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PICList Thread
'[PIC] Traction Control / Wheelspeed sensing'
2004\10\18@132102 by Andy Meng

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I am working on a wheel speed sensing and traction control system as part of
our schools Formula SAE race car. Basically, it has more power than traction
(about 80HP/500 pounds) and we don't have much time to practice driving it,
so the more help we can get in the traction department, the better
(especially in the acceleration/drag race event).

The basic principle is to compare the speeds of the front (non driven) and
back (driven) wheels to determine how much slip/loss of traction there is.
Apparently, about 10% slip is best. If the back wheels are spinning too
fast, the throttle is cut electronically (strict throttle by wire is
prohibited for obvious reasons, but just cutting fuel/spark isn't) to limit
the slip (and hopefully maintain it with decent control).

I plan to use a CAN bus around the car, both for the wheel speed sensing,
and other data transfer (shifting information, other sensors in the future).
I have designed a wheel speed sensor (there will be 4 copies of it) that
will mount on the kingpin assembly - it has a hall effect sensor, PIC10F or
12F (timing pulses), MCP2515 CAN controller, and a CAN transceiver IC that
hangs on the bus. Hopefully the CAN transceiver will make it pretty easy -
all it needs is SPI data from the PIC.

The other end will be a bit more difficult. I still haven't worked out the
algorithms yet. I think it will likely consist of a similar CAN interface
feeding a 16F or 18F PIC (some of the 18's also have a built in CAN
interface). This PIC will have 4 outputs so it can individually cut
cylinders. I think this PIC should probably be done in C to make it easier
to try new algorithms.

I would appreciate comments on the system outlined above.

What I am wondering about the most is the feedback loop. I don't have much
experience dealing with feedback systems. What I am thinking is that it
would probably be a good idea to measure the loop gain and bandwidth of the
system once we have the hardware put together. We need to know how fast the
engine will respond to cutting differing amount of cylinders for a given
amount of time (it seems that this will change with engine speed and load
also). Or is this too much to keep track of and it would be better just to
do a trial and error algorithm?

Thanks for any help/advice you can give.

Andy Meng

LeTourneau University
Formula SAE Race Team

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2004\10\18@184955 by Andrew Warren

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Andy Meng <spam_OUTpiclistTakeThisOuTspammit.edu> wrote:

> I am working on a wheel speed sensing and traction control system
> as part of our schools Formula SAE race car. Basically, it has more
> power than traction (about 80HP/500 pounds) and we don't have much
> time to practice driving it, so the more help we can get in the
> traction department, the better (especially in the
> acceleration/drag race event).

   Andy:

   If you can give him a dozen-or-so runs to find the right launch
   RPM and shift point(s), I think your unassisted driver will
   perform as well or better than your electronic traction-control
   system.

   If you don't have enough practice time for him to learn how to
   launch the car, you certainly don't have enough practice time to
   tweak and tune a traction-control system.

   But, since you probably want to have something to do between now
   and the end of May...

> The basic principle is to compare the speeds of the front (non
> driven) and back (driven) wheels to determine how much slip/loss
> of traction there is. Apparently, about 10% slip is best.

   10% is only a general rule-of-thumb; the optimum amount of slip
   could vary between half and twice that value.  Tire compound,
   construction, temperature, and condition all affect it, as does
   the surface you're driving on.

   Also... Comparing front-wheel RPM to rear-wheel RPM is easiest
   if a) tire deformation doesn't significantly change the rolling
   circumference of your rear tires, b) you're not cornering, c)
   you have a locked differential.

   I wouldn't expect you to have a problem with tire deformation and
   I assume that you're only planning to use the traction-control
   system for the straight-line acceleration event, not for the
   autocross... But since you probably have a limited-slip or (yuck)
   open diff, you need to think about how you'll measure wheelspeed
   when your rear wheels aren't spinning at the same rate.

> If the back wheels are spinning too fast, the throttle is cut
> electronically (strict throttle by wire is prohibited for obvious
> reasons, but just cutting fuel/spark isn't) to limit the slip (and
> hopefully maintain it with decent control).

   Consider retarding the ignition timing instead.

> I have designed a wheel speed sensor (there will be 4 copies of it)

   How many pulses per revolution?  More pulses gives you the
   potential for quicker response, but also could lower your timing
   resolution and make your front-to-rear ratio calculation less
   accurate.  Have you done the math to find a good compromise?

{Quote hidden}

   Have you added up all the timing to make sure that you can do
   everything (read the wheelspeed, transmit the data to your
   central PIC, process it, reduce/increase power) fast enough?

   Four sensors with embedded microcontrollers sending data
   asynchronously, plus a central PIC that has to act on that data,
   is a complicated system.  Think hard about ways to reduce the
   complexity.

> I would appreciate comments on the system outlined above.

   If it were me, I'd simplify the output side of the system by
   choosing to reduce engine power by retarding the timing; it's
   fast, effective, and less intrusive with fewer unwelcome
   side-effects than cutting fuel or spark. It also requires only
   one output instead of the four that you're planning on.

