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How do EBD algorithms work?


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#1 giskard

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Posted 11 March 2013 - 15:27

Hi,

Can someone tell me how Electronic Brake Distribution works vs a mechanical prop valve? (In a street car)

I understand that the mathematically ideal rear vs. front proportion is a sort of lay-down parabola, and that a mech. prop valve approximates that with 2 linear segments. I also know that with EBD, the prop valve is gone, and the ABS valves are used to do the proportioning.

My specific Q's:

- Does the EBD algorithm then target a specific proportioning curve?
--- or does it do something silly like wait for the fronts to start to slip then add rear line pressure?

- (I understand that EBD may have a means of changing the proportioning depending on inputs such as rear suspension deflection, yaw rate, steering wheel angle, or other inputs from the stability control system)

- Are ABS valves capable of reducing (or adding) line pressure smoothly to yield a given target curve without pulsation?
--- or does it necessarily pulsate?


P.S. I forgot to select "email when replied to". How do I change it after posting?

Edited by giskard, 11 March 2013 - 15:28.


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#2 giskard

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Posted 11 March 2013 - 20:09

Thanks for the curves. That curve suggests that EBD is able to follow the "ideal" curve. So in addition to my original questions:

- Is that curve baked in?

Edited by giskard, 11 March 2013 - 20:19.


#3 Greg Locock

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Posted 11 March 2013 - 22:03

I know a bit more about ESC than EBD, but given they are programmed by the same people, I think it'll work like this:

The important thing about EBD is that is it working in the linear range so sensing lockup of a wheel is far too late.

So, they start with a simple model of the car, cgz, wb, track, cgx. They might even build a weight model of the car based on fuel level, seat belts fastened, and any height or pitch sensors that the car has. Then they build a table of 'ideal' proportioning based on that. Then they use feedback from the ABS tone wheels to check that all four wheels are decelerating at the same rate. As the longitudinal g increases then the ABS can take over , btu again the first guess is derived from the simple vehicle model.



#4 giskard

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Posted 12 March 2013 - 03:33

Thanks. So my only unanswered question is:

- Are ABS valves capable of reducing (or adding) line pressure smoothly to yield a given target curve without pulsation?
--- or does it necessarily pulsate?

#5 Greg Locock

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Posted 12 March 2013 - 03:52

Thanks. So my only unanswered question is:

- Are ABS valves capable of reducing (or adding) line pressure smoothly to yield a given target curve without pulsation?
--- or does it necessarily pulsate?


I'm guessing no pulsation is necessary. When my ESC kicks in it is so gentle as to be unnoticable, for example. That mey be because of 2 reasons (a) I don't have my foot on the brake pedal at the time, and (b) the interventions with ESC are probably fairly gentle, say 0.2g equivalent on one wheel, ie 0.05 g in a total vehicle sense.

#6 murpia

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Posted 12 March 2013 - 22:02

- Are ABS valves capable of reducing (or adding) line pressure smoothly to yield a given target curve without pulsation?
--- or does it necessarily pulsate?

I looked into this briefly recently, using some of the VW group 'self study' training that can be found online. My understanding is that some pressure can be reduced without pulsation, but beyond a certain point the displaced fluid must be returned to the pressure side of the ABS valves, which is done via a pump (causing the pulsation).

So the answer to your query is 'it depends'. Having driven a car with EBD for many years I never felt any strange pedal feel unless during an ABS event, so I suspect the EBD authority is below that which requires the return of fluid. Whether this means a different hydraulic circuit in the ABS module to the self-study guides, I don't know.

Regards, Ian

#7 giskard

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Posted 15 March 2013 - 15:07

OK so could someone corroborate:

- EBD only reduces rear brake line pressure and leaves front pressure unmodified (up to the point where ABS kicks in of course)
- Rear brake line pressure reduction curve is baked into the EBD algorithm (and possibly changing according to inputs such as rear static ride height)
- Said pressure reduction up to near max braking (i.e. no tire slippage) can be performed by the ABS valves, without pulsation


#8 mariner

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Posted 17 March 2013 - 11:21

As an historical reference the DS Citroen had fully dynamic F/R proportional braking in 1955.

