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Active suspension and skyhook damping

Active suspension and skyhook damping

(OP)
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.

RE: Active suspension and skyhook damping

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


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Active suspension and skyhook damping

Quote (http://en.volkswagen.com/en/innovation-and-technol...)

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".

RE: Active suspension and skyhook damping

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!

RE: Active suspension and skyhook damping

Magnetic dampers are about as individually adjustable as it gets.

RE: Active suspension and skyhook damping

...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


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Active suspension and skyhook damping

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!

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

RE: Active suspension and skyhook damping

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

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Active suspension and skyhook damping

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#/b41b...

RE: Active suspension and skyhook damping

(OP)
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.

RE: Active suspension and skyhook damping

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.

RE: Active suspension and skyhook damping

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.

RE: Active suspension and skyhook damping

(OP)
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....

RE: Active suspension and skyhook damping

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.

RE: Active suspension and skyhook damping

(OP)
Ugh, how embarassing...thank you for the explanation gt6r2.

RE: Active suspension and skyhook damping

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 [i]sequence[i] 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?


RE: Active suspension and skyhook damping

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


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Active suspension and skyhook damping

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?

RE: Active suspension and skyhook damping

fast4 - which McLaren ?

RE: Active suspension and skyhook damping

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.

RE: Active suspension and skyhook damping

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Active suspension and skyhook damping

Quote:

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
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RE: Active suspension and skyhook damping

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......)

RE: Active suspension and skyhook damping

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".

RE: Active suspension and skyhook damping

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Active suspension and skyhook damping

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.

RE: Active suspension and skyhook damping

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.

RE: Active suspension and skyhook damping

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".

RE: Active suspension and skyhook damping

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

RE: Active suspension and skyhook damping

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Active suspension and skyhook damping

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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