2

Author Topic: The right motor for the job  (Read 1196 times)

0 Members and 1 Guest are viewing this topic.

Offline gathemTopic starter

  • Jr. Member
  • **
  • Posts: 9
  • Helpful? 0
The right motor for the job
« on: May 25, 2012, 01:13:00 PM »
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!

Offline tomcharley

  • Jr. Member
  • **
  • Posts: 19
  • Helpful? 0
Re: The right motor for the job
« Reply #1 on: May 25, 2012, 02:19:24 PM »
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

Offline gathemTopic starter

  • Jr. Member
  • **
  • Posts: 9
  • Helpful? 0
Re: The right motor for the job
« Reply #2 on: May 25, 2012, 04:43:29 PM »
Thank you!

Offline gathemTopic starter

  • Jr. Member
  • **
  • Posts: 9
  • Helpful? 0
Re: The right motor for the job
« Reply #3 on: May 25, 2012, 11:30:36 PM »
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"

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.

Offline gathemTopic starter

  • Jr. Member
  • **
  • Posts: 9
  • Helpful? 0
Re: The right motor for the job
« Reply #4 on: May 26, 2012, 07:05:41 PM »
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: [Select]
At least 153 RPM (for 4fps) and a torque of at least 160 ozf-inch with 6" wheels.
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.

Offline tomcharley

  • Jr. Member
  • **
  • Posts: 19
  • Helpful? 0
Re: The right motor for the job
« Reply #5 on: May 29, 2012, 12:47:51 PM »
Dear Gathem,

  Just from looking at the links you posted, I think you picked some good motors and a good motor controller.  Just be careful that you keep the voltage under 16V and the average current around 13A so that you don't break your controller.  Considering that the motor takes 3.6A without any load, and it stalls at 130A, you'll probably need to be most wary of the current draw.  If you figure that you're using 3.6A at 0 oz-in, and 130A at 2500 oz-in (both at only 14V), you can do a rough interpolation to figure out what current you'll be drawing for the average torque that you'll need.
  I'm not sure why you need it to go a full 5mph.  In general, people are understanding of robots, so they will go a bit slower than normal to follow one.  It is also important to consider safety.  Your robot will need to sense and react to objects in its environment, and accurate, reliable sensors usually don't have great range.  You don't want a 60lb mass of metal flying along at 5mph to hit some little old lady standing in the hallway.  Also, if you reconsider your need for speed, you won't strain your motor controller or cut into your battery life as much.
  Good job finding such inexpensive, powerful motors!  I have found that motors tend to be the most expensive part of my projects.  You really have to dig to find good deals since everyone posts different stats and prices.

Good luck! Keep us updated!
~Tom

Offline gathemTopic starter

  • Jr. Member
  • **
  • Posts: 9
  • Helpful? 0
Re: The right motor for the job
« Reply #6 on: June 11, 2012, 12:38:55 PM »
I ordered those motors but the company actually waited 2 weeks and canceled my order without ever contacting me. In that time I found another solution which is about half the cost for substantially more torque.

I bought 2X black and decker 12V drills rated at 200 in-lb of torque ($40 each). Upon taking them apart they are some nice motors with a solid planetary gear box. For the motor controller I picked up a 12v 30A brushed motor electric speed controller used on very old school RC planes ($20)

So for a grand total of $60 (each) I now have motors rated at 200 lbs-in as opposed to the $125(each) I was going to spend on 300 oz-in setup.

They work fantastically.

I thought I would share my experience in being ultra cheap successfully

 


Get Your Ad Here

data_list