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Old 19th August 2015, 09:34   #601
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Default Re: The ABS discussion thread

I have labelled the basic components of an ABS / ESC unit.

There seems to be some confusion regarding role of pump / motor.

Motor drives the pump which returns fluid to the reservoir.

Image courtesy: Google / Bosch China

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Viju
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Old 19th August 2015, 19:37   #602
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Default Re: The ABS discussion thread

Quote:
Originally Posted by DerAlte View Post
The ABS oil pump. Keeps the pressure up when the valves act, yes. You are right, without it the pedal will gradually go down. The pump works only when there is positive pressure on the pedal, not all the time.

It is also associated with EBD (it is assumed that ladies can't stomp on the brake pedal hard enough) to improve line pressure when it is required - in this case brake pedal is pressed 'fast'.
Sorry for back-to-back posts as I am past my edit-time.

Der Alte

You seem to have misunderstood the role of the pump in the ABS hydraulic unit.

ABS functioning cycle is quiet simple.
It is majorly* divided into:

* - I say majorly divided because there are many transition phases between the major phases which entirely depends on the software and adaptation of each ABS maker. For obvious reasons, I cannot get into those in a public forum as I love my job and would like to keep it for a long time to come.

1. Apply:
Pressure application - pressure applied on wheel brake is the pressure applied by the driver. No additional pressure is applied by ABS at any point of time.

2. Hold:
When this pressure is too much (causing wheel slip beyond a preset threshold), the solenoid valve in the inlet circuit (of that particular wheel) closes to avoid any increase in pressure.

Inlet circuit is from Master Cylinder Reservoir ---> Wheel Cylinder.

3. Release:
If even after holding the pressure, the wheel continues to go into slip, the solenoid valve in the outlet circuit is opened to enable return of brake fluid from the Wheel Cylinder (to a Low Pressure Accumulator). Since the pressure at Wheel Cylinder is higher than Accumulator, the fluid flows on its own to the accumulator.

Depending on the wheel slip condition, Apply - Release - Hold cycles are executed in varying sequence till that particular wheel is within acceptable slip behaviour.

The fluid from the accumulator needs to be pumped back to the reservoir / inlet circuit. This is the ONLY function of the pump in an ABS unit. To drive this (reciprocating) pump, there is an electrical motor.

In a passenger car, there are four wheels, hence four brake circuits; one inlet valve and one outlet valve per circuit, and therefore total 8 valves.

Since two of the circuits are usually connected (X or H configuration), there are only two pumps (1 pump for 2 wheels) which is run by a single electric motor.

EBD:

EBD is Electronic Brakeforce Distribution and it is only a software function which substitutes a Proportioning Valve found in non ABS-equipped vehicles (some older vehicles like Estilo used to come with ABS without EBD, but proportioning valve).

Proportioning valve as you would know is used to reduce the brake pressure to the rear wheels since if equal pressure is applied to front and rear axles, the rear wheels will lock first (due to load transfer during braking) and the car will go into a spin. Front wheel lock is always preferred to rear wheel lock while designing passenger car brakes.

Interestingly, the pump motor has no function at all during functioning of EBD. EBD usually works just by Hold cycle and very rarely goes to Reduce cycle. Therefore, no fluid is returned to the Accumulator and therefore, the pump is not required to operate. EBD works purely by actuation of the valves (99% of the time).

Hence, even if the ABS motor fails, only the ABS warning lamp will come on and Brake Warning Lamp will not come on (meaning base brakes will continue to function properly). If valve failure occurs, base brake fails.

What you are talking about is EBA or HBA (depending on supplier) meaning Electronic / Hydraulic Brake Assist. It replaces the Mechanical Brake Assist provided in vacuum boosters (e.g. Swift Diesel). EBA / HBA is a function of ESP (Electronic Stability Program) and not ABS.

Quote:
Originally Posted by im_srini View Post
Besides, if the only pressurizing mechanism is the foot, then each time the ABS module releases pressure, won't the pedal too depress a bit ?
im_srini

Pedal gets depressed if and when the driver presses the pedal. When the fluid is released, basically the circuit through which the fluid passes to the wheel cylinder (which is your input) is temporarily closed. So you feel a pressure against your foot. Since this is done several times a second, it is felt in the form of a pulsation.

The quantity of fluid released is so less (the total capacity of the low pressure accumulator itself is a few millilitres) that the pedal will not get depressed as you think.

Contrary to what many people believe, not all cars in all conditions give huge pulsations on the pedal every time the ABS kicks in. With the modern systems and analog controlled valves, the pressure pulses can be smoothed out to an extent where minor ABS interventions would go unnoticed by normal drivers. This is a 'comfort feature' of ABS and is demanded by most OEMs.

That is all I can say with respect to basic functioning of ABS without getting too technical.

