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'[EE]: Looking for method to differentiate gravityf'
2005\08\31@131736 by olin piclist

face picon face
David Van Horn wrote:
> You can measure pressure in the brake lines though, which should do the
> job.

Theoretically that would work, but that's one part of a car you shouldn't
mess with.  I would look for ways to detect how far the pedal has been
pressed or some other part of the linkage has been moved without breaking
any connections or getting "into" the system.


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2005\08\31@132552 by Mauricio Jancic

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face
Mmm if you have one measurement axis parallel to the road, and the other
perpendicular to it, then, when you are down the hill, you'll see a
acceleration smaller than 1G on the perpendicular axis.

And you'll also see that acceleration decrease when the car is breaking... I
think I saw an acceleration sensor with those two perpendicular axis...

Is anything wrong with that?

Mauricio Jancic
Janso Desarrollos - Microchip Consultants Program Member
spam_OUTinfoTakeThisOuTspamjanso.com.ar
http://www.janso.com.ar
(54) 11 - 4542 - 3519

2005\08\31@133011 by M. Adam Davis

face picon face
If you do need to integrate into the brake system a trailer brake
controller would be a good choice for safety.

-Adam

On 8/31/05, Olin Lathrop <.....olin_piclistKILLspamspam@spam@embedinc.com> wrote:
{Quote hidden}

> -

2005\08\31@134308 by M. Adam Davis

face picon face
That should work, though there are two factors that may cause problems:

1) G is not constant all over the earth.  This could probably be
calibrated at engine turn-on with no problems.  The difference may
even be small enough in the locations the device is expected to be
used that one might not need to consider it at all.
2) If the car is on a banked surface then the device may think the car
is going downhill since the perpendicular will read less than 1G.  In
this case the light would not come on if you did brake slowly.  A
three axis accelerometer would remedy this.  Might only matter at the
race track and some highway entrance and exit ramps.

-Adam

On 8/31/05, Mauricio Jancic <infospamKILLspamjanso.com.ar> wrote:
{Quote hidden}

> -

2005\08\31@134341 by PicDude

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face
On Wednesday 31 August 2005 12:18 pm, Olin Lathrop scribbled:
> David Van Horn wrote:
> > You can measure pressure in the brake lines though, which should do the
> > job.
>
> Theoretically that would work, but that's one part of a car you shouldn't
> mess with.  I would look for ways to detect how far the pedal has been
> pressed or some other part of the linkage has been moved without breaking
> any connections or getting "into" the system.


I'd agree, but the braking system is quite reliable and easy to mess with.  In
fact, the brake light switch is quite often a pressure switch itself.  Most
cars nowadays (built in the last few decades) also come with a split braking
system such that if pressure is lost in one half, the prop valve will shut
off that side and the other 2 wheels will still brake.

Cheers,
-Neil.


2005\08\31@161648 by David Van Horn

picon face
> Surely you are braking when the brakelight is turned on, which you can
> detect, since you are attached to the brakelight - or am I totally out
> to lunch?

I understood that he wanted something proportional to the amount of
braking being applied.




2005\08\31@161835 by David Van Horn

picon face
> 2) If the car is on a banked surface then the device may think the car
> is going downhill since the perpendicular will read less than 1G.  In
> this case the light would not come on if you did brake slowly.  A
> three axis accelerometer would remedy this.  Might only matter at the
> race track and some highway entrance and exit ramps.

Would it work in San Francisco?

:)




2005\08\31@163845 by David P Harris

picon face
David Van Horn wrote:

>>Surely you are braking when the brakelight is turned on, which you can
>>detect, since you are attached to the brakelight - or am I totally out
>>to lunch?
>>    
>>
>
>I understood that he wanted something proportional to the amount of
>braking being applied.
>
>
>
>
>  
>
No, he asked how to distinguish braking from going downhill.  If the
brake pedal is pushed, then braking is occurring and the brake-light
comes on.  I am assuming that he wants to install a 'smart-bulb' at the
brake-light, so am assuming that the braking signal is present.
David


2005\08\31@164045 by olin piclist

face picon face
Herbert Graf wrote:
> Consider a rocket with a force meter on the nose, and one on the tail,
> and the rocket is on the earth pointed straight up. On the pad the two
> meters would differ in reading, since the one on the tail is closer to
> the earth then the one on the nose.

Correct in theory, but now figure out the difference in practise, even for a
large rocket.  The radius of the earth is about 6.38Mm.  Let's say the large
rocket is 100m tall.  That yields a distance ratio of 1.0000157 between top
and bottom.  Gravity is proportional to 1/R**2, so the gravity ratio is
1.0000314, or about 31ppm.  Measuring accelleration to that accurracy is NOT
easy.


