### Author Topic: RMF calculator http://www.societyofrobots.com/RMF_calculator.shtml  (Read 2343 times)

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#### Deffie

• Beginner
• Posts: 4
##### RMF calculator http://www.societyofrobots.com/RMF_calculator.shtml
« on: June 23, 2008, 02:06:21 PM »
Lets suppose i have this motor:

450W 24v
2500 rpm 1.9 nm
reduced by 35:1:
71 rpm 66 N/mt 6.7 kg/mt without considering losses

And lets plan our robotic platform:
Mass: 30 Kg
1.1 Mt/sec 3.9 km/h
0.5 m/sec accel.
75 % Efficency (the default)
10 degrees incline angle
0.3 meters wheel diameter
4 powered wheels

Rmf says:
12.6 kg/m/sec RMF
33 kg*m/sec
70 rpm motor rotation speed

And my motor:
6.7 kg/mt
71 rpm
7.9 RMF

Now:
One of these 450W drives an electric tricycle over 20 Mph with a man on it, which can weight 100kgs;
i'm using four of them for 30kg and RMF calculator says it is not possible.

Theres something wrong...help!

#### bukowski

• Robot Overlord
• Posts: 219
##### Re: RMF calculator http://www.societyofrobots.com/RMF_calculator.shtml
« Reply #1 on: June 23, 2008, 10:33:56 PM »
Im not seeing where you calculated the 35:1 reduction...

#### Deffie

• Beginner
• Posts: 4
##### Re: RMF calculator http://www.societyofrobots.com/RMF_calculator.shtml
« Reply #2 on: June 24, 2008, 12:46:17 AM »
Quote
2500 rpm 1.9 nm
reduced by 35:1:
71 rpm 66 N/mt 6.7 kg/mt without considering losses

2500/35=71 rpm
1.9*35=66 Nm / 9.8 = 6.7Kg/m

This is how i calculated gearbox reduction but maybe i'm wrong...

#### Deffie

• Beginner
• Posts: 4
##### Re: RMF calculator http://www.societyofrobots.com/RMF_calculator.shtml
« Reply #3 on: June 25, 2008, 10:37:51 AM »
Ping!

Where is RMF calculator creator ?

• Supreme Robot
• Posts: 11,632
##### Re: RMF calculator http://www.societyofrobots.com/RMF_calculator.shtml
« Reply #4 on: June 25, 2008, 01:20:19 PM »
Quote
One of these 450W drives an electric tricycle over 20 Mph with a man on it, which can weight 100kgs;
i'm using four of them for 30kg and RMF calculator says it is not possible.
The calculator doesn't say it isn't possible to drive a man around, only that it cannot go at the specs you entered. For example, did you measure the acceleration or incline angle or efficiency for that 20mph run?

Also, where did you get the rpm/torque data for the motor? To use it properly, you need to get it from a rpm to torque curve on the motor datasheet . . .

For example, you wrote the motor goes at 71rpm with a wheel diameter of .3 meters. If you do the math, that only comes out to 2.5 mph.

#### Deffie

• Beginner
• Posts: 4
##### Re: RMF calculator http://www.societyofrobots.com/RMF_calculator.shtml
« Reply #5 on: June 28, 2008, 04:58:00 AM »
The calculator doesn't say it isn't possible to drive a man around, only that it cannot go at the specs you entered. For example, did you measure the acceleration or incline angle or efficiency for that 20mph run?

Please reread my post, i'm not aiming at 20mph, but 1.1 mt/sec -> 2.4 mph, 3.9 km/h which is about a walking human.

Quote
Also, where did you get the rpm/torque data for the motor? To use it properly, you need to get it from a rpm to torque curve on the motor datasheet . . .

I've got them from the motor's datasheet

Quote
For example, you wrote the motor goes at 71rpm with a wheel diameter of .3 meters. If you do the math, that only comes out to 2.5 mph.

2.4 mph is just what i was expecting, Waaaaay similar to what you calculated

Take a look at this scooter http://www.electrikmotion.com/gt450main.htm , it is equipped with a 450w motor and with a man on it (80kgs ?) does 15 mph, lets assume it is optimstic, lets say 10 mph; why i cannot do that with four 450 watt motors and 30 kgs?

I just want to understand, i'm not saying your applet is wrong, i feel it useful and i just have to size motors for my project

Thank you,
Giacomo.