the damping force in compression and rebound can be configured separately, something which is necessary for achieving good secondary ride comfort and for minimising vertical wheel motion even in the soft damper hardware characteristic.
Additional distance sensors between the wheel and the body and vertical damper acceleration sensors are used. The latter are directly integrated into the actuator control units. The four actuator control units and the master unit are connected to the FLEXRAY bus system of the car.
The system logic works with a sky-hook algorithm and is divided between the actuator and master control units. An observer is used to estimate the body and wheel movement based on the damper vertical acceleration and the distance sensors between the wheel and body. An additional roll stabilisation system is implemented according to the configuration of the car.