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2005\08\31@170056 by Rick Thompson

picon face

> I understood that he wanted something proportional to the amount of
> braking being applied.


How about simply putting a strain gage on a brake line?  (a very small one)



2005\08\31@180341 by Oscar T.

picon face
I don't see the relevance of gravity.

The earth's attraction, the gravity force F = m*g,  is almost
compensated by the reaction of the earth's surface, the resultant of
both is the centripetal force F = m*w^2*R, where  w is the angular
velocity ( = 2*pi/(24*60*60) ) and R is the earth's radius. If you do
the math the centripetal force is almost zero (actually 0.0000337
m/s^2), and that is what an accelerometer should measure along the
axis perpendicular to the earth's surface. No matter if it's downhill
or 'uphill' as long as it is steady.

By far the acceleration/deceleration of the car relative to the
earth's surface/road is what an accelerometer will measure.


Oscar

2005\08\31@182335 by David Van Horn

picon face
> By far the acceleration/deceleration of the car relative to the
> earth's surface/road is what an accelerometer will measure.

Well, if you manage to exceed 1G on a flat road, let me know. :)

How do you tell the difference between a car nose down on a hill, and a
car on flat land braking a bit, or a car nose up on a hill, braking
harder?




2005\08\31@184413 by Jinx

face picon face
> How do you tell the difference between a car nose down on a
> hill, and a car on flat land braking a bit, or a car nose up on a hill,
> braking harder ?

I don't know, how do you tell the difference between a car nose
down on a hill, and a car on flat land braking a bit, or a car nose
up on a hill, braking harder ?

Oh wait, you didn't go "I say I say I say" first and this isn't 1923

You'd definitely need sensors such as accelerometers and some
filtering s/w. You would have to know the orientation of the car
and look for sudden hard braking, whilst not responding to short
unimportant events like humpback bridges, speed bumps, getting
into driveways even

To be honest, I can't see how the DataBrake I mentioned in the
other post will make a huge difference. Firstly, every driver
following such a car would have to know the significance of the
hazards coming on. Secondly, it's a driver behaviour thing anyway.
People follow too closely to avoid rearenders no matter how
smart brake lights are

2005\08\31@193231 by Oscar T.

picon face
2005/9/1, David Van Horn <EraseMEdvanhornspam_OUTspamTakeThisOuTmicrobrix.com>:
> > By far the acceleration/deceleration of the car relative to the
> > earth's surface/road is what an accelerometer will measure.
>
> Well, if you manage to exceed 1G on a flat road, let me know. :)
>

It 's not about to exceed 1G. It's about to exceed  0.0000337/9.8 =
0.0000034 G.

Cheers.


'[EE]: Looking for method to differentiate gravityf'
2005\09\01@004357 by Herbert Graf
flavicon
face
On Wed, 2005-08-31 at 16:40 -0400, Olin Lathrop wrote:
> Herbert Graf wrote:
> > Consider a rocket with a force meter on the nose, and one on the tail,
> > and the rocket is on the earth pointed straight up. On the pad the two
> > meters would differ in reading, since the one on the tail is closer to
> > the earth then the one on the nose.
>
> Correct in theory, but now figure out the difference in practise, even for a
> large rocket.  The radius of the earth is about 6.38Mm.  Let's say the large
> rocket is 100m tall.  That yields a distance ratio of 1.0000157 between top
> and bottom.  Gravity is proportional to 1/R**2, so the gravity ratio is
> 1.0000314, or about 31ppm.  Measuring accelleration to that accurracy is NOT
> easy.

And, if you had bothered reading my post in this thread PRIOR to this
post you would have seen me say:

"Yes, there is a difference, it's just really hard to measure."

TTYL

-----------------------------
Herbert's PIC Stuff:
http://repatch.dyndns.org:8383/pic_stuff/

2005\09\01@075330 by olin piclist

face picon face
Oscar T. wrote:
> By far the acceleration/deceleration of the car relative to the
> earth's surface/road is what an accelerometer will measure.

Totally wrong.  It will also measure 1g up, the apparent accelleration due
to gravity.  In other words a "stationary" accellerometer sitting on my desk
will measure about 9.8m/s**2 upwards.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\09\01@082826 by olin piclist

face picon face
> I see no way to differentiate between braking and going downhill. Any
> ideas ?

