Pot readout on joysticks - deadspace at outer limits

Discussion in 'General Electronics Chat' started by photocs, Jun 1, 2013.

  1. photocs

    Thread Starter New Member

    Jun 1, 2013
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    Hey everyone!

    I'm hoping someone can shed some light onto a project I'm currently working on. I'm building a motion control rig (for cameras) and have two joysticks which control stepper motors.

    I'm a bit dissapointed in the performance of the joysticks because, even though the joystick physically moves at angles ±45°, I can only get a reading off the pots between the first ±10° -- which yields a sort of 'deadband' or 'deadspace' for the outer ±35° where the readout values are stuck at max/min. This makes it very difficult to actually use the joysticks because you aren't using all the travel distance to choose different values on the pots (i.e. they are highly sensitive to change).

    So, my questions are:
    - am I missing something or am I doing something stupid? I've got 5V feeding the pots and am reading the analog values through an analog pin (10-bit) on the Arduino Due
    - are the pots just too cheap and should i consider switching them out for others that perform better? I can open up the joysticks and i'm thinking i could find some pots to put in there with the help of my trusty dremel...
    - if I should consider switching them out, could you recommend a proper pot to use?

    thanks everyone!
     
  2. LDC3

    Active Member

    Apr 27, 2013
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    We will need to see the wiring diagram to be certain you didn't connect up something incorrectly.
    Can you measure the resistance from one end of the pot to the wiper as you move the joystick from one end to the other to see if it continuously changes?
     
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  3. photocs

    Thread Starter New Member

    Jun 1, 2013
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    Good idea, I'll check the resistance across the full travel of the pot just as soon as my brother comes back with my volt-meter. I'll report in then!
     
  4. photocs

    Thread Starter New Member

    Jun 1, 2013
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    Hi LDC3,

    I've done some testing with an analog volt meter. I should note though, which I hadn't in the original post, that the actual position of the pots can be adjusted by unscrewing a small screw and twisting the pots; this way you can center the pots. I had previously done this by watching the reported analog values from the microcontroler and twisting the pot so that it showed a value around 500 (half way between 1023).

    Now, when reading resistance between the +5V and the GND, I can see about 4k Ohm, the pots are rated as 5k....

    When reading between the +5V and the wiper I see the following behavior:
    - with stick in middle -> reading 4.5k Ohms
    - push stick right -> resistance lands on 2k Ohms with no deadband towards the end of travel
    - push stick left -> resistance moves towards 3.5k Ohms with a large amount of deadband at the end of travel

    I've also attached a sketch of the wiring diagram. In this case, a single joystick with two pots. 5V is connected to each of the in pins, grounds are connected together, and then the out is read through analog pins on the microcontroller.
     
  5. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    That is very strange that you don't get either an increasing or decreasing resistance from one side to the other side. Instead you get:
    2K on the right, 4.5K in the middle, and 3.5K on the left
    Also, since the pots are designated as 5K and you are only getting 4K, I would say that the pots are well used and need to be replaced.
     
  6. photocs

    Thread Starter New Member

    Jun 1, 2013
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    Strange indeed, especially since the readout on the digital side doesn't show this. I think best to move on to some higher quality pots, do you have any recommendations for replacements? Obviously you don't know the physical size that I need, but what are some specs? 5k? Linear? Are there high quality pots?

    Thanks for the help!
     
  7. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    You would want a 5k linear pot. The better ones use wire as the resister.
    I'll take a look later when I have more time.
     
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    When you specify these measurements are you moving the pots by hand, the full 300 degrees of motion or are you taking these measurements by moving the joystick? Could be that the joystick does not deliver the full 300 degrees of pot movement for the full 90 degrees of joystick movement due to improper gearing, I have seen this before.

    Another possibility is that at some point, by some means, you have exceeded the 300 degrees of permissible travel of the pot and come all the way around to the other side (some cheaper pots can achieve this with very little, almost unnoticeable force).
     
  9. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    I have had incredible reliability from wirewound pots. Even when they get dirty, they clean up as good as new with some contact cleaner spray. The cermet (ceramic/metalic) pots have excellent specs, but I haven't used them. I wouldn't even try using a cermet pot unless I needed a resistance range that I can't get in wirewound.
     
  10. photocs

    Thread Starter New Member

    Jun 1, 2013
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    The stick is hooked-up directly to the pots, so I doubt it's the gearing; but to your second suggestion, maybe they are installed (in the joystick) incorrectly. I should just take them out completely and test them.
     
  11. photocs

    Thread Starter New Member

    Jun 1, 2013
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    Hmm, looks like wirewound pots are the way to go; any favorite places you get them from?
     
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