Converting N mm to Kg M

[Adramalech]

I'm looking for a couple of dc motors, and I am using the motor RMF calculator.
All the motors I can find have the torque in N mm, and I don't know how to properly convert it into something the
calculator can use (Kg m/lb ft).

Does anyone know how to do this?

p.s. I can't get my motors from American sites because the shipping time+costs to Europe aren't necessary,
and the dutch sites have it in N mm,

[Razor Concepts]

Google "X newton millimeters to foot pounds"

where X is the number in N mm

[Adramalech]

Thanks, I thought I was wrong because the numbers were very low.

The thing is, there are these electric motors which cost 50 euros, and they suck, with 270 N mm (0.2 lb ft) and 30000 rpm,
they can't even nearly power a robot of 25 kg with a normal speed, according to the RMF calculator.

I just thought this hard to believe,

[nottoooily]

You can check for mistakes by seeing if the power is reasonable and compare to other motors, electric power, light bulbs, etc.

Power = torque * angular velocity

Pick a point on the speed-torque curve where you expect to operate at. Maybe you were using a speed and torque that's really innefficient??

Thanks, I thought I was wrong because the numbers were very low.

The thing is, there are these electric motors which cost 50 euros, and they suck, with 270 N mm (0.2 lb ft) and 30000 rpm,
they can't even nearly power a robot of 25 kg with a normal speed, according to the RMF calculator.

I just thought this hard to believe,

[waltr]

Thanks, I thought I was wrong because the numbers were very low.

The thing is, there are these electric motors which cost 50 euros, and they suck, with 270 N mm (0.2 lb ft) and 30000 rpm,
they can't even nearly power a robot of 25 kg with a normal speed, according to the RMF calculator.

I just thought this hard to believe,

30,000 rpm wouldn't do much good directly driving a robot wheel. Is this what you tried to calculate?
If you gear reduce to 100 rpm (300:1) the torque increases by almost 300 to 60 lb-ft. Now that sounds like it should move a 55 lb robot.

[Admin]

30000 rpm?!

You want a motor thats geared down (has a built in gear box). Something like 1000 rpm is much better.

[Adramalech]

I've been searching for geared motors for a while now, but the "strongest" I've been finding,
(claiming they are for the big RC bots) are 25 kg/cm, 0.25 kg/m.. this results in an RMF of about 5.
Am I missing something here? it appears that the stronger type of motors are still unbelievably weak.

[waltr]

There are many stronger motors available. Here are a few:
http://banebots.com/c/MP-36XXX-540
http://www.robotshop.com/lyn-ghm-16-3.html
http://www.robotpower.com/products/magnum775_info.html

http://www.robotmarketplace.com/products/TW-TWM3M.html
http://www.robotmarketplace.com/products/motors_geared.html

With additional searching I could find more. I know the local train uses DC motors that move a multi-ton passenger car.

[nottoooily]

25kg-cm is heaps. With one motor and a 1cm radius wheel, you could drive your 25kg robot up a vertical slope, well at least hold it up. You could carry a person with a couple of those.

(claiming they are for the big RC bots) are 25 kg/cm, 0.25 kg/m.. this results in an RMF of about

[Admin]

25kg-cm is heaps. With one motor and a 1cm radius wheel, you could drive your 25kg robot up a vertical slope

ehhhh two problems . . .

1cm radius wheel, not much clearance with that . . .

Also, it'd only hold the person with a 1 cm radius wheel if the person weighed 25kg or less
(unless the slope is less than 20 degrees or so vertical)

[Adramalech]

So something like 200 rpm 6 KG/cm torque will be able to move a 25kg robot with (2/4 powered) 10cm wheels?
I'm just confused because the calculator says absolutely not.

[Admin]

Depends on the slope, the required velocity, required acceleration, and your efficiency. Reduce your requirements, and it might be able to do what you want.