   I'd simplify the input side, too.  The entire system can be made
   ENORMOUSLY simpler if you find a way to sense slip without
   measuring wheelspeed.  Such a way exists, but I don't want to
   ruin the surprise for you.

   Good luck...

   -Andy

=== Andrew Warren -- .....aiwKILLspamspam@spam@cypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

____________________________________________

2004\10\18@201947 by Alex Harford

face picon face
On Mon, 18 Oct 2004 15:53:54 -0700, Andrew Warren <aiwspamKILLspamcypress.com> wrote:
>
>
>     I'd simplify the input side, too.  The entire system can be made
>     ENORMOUSLY simpler if you find a way to sense slip without
>     measuring wheelspeed.  Such a way exists, but I don't want to
>     ruin the surprise for you.

GPS for the groundspeed, vs engine RPM (and you'd have to know the
gear, and tire diameter as well).

:)

Alex
____________________________________________

2004\10\18@233925 by John Pearson

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I thought about employing a speedometer and tach, plotting the relationships
for comparison. I only needed it for first gear so that simplified things.

If all else fails and you end up using your right foot for your traction
control, try something I learned on street radials: Get the car moving, then
lift for a split second. Once you lift you can floor it without wheelspin.

John
{Original Message removed}

2004\10\19@034847 by hael Rigby-Jones

picon face


>-----Original Message-----
>From: .....piclist-bouncesKILLspamspam.....mit.edu [EraseMEpiclist-bouncesspam_OUTspamTakeThisOuTmit.edu]
>On Behalf Of Andrew Warren
>Sent: 18 October 2004 23:54
>To: Microcontroller discussion list - Public.
>Subject: Re: [PIC] Traction Control / Wheelspeed sensing
>
>    I'd simplify the input side, too.  The entire system can be made
>    ENORMOUSLY simpler if you find a way to sense slip without
>    measuring wheelspeed.  Such a way exists, but I don't want to
>    ruin the surprise for you.

I'm not really familiar with traction control systems, so I'll take a guess:
Measure the rate of change of engine speed.  With 100% traction the rate of
change will be at a maximum in 1st gear, anything higher than this indicates
a loss of traction.  Am I close or miles off?

Regards

Mike

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2004\10\19@055039 by John Snider
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> > Andy Meng <piclistspamspam_OUTmit.edu> wrote:
> > <snip>
> > I have designed a wheel speed sensor (there will
> be 4 copies of it)
> > it has a hall effect sensor, PIC10F or 12F
> (timing pulses), <snip>
--------------------------------
Most autos use a gear tooth sensor (such as the ATS672
http://www.allegromicro.com/datafile/0672.pdf , ~$7 US
at Newark). Three pins: power, ground, and open
collector ouput, magnet in package, digital clean up
of signal, zero speed sense, compensates for
temperature and air gap, etc...
____________________________________________

2004\10\19@085735 by Andy Meng

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WARNING: LONG POST/REPLY.

>     If you can give him a dozen-or-so runs to find the right launch
>     RPM and shift point(s), I think your unassisted driver will
>     perform as well or better than your electronic traction-control
>     system.
>
>     If you don't have enough practice time for him to learn how to
>     launch the car, you certainly don't have enough practice time to
>     tweak and tune a traction-control system.
>
>     But, since you probably want to have something to do between now
>     and the end of May...

I agree with you to some extent. However, we have about five drivers (which
I think is too many - but that's not my decision) and bringing them all up
to the same point would probably be a bit difficult given their level of
experience. Also, like you said, we (two other EE's and myself) have been
assigned the task of building a traction control system.

>     10% is only a general rule-of-thumb; the optimum amount of slip
>     could vary between half and twice that value.  Tire compound,
>     construction, temperature, and condition all affect it, as does
>     the surface you're driving on.

I understand that - ideally, we will have a pot that will allow us to adjust
the amount of slip for different conditions such as tire temp, rain
tires/slicks, etc.

{Quote hidden}

Tire deformation should be pretty close front to back, unless the back is
spinning out of control (ever seen top fuel/funny car tires at launch?) -
and if they are spinning that much faster, it will probably be far greater
slip percentage than the cutoff so it should deal with this OK. We have a
Zexel-Torsen diff, which I understand to be limited slip, torque balancing
(still curious about how that works... although thinking about it I guess a
limited slip diff is probably torque balancing by definition).


>     Consider retarding the ignition timing instead.
[order changed]
>     If it were me, I'd simplify the output side of the system by
>     choosing to reduce engine power by retarding the timing; it's
>     fast, effective, and less intrusive with fewer unwelcome
>     side-effects than cutting fuel or spark. It also requires only
>     one output instead of the four that you're planning on.


This is the main part of the post that is getting me thinking. I can't think
of a way to do this with the stock ECM, other than maybe delaying the
ignition pulse generator and camshaft position sensor. As far as one-line
control, I would prefer to have a variable amount of effect, i.e, with
control of each cylinder, so we can cut a different number of cylinders
depending on how much we need to slow down the engine. It would probably be
possible to retard the ignition a variable amount, if I knew how to do it at
all. What are the side-effect of cutting fuel/spark you are referring to? By
the way, we are using an F4i engine (CBR600) - the coils appear to be
integrated into the monster (~1" dia x ~5" long) plug caps, so switching
these signals should not be as difficult as cutting a typical high voltage
spark plug line.

