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Author Topic: Calculations for motor check  (Read 1298 times)

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Offline baalexanderTopic starter

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Calculations for motor check
« on: November 10, 2009, 11:27:03 PM »
I was hoping y'all could verify that I'm on the right track for motor calculations. I expect these numbers to be reasonable, but not exact (for example, friction isn't included).  The robot will be similar to a tilted mover's dolly, weigh about 22.6 kg (roughly 50 lbs), and be powered by two motors. The wheels right now are inline skate wheels, but I may have to get bigger wheels if I can't find a pair of motors to match the RPMs.

robot weight: 22.6 kg
max incline: 20 degrees
velocity: 167 cm/s (or 100 m/min, just above the average speed of a walking male)
acceleration: 80 cm/s^2 (half of velocity, advice from Robot Dynamics tutorial http://www.societyofrobots.com/mechanics_dynamics.shtml)
acceleration from incline: 335.5 cm/s^2 (from 981 cm/s^2 * sin (20 * pi / 180))
total acceleration: 415.5 cm/s^2

wheel height: 7.6 cm (inline skate wheel)
wheel circumference: 23.876 cm (pi * diameter, so pi * 7.6 cm)

force: 93.9 N (mass * acceleration, so 22.6 kg * 415.5 cm/s^2)
total torque: 3.5682 N m (force * wheel radius, so 93.9 N * 3.8 cm)
torque per motor: 1.7841 N m (torque / 2)

rpm: 419 rpm (velocity / wheel circumference, so (10,000 cm / min) / 23.876 cm)

I put these numbers into the RMF calculator (http://www.societyofrobots.com/RMF_calculator.shtml) and received an RMF score of 54.3 kg*m*rps.

I'm concerned because if I put the calculated torque (.18 kg*m, from 1.7841 N m) and speed (420 RPM) into the Motor Characteristic section, I get an RMF score of 1.26 kg*m*rps, not even close to the 54.3 kg*m*rps.

Offline hopslink

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Re: Calculations for motor check
« Reply #1 on: November 11, 2009, 04:50:57 AM »
...

I'm concerned because if I put the calculated torque (.18 kg*m, from 1.7841 N m) and speed (420 RPM) into the Motor Characteristic section, I get an RMF score of 1.26 kg*m*rps, not even close to the 54.3 kg*m*rps.

Stay cool 8) you don't re-use your calculated torque and speed in the motor section, you use the loaded torque and speed values from the motor datasheet.

In essence what you have calculated in the robot section is the minimum input power required to move your robot on your specified terrain with the dynamic performance you require. In the motor section you check a potential motor to see if it's output meets that minimum requirement. You are looking for a motor RMF value greater than your calculated robot RMF plus any inefficiency from friction, drive type, terrain etc. 

Offline baalexanderTopic starter

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Re: Calculations for motor check
« Reply #2 on: November 13, 2009, 12:06:40 AM »
After looking at motors on Robot Market Place, Pololu, and others from the parts list section, I've decided it's okay for the robot to be a bit slower and lighter (most of the heaviness depends on the load the bot will carry, and therefore is adjustable). Given that, I'm thinking this motor at Pololu: http://www.pololu.com/catalog/product/1104. My thoughts are if I shave the weight down to under 15 kg, the torque required per motor will be about 1.2 N-m, inline with the Pololu motor.

The 200 RPM speed is no-load, and still half of my original goal of 420 RPM. I'm okay with a slower robot, should be easier to work with, but is there a rule of thumb for no-load RPM to real world RPM (it can be a *rough* rule of thumb since there's plenty of factors :) )?

Any thoughts on this motor or other things I should be considering?

« Last Edit: November 13, 2009, 12:32:47 AM by baalexander »

Offline hopslink

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Re: Calculations for motor check
« Reply #3 on: November 14, 2009, 05:42:02 PM »
This seems like a good DC motor torque/speed curve tutorial .

It looks like the motor you link to will not be powerful enough for a 15kg robot (with anything like the performance you want). That said I am no expert :-[ , so if anyone knows better please call me on this as I'm learning too.

Offline baalexanderTopic starter

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Re: Calculations for motor check
« Reply #4 on: November 16, 2009, 03:43:13 PM »
Thanks for the link hopslink, that was quite useful!

I've found finding the right motor that balances torque for a 20+ kg bot with the RPM desired surprisingly difficult. Some of the sites I've tried if anyone is interested are: robotmarketplace.com, robotstore.us, polulu.com, lynxmotion.com, and several hobby sites.

Still, I've learned a fair amount from this, so am not complaining. I went ahead and ordered a torquier but slower version of the motor. I figured it's best to get something down and working than spend too much time finding that perfect motor.

 


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