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Maximum useage of a Potentiometer

  1. Sep 14, 2005 #1
    I am looking to create a set of racing simulator pedals, similar to these.

    I have identified the best potentiometer to use - this one. As you can see the maximum range of the meter is 340 degrees. When the pedals are constructed the potentiometer will be attached at the fulcrum and although the meter measures right through from 0 to 340 degrees, when attached in this way it will only be able to measure approximately 90 degrees or whatever the pressing range of the pedal will be - obviously this cannot be too much.

    However the less range the potentiometer can measure, the less accurate it is going to be. There is a away to extend the range of the pedals by having a series of gears. Am I right in saying:

    The pedal can be attached to a rod which is attached to a large cog. The large cog is connected to a small cog, which turns the potentiometer and hence, depending on the ratio of the gears can be turned right through 340 degrees when the pedal is only turned 90 degrees.

    If this is true - what gear ratios do I need taking that the pedal will go from 0 to 90 degrees?

    Also - do you know where I could by gears like this?!

    Many thanks,

    Nick.
     
  2. jcsd
  3. Sep 14, 2005 #2

    FredGarvin

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    I think you should re examine the need to do all of this gearing. Ask yourself if the added complexity and work that will be involved (which will also introduce innacurracy into the measurement) is really worth it? What kind of accuracy do you really need? What kind of constraints have you deciding on that particular potentiometer? Could you look at others? How about...

    http://www.omega.com/pptst/RP101.html
     
  4. Sep 14, 2005 #3

    Cliff_J

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    Gearing involves backlash (the 'slack' or looseness between gears) that will far exceed the error in a decent quality pot. Then you'll need some sort of backlash solution like spring loaded double gears or something similar to eliminate the error.

    For this application, a 'precision' pot can be had cheaply that should be plenty accurate for your needs. Take the model 534/536 in the lower right corner of this page:

    http://www.mouser.com/catalog/623/480.pdf

    For $9+shipping you have a pot that should be accurate to within 1/8" at the end of a 6" pedal assuming you have a good backlash-free connection from the pedal to the pot.

    Precision only gets more expensive from there and then there's always encoders that can become really expensive and you'd need to have the circuits to count the pulses and determine position from that. The pots used by Mercedes in their road cars and race cars aren't that super-precise either, and while they use a bellcrank to increase the movement of the pot you could copy that too if needed. From what I've seen, they use maybe 150 degrees of pot rotation on their application - think of the pivot of the pot being mounted above the pedal and hooked to it with a small arm and a crank - the pedal moves 45 degrees but since the arm is connected say 2" from the pivot it results in a lot more travel on the <1" crank attached to the pot. Doubt you'd need it, just don't buy a cheap pot as the quality can get really bad with some...

    An example to look at of the linkage to increase pot rotation, the BMW system, the gear ratio can be approximated as just the ratios of the radius from the mounting points (although this is not linear).
    http://e46m3performance.com/tech/throttle/
     
    Last edited: Sep 14, 2005
  5. Sep 14, 2005 #4
    Pot's are not finite step devices like rotory switches. They continuously change which means you can amplify a signal from the pot to suit your desired operating range and not lose accuracy. If you used a 10K pot then you'd have just under 30 ohms/degree of travel thus 90 degrees would give a total resistance band of 2.6K. Lets say you run 5 volts from pin 1 to three and offset the pot by an initial 100 degrees so the pot operates between 100 and 190 degrees. You can see that the voltage at pin 2 with the peddal not depressed(assuming not depressed is 100 degrees and fully depressed is 190 degrees) would be 3.5V. Fully depressed you'd have a value of 2.2V thus a voltage band of 1.3V. 1.3/90 yields 0.014V/degree. 14mV can easily be amplified with little error added. You need to think about amplifier design for this problem if you go a direct connect route. Also, are you using this as an analog signal or do you plan on converting it to a digital signal?
     
  6. Sep 15, 2005 #5
    I plan to connect it to this.

    What would the the best option - to go for a high quality pot? rather than a gearing system?

    What do you think of this system?

    Also, what would be the post efficient method of connecting the pedal to the pot? If i were to use a bellcrank - how can i calculate the best place to fit it to the pedal?
     
    Last edited: Sep 15, 2005
  7. Sep 15, 2005 #6

    Cliff_J

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    The system you linked to would be kind of technical to set it up, I guess I'd error on the side of the KISS (Keep It Simple Silly) principle.

    Any local RC shop should have plenty of parts, I only grabbed some of the first ones that came up and you'd want to cut some of them. Anyways, here is a plastic bellcrank pre-drilled and with 4 arms in case you'd ruin one you could use another and cut off the ones you don't need.
    http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LX0865&P=0

    You could glue one of those to the end of the pot's shaft, now you have lots of attachment points. Then you need some ends and some shaft:

    http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXD609&P=7
    http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXD871&P=ML

    Now you could drill some holes on the pedal and hook up the link at various points to see if you can get the right balance of pedal movement to get the pot to move kinda close to just under a half-circle without any binding. 120 to maybe 150 degrees would be the most you could get, but as faust9 pointed out this should be plenty.

    Construction will be likely the harder part of this. According to an article in a car mag, the general rule of thumb for an accelerator pedal is to have it move about 4". Depending on how you construct yours, you may want to build that first and have the pedal movement and mechanical stops in place before attaching the pot via the linkage.
     
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