finding suitable motors
I am trying to find suitable motors for a robot. I have used parallax's continuous rotation servos, but they are not quite fast/strong enough for the robot I am making. I expect that it will weigh roughly 5 lbs. and I would like it to move about the speed of a roomba. I know the speed/torque is a function of the wheel diameter as well at the motors torque/RPM ratings. I am looking through Jameco's DC gearmotors (they have ALOT) and I can not get an idea of how fast 10 rpm is, or how powerful 1200 g/cm is. How do the rest of you figure this stuff out? what is the difference between stall torque and no load torque? why is the sky blue? ... uh, anyway if someone could help me out here, I would appreciate it.
Thanks,
Justin W.
Thanks,
Justin W.
Comments
Cant help with the roomba thing because I have no idea how fast a Roomba is, in your case except for driveline drag torque isnt much of an issue unless you have really deep carpet. Torques only major effect on hard flat ground with a decent drive system is going to be rate of acceleration.
1200 g/cm means the motor generates 1200 grams of force with a lever 1 centimeter long from the center of the shaft.
1200 g/cm on a 4 inch wheel,·2 inches (half the wheel dia to get the distance from centerline to the ground) times 2.54 to convert to·centimeters equals·5.08 centimeters, so your lever is·5.08 times the one used in the rating. 1200 divided by·5.08 equals 236.22 grams of force at the ground. Multiply grams times .04 to get ounces and you get 8.33 ounces of force at 125.6 inches a minute. Pretty slow but with very respectable force. Once you get this far speed and force are effected by the same amount, so if you decide to double the speed to 250 inches a minute, with that motor torque you should get about 4 ounces of force or forward thrust, which depending on the bot size, driveline drag, surface may be plenty.
Hope that helps
Chris
Electric motors run smoothly at thousands of RPMs, but robots want motion that is in the hundreds or tens of RPMs.
The motors need that momentum to do so. It also helps to generate cooling via the spin creating air flow.
Thusly, the whole issue is a package of gears and motors together at an appropriate voltage for control. The gears tend to waste some power with friction, but provide the necessary momentum at the right RPM.
Lower voltages and high motor RPMs are more optimal. While higher voltages issue more power, they also wear on relays or other switching devices via sparking and heat generation.
Mostly for robotic motion, you want to gear down to 60 - 200 rpm with motors that will really push. You can always de-rate a faster motor by using a lower DC voltage or PWM power.
You need a real comparative benchmark. I suspect Watts is most useful. Amperage or coil resistance is easily converted into watts. Most motors run at about 70 to 80 percent efficency of their electical consumption. So a 10 watt motor will produce 7 to 8 watts of useful work.
Take a look at a 1/4 or 1/10th horsepower motor for its wattage. I suspect you will notice that most of the motor are not providing much work at all.
Grams per centimeter is a bit confusing as someone else might provide grams per millimeter and so on.
Additionally, if you are packing any weight, you may want the wheels to be on their own weight bearing axle rather than be supported entirely by the motor shaft. Why wear out the motor via the additional weight?
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Kramer: yeah, all the motors I am looking at are gearhead motors because my previous experiences in building my own gearbox have left alot to be desired. 60-80 RPM seems to be my target speed here ,and considering 4" wheels are what I am using, that will move the robot at a pretty good speed.
I found jameco's motor selection interesting... I looked at the motor with the best torque to speed ratio I could find, and they are no better than the Parallax continuous rotation servos, which would be an advantage simply because I would not need a driver circuit. However, as I said, I am looking for something stronger, and the only way those would work comfortably (with my design as well as torque requirements) is if i changed the robot to a smaller, squarer design, which I suppose is no big deal, but I was hoping to keep my original size.
My robots ideal specs are as follows: 10"x10" octagonal base, weight will probably be no more than 5 lbs. and it will be no more than 4" tall. I just want motors that can move that over small obstacles such at wires etc, but it will mostly be flat ground.
-Justin
<Edit> Kramer the problem with using wattage to determine power is that in the inexpensive motors the efficiency varies widely and usualy isnt available as a specification. My personal approach though has pretty much always matched what you just described if I needed a motor for something. It was massively overrated and then just pulled back to what I need. Now if I get time to try and figure out how to spin a 3 phase brushless I'll have it made>
Post Edited (ChrisP) : 12/12/2005 7:41:09 AM GMT
One of the more recognized names in DC PM gear motors is Pittman. One can often find them in the surplus market, in many different sizes, voltages, and gearing. Don't be concerned about some of the odd operating voltages, as they are merely "design voltages" and neither represent the maximum, nor the most efficient voltage for all operations. It merely represents the designated operational voltage for the operation and use for which it was originally designed and intended.
Said differently, there is generally no problem in operating a (say) 19.1 VDC motor on 24 VDC, on a non-continuous basis. So too, a (say) 14.5 VDC motor will usually operate nicely, with slightly reduced torque, on 12 VDC. Most of these motors are built for commercial and industrial applications, and thus have some real "guts" to them.
Speaking of "guts", here is a complete re-work of a Pittman DC gearhead motor, by the Dallas Personal Robotic Group (DPRG), with the addition of a homebrew tachometer and motor direction encoder!
http://www.dprg.org/projects/2001-09a/
Excellent project, and VERY WELL layed out. The DPRG is noted for their project excellence, including plenty of visual aids (pictures and graphics), and the easy to follow, step-by-step instructions.
Although that doesn't directly answer your original question, I hope it was helpful to see the insides of a decent DC PM gearmotor and what can be done with them, without too much work.
Regards,
Bruce Bates
http://www.lynxmotion.com/Category.aspx?CategoryID=11
http://www.robotcombat.com/marketplace_ant_motors.html
Various motors that may be of use