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Connecting pots to wiper motors

Discussion in 'DIY Motion Simulator Building Q&A / FAQ' started by egoexpress, Dec 11, 2010.

  1. egoexpress

    egoexpress Active Member

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    Just a thought...

    Some of you guys simply did connect the pots to the leaver.
    Nano' simulator
    [img600px]http://img571.imageshack.us/img571/6830/posicin.jpg[/img600px]

    And some of you did drill a hole into the housing, and screwing a cog-wheel on the motor shaft, which itself is connected to a multiturn pot by another cog-wheel and corresponding gear ratio.

    ElektronikJack' simulator
    [img600px]http://www.x-simulator.de/forum/download/file.php?id=145&mode=view/IMAG0002.JPG[/img600px]

    I wonder how much the method of connecting the pots to the wiper motors affects the quality of motion.

    I'd guess connecting the pot to the motor shaft would give a better result, though it is probably more difficult to tinker.

    Regards
  2. Frakk

    Frakk Active Member

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    Motion quality depends on many factors, the method of pot mounting will have very little effect in the end.

    Multi turn pots are usually more expensive and higher quality than single turn potentiometers. Given the same precision and quality for both single and multi turn potentiometers, the end result will be the same, or very close.

    The only advantage of using multi turn pots with gearing is that the full travel of the potentiometer can be utilized. This will ensure that the full A2D range of a controller is used, 0-1024 with a 10bit conversion for example.

    As an example: 10bit A2D resolution, seat mover setup where the servo arm moves the seat directly.
    The maximum usable travel of a servo arm is 180°, where no linear movement is generated as it get's close to 0° and 180°. Linear movement repeats every 180° and is sinusoidal with respect to shaft position.

    1) 270° single turn potentiometer is used directly on the output shaft.
    180°/270° = 66.6% of travel used. After the conversion we will have ~683 steps within this 180° movement range.
    This means 0.26° resolution.

    2) 10 Multiple turn pot is connected on on the motor shaft. there is a 100:1 reduction at the worm gear, 180° servo arm travel.
    (180°/360°) * 100 = 50 rotations of the motor to produce 180° at the output shaft.
    A 5:1 gear ratio will give 10 revolutions on the pot shaft, utilizing 100% of the A2D resolution: 1024 steps within the 180° movement.
    This means 0.17° resolution.

    Over all, the gain is an extra 0.09° POSITIONING accuracy. I'm sure that only the worm gears will have backlash far greater than this.

    The other part of the discussion is that the motor driving signal is calculated from position and the data coming from X-Sim. Generally lower resolution will give less # of different speeds the motors will turn, but there are ways to improve this, and after all the human body won't notice the difference above a certain level.


    In the end, it will not matter that much if it is a single or multi turn pot and how it is mounted.
  3. estanislaolopez

    estanislaolopez Member

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    My Motion Simulator:
    2DOF, DC motor, SimforceGT
    While my simulator is exactly like the photo belongs to the Nano simulator , also from Argentina, which helped me with their ideas ...

    Gracias Nano !!!!

    Regards
  4. Nano

    Nano New Member

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    Ok estanislaolopez.
    Mi inglés es fantástico!!! :yippiee:
    Regards.
    Nano
  5. egoexpress

    egoexpress Active Member

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    Well, I did relate to this post, assuming it belonged to you ;P
    post28444.html#p28444

    Anyway, back to topic!

    @Frakk
    I am still not convinced 100%, that the difference is just marginal.
    In my opinon, the play in the worm gear has more negative impact if the pot is connected to the leaver, then if pot is connected to the motor shaft, regarding hitting the target position and resting there smoothly. I think especially fast and short rumbling effects could be played back better, if there is no play between motor shaft and pot. If the worm gear has much play, the motor is imo more likely to overshoot target position, as it gains to much accelleration before the pot recognises that the motor is in movement.
    But on the other hand, I may be wrong...

