Choosing a motor

[Warhawk87]

Hey everyone,

I read Admins tutorials on motors and torque, but im afraid it is not quite helping me to pick a motor.   After using the RMF calculator, i got a value of 120, and a lot of the decent motors that I thought were good turned out to be around 60.  Also, on torque, what is the minimum amount of torque to get a 5 pound robot to move?  Im planning on using a 6-7.2V battery to power it if that matters also....

Thanks for the help!

[MechHead]

How many wheels are going to be powered and what diameter are the wheels?  If only 2 wheels are powered, the motors need to be roughly twice as strong than if 4 wheels are powered.  Also the bigger the diameter of the wheel, the more torque needed, so there is no set minimum torque.

[Warhawk87]

I was thinking a 1 1/2 inch wheel radius was a good size and yes i am only powering 2 wheels

[Soeren]

Hi,

[...] Also, on torque, what is the minimum amount of torque to get a 5 pound robot to move?

Assuming two motors and 1.5" wheels, each needs to have at least 30 ozf-in just to make it barely move on a level plane if you want speed and hill climbing abilities, add torque.
This is the torque on the motor axle, i.e. after the gearbox, the motor itself will have lower torque at a higher than useable speed.
Dismissing losses... If you use a gearbox reducing speed by 10, it will increase torque with the same factor (10).
In the real world there is frictional losses to overcome as well, so use a slightly more powerfull motor than calculated.

Im planning on using a 6-7.2V battery to power it if that matters also....

It doesn't. Some 3V motors will be way more powerfull than some 12V motors. It's input power and efficiency that sets the game.

[Warhawk87]

Thank you for your reply!

Does it matter the size of the motor at all?  Some are 32mm, 42mm, 52mm etc.... I dont think it should but just making sure

[Warhawk87]

Ive been searching for 40 oz-in torque motors at 12V and i cant seem to find anything at all...im a little stuck... advice on where to find an affordable motor like this?

[Warhawk87]

Ok I think i may have found a good one, so tell me if this is a good matchup...

This motor - http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_155012_-1
With this battery - http://www.all-battery.com/12v2600mahnimhbatterypack11620.aspx
On a roughly 5 pound robot

any more info just ask and ill be happy to provide....I dont know if the motor is any good because i dont know what all the numbers and ratings mean so if you guys could just tell me if its good and why its good, it would be much apreciated!

[waltr]

Is the output RPM ok for your application? I didn't see that spec.

[MechHead]

Motor size generally does effect the power of the motor, as larger motors generally are more powerful, but not always.  Also one number for a size of a motor doesn't mean that much, because the motor could be a small diameter, but very long, or could be a pancake motor (short but wide).
I don't think that you want this motor, the torque is more than adequate, but it says its RPM is 2.  So your robot will move at a really slow crawl.

[Warhawk87]

I don't think that you want this motor, the torque is more than adequate, but it says its RPM is 2.  So your robot will move at a really slow crawl.

As you said, there is torque to spair, so couldnt I fool around with gears and get the RPMs up? Or would I not be able to trade off enough to make a difference?

[Warhawk87]

For about the same price, this motor http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_253534_-1 kicks all the other motor's rear ends.... the torque is really high (7500 g-cm or 104 oz-in) and the speed is higher (24 RPM)

But a motor like this at the same price of much inferior motors is too good to be true... so what are the drawbacks?

[MechHead]

The only drawback I can see is that it draws more current.  Sometimes motors are just not priced logically.

You might be able to open up the gearbox and rearrange the motors, I don't know for sure (I've only messed around with planetary gearboxes on motors) but I don't think you would be able to get anything useful out of it.  If you want 40 oz-in and the motor has roughly 80, you would only end up with 4 RPM still.  24 RPM from this other motor is still pretty slow, but I don't know how fast you want your thing to move, but with only 1.5" diam wheels, its pretty slow.

You might like something like this motor:
http://www.robotmarketplace.com/products/0-BHG31.html

Similar price, but much more power and speed.

