From: eliot@stalfos.engr.washington.edu (eliot)
Subject: clarification on antilock brakes (was More on anti-lock brakes
Organization: clearer than blir
Date: Wed, 18 Nov 1992 00:20:50 GMT

In article <1992Nov17.214704.16714@CSD-NewsHost.Stanford.EDU> sankar@Xenon.Stanford.EDU (Sriram Sankar) writes:
>> What you have described is called a three channel system. It treats
>> the two back wheels as a single entity and cannot independently
>> control the braking of the rear wheels. If one starts to slip, braking
>> force is reduced at both wheels and you lose some braking ability.

>> Most passenger car systems are four channel - they have separate wheel
>> speed sensors for each wheel and can modulate the hydraulic pressure
>> at each wheel.

you will be surprised to learn that some high priced japanese cars use
3 channel systems.. the infiniti j30 if i'm not mistaken.  they even
brag about it in their ads!!!

>> The ABS system on most cars usually have 3 - 4 hydraulic circuits.
>> Each front wheel gets one, and depending on the car, the rear wheel
>> gets either one per wheel or one total.  Each circuit is equipped with
>> a separate actuator which can release hydraulic pressure in just that
>> circuit.

>My car is a 1992 Saab 9000 with three hydraulic circuits for the brakes.
>I learned yesterday that the 1993 Saab 9000's come in two forms - one
>with three hydraulic circuits (just like the 1992), and the other with
>just two hydraulic circuits (one for each front wheel and the diagonally
>opposite rear wheel).  The three circuit version is used with models with
>traction control.  Also, the 1992 Honda Accord user manual says that
>it has two hydraulic circuits for the brakes.

>So, if the responses above are true, then:

>1. The Saab 9000 ABS which someone stated is behind the times is going
>   further behind the times.  I can't believe that Saab would do this
>   especially since they already have it and one of their main selling
>   points is safety.

>2. If the Honda info is correct (I'm positive about the Saab info, but
>   I don't trust a user manual to give accurate technical facts - so I'm
>   not sure about Honda), then the Honda ABS is very primitive.

i think there is mass confusion here on the term "hydraulic circuit".
let's get back to basics.

let's consider a car without abs, but with hydraulically actuated
brakes on all 4 wheels, either discs or drums or both.

obviously, at the very least each brake requires a hydraulic line to
it.  now, there is only one brake pedal and 4 braked wheels, so
there's got to be some "hydraulic circuit" to divide up the braking
effort.  in one sense of the word, this is a "hydraulic circuit".

now consider the scenario where the hydraulic brake line bursts.  you
lose hydraulic pressure to all 4 wheels.  obviously a really dangerous
situation.  all cars today have some form of redundancy in its
"hydraulic circuits".  e.g. in the "dual diagonal" kind, there are two
independent brake circuits, one braking left-front/right-rear and the
other braking right-front/left-rear.

when all is ok, braking pressure will first be divided up between the
two circuits, which in turn divides it up between the 2 wheels that
make it up.  if a brake line on one circuit breaks you will still have
one front wheel and one rear wheel of the other circuit.  the cheaper
cars use this and front what you quote, it seems that this is what the
honda has.

those makers who are more concerned with safety have a "dual
triangular" circuit (or whatever the name is) where each circuit
comprises of 2 front wheels and one rear wheel.  if one circuit fails,
you still have 2 front wheels and one rear wheel.  i suspect that this
is what the saab has.  i know for a fact that volvo has this system.

enter abs.

abs detects lockup by comparing speed differences between wheels.  the
4 channel kind is where speed differences between all 4 wheels are
compared.  the 3 channel kind is (correct me if i'm wrong) where the
aggregate speed of the 2 rear wheels are treated as one and the 2
front wheels are compared.  sensors send speed information to a cpu.
this information is sent electronically through wires and not
hydraulically.  (just want to drive home the point that the brake
circuits are totally unrelated to abs)

so when a locked wheel (or set of wheels) is identified, it is pumped.
this pumping is done at the brake actuator.  in other words, abs
pumping is independent of how the redundant brake circuits are
organized.  as long as there is one unbroken hydraulic brake line to
the actuator, abs will pump.  you can have a car with a single
hydraulic circuit whereby if it breaks, you lose brakes altogether
coupled with 4 channel abs, where each wheel can be pumped
independently.  or to quote an absurd example, you can theoretically
have a car with dual quadruple hydraulic circuits (i.e. 100%
redundancy, lose one circuit, still have 4 wheel braking) and a dual
channel abs where only pairs of wheels are pumped by abs.

to summarise, the key point to understand is to think of the car
without abs first, how the brake lines are organized with the safety
redundancy then think about implementing abs on top of it.  hydraulic
circuits and abs channels are completely different layers altogether.


eliot
Newsgroups: rec.autos.tech