    The best method, besides quadrature encoders of course, would be IMO to connect the pot to the motor shaft via a rubber belt and pulley wheels, to eliminate play even more successfull.

    Regards
  6. frviana

    frviana New Member

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    How did you connect your Pot? Do you have any pictures you can share?
    I've just finished testing my sim and is now working and I have to start attaching the pot to the right place but I just can't find a good way to do it.
  7. riton

    riton Active Member

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    with a potentiometer on the shaft of the gearbox and a multiplication
    by suitable gearing, the accuracy is better than directly to the potentiometer on the axis.

    there are 360 degree potentiometers with better accuracy, the race is more important than a 270 °

    What do you think of this solution?
    potentiometer 360 °, 340 ° electrical travel and gear reduction. 100% potentiometer
    180 ° motor= 100% potentiometer 340 °

    in some cases, the motor speed does not make potentiometer, there is too much speed.

    there is also the risk of breaking the pot if full revolutions of the engine.

    360 ° knobs have no mechanical stop
  8. bsft

    bsft

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    Yes, pot mounting can be a bit tricky
    [​IMG]
    [​IMG]
    This is a pic or two of what I did.

    In regards to potentiometer mounting, I noticed that some people connect the shaft of the pot direct to the shaft motor. Only one problem, most single turn pots are a 270 deg rotation, whist a motor is 360 deg rotation. If by some chance you have a feedback disconnect, the motor may spin freely and break the pot. The pot may still work, but it would have issues.
    Some people use a lever type system, so as the pot moves between 90 and 180 deg rotation. Use plastic arms.
    I made a lever system and set it up, then with the motor running I moved the pot on the bracket to a point where it moved ok, then I screwed it down

    Heres a video, the levers are now different but you may get the idea.
    http://www.youtube.com/watch?v=Kr2QqH55byc
  9. bsft

    bsft

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    There is a simple and cheap way to fix backlash from worn gears in wiper motors.
    Heres what I did, I attached a compression spring of considerable force ( no I do not know how hard), I know its a STRONG accelerator or handbrake cable spring. I fitted to the other side of the motor. I have backlash in my motors. It creates a consistent load on the motor gears in one direction, despite the motor changing direction. The springs cost me $5 each and they work. I chopped out 2 motors before I fixed the problem.
    [​IMG]
    [​IMG]
    Also it can be seen in this video.
    http://www.youtube.com/watch?v=hu6BfD9jqaE
    A suggestion would be to try this first, as it may save you a fortune in re-development.
    :cheers:
  10. jyrki.j.koivisto

    jyrki.j.koivisto New Member

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    When using any analog converter one wants the full or almost full swing of the voltage. When mounting pots make sure that the potentiometers rotation is almost fully used (rest is for safety margin). That way the analog signal coming from the pot has greater amplitude and AD-converter can better distinguish different pot positions. There is no sense for example try to use a 12 bit converter and try to detect a signal change from 1.23 min to something like 2.31 volts, AD loses it's accuracy that way. Noise would also make things worse as there may be some spikes on the pot line as motors generate signal noise on lines. One should also use shielded cable and only connect the shield on the PCB side. Some noise reduction can also be made by connecting small capacitor across the signal line and ground, some diodes could also be used to protect from over voltages and spikes.
  11. frviana

    frviana New Member

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    Thanks a lot for the tips. It has been a challenge for me to figure out the best way to mount the pots. I was going the round pot direction but this makes much more sense. I guess I have to make a couple stops now at the electronic shop and the RC Hobby shop. Thanks again.

    When I finish mounting and testing everything I will post my final pictures.
  12. frviana

    frviana New Member

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    Sirnoname,

    I went out and bough a couple sliding potentiometer, RC plane rods and ball bearing joints. I mounted exactly like the picture but it didn't work well. Even when the rod connection on the motor is very close to the pin it still provides a long run on the rod and it causes to push to further and back on the pot beyond its limit. I would have to find a longer sliding potentiometer. The other thing I noticed is that the sliding pot that I have is not very precise. When the pot sliding is on the middle it gives me about 45 of 255 and before reach the end is already on 255.