> > I have designed a wheel speed sensor (there will be 4 copies of it)
>
>     How many pulses per revolution?  More pulses gives you the
>     potential for quicker response, but also could lower your timing
>     resolution and make your front-to-rear ratio calculation less
>     accurate.  Have you done the math to find a good compromise?

I had already made up an Excel spread sheet so I can compare the time
differences and timer counts under different conditions - it has inputs for
speed, tire dia, number of magnets, PIC clock, and timer prescaler. With
four magnets and the PIC running at 4MHz (8:1 prescaler), there are roughly
55800 counts (446 ms pulse interval) per pulse at 2 MPH, and 1600 counts
(13ms interval) at 70MPH (faster than we will probably ever go (at least in
competition) and also probably out of the realm of traction control
usefulness). At a more useable speed of 30MPH, there are ~3700 counts per
pulse, with a difference of 128 counts from 29MPH. To me, this seems to
provide a workable range of numbers. Anywhere above 5MPH, the response time
from the wheel speed sensors is under 180ms.

>     Have you added up all the timing to make sure that you can do
>     everything (read the wheelspeed, transmit the data to your
>     central PIC, process it, reduce/increase power) fast enough?
>
>     Four sensors with embedded microcontrollers sending data
>     asynchronously, plus a central PIC that has to act on that data,
>     is a complicated system.  Think hard about ways to reduce the
>     complexity.

A part of the reason for this complexity is to allow the system to be
extendable in the future. Eventually (in following years) we would like to
have more sensors for different systems, data aq (which it would be nice to
have wheel speed data for), etc. I want to create a system that can be built
on in the future, something that hasn't been done in the past - I'll be
around for two more years and I don't think the EE team should be repeating
the same work every year. Having separate processors for timing lessens the
work that the main processor has to do, as well as makes the information
available for future systems. With a CAN bus speed of 500kbps (not set in
stone yet), we should be able to get pretty fast (<200uS) transfer of a
57-bit (two data byte) CAN packet.

{Quote hidden}

As I mentioned before, having the wheel speed sensors in place would provide
some valuable information for other things later on (although that isn't
100% necessary). I was trying to think of ways to sense slip other than
wheel speed determination and couldn't think of any. Since we had to start
the design, I went with it. I guess another way would be to sense
acceleration and try to get the best acceleration (start cutting power after
you go over an acceleration peak). It seems like it would be hard to know
when you could stop cutting it, because the indication would be relative.
Another poster mentioned basically taking the derivative of engine speed and
if it was too great for what you knew available traction could support - you
could also have a pot to adjust the desired maximum engine derivative. The
problem that comes to mind with this is that it would change as you got into
higher gears and with different loads.

Thank you very much for your insights. It sounds like you have some
experience with this problem or at least the competition, and that is
appreciated. For background, we have been building cars since 2002, when we
placed 12th overall. The next year we didn't finish the endurance race due
to wiring problems. Last year we did not go to competition because we did
not register before the 140 team limit was reached, but we still built a car
and competed in a local SCCA event. However, this car also had wiring
problems. Another key task (far more so than traction control) this year is
getting a solid electrical system. We have several plans for how to
accomplish this - mainly by planning for it and taking the time to do it
right - something past teams have not done.

Thanks again,
Andy

> === Andrew Warren -- @spam@aiwKILLspamspamcypress.com
> === Principal Design Engineer
> === Cypress Semiconductor Corporation
> ===
> === Opinions expressed above do not
> === necessarily represent those of
> === Cypress Semiconductor Corporation
>
> ______________________________________________

2004\10\19@090356 by Kev Pearce \(kevp.com\)

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Most production car traction control systems use the pick up signals rfom
the ABS wheel speed sensors.
These use toothed wheels with anything from 60 - 80 teeth giving a frequency
signal output or even an analogue voltage.
My point is that counting the 4 wheels nuts or mounting bolts is not going
to give a signal that can sense the changes in wheel rpm fast enough to do
anything about it.

You might want to visit a scrap/junk yard and get some ABS toothed wheels
and sensors or a car and a play with them.

This is something I'm looking at doing in the future on my kit car but I am
planning to get the ABS in first so I can just use the signals...


Kev/.


____________________________________________

2004\10\19@090518 by Kev Pearce \(kevp.com\)

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> If all else fails and you end up using your right foot for your traction
> control, try something I learned on street radials: Get the car moving,
then
> lift for a split second. Once you lift you can floor it without wheelspin.

Techcically known as a 'Granny Start'!!!

Kev/.



____________________________________________

2004\10\19@094806 by hael Rigby-Jones

picon face


{Quote hidden}

That is a bargain compared to the simple inductive ABS sensors that cost
five times that from main dealers!

