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Active suspension and skyhook damping

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ttx40

Automotive
Apr 27, 2012
12
I've become really interested in fully active suspension systems lately; it seems like a good corollary to learning about conventional passive suspension. Now, I don't have a background in control systems or anything like that, so my current understanding is almost nil. One concept I keep coming across is skyhook damping, where a damper (that is attached to the sprung mass?) is fixed at the other end to the 'sky', or inertial space. Could anyone tell me what that really means, or rather, how it functions? I have had a look at a few papers, but most talk about LQR's and different control strategies.

Thanks all.
 
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Just saw the Top Gear review of the new McLaren. It's got magneto-adjustable dampers and a computer-controlled roll bar. Near as I can tell it still has good old springs in the mix. At what point do we consider something like this "fully" active?
 
I would think fast 4 door is referring to the MP12, though I thought this ran some variant of the Kinetic system...I may be wrong though. I would NOT characterize that as an active system as described by fast4door.
 
The marketing definition of active is anything they can get away with,the engineering definition, as discussed above is that the system can add (reasonably large) amounts of power to move things around as necessary. You could argue that a simple height adjusting system is sort of active until you put in some lower limits on bandwidth or power.

So far as I know nobody has actually released a high bandwidth high power active system as a production car option. There have been various low bandwidth low power systems, which obviously did not set the world alight, and the more advanced Kinetic system lends itself to incremental steps in that direction, since the shocks are replaced by rams, and so the remote valve block could include a pump. Kinetic were at least considering their options when they were bought out.

Automatically controlled variable valving in shocks and variable stiffness sta bars aren't active, they are adjustable passive, which is usually classified as "semi active" or perhaps more sensibly "adaptive".

Incidentally a typical production cars's shock absorbers can absorb up to 1.5kW quite easily, so any so called active system that can't generate several horsepower is not even playing the main game.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
There have been various low bandwidth low power systems, which obviously did not set the world alight

This sounds like the Citroen D series hydro pneumatic system. The car would roll into a corner and have self levelled by about 1/2 way through it, way back in 1955.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
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I'm glad Pat brought up the DS19, which I'm sure everyone agrees fits the definition of "active". That was fifty eight years ago, and totally without benefit of silicon engineering. My chapeau doffs to M. LeFebre yet again.

With respect to passive systems, the problem amounts to intelligent energy storage, something that combined hydraulic/pneumatic systems can do rather well. (my chapeau......)
 
I've been following this discussion with interest. I am only slightly familiar with current active, semi-active and "adjustable passive" systems, but I'd like to throw couple of ideas into the mix. First, shock absorbers or dampers, normally thought of as passive components have been proposed as energy generating systems to power whatever, including suspensions. Gas shocks are or can be hydraulic pumps, accumulators, and rams -depending on the control of the valving.
The power of a system based on this source of energy would be low, but I disagree that you can draw a line based on power and/or bandwidth to make a definition of active. An ideal could be a springless, damperless system similar to the systems in Formula 1 in the '90s, but something less than that could still be called active. If anything it ought to depend on some standard of system performance.
Additionally, in racing they also outlawed multiply articulated suspension systems and moveable pickup points, active or passive, that would vary wheel camber, chassis offset and other parameters. Perhaps variable control of any of these could be considered parts of active suspension. After all, it is not just the vertical motion of chassis and wheels that are defined by the "suspension".
 
Yup, you can write your own definition of any word or phrase you like. The acid test is, will other people use it?


If your suspension has variable geometry, why call it active? Why not call it, just for grins, a variable geometry suspension?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
Greg, I recognize that conventions are important so that we all know what we are talking about. The engineering definition you gave is rather vague. Is there an official SAE definition?

I think it is also important to make conventional definitions conceptual and not only descriptions of observed systems. That way we avoid arbitrary, "coloquial" defintions or definitions that are only contemporary or applicable only to a small subset or are subject to quick obsolescence. I'm not trying to define anything, but you have suggested a modification to the (presumed) definition to include a lower limit on power. I think this is particularly arbitrary what with the vast range of power needed for very broad ranges of vehicle weights, vehicle speed regimes and terrain roughness -from 35mph electric grocery getters to 200mph sports cars to pickups and heavy SUVs that may travel dirt roads. Also, power levels should be expected to drop, perhaps drastically, with improvements in technology. IMHO, we should not take what is a basic figure of merit -low power requirement- and use it as a disqualifier in the definition.

Also, I wanted to point out that "suspension" is normally taken to be the system that holds the wheel to the chassis and controls its motion. There are 6 degrees of freedom of movement, in some systems controlled each by a distinct suspension link. The Z-axis, normally controlled by spring and shock, is only one degree. "Active suspension", as you seem to define it, is only Z-axis control. But "active suspension" that controls ride AND handling would HAVE to include at least some of the other degrees of freedom. It sounds like systems that involve active control of the other suspension functions than the Z-axis are excluded in your definition.
 
Airpower, I think you've missed the context of the O.P. We are discussing "Active suspension and skyhook damping". I'm sure it can be argued that X and Y skyhooks are possible but given the common desire of the driver to change direction and speed, they are probably not the most useful strategies.
 
gt6racer2, re Skyhook damping you are right, part of my post is not relevant, but the minimum power issue is relevant as it relates to the definition of "active suspension".
 
To me, an active system must be able to add significant energy to the system; enough to pull a wheel up over an irregularity in the road surface so as to maintain a consistent contact patch load. This has obvious limitations (suspension stroke being primary among them).
As far as handling goes, tire forces are a direct function of the load they support. As such, handling would certainly be affected by the action of an active system.
My question, and it's probably a stupid one, is one of tires. Say a fully active system is produced for a road car. What would be the effect of making drastic changes to tire cornering stiffness, and/or vertical rate. I realize vertical tire rate is a function of inflation pressure, camber, etc., but I am curious about cornering stiffness. I suppose the system could always rely accelerometer info.....

Thanks for any thoughts, corrections.

Wil
 
I have never seen a general formal definition of active systems. For suspensions there is a 10 page discussion in Formula 1 Technology by Peter Wright SAE R230. But it is not definitive.

One of the issues with active suspensions, and modern ESC systems, is that they can mask non-linear behaviour of the tire, so to some extent they can and do compensate for changes in cornering stiffness and so on.

For instance if one tire were going flat an ESC system would see the higher slip angles, and would brake the rear wheels (typically) to bring the vehicle yaw velocity back into line with the steering wheel angle.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
wiloberlies, as far as I know, all active systems that are computer controlled are globally closed-loop. They are feedback systems in which all elements that affect behavior are inside the loop. That would include tires. A feedback system will seek to produce some reference output regardless of what goes on inside the loop, so changes in components, wear, etc will have little effect on the output behavior. The limits of effectiveness for feedback systems depend primarily on the amount of feedback which in turn depends primarily on the gain (power in this discussion) and bandwidth of the system in open-loop (before feedback is applied).
So tires with stiffer construction, quicker response, harder ride and more grip than the designed for tires will tend to have those characteristics effectively neutralized and not affecting the output, probably except for the higher grip, -unless the computer looks for superior tire characteristics and exploits them.
This is not different than engine control which is a closed-loop system under normal operation, so that spark plug wear does not effect performance until they are far gone. And like the engine computer, the suspension computer will be reprogrammable. I can envision people selling handling chips for active suspensions to go with their wheel-tire packages.
 
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