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very active suspension MB style


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#1 mariner

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Posted 14 November 2011 - 08:50

A brief description of the suspension for the next MB S class.


"The big news, however, is the appearance of Mercedes’ heavily touted Magic Ride Control system. First unveiled on the company’s F700 concept car, it uses cameras to detect the state of the road and alter the tuning of the springs and damping to provide what the German car maker’s engineers describe as “optimal ride comfort”. A development of the existing S-class’s Active Body Control system, it can alter the characteristics of each individual wheel, through a fast-reacting air spring system, to provide constant ride height and virtually no lean in corners."

A more detailed , and highly complementary description is here

http://www.caranddri...l-system-driven

The camera resolution claimed is amazing 10mm at 14 metres ahead.

Question, folks - do you think it will be fundementally better than full ( Lotus) active as it has full "sky hook" damping?

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#2 Greg Locock

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Posted 14 November 2011 - 11:22

The camera resolution claimed is amazing 10mm at 14 metres ahead.

Question, folks - do you think it will be fundementally better than full ( Lotus) active as it has full "sky hook" damping?

I think it is a big enhancement, anticipation means you can provide the minimum necessary force for a long time, rather than a big push NOW to get the wheel where it needs to be. So for comfort at least, yes.



#3 maxay1

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Posted 14 November 2011 - 11:47

At the expense of hub (wheel hop) control?

#4 cheapracer

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Posted 14 November 2011 - 12:02

As for anti roll surely GPS is now accurate enough to predict a left hander or right hander coming up and prepare the suspension considering speed etc?

#5 Tony Matthews

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Posted 14 November 2011 - 12:09

James Allington, my one-time employer, had a dachshund that could predict left- and right-hand bends and would lean left or right before actually necessary...

#6 DaveW

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Posted 14 November 2011 - 14:04

A development of the existing S-class’s Active Body Control system, it can alter the characteristics of each individual wheel, through a fast-reacting air spring system, to provide constant ride height and virtually no lean in corners."


My best guess, after a little research, is the the "ABS" comprises a series hydraulic actuator placed at each "corner" in series with a conventional damper/spring, rather similar to the system designed by Mumford and trialed on a Jaguar research vehicle in 1980's (sorry I haven't bothered to look for a reference to that). Such systems are attractive, but leave a number of issues to be resolved. These include how to make good the loss in (potential) suspension stroke, and how to prevent the controlled system "fighting" the passive system.

Passenger "comfort" for symmetric inputs is (at least potentially) very dependent upon available suspension stroke. I guess that "preview" would be a good way of ensuring that the actuators are in place for road ahead, thereby reducing stroke consumption (courtesy of a few geometric calcs - made slightly more difficult when the cameras are mounted on the sprung mass). It would also help to "decouple" the active & passive elements.

I haven't mentioned roll, pitch or warp control. The "ABS" has the potential to improve those responses, of course.

My research suggested the "ABS" iteration rate is around 100 Hz. This would imply a system bandwidth of not much more than 10 Hz. belying, I think, the rather more esoteric claims of the referenced article.

As an afterthought, the system appears, logically, to have more in common with the Citroen hydro-pneumatic system than the Lotus system. I can't speak for Tony's dachshund.

Edited by DaveW, 14 November 2011 - 14:13.


#7 Tony Matthews

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Posted 14 November 2011 - 15:22

I can't speak for Tony's dachshund.

I would have thought that a camera system would find it hard to differentiate between a gentle slope height x and a steep slope of the same height, even with a stereo system the width of the car. particularly with changing light and weather conditions. However, face recognition on mobile phone cameras is pretty clever. I think radar has been tried in the past.

The dachshund travelled on my lap, front paws on my knees, so doubly unsprung...

#8 DaveW

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Posted 14 November 2011 - 15:56

I would have thought that a camera system would find it hard to differentiate between a gentle slope height x and a steep slope of the same height.


Also between a brick, tin can, & a paper bag....


#9 mariner

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Posted 14 November 2011 - 16:16

Thank you for all the comments.

Like I keep saying I am not an enginner hence dumb questions/comments.