Mike

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____________________________________________

2004\10\19@102752 by Peter L. Peres

picon face

On Mon, 18 Oct 2004, Alex Harford wrote:

> On Mon, 18 Oct 2004 15:53:54 -0700, Andrew Warren <TakeThisOuTaiwEraseMEspamspam_OUTcypress.com> wrote:
>>
>>
>>     I'd simplify the input side, too.  The entire system can be made
>>     ENORMOUSLY simpler if you find a way to sense slip without
>>     measuring wheelspeed.  Such a way exists, but I don't want to
>>     ruin the surprise for you.
>
> GPS for the groundspeed, vs engine RPM (and you'd have to know the
> gear, and tire diameter as well).

No, drive torque drops or becomes near constant instead of increasing with
rpm. This causes the engine rpm to increase (fast). One can also measure
car acceleration vs. drive shaft torque using a horizontal accelerometer.
Just measuring and limiting the drive shaft torque is probably sufficient
as a first approach.

Peter
____________________________________________

2004\10\19@105451 by Andy Meng

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face
I like that idea. It could possibly be as simple as strain gauges on the
diff mounts. It seems to me that the torque required to break the tires free
would be independent of speed. On the other hand, I don't know that this
would offer information much different from an accelerometer - the torque is
going to drop when the wheels start spinning since kinetic friction is less
than static friction. Once the wheels are broken free it's pretty easy to
keep them spinning. An accelerometer would give similar data, a peak as the
tires were on the edge of traction (or in the optimal slip area), then
falling off.

One conclusion I have come to is that I should have posed this question a
month ago.

Andy


> No, drive torque drops or becomes near constant instead of increasing with
> rpm. This causes the engine rpm to increase (fast). One can also measure
> car acceleration vs. drive shaft torque using a horizontal accelerometer.
> Just measuring and limiting the drive shaft torque is probably sufficient
> as a first approach.
>
> Peter

____________________________________________

2004\10\19@110553 by Bob Axtell

face picon face
I once acted as a consultant (feasibility study) to someone who wanted
to design a reliable
NASCAR speedometer for INSIDE the car (to prevent tickets when going
into/out of
PIT ROW or when under YELLOW flag). I installed a few pieces of custom test
equipment on a test race car and ran it for a few days. Here's what I
found out:

1. The tires wear so rapidly, and changed shape so rapidly, that using a
fixed mechanism was
never better than 10% accurate. In particular, the right and left tires
wore differently, and
sometimes were deliberately inflated differently.

2. Since speed is the measurement of the distance per unit time of the
car going FORWARD
over the ground, the slippage when the car cornered is significant.

3. The best (and present) method is to detect the car between posts,
which provides a reliable AVERAGE
speed. But that does NOT provide instant speed values.

Along with my mechanical engineer consultant associate, we decided:

1. No mechanical mechanism would provide a reliable source of speed
measurement.

2. Either Ultrasonic Audio or RF doppler "radar" could be used if the
sensors were mounted under
the car, and would bounce off the pavement. The reflected signal would
be frequency-shifted due to
the pavement. Its like normal radar except that the sensors were seperated.

3. We decided that Ultrasonics would be cheaper but RF would be the most
accurate. The Audio source
would have to be intense to not be drowned out by the noise of the car.

It was a fun project. But it took forever to get paid....

--Bob


John Pearson wrote:

>I thought about employing a speedometer and tach, plotting the relationships
>for comparison. I only needed it for first gear so that simplified things.
>
>If all else fails and you end up using your right foot for your traction
>control, try something I learned on street radials: Get the car moving, then
>lift for a split second. Once you lift you can floor it without wheelspin.
>
>John
>{Original Message removed}

2004\10\19@114040 by Andy Meng

flavicon
face
Hmmm... this concerns me. I had thought that monitoring wheel speed and
updating the engine control every quarter wheel revolution (actually from 4
magnets located inside the wheel, but anyway...) would be enough. 60-80
sounds really high. Interestingly, I just plugged 80 pulses per revolution
into my spreadsheet and it indicates that I will still have plenty of timer
resolution (I guess that's not too surprising when monitoring something
mechanical with a PIC) - about 9000 timer counts between pulses at 5MPH
going to 637 pulses at 70MPH. Of course, the big problem with that (under
the current scheme) is the time to send the data and process it. Then add in
the fact that even at 10K RPM, each revolution takes 6ms (although with a 4
cyl you have a pulse every half revolution). If you are making any changes
much faster than 5 or so ms, it is probably being wasted anyway, so you
could toss out a lot of the pulses.

These sensors are inductive? So basically there is a metal gear, and the
sensor senses each tooth?

It's kind of interesting doing a lot of thinking when writing emails - I've
been doing it a lot lately.

Thanks for the comments.