    My challenge now is to find a very precise and long sliding potentiometer which I think will be very hard.

    Any ideas or tips? I will try to post a picture or little video of my attempt.

  13. bsft

    bsft

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    Different people use different pot configurations. Some people like sliding, some like single turn, some like multiturn pots. Each will need a different leversystem to set them up. A suggestion would be to either make yourself some simple lever arms from plastic lengths and experiment. Thats what I did, until I found a good way of turning the pot on the motor lever arm, or for you, getting the right movement. My levers do not look flash, but they work.
    I use a single turn that now moves about 120 deg, I think. I just set the feedback in the JRK utility and off I went. I do not know if making the pots turn more would make a real difference. But thats me. The pot is there to provide feedback to tell the program where the motor is. My guess is that as long as the pot turn - slide is not too short, then set it and test the motion.
  14. frviana

    frviana New Member

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    Thanks for the tips. Tomorrow I will be doing a lot of tests with rotating pots to see what I can get. I think you are right, I just have to try different arms and modes until I find something that works for me.

    Thank you.

  15. frviana

    frviana New Member

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    Ok, tried a few models and the best one I could get so far is the one on the link below. I would really appreciate some advices because I don't think this one is final and it has been the biggest challenge for me.

    The sliding pot didn't work at all. I couldn't find one long and precise enough.
    I have found more options with the rotational pot.

    On my test I managed to get full 0-255 however I can't get the middle of the turn to be 50% the top to 100% and bottom to 0 so I believe this is not the right setup but its a way there.

    I saw the design with two arms where the Pot is connected to one arm and the seat connected to the other but the problem I see on this design is that if the motor spins around it would break the whole thing.

    Here is a link for my video and I would really appreciate comments and suggestions. Setting this up has proved to be much harder then I thought.

    http://youtu.be/0bLxpzmNpyI

    I would really appreciate any comments or suggestions. This has proved to be much harder then I thought.
  16. Frakk

    Frakk Active Member

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    Linear pots mounted to rotating arms = loss of linearity, the feedback will be inaccurate.
    Pots connected to rotating arm with a rod = loss of linearity, the feedback will be inaccurate.
    This means: at different positions, the same movement will cause less or more movement in the feedback. :thbd:

    If you want to keep any sort of accuracy, the feedback has to be mounted to: 1) motor shaft, or 2) output shaft

    I mounted it to the back to avoid breaking it and for easy adjustment of the 0 position.

    Attached Files:

  17. frviana

    frviana New Member

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    Couple questions:

    1. What happens if the motor spins around (if xsim crashes or you turn the power on before opening the xsim my motor spins around. Wound't this break the pot?

    2. How did you get the shaft to also come from the back? My shaft is only on the front.


  18. kubing

    kubing Member

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    I am not sure what type of controller do you use. I thought most of developer puts this in the first place in their design. no matter what happen to PC side , crash , burn , earth quake they will not spin more than its max position. unless you cut the pots wire accidentally. :cheers:
  19. frviana

    frviana New Member

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    I'm using K8055 + Sabertooth 2x12.

  20. Frakk

    Frakk Active Member

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    1. What happens if the motor spins around (if xsim crashes or you turn the power on before opening the xsim my motor spins around. Wound't this break the pot?

    The seat goes up and down, the pots starts slipping on the shaft as they reach their stops.


    2. How did you get the shaft to also come from the back? My shaft is only on the front.

    I drilled the back cover and drilled into the plastic worm gear to fit the pot's shaft snugly, but not too tight to let it slip.


    Instead of doing nothing? Something is always done... :D
    You can try and reduce the sine deviation, at the end of the day you still end up with: 1) more complicated mechanics, 2) worse reliability, 3) still inaccurate feedback.