Robotic Fertilizer Spreader & Motor Control Circuit

My name is Blake Nolingberg and I'm a student at the University of Houston's College of Technology. My senior project group is trying to
make a robotic fertilizer spreader. And we are hoping some of you might be able to help us with a question we have.
We are building our project from a donated golf cart. The current problem we are facing is finding a cheap motor control circuit for a 36v golf cart motor. Our logic is very simple. We do not need lots of motor control. The robot will only be made to operate in square or rectangular spaces at a relatively constant speed. We need it to travel in a straight line, turn (each turn will be identical), and stop. We do not plan to vary the speed and other functions that expensive motor control circuits offer. We plan on making the unit turn w/ a different servo motor as well. The only thing our motor control circuit needs to do is make the motor start and stop when the microprocessor instructs it to.
Do you have any ideas about where to find or how to build such a motor control circuit? Are there any other ways to control a DC motor besides PWM?
Thanks a lot, Blake
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You may find www.4qd.co.uk interesting - there is quite a lot of basic stuff about pwm drives ...

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Why not use whatever controller the golf cart came with?
Mark

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Blake,
I agree if all you want is go/stop then you can probably just tweak the existing controller attached to the throttle either electronically or mechanically with a solenoid. Another way to go might be a simple relay like a starter relay from a car that can handle lots of amps.
I'm in the Houston area and would be glad to come out and see what you guys have going on.
Good luck!
--Steve
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Thanks a lot for all the good prompt responses. I would like to use the motor controller already on the cart. The problem is that the carts were made in the 80's and I wouldn't know where to start as far as interfacing them to recieve signals from our micro controller. We are using an Atmel 8051. Thanks again everyone. This newsgroup is proving to be one of our best resources.
-Blake
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It will almost certainly be easier to hook into the existing controller than to roll your own. And MUCH less expensive. You might want to try contacting the manufacturer for information.
Mark

guys
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Thanks Mark. I'm really hoping we can figure out how to interface the existing controller to our 8051. You might see further posts on this newsgroup if we run into problems with that. I'm not sure what model of golf cart my team members wound up getting but I know a couple of the manufactures we were looking at are now defunt. I know one was Davis 500, which is no longer in existance. Hopefully we can find information somewhere.
Thanks again guys, Blake

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Some golf carts have useable motor controls. The throttle pedal may be a low power pot... a bit o' tweakin and you could emulate the pot... Perhaps a servo/mechnical solution will be cheaper than an adequately high powerd H-bridge thingy.. (make a robotic widget to move the pedal)
maybe this will help.. http://home.interlog.com/~dgv /

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