Andy


> Most production car traction control systems use the pick up signals rfom
> the ABS wheel speed sensors.
> These use toothed wheels with anything from 60 - 80 teeth giving a
frequency
> signal output or even an analogue voltage.
> My point is that counting the 4 wheels nuts or mounting bolts is not going
> to give a signal that can sense the changes in wheel rpm fast enough to do
> anything about it.
>
> You might want to visit a scrap/junk yard and get some ABS toothed wheels
> and sensors or a car and a play with them.
>
> This is something I'm looking at doing in the future on my kit car but I
am
> planning to get the ABS in first so I can just use the signals...
>
>
> Kev/.
>
>
> ______________________________________________

2004\10\19@115247 by John Pearson

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face

----- Original Message -----
From: "Andy Meng" <RemoveMEAndyMengspamTakeThisOuTletu.edu>
To: "Microcontroller discussion list - Public." <piclistEraseMEspam.....mit.edu>
Sent: Tuesday, October 19, 2004 7:06 AM
Subject: Re: [PIC] Traction Control / Wheelspeed sensing


> I like that idea. It could possibly be as simple as strain gauges on the
> diff mounts. It seems to me that the torque required to break the tires
free
> would be independent of speed.

Keep in mind weight transfer. Off the line, weight transfer to the rear
wheels will be greatest. As speed increases, weight transfer to the rear
wheels will lessen. Then there is chassis torque, the twisting of the
chassis and drive train.


On the other hand, I don't know that this
> would offer information much different from an accelerometer - the torque
is
> going to drop when the wheels start spinning since kinetic friction is
less
> than static friction. Once the wheels are broken free it's pretty easy to
> keep them spinning. An accelerometer would give similar data, a peak as
the
> tires were on the edge of traction (or in the optimal slip area), then
> falling off.
>
> One conclusion I have come to is that I should have posed this question a
> month ago.
>
> Andy
>
>
> > No, drive torque drops or becomes near constant instead of increasing
with
> > rpm. This causes the engine rpm to increase (fast). One can also measure
> > car acceleration vs. drive shaft torque using a horizontal
accelerometer.
> > Just measuring and limiting the drive shaft torque is probably
sufficient
> > as a first approach.
> >
> > Peter
>
> ______________________________________________

2004\10\19@115508 by hael Rigby-Jones

picon face


>-----Original Message-----
>From: EraseMEpiclist-bouncesspammit.edu [RemoveMEpiclist-bouncesEraseMEspamEraseMEmit.edu]
>On Behalf Of Andy Meng
>Sent: 19 October 2004 15:53
>To: Microcontroller discussion list - Public.
>Subject: Re: [PIC] Traction Control / Wheelspeed sensing
>
>
>These sensors are inductive? So basically there is a metal
>gear, and the sensor senses each tooth?
>

Yes, they are inductive and therefore a PITA to interface to, as the
amplitude increases with wheel speed and the waveform changes polarity when
the tooth moves away from the sensor. Your interface circuit has to be able
to deal with all this to reliably detect the crossing point.  IC's
specifically for this application exist of course, but the hall-effect
sensor refered to in another post looks like a much nicer solution.

Regards

Mike

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____________________________________________

2004\10\19@131315 by Milosz Kardasinski

picon face
> >These sensors are inductive? So basically there is a metal
> >gear, and the sensor senses each tooth?
> >
>
> Yes, they are inductive and therefore a PITA to interface to, as the
> amplitude increases with wheel speed and the waveform changes polarity when
> the tooth moves away from the sensor. Your interface circuit has to be able
> to deal with all this to reliably detect the crossing point.  IC's
> specifically for this application exist of course, but the hall-effect
> sensor refered to in another post looks like a much nicer solution.

A VR sensor is the preferred method...all OEM's stopped using hall
effect stuff in the early 90's. Attaching magnets to stuff might pose
a problems with respect to balance and other aspects of the mechanical
system.

I've been working with VSS sensors on automobiles lately and have
realized that they are robust little sensors. The LM1815 is a very
nice IC to use and solves all your noise and signal issues.
____________________________________________

2004\10\19@142035 by John Pearson

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----- Original Message -----
From: "Kev Pearce (kevp.com)" <RemoveMEemail.mespam_OUTspamKILLspamkevp.com>
To: "Microcontroller discussion list - Public." <RemoveMEpiclistTakeThisOuTspamspammit.edu>
Sent: Tuesday, October 19, 2004 6:05 AM
Subject: Re: [PIC] Traction Control / Wheelspeed sensing


> > If all else fails and you end up using your right foot for your traction
> > control, try something I learned on street radials: Get the car moving,
> then
> > lift for a split second. Once you lift you can floor it without
wheelspin.
>
> Techcically known as a 'Granny Start'!!!

Ah yes! But very consistant. One less variable. A little practice and it
becomes hardly noticable.

>
> Kev/.
>
>
>
> ______________________________________________

2004\10\19@143843 by Andy Meng

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It sounds like an interesting idea, but does it work well dependent of
engine power? It seems like it wouldn't work as well on a powerful, light
car... I'll have to try it on the way to work in a few minutes. :-)


Andy


> > > If all else fails and you end up using your right foot for your
traction
> > > control, try something I learned on street radials: Get the car
moving,
> > then
> > > lift for a split second. Once you lift you can floor it without
> wheelspin.
> >
> > Techcically known as a 'Granny Start'!!!
>
> Ah yes! But very consistant. One less variable. A little practice and it
> becomes hardly noticable.

____________________________________________

2004\10\19@144156 by Andy Meng

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How does this work? A metallic toothed wheel with an inductive pickup next
to it? Does the pickup need a magnet on one side (I think the hall effect
devices work this way), or is the coil used as a variable inductor in a
resonant circuit?

