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'Need way to accurately measure distance with PIC o'
2000\03\22@151428 by Jon Petty

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I

I have an application where I need way to accurately measure distance with
PIC or BS2. I need to measure pedal travel on different vehicles. The
distance measurement need to be repeatable. So each time the measurement
device is attached to the pedal the same measurement is made.

To explain further, say a linear potentiometer is used. If the potentiometer
is angled to contact the floor at different places the pedal travel
measurement will be different. That is something I need to eliminate. An
ideal solution would be a non contact method that would measure the same way
each time the device is attached to the pedal.

The measuremnet needs to be accurate to an 1/8" and repeatable. The cost of
raw measurement components should be less then $20.

What would the best measurement method be?

Any ideas or other suggestions?

Thanks

Jon

2000\03\22@155135 by Mark Newland

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You talking about angular distance or the length of the TOP of the pedal towards
the floor.  On different vehicles I suspect that the length of the pedal would
be different and therefore the TOP of the pedal would travel different distances
even with the same angular distance.

Jon Petty wrote:

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2000\03\22@162835 by Mark Willis

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I'm not sure I understand the problem completely <G>  What's the maximum
pedal travel?  What is the minimum distance you have between the two
mounting points you're using?  You need to choose, whether you're
measuring an angle, a distance, a ratio (as in % of pedal depression
distance or angle, instead of an absolute distance or angle), or ???
It'd make the most sense to me, to measure a ratio here, I think (but -
I don't know your application <G>)

Several ideas for you:

Use mounts that preclude a problem (i.e. epoxy or superglue a pair of
fixed metal dowel pins, one onto the pedal and one onto the floor, you
then have a repeatable mounting system, and can retain a removable
sensor - at the price of good linear pots, kind of nice.)  The general
idea being to fix whatever you're measuring, precisely and repeatably.
You want to fasten the sensor in place when in use (clip, screws, wire
ties, rubber bands, ???)

Could use any of a number of sensors for this - linear pot (Systron
Donner used to make decent low-friction ones of these), gray-coded
optical sensors, all kinds of solutions.  You can make the optical
sensor board yourself (pc board with the pattern on it), nothing
requires the optical sensors to be aligned alongside each other BTW,
with 1/8" granularity you should have no problems.  Dry Transfer method,
maybe?  Can make a rotary encoder or a linear one here.

Also can generate a "Click" with a piezo and listen for it with another
piezo to determine distance, it's been a while since I saw that done so
I don't remember the "gotchas".  Can be rather cheap - I found some
piezos for 10 cents the other day, I'd like to find more at that price
though.

Can gear a multi-turn pot up so you measure pedal angle and a say 15
degree pedal movement, generates 2 turns of pot stem movement;  Think of
how long the pot'll last here, of course, as trim pots are hardly
designed for continuous adjustments.  Gear lash should be below the
noise floor with 1/8" pedal movement granularity.  One glaciologist I
know used a carbide-tipped saw blade on a multi-turn pot to get the
first accurate real-time glacier movement measurements ever done over a
long period - SMART guy, it's always a real pleasure working with
someone who thinks well <G>

 Mark

Jon Petty wrote:
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