One thing that went through my mind on this is at a very simple level. The Lotus full active was likened ( accurate or not) to a skier moving his/her legs up and down in response to the varying loads so as to keep the motion at head height roughly level and minimise the variations in shock loading on the legs and skis ( i.e the tyres). This sort of fits in with Dr Fred Lanchester's original idea of setting the spring frequencies so as the head replicated the motion of a walking man i.e gently up and down but no fore/aft pich.

I see the Lotus active as replicating the action of a walking person by responding very fast to perceived loads whereas the MB system requires much more processing to arrive at the same effect. Therefore it is only superior if the reaction time of full active cannot be made fast enough to give level ride/no roll. I don't know if active plus latest electronics is or is not too slow.

I understand Greg's point about more time but I would have thought that was processing time not suspension movement time for most of the 14 metre forward view as ( processing lag aside ) you can't actually move the suspension up for a bump until it has got to that bump ( if that makes sense)


#10 Greg Locock

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Posted 14 November 2011 - 18:41

That's the advantage of skyhook damping, you lift the wheel up over the bump, instead of the whole car.

Incidentally Tony's point about the field of view of the camera is a good one as well, for low frequency bumps it'd be fine, but for potholes the extent of the bad news is only revealed rather late.


#11 DaveW

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Posted 14 November 2011 - 21:08

An hydraulic actuator (neglecting time delays), responds with a velocity proportional to the current used to drive the servo valve. The output of a vertical accelerometer mounted on the upright, integrated (velocity), scaled appropriately, can be used to drive an actuator (replacing the spring/damper combination) to have a vertical closing velocity that exactly matches the vertical velocity of the upright. The result is that any continuous input with an amplitude less than the stroke of the actuator can be negotiated with no change in force applied to the sprung mass (Greg, I'm not sure that would call that "skyhook" damping).

However, drive such a set-up at a hump-backed bridge & the ride is destroyed once the actuator runs out of stroke. This is the situation that requires preview, or a self-centering algorithm, &/or simulated springs.

All very idealized (& not something one would want do to), but is does demonstrate (hopefully) that preview is not actually required for "perfect" ride over limited amplitude continuous inputs. It was, incidentally, possible to demonstrate the phenomenon with the Lotus Active suspension.

Applying those thoughts to the MB system, preview allows the adoption of lower rate springs (& dampers) than would be possible otherwise, & that will (or should) improve ride.


Edited by DaveW, 14 November 2011 - 21:43.


#12 MatsNorway

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Posted 15 November 2011 - 20:04

The camera resolution claimed is amazing 10mm at 14 metres ahead.


Must be great in snow... Camera goes either Oh shit! can`t see!. snow on the bumper.. go to passive mode. or AaaaAAawaHAH big ****ing bump better swallow it! when its actually just a puddle of snow...

#13 pugfan

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Posted 15 November 2011 - 22:25

An hydraulic actuator (neglecting time delays), responds with a velocity proportional to the current used to drive the servo valve. The output of a vertical accelerometer mounted on the upright, integrated (velocity), scaled appropriately, can be used to drive an actuator (replacing the spring/damper combination) to have a vertical closing velocity that exactly matches the vertical velocity of the upright. The result is that any continuous input with an amplitude less than the stroke of the actuator can be negotiated with no change in force applied to the sprung mass (Greg, I'm not sure that would call that "skyhook" damping)


This is a little off-topic but on a thread a while ago, I think it was you DaveW that mentioned on an active Lotus F1 (I think) that initially the system did not have warp stiffness and that you persuaded to put it in and I've been meaning to ask for a while if you could expand on that?


#14 Catalina Park

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Posted 16 November 2011 - 07:15

James Allington, my one-time employer, had a dachshund that could predict left- and right-hand bends and would lean left or right before actually necessary...

I had a German Shepard that used to duck when I was driving under bridges.

#15 DaveW

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Posted 16 November 2011 - 07:21

This is a little off-topic but on a thread a while ago, I think it was you DaveW that mentioned on an active Lotus F1 (I think) that initially the system did not have warp stiffness and that you persuaded to put it in and I've been meaning to ask for a while if you could expand on that?


Here is the post you refer to, I think. The warp mode is not a "degree of freedom" in quite the same way as the other modes (has compliance, but no mass) - that is my only excuse for omitted it. The control hardware was analogue (No PC's or DSP's were available at the time) so the structure of the control loops was physically hard wired. It followed that adding the warp freedom required additional circuitry, a soldering iron & a day or two's checking.

