Active suspension was a topic of great research interest near the end of last
century. Ultimately broad bandwidth active systems were found to be too
expensive in terms of both energy and financial cost. This past work, developing
the ultimate vehicle suspension, has relevance for today’s vehicle designers
working on more efficient and effective suspension systems for practical
vehicles. From a control theorist’s perspective, it provides an interesting case
study in the use of “practical” knowledge to allow “better” performance than
predicted by theoretically optimal linear controllers.
A brief history of active suspension will be introduced. Peter Wright, David
Williams, and others at Lotus developed their Lotus modal control concept. In a
parallel effort, Dean Karnopp presented the notion of inertial (Skyhook)
damping. These concepts will be compared, the combination of these two
distinctly different efforts will be discussed, and eventual vehicle results
presented.
Most of the contemporary literature treated active suspension as a theoretical
vibration isolation problem, but handling improvements from active suspension
were even more impressive. Handling and actual hardware considerations motivated
a confluence of both primary approaches. This innovative implementation of a
control algorithm preserving features of both Lotus modal control and inertial
damping is discussed, and compared with theoretical optimal controllers.
Finally, a surprising fundamental performance limit of the modal inertial
damping algorithm is discussed, and a solution presented.
