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Author Topic: new to motors... please suggest the ratings..  (Read 694 times)

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Offline extreme.alyTopic starter

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new to motors... please suggest the ratings..
« on: February 14, 2013, 07:49:59 AM »
I'm making a robot for a competition in which the robot have go thru a maze and pot three different coloured balls (balls already placed on the robot) in the respective coloured boxes..

i think two motors (for driving purpose) will do the job.. one for left, and other for right.. i'll be using two 12v batteries, one to power up the ics (obviously after stepping it down to 5v) and the other for the two motors..

one more thing.. i'm thinking of having a 4-wheeled robot.. 2 on each side.. and use a chain to drive both wheels with a single motor..

if we suppose that my robot weighs 5-6kg, what motors i should use? i need torque and a little speed too.. i need it to be quick to start and i also need it to stop instantly (obviously when signals are received from microcontroller).. please help!

thank you :)

Offline newInRobotics

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Re: new to motors... please suggest the ratings..
« Reply #1 on: February 14, 2013, 07:52:45 AM »
We need wheel radius to give You torque estimate :)
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Offline jwatte

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Re: new to motors... please suggest the ratings..
« Reply #2 on: February 14, 2013, 02:13:40 PM »
How big is the maze? It sounds to me as if you could solve this with way less than 6 kg even for a pretty big maze (say, two-foot distance between walls.)

I think any of the Pololu 25D or 37D gear motors would work fine. Typically with mid-to-lower gear ratios to get some speed -- between 1:20 and 1:50.

Also, you'll want to look into getting motors (or wheels) with encoders, to make sure you can tell when you're driving straight!

If you have one motor per side driving, you don't need a chain to drive both wheels. That just adds complexity. Unless the maze has hazards like oil slicks or ice patches or whatever...

Offline extreme.alyTopic starter

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Re: new to motors... please suggest the ratings..
« Reply #3 on: February 15, 2013, 09:27:38 AM »
We need wheel radius to give You torque estimate :)
its around 3.5 - 4 cm...

How big is the maze? It sounds to me as if you could solve this with way less than 6 kg even for a pretty big maze (say, two-foot distance between walls.)

I think any of the Pololu 25D or 37D gear motors would work fine. Typically with mid-to-lower gear ratios to get some speed -- between 1:20 and 1:50.

Also, you'll want to look into getting motors (or wheels) with encoders, to make sure you can tell when you're driving straight!

If you have one motor per side driving, you don't need a chain to drive both wheels. That just adds complexity. Unless the maze has hazards like oil slicks or ice patches or whatever...
the distance between walls is 50cm..well i really dont know how much its gonna weigh with all the motors and the body..
if not chain, how do u suggest i drive both wheels with a single motor..?

Offline jwatte

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Re: new to motors... please suggest the ratings..
« Reply #4 on: February 15, 2013, 12:41:59 PM »
Quote
if not chain, how do u suggest i drive both wheels with a single motor..?

You said you'd use two motors. I suggest driving the left rear wheel with one motor, and the right rear wheel with the other motor, and leaving the front wheels un-driven. If you can add servo-based steering, you'll be better at turning, but you can probably do without it.

Or just put in four motors. Motors are cheaper than transmissions and linkages these days... Look at, for example, the Wild Thumper series for design ideas.

Offline extreme.alyTopic starter

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Re: new to motors... please suggest the ratings..
« Reply #5 on: February 16, 2013, 08:18:41 AM »
Quote
if not chain, how do u suggest i drive both wheels with a single motor..?

You said you'd use two motors. I suggest driving the left rear wheel with one motor, and the right rear wheel with the other motor, and leaving the front wheels un-driven. If you can add servo-based steering, you'll be better at turning, but you can probably do without it.

