cheapo strain gauge

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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



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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



Thanks for the reply.

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Is there anything off the shelf that would do all this? Hmmm maybe I could
just use the internals of a set of scales..

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That's possible but maybe a second option at the moment.
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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?

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are you sure? I thought thisd was connected to a joystick port...

Bye.
   Jasen

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It's via usb. The existing pedal starts at approx 4.7 volts and goes down to
0.5 volts with the pedal depressed.

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Seems easy enough...   just add a unity gain inverting amp to the output of that
circuit

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the
We've slightly trimmed the long signature. Click to see the full one.
Re: cheapo strain gauge


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Won't that make it negative voltage?

Michael



Re: cheapo strain gauge


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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

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I've done more than that, here's the complete setup :-)

http://mikesdriveway.com/racesim/assembled.jpg

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Does the pin move?

Michael



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A few millimetres. depends how tight you get the wires, if you put
power through them they expand (eg: heat the bottom ones while tightening the
top)

you don't get a very strong signal from it, it'll need quite a bit of
amplification.

Bye.
   Jasen

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I was overthinking it. :-)



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I noticed that.

Re: cheapo strain gauge


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incidentally try adding a subby under the seat .

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I have. I've also got a bass shaker from jaycar that is bolted in but not
hooked up as I need to organise an amp for it. I could get a car amp but
would need 12V and the power supply would be a little pricey.



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Jaycar have some decent amps  check the web

Re: cheapo strain gauge



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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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