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Select a robot motor

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Hi there,

I'm constructing a mobile robot, and need some advice concerning the choice of motor. Because of the type of construction, I need to drive every wheel with it's own motor. The motors can then be smaller.

I would like to run it all from car batteries (12V), but what type/size of motor should I use, in order for the motors to each run at a relative speed (not just 1-2 mph) and also to carry at least 100 lbs pr. wheel?

The price must also be relative low :D
 
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tytower

Banned
Have a look at ardino web sites such as "Maker" magazine and Toysdownunder.com , get that right because downundertoys.com gives something entirely different.

Google Ardino , there are some cool cheap motors about and Arduino is a processor board with free open source software available.
 
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mneary

New Member
You'll find the control system very "entertaining" when you have four independent motors. Getting all four of them running at exactly in unison is going to require some sort of feedback system.

There are two primary considerations for your motor: Power and Speed.

The motor power is complicated. Estimate how fast you want your bot to accelerate. Say you choose 1 m/s as your top speed. You want to achieve that in 2 seconds. You estimate your finished bot will have a mass of 200 kg (440 lb).

How do you choose a motor from that mess? Take out a physics book, and put those numbers together to get a result in Joules (watt-seconds). (really!) Figure a DC motor with gearbox is about 15-25% efficient unless you have a data sheet that says otherwise.

If the only motor spec you have is "stall current" you should initially plan to run at no more than 10% of this figure. But design the DC controller to handle the whole thing.

Then you choose a drive train starting with motor(s) powerful enough and a reduction ratio suitable to drive your wheels at your top speed.
 
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Mr RB

Well-Known Member
Hmm, 400 lbs? Car batteries? 1 metre/second? Cheap construction?

Should we call the lawyers first to give them some time to prepare the lawsuit before he switches the robot on?
:D
 

giftiger_wunsch

New Member
I'm not an expert but wouldn't even a car battery run flat pretty quickly with that load at that velocity? :eek: It's more or less comparable to a drivable lawnmower, which typically run with a petrol-driven engine...
 
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tytower

Banned
Oatley Electronics, Australia have some motors for this type of stuff and controllers but ,for that weight and speed ,I doubt it. Thats pretty demanding

Starter motor types would be too slow and gearboxes would be needed

Here is one at 32V for 750 watts but at $180 a piece
 
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mneary

New Member
I just pulled those numbers out of..... thin air.

I'm not an expert but wouldn't even a car battery run flat pretty quickly with that load at that velocity? :eek:
Turns out, once you achieve steady state, your power consumption also flattens. 50% of your battery power is in acceleration; i.e. accumulating kinetic energy in the load.

Hmm, 400 lbs? Car batteries? 1 metre/second? Cheap construction?

Should we call the lawyers first to give them some time to prepare the lawsuit before he switches the robot on?
:D
My friend in his wheelchair weighs at least 400 lbs. and goes all day on a charge. 1m/s is about 3 miles per hour, which for me is a brisk walk.

He didn't say if his bot would be ten grams or 200kg. Same arithmetic. I hope I didn't frighten him.
 
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Mr RB

Well-Known Member
1 metre/second is FAST for a big robot, he did say 100lbs per wheel x4 wheels. Industrial robots in the "hundreds of pounds" range usually are quite slow for safety reasons or they have large painted off areas to run in where humans are not allowed.

A 400lb bot running into your leg at 1 m/s and full power would be extremely memorable. :)

Anyway the bearings will cost as much as the motors, you can get wheelchair style worm drive DC motors cheap enough but the shaft is not rated to carry 100lbs radial load even for intermittant use, never mind continuous use. He will need bearing blocks, shafts etc.

A good source of heavy robot gear is;
The Robot MarketPlace & Team Nightmare - BattleBots & Combat Robotics Information

they have the big motors, wheels, bearings etc for large bots but they do seem to have become more expensive lately than they used to be.
 

giftiger_wunsch

New Member
A 400lb bot running into your leg at 1 m/s and full power would be extremely memorable. :)
Indeed, that's ~180kg.m/s which is a lot of momentum... equivalent to quite a hefty swing from a baseball bat :eek: or a full-speed tackle from a rugby player :D better hope it has soft bumpers :p
 
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