X axis problem

Discussion in 'General Electronics Chat' started by sowjoe, Mar 11, 2013.

  1. sowjoe

    Thread Starter New Member

    Oct 6, 2012
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    I'm looking for the simplest way to build a gantry that travels back and forth on the X axis only without it being hooked up to a computer. It will be belt driven and controlled by an 18v dc drill motor.

    I was thinking about using an "on-off-on" switch at each end of the gantry slide to reverse the direction of the gantry, but I think the motor won't last long if I do that. I need something to slow the motor at each end before it reverses direction.

    I've thought about Hall sensor's, 555 timers, limit switches etc., but they seem complicated being that I don't know much about them. Just want something simple.

    Any idea's?
     
  2. JayArr

    New Member

    Mar 11, 2013
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    Put the off switch at approx 90% of the travel and put a spring to slow down the inertia for the last 10% before reversal.
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    Is this thing going to just be travelling back and forth with no concern of speed regulation or stopping at a particular point? Does it not matter how jerky the motion is while it is reversing direction or exactly (and how repeatably) it reverses at the same point each time? What is the approximate period of the motion (the time between when it is at the same point going a particular direction and the next time it is at that same point going that same direction)?
     
  4. sowjoe

    Thread Starter New Member

    Oct 6, 2012
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    Yes, it will be just traveling back and forth.

    Speed can be adjusted using the drill motor trigger or another device. I'm open for suggestions.

    There will be no stopping in the movement. The carriage will travel from one end of the gantry to the other with on stopping.

    I was concerned with the "jerkiness" and/ or torque placed on the motor when reversing.... any idea's?

    The approximate period of time that the carriage will traverse from a point and back I would guess would be 2 seconds. But, this can be increased by the drill motor switch trigger or some other device.
     
  5. WBahn

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    Mar 31, 2012
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    Just 2 seconds? One second from one end of the track to the other? How long is the track? Unless it's very short, you are going to need a pretty substantial acceleration at the ends to stop it and get it going the other way. How heavy is the total amount of mass that is going to be in motion?
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Déjà vu. We've had this question before.
    How about a rotating wheel with an off-center post
    or cam-like mechanism?
     
  7. WBahn

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    Mar 31, 2012
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    I'm not picturing the physical arrangement you have in mind. What is the cam's role? What is it mounted on? What is riding on it (or it riding on)?
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
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    Crankshaft and connecting rod.
     
  9. WBahn

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    Mar 31, 2012
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    Okay, I see what you're getting at. The applicability of that would seem to hinge on the answer to one of my questions, which is how long is the travel for this gantry mechansim.
     
  10. sowjoe

    Thread Starter New Member

    Oct 6, 2012
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    The gantry rails will be 48 inches long.

    I think what I'm going to do is build a test model using one on-off-on switch. The 2nd on will reverse the polarity of the dc motor thereby reversing the direction of the carriage. Simple and easy.

    If I have a problem with reverse-direction smoothness I could incorporate some type of soft-start motor controller too ....
    http://www.skpang.co.uk/catalog/pololu-highpower-motor-driver-18v25-cs-p-662.html

    Here is the belt drive design I want to use....

    http://bell-everman.com/products/lin...elt-linear-sbl

    Video.... http://bell-everman.com/products/lin...elt-linear-sbl
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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    How far apart will the two gantry columns be? If they are very far apart, skew may become a problem if only one side is being driven. As for smoothness of reversing, limit switches could be placed to signal a "slow down zone" which would trigger a decel ramp and upon reverse, an accel ramp. It sure seems like a micro-controller is in your future.
     
  12. JayArr

    New Member

    Mar 11, 2013
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    If you really want smoothness and reversability then look into stepper motors, they'll solve your reverse problem in an instant! You can buy power paks with all the circuitry inside and a simple terminal for CW/CCW change the voltage on the pin and the motor reverses without any delay, lag or difference in speed.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    So you plan to have something that is moving four feet in one direction, reversing direction, coming back four feet, and reversing again and that's gonna take about two seconds.

    Let's assume that you have it moving harmonically (sinusoisally) so that you have a smooth reversal at each end.

    That means that the position is going to be given by

    x = 2ft sin (2∏t/2s)
    = 2ft sin (∏s^-1 * t)

    The velocity would then be
    v = 2∏ft/s sin (∏s^-1 * t)

    and the acceleration would be
    a = 2∏^2 ft/s^2 sin (∏s^-1 * t)

    So your peak velocity would be about 6.3 ft/s and your peak acceleration would be about 20 ft/s^2 or about 0.6g.

    Those numbers are probably pretty manageable, depending on how massive the moving load is. But if you want a more constant speed for most of the travel, then you going to have to accelerate a much large delta-v over a much shorter time period and will quickly get into the realm of excessive accelerations that can cause all kinds of problems.
     
  14. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Many CNC mills rapid traverse in the 10ft/s (120in/s is how they are rated) range, and stop within .001 inch repeatability.

    And most plotters, ink jet printers and drag knife style cutters use a single belt drive like the O/P is talking about.
     
    Last edited: Mar 14, 2013
  15. WBahn

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    Mar 31, 2012
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    Yes, but in both cases there is a lot more involved than the simple kind of setup that the OP is trying to stick with. In both cases, motion trajectories are used to keep accelerations in firm control and under maximum levels. For instance, a CNC mill is not going to be moving at 10ft/s when it is 0.001 inch away from where it is supposed to stop since that would reguire an acceleration on the order to 20 thousand gees. So it must start slowing down well before it gets that close to its destination.
     
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  16. sowjoe

    Thread Starter New Member

    Oct 6, 2012
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    I like the sound of this.... if I use a stepper motor I will need a driver, motor controller and various other things, does that mean I'll have a 2 foot box to hold all these components attached to the end of this thing? I would like to keep it streamlined... less is more.

    sowjoe:
    I have seen some diy belt driven cnc machines that slow down very little when they get close to their ends. So I think it's possible.

    WBahn:
    ... how long is the travel for this gantry mechansim.

    sowjoe:
    48 inches

    MrChips:
    How about a rotating wheel with an off-center post or cam-like mechanism?

    sowjoe:
    Don't think that will work. The carriage travel is 48 inches.
     
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