The invention is based on a method and an apparatus as generally set forth hereinafter. Means for controlling the spring stiffness, for instance of a motor vehicle, are known (German patent No. 16 30 058); in this known apparatus, two work chambers of a shock absorber or telescoping spring are connected via external lines to an apparatus comprising a pump and two reservoirs. Only one-way check valves are disposed in the connecting lines to the telescoping spring. However, with this type of apparatus, the damper stiffness of this kind of shock absorber cannot be varied, because to do so energy must be supplied from outside--via the pump--which takes a relatively long time and means that there is a certain energy demand. Controlling the damper stiffness in a shock absorber is also known from German Offenglegungsschrift No. 33 04 815. Suspension systems of present-day vehicle types, in particular passenger vehicles, are typically optimized to an average operational case in terms of the spring stiffness and damper stiffness, with parameters being structurally fixed and remaining unchanged, except for effects associated with aging, during driving. Since in extreme operational cases, such as an empty or fully loaded vehicle, or with varying vehicle movement parameters (rapid cornering, braking, acceleration, smooth highway driving, and the like), optimal suspension or damping of the suspension system is not attained in all such operating situations, it is also already known to switch over among a plurality of damper settings or spring stiffnesses. This prevents long-term adaptation of the suspension system (and possibly the damper system, which is either arbitrarily integrated into the suspension system itself or is a component thereof), and especially it is impossible to make an automatic, finely-tuned adjustment to various road conditions or various kinds of driving, because a switchover in the characteristic curves of the suspension and/or damping system can be made only in stages, typically between only two operating states.
It is also known (U.S. Pat. No. 3,807,678), in a suspension system involving two masses, one of which may be one or more wheels of a vehicle and the other the vehicle body, to dispose a standard, passive compression spring between the two masses, which is called a passive isolating element and has a so-called active damper switched parallel to it. This damper, in which a piston slides in a cylinder and divides it into two work chambers, is considered to be active because an intervention is made into the damping properties, that is, into the positive volume displacements of the pressure medium in the various working halves of the damper by control means, in a so-called active manner. To this end, the two working chambers are connected crosswise and parallel to each other via opposed valves allowing a flow of pressure medium in only one direction; the amount of pressure medium then allowed to pass through these valves then also becomes "active" by appropriate control of the valves by means of suitably prepared sensor signals. Because in this known suspension system, the spring itself is entirely passive, but the damper is conceived of as being active in terms of its properties, the overall system in this patent is called a semiactive system. However, this term is not semantically related to the dampers of the present invention, which without reference to suspension systems not taken into account are themselves designated as so-called semiactive dampers, for reasons to be explained hereinafter.
It is also known, in wheel suspensions in vehicles, to provide so-called active damping means (see the article, "Active Damping in Road Vehicle Suspension Systems", published in the periodical, Vehicle System Dynamics, 12 (1983), pages 291-316). This publication is referred to also because it includes basic concepts, in theoretically detailed form, applicable in particular to active damping properties.