Hope this helps in some way.
Cheers
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Old 20th August 2015, 01:44   #603
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Default Re: The ABS discussion thread

Quote:
Originally Posted by Viju View Post
Contrary to what many people believe, not all cars in all conditions give huge pulsations on the pedal every time the ABS kicks in. With the modern systems and analog controlled valves, the pressure pulses can be smoothed out to an extent where minor ABS interventions would go unnoticed by normal drivers. This is a 'comfort feature' of ABS and is demanded by most OEMs.
That bit is particularly interesting to me, as I always thought that lack of felt-in-the-foot pulse meant no ABS operation.

As an opinion thing, I wonder if there isn't a point to giving abs-is-functioning feedback to the driver. If it is happening regularly it is a suggestion that one's driving style could do with a little moderation.
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Old 20th August 2015, 07:18   #604
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Default Re: The ABS discussion thread

Quote:
Originally Posted by Thad E Ginathom View Post
That bit is particularly interesting to me, as I always thought that lack of felt-in-the-foot pulse meant no ABS operation.

As an opinion thing, I wonder if there isn't a point to giving abs-is-functioning feedback to the driver. If it is happening regularly it is a suggestion that one's driving style could do with a little moderation.
If you read my carefully chosen words () again, I have mentioned ''no huge pulsations'' and ''minor ABS interventions''. I wouldn't say, complete lack of felt-in-the-foot pulses during ABS operation. I wanted to say that they can be smoothed out to avoid discomfort to the driver (meaning driver can feel it, but not to an extent where he would complain to the dealer).

Of course, as you would know, pedal pulsations are not the only way of noticing ABS in operation as in most cars (except may be the ''built-like-a-tomb'' and ''costs-a-lot-of-money'' kind of cars), ABS operation noise (valve actuation noise mainly - goes a bit like a faint tak-tak-tak) from the engine bay is mostly audible to the driver.

These decisions (limits of smoothing out the pulses, noise intrusion allowed into the cabin) are taken by OEMs from their experience / market feedback / benchmarking; and the ABS makers do their best to meet the OEM specifications.

Cheers
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Old 20th August 2015, 17:23   #605
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Thumbs up Re: The ABS discussion thread

Quote:
Originally Posted by Viju View Post
...
No additional pressure is applied by ABS at any point of time.
...
The fluid from the accumulator needs to be pumped back to the reservoir / inlet circuit.
This is the ONLY function of the pump in an ABS unit.
...
When the fluid is released, basically the circuit through which the fluid passes to the wheel cylinder (which is your input) is temporarily closed.
...
The quantity of fluid released is so less (the total capacity of the low pressure accumulator itself is a few millilitres) that the pedal will not get depressed as you think.
...
Hello Viju, many thanks for the details, did not know there was an accumulator in the ABS circuit - is the accumulator a cavity within the cubical end of the ABS module or is it separate ?
Does the ABS pump works against the pressure applied by the driver & force brake fluid into the circuit between the master cylinder & the brake calipers ?
Or, does it drain the fluid in the accumulator into the unpressurized fluid in the brake fluid reservoir on top of the master cylinder ?

Quote:
Originally Posted by Viju View Post
That is all I can say with respect to basic functioning of ABS without getting too technical.
I'm sure a lot of us on the forum wouldn't mind the gory details, thanks once again for the information.
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Old 20th August 2015, 17:28   #606
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Default Re: The ABS discussion thread

Quote:
Originally Posted by Viju View Post
... You seem to have misunderstood the role of the pump in the ABS hydraulic unit. ...
Thanks. Have I? Hmmm. I don't think so, but then my geriatric brain does funny things sometimes so I wouldn't be surprised.

BTW, you haven't still answered my last question about "reference velocity v/s wheel slip".
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Old 20th August 2015, 17:55   #607
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Default Re: The ABS discussion thread

Quote:
Originally Posted by Viju
I have mentioned ''no huge pulsations'' and ''minor ABS interventions''. I wouldn't say, complete lack of felt-in-the-foot pulses during ABS operation
Thanks for the clarification on that.

Actually, I am amazed at the extent to which people say they have experienced ABS: I regard it is an emergency function (like an airbag, for instance) that one should never experience at all in normal driving
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Old 20th August 2015, 20:10   #608
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Quote:
Originally Posted by DerAlte View Post
Thanks. Have I? Hmmm. I don't think so, but then my geriatric brain does funny things sometimes so I wouldn't be surprised.

BTW, you haven't still answered my last question about "reference velocity v/s wheel slip".
DerAlte, you obviously seem to have deep knowledge of ABS functioning and logic. I am still at the bottom of the learning curve. I intentionally ignored your question since I did not want to go deeper into details of the algorithm. You may just consider that I am ignorant about it. Since you seem to have access to it, why not post the relevant portions so that others also learn more about ABS operation?

Regarding your understanding about the functioning of the pump, if you think it is correct, good for you! If I remember right, you had also stated earlier that there is no motor in the ABS unit. Right?

