Control of a low frequency active suspension

Automotive suspension design requires a compromise between passenger comfort and vehicle control. The introduction of active elements into the suspension allows the compromise to be redefined, which can bring improvements in performance, however limitations still exist due to finite suspension travel, and invariant points in the suspension transfer function. The primary requirements of the suspension are that it should minimise the vertical accelerations in the body to increase passenger comfort, minimise the dynamic tyre load to provide maximum road holding while cornering and braking, and react to load changes on the body that occur during manoeuvring, and when the passenger load changes. Two forms of active suspension are commonly recognised: in the high bandwidth active suspension the actuator sits in parallel with the road spring, between the body and the unsprung mass, and must control both the body, with a frequency around 1 Hz, and the unsprung mass with a frequency around 12Hz; in the low bandwidth active suspension the actuator sits in series with the road spring and controls the body motion, while the unsprung mass is controlled by the passive damper. This paper describes the design and development of a low bandwidth active suspension suitable for a luxury, or executive motor car.