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PICList Thread
'[PICLIST] [PIC-App, EE]: Speed and Distance Measur'
2001\02\02@082700 by Bourdon, Bruce

A friend of mine is into (mostly stock) tractor pulls, and he actually
volunteers a lot of time and effort to help out with many of the areas

He is interested in learning about the PIC and is thinking making a device
that could be used to help display the tractors speed and distance as it
travels down the track.

We've tossed several ideas back and forth, and would like to hear any ideas
or similar applications that anyone has attempted.

A few notes:

* the event is public, with a significant level of noise coming from the
crowd, PA system, tractors & engines, etc..

* distance is the main criteria by which pulls are judged.

* a full pull (max distance) is typically about 200 feet.

* the course is straight, and movement on the course is controlled (i.e.
roped of area - public cannot walk onto path).

* the towed sled can be modified, adding reflectors, sensors, etc..

* the speed is a significant bonus, as the max speed gives a relative figure
of merit of the tractors horsepower (a published figure from the factory) -
making it easier to spot cheaters (dynomometers are rarely provided).

* the speed is on the order of a few miles per hour, therefore any acoustic
methods should attempt to cancel air movement (which could easily exceed
this value).

* the towed sled has a standard truck rear end and differential which turns
a drive shaft to a transmission which turns a shaft that a cable is attached
to and winds up on - pulling a weighted box up an incline - all mounted on
the "sled."

* distance resolution should be sub-foot (i.e. tenths of foot).

* speed resolution should be at least tenths of a mph, though this is not
critical to the project.

The ideas that we've seriously considered include:

- mounting sensors (hall-effect or opto-but shileded from dust..) on the
drive shaft of the towed sled

- mounting onto the sled, a swingarm with a wheel that has sensors to
measure the distance and have a pic take this, calculate speed and display
or transmit this inofrmation to a remote pic/display.

- mounting a flat reflector on the sled, and using ultrasonic pulses to
measure the distance, calculate speed, etc. (the advantage to a remote
ultrasonic tranciever is that it any component of the wind that is in the
direction of the sleds path will be cancelled by the other "half trip").
Disadvantage: extreame distance and environmental noises.

- aquiring a radar gun and modifying it to provide a pic with the speed,
have the pic accumulate distance by combining speed measurements with time.
Disadvantages: resolution of most radar guns probably not sufficient for
speeds involved, distance measurement questionable, cost, etc..

- laser rangefinder and speedometer... possibilities include measuring the
time of travel (about a foot per clock at GHz frequencies), triangulation,
counting the audible "pops" from a setup that has the beam split, one
portion travels to the sled and back and is then recombined with the other
beam and then passed to an opto sensor & amplifier then pic counter (this
would count each time the distance moved through one wavelength as the
relative phases combined to modulate the beam at the sensor).

- placing trancducers/sensors along the track with complimentary
sensors/transducers on the sled, with the pic monitoring distances between
each along the way...

- purchase a large quantity of pics on a tape reel, mark the tape in


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2001\02\02@090312 by Simon Nield

300m coil of lightweight rope (or steel cable ?) on a 'lightly' spring loaded reel. Attach one end
of rope to some bit of vehicle with a D clip (like for climbing).
fit biphase encoder to drum to mearure angular displacement.
add pic to read biphase data (suggest biphase waveform is processed by programable logic / discrete
counter / ic made for the job as it will give you less headaches that way)

hit big reset button when vehicle is on start line.

when 200m has been payed out, stop the clock.

pic or whatever takes distance and time and calculates speed.

(will need some filtering to cope with jerky movement of cable drum if real time speed readout is

will need to be a bit careful when coiling the cable back in (to avoid decapitation) - probably not
too difficult to come up with a simple mechanism to wind the cable back on to the bobbin neatly and
slowly. may need to clean cable as it is wound up... pass it through a washer / air blaster ?


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2001\02\02@111727 by Robert Rolf

picon face
Laser Radar gun. (LTI Inc).
Gives range to meters and speed to tenths km (option setting).
Some versions (20/20? ) have serial port out with for data logging
purposes (statistics). Basically Time of Flight measurement using IR
and interference filter on detector. Check 'Car and Driver' web site for
technical details (they reviewed several units to see how best to
'fool'/'jam' them).

"Bourdon, Bruce" wrote:
{Quote hidden}

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2001\02\05@191204 by Gennette, Bruce

A simple, stand-alone solution is a spring loaded wheel behind the sled.

As the sled is dragged forward the wheel turns.
As the wheel turns rotary encoders send pulses to a PIC.
The PIC calculates instantaneous speed and distance from the pulses.
The PIC either drives a local display or sends the data out via a short
range radio link.

Simple, cheap and (if 2 encoders and PICs are fitted to each wheel) highly


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