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'Help on measuring speed'
1998\10\09@161449 by

Here is the problem:
I need to measure the speed of a truck and it need to be simple and cheap.

Range: 0 to 10 miles/h

My solution: Use a laser pointer and a photo detector, the weel will
interrupt
the laser. A PIC measure the time of the interruption, I know the size of
the
weel, the PIC calculate the speed and show on a 2 digit display.

My question:Can the laser pointer be on 24h/day? Any suggestions?

Regards,

Octavio
=======================================================
Octavio Nogueira - nogueiramandic.com.br ICQ# 19841898
>From the creator of ProPic,   ProPic 2  now much better
*        http://members.tripod.com/ProPic             *
* PIC Programmer for Windows with down to earth price *
=======================================================

>My question:Can the laser pointer be on 24h/day? Any suggestions?

Assuming you know the wheel size, at the height that the beam is, then
you can do this.
You'd be better off to use two beams, and measure the time between
breaking beam A and beam B.
That removes the size of the object as a variable.

Narrow beams have problems with bugs, rain, birds, and heat lensing.
You might think about a simple lens system to widen the beam to an
inch or two, then re-condense it on the other end.  This makes it
impossible for small things to block the beam.

You might try an alternative, grab a cheap microwave burglar alarm,
and take the speed from the Doppler output.

part 0 504 bytes
A laser pointer probably isn't the best method for sensing motion.
For detecting speed, you might be better off with a pre-made photo-
beam assembly.  One common type is a U-shaped gizmo with an IR led
and an IR photo-transistor pre-aligned and pre-focused on either
side of the slot.  If you can have another wheel on the same shaft
as the "real" one and have the edge of that smaller wheel sit in
the sensor's gap, then you should be able to easily and reliably
detect the wheel's rotation.

> 0 to 10 mph

That is very slow. Try to gain access to *both* front wheel's brake drums
and use two proximity sensors near the vent holes in the hubs imho. You
need to OR the pulses in the micro and you need to measure period, not
frequency at that speed.

> laser pointer, 24/24

Only for the 1st 24 hrs unless you derate the current ;). Also temp range
will temporarily weaken the beam very much as it heats up. Beyond 70 deg.
C case most laser pointers are worse than red LEDs. Try to spray freeze on
a working laser pointer some day (not on the lens, it will fog anyway).

Peter

As I understood it, he wanted to measure the speed of vehicles on a
road, not to attach something to the vehicles themselves.

Opto's might get dirty and can be affected by external light.
Other alternatives is a magnet on the axis or wheel with a Hall sensor
(this is used by some car computers).
Or ultrasonic TX/RX that bounces off the road (Doppler effect). Might be
affected by road conditions. Maybe bounce it off the wheel at an angle.
Somebody mentioned a proxy sensor, also a good idea, not sure how fast a
proxy can work though.

Quentin

>Assuming you know the wheel size, at the height that the beam is, then
>you can do this.
>You'd be better off to use two beams, and measure the time between
>breaking beam A and beam B.
>That removes the size of the object as a variable.
>
>Narrow beams have problems with bugs, rain, birds, and heat lensing.
>You might think about a simple lens system to widen the beam to an
>inch or two, then re-condense it on the other end.  This makes it
>impossible for small things to block the beam.
>
>You might try an alternative, grab a cheap microwave burglar alarm,
>and take the speed from the Doppler output.
>

That would be great, much better than the laser because I could measure
the speed several times for the same truck.
As a mather of fact I need to measure the speed and inform to the driver,
this
thing will go to a dynamic scale in a road, the scale is already running but
I
need the truck to maintain a constant speed, so I need to measure it and
inform
back to the driver.
I will put a big display near the scale and the measuring device 60ft
before, If I could
show the speed several times it would be easier for the driver.