You seem to be hung up in the process and forgot to do a reality check.
First, going up or down hill by itself isn't relevant.  Your implicit
assumption is that you are going at a constant speed down that hill.  If
you're coasting there will be no front/back accelleration.  If you are in
fact going down the hill at a constant speed, then you *are* braking, either
with the brake or some other means like with a low gear.  Does it really
matter which if you are trying to alert cars behind you?  Will they be less
likely to rear end you if you are braking with a low gear than with the
brake, even though the resultant car motion is the same?

Another simple reality check you have missed is the magnitude of the
deccelleration when going down a hill at constant speed.  6% grade is
considered a "steep" hill for a normal paved road where speed is expected to
be several 10s of miles/hour.  Such a 6% down grade will have truck warning
signs at the top and will require using low gear or braking most of the
time, else your speed will get unacceptably high in just a few seconds (you
will pick up about 13 MPH in 10 seconds).  Now 6% grade is just what it
says, so inside a car going down that hill at constant speed it will feel
just like braking at 0.06g.  This is rather mild braking, and probably not
far from the noise floor for an accellerometer bouncing around in a car
anyway.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\09\01@084503 by Russell McMahon

face
flavicon
face
>> By far the acceleration/deceleration of the car relative to the
>> earth's surface/road is what an accelerometer will measure.

> Totally wrong.  It will also measure 1g up, the apparent
> accelleration due
> to gravity.  In other words a "stationary" accellerometer sitting on
> my desk
> will measure about 9.8m/s**2 upwards.

He qualified his comment with the cavea "perpendicular to the earth's
surface", which is liable to be the useful direction for a single axis
accelerometer to be aligned when attempting to measure braking.

He was, if I understand him correctly, incorrect to the extent that
the surface slopes relative to a line from the centre of the earth -
ie local vertical. As I know you are aware, on  a hill a horizontal
accelerometer will measure Sin(X) * g where X is the angle of the
slope.

Putting together what several people have already said.

- Brake light signal will be useful.

- A fairly sure fire method to detect braking is to vector sum
vertical and horizontal components relative to the car. With no
braking these should always be ABOUT 1g in vector sum regardless of
slope. If the sum exceeds one g it will be due to braking except that
transient conditions (rate of change of acceleration/deceleration)
will produce transient signals



       RM

2005\09\01@085458 by olin piclist

face picon face
Russell McMahon wrote:
> He qualified his comment with the cavea "perpendicular to the earth's
> surface", which is liable to be the useful direction for a single axis
> accelerometer to be aligned when attempting to measure braking.

But apparent accelleration due to gravity *is* perpendicular to the earth's
surface on average.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\09\01@090118 by Michael Rigby-Jones

picon face


>-----Original Message-----
>From: piclist-bouncesspamspam_OUTmit.edu [@spam@piclist-bouncesKILLspamspammit.edu]
>Sent: 01 September 2005 13:56
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE]: Looking for method to differentiate
>gravityfromacceleration
>
>
>Russell McMahon wrote:
>> He qualified his comment with the cavea "perpendicular to
>the earth's
>> surface", which is liable to be the useful direction for a
>single axis
>> accelerometer to be aligned when attempting to measure braking.
>
>But apparent accelleration due to gravity *is* perpendicular
>to the earth's surface on average.


I suspect the OP meant tangential...

Mike

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2005\09\01@094012 by Howard Winter

face
flavicon
picon face
Olin,

On Thu, 1 Sep 2005 07:54:16 -0400, Olin Lathrop wrote:

> Oscar T. wrote:
> > By far the acceleration/deceleration of the car relative to the
> > earth's surface/road is what an accelerometer will measure.
>
> Totally wrong.  It will also measure 1g up, the apparent accelleration due
> to gravity.  In other words a "stationary" accellerometer sitting on my desk
> will measure about 9.8m/s**2 upwards.

Absolutely right - a G-meter in an aircraft will read +1 when it's parked, which surprises most student pilots
the first time they see it.  There are telltales that show the max and min achieved since they have been
reset, and having them both within 0 and +2G indicates it was a smooth flight and a decent landing!  :-)

Cheers,



2005\09\01@123754 by gacrowell

flavicon
face


I'd suggest trying rate of change of velocity.
Velocity measured from wheel rotation or some sort of ground motion
sensor.

Gary Crowell

2005\09\01@131600 by olin piclist

face picon face
gacrowell@micron.com wrote:
> I'd suggest trying rate of change of velocity.