Quote:
Originally Posted by im_srini View Post
is the accumulator a cavity within the cubical end of the ABS module or is it separate ?


Does the ABS pump works against the pressure applied by the driver & force brake fluid into the circuit between the master cylinder & the brake calipers ?


I'm sure a lot of us on the forum wouldn't mind the gory details, thanks once again for the information.

im_srini, I will try and post a picture of the accumulator tomorrow. In the picture I posted earlier, it was not visible enough to mark.

Yes, the pump works against the pressure as there is no separate line to feed fluid back into the reservoir.

Even I know a lot of people would like to know more, but my knowledge is limited (at least what I can openly share) and am really not in a mood to get into a debate with more knowledgeable people already contributing to this thread.

I like to explain things which I am clear about, in simple words which everyone can understand, rather than talk using unnecessarily complex terms about things which I don't have complete knowledge of.

Cheers
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Old 21st August 2015, 13:44   #609
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Default Re: The ABS discussion thread

Quote:
Originally Posted by Viju View Post
These decisions (limits of smoothing out the pulses, noise intrusion allowed into the cabin) are taken by OEMs from their experience / market feedback / benchmarking; and the ABS makers do their best to meet the OEM specifications.
Would be interesting to know how they dampen out these pulses. Do you work on 2 wheelers or 4 wheelers?

Quote:
Originally Posted by Viju View Post
I like to explain things which I am clear about, in simple words which everyone can understand, rather than talk using unnecessarily complex terms about things which I don't have complete knowledge of.
Viju, a lesson which I learned during my stint in India : "if you can't convince people, confuse them".

Leave it, Prost !
Spike

Last edited by GTO : 30th September 2015 at 08:41. Reason: PM
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Old 21st August 2015, 15:15   #610
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Default Re: The ABS discussion thread

Quote:
Originally Posted by Viju View Post
... I intentionally ignored your question since I did not want to go deeper into details of the algorithm. ...
There was no intention to go deeper into the algorithm. It is just that your usage of specific terms "reference velocity" and "wheel slip", well, went over my head so to say! Till then, neither terms were something that, with whatever little I knew of ABS, were part of ABS decision making.

Quote:
Originally Posted by Viju View Post
... Regarding your understanding about the functioning of the pump, if you think it is correct, good for you! If I remember right, you had also stated earlier that there is no motor in the ABS unit. Right? ...
You seem to be taking umbrage at something I am not able to fathom. Let me not make the situation worse by saying something more.

Just to clarify (maybe you were short of time when you read my earlier post), here is what I wrote earlier:

Quote:
Originally Posted by im_srini View Post
... Also, does the ABS motor actually modulate brake pressure ( as in threshold braking ) or is it that it just pulses ( relieves pressure & then pressurizes ) at some pre-defined frequency ?
Quote:
Originally Posted by DerAlte View Post
... No 'motor' - it is just a set of electronically controlled hydraulic valves. ...
I am surprised that you interpreted my answer to mean "There is no motor in the ABS unit". @im_srini was referring to functionality v/s braking method. 'ABS motor', in that context, would not be relevant.
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Old 21st August 2015, 15:34   #611
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Default Re: The ABS discussion thread

Quote:
Originally Posted by SPIKE ARRESTOR View Post
Leave it, Prost !
Spike
Spike

Totally agree with what you said. I am off this thread.

PS: I have worked on both, though I work exclusively on motorcycles right now. It may change any time depending on my employer's requirements.

Quote:
Originally Posted by DerAlte View Post
There was no intention to go deeper into the algorithm. .............. in that context, would not be relevant.
Respected DerAlte

You were right. I was wrong.

Cheers

Last edited by GTO : 29th September 2015 at 12:42. Reason: Quoted post edited
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Old 21st August 2015, 19:34   #612
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Default Re: The ABS discussion thread

Quote:
Originally Posted by SPIKE ARRESTOR View Post
"if you can't convince people, confuse them".
Naughty. Very naughty.
Let the young enjoy their innocence as long as they can.

Regards
Sutripta
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Old 23rd August 2015, 19:58   #613
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Quote:
Originally Posted by Sutripta View Post
Let the young enjoy their innocence as long as they can.

Which does beg the question, why anybody would want to grow up?
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Old 23rd August 2015, 21:39   #614
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Originally Posted by Jeroen View Post
Which does beg the question, why anybody would want to grow up?
Not want, need. Survival demands it.

Anyway lets stick to things automotive/ technical.

Regards
Sutripta
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Old 15th September 2015, 10:36   #615
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Default Re: The ABS discussion thread

Very informative thread indeed.

Anyone has seen this item on ebay? http://www.ebay.in/itm/252069203990
The price looks too good to believe that it will really work. They say its universal and you can DIY.

Sorry if someone had already posted this earlier.
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