The semi-active suspensions described above control body motions by controlling the rate at which the suspension is compressed. A fully active system can counteract body motions by actually forcing the suspension to extend in response to measured and anticipated vehicle motion. Such systems have been used on several production cars but aside from Citroen, they haven't been very successful in marketing terms. Coil springs are used at each corner. The shock absorbers are actually hydraulic rams called actuators that are pressurized by an engine driven hydraulic pump.
Pressure is directed with a system of control valves, check valves, reservoirs and accumulators.
Pressure in the system can range from zero to more than 1400 psi. In many ways, active suspension is simpler than the other systems because it doesn't need exotic shock absorber or air spring technology, just four hydraulic rams and the attendant valves and plumbing. There is the engine driven pump, oil cooler and reservoir, but all that is fairly low-tech too. The real sophistication is in the sensors and operating program. With all this information, the control unit calculates vehicle motion and momentum to control not only ride height and quality, but also roll, pitch and yaw.