[nottoooily]

Yea I was just trying to illustrate how strong it was. Another problem is how you get traction on a vertical slope! Well actually that would solve the ground clearance problem if you made it a cable car The person was a seperate thing.

25kg-cm is heaps. With one motor and a 1cm radius wheel, you could drive your 25kg robot up a vertical slope

ehhhh two problems . . .

1cm radius wheel, not much clearance with that . . .

Also, it'd only hold the person with a 1 cm radius wheel if the person weighed 25kg or less
(unless the slope is less than 20 degrees or so vertical)

[nottoooily]

You must be misinterpreting something. Where's this RMF calculator? And what numbers did you put in?

So something like 200 rpm 6 KG/cm torque will be able to move a 25kg robot with (2/4 powered) 10cm wheels?
I'm just confused because the calculator says absolutely not.

[waltr]

The RMF calculator is under calclators on the tutorial page.

Adramalech,

I did run the RMF cals:
mass = 25kg
speed = 7 m/s (~15mph)
accel = 1m/s/s
effe = 75%
grade = 5°
wheels = 2 x 400mm dia
RMF = 113 kg-m-rps
@ 334 rpm
This requires a motor/gearbox with an output of:
torque = 21 kg-m @ 334 rpm
for an RMF of 117

So your calcs sound correct.

If I did the calculations correctly the motor needs to be 0.3 HP (Horse Power).
These are available but not from the common robot parts stores.

I have been looking for motors that will move a 25kg robot. Wheelchair motors will do this so some searching will find them.
http://www.coolrobots.com/builders/newbie.html#motors
Also looked for what is being used for the larger Battle-Bots. Here is one source:
http://www.npcrobotics.com/products/index.asp

Hope this helps.

[nottoooily]

I think a good way to make the calculator always say "NOWAY" is to demand all of
- acceleration
- speed and
- hill climbing
at the same time. Each of these things requires a load of power so most vehicles are designed to accelerate fast on the flat, climb a steep hill slowly, and drive fast without accelerating.

Remember it won't be accelerating when it's at top speed! Don't ask for 1m/s^2 acceleratation and 1m/s speed in the same calculation if you don't want it to shoot away out of control.


But Ouch! I played with that RMF calculator, and it's great it give you the power.
But you have to multply the RMF in kg-m-rev/s  by 8.7 to get watts.
Or multiply the RMF in ft-lb-rev/s by 2pi to get ft-lb/s which google can convert to watts or hp or whatever.

But yea Adramalech, what other requirements do you have?
5deg slope = steep wheelchair ramp
1m/s = hold a ticking clock to your ear while you walk along to get a feel for speed

[Adramalech]

Well the requirements now are moving 25 kg at about 2m/s, with 0.5m/s^2 acceleration, 4 powered wheels of 15 cm, 75% efficiency and 10 degree angle.
I see now, I need to lower the weight anyway, because it would be too expensive if I need to buy some 100$ motors.
The problem is, the motors I have available,  are about 25 Kg/cm, so 0.25 Kg/m, and 300 rpm.
That resulted in a RMF of 1.25, while i needed about 20.

[Admin]

another place to find powerful motors:
http://npcrobotics.com/products/viewcat.asp?cat=20&mode=gfx

25 kg at about 2m/s, with 0.5m/s^2 acceleration

For a robot that heavy, 2m/s is scary (unless its a battlebot, as scary is a good thing).

Also, if your robot is going 2m/s, with 0.5m/s^2 acceleration, that means it'll take 4 seconds to stop if its going at full speed. I'm not sure what you're building, but I see this as a problem . . . you'll need mechanical breaks for safety reasons, for sure!

[nottoooily]

Well the requirements now are moving 25 kg at about 2m/s, with 0.5m/s^2 acceleration, 4 powered wheels of 15 cm, 75% efficiency and 10 degree angle.

