FIELD OF THE INVENTION
The invention relates to a method and an arrangement for controlling (open loop or closed loop) the damping force for at least one vibration damper of a vehicle wheel suspension.
BACKGROUND OF THE INVENTION
For reasons of driving safety, it is desirable to undertake the electric control of a vehicle damper in such a manner that the driving safety remains ensured even when there is a failure of the damper control because of a defect in the control unit or in the case of a fault with the current supply. This greater driving safety is more likely provided with a harder damping than with a softer damping. For this reason, the control should basically be so designed that the harder damping characteristic becomes effective when there is a fault of the control; whereas, the damping by means of the electric control reacts softer and therefore with greater comfort.
It is known to control semiactive dampers in accordance with the skyhook method. In the skyhook method, a desired damping force is computed which is proportional to the body velocity. In electrically adjustable dampers, a desired current is computed (in general, with an inverse damping characteristic field) from the desired damping force and the determined damping velocity and, with this desired current, an electrical valve changes the damping force in the damper. Because of the characteristic of a passive damper, only the least damaging damping force (the smallest possible damping) is adjusted when there are mutually opposite signs of damping velocity and required damping force.
The parameters of the skyhook controller (especially the proportional coefficient with which the vehicle body velocity is evaluated) or, more generally, the parameters of a characteristic field with which the body velocity is evaluated, are optimized usually with respect to the following: a low body excitation, low wheel load fluctuations and additional subjective criteria such as vehicle handling based on the evaluation of many vehicle maneuvers and these parameters are fixedly adjusted. For a fixed adjustment, a compromise must always be made between the driving comfort and the vehicle handling characteristics. A significant quality advantage is obtained when the control system recognizes specific driving maneuvers wherein the basic matching of the dampers is not used but (adapted in correspondence to the specific driving maneuver) the optimal damping is adjusted in a targeted manner. In the literature, this is done, for example, by changing the proportional coefficient of the body velocity.
The skyhook controller has the disadvantage that the damping force is continuously switched between hard and soft in dependence upon the damper velocity. This can, for example, lead to unwanted acoustic problems. A further disadvantage of the skyhook controller is that the damper automatically adjusts to “soft” when the body velocity is not present. This can lead to unwanted velocity movements (for example, at high vehicle speeds or braking and/or steering maneuvers) and therefore to a deterioration of the driving comfort.
The skyhook method is disclosed, for example, in German Patent 4,135,525 and German patent publication 4,115,061.
SUMMARY OF THE INVENTION
It is an object of the invention to so improve the driving of a damper, which is adjustable in accordance with a standard control method, especially the skyhook method, so that unwantedly occurring body movements in the above-mentioned driving situations are suppressed as early as possible and as much as possible.
The method of the invention is for controlling the damping force for at least one vibration damper of a vehicle wheel suspension of a vehicle. The method includes the steps of: providing a damper for generating a damper force and the damper having a damper characteristic adjustable within an adjusting range for the damping force delimited by upper and lower limit values; detecting a first signal representing the vertical velocity at a part of the vehicle chassis as a suspended mass; detecting a second signal representing the vertical velocity of the unsuspended wheel mass; computing an input control signal based on the first and second signals for adjusting the damper characteristic within the adjusting range in accordance with a standard control process; discretely or continuously adjusting the damping characteristic to the input control signal; and, changing the adjusting range in specific driving situations so as to cause one or both of the upper and lower limits to be raised or lowered.
According to the method of the invention, this does not take place only via a reduction of the control in the manner that the maximum value of the actuating variable is reduced in especially previously defined driving situations and the minimum value of the actuating variable in increased; instead, and in accordance with the invention, a narrowing as well as a widening of the adjusting range is detected. In this way, the situation is taken into account for the driving conditions in which the vehicle body velocity is zero, the dampers of the vehicle are adjusted to soft (and therefore an adjusting range of zero is adjusted) and, starting from this driving condition, the adjusting range is widened when a body acceleration is indicated because of the driving condition. Furthermore, with the invention, the possibility is provided that a wide actuating range is narrowed.
The basic idea of the invention is that the adjusting range of the damper is adapted to the specific driving situation and already then when this driving situation is announced, for example, with the actuation of the brake pedal.
The damping forces, which are generated by the skyhook algorithm in dependence upon the body and damper velocities, are changed via the above changeability of the actuating range of the skyhook algorithm in dependence upon these signals. The damping forces are changed within the technical capability of the particular damper for the duration of the critical driving situation with respect to a possible soft setting or a possible hard setting.
In accordance with a preferred embodiment, the changeability of the maximum value and/or minimum value is coupled to driving dynamic variables, that is, a change of the control range can be coupled to certain pregiven vehicle measured variables.
Signals, which announce or indicate a change of the vehicle longitudinal deceleration, are, for example: changes of the brake pressure, changes of the brake light signal, driver command torque, engine drive torque or accelerator pedal position.
Signals which announce or indicate a change of the vehicle transverse acceleration are, for example: changes of the measured transverse acceleration or the transverse acceleration computed from the steering angle and the vehicle speed.
When determining whether action is required via the control electronics, which are connected to the corresponding sensors, the adjusting variables can be adjusted in the direction of a damping hard adjustment or a damping soft adjustment depending upon weighting. The adjustment is gradual or time delayed, that is, the adjustment is not abrupt.
With the aid of limiting the adjusting range in accordance with the invention, a significantly improved damping of the vehicle velocity is provided especially for acceleration operations in the longitudinal and transverse directions.