It seems to me that by attaching small (1/4" dia x 1/8" thick) magnets
equidistant around the rim, the balance effect would be minimized... Is
there a reason other than this that cars have switched to a VR system?

Thanks,
Andy

{Original Message removed}

2004\10\19@163747 by Roland

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At 01:53 PM 19/10/2004 -0400, you wrote:
>How does this work? A metallic toothed wheel with an inductive pickup next
>to it? Does the pickup need a magnet on one side (I think the hall effect
>devices work this way), or is the coil used as a variable inductor in a
>resonant circuit?
>

I've interfaced to truck VSS sensors. They already have a magnet built in,
so the passing tooth induces a field. Very simple to interface, two wires,
just limit the ac coming in, and convert to ttl.

Don't know much about racing cars, but I would imagine it'd be easier to
control spin on the wheels using the brake, as in opposite to abs. (limited
slip)

Basically monitor two systems independently;
propshaft speed(use one wheel speed) related to engine power/throttle in
one relationship

back wheel speeds related to each other and brake control on fastest wheel
if more than say 10% of other.
it's a lot easier/quicker to limit a wheel than drop the engine power and
bring it up again? This control  would be easier using an op-amp balance.

Regards
Roland


____________________________________________

2004\10\19@190901 by John Pearson

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----- Original Message -----
From: "Andy Meng" <EraseMEAndyMengspamspamspamBeGoneletu.edu>
To: "Microcontroller discussion list - Public." <RemoveMEpiclistKILLspamspammit.edu>
Sent: Tuesday, October 19, 2004 10:50 AM
Subject: Re: [PIC] Traction Control / Wheelspeed sensing


> It sounds like an interesting idea, but does it work well dependent of
> engine power? It seems like it wouldn't work as well on a powerful, light
> car... I'll have to try it on the way to work in a few minutes. :-)
>
>
> Andy
>
Who can say. I used it on a 2300 lb front engine, rear wheel drive Mustang
5.0 with street radials. It's all in the timing and duration. Your results
may vary.

{Quote hidden}

> ______________________________________________

2004\10\19@193401 by Andrew Warren

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John Pearson <piclistSTOPspamspamspam_OUTmit.edu> wrote:

> > Get the car moving, then lift for a split second. Once you lift
> > you can floor it without wheelspin.
> ....
> A little practice and it becomes hardly noticable.

   Hardly noticeable anywhere but in your ET, you mean...

   -Andy

=== Andrew Warren -- spamBeGoneaiwSTOPspamspamEraseMEcypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

____________________________________________

2004\10\19@194355 by Bill & Pookie

picon face
For a speedometer how about a "drag mouse".  as entering pit lane, drop and
drag an optical mouse along the payment.

For slippage, one optical mouse sensing each wheel/tire.

Bill

Current project is using a pic as the controller for a plotter using an old
Etch-a-Sketch.

{Original Message removed}

2004\10\19@224959 by John Pearson

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----- Original Message -----
From: "Andrew Warren" <KILLspamaiwspamBeGonespamcypress.com>
To: "Microcontroller discussion list - Public." <EraseMEpiclistspamEraseMEmit.edu>
Sent: Tuesday, October 19, 2004 4:38 PM
Subject: Re: [PIC] Traction Control / Wheelspeed sensing


{Quote hidden}

Yes, but not nearly as much as spinning the tires.

> === Andrew Warren -- spamBeGoneaiwspamKILLspamcypress.com
> === Principal Design Engineer
> === Cypress Semiconductor Corporation
> ===
> === Opinions expressed above do not
> === necessarily represent those of
> === Cypress Semiconductor Corporation
>
> ______________________________________________

2004\10\20@000511 by Andrew Warren

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Andy Meng <.....piclistspam_OUTspammit.edu> wrote:

> we have about five drivers (which I think is too many - but that's
> not my decision) and bringing them all up to the same point would
> probably be a bit difficult given their level of experience.

   Nah, just let your best driver find the right launch and shift
   RPMs, then have him tell the others.  Once they know where to
   hold the revs and when to shift, even your worst drivers should
   do fine.  I mean, GIRLS do ok on the dragstrip, so how hard can
   it be?

   [That was a joke...]

> ideally, we will have a pot that will allow us to adjust the amount
> of slip for different conditions such as tire temp, rain
> tires/slicks, etc.

   That's only "ideal" if you can know exactly how those variables
   affect the optimum setting.  I'm not sure you can.

> Tire deformation should be pretty close front to back, unless the back
> is spinning out of control (ever seen top fuel/funny car tires at
> launch?)

   Fortunately for you, your rear tires won't do that.  The
   deformation I was referring to isn't the wind-up and ballooning
   that you see with drag slicks, but the smaller changes that can
   come from weight transfer...  But with your engine's torque, I
   don't even think that'll be a big issue.  

> We have a Zexel-Torsen diff, which I understand to be limited slip,
> torque balancing (still curious about how that works...