[Conscripted]

How about something like this? http://www.solarbotics.com/products/gm9/

Conscripted

[Soeren]

Hi,

I don't think that you want this motor, the torque is more than adequate, but it says its RPM is 2.  So your robot will move at a really slow crawl.

As you said, there is torque to spair, so couldnt I fool around with gears and get the RPMs up? Or would I not be able to trade off enough to make a difference?

Before you go disappoint yourself badly, please reread what I wrote:
  "Assuming two motors and 1.5" wheels, each needs to have at least 30 ozf-in just to make it barely move on a level plane if you want speed and hill climbing abilities, add torque".

You need far more torque unless you want to push start your vehicle or use a calender for a stopwatch timing it.
You want... Cheap, Geared, Powerfull
Don't go to robotics sites to get that - go to surplus sites, where brand new motors from eg. a closed down production run are going for cheap. There you will be able to find more powerfull motors cheaper than the one linked to allready.

A few examples:
http://www.surpluscenter.com/sort.asp?keyword=GIDD&catname=electric&PAGELEN=10000
http://www.surpluscenter.com/sort.asp?keyword=GRDD&catname=electric&PAGELEN=10000
http://www.surplustraders.net/a/0184.shtml

Find more here:
http://www.google.com/search?q=gearhead+motor+surplus&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:da:official&client=firefox-a

Before you select a motor, calculate what speed you'd
 a) Need
 b) Like
And what voltage range is:
 a) needed
 b) can be tolerated
And what current consumption is
 a) ... You know what goes here 
Etc.

Then selecting useable candidates for price comparison is quite easy

(Remember to calculate  the motors at your door .i.e. inclusive shipping when you compare)

[Warhawk87]

Thanks a lot for the links everyone and sorry that I misunderstood your post Soeren, I thought +10 torque would be adequate but I dont know how much torque is equal to what so Im just shooting in the dark....  Thanks for the advice on where to buy them, that makes a lot of sense, and ill check your links and do some more research and post back here with any questions... With that said, how many RPM with 1.5'' wheels is good to get the robot moving at a decent speed?  I dont know how much speed is decent so I'll leave that for your interpretation.

Thanks!

[Warhawk87]

I really like the motor that MechHead posted, http://www.robotmarketplace.com/products/0-BHG31.html, I assume 200 RPM is good?  And with the battery i posted above, is a 700 mA draw too much?

[waltr]

With that said, how many RPM with 1.5'' wheels is good to get the robot moving at a decent speed?  I dont know how much speed is decent so I'll leave that for your interpretation.

That is for you to determine.
Try laying out a fixed distance and then roll a ball so that it covers this distance. Do it again and again adjust how hard you roll the ball until it 'looks' to be the top speed you want your Bot to go. Time how long it takes the ball to go the fixed, measured, distance. Now you can calculate the desired speed and also can calculate the required wheel shaft RPM with the wheel diameter you choose.

[Conscripted]

I really like the motor that MechHead posted, http://www.robotmarketplace.com/products/0-BHG31.html, I assume 200 RPM is good?  And with the battery i posted above, is a 700 mA draw too much?

700mA was for a 12 volt motor with no load. You will need to look at the current requirements for stall as well. Stall current for this motor at 12VDC is 3.5Amps. Your actual requirements will probably be somewhere in the middle. Motor stall is when there is so much resistance on the motor shaft that it can no longer move. The current requirements will change for any motor depending on how hard they have to work.

What motor controller are you planning on using for this project?

Conscripted

[Warhawk87]

What motor controller are you planning on using for this project?

Conscripted

I was planning on building my own H bridge with transistors to control my motors

And with the expected current draw under a load, can my battery handle it?

[Warhawk87]

Ive gotten most of my supplies now but I'm still missing motors... Never thought one component would be such a headache! How long and wide is a normal motor shaft? I just need an average to work with so i dont get anything TOO small... I dont mind too big... But how do i lock the wheels in once they're on the shaft?