### Author Topic: Motor RMF calculator question  (Read 4818 times)

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#### MvB

• Beginner
• Posts: 6
##### Motor RMF calculator question
« on: April 28, 2011, 04:25:30 PM »
Hi everyone,

I am starting to build my first robot. I used the RMF calculator on this website to verify the motor calculations. With an efficiency of 100% the Result RMF and Motor RMF should have the same value, but it is about 30 times smaller. Something has gone wrong, but I really can’t find the mistake. (Pololu 29:1 metal gearmotor specifications are used for the calculation.) This are the values:

d = 0.20                 [Wheel diameter, m]
rpm = 350                 (from motor specs)
v = pi*0.2*350/60 = 3.67    [max velocity, m/s]
T = 1.55            [Total torque, Nm] (from motor specs)

a = F/m = 1.94         [acceleration, m/s^2]
F = T / (0.5*d) = 15.5   [Force, N]
m = 8            [Mass, kg]

Desired Robot Inputs:
Mass                    8      kg
Desired Velocity              3.67      m/s
Desired Acceleration      1.94      m/s^2
Expected Efficiency      100%
Incline Angle         0       degrees
Wheel Diameter              0.2      m
# of Powered Wheels      2       wheels

The Results RMF turns out to be 14.9 kg*m*rps

Enter Motor Characteristics:
Torque            0.078      kg*m
Speed            350      rpm

Motor RMF = 0.467   kg*m*rps

Can someone help me out? Unfortunately I really don’t understand why the RMF’s aren’t the same. Probably it is somtehing very simple…
Thanks!
« Last Edit: April 28, 2011, 04:28:21 PM by MvB »

#### Tigr

• Beginner
• Posts: 2
##### Re: Motor RMF calculator question
« Reply #1 on: April 28, 2011, 04:33:35 PM »
Hello!

I haven't even started building my first robot yet, so I don't really know much about robots or RMF, but maybe the 1/30 factor has something to do with the 29:1 ration on the motor?

Good luck!

#### MvB

• Beginner
• Posts: 6
##### Re: Motor RMF calculator question
« Reply #2 on: April 28, 2011, 04:45:24 PM »
Hi Tigr,

I don't think the 29:1 ratio has anything to do with the RMF, because the rpm and torque are based on the output axle.
For example: the 50:1 motor ratio results in a 1/3.3 factor. The gearing itself makes no difference in that.
But thanks for the suggestion

#### waltr

• Supreme Robot
• Posts: 1,944
##### Re: Motor RMF calculator question
« Reply #3 on: April 28, 2011, 06:46:02 PM »
The first calculation of RMF is what is required to have the Bot move as specified.

The second is what the selected motor's RMF actually is.

What you need to do is find a motor that has an RMF equal to or larger than the RMF needed to move the Bot.
Or, change the Bot's specs, weight, acceleration, speed, until the RMF is less than the motor you choose.

#### MvB

• Beginner
• Posts: 6
##### Re: Motor RMF calculator question
« Reply #4 on: April 29, 2011, 12:53:25 AM »
Hi Waltr,

You're right, but that is exactly my problem. With the values I used, the Motor RMF and Results RMF should be the same.
I didn't enter the desired velocity and acceleration, but the maximum velocity and acceleration this motor can provide.
I've done the calculation mentioned above over and over again, but I really can't find the mistake.

« Last Edit: April 29, 2011, 12:55:48 AM by MvB »

#### waltr

• Supreme Robot
• Posts: 1,944
##### Re: Motor RMF calculator question
« Reply #5 on: April 29, 2011, 08:55:13 AM »
No, they shouldn't be the same unless you change the BOT's specs.
You are spec'ing an 8kg bot!!!!
Try decreasing the Bot's mass to about 0.4kg and decrease the acceleration to 1m/s then see what the RMF required is.

Now they are close enough to be a match.