   Torsens are really nice.  Your diff has its own web page:

       http://www.torsen.com/fsae/fsaefaqframes.htm

   but there isn't much "how it works" info there.  Try:
   
       www.sonic.net/garyg/zonc/TechnicalInformation
       /TorsenDifferential.html


> > I'd simplify the output side of the system by choosing to reduce
> > engine power by retarding the timing; it's fast, effective, and
> > less intrusive with fewer unwelcome side-effects than cutting
> > fuel or spark. It also requires only one output instead of the
> > four that you're planning on.
>
> This is the main part of the post that is getting me thinking. I
> can't think of a way to do this with the stock ECM, other than
> maybe delaying the ignition pulse generator

   That seems like a pretty good way...

> As far as one-line control, I would prefer to have a variable
> amount of effect, i.e, with control of each cylinder, so we can
> cut a different number of cylinders depending on how much we need
> to slow down the engine.

   If you have a distributor, you only need one line to control all
   four cylinders (and if not, there's probably still a single wire
   somewhere in your ignition system that pulses -- or shorts to
   ground or whatever-- for each cylinder's spark).

   Delaying that signal retards your timing.  The amount of delay
   can be adjustable, and you can choose spark-by-spark whether to
   delay the timing at all (although you'll probably find that
   there's no need to have control of each cylinder individually).

> What are the side-effect of cutting fuel/spark you are referring
> to?  By the way, we are using an F4i engine (CBR600)

   Cutting fuel (at least on your motor, which has an injector per
   cylinder) isn't TOO bad; airflow through the induction and
   exhaust system suffers, but you don't care much about that, since
   you're trying to reduce power anyway. With injectors that are
   shared among multiple cylinders, cutting fuel is more dangerous,
   since you run the risk of loading a cylinder with a lean mixture
   that'll detonate.  On a carbureted engine, cutting fuel probably
   wouldn't be worth attempting to do.

   Cutting the spark has the potential for bad side-effects, as
   well.  Unburnt fuel can collect in the cylinder, wash down the
   cylinder walls, and blow past the rings into the crankcase where
   it'll dilute your oil and/or build up in the form of explosive
   vapor.  Fuel that doesn't end up in the crankcase will get
   pumped into your hot exhaust system to burn there.  None of this
   makes your car go faster or last longer.

   Of course, your motor isn't going to spend a lot of time in the
   reduced-power state -- and it doesn't have to last a long time
   anyway -- so these are mostly academic arguments... But in
   general, retarding the timing is safer and smooother than
   cutting fuel or spark.

{Quote hidden}

   With what you've said above and in your other emails, you
   already have all the building blocks to come up with a
   slip-sensing method that doesn't require wheelspeed
   measurements... But if it doesn't come to you, just say so and
   I'll post it.

> Another key task (far more so than traction control) this year is
> getting a solid electrical system.

   Yeah, it's funny how the high-tech stuff (like a 500kbps data
   link from each wheel, for instance) is easy and reliable, but
   what makes you lose races are the dumb low-tech mistakes like
   failing to crimp AND solder connectors, using spade lugs instead
   of rings, forgetting to make everything waterproof, letting wires
   brush against the sharp edges of holes you've punched in
   sheetmetal, etc.

   Best of luck.

   -Andy

=== Andrew Warren -- TakeThisOuTaiw.....spamTakeThisOuTcypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

____________________________________________

2004\10\20@042351 by hael Rigby-Jones

picon face
>-----Original Message-----
>From: TakeThisOuTpiclist-bouncesKILLspamspamspammit.edu [.....piclist-bouncesspamRemoveMEmit.edu]
>On Behalf Of Milosz Kardasinski
>Sent: 19 October 2004 18:13
>To: Microcontroller discussion list - Public.
>Subject: Re: [PIC] Traction Control / Wheelspeed sensing
>
>A VR sensor is the preferred method...all OEM's stopped using
>hall effect stuff in the early 90's. Attaching magnets to
>stuff might pose a problems with respect to balance and other
>aspects of the mechanical system.
>

The hall effect sensors mentioned actually have a magnet built in, so they
are VR sensors, but use a hall effect device instead of a coil, and have all
the signal conditioning built in, and best of all they seem to be very
inexpensive (~$7 was quoted).  I suspect a simple coil based VR sensor would
be much cheaper to manufacture and more robust, but I don't know of any
sources for low quantities at good prices.

Regards

Mike

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2004\10\20@071505 by Peter L. Peres

picon face

On Tue, 19 Oct 2004, Andy Meng wrote:

> These sensors are inductive? So basically there is a metal gear, and the
> sensor senses each tooth?

They are differential gmr sensors with a bias magnet built in. Old ones
may have been the oscillator type. They have to get close to the teeth and
stay that way while the wheel experiences violent accelerations from road
umps etc.

Peter
____________________________________________

2004\10\20@074941 by hael Rigby-Jones

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{Quote hidden}

All the ABS sensors I have changed were simply VR sensors using a coil and a
permanent magnet, no active components at all.

