Active suspension and skyhook damping

[ttx40]

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.

 

[GregLocock]

Skyhook damping uses the ground (or the sky) as the reference for one end of the damper. Conventional dampers have the wheel at one end and the body at the other, so the relative velocity of the two is the control variable.



Cheers

Greg Locock


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[JackAction]

http://en.volkswagen.com/en/innovation-and-technology/technical-glossary/skyhook-daempfung.html[/URL]]A skyhook damping system has sensors which register the acceleration of the body and wheels and equalise them by adjustment of the dampers. It adapts to the driving conditions and minimises any roll and pitch tendencies.

The suspension can therefore cope with any pothole and any bump on the road surface.

These outstanding suspension characteristics are comparable with a vehicle body suspended on an actual "skyhook".

 

[sbozy25]

I have always been told that a truly fully active suspension has yet to be created. The experts I have spoken with say the theory is there, but in practice, everyone is just selling a really high quality semi-active design....

Cabbages, knickers, It hasn't got A BEAK!

 

[GregLocock]

So in what way were the various Lotus systems /not/ fully active?

Cheers

Greg Locock


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[patprimmer]

Just to lighten things up, I would have thought a Skyhook suspension might create a Horror Movie.

You probably need to be old and Aussie to get it though.

Regards
Pat
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[GregLocock]

Only the very high performance active systems operate at typical wheelhop frequencies, and I suspect they don't try to damp them.

That's probably not what you meant.

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Greg Locock


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[AeroFC]

Magnetic dampers are about as individually adjustable as it gets.

 

[GregLocock]

...except for the various fully active systems. Has this forum suddenly turned into "Automotive suspensions for people who learned everything they know by browsing catalogs at Super Cheap Autos"?

Cheers

Greg Locock


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[sbozy25]

I actually have an email between myself and a good friend who is a vehicle dynamics professor at VT. Here is a paste of his response to me on active vs semi active


Active Suspension: Any suspension that can move the two ends of the suspension independent of the relative velocity or displacement across it. The most obvious example is when we have a hydraulic actuator that is installed in place of the damper and can push against both sprung and unsprung masses independent of the relative velocity across them. Obviously a passive shock will not be able to do so.

Semiactive Suspension: A real-time adjustable suspension that generates a force that is a function of the relative velocity or displacement (or both) across the damper. Most dampers (or suspensions) in use currently fall in this category. Although we are able to change the damper setting, ultimately the force across the damper is governed by the relative motion of the sprung and unsprung masses. Unfortunately, the term "active" is often misused for this class of systems, mostly as a marketing gimmick. I have heard companies saying that the term "semiactive" sounds like something is not complete (or half-baked)! They play with the word "active"and call their system by that name because they can "actively" (read that as in real time) adjust the damping characteristics.

When I think of these systems (or any other), I like to boil them down to the first energy principles. If we can add energy to the system in a meaningful way (such as a having a hydraulic actuator) then it is an active system. All others are semiactive!


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[GregLocock]

And by your profs definition both the Lotus and (I think) Bose systems are fully active.

I'm actually not that fussed if the basic weight bearing is via normal springs, that does not compromise the 'active' nature

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Greg Locock


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[gt6racer2]

Typically for me, the difference between semi active and active is the ability of the active suspension to generate a (significant) force in the same direction at its motion - ie operate in all four quadrants of the force/velocity plot. I'd use the term "fully active" when describing a system where it also supports the vehicle static weight. ( The latter case is really not so important since the rate(stiffness) of the support spring can be very low if the suspension is active ).
On skyhook control, this is just one criteria of a suspension control strategy - whereby (as noted above) we try to keep the car body still vertically and rotationally (pitch, roll) with respect to an imaginary "sky-hook". Sky hook must act in conjunction with other control criteria such as wheel vibration control (as Greg said, hard to do unless switching is very fast), end stop control (suspension travel is finite, and can hence conflict with skyhook) and other "overrides" such a braking, steering or other driver input.
To my mind, (and I'll admit some bias) the cleverest idea to get into the 3rd and 4th quadrants is the technology named as Vehicle Dynamics International magazine "Innovation of the Year" ACOCAR.

http://viewer.zmags.com/publication/b41bd0bd#/b41bd0bd/20

 

[ttx40]

Thanks for the info everyone...Gt6racer2 I'm curious about your definition of an active system in particular. I'll have to think about that further to understand fully, but thank you for posting that.
It seems the heart of the whole thing is the control laws that are used; I know the Lotus system was designed around suspension modal displacements, I'm sure there are other methods. If anyone has information regarding this that they can share, that would be really interesting.

Thanks again.

 

[RigTest]

gt6racer: we must label our quadrants differently, as I would refer to the 2nd and 4th quadrants (Force-Velocity) as the active-only regions (energy input) and the 1st and 3rd quadrants as the passive regions (energy dissipated).

These 'semi-active' systems where damper or spring characteristics can be rapidly changed I prefer to call 'adaptive'. They are still passive elements that can only dissipate or store/release energy, not supply it; this is equivalent to a component producing force against (passive) or with (active) relative velocity. Active systems have certainly been created and used as mentioned above.

Groundhook is a similar control strategy to Skyhook except the unsprung mass is "connected" to the sky/ground rather than the sprung. This strategy seeks to minimize motion of the unsprung mass as it is disturbed. A hybrid combination of Skyhook and Groundhook can be implemented as well.

 

[fast4door]

Bose abandoned their efforts into active suspensions because of cost, packaging, etc. Now they're making semi-truck seats, and they also parlayed the linear actuators into materials testing. The truck seats would be the bomb - I took a ride in a semi truck passenger seat (rigidly bolted to the cab floor) that felt like I was getting bucked off a horse for 8 hours.

 

[ttx40]

I assume the quadrants being referred to are those on a damper f vs. v diagram? I'm not following the energy input and energy dissipated reference: I thought the area contained in the trace is work done or energy dissipated by the damper....

 

[gt6racer2]

Rigtest- yes- we're on the same page ( just different quadrants of it !).
ttx - the basic function of a damper is to generate a force that opposes a motion. If the damper rod moves in, the force will always be pushing out, and when it moves out, the force will always be pushing in. ( neglecting gas pressure, compliances etc . ). If you look at this relationship in a force velocity plot, you will see that it can only describe points in two quadrants that are diagonally opposed. The other two quadrants would require a force in the same direction as the motion, which can only be achieved if we add energy.

 

[ttx40]

Ugh, how embarassing...thank you for the explanation gt6r2.

 

[wiloberlies]

That's a neat way of thinking about the relation/difference between active and passive suspensions. Just to check my understanding, I'm accustomed to the displacement vs. force diagram having four quadrants: the first is the upper left, continuing in clockwise fashion (such that 4 is directly below 1).
Q1 has the shaft moving in, relative to the damper body, so the force is obviously 'pushing out'. I guess in order to preserve the sequence in gt6r2's post above, then yes, it holds true when applied to Q3, not Q2. Is this the logic behind Q2 and Q4 being the active, or energy input regions?

 

[GregLocock]

I think you've got it right. A conventional unpowered damper can, crudely, only absorb power, so it will produce a force resisting motion at all times. That is, in a convenient frame of reference, positive velocity*positive force=positive power absorbed, and negative force * negative velocity=positive power absorbed,

Obviously if you reverse the sign of just the force or just the velocity, you get negative power, ie the damper is adding power to the rest of the system.

Cheers

Greg Locock


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