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>> BTW ABS doesn't work below about 5 or 10 mph...
>
> Really?
>
> Any specific reason?
I imagine it's because if the ABS system sees all four wheels go from 5mph
to 0mph it can't tell whether you've actually come to a stop or you are
skidding on ice. At 50mph it's obvious if the wheels all stop within a
fraction of a second you are skidding, at low speeds it's not so easy to
work out.
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scott wrote:
>>> BTW ABS doesn't work below about 5 or 10 mph...
>>
>> Really?
>>
>> Any specific reason?
>
> I imagine it's because if the ABS system sees all four wheels go from
> 5mph to 0mph it can't tell whether you've actually come to a stop or you
> are skidding on ice. At 50mph it's obvious if the wheels all stop
> within a fraction of a second you are skidding, at low speeds it's not
> so easy to work out.
Mmm, OK.
I must admit, I've always wondered how the hell ABS actually works.
But not as much as power steering. This, as far as I can tell, should be
logically impossible. And yet it apparently works. How baffling!
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Invisible wrote:
> Uh... WTF? It's not exactly like the Arctic out there! :-P
There are large numbers of people who have difficulty driving in
anything more slippery than rain. This is most likely in areas which
are known for the mildness of their winters.
Add to this the general temperament of the government to view itself as
some kind of mother to the people, and you get urgent warnings telling
people to regard as wild uncontrollable dangers things that in fact
present a small and manageable risk.
I grew up in Michigan, and can tell you that even as experienced with
the snow as they are there, the first significant snowfall of the winter
is a refresher course in winter driving for just about everybody.
Regards,
John
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> I must admit, I've always wondered how the hell ABS actually works.
A speed sensor on each wheel, if one drops too quickly compared to the
others, or they all drop too quickly than what is "usually possible" a
solenoid is opened that releases the brake pressure until the wheels speeds
are back to normal, then the solenoid is closed. The process might then
repeat if the driver is still pressing the brake pedal hard - this is the
"pulsing" sensation you feel.
Note that modern cars are much more complex, with ABS being part of a much
larger control system, capable of regulating the braking force to each
individual wheel (to allow each wheel to be giving the maximum stopping
power, and to prevent skids, wheel-spin etc).
> But not as much as power steering. This, as far as I can tell, should be
> logically impossible. And yet it apparently works. How baffling!
There's a part in the steering system that twists or bends by a specific
amount depending on how hard you tug at the wheel. A set of valves then
controls some pressurised hydraulics based on how much force you are putting
into the wheel, which helps you steer.
Newer cars have a torque transducer and an electric motor with some clever
software. This saves on fuel as there is no hydraulic compressor having to
be run the whole time (usually you only need power assist at low speeds).
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>> I must admit, I've always wondered how the hell ABS actually works.
>
> A speed sensor on each wheel, if one drops too quickly compared to the
> others, or they all drop too quickly than what is "usually possible" a
> solenoid is opened that releases the brake pressure until the wheels
> speeds are back to normal, then the solenoid is closed. The process
> might then repeat if the driver is still pressing the brake pedal hard -
> this is the "pulsing" sensation you feel.
>
> Note that modern cars are much more complex, with ABS being part of a
> much larger control system, capable of regulating the braking force to
> each individual wheel (to allow each wheel to be giving the maximum
> stopping power, and to prevent skids, wheel-spin etc).
...not to mention traction control... ;-)
>> But not as much as power steering. This, as far as I can tell, should
>> be logically impossible. And yet it apparently works. How baffling!
>
> There's a part in the steering system that twists or bends by a specific
> amount depending on how hard you tug at the wheel. A set of valves then
> controls some pressurised hydraulics based on how much force you are
> putting into the wheel, which helps you steer.
>
> Newer cars have a torque transducer and an electric motor with some
> clever software. This saves on fuel as there is no hydraulic compressor
> having to be run the whole time (usually you only need power assist at
> low speeds).
The part I can't figure out is that, as far as I can tell, hooking up a
motor that turns the steering wheel in the same direction as you're
trying to turn it should generate a positive feedback loop. I can't
figure out how turning the wheel doesn't cause the powerful motors to
rip the wheel out of your hands and put the car into full lock before
snapping the steering column in half...
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John VanSickle wrote:
> There are large numbers of people who have difficulty driving in
> anything more slippery than rain. This is most likely in areas which
> are known for the mildness of their winters.
I did notice that in the videos of people pushing cars, all the cars
seemed to be attempting to drive uphill at no more than walking pace -
shorly not a sensible thing to do in snow?
> Add to this the general temperament of the government to view itself as
> some kind of mother to the people, and you get urgent warnings telling
> people to regard as wild uncontrollable dangers things that in fact
> present a small and manageable risk.
Yeah, maybe. I guess the government doesn't want to hear the whole "why
didn't you warn people?" thing, or "why didn't you take action?". (OTOH,
it didn't seem to be a problem for New Orleans...)
> I grew up in Michigan, and can tell you that even as experienced with
> the snow as they are there, the first significant snowfall of the winter
> is a refresher course in winter driving for just about everybody.
I can remember 1995 or so, there was a lot of snow one day. About 20 cm
or so. Not huge, but a lot. Nobody seemed to really pay much attention.
A few people got slightly stuck or something, but nobody considered it a
big deal. Yet today, it's a catastrophy... hmm.
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> ...not to mention traction control... ;-)
Er yeh, that's what I meant by wheel-spin. OOC I looked up the instructions
for how a dealer fits the "USB Audio" upgrade to my car (I was just
wondering if the cables were already there and I simply needed to fit a
suitable plug - sadly not). Anyway, it appears my car has a network of
fibre optic cables o_O. To fit the upgrade, apparently involves ripping out
half the car, making some connection to a box of opto-electronics in the
boot, then fitting a "USB hub" under the drivers seat and fitting the USB
socket in the centre console, and of course fitting cables between all of
them. The 250 pound price tag doesn't sound so bad now!
