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- Michael C
February 7, 2007, 10:58 pm

I've got a steering wheel and set of pedals for the computer (see link
below). I'd like to modify the brake pedal because it doesn't work like a
normal brake pedal, it works just like the clutch or accelerator going all
the way to the floor. I presume the best option is a strain gauge. I've
phone a couple of places and they can sell me something for over $200 but
the set of scales I have in the bathroom has 4 of the things and it was
something like $80. Where would I get something like that? How hard would it
be to interface with the pedals that currently use a pot?
http://mikesdriveway.com/racesim/pedals.jpg
Thanks,
Michael

Re: cheapo strain gauge

it
Using a strain gauge is non-trivial - from my vague knowledge, it requires a
wheatstone bridge, compensation and probably some filtering and
amplification to make it compatible with your application.
What might be easier is to restrict the travel of the brake pedal and use a
spring or torque rod to add the required resistance, then either use the
existing pot with a linkage to magnify the range of movement, or use a
different size pot and resistors to match up the range of resistance.

Re: cheapo strain gauge

I've found these that are cheap enough and look easy to design a circuit
for.
http://www.tekscan.com/flexiforce/specs_flexiforce.html
I just need the voltage out of the circuit to go the other way, ie more
pressure = less voltage
http://www.tekscan.com/images/FlexiSampleCircuit.jpg
Is there an easy way to do that?


Re: cheapo strain gauge

just get a strong spring (say a surplus valve spring from an old car engine)
and a short slide pot
you'll need to bolt your chair to the floor.
I saw a homemade load cell once, the guy had arranged resistance wire like
guy ropes to support a pin at both ends
force
||
|| HHHH "pin"
\/
ZZZZ mounting bracket
HH
/HH\ \// resistance wire (4 each end)
/ HH \
/ HH \
ZZZZ HH ZZZZ
\ HH /
\ HH /
\HH/
HH
the 4 upper wires were connected in series to form a single resistor
as were the 4 lower wires,
the upper and lower resistances formed two legs of a wheatstone bridge.
--
Bye.
Jasen
Bye.
Jasen

Re: cheapo strain gauge

You should be able to get unbonded strain gauges for way less than $200. You
can then devise a mechanical setup that stresses a piece of metal as you
apply force. Use 2 gauges, glue one on one side of the metal and one on the
other but close to each other, so that they respond in opposition and apply
some temperature compensation. Then put them in a bridge with a power supply
of a volt or two.
Strain gauge suppliers generally specify some sort of special glue. I've
done this using ordinary araldite and it works fine.
You'll need a fairly senstive amplifier, with low drift. Typical gains would
be upwards of 1000 if you want an output of a volt or more. I happened to
have a nice AD chip free, if you use a common opamp you'll need to be
careful in terms of stability. Someone here can surely advise.
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