It used a roller and balance bar assembly under the foot pedal which adjusted the F/R ratio continously according to the vehicle weight distribution. The distribution was known from the ride height sensors in the hydraulic suspension sytem.

You could also remove the rear wings/fenders by using the jack handle but it was an odd car!

#9 Tony Matthews

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Posted 17 March 2013 - 13:25

You could also remove the rear wings/fenders by using the jack handle but it was an odd car!

A very interesting car, and as a side note, I saw one in King's road, Chelsea, body and all brightwork painted matt black. It looked sensational, and that was in 1973/4, and matt/satin finishes are regarded as daringly new now...

#10 desmo

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Posted 17 March 2013 - 15:59

My grandparents had a DS in the late sixties they'd bought in France and brought back, it really was a fabulous car in many ways, certainly advanced for its day. I can however remember the police not being happy about the typically French yellow headlights. The ride was pluuuuush and I imagine the aero drag was competitive with anything today. This was back when Citroën built cars that were actually substantively different than the norm.

#11 Tony Matthews

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Posted 17 March 2013 - 19:50

and a photo of the brake pedal assembly as mentioned by Mariner:

Posted Image

At first glance I thought it was a hydraulic clay-pigeon thrower...

#12 desmo

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Posted 18 March 2013 - 01:54

Excellent. I noticed Magrès, the man behind the electrohydraulic wizardry, was hired as an unskilled worker and trained within the company. Can you even imagine a company investing in worker training to that degree today? Modern corporations want the workers themselves and the government to pay the bill for their training so those workers become disposable commodities as the companies have nothing substantive invested in them.

#13 rory57

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Posted 18 March 2013 - 10:17

Magrès, the man behind the electrohydraulic wizardry,


Nothing electro about Citroen Hydraulics until Activa etc much later.
I have never driven a DS but the brakes of Citroens GS,CX and BX have no peers in my experience.
Such a shame that once great company is reduced to flair-free, progress-free junk like Saxo, C2 etc.

#14 Magoo

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Posted 18 March 2013 - 13:45

Excellent. I noticed Magrès, the man behind the electrohydraulic wizardry, was hired as an unskilled worker and trained within the company. Can you even imagine a company investing in worker training to that degree today? Modern corporations want the workers themselves and the government to pay the bill for their training so those workers become disposable commodities as the companies have nothing substantive invested in them.


Paul Mages is one of my heroes.

Another: John Wandersee. Not far into his campaign to build a mass volume, high-quality, low-price car, Henry Ford perceived that advanced steel alloys were crucial. Ford, Wills, and a few others were standing on the shop floor pondering how they could obtain one of the world's leading metallurgists for their own use when Ford pointed to a guy sweeping the floor nearby, one of Ford's first employees, and said, "Let's make Wandersee a metallurgist." And they did. Wandersee became Ford's chief metallurgist, one of the sharpest in the world, and was one of the company's key technical people for decades.

The story is often misunderstood or mistold to indicate that Ford had a disdain of formal knowledge. More accurately, he had a disdain of experts. Of the multiple innovations that made the Model T possible, Ford once said, "At some point we found it necessary to dispense with the services of experts."


#15 desmo

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Posted 18 March 2013 - 13:52

Nothing electro about Citroen Hydraulics until Activa etc much later.


Of course. I knew that.




#16 giskard

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Posted 07 November 2013 - 20:20

Why are dual-master setups with balance bars used in club racer type cars?

AFAICT they don't have a breakpoint in their rear pressure characteristic like prop valves do (left diagram below).

Wouldn't that make them inferior to a properly set up proportioning valve?

 

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