What a great idea!  And here we were all wasting time talking about
measuring accelleration.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\09\01@135822 by David Van Horn

picon face


> -----Original Message-----
> From: KILLspampiclist-bouncesKILLspamspammit.edu [RemoveMEpiclist-bouncesTakeThisOuTspammit.edu] On
Behalf
{Quote hidden}

Superb olin, just superb!




2005\09\01@144756 by Mauricio Jancic

flavicon
face
>> I'd suggest trying rate of change of velocity.

>What a great idea!  And here we were all wasting time talking about
>measuring accelleration.

That's just hilarious... or however you spell it...

Mauricio Jancic
Janso Desarrollos - Microchip Consultants Program Member
TakeThisOuTinfoEraseMEspamspam_OUTjanso.com.ar
http://www.janso.com.ar
(54) 11 - 4542 - 3519

2005\09\01@160855 by Peter

picon face


On Thu, 1 Sep 2005, Gordon LaPoint wrote:

> On Wed, 2005-08-31 at 18:26, David Van Horn wrote:
>
>> Well, if you manage to exceed 1G on a flat road, let me know. :)
>>
> AAFD class drag race cars do .25 mile standing start in about 4.7
> seconds, reaching a peak speed of >300MPH, exceeding 4G for some of the
> race, on a flat road.

According to v = a*t 0.25 miles to 300MPH in 5 seconds corresponds to 27
m/sec^2 (~3G). Horizontal acceleration above 1G is not possible without
anti-g (or pro-g) devices or reactive propulsion imho.

More realistic figures are e.g. 0-100km/h in 9 seconds which from above
yields 2.8 m/sec^2 (~0.3G). 0-100km/h in 4.5 seconds is close to the
limit of adherence of the rubber tyres to the ground. It's still only
~0.6G.

An aircraft that takes off at about 160mph (71 m/sec) and needs 1200
meters to do that pulls ~0.2G's horizontal (a = v^2/(2*x)).

Peter

2005\09\01@161719 by Mark Jordan

flavicon
face
On 1 Sep 2005 at 13:17, Olin Lathrop wrote:

> RemoveMEgacrowellspamTakeThisOuTmicron.com wrote:
> > I'd suggest trying rate of change of velocity.
>
> What a great idea!  And here we were all wasting time talking about
> measuring accelleration.
>

       How could that be done without messing with the engine or wheels?

       Mark


2005\09\01@164747 by Rich Mulvey

flavicon
face
Mark Jordan wrote:

{Quote hidden}

   Ummmm.... I'm not sure how to put this gently, but I think you might
need to look at http://dictionary.reference.com/search?r=2&q=acceleration.

- Rich


2005\09\01@170309 by gacrowell

flavicon
face


> On 1 Sep 2005 at 13:17, Olin Lathrop wrote:
>
> > EraseMEgacrowellspammicron.com wrote:
> > > I'd suggest trying rate of change of velocity.
> >
> > What a great idea!  And here we were all wasting time talking about
> > measuring accelleration.
> >
>
>        How could that be done without messing with the engine
> or wheels?
>
>        Mark

Well, before Olin clipped my post for the humor value, I'd suggested a
wheel sensor (without realizing the original poster was looking for a
stand-alone installation), or some sort of ground motion sensor.  I have
no idea what that sensor might be, other than perhaps some variation on
an ultrasonic flowmeter.

Gary

2005\09\01@172043 by David Van Horn

picon face
> Well, before Olin clipped my post for the humor value, I'd suggested a
> wheel sensor (without realizing the original poster was looking for a
> stand-alone installation), or some sort of ground motion sensor.  I
have
> no idea what that sensor might be, other than perhaps some variation
on
> an ultrasonic flowmeter.

You could do Doppler microwave underneath the car without too many
problems.
Cosine error would have to be dealt with, but that ends up as a
constant.
Then you just look at how fast you are slowing down, and handle the
brake lights appropriately.

You have the advantage of knowing the speed, as well as the delta speed,
where pretty much everything else discussed of late has significant
complicating issues, or is measuring something indirect, like brake
pressure or pedal position.



2005\09\02@002354 by Carey Fisher - NCS

face picon face
  > -----Original Message-----
  > From: RemoveMEpiclist-bouncesEraseMEspamEraseMEmit.edu
  > [RemoveMEpiclist-bouncesspam_OUTspamKILLspammit.edu]On Behalf Of David Van Horn
  > Sent: Thursday, September 01, 2005 5:24 PM
  > To: Microcontroller discussion list - Public.
  > Subject: RE: [EE]: Looking for method to differentiate
  > gravityfromacceleration
  >
  >
  > > Well, before Olin clipped my post for the humor value, I'd
  > suggested a
  > > wheel sensor (without realizing the original poster was looking for a
  > > stand-alone installation), or some sort of ground motion sensor.  I
  > have
  > > no idea what that sensor might be, other than perhaps some variation
  > on
  > > an ultrasonic flowmeter.
  >
  > You could do Doppler microwave underneath the car without too many
  > problems.
  > Cosine error would have to be dealt with, but that ends up as a
  > constant.
  > Then you just look at how fast you are slowing down, and handle the
  > brake lights appropriately.


