ABS - exactly how does it work?

OK, I know how it works... however, maybe I'm not so sure of the details. If I understand correctly, sensors sense 'lock' up and release brakes.... what
exactly makes the sensors realize that the car has or is about to stop and the wheels are not simply locking up.... what am I missing to distinguish the two situations.... ie how do the brakes 'know' that the wheels have locked up or that they've stopped? ..... push came to shove I could'nt figure it out
cheers, guenter
`
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Guenter Scholz wrote:

Here's my guess. During brake application, it's looking for uniform decrease in speed from all 4 wheels. If it sees one wheel suddenly decrease in speed compared to the other wheels, it knows that wheel is starting to skid and releases pressure on it until it comes back to the speed of the other wheels. In a normal braking situation, there will be a continuous progression from all wheels spinning at virtually the same speed to all wheels stopping at the same time.
Now I guess the pathological case could be where you're going at 50 and suddenly hit the brakes so hard that all 4 wheels lock up and stop turning simultaneously. How it knows that is an all 4 locked wheel condition vs that the vehicle has stopped is an interesting question. Perhaps it's done by knowing that the car couldn't have stopped from 50 in just x secs?
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On Sat, 19 Aug 2006 00:07:52 +0000 (UTC), snipped-for-privacy@sciborg.uwaterloo.ca (Guenter Scholz) wrote:

http://auto.howstuffworks.com/anti-lock-brake.htm WY
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And of course it is based on the good old fashioned Teutonic atitude that crickets live only to keep man happy and safe. So it has a strong psychological component also. Hence it it VERY expensive to work on as the wrench need to be well versed in sonic physics, psychology, and very small animal husbandry. As we all know, crickets are sensitive to temperature. Specificaly, their output of sonic energy is proportional to their temperature, with each cricket having a different constant of proportionality which introduces even further complexity and expense into this system. What happens is that a little cricket house gets built on each axel of the kar. This house is sort of like the houses used in reality shows in that there are sonic pick-ups in each room. As the velocity of the kar increases, the friction on the bottom of the house increases and so does the sonic output of the cricket. This is picked up and transmitted to the central computer which has to be factory calibrated for the different cricket sound frequencies, intensity, and basal rates of emission. Then any deviation from the normal is simply converted to pressure on the brakes in that wheel. Of course there are some problems with the colder climates where these may be used, but that is covered by the use of the famous freezer crickets of the Swedish bikini team. In fact, it was the late development of these crickets in Sweden which delayed this safety development by several decades.
mcbrue explainingly under the bridge in the trailer down by the river
96 S420
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I didn't think Germans knew what cricket is, let alone know the rules. That there is a chain* between two wickets and, you know, when you're in you can be bowled out... that sort of thing...
DAS
*chain = 22 yds
For direct contact replace nospam with schmetterling
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Americans are far more knowledgable than Germans.
We at least know the wicket should not be sticky although we also do not know what wickets are.
What I find interesting is that they took ABS and incorporated it into the Electronic Stability Program ( ESP ) and with the Sprinter they now have Adaptive ESP which adapts to whatever load the vehicle has.
In my case the vehicle load will probably be 1 or more all terrain vehicles, motorcycles, coolers, large speakers, mattresses and other assorted camping equipment.
Additionally, who is Jason Barlow and why does he talk funny?
http://www2.mercedesbenz.co.uk/content/media_library/unitedkingdom/mpc_unitedkingdom/vans/products/new_vans/sprinter/Sprinter_Jason_Barlow_video_15MB_mov.object-Single-MEDIA.tmp/Sprinter_Jason_Barlow_15mb.mov
.
.
Dori A Schmetterling wrote:

