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'[Way OT] ABS brakes.'
1998\01\23@071227 by Oyvind Kaurstad

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This is way off topic, but it is related to electronics, and
could theoretically be related to PIC's... :-)

Modern cars have ABS brakes, which is a system made to
prevent the wheels from locking up.

This system have sensors on the wheels that senses whether
it is rotating or not. Usually it is sensing on a toothed disk of
>some kind. (Probably some kind of hall-effect gadget)

This is perfectly suited to sense if there is rotation or not.

But here comes the interesting part:

How can a car with ABS brakes come to a stop in a hill?

Why don't the ABS-system release the brakes when the car stops?

And now you might say that the ABS-system is deactivated below
a certain speed, which is absolutely correct.

But what if a car standing still starts to slide? Will the ABS-system
be able to release the brakes?

Just wondering....


-Oyvind

1998\01\23@081106 by Steve Baldwin

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> This is way off topic
No kidding !

> How can a car with ABS brakes come to a stop in a hill?
>
> Why don't the ABS-system release the brakes when the car stops?

The ABS system doesn't do the braking. Your foot does.
The ABS uses various methods to effectively make you take your foot off the
brake for very short periods.
One way is to pulse a very high pressure onto the line for a short time
which pushes your foot off the pedal.
There are other methods but they get a bit convoluted.

Steve.

======================================================
 Very funny Scotty.  Now beam down my clothes.
======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680                email: spam_OUTstevebTakeThisOuTspamkcbbs.gen.nz
New Lynn, Auckland           ph  +64 9 820-2221
New Zealand                  fax +64 9 820-1929
======================================================

1998\01\23@090229 by Oyvind Kaurstad

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>> This is way off topic
>No kidding !

But I did warn you, didn't I?

>> How can a car with ABS brakes come to a stop in a hill?
>>
>> Why don't the ABS-system release the brakes when the car stops?

>The ABS system doesn't do the braking. Your foot does.

I didn't say the ABS-system do the braking. I said RELEASE the brakes.
There's a big difference.

>The ABS uses various methods to effectively make you take your foot off the
>brake for very short periods.
>One way is to pulse a very high pressure onto the line for a short time
>which pushes your foot off the pedal.

I don't think that's how it's done. If you put very high pressure on the
line you will also apply this pressure to the brakes, and they would
brake even harder. But that's not the issue here. I'm not interested
in the mechanic part of this, what I'm interested in is the electronic part.

How will the ABS system sense speed when the wheels are locked up?

I'm not sure it does.

And if this is correct, it is possible that the system could be fooled.
If a person brakes in panic and the road is *very* slippery the ABS will
start functioning. The speed is reduced and falls below the limit of
>which the ABS will deactivate (and the wheels are locked).
By coincidence the road starts to decline more at this point, and the
speed of the car increases again. But since the ABS was deactivated and
the wheels locked it will not start functioning again until the brake
is released manually so that the wheels get rolling again.

Ok, I know this is a bit weird, but I'm interested in knowing if this
is true or not.

Feel free to take this thread private if you like.


-Oyvind

1998\01\23@093315 by alex_holden

picon face
This reminds me of a problem one of the prototype Range Rovers (one of
the first vehicles to have ABS) had.
Here's the scenario:
You're off road, climbing a steep hill. You lose traction and the wheels
start to spin like mad. At this point, the ABS controller thinks you're
travelling quite fast.
You slam your foot on the brakes, and the wheels stop spinning. The ABS
controller thinks 'Hey up, the wheels have locked', and releases the
brakes again. You go careering backwards down the hill.

This actually put the release of the model back several months, as they
could not fix the bug in the existing unit, and had to design a new one
from scratch (this was in the days when ABS units were extremely high
tech and phenomenally expensive).

Oyvind Kaurstad wrote:
{Quote hidden}

--
------------------- Linux- the choice of a GNU generation.
--------------------
: Alex Holden- Caver, Programmer, Land Rover nut, and Radio amateur (M1
CJD). :
-------------- www.geocities.com/CapeCanaveral/Lab/1532/
---------------

1998\01\23@100139 by unthiti Patchararungruang

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The ABS system does not operate by detecting the speed of car's wheels. I
detects the  deceleration rate. If the deceleration of the wheels is too
high, it sends signal to release the breaks.