#### MvB

• Beginner
• Posts: 6
##### Re: Motor RMF calculator question
« Reply #6 on: April 29, 2011, 09:49:41 AM »
Thanks for your reply. With m = 0.4 kg and a = 1 m/s^2 they are indeed close enough:
RMF required: 0.383
RMF motor: 0.455

But I still don't understand where I made a mistake; with a motor speed of 350 rpm and 1.55 Nm torque, the acceleration is 1.94 m/s^2 and the top speed 3.67 m/s. That's just a simple calculation without using the RMF calculator.
I hope you can explain a little more.

EDIT: I think I have found the mistake
I assumed a 1.55 Nm constant torque. But I forgot it isn't constant... This makes the calculation a little bit more complicated, I can probably easier make a Matlab file to choose the right motor.
Thanks everyone!
« Last Edit: April 29, 2011, 10:30:26 AM by MvB »

#### waltr

• Supreme Robot
• Posts: 1,944
##### Re: Motor RMF calculator question
« Reply #7 on: April 29, 2011, 10:26:55 AM »
The required torque is dependent on the MASS that needs to be ACCELERATED.
Torque is a rotational FORCE and can be approximated in a linear motion with the common equation: F = ma.

You were trying to move a large Mass (8kg) at a high acceleration (1.67m/s/s) with too small of a Torque (Force).
If you built the Bot with that Mass and those motors it would just sit there and the motors would be stalled and over heat.

The two RMF calculations are independent and you need to change parameters until they converge.

Try changing the other parameters for the Bot like wheel diameter, incline, acceleration and mass to see have each affects the required RMF.
Also, did you read the motor tutorial that explains the reasoning and calculation behind the RMF value?

#### MvB

• Beginner
• Posts: 6
##### Re: Motor RMF calculator question
« Reply #8 on: April 29, 2011, 02:27:27 PM »
You replied just before I edited my last message I see.
Maybe I didn't explain the problem good enough; I know what these formulas do and how to use them. (I've used them hundreds of times during dynamics college.)

This is the mistake: I assumed a 1.55 Nm constant torque, while I should have used something like this:
T(v) = Tstall - (Tstall / rpmmax) * rpm = 1.55 - (1.55/350)*rpm

I'm going to make some graphs to choose the right motor. Thanks for all the replies.

#### waltr

• Supreme Robot
• Posts: 1,944
##### Re: Motor RMF calculator question
« Reply #9 on: April 29, 2011, 03:10:43 PM »
Ahh...motors and torque is not easy to get your head around. Looks like you are making progress however.

Here is a pretty good tutorial on motors:
http://www.micromo.com/dc-motor-tutorials.aspx

Hope these help.

• Supreme Robot
• Posts: 11,703
##### Re: Motor RMF calculator question
« Reply #10 on: May 06, 2011, 10:42:04 AM »
8kg at 3.67 m/s at 1.94 m/s^2  ?!?!?

Thats like instant death if you get hit by that thing . . . not even battlebots can move that much mass that fast!

By comparison, my ERP is like 1.5kg at max 0.8 m/s at 1.2 m/s^2. Unless you are making a death machine, your numbers are waaayyyy too high lol.

#### MvB

• Beginner
• Posts: 6
##### Re: Motor RMF calculator question
« Reply #11 on: May 06, 2011, 01:25:26 PM »
Yeah the acceleration seemed too good to be true. Although it would be an interesting challenge, it's not the intention to make a death machine lol.

With two of these motors and the steering servo I think the robot will be powerful enough (other suggestions are always welcome of course):
http://www.superdroidrobots.com/shop/item.aspx?itemid=840
http://www.superdroidrobots.com/shop/item.aspx?itemid=670

I've read the tutorial on motors, but most shops do not provide enough specifications to make very detailded calculations.
Here is some more information too, for people who are interested: http://www.islproducts.com/designnotes.
« Last Edit: May 06, 2011, 01:26:48 PM by MvB »