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Making a continuous rotation servo not so continuous. — Parallax Forums

Making a continuous rotation servo not so continuous.

UghaUgha Posts: 543
edited 2009-01-12 00:30 in General Discussion
I'm working on adding arms for my robot and i've hit a snag.

I'm using two parallax continuous rotation servos for shoulder joints because... well... its what I have.

It's a bit pricy to pay $35+ for standard servos just for the robot's arms so I'd like to figure out a way
to use the servos I have.

My problem is the arms are impossible to control because I can't monitor the position properly.
I need the arms to be able to reach full down, forward and full up... that's it.

I've tried to puzzle out exactly how to do this without having to buy anything else.

An encoder disc would work, but that'd be quite ugly and hard to do on my setup.
I don't have a hall-effect sensor, so magnets are out.
A pot glued to the arm and suspended by some kind of shoulder plate is out because of the setup.

I've got a couple IR leds and receivers, but not enough for all three positions on both arms.

I'm toying with the idea of a shielded LED mounted on the arm and three CDS cells at the various
locations on the torso, but its a rather crude idea and it'll be ugly and use up almost all my CDS cells.

I wish there was some way to put an encoder INSIDE the servos but there doesn't seem to be enough
room in the top half (where the gears are)... is there enough room in the bottom half? (I've been too
nervous to take out all the gears).

Any suggestions will be greatly appreciated.

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-01-07 19:25
    Save your continuous rotation servos for another project and just spend the 35 bucks. By the time you get something to work with the CR units, you may ending up spending that much anyway, not to mention the time involved. (OTOH if your underlying objective is to learn about position control rather than just getting something to work, this would be a good exercise!)

    -Phil
  • sailman58sailman58 Posts: 162
    edited 2009-01-07 20:11
    Using the new Pico servo would save you a couple of bucks. You could also save a few bucks by changing brands and buying form another vendor.
  • P!-RoP!-Ro Posts: 1,189
    edited 2009-01-08 23:48
    I would try taking it apart first before wasting your money on such an expensive servo. Sometimes when they modify it they leave the pot in it, but they don't have it attached to the gears. Also, since you were worried about taking out the gears, you should be able to pop of the back cover without removing them. If there isn't a back cover, go ahead and take out the gears, just be sure to place them somewhere clean so they don't get dirty, and try not to rub off the grease. Good luck!

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    PG
  • UghaUgha Posts: 543
    edited 2009-01-09 02:52
    I went ahead and took one of the servos apart... it wasn't that horribly hard to get the gears back in the right order once i realized that one of the shafts was mounted to the TOP of the
    gear section instead of the bottom...

    Unfortunately, there's no pot in there and while there is a good bit of room available in the bottom section (where the motor and electronics are), there's no way to put an IR encoder or anything
    of the like onto the shaft of the motor.

    I considered trying to add an IR pair I carved out of an old mouse to the gear section, I figured the grease would be a problem... also the IR pair is too large.

    I'm still toying with ideas to add a pot or encoder to the servo... I'd welcome any additional suggestions.
  • SRLMSRLM Posts: 5,045
    edited 2009-01-09 06:00
    Make your own servo out of a DC motor and your encoder.
  • kelvin jameskelvin james Posts: 531
    edited 2009-01-09 06:54
    homepage.mac.com/sigfpe/Robotics/encoder.html


    **
  • sailman58sailman58 Posts: 162
    edited 2009-01-09 15:24
    you are talking about $15 ot do the modifications. For that $15 you can almost get two new servos with a little bit of shopping.

    Ron
  • P!-RoP!-Ro Posts: 1,189
    edited 2009-01-11 04:00
    If you still would like to go with a pot, there is probably a resistor put in place of it. Double check and see if you can find it. If you can, just get some pots and put it in it's place. There sould be a gear with a large space in it for sticking the pot. Just glue it to the gear once the pot is soldered in.

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    PG
  • UghaUgha Posts: 543
    edited 2009-01-11 15:55
    That's an interesting concept Pi Guy... but doesn't the servo need to have a gear with a "stop" on it?
    At least thats what I gather from the modifying servos tutorials I've seen on the 'net... you have to clip off the stop in order to make it continuous and I don't
    see a way to put the stop back.

    Kelvin: That's a great site... although it doesn't help my current situation, it's really interesting and I'm sure I can use it in the future, thanks!

    Sailman: I'm curious... where can you get a servo with the same power/connector as a parallax servo for half the price? (PM might be best to answer this)
  • P!-RoP!-Ro Posts: 1,189
    edited 2009-01-12 00:30
    "Doesn't the servo need to have a gear with a "stop" on it?" That is just to make sure you don't rip out the pot from too much force when it hits the end of it's range. If you program it correctly it won't be a problem. However, if you are still worried about it you can use some glue and fill in a few teeth on one of your slower gears. Every time the motor turns that far the gear won't·allow it to move anymore since the teeth can no longer mesh. Just be careful, though. If you glue it wrong your servo may not move like it should. Good luck Ugha.

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    PG
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