Motor recommendation

[SHHSrobotproject]

Dear Sirs,

I was hoping you could help me determine a range of DC gearmotor specifications needed to run our robot.
It’s an all-terrain (mainly dirt and grass) outdoor robot with a total weight of no more than 200lbs (probably 150lbs) that has to run at a top speed of 15mph. The robot is a ’tank-drive’ design (2 sets of wheel and DC Permanent-Magnet gearmotor combinations and 1 front caster wheel) running on two 12V batteries. The robot will be roughly 4 feet long, 2 feet wide and 4 feet tall with most of the weight (battery and motors) in the back and bottom of the bot. The tires will probably be no less than 12 inches in diameter.

If possible, could you please fill in the chart below:
Shaft Speed: At least and no more than rpms
Continous Torque: At least and no more than in-lbs
Continous Current: At least and no more than amps
Continous Output Power: At least and no more than hp
Overhang load: At least and no more than lbs

How would the specs change if we needed to run the robot at greater or lesser max speeds (10mph, 12mph, 20mph)?
How does the total weight of the robot affect the specs or performance the motors?
Would the cost of a motor differ significantly if I wanted a robot that was 5mph faster or one that weighed 50lbs more?
How does the wheel diameter affect the specs?
We’ve already researched a number of motor manufacturers, do you have any suggestions on what models we should use?




What about speed controller suggestions?
Do you suggest using 1 or 2 front caster wheels?
What material and gauge/thickness would you use to build your frame? It’s not a battle-bot, so it should not need to withstand heavy beatings.

[dyarker]

5280ft/mi * 12 = 63360in/mi
15mph * 63360in/mi = 950400in/hr
950400in/hr / 60min/hr = 15840in/min

Circumference of 12in wheel:
12in * pi = 37.7in

15840in/min / 37.7in = 420.16RPM

if max motor RPM is 3000, then overall gear ratio is:
3000RPM / 420.16RPM = 7.14:1 (seems too low unless you have monster motors. Lower MPH might be more realistic.)

Substitute your wheel diameter, max speed and max motor RPM.

Torque is trickier due to friction and drag. But max torque needed from the motor is greater than torque needed to maintain max speed because you need to accelerate up to max speed. (torque determined by desired acceleration, weight and friction.)

As an example starting quess, let's say the steepest incline you want the robot to climb is 30°; then calculate the stall torque at 45°. That should give enough torque to accelerate on flat ground.
(I might get the calc wrong here, but someone will correct it)

150lbs * sin45 = 106lbs
106lbs * 6in (wheel [bold]radius[/bold]) = 636in-lbs <<<<correction
636in-lbs / 7.14 = 89in-lbs motor stall torque <<<<corrected

(anyone, the motor torque looks ridiculous to me! Is the calc wrong, or is 15MPH too fast?)

Cheers,

[SHHSrobotproject]

Thanks!!! very helpful

[dyarker]

made a correction in torque calc; it should be force time wheel radius, not diameter.

I guess everybody is too busy complaining about the spam.

Cheers,

[slamer]

Heres some links that should get you rolling

Click on the tools link on this page for an online motor and battery sizing calculator

http://www.architeuthis-dux.net/tcr-home.asp

Or you can pull out your calculator and do just a little bit of simple math

http://www.teamkiss.com/newkiss1220.html

www.teamkiss.com

[Droidwerkz]

i have found that old c-band satellite dish azimuth postioners (jack-shafts) which are linears actuators use a nice 36 volt high torque motor to drive the jack-screw, what i do is drive around town, looking for old c-band satellite dishes, if i find one , i talk to the owner, most people don't use them anymore , and consider them an eyesore. offer to cut it down at ground level , and haul it for free, most people will consider this a favor! (use a sawzall with a fire/rescue grade blade at the 4" pipe at ground level. ) once you have cut it down , cut up the aluminum mesh dish, and once you have hauled it off , sell the aluminum dish to a local scrap yard, and you will get about $15.00 for the scrap aluminum, and you will get a nice high torque motor that is worth about $50.00, or $100.00 or more for the linear actuator included. the motor also has a built in position sensor, either a potentiometer or optical encoder. If you are really crafty , you can charge the folks for the removal , (I've earned up to $80.00 for about an hour's worth of work doing this sort of thing) Buy if you don't have a pickup truck , or cargo van , don't even bother. Even after dissasembly, the scrap won't fit into a typical car or station wagon. Also , if the dish is mounted a good distance from the house, and they don't want the wire anymore, there is a nice 16 gauge pair of wires , servo feedback wiring (typically 22 gauge,) and an rg-6 or rg-59 coax cut it off at the house, and take it too....hey, free parts are free parts, and copper wire sells as scrap also , so if you don't want it , seel it to the guy who buys the aluminum!