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I have to get a real usenet feed - google truncated the link!
Here it is again with newlines inserted so you have to remove them....
http://www2.mercedes-benz.co.uk/content/media_library/unitedkingdom/mp c_unitedkingdom/vans/products/new_vans/sprinter/Sprinter_Jason_Barlow _video_15MB_mov.object-Single-MEDIA.tmp/Sprinter_Jason_Barlow_15mb.mov
If it still does not display, hit reply and the entire link will display.
.
greek_philosophizer wrote:

http://www2.mercedesbenz.co.uk/content/media_library/unitedkingdom/mpc_unitedkingdom/vans/products/new_vans/sprinter/Sprinter_Jason_Barlow_video_15MB_mov.object-Single-MEDIA.tmp/Sprinter_Jason_Barlow_15mb.mov
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Jason Barlow is the editor for Car Magazine as well as car reviewer for various media.
greek_philosophizer wrote:
[snip]

http://www2.mercedesbenz.co.uk/content/media_library/unitedkingdom/mpc_unitedkingdom/vans/products/new_vans/sprinter/Sprinter_Jason_Barlow_video_15MB_mov.object-Single-MEDIA.tmp/Sprinter_Jason_Barlow_15mb.mov
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Well then that settles it.
If he likes the Sprinter then it must not be too bad a vehicle.
Now if I could only get DaimlerChrysler to take my money for a 2007....
.
OM wrote:

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Many thanks, good explanation there. What they don't make to clear is how they ascertain that the wheel has slowed down too fast. quite easy to do though, likely some kind of induced voltage which increases with rate of deceleration of the cog wheel/sensor ....
On ice, I assume, this mechanism would need some kind of fine tuning....
cheers, guenter

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Guenter Scholz wrote:

I've always pictured it to be much like a mouse movement sensor (you know, the old kind with the ball).
http://www.picotech.com/auto/waveforms/abs_generic.html
I guess a digital circuit would count the pulses and if it has to wait too long for the next pulse, the wheel must have stopped. If you combine this with knowledge about the maximum regular braking effect of the car the ABS controller can predict from the two last pulses how long it should be before it receives the next pulse. If that pulse is late, the wheel must have blocked and the system reduces the brake pressure to that wheel until it sees the next pulse.
Doesn't sound to difficult once you get it in the digital domain ;)
Ximinez
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Guenter Scholz wrote:

you're exactly right - it's called a Hall effect sensor ...
http://en.wikipedia.org/wiki/Hall_effect#Wheel_rotation_sensing
atb
Steve
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Well, not quite.
A Hall effect sensor is used to measure magnetic field strength, and there is nothing magnetic involved.
What's used is a socalled proximity switch. It consists of a coil, which is part of an oscillator circuit. When a metal object gets close to the coil, the frequency of the oscillator will change slightly depending on the distance and the metal. By monitoring the frequency, the internal circuit will determine whether metal is present or not, thereby swtching the output on or off.
There is a "toothwheel" attached to the wheel hub, and the proximity switch is facing this with a small distance. When the wheel turns, the teeth of the toothwheel will pass the switch with different distance between top and bottom of the teeth, causing the output to pulse according to the speed of the wheel.
/Jens
drd wrote:

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Thanks for ALL the replies.... much appreciated. I now understand ABS :-) Had the wheel off and see the ring gear at the end of the universal... the proximity counter is there to measure the frequency... I guess the chip that does the evaluation of the rpm and deceleration is somewhere under the dash.
cheers, guenter
ps I am suprised that some do not use this for acceleration control to keep from burning rubber.... probably I'm just not aware... not sure I would want this myself though ;-)

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Guenter Scholz wrote:

I'm sure sure Jens will be able to confirm and elaborate ;-) (as he does actually know what he's talking about ... )
atb
S
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control system called ASR (Antriebs-Schlupf-Regelung). It is using the ABS sensors, using brakes (as an immediate and fast control) as well as engine power control to stop wheel slip.
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I still like the concept of the Swedish Bikini Team keeping those poor little freezer crickets nice and warm! Rotating spokes indeed! Ole Tom B and I proved back in High School that the proximity sensors could be built but cost too darm much to do any good in scare circuits placed around the chicks places to hang.
mcbrue capacitatively under the birdge in the trailer down by the river
96 S420
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PS: Many ABS errors are caused by oily dirt between the teeth, so the proximity switch will not pulse correctly according to the speed. The ABS computer will detect this as an error and go to fail safe mode. So, if the ABS light turns on, try first to clean ABS rings (toothwheels) on all wheels.
/Jens
Jens wrote:

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