A quick resume of the Lotus Active Suspension System>
Lets start with xpensive's proposal:
xpensive wrote:If you imagine an infinitely fast and corner-independent servo-assisted hydraulic suspension with feed-back, or "is-value", from four ground-clearance sensors in each corner of the car, then the chassis flex wouldn't matter much unless the system is programmed for keeping the same ground-clearance in all the corners.
As I noted above, that is a good description of the Lotus System. I only need to add four coil over springs (used for fall back in case of a failure and for sharing the load with the actuators to reduce power consumption).
Now, each suspension actuator was controlled by an EHSV (electro-hydraulic servo valve) whose function was to control the actuator to move with a linear velocity proportional to the current driving the valve. It is fair to state that the drive current/velocity was not exactly invariant, but it was close enough not to matter over a frequency range of, perhaps, 100 Hz.
With that information, it is possible to imagine an accelerometer attached to the upright (the wheel side of the actuator) measuring the vertical component of upright acceleration. If the signal generated by the accelerometer is integrated, to produce a signal proportional to absolute velocity, and that velocity is scaled to generate current that is used to drive the "local" EHSV, then the actuator can be made to faithfully (more or less) move to exactly reflect the velocity of the upright. In that case, the actuator will transmit no additional load to the sprung mass, regardless of upright motion (with caveats). The sprung mass will then remain stationary in space. That should satisfy xpensive's proposal (again with caveats), and the sprung mass could be made of cheese, or anything else that springs to mind.
That is not a bad starting point for thinking about Active Suspension. It could be made to work in that way, but it would not form a realistic suspension solution for various reasons:
1. It would extract no energy from the tyre/unsprung mass mode, which would therefore be severely underdamped.
2. It would dissipate no energy from unsprung mass motion.
3. It would quickly run out of actuator travel (drive straight into hillsides, etc.)
4. It would do nothing to control load distribution (such as that mentioned by Tim Wright).
It is an interesting thought experiment, however, because it demonstrates (hopefully, & again with caveats) that "preview" is not absolutely necessary for a good active suspension.