Octavio
=======================================================
Octavio Nogueira - nogueiramandic.com.br ICQ# 19841898
>From the creator of ProPic,   ProPic 2  now much better
*        http://members.tripod.com/ProPic             *
* PIC Programmer for Windows with down to earth price *
=======================================================

On Sat, 10 Oct 1998, Octavio Nogueira wrote:

> That would be great, much better than the laser because I could measure
> the speed several times for the same truck.  As a mather of fact I need
> to measure the speed and inform to the driver, this thing will go to a
> dynamic scale in a road, the scale is already running but I need the
> truck to maintain a constant speed, so I need to measure it and inform
> back to the driver.  I will put a big display near the scale and the
> measuring device 60ft before, If I could show the speed several times it
> would be easier for the driver.

AH ! Why didn't you say it has to be on the road, not on the truck ? To
measure slow speed in the road, bury a set of closely spaced
pressure-sensitive hose switches across the road. If it's hardcover a
piezo strip will do better. The distance between them is given by their
speed vs. vehicle speed.

The other way to get such a slow speed read out is a modified ex-police
laser gun shooting from front or back. This one avoids digging the road
up. This can show speed continuoulsy as long as the truck is in range. You
can also mount the gun on the truck and have the readout near the driver.
It depends on what it bounces from though.

Peter

>AH ! Why didn't you say it has to be on the road, not on the truck ? To
>measure slow speed in the road, bury a set of closely spaced
>pressure-sensitive hose switches across the road. If it's hardcover a
>piezo strip will do better. The distance between them is given by their
>speed vs. vehicle speed.
>

Where I can get a piezo strip?

Octavio
=======================================================
Octavio Nogueira - nogueiramandic.com.br ICQ# 19841898
>From the creator of ProPic,   ProPic 2  now much better
*        http://members.tripod.com/ProPic             *
* PIC Programmer for Windows with down to earth price *
=======================================================

>>You might try an alternative, grab a cheap microwave burglar alarm,
>>and take the speed from the Doppler output.
>>
>
>That would be great, much better than the laser because I could
measure
>the speed several times for the same truck.

Well, 10Ghz microwave alarm heads can be had for very little \$ at ham
swap meets.  Some door openers are also microwave, either 10 or 24Ghz.
You'll need a stable, quiet gain amp to amplify the Doppler output,
but the fact that the target is the front end of a truck will help a
great deal.

Try piezo cable.  It's made to be buried and should last a long time.  If I
remember correctly Beldon Wire makes some.

Alan
{Original Message removed}
On Sun, 11 Oct 1998, Octavio Nogueira wrote:

> >AH ! Why didn't you say it has to be on the road, not on the truck ? To
> >measure slow speed in the road, bury a set of closely spaced
> >pressure-sensitive hose switches across the road. If it's hardcover a
> >piezo strip will do better. The distance between them is given by their
> >speed vs. vehicle speed.
> >
>
> Where I can get a piezo strip?

Oops. Try someone who supplies sonar gear. Otherwise, for testing, you can
sandwich many piezo discs between 2 conductor strips (copper tape) using
electro-conductive epoxy. The discs need to be 10-20 cm apart and be sure
the strip does not short out. Use a punched-out plastic tape as insulator.
This needs to be buried 2-5 cm deep in concrete or asphalt. When you drive
a car over it you get over 50 Volts output, at high impedance. The piezo
strip is less sensitive, flexible and made mostly of proprietary piezo
plastic.

imho, if you don't know where to get the piezo strips, get the hose
switches instead. I don't know who makes these but they are extensively
used by city transport depts. to make traffic surveys. It's a closed
rubber hose filled with gas or liquid with a pressure switch sensor or
piezo at one end. When you drive over it the pressure propagates to the
sensor through the liquid. I guess one could make such a sensor from gas
hose (thick walls) filled with light oil and closed with a pressure sensor
at one end (or piezo disc in suitable fixture).

there goes my next long mail to this list. I wish I could make my wording
more terse ;)