Mike

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2004\10\20@104522 by Peter L. Peres

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On Wed, 20 Oct 2004, Michael Rigby-Jones wrote:

>> -----Original Message-----
>> From: TakeThisOuTpiclist-bouncesspamspammit.edu [piclist-bouncesEraseMEspammit.edu]
>> On Behalf Of Milosz Kardasinski
>> Sent: 19 October 2004 18:13
>> To: Microcontroller discussion list - Public.
>> Subject: Re: [PIC] Traction Control / Wheelspeed sensing
>>
>> A VR sensor is the preferred method...all OEM's stopped using
>> hall effect stuff in the early 90's. Attaching magnets to
>> stuff might pose a problems with respect to balance and other
>> aspects of the mechanical system.
>>
>
> The hall effect sensors mentioned actually have a magnet built in, so they
> are VR sensors, but use a hall effect device instead of a coil, and have all
> the signal conditioning built in, and best of all they seem to be very
> inexpensive (~$7 was quoted).  I suspect a simple coil based VR sensor would
> be much cheaper to manufacture and more robust, but I don't know of any
> sources for low quantities at good prices.

imho you don't want one of those for his application. Traction control
must work at ~0 wheel speed.

Peter
____________________________________________

2004\10\20@104525 by Peter L. Peres

picon face

On Wed, 20 Oct 2004, Michael Rigby-Jones wrote:

{Quote hidden}

And none of them work for traction control, f.ex. as neeed for starting
safely on ice, since they give 0 signal at 0 wheel speed, and that's when
you need traction control most ...

Peter
____________________________________________

2004\10\20@113750 by hael Rigby-Jones

picon face


{Quote hidden}

Why would this not work at zero wheel speed?  Hall effect devices work fine
with static magnetic fields.  Also I am a little unsure as to why this would
be a problem anyway, zero output = zero wheel speed. Also surely zero wheel
speed is not a common condition for traction control, ABS perhaps...

Regards

Mike

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____________________________________________

2004\10\20@121608 by Bob Blick

face picon face
>> ideally, we will have a pot that will allow us to adjust the amount
>> of slip for different conditions such as tire temp, rain
>> tires/slicks, etc.

I'm not sure how much of this would be track-legal. Most anything with
feedback is not allowed, at least in drag racing.

All you need is a set of Mr. Gasket Traction Bars anyway :)

Cheerful regards,

Bob

____________________________________________

2004\10\20@131633 by Andy Meng

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The reason we can't do that is because it would require electronic brake
activation. That would likely be heavy and it would add complexity to the
system that we would like to avoid.

Andy

{Original Message removed}

2004\10\20@142248 by Andy Meng

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This is for the Formula SAE competition - traction control is legal.

Andy

----- Original Message -----
From: "Bob Blick" <RemoveMEbblick@spam@spamspamBeGonesonic.net>
To: "Microcontroller discussion list - Public." <.....piclist@spam@spamEraseMEmit.edu>
Sent: Wednesday, October 20, 2004 12:16 PM
Subject: Re: [PIC] Traction Control / Wheelspeed sensing


{Quote hidden}

> ______________________________________________

2004\10\21@095733 by Alan B. Pearce

face picon face
>One conclusion I have come to is that I should
>have posed this question a month ago.

Ahh, I can see this university is teaching you something :)))))))

____________________________________________

2004\10\21@101619 by Peter L. Peres

picon face

On Wed, 20 Oct 2004, Michael Rigby-Jones wrote:

{Quote hidden}

You said (I think) that most wheel sensors you know were coil type VR. A
coil type VR sensor does not sense at zero speed. A GMR one does, and an
active VR one does too (with a lm or other oscillator chip). The highest
available torque appears in the lowest gear, at standstill, when the car
starts moving, and also in low gears, when the engine is revved up into
its maximum torque zone (which is at 1/2 to 3/4 of max rpm for most
untuned gas engines). So to have efficient traction control you want to
know torque and speed at very low wheel speeds (essentially zero). Even
more so in drag racing (op's target) where the goal is maximum
acceleration.

Peter
____________________________________________

2004\10\21@142309 by Peter L. Peres

picon face

On Wed, 20 Oct 2004, Michael Rigby-Jones wrote:

>> simple coil based VR sensor would be much cheaper to manufacture and
>> more robust, but I don't know of any sources for low quantities at good
>> prices.
>>
>> imho you don't want one of those for his application. Traction control
>> must work at ~0 wheel speed.
>>
>> Peter
>
> Why would this not work at zero wheel speed?  Hall effect devices work fine
> with static magnetic fields.  Also I am a little unsure as to why this would
> be a problem anyway, zero output = zero wheel speed. Also surely zero wheel
> speed is not a common condition for traction control, ABS perhaps...

A VR sensor of the kind described is passive, it consists of a coil and a
magnet. The output voltage is nearly zero at low speed. The active type of
VR sensor (using lm*** and mitsumi chips) does give signal at very low
speed but may be more expensive than differential gmr. The only reason I'd
think of to use active VR instead of gmr is low temperature behavior. But
they have that fixed now.

Peter
____________________________________________

2004\10\22@034619 by hael Rigby-Jones

picon face


{Quote hidden}

Peter, the quotes above are a bit out of context, the second post from me
above was refering to an active sensor using a hall effect device coupled
with signal conditioning circuitry.

Regards

Mike

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