The Lotus 92 system was also analogue, but this time we replaced the scaling potentiometers with slightly more sanitary multiplying DAC's driven by a low powered 8 bit processor. Later in the year, the first TI DSP became available, & all later active systems were controlled digitally.

I can't think of much more to add.



#16 Tony Matthews

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Posted 16 November 2011 - 07:53

I had a German Shepard that used to duck when I was driving under bridges.

You should have let him ride inside, not on the roof rack...

#17 pugfan

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Posted 16 November 2011 - 23:04

I can't think of much more to add.


Bruce Maclaurin of RARDE soon pointed out that it might work better if it was included, & he was right


De-coupled suspensions are a favourite thought experiment of mine so would you be able to elaborate on how it worked better?

I can see how pure warp (2 opposite wheels going up, other 2 going down) will have no effect on the sprung mass so, intuitively, not including it would be better for ride and contact patch load variation?


#18 DaveW

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Posted 17 November 2011 - 23:34

De-coupled suspensions are a favourite thought experiment of mine so would you be able to elaborate on how it worked better?

I can see how pure warp (2 opposite wheels going up, other 2 going down) will have no effect on the sprung mass so, intuitively, not including it would be better for ride and contact patch load variation?


Warp deflections allow the wheels to maintain (good) contact with the road surface through camber changes & also control the way turning moment is reacted by the front & rear axles (i.e. allow the lateral balance of the vehicle to be controlled).

Modal suspension algorithms allow much set-up flexibility, including properties that can't easily be achieved passively. It is very easy to control roll & pitch deflections during manoeuvres. Was it better than a more conventional strategy? I don't know, but in the 7 years that I was involved nobody to my knowledge was moved to implement a more conventional alternative.



#19 pugfan

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Posted 18 November 2011 - 05:25

Warp deflections allow the wheels to maintain (good) contact with the road surface through camber changes & also control the way turning moment is reacted by the front & rear axles (i.e. allow the lateral balance of the vehicle to be controlled).


I can see how that would have made a big difference to a high powered rear wheel driven car.

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#20 munks

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Posted 18 November 2011 - 15:19

You should have let him ride inside, not on the roof rack...


Ouch, a Mitt Romney joke from your side of the pond?

#21 Lee Nicolle

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Posted 21 November 2011 - 10:36

And what hapens when the camera is blinded by mud? Or do you only drive on bitumen roads when it is not raining.Or dark. My human brain cannot process all the info when driving, and I cannot process when I cannot see.
Or is this like steam turbine aeroplanes. Or electric vehicles. Theoretically able to be done but totally impractical.
I think MB should employ Tonys dog!

#22 Tony Matthews

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Posted 21 November 2011 - 13:19

My human brain cannot process all the info when driving, and I cannot process when I cannot see.

You don't have to process anything, Lee, the info from the camera/s goes straight to the suspension CPU. As to mud and dark - that's another problem! Active infra-red would sort out the dark, though.

#23 24gerrard

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Posted 21 November 2011 - 13:50

You don't have to process anything, Lee, the info from the camera/s goes straight to the suspension CPU. As to mud and dark - that's another problem! Active infra-red would sort out the dark, though.


I thought Merc owners only had licenses for VMC.

#24 Greg Locock

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Posted 21 November 2011 - 23:01

Jaguar pointed out that if there isn't enough light for the cameras to work then there wasn't enough light for your eyes to work. That was one of their arguments for pursuing vision based systems rather than radar/IR.

#25 Lee Nicolle

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Posted 22 November 2011 - 03:07

Jaguar pointed out that if there isn't enough light for the cameras to work then there wasn't enough light for your eyes to work. That was one of their arguments for pursuing vision based systems rather than radar/IR.

There is frequently not enough light for our eyes to work properly. And when dark and rainy even less. That is why more accidents happen in those conditions.
This winter I found a large hole where the road had washed away on an edge.A metre!
Luckily I was only travelling about 50kmh[80 limit road] and I only got the edge of it. Pitch dark and drizzling and I did not even see it. This on a road I travel semi regularly. Luckily I tend to hog the road centre in those conditions.
I doubt a camera would have seen it either!!