-Sunthiti Patchararungruang

On Fri, 23 Jan 1998, Oyvind Kaurstad wrote:

{Quote hidden}

1998\01\23@111041 by Mike Keitz

picon face
On Fri, 23 Jan 1998 13:11:35 +0100 Oyvind Kaurstad
<.....oyvind.kaurstadKILLspamspam@spam@NOFAC.ABB.NO> writes:

>How can a car with ABS brakes come to a stop in a hill?

It doesn't seem like it would matter whether the road is level, uphill,
or downhill, just whether the tires are sliding (because the brakes
locked) or not.
>
>Why don't the ABS-system release the brakes when the car stops?

Some of the early ones did.  There was an attempt to require ABS brakes
on heavy trucks in the 1970's before the technology was ready.  There
were many failures of this sort, which caused collisions that would not
have occurred with standard brakes.  After that disaster, only now (after
years of proving in cars) are ABS brakes being fitted to trucks again.

>And now you might say that the ABS-system is deactivated below
>a certain speed, which is absolutely correct=2E

I think they also look for things like one wheel stopped while the others
are turning, a sudden decrease in the speed of a wheel, etc.  There are
also a lot of self-checks to see if a sensor or other part has failed.
Usually the system lights a warning light and reverts to standard brakes
if trouble is detected.

>But what if a car standing still starts to slide? Will the ABS-system
>be able to release the brakes?

Why would you want to?  The best way to keep a car still would be to lock
the brakes.  If the tires don't have enough static friction on the road
to keep the car from moving, releasing the brakes would only make it move
faster.

1998\01\23@115540 by Wayne Foletta

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Oyvind:

Modern ABS systems use hydraulic pressure, temperature, and each wheel's
rotational velocity to compute a skid condition before it happens. The
brake hydraulic pressure is modulated and or reduced to equalize the
rotation of all wheels. Below a certain speed the ABS affect is near
zero (as when stopped or slowing to a stop from a low speed).
Many systems (like the Boeing 747 I worked on in the early 70's) use
cross or diagonal wheel (or wheel carriage or bogie) systems such that
even with a system error the braking fails to a balanced set of normally
braked wheels.

- Wayne Foletta
BMI - Saratoga, CA

{Quote hidden}

1998\01\23@132428 by Alvaro Deibe Diaz

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  This is a very interesting topic to me. Where can I find more
information about the ABS? And the VSC? How did people at LEXUS mixing ABS
and VSC? I'll be working on this subject soon, so I'll need all the
information I can. There are some interesting articles in the SAE web
(http:\\http://www.sae.org), but I need more information.

  Thanks.

-----Mensaje original-----
De:     Wayne Foletta [SMTP:EraseMEwaynespam_OUTspamTakeThisOuTELECTROTEK.COM]
Enviado el:     viernes 23 de enero de 1998 17:55
Para:   PICLISTspamspam_OUTMITVMA.MIT.EDU
Asunto: Re: [Way OT] ABS brakes.

Oyvind:

Modern ABS systems use hydraulic pressure, temperature, and each wheel's
rotational velocity to compute a skid condition before it happens. The
brake hydraulic pressure is modulated and or reduced to equalize the
rotation of all wheels. Below a certain speed the ABS affect is near
zero (as when stopped or slowing to a stop from a low speed).
Many systems (like the Boeing 747 I worked on in the early 70's) use
cross or diagonal wheel (or wheel carriage or bogie) systems such that
even with a system error the braking fails to a balanced set of normally
braked wheels.

