Society of Robots - Robot Forum
Mechanics and Construction => Mechanics and Construction => Topic started by: sbonkosk on December 11, 2010, 10:08:15 AM
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Im currently working on a project for a lawn mower robot and need some help with motor selection and mounting of the motors.
We're not sure to go about using 2 motors out back and a free moving wheel up front, or 4 wheel drive. We estimate the robot to be about ~20 lbs or less, and our goal is 1 foot/second speed.
We have no experience building a mechanical system like this so we messed up the first time around. We built a frame, mounted the motor to the frame, and then the hub to the motor shaft which connected to the wheel. That was it. This ran for a little bit but then 2 of the motors seized because of the weight being supported by only the motor shaft.
So we're trying to figure a new approach to actually mounting the wheels/motors so that the weight wont destroy the motors, and also looking into possibly new motors because we're returning the 2 that are currently still working.
These were our original 4 motors
http://www.robotmarketplace.com/products/0-B231.html (http://www.robotmarketplace.com/products/0-B231.html)
We're thinking of now getting 4 of these instead
http://www.pololu.com/catalog/product/1107 (http://www.pololu.com/catalog/product/1107)
Can somebody recommend a better motor choice for our application? 12v, 1ft/second speed, going through grass, robot weight ~20 lbs or less, low cost.
Should we go with a 2 wheel or 4 wheel design? Would either be fine?
Also, we're now thinking of mounting the motors like so:
http://yfrog.com/f/evimag0386aj/ (http://yfrog.com/f/evimag0386aj/)
Will this avoid the weight of the robot being forced on the motor shaft?
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Hi,
Getting a weaker motor sure won't help.
2 motors with a torque each of 18 kg-cm on a 9 kg (20 lbs) robot means that your wheels should be quite a bit smaller than 8 cm (about 3-1/4") in diameter to be able to barely move on a smooth and level floor.
Small wheels won't do well on grass (which in itself takes more torque than a level floor).
You should go for at least 8" diameter wheels and make wheel axles that is driven from the motors (just get much more powerful motors) by way of a bicycle chain or timing belt. The axles could be supported each side of the wheels to avoid a non-vertical load, or at least use two bearings - one where the axle exits towards the wheel and one closer to the center line of the robot (and a strong axle).
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Hi,
Getting a weaker motor sure won't help.
2 motors with a torque each of 18 kg-cm on a 9 kg (20 lbs) robot means that your wheels should be quite a bit smaller than 8 cm (about 3-1/4") in diameter to be able to barely move on a smooth and level floor.
Small wheels won't do well on grass (which in itself takes more torque than a level floor).
You should go for at least 8" diameter wheels and make wheel axles that is driven from the motors (just get much more powerful motors) by way of a bicycle chain or timing belt. The axles could be supported each side of the wheels to avoid a non-vertical load, or at least use two bearings - one where the axle exits towards the wheel and one closer to the center line of the robot (and a strong axle).
So would you recommend a 2 wheel drive design or 4 wheel drive? I would think for our case a 4 wheel system would provide better traction and its important since we'll be moving through grass, but Im not sure what benefits a 2 wheel system would offer.
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Hi,
I'd definitely go for a 4 wheel construction.
If you create an outer frame with the wheels and all, you can have the cutter mechanism in the middle being able to move up and down to both adjust cutting height and perhaps even to keep a distance if you're going over a hump (or the cutter could be directly in line with one of the axles).