Questions about the 12V motor mount kit with encoders

Gabriel

I have a few questions about the 12V Motor Mount Kit with encoders.
1. Can the resolution of the encoders be increased by changing the black spoked wheel to get something like 1 degree precision?
2. How big is the slop in the motor gears? About 1 degree or more?

I am thinking of getting a kit to build a balancing robot. Is this feasible or not? Will the gears strip from the too many direction changes? I estimate the robot will weigh about 15kg.

Thanks in advance.

W9GFO

I doubt the gears would strip. I believe it uses a worm drive which is very robust.

To increase the resolution I think you would need a larger diameter encoder disk - so that the tabs remain the same size and distance apart, just more of them.

Otherwise, maybe a replacement disk with double the number of tabs (each half the normal width) would work but may require more careful alignment - don't know if that would work or not, seems plausible.

Rich H

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The Simple Servo Tester, a kit from Gadget Gangster.

erco

The best thing anyone can say about worm gears is that they are compact. They are notoriously inefficient for power transmission, both in power wasted and mechanical wear, as compared to spur gears. There is 100% sliding contact in worm gears (which generates the wear), where spur gears have much more of a relative rolling motion. Worm gears are inherently self-locking, which is desirable in some, but not all applications. That is, they don't coast very much when power is removed, and you can't "back-drive" them. In a balancing bot, the drive direction changes constantly and frequently, so there will be accelerated wear and higher power requirements. You'll have to carefully ramp your drive voltage up and down as it reverses direction to avoid locking your wheels and skidding your tires on the ground to achieve stability. IMHO, a spur gear transmission would be much more desirable.

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Roy Eltham

I can turn the wheeel on my motor mount and wheel kit by hand fairly easily. I don't think I could do that if it was a worm gear setup, unless it has some sort of clutching or slipping setup, but it sounds and feels like I am turn the motor shaft when I turn the wheel.
Maybe it's something more like this inside there? http://www.ul.ie/~nolk/Ken8.gif

They also do have a little bit of "play" in them, just a few degrees though.

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Tom C

Roy,

Trust erco, they are worm gear windshield wiper motors adapted for a robotics application. I have a set and they have worked fine for my robotics application. See the Parallax article in Robotics mag.

Regards,

TCIII

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If you are going to send·a Robot·to save the world, you·better make sure it likes it the way it is!

erco

@ Roy: I was quite impressed by smoothness of these motors in the videos of your Parallax wooden platform robot. I think that is the perfect application for these motors. They won't see extremely high mileage, and they deliver a compact drive with good accuracy in a non-balancing diffdrive robot. It is interesting that you can backdrive them, that's usually not the case with worms.

I posted because the OP asked specifically about a balancing robot, and also about how much slop was in the wheels. In the case of a balancing robot where the wheels are constantly reversing direction (especially standing still), and there will be ten (probably more) times the number of fwd/reverse cycles of a non-balancer. The worm gear's sliding motion will wear faster than spur gears and the "slop" will increase at an accelerated rate in a balancer. Increasing gear backlash will introduce new stability problems.

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·"If you build it, they will come."

Roy Eltham

Tom C, I've did some searching before I posted that, and not all windshield wiper moters use worm gears, but they do all seem to have some form of slip/clutch setup on the main output gear/shaft. Anyway, I took the back plate off of one of mine and it is a worm gear drive, and when you turn the wheel it's actually driving the worm gear to spin. It's got a steeper angle in the worm gear than I've seen before which is allowing it to be driven by the big drive gear.

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W9GFO

My wheelchair motors, which I presume use worm gears, are also easily back driven (once the electromagnet brake is removed). It has about 2.7 degrees of play. The drive ratio is 32.25 : 1.

This is the best illustration of the gearbox I could find.



Rich H

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The Simple Servo Tester, a kit from Gadget Gangster.

Post Edited (W9GFO) : 5/27/2010 6:40:01 PM GMT

Gabriel

Thanks for your responses guys.

I have been building Lego robots (and "sheet metal" robots) so I know exactly how a worm drive works, as I have used this setup in my Lego robots many times. I am working now on several different projects and a balancing robot is on the way. But I still haven't found proper motors for it. I want strong enough motors that do not use planetary gearboxes (they are notoriously noisy) with a back shaft or built in encoders. I would like to use 6" or 8" wheels and the estimated weigh of the robot is about 30 lb and about 3 feet tall. It's always good to have some spare power to be able to climb an incline like the handicapped inclines we find at the entrance of the buildings.

If any of you guys have experience with motors and can recommend me something suitable I would really appreciate it.

Thank you in advance for your recommendations.