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Thread: Power Scooter to Lawnbot

  1. #1
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    Question Power Scooter to Lawnbot

    I need some advice on my project. I am building a lawnbot out of a used power scooter. So far I have shortened the chair by cutting off the front steering deck and removed the seat. I have 2 large caster wheels attached to the front, as I will be steering the bot by using the drive wheels. The drive wheels have differential drive, so both receive power from the drive motor. The way I intend to turn the bot is by turning on and off the brakes located at each wheel. When the bot needs to turn right, the right brake will be applied. This will cause the power from the main motor to be delivered to the left wheel, causing it to turn to the right. The opposite will happen when turning left.

    At the bottom of the bot, I installed a guard all around the bottom to prevent anyone from getting cut from the cutting blade. The cutting blade is made out of plexiglass shaped in a wheel with blades attached to the edge. The motor is driven by the 24volts at 6000rpm. I may put a speed control on the blade, because I think it turns way too fast. I will also install a safety mechanism that will detect when the bot is turned sideways so that it can turn off the blade motor.

    Later on, I will decide what controller to use. I thought perhaps a PIC 16F877a with sensors for a burried line detector to keep it in my yard.

    My problem is that I need an idea on how to install brakes at each wheel. The bot uses 24 volts for the drive motor, so this is what I have to work with. I thought of using pinball coils to energize and bring in the brake at each wheel. I don't know if the coil will be strong enough to provide enough braking power, but perhaps using a lever will increase the pressure the brake will apply. I tried the coils and they do get hot after a few seconds, so I will have to work out a circuit that will apply the full 24volts at first, then reduce it to say 12 volts to hold the coil in.

    Does anyone have experience with this type of build? Any ideas would be greatly appreciated.
    Sidster . . . . . :^)

  2. #2
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    Re: Power Scooter to Lawnbot

    >> The drive wheels have differential drive, so both receive power from the drive motor.

    Sounds to me like the drive wheels are not differential drive. Sounds like a gearbox. Both wheels turn the same speed without independent control. So it has a differential gearbox. If it was differential drive, then you would just pulse one side or the other and it never would have had a steering mechanism, right?

    Generally those things have a limited slip differential. Are you concerned with overheating the LSD and blowing it up?
    I Void Warranties�

  3. #3
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    Re: Power Scooter to Lawnbot

    I checked and the unit doesn't have a limited slip differential. Sorry for the wrong terminology, it is a differential gearbox drive for the back wheels. I did try stopping the left wheel and the bot did turn left with no problems to the driver board. By the way, I am using the driver board that came with the scooter and it works fine. My only hangup now is the braking method. Any ideas?
    Sidster . . . . . :^)

  4. #4
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    Re: Power Scooter to Lawnbot

    Maybe bike disc brakes http://www.cambriabike.com/shopexd.a...CAL+DISC+BRAKE this is the first thing that came my mind. These are $20 bucks a piece just need a motor to pull the cable, and a mount to the wheel. I don't know if they would give enough stopping power or if this is even what you were thinking about.
    Also I have been seriously thinking about making a mower myself. I really hate to mow . Keep us posted !

  5. #5
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    Re: Power Scooter to Lawnbot

    Bicycle disc brakes are an excellent idea. I've trusted my life to them to bring a 350lb tandem to a stop from 70+mph faster than a car can, and to maintain speed on gnarly descents. A servo would probably be a better bet for gently actuating them, and hydraulic is a must-have.

    Good suggestion!
    I Void Warranties�

  6. #6
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    Re: Power Scooter to Lawnbot

    I will have pictures tomorrow for everyone to see my progress so far. I forgot to mention that I will have prox switches at each wheel that will receive 5 signals per rotation. Eventually, the CPU controlling the bot will monitor the wheel movement and adjust the braking to each wheel thus keeping them in sync and going straight when that is what it needs to do.

    I don't see how bicycle brakes will work since the wheels are much thicker than bicycle tires.

    I have attached a power point to show how the braking will work to allow the bot to turn. I just need ideas whether to do it this way. If anyone has a better idea please let me know.
    Last edited by Sid723; 09-02-2009 at 02:05 AM. Reason: Attachment not added.
    Sidster . . . . . :^)

  7. #7
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    Re: Power Scooter to Lawnbot

    Hydraulic disc brakes do not fit around the wheel. They function the same concept that stops cars, trucks, airplanes, and space shuttles - I think they'll do fine for a lawnmower.
    I Void Warranties�

  8. #8
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    Re: Power Scooter to Lawnbot

    You could try pocket bike wheels.
    They have the disk brake bolted on already and a sprocket if needed
    also about the same size as you scooter wheels.
    most use a cable to make them work.
    Saves a bit of hasle mateing the disk to your wheels.
    If you have a importer near you they can be picked up for about the
    same as the bicycle ones but you also get the cooler looking wheels and
    tyres.
    Next on pimp my lawnmower
    If your scooter is chain drive its real easy to do.
    Still need something to pull the cable.
    Necessity is not the mother of invention.
    laziness is.

  9. #9
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    Re: Power Scooter to Lawnbot

    The solinoids from car starter motors have some serious grunt and are used to beening hot.
    Just watch your fingers as they bite so they should work to pull a cable.
    Might be to much load but worth a try.
    Necessity is not the mother of invention.
    laziness is.

  10. #10
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    Re: Power Scooter to Lawnbot

    Quote Originally Posted by Sid723 View Post
    I forgot to mention that I will have prox switches at each wheel that will receive 5 signals per rotation. Eventually, the CPU controlling the bot will monitor the wheel movement and adjust the braking to each wheel thus keeping them in sync and going straight when that is what it needs to do.
    How big are your wheels? (Diameter-wise). Generally, you want a fairly high encoder count in order to do speed regulation, otherwise you won't have the necessary resolution in your count in order to compare speeds. Just something to think about, in case you haven't dealt with feedback loops before.

    -Fergs

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