> The part I can't figure out is that, as far as I can tell, hooking up a
> motor that turns the steering wheel in the same direction as you're trying
> to turn it should generate a positive feedback loop. I can't figure out
> how turning the wheel doesn't cause the powerful motors to rip the wheel
> out of your hands and put the car into full lock before snapping the
> steering column in half...
If you simply program the motor to apply the same torque you are applying
with your hands to the wheel, then there is no feedback, it will just feel
like it takes half the force to turn it. Of course in reality it is more
complex, the multiplier is probably dependant on speed, and maybe other
factors like how sharp you are turning. And maybe there is some limit the
torque you apply needs to go above before the motor kicks in?
Something like:
If AppliedTorque > 10 Then
MotorTorque = (AppliedTorque - 10) * CalculateAssistFactor( VehicleSpeed ,
SteerAngle )
Else
MotorTorque = 0
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scott wrote:
>> ...not to mention traction control... ;-)
>
> Er yeh, that's what I meant by wheel-spin.
ABS turns the brakes off if you brake too hard, traction control turns
the drive off if you accelerate too hard. At least, as far as I know... ;-)
> OOC I looked up the
> instructions for how a dealer fits the "USB Audio" upgrade to my car (I
> was just wondering if the cables were already there and I simply needed
> to fit a suitable plug - sadly not). Anyway, it appears my car has a
> network of fibre optic cables o_O.
Well... less suseptable to EMI and corrosion?
> To fit the upgrade, apparently
> involves ripping out half the car, making some connection to a box of
> opto-electronics in the boot, then fitting a "USB hub" under the drivers
> seat and fitting the USB socket in the centre console, and of course
> fitting cables between all of them. The 250 pound price tag doesn't
> sound so bad now!
LOL! Yeah. Apparently modern cars have quite big wiring looms inside
them now. I remember one guy did some work on his own car, and then had
to pay for an accredited shop to confirm that the airbag detonation
system is still operating within spec.
Seriously though, you'd think they'd just put lots of spare cable in
there so you can add new connections without rearranging half the car...
>> The part I can't figure out is that, as far as I can tell, hooking up
>> a motor that turns the steering wheel in the same direction as you're
>> trying to turn it should generate a positive feedback loop. I can't
>> figure out how turning the wheel doesn't cause the powerful motors to
>> rip the wheel out of your hands and put the car into full lock before
>> snapping the steering column in half...
>
> If you simply program the motor to apply the same torque you are
> applying with your hands to the wheel, then there is no feedback, it
> will just feel like it takes half the force to turn it. Of course in
> reality it is more complex, the multiplier is probably dependant on
> speed, and maybe other factors like how sharp you are turning. And
> maybe there is some limit the torque you apply needs to go above before
> the motor kicks in?
Oh, it's programmable? I thought it was a purely mechanical system...
Even so, how can you "sense" the amount of force the user it putting
into the system seperately from the amount of force the power assist is
putting into it? I can't think of a way of doing that.
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> ABS turns the brakes off if you brake too hard, traction control turns the
> drive off if you accelerate too hard. At least, as far as I know... ;-)
Newer cars can apply the brakes too programmably, it's much faster than
cutting the fuel and more accuracte, and when you are about to skid out
sideways on a slippery road fast reactions are what you need.
> Well... less suseptable to EMI and corrosion?
Yeh the EMI issue is probably the biggest reason, especially now that they
seem to be wanting to put more stuff in the back of the car to keep the
weight distribution 50/50, it means you have long wires which are always bad
from an EMI point of view.
> Seriously though, you'd think they'd just put lots of spare cable in there
> so you can add new connections without rearranging half the car...
Guess it's cheaper not to, and then charge more for people who want to do
stuff afterwards.
> Oh, it's programmable? I thought it was a purely mechanical system...
Older cars it was mechanical, yes, programmable (once!) by the positioning
of the hydraulic valves :-) Newer cars it's all electronic controlling a
motor in software.
> Even so, how can you "sense" the amount of force the user it putting into
> the system seperately from the amount of force the power assist is putting
> into it? I can't think of a way of doing that.
Measure how much the steering column is twisted (ie rotation at one end
minus rotation at the other end) between the steering wheel and where it
connects to the rack/motor/hydraulics. This will be directly proportional
to the torque the driver is applying to the wheel, no matter what the motor
or the wheels are doing.
In mechanical systems, this twist would cause hydraulic valves to open and
assist the rack moving, in electronic systems there is probably just a
strain gauge on the steering column that goes through an ATD to the
software.
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scott wrote:
>> Well... less suseptable to EMI and corrosion?
>
> Yeh the EMI issue is probably the biggest reason.
There would seem to be quite a lot of electronics in a car - especially
a big alternator and a set of sparkplugs. That's gotta kick out a out of
EMI, one would think...
>> Even so, how can you "sense" the amount of force the user it putting
>> into the system seperately from the amount of force the power assist
>> is putting into it? I can't think of a way of doing that.
>
> Measure how much the steering column is twisted (ie rotation at one end
> minus rotation at the other end) between the steering wheel and where it
> connects to the rack/motor/hydraulics. This will be directly
> proportional to the torque the driver is applying to the wheel, no
> matter what the motor or the wheels are doing.
>
> In mechanical systems, this twist would cause hydraulic valves to open
> and assist the rack moving, in electronic systems there is probably just
> a strain gauge on the steering column that goes through an ATD to the
> software.
I was under the impression that the rod is ridig, and therefore can't
twist (only rotate).
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