- Wayne Foletta
BMI - Saratoga, CA

{Quote hidden}

1998\01\23@180931 by Steve Baldwin

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> >> How can a car with ABS brakes come to a stop in a hill?
> >> Why don't the ABS-system release the brakes when the car stops?
>
> >The ABS system doesn't do the braking. Your foot does.
>
> I didn't say the ABS-system do the braking. I said RELEASE the brakes.
> There's a big difference.
>
> >The ABS uses various methods to effectively make you take your foot off
the
> >brake for very short periods.
> >One way is to pulse a very high pressure onto the line for a short time
> >which pushes your foot off the pedal.
>
> I don't think that's how it's done. If you put very high pressure on the
> line you will also apply this pressure to the brakes, and they would
> brake even harder. But that's not the issue here. I'm not interested
> in the mechanic part of this, what I'm interested in is the electronic
part.
>
> How will the ABS system sense speed when the wheels are locked up?

Taking your last question first.
Simply, it doesn't. It can't.
The sensing of a skid is done by looking at the change in speed. If it
measures the speed of the wheel at 30mph and then 100ms later finds that
the speed of the wheel is now 0mph, it reasons that this doesn't obey
certain laws of physics. Therefore the wheel is skidding.
It's not practical to try and determine what the optimum braking pressure
is going to be. There are too many external variables.
Tyre condition, road surface, a deep puddle 1" in front of the wheel, lines
painted on the road, bumpy surface, and so on.
Regardless of all these, if the speed of the wheel drops suddenly at a high
speed, it can't be doing effective braking.
If the initial speed is very low (say 2mph), the ABS leaves the situation
alone. At that speed the difference in stopping distance between a locked
wheel and one with ideal slip is minimal.
So now there are two simple rules to our algorithm.
1) If the last speed I measured was less than x, don't do anything.
2) If the last speed I measured was greater than x and now it is zero, do
something.

This simple set of rules can be expanded to compare with other wheels and
so on.

Now, what can it do.
We know that the wheel is now at speed=0 so applying more brake pressure is
going to make no difference to the wheel. It can't go any slower. But,
putting more pressure on the line will force the pedal back up against the
drivers foot and lift it up a little bit. When we take our extra pressure
away again, the drivers foot is no longer pressing on the pedal as hard as
it was before it caused the lockup to occur. ie. less braking pressure.
Less braking pressure allows the wheel to start turning again and we can
repeat the process.

This is the system often used in passenger cars where you can feel the
pedal doing odd things. The ABS actuator is inserted in the brake lines of
an otherwise standard system.
The advantage of doing it this way is that it is fail safe. If the ABS
malfunctions, you still have a complete standard braking system.

There are other systems used on things like trucks where the standard brake
system relies on pneumatic servoing. There are also other hydraulic systems
that use a very convoluted means of removing pressure from the brake line.


Steve.

======================================================
 Very funny Scotty.  Now beam down my clothes.
======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680                email: RemoveMEstevebTakeThisOuTspamkcbbs.gen.nz
New Lynn, Auckland           ph  +64 9 820-2221
New Zealand                  fax +64 9 820-1929
======================================================

1998\01\23@223205 by Dean

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face
Hey ABSters,

>>The ABS uses various methods to effectively make you
>>take your foot off the brake for very short periods.
>>One way is to pulse a very high pressure onto the line
>>for a short time which pushes your foot off the pedal.

>I don't think that's how it's done. If you put very high pressure on the
>line you will also apply this pressure to the brakes, and they would
>brake even harder.
>But that's not the issue here. I'm not interested
>in the mechanic part of this,

Just in case someone else might be.
The push you feel is the pressure that is being released from the
wheel that is at a different speed than the others and is being
reintrodeced upstream of the wheels fluid cutoff device, which is
on the same side of the pressure line as your foot is. There is
most likely a cutoff solinoid on the line to each wheel and is
closed when pressure is about to be released so pressure can
be held on the other wheels.


>what I'm interested in is the electronic part.
>How will the ABS system sense speed when the wheels
>are locked up?

>I'm not sure it does.

I think you are right, if all the wheels are locked up then the
vechile is, probably, stationary, because if the ABS is doing its
job any one wheel will not ever be locked up alone. Sure it might
chirp on dry tarseal but not because its locked up but because its
doing 50 kmh for a split second and the car is doing 100 kmh. To
my knowledge, up until 2 years back I was a Toyota mechanic,
no Toyota has any other speed sensors for the BRAKE SYSTEM
other than at the wheels to tell rotating speed and maybe it might
use the Vechile Speed Sensor which is usually either at the gear
box end of the speedo cable/pickup or inside the car in the
speedo dash assembly. No accelerometer used in the brake
system, yes they are used else where.