Peter

Octavio Nogueira wrote:
>
> >AH ! Why didn't you say it has to be on the road, not on the truck ? To
> >measure slow speed in the road, bury a set of closely spaced
> >pressure-sensitive hose switches across the road. If it's hardcover a
> >piezo strip will do better. The distance between them is given by their
> >speed vs. vehicle speed.
> >
>
> Where I can get a piezo strip?
>
> Octavio
> =======================================================
> Octavio Nogueira - nogueiramandic.com.br ICQ# 19841898
> >From the creator of ProPic,   ProPic 2  now much better
> *        http://members.tripod.com/ProPic             *
> * PIC Programmer for Windows with down to earth price *
> =======================================================
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On Sun, 11 Oct 1998, Dave VanHorn wrote:

> Well, 10Ghz microwave alarm heads can be had for very little \$ at ham
> swap meets.  Some door openers are also microwave, either 10 or 24Ghz.
> You'll need a stable, quiet gain amp to amplify the Doppler output,
> but the fact that the target is the front end of a truck will help a
> great deal.

Isn't there a problem with outdoor mounting these things ? It would have
to be hanged above the road somehow and sit in the full sun all day.
Sounds like milspec parts to me.

Peter

>Isn't there a problem with outdoor mounting these things ? It would
have
>to be hanged above the road somehow and sit in the full sun all day.
>Sounds like milspec parts to me.

Umm. where are supermarket door openers (the ones that see you coming
from the outside) mounted?

There is no problem with placing microvaves outdoors.  This
application isn't sensitive to the occasional false reading caused by
a sheet of rain. All they need is a rainproof enclosure, they'll be
fine.

On Mon, 12 Oct 1998, Dave VanHorn wrote:

> >Isn't there a problem with outdoor mounting these things ? It would
> have
> >to be hanged above the road somehow and sit in the full sun all day.
> >Sounds like milspec parts to me.
>
>
> Umm. where are supermarket door openers (the ones that see you coming
> from the outside) mounted?

Under the courtesy apron ;) Maybe it's the climate but hereabouts a black
box placed in the sun reaches 90 degrees C easy. Bank on 110 degrees.
Brazil is also not known for a cold climate...

> There is no problem with placing microvaves outdoors.  This
> application isn't sensitive to the occasional false reading caused by
> a sheet of rain. All they need is a rainproof enclosure, they'll be
> fine.

Maybe but whatever you add to it must withstand this too. -50/+125 C parts
are the minimum for this imho. How much for a 10 uF/25V decoupling cap
speced at 125 deg. C ?

I admit that using doppler is elegant ;) After you make it work reliably,
that is <g>

Peter

I know that they are doing this in the states, at the border inspections
(port of entry's) between states.  I have a buddy that works for the
the other day.  The scales can measure the weight of the trucks if they
are going slow enough.  They can also be set up as portable scales as
well.

>Maybe but whatever you add to it must withstand this too. -50/+125 C
parts
>are the minimum for this imho. How much for a 10 uF/25V decoupling
cap
>speced at 125 deg. C ?
>
>I admit that using doppler is elegant ;) After you make it work
reliably,
>that is <g>

BTDT. I installed alarms for a living for about 5 years, high end.

I'll agree that a given unit might have problems with the far
extremes, but in general, I wouldn't anticipate a problem. I don't
think 125C is going to happen in real life, no matter WHERE you are,
likewise -50C.
Doppler uwave also tracks it's own transmitter automatically, so drift
isn't a problem, and the waste heat from the gunn device keeps them
nice and cosy on cold nights.

I wasn't aware that he wanted to clock trucks on Venus or Pluto. For
anything you're likely to run into (in regions of the earth where
there are trucks), a standard commercial unit should work fine.  I
doubt most trucks could handle -50 to +125C either.

Reliability isn't much of an issue here. It has to function, but if
the reading is off by 5%, who cares? His worry is to get the truck
below some specific speed so that the scales can read accurately. If
the unit occasionally sees a reflection and indicates a target where
none exists, so what?  If the speed occasionally dithers, this isn't
going to be a major issue.

I guess I was assuming that the unit wouldn't be mounted at the focus
of a solar collector or some other ridiculous place.  A ventilated
metal or plastic enclosure that will keep the rain and bugs out should
be fine.

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