Active Suspension
Due to its low power/weight ratio and its high power density, a hydraulic cylinder is suited as a displacing element for the active control of the ride-level (see figure).
Cylinder pressure is controlled by means of a quick-acting servo valve, which receives the control signals from an electronic control unit. Various information about the driving condition picked up by sensors on the vehicle can be fed to the control logic. The active adjusting force can be controlled in relation to the wheel or body acceleration or by the pre-determination of the road irregularities.
Unlike a fully active suspension, the active version of the hydropneumatic suspension also provides a spring action without inflow and outflow of hydraulic oil. The control valves supply or withdraw oil only if the system recognizes that it can improve the vehicle handling by an active intervention. The advantage here is that even under extreme driving conditions, the energy input required is significantly lower.
In lower part of the figure shows the spectral power densities of active hydropneumatic and fully active suspension are compared with those of passive systems. Suspension comfort as well as road grip of the wheels are clearly improved over a wide range of frequencies.
Realization of motor vehicles with significantly improved suspension comfort is technically possible. Drawbacks are seen in the higher design expenditure and the energy requirements, which for a standard-size car amounts to between 7 kW (active hydropneumatic suspension) and 20 kW (fully active suspension).