Are you sure you need to accelerate at that speed? That means at 2m/s it's still accelerating up to an even higher speed. It also means you'll get more than 0.5m/s^2 at low speeds, such as starting up. If 2m/s is the top speed then put acceleration=0 in the calculator, that reduces RMF from 20 to 15 kg.m.rps.

Also I just noticed that calculator ignores rolling resistance. You might need even more power if you've got soft wheels or ground.

Have you considered windscreen wiper or power window motors? Used ones are practically free. Whenever I find a dumped car I try to take those bits Windscreen wipers have a pretty fantastic torque and the speed would be ideal.

By the way torque has units of force*length, not force/length.

I see now, I need to lower the weight anyway, because it would be too expensive if I need to buy some 100$ motors.
The problem is, the motors I have available,  are about 25 Kg/cm, so 0.25 Kg/m, and 300 rpm.
That resulted in a RMF of 1.25, while i needed about 20.

[nottoooily]

For a robot that heavy, 2m/s is scary (unless its a battlebot, as scary is a good thing).

I reckon! Not to mention flipping over on corners

[Adramalech]

Thanks for all the information and advice everyone,

The idea was to build a battlebot indeed, but we have decided to build something lighter and less powerfull to gain more knowledge about building robots.
I have found the motors required to build the new one, maybe later we'll try to build the heavy bot, if our new robots succeeds

I haven't considered alternative motors scrapped from somewhere, I'll look into that later on.

And I know it's force*length, I'm just used to type x/x/x

New specs are about 5 kg, moving at 2m/s (calculated with 0m/s^2), 4 powered wheels of 15 cm, 75 efficiency, 10 degree angle.
motors specs of 0.25 kg/m and 300 rpm are good enough for now.
We thought of buying a lead acid battery for the low costs, but I'm not sure if it's smart to do. So are we really doing something stupid here or is it ok?

[nottoooily]

Cool. For me the biggest problem with lead acid is that you have to keep it charged. So if you put the thing away for a year you come back to an damaged battery. But it's so easy to charge that can be an advantage.

New specs are about 5 kg, moving at 2m/s (calculated with 0m/s^2), 4 powered wheels of 15 cm, 75 efficiency, 10 degree angle.
motors specs of 0.25 kg/m and 300 rpm are good enough for now.
We thought of buying a lead acid battery for the low costs, but I'm not sure if it's smart to do. So are we really doing something stupid here or is it ok?

[Admin]

calculated with 0m/s^2

A robot that can't accelerate can't move

We thought of buying a lead acid battery for the low costs, but I'm not sure if it's smart to do. So are we really doing something stupid here or is it ok?

Don't confuse price with value per dollar. If the price is half, but the features are 1/4th, is it still worth buying?

We need to know more about your requirements before we can tell you what battery is best. That said, for the small/slow robots I like to make, NiMH is definitely the best value per dollar spent.

[Adramalech]

calculated with 0m/s^2

A robot that can't accelerate can't move

We'll just kick it in the back , nah it can accelerate, not too fast, but we don't ask much of it,

We thought of buying a lead acid battery for the low costs, but I'm not sure if it's smart to do. So are we really doing something stupid here or is it ok?

Don't confuse price with value per dollar. If the price is half, but the features are 1/4th, is it still worth buying?

We need to know more about your requirements before we can tell you what battery is best. That said, for the small/slow robots I like to make, NiMH is definitely the best value per dollar spent.

I have done the calculators on this one too, we do have the best (good enough)  value per dollar for our bot, I was just wondering if lead-acid wasn't too problematic or something.

[hopslink]

Lead acid is not well suited to robotics applications for the following reasons:

Poor tolerance of deep cycling - you can get deep cycle lead acid batteries however these are not cheap. Expect a hit in battery performance/lifetime after relatively few deep cycles. OK if you have short duration / high current application (like starting a car) with plenty of opportunity to recharge.
Low energy density (capacity/size ratio).
High weight/capacity ratio. You will take a performance hit or have to spend a lot more on motors/drivers.
Long charge times.

See the Battery University.