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Does this pot exist?

Discussion in 'Electronic Basics' started by steamer, May 10, 2005.

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  1. steamer

    steamer Guest

    --Howdy! I've been fiddling around with using a pot to position a
    "servo". I'm using the teeny weeny one from Parallax that needs to be
    turned with a screwdriver. The application is to drive one of those
    surplus windshield wiper motors; I'm using the PWM output to send signals
    to a Victor motor controller and this is working very well.
    --The trouble is determining, with certainty, that the motor is in
    "neutral", so to speak. What I'm trying to do is use the motor as a winch
    to move a small load up and down on an art car I'm building. The winch
    will position the load one way when the vehicle is in motion and in
    another when it's parked. Although it's tempting to use a DPDT switch, I'd
    prefer to control the rate of the load's position change with a big, easy
    to grasp knob on a pot that could be located a few feet from the Stamp.
    What's needed is a tactile feedback of some kind so that I know when the
    motor is in "neutral", so that the motor isn't creeping. Text and photos
    of project to date at: www.nmpproducts.com/artcar.htm.
    --TIA,
     
  2. Tom Biasi

    Tom Biasi Guest

    Hi,
    Say more about the pot that is working now. Is it a multi turn pot?
    What resistance? The motor will be in 'neutral' when the voltage across the
    motor is zero volts.
    Tactile feedback is another story.
    Tom
     
  3. Chris

    Chris Guest

    Lookin' good, Ed. The model sitting in the cart in Fig. 2 is very
    complimentary to your project -- easy on the eyes, and a definite plus
    for the Art Cart team.

    Have you considered using a "rocker pot" joystick? You can easily
    program in a big enough dead zone for any variability in supply voltage
    and pot manufacturing. This will definitely give you a "neutral"
    position with feedback -- you just let go of the joystick or let it
    spring back to its center position, and the motor stops. You would be
    using the pot to determine motor speed and direction rather than
    absolute position. You also might just use a couple of microswitches
    on the winch to determine end of travel positions (this prevents winch
    overtravel and/or overloading the motor). If you were interested in
    saving a few clams (and possibly provide some flair of a commercial
    sort to your project) you might want to look at some old joysticks (you
    can feel if they're just switches by the "click" on end of travel --
    stay away from those), and only use the X- or Y-axis (depending on
    whether it's going up/down or side-to-side). Many of the older ones
    were made to be rugged, have a good plastic/rubber boot over the stick,
    and might withstand an onslaught of sand pretty well.

    Good luck
    Chris
     
  4. John Smith

    John Smith Guest

    I've been lazy and haven't looked at your link.

    But...

    Have you considered a potentiometer with a built-in switch, like those used
    on small transistor radios which control the on-off (switch) AND the volume
    (potentiometer)? Should be really easy to get hold of.
     
  5. steamer

    steamer Guest

    : Hi,
    : Say more about the pot that is working now. Is it a multi turn pot?
    --Single turn; maybe 270 degrees total movement; I'll have to
    check to be sure tho..
    : What resistance? The motor will be in 'neutral' when the voltage across the
    : motor is zero volts.
    100k ohm, if I read the meter correctly. Yah, that's the
    kicker: the motor in "neutral" is still heating up, methinks. However
    I crossposted to a robotics group and I found an answer in the form of
    a "deadband" that I can program into the Stamp.

    : Tactile feedback is another story.
    --Yeah; still needed tho...
     
  6. steamer

    steamer Guest

    --Re: rocker pot: neat idea! Must remember to put it where it
    can't be bumped accidentally tho..
     
  7. steamer

    steamer Guest

    : I've been lazy and haven't looked at your link.
    : But...
    : Have you considered a potentiometer with a built-in switch, like those used
    : on small transistor radios which control the on-off (switch) AND the volume
    : (potentiometer)? Should be really easy to get hold of.
    --Thanks; a viable Plan "B".
     
  8. Chris

    Chris Guest

    Hi, Ed. You're right -- accidental jostling of joysticks can result in
    big problems.
    applications, I'd like to suggest a "dead man" footswitch or pushbutton
    located somewhere else on the control panel. The joystick will only
    work while the other button is being pressed. That prevents
    inadvertent motion, because the operator has to do "something wrong"
    with both hands or one hand and one foot simultaneously.

    It's far better to use the "dead man" switch independently of the
    processor, rather than just using it as a Stamp input. I would
    consider tying the switch/button into the power bus for the motor
    drive, using it to drive a relay which does, or using the switch as an
    "inhibit" input to your drive.

    If you're going with a relay, one good pushbutton choice would be the
    All Electronics PB-211 red jumbo pushbutton. You'd recognize it as an
    arcade game button, and the switching mechanism is physically separated
    from the pushbutton as a microswitch below the plunger mechanism. That
    will help with the sand. Somehow, grit always wants to slide in on the
    sides of the switch. Another option is to get a pushbutton with rubber
    boot, but that will be quite a bit more expensive.

    Assuming you've got 12VDC available, you might want to try these for
    your Art Cart:

    "Rocker Pot" Joystick -- check around -- there are a lot of old ones
    available in garage sales and attics. I once got one for free as a
    "throw in" with purchase of an old "transistorized" table radio at a
    garage sale.

    Relay: All Electronics RLY-351 -- $2.40 ea.

    12VDC 88 ohm coil, N.O. contacts are rated to switch 30A. Bulkhead
    mount, faston-type tab connectors

    Switch: All Electronics PB-211 -- $3.75 ea.
    With 3 amp SPDT microswitch

    http://www.allelectronics.com/

    No minimum order at All Electronics -- be sure to sign up for their
    catalog mailing list.

    Good luck
    Chris
     
  9. Rich Grise

    Rich Grise Guest

    Look at some Radio Control stuff - I think the transmitters for radio
    controlled cars have a small steering wheel with a spring-return to
    center. Check out your local hobby shop.

    Good Luck!
    Rich
     
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