>And if this is correct, it is possible that the system could be fooled.
>If a person brakes in panic and the road is *very* slippery the ABS will
>start functioning. The speed is reduced and falls below the limit of
>which the ABS will deactivate (and the wheels are locked).

I think the speed limit for ABS cutout is very close to 0 kmh. If it
was disabled at say 5 kmh then as soon as the car dropped below
this, under heavy braking, the wheels would lock, and I haven't seen
an ABS car leave black marks on the road or heard a wheel skid
under heavy braking just as it comes to a stop.

>By coincidence the road starts to decline more at this point, and the
>speed of the car increases again. But since the ABS was deactivated and
>the wheels locked it will not start functioning again until the brake
>is released manually so that the wheels get rolling again.

You are probable right here too, if I understood what you said.
It would be the same as if you hooked a tow rope up to the
car in front and stood hard on the brakes. If the tow car could
pull you forward I'd expect your wheels to stay locked until the
brakes were released then on reapplying them the ABS should
start functioning again.

>Ok, I know this is a bit weird, but I'm interested in knowing if this
>is true or not.

[sniped from another posting]

>Here's the scenario:
>You're off road, climbing a steep hill. You lose traction and the wheels
>start to spin like mad. At this point, the ABS controller thinks you're
>travelling quite fast.
>You slam your foot on the brakes, and the wheels stop spinning. The ABS
>controller thinks 'Hey up, the wheels have locked', and releases the
>brakes again. You go careering backwards down the hill.

I can't work out how they got around that problem.

Dean.

(-;     Forget about World Peace . . . Visualize Using Your Turn Signal!

;-)

1998\01\24@064836 by wwl

picon face
On Fri, 23 Jan 1998 15:00:43 +0100, you wrote:

>>> This is way off topic
>>No kidding !
>
>But I did warn you, didn't I?
>

>How will the ABS system sense speed when the wheels are locked up?
>
>I'm not sure it does.
>
>And if this is correct, it is possible that the system could be fooled.
>If a person brakes in panic and the road is *very* slippery the ABS will
>start functioning. The speed is reduced and falls below the limit of
>which the ABS will deactivate (and the wheels are locked).
My understanding is that the major de-braking criterion is when ONE
wheel suddenly slows, indicating lock-up - it is unlikely that all
wheels will lock at exactly the same time. This would avoid most
problems of false detection due to normal decelerations.

On my car, if I turn very hard & brake hard at the same time (e.g.
turning into a driveway a bit too quickly), the ABS sometimes gives a
few clicks, presumably due to the difference between wheel speeds on
either side of the car due to the sharp turn.

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'[Way OT] ABS brakes.'
1998\02\09@024332 by Michael Ollendieck
picon face
>>But what if a car standing still starts to slide? Will the ABS-system
>>be able to release the brakes?
>
>Why would you want to?  The best way to keep a car still would be to lock
>the brakes.  If the tires don't have enough static friction on the road
>to keep the car from moving, releasing the brakes would only make it move
>faster.
>
>
I don't think this is entirely correct.  The coeffient of static friction
is greater than the coefficient of kinetic friction.  If the car is parked
and not sliding then obviously this is static friction.  Once the car
begins to slide, you now switch to kinetic friction because the tire's
surface is moving relative to the road.  Theoretically, to best try to
regain control, the brakes should release so that the tires can grip the
road and convert back to static friction (the spinning wheel that is
rotating at the same speed as the road it is transversing is working with
static friction whereas the wheel that is rotating at a different speed
than the road is working with kinetic friction).  Then once the wheels are
rotating at the same speed as the road, it should try to brake without
locking up.

Of course practically speaking if the brakes can't hold the car when it is
parked, such as an icy road that is sloped, there won't be much the braking
system can do to save the day.

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