I know this doesn't help much but...
(never stopped me or several others here before)

There is No Gravity...the Earth Sucks!!!

2005\09\02@013646 by Russell McMahon

face
flavicon
face
>> I'd suggest trying rate of change of velocity.

> What a great idea!  And here we were all wasting time talking about
> measuring accelleration.

One could reasonably infer, with a prospect of being in error, that he
meant rate of rate of change, which may well be useful.


       RM

2005\09\02@033400 by PicDude

flavicon
face
On Thursday 01 September 2005 04:03 pm, RemoveMEgacrowellTakeThisOuTspamspammicron.com scribbled:
> Well, before Olin clipped my post for the humor value, I'd suggested a
> wheel sensor (without realizing the original poster was looking for a
> stand-alone installation), or some sort of ground motion sensor.  I have
> no idea what that sensor might be, other than perhaps some variation on
> an ultrasonic flowmeter.
>
> Gary

Not a bad thought -- he could use somthing like a pitot tube (that measures
airspeed) on an airplane.  It compares ram pressure (air being forced into
the tube), against static pressure and determines airspeed.  There are other
factors (such as density altitude), but this could probably be a non-factor
here for this non-precision app.

Cheers,
-Neil.



2005\09\02@033538 by PicDude

flavicon
face
On Thursday 01 September 2005 04:24 pm, David Van Horn scribbled:
> You could do Doppler microwave underneath the car without too many
> problems.
> Cosine error would have to be dealt with, but that ends up as a
> constant.
> Then you just look at how fast you are slowing down, and handle the
> brake lights appropriately.


Curious -- is this how an optical mouse works?
I think I'll go google for a bit...

Cheers,
-Neil.


2005\09\02@073629 by olin piclist

face picon face
PicDude wrote:
>> You could do Doppler microwave underneath the car without too many
>> problems.
>
> Curious -- is this how an optical mouse works?
> I think I'll go google for a bit...

No.  Modern optical mice have essentially a low resolution camera in them
that looks at the texture and inevitable imperfections of the surface.
Successive frames are compared to determine the distance moved between
frames.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\09\02@083702 by Jinx

face picon face
> > Curious -- is this how an optical mouse works?
> > I think I'll go google for a bit...
>
> No.  Modern optical mice have essentially a low resolution
> camera in them that ...................

http://computer.howstuffworks.com/mouse4.htm

2005\09\02@092100 by David Van Horn

picon face
> Curious -- is this how an optical mouse works?
> I think I'll go google for a bit...

No, mice use little TV cameras, and take pictures. At least that's what
I'm told.  Anything video-optical is going to have a lot of trouble
living underneath a car.


2005\09\02@163903 by Peter

picon face

The op asked for a self-contained device that must be inexpensive.
Measuring G to about 5% would do that. It could be done with
accelerometers or with an inexpensive pendulum (can be liquid - f.ex.
mercury switch or liquid level sensor, or a free-swiging pendulum with
2dof).

http://hyperphysics.phy-astr.gsu.edu/hbase/pendl.html

shows the formula for the period of a large amplitude pendulum. A 2cm
pendulum will have a period of about 0.3 seconds and at 15 degrees swing
deviates by less than 0.5% from the small amplitude pendulum period.

With 0.1G acceleration (~1m/sec^2) the period changes by more than 2%.
The pendulum measures absolute acceleration since it is free-swinging in
2dof. So it cannot differentiate between acceleration and deceleration
(but and-ing its output with the brake signal would solve that - for
example if the power comes from the brake light wire). It could be
sensed with a hall or optical sensor. It would 'bob' all the time due to
the car moving and people in it moving. It could provide the basic blink
rate (~3Hz) and this could be multiplied up as needed.

A more expensive solution is to use mems accelerometers and measure
sqrt(Gvert^2 + Gx^2 + Gy^2). The sqrt can be lost for math
simplification. Then detecting 0.01G should be possible using
accelerometers that are stable to 0.5% or so (S/N 46dB ?) after
filtering. No ?

Peter

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