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Thread: Power Scooter to Lawnbot

  1. #11
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    Re: Power Scooter to Lawnbot

    As promised, here are the photos of my lawnbot so far:

    Attachment 1460
    Attachment 1461
    Attachment 1462
    Attachment 1464

    The blade design: I actually modeled it after a method I saw on other lawnbots sold comercially.

    How big are your wheels? (Diameter-wise).
    The wheels are 10.5 inches in diameter. If you look closely at the side view of the wheels, you will see 5 circles. This is what the prox switch will sense as the wheel rotates.
    Attachment 1463

    I would like to put the braking mechanism on the main deck where the motor is located. As you can see, I don't have a lot of room for hydraulics. Besides, it doesn't take much to stop the wheels from moving. The brake I showed ( in the previous power point) will have a rubber pad that will slow or stop each wheel. Remember, only one wheel will brake at any one time. This is only to steer the bot left/right. Also for stopping both wheels at the same time, the power scooter already has dynamic braking that is incorporated in the motor controller.

    The last picture shows the cover on the bot. This is actually the same cover that came with the power chair/scooter. It looks very nice when all put together.
    Sidster . . . . . :^)

  2. #12
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    Re: Power Scooter to Lawnbot

    To easy
    Those five circles are the perfect place to mount the disk brake disks, just drill
    and tap might have to pack out for clearance then a couple of brackets for
    the calipers, done.
    And if your lawn is like mine (read jungle) you might want to get bigger casters for the front .
    Oh and put a fake weapon on the front just to freak the neighbors out
    Looks good so far.
    also most disk brakes have holes in them for cooling around the work surface so you can
    get a better resolution for the encoder than the five points you want to use now.
    Necessity is not the mother of invention.
    laziness is.

  3. #13
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    Re: Power Scooter to Lawnbot

    Five points for a wheel encoder? I suspect that's an impractical application if you want to keep it in the same county and not spinning in circles half the globe away. In a perfect world that never has any wheel-slip, that might almost be adequate. In the real world, I think you're a _few thousand_ short per wheel.
    I Void Warranties�

  4. #14
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    Re: Power Scooter to Lawnbot

    PRE-QUALIFIER: I am a nub.
    If you are going to use the wheel breaks for steering will that not mess with trying to encode the wheels to turn at the same rate (speed)? If you purposely stop a wheel will the motor not try to turn that wheel faster or make the other wheel stop to stay in the same location??
    I like your idea of pushing against the tire I just know that it is practical or good for the tire
    Are the drive shafts keyed? I can't tell from the pics. If so than RevBryce's comment regarding mini bike brakes might be a cheaper route than mountain bike disk breaks.

  5. #15
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    Re: Power Scooter to Lawnbot

    +Rep for the Noob. That can be addressed to some extent in software, but if the wheel doesn't pivot perfectly in place (and how could it on uneven terrain) you are absolutely correct in your concern.
    I Void Warranties�

  6. #16
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    Re: Power Scooter to Lawnbot

    If you look at the link on the first page it shows a disk with 30 holes in it.
    A long way from perfect but getting better for the encoder.
    Realy encoder wheels ar`nt that hard to make with enough slots to make it
    work ,i`ve seen somwhere how to print them so thats easy.
    as another thought could the insides of a stepper motor be used with a
    hall effect sensor,hmmm mounting it on the shaft could be a problem,but
    it should work . maybe a bit slow to read.
    Necessity is not the mother of invention.
    laziness is.

  7. #17
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    Re: Power Scooter to Lawnbot

    OK , I see the light about the encoder. Problem is being outside in the dirt and grass, I cannot use a photo encoder. I would like to stay with the prox sensor I have, which is about 1/4" in diameter itself. The wheel diameter is 10.5 inches and I can make a plexiglass encoder wheel to attach to the side of the wheel that will have metal sensor points that the prox can detect. I just can't make the sensor wheel too large where it will interfere with the braking mechanism. With that, I have come up with these calculations:

    With an encoder wheel of 6" and sensor points spaced 1/2" apart around the wheel I will have:
    6 * 3.1415 approximately 19 " circumference around the disk.
    19" / 0.5 inch (per sensor point) = 38 sensor points

    Which comes out to : 360 deg / 38 = 9.5 degrees per sensor input or
    (10 " wheel * 3.1415 = 32 inches per revolution)
    32 inches / 38 (signals) = 0.84 inches per sensor input.

    That to me doesn't seem all that bad.

    If you are going to use the wheel breaks for steering will that not mess with trying to encode the wheels to turn at the same rate (speed)? If you purposely stop a wheel will the motor not try to turn that wheel faster or make the other wheel stop to stay in the same location??
    The CPU on board will help to correct the drift if it needs to go straight. When it needs to turn, the CPU will apply the brake to one of the wheels (depending on which way it wants to turn) and count the appropriate amount of sensor inputs on the other wheel before turning the brake off. This is also good for alerting me when there is a problem.
    For instance, the CPU can turn a brake on at say the left wheel, then if it detects too many sensor inputs for that same wheel, it should stop and alert me that something went wrong. This would indicate a brake failure. Good for troubleshooting.
    Last edited by Sid723; 09-03-2009 at 11:10 AM. Reason: Typing error.
    Sidster . . . . . :^)

  8. #18
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    Re: Power Scooter to Lawnbot

    0.84 Inches is fine for stopping at a certain distance traveled, but you mentioned trying to drive in a straight line. The problem with keeping two motors going the same speed is that you'll need a fairly high update rate on your feedback controller, and you need a fairly high density of pulses to be able to determine error of speed.

    Not sure how fast your mower will be driving, but lets make our example easy and guess 1ft/sec (12"/sec). That's 14 counts per second. If you want to have a 10hz feedback loop, you only get 1 or 2 counts per iteration of the loop, that's not nearly enough: if you read 2 you're going too fast and so you slow down, then you see 1 count, you speed up, etc, etc, and the bot drives all over the place except straight. Even with a 1hz feedback loop (and that low of a rate will probably veer and drift all over the place and oscillate like crazy), you'd only get 14 counts per iteration of the loop, which isn't a whole lot of data. Anything less than a 1hz update rate on your feedback loop is almost certain to drift off course...

    -Fergs

  9. #19
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    Re: Power Scooter to Lawnbot

    ^^^ Practical Robotics Defined right there.
    I Void Warranties�

  10. #20
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    Re: Power Scooter to Lawnbot

    Again not perfect but how about multiple sensors?
    Necessity is not the mother of invention.
    laziness is.

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