Or just put in four motors. Motors are cheaper than transmissions and linkages these days... Look at, for example, the Wild Thumper series for design ideas.


if i let two front wheels undriven, i'll have issues it making 90 degree turns while standing.. :s can you suggest what exact motors should i use? a picture will help :P

Offline newInRobotics

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Re: new to motors... please suggest the ratings..
« Reply #6 on: February 17, 2013, 03:13:22 PM »
its around 3.5 - 4 cm...
If I were You, based on numbers provided (r = 4cm, m = 6kg), I'd go with at least 4.71 N-m motors.

To calculate RPM of the motor use formula: RPM = DesiredVelocity / WheelCircumference.

As for 4WD, why not to go with 2 drive wheels and replace other 2 wheel by a caster wheel, that way You can perform fluid turns.
« Last Edit: February 18, 2013, 01:20:36 AM by newInRobotics »
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Offline jwatte

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Re: new to motors... please suggest the ratings..
« Reply #7 on: February 17, 2013, 11:32:18 PM »
Right, there are two options:

1) Use four wheels like a model car. Either steer the wheels in front, or drive each wheel individually and do "tank steering." The Wild Thumper does the latter -- check it out here: http://www.pololu.com/catalog/product/1565
That, in turn, uses these kinds of motors: http://www.pololu.com/catalog/product/2274

2) Use two big wheels, and two smaller wheels that are just "limp" and ideally pivot. Casters, balls, cart wheels, whatever you want to call them. Turning the big wheels different directions will pivot the robot in place. With only two motors, this is the easiest to implement.

3) You probably want motors with built-in encoders, so you can tell whether one wheel is going faster than the other and compensate, to go straight.

4) You don't need the full weight * radius amount of torque, as long as you don't want to accelerate wildly (and can make soft start/stop part of your control.) Wheels turn very easily, at least if they are narrow and hard.

If you haven't gotten any motors yet, those motors are a fine recommendation for 6V-8V systems. If you are doing a 12V-16V system, try these instead: http://www.pololu.com/catalog/product/1444

You will need some motor controllers to drive them, too. And a microcontroller to tell the motor controllers what to do. Feel free to ask again if you don't know where to go from here!

Offline extreme.alyTopic starter

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Re: new to motors... please suggest the ratings..
« Reply #8 on: February 24, 2013, 06:46:16 AM »
Right, there are two options:

1) Use four wheels like a model car. Either steer the wheels in front, or drive each wheel individually and do "tank steering." The Wild Thumper does the latter -- check it out here: http://www.pololu.com/catalog/product/1565
That, in turn, uses these kinds of motors: http://www.pololu.com/catalog/product/2274

2) Use two big wheels, and two smaller wheels that are just "limp" and ideally pivot. Casters, balls, cart wheels, whatever you want to call them. Turning the big wheels different directions will pivot the robot in place. With only two motors, this is the easiest to implement.

3) You probably want motors with built-in encoders, so you can tell whether one wheel is going faster than the other and compensate, to go straight.

4) You don't need the full weight * radius amount of torque, as long as you don't want to accelerate wildly (and can make soft start/stop part of your control.) Wheels turn very easily, at least if they are narrow and hard.

If you haven't gotten any motors yet, those motors are a fine recommendation for 6V-8V systems. If you are doing a 12V-16V system, try these instead: http://www.pololu.com/catalog/product/1444

You will need some motor controllers to drive them, too. And a microcontroller to tell the motor controllers what to do. Feel free to ask again if you don't know where to go from here!


thabk you so much! it really helped me out!
-well i cant use more than 2 motors for driving purpose.. its in the rules..
-casters.. i heard that the arena that they make is not perfectly levelled (friend's experience), so robots with casters had issues there, like stuck on small bumps.. thats why we dropped that idea..

i'm thinking of buying this motor: http://www.evselectro.com/motors-_vdc-motor-2299
well, do i need to have external gears to reduce the speed to my required one? or is PWM good enough?

Offline jwatte

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Re: new to motors... please suggest the ratings..
« Reply #9 on: February 25, 2013, 03:04:32 PM »
You need a gearbox. Ideally, a motor with a built-in gearbox, like the ones from Pololu.

 


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