Mechanics and Construction > Mechanics and Construction
The right motor for the job
gathem:
Hello,
I have made the mistake of buying inadequate motors in the past and for this build I cannot afford to buy the wrong motors.
I am building a robot and I am hoping to find some suggestions on motor options. The attachment should be pretty clear as to what my goals are.
Quick overview:
2 geared motors + motor controller + arduino
2 casters.
50-60 lbs of weight
very big (up to 40lbs alone) battery. (For making the CG very very low, and to give it a very long battery life)
I was planning on putting 2 12V lead acid batteries in series to make 24V.
The goal: I want my robot to be able to move at a similar speed as a human walks (I can tone it down in the code but it must be able to go at least that fast). I want to be able to move it very short distances (< 3 inches)
I am thinking of this motor http://www.robotcombat.com/products/ML-42-24.html
I know it has 90lbs/in stall torque but I really dont know if that is enough.
Also I think 240rpm (with 4-6" wheels) should be able to keep up with a persons walking speed although I am unsure of this.
I was thinking of using this motor driver http://www.dimensionengineering.com/products/sabertooth2x12
I really cannot afford to spend $220 on this setup to find out that I picked the wrong motors.
Can anyone provide any feedback on my intended selections?
Thanks!
tomcharley:
Dear Gathem,
I think the tutorial that you want is here: http://www.societyofrobots.com/mechanics_dynamics.shtml
Just in terms of speed alone, you'll probably want to have your robot be capable of about 3.5mph (at least 3mph if you don't mind slowing some people down a bit). 240rpm with 6" diameter wheels will only get you about 1.4mph, so you'll want something a little better than that.
Your torque calculations will depend a bit on the terrain as well as wheel size and robot weight. You'll need to think about your acceleration needs as well. The tutorial above should show you how to customize your calculations.
~Tom
gathem:
Thank you!
gathem:
I think there is an error in one of our calculations. I read both the dynamic and static robotics tutorials and appreciate the link to the info!
--- Quote ---3 ft/s = diameter * pi * 100rpm
3 ft/s = diameter * pi * 1.67rps (rotations per second)
diameter = 3 ft/s / (3.14 * 1.67 rps)
diameter = 0.57 ft, or 6.89"
--- End quote ---
I want to go 5mph ideally which is 7.33 fps
7.33(ft/s) = diameter * 3.1415 * 4(rps);
7.33 = 12.566 * diameter -> diameter = 0.5833 ft OR 6.99 inches.
So it looks like 7inches at 240 is the ideal combination
But I did further calculations and it seems like I am falling short on the torque.
The rated torque is 12lbs /in or 1ft/lbs.
The robot itself weighs almost nothing, but the battery is going to weigh 20+ lbs. Based on the tutorial it seems like this motor would not be suited for any robot above 5 lbs :o
This is NOT a battle bot, it is designed for level surfaces. The biggest challenge it will handle is carpet.
Can anyone recommend a good motor < $100?
I was really hoping to keep the cost of the 2 motors + motor controller at < $250.
gathem:
At this point I am just confused. I think I have looked at every robot suitable motor from every robot parts supplier out there as well as kicked the wheels on some used electric scooters on CL.
I read a post on here which stated the following in regards to a 30 lbs R2D2 robot
--- Code: ---At least 153 RPM (for 4fps) and a torque of at least 160 ozf-inch with 6" wheels.
--- End code ---
The motor above fits that torque description, but it seems too weak. I just cant press the buy button on it.
After spending waaaaay too much time worrying about this
I decided to go with
2X http://www.diybin.com/products/Robot-Power-Magnum775-Planetary-Gearmotor.html
1X http://www.diybin.com/products/Pololu-Qik-2s12v10-Dual-Serial-Motor-Controller.html
The motors torque ratings are different on every website I check
I believe these are 300oz-in. @ 14V (capable of going up to 16V for more)
and 720RPM (I will have to re-do the math to calculate the right wheel size to achieve 7.33fps)
I really really hope that I chose the right combination because I cant afford to have chosen the wrong one.
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