Motor selection

Hi All,
I'm in the planning stages of a new robot base. My last robot used servos for the drive wheels. This time, I want to use H-bridge motor control. I
want this robot to be very maneuverable and capable of a wide range of speeds from a slow smooth crawl up to a maximum speed of 3 ft per second.
I had hoped to use 2.5 inch drive wheels but doing the math I would need a motor output speed of 275 rpm (loaded) for the desired robot max speed which I'm not even sure if such a motor or gearbox is available, system voltage is 7.2V nominal. I wish to keep it a fairly small tabletop sized base so nothing huge.
I would also like the motor to have enough torque to accelerate the robot from a dead stop to full speed in a couple of seconds at the most.
I would appreciate suggestions and or specific part numbers for motors and or gearboxes or weblinks also. Most of the gearbox motors available on many of the robot websites are far too slow and many don't have even have the torque of a bottom end servo. Thanks.
-Dave
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There are a number of fairly nice Globe and Pittman DC gearmotors on the surplus market to choose from. You likely won't find a 6 or 7.2 volt motor that does what you need, but it's okay to run a 12 volt motor at 7.2, in which case you'll get about half the speed and torque. So for such a motor, and assuming about 250-300 rpm target speed, a 600-700 rpm 12 volt motor should be ideal (for speed).
Indeed, All Electronics sells such a motor for $14.95. It's a very nice Globe 12 volt 650 rpm gearmotor, which I have used in a number of applications. It's their stock ID: DCM-208. Torque on these is unknown, but I've powered 3-4 pound robots with them.
Also, consider a 12 volt separate supply for your motors. You can run motors at a higher voltage than your system voltage. Most bridges are fine with this, and in fact, any DMOS-based bridge will insist on 10 volts or more. You can use a separate 12 volt gel cell battery, or something.
You might as well use wheels larger than 2 1/2" inches, as even at half voltage these motors should have fairly decent torque. With 3 1/2" wheels, and assuming a conservative 250 rpm top speed, you're looking at 4.1 rps, or about 45" per second. That's no load speed, so whether your robot, at whatever weight it's going to be, can hit your 36"/sec target is yet to be seen.
If you do run the motor at 12 volts, you'll have plenty of headroom to play with. This is always a good idea in motor selection.
-- Gordon Author: Constructing Robot Bases, Robot Builder's Sourcebook, Robot Builder's Bonanza
Rylos wrote:

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Thanks Gordon. I forgot to mention initially that I would rather ere on the side of overpowered but your suggestions pretty much took that into account already. I 'll check them out.
BTW, any plans on coming out with a 3.5 inch double O-wheel? I used your doubles for my last robot and they're terrific. We spoke once before on the phone about the spreading problem you thought would be an issue on the double wheels for large diameter. I just wondered if you've gave it anymore thought, I'd buy them. Best regards,
-Dave

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I have a number of new products in development, and 3.5" double o-wheels are one of them. I have some nylon 4-40 screws on order for use on the the outer diameter of the wheel. I want to use nylon rather than steel for the weight. I'm now using the 2.5" D-O wheels exclusively with the Scooterbot and Octabot, and I like them better than the original polyurethane skate wheels I used. Lighter for one thing.
The new stuff should be ready in more than enough time for fall semester...mid August at the latest.
-- Gordon http://www.budgetrobotics.com
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