Gearhead motor with high rated load

[SeagullOne]

Hello, everyone!

Work on my robot is progressing. My deadline for its completion is sometime before November. However, I've come across a pickle.

I need a good motor to support the robot's weight. I've tried to find the lightest possible materials for it, including batteries and frame. I need two identical gear head motors that run on 12V, have a 4mm or 6mm diameter shaft, and can carry a rated load of something around 15 to 20 lbs each, preferrably also good for instant reverse, but that's not too necessary. The rpm should also be something around 60 rpm at the rated load, no lower than 30 rpm. I've looked all over for gear head motors like these, but no luck. All the gearhead motors I find are either not powerful enough, way too powerful (industrial), or require more voltage to run, or are too slow, like 14 rpm at no load.

Can anyone point me in a direction that would help my application? ?Thanks.

[Admin]

You forgot to mention required torque

Did you calculate it yet?
http://www.societyofrobots.com/mechanics_dynamics.shtml

preferrably also good for instant reverse, but that's not too necessary

The bigger the motor, and the higher the voltage, the less instant the reverse will be. Motor inductance (resistance to reversing in this case) goes up by the cube with motor size and square of voltage. At 12V and your possibly high torque, I'd expect a huge voltage/current spike when you reverse these motors . . .

(someone correct me if Im wrong on this, Im rusty on my physics and too lazy to double check the scaling laws)

[SeagullOne]

Great link, Admin . Really helpful.

I haven't exactly begun building my robot yet, but with the supplies and materials I have, I'm making a safe estimate that it will weigh anywhere from 25 to 30 lbs (Hopefully less). My robot will be about three feet tall, with most of its weight at the base. So the motors I should use: I'm still getting to know how to calculate torque...I'd probably be safe investing in a motor with a torque of...maybe 400oz/in or higher. Maybe even definitely higher than 400oz/in. The wheel rotations per minute (it will run on treads. does this make things difference than normal wheels?) should be about 65 rpm. Its a 3.75 diameter wheel.

Hmmm...is there some kind of motor that can carry such torque at the right speed and be able to reverse instantly? Some kind of modified servo, perhaps? Like the titanium geared hi torque Hitec robot servos? They only use 6V tops. If I bought four of them and used two per wheel (They have a standing torque of 433 oz-in at 6V...standing torque is the maximum torque the motor can handle at the rated load, right?), and modified them for continuous rotation, I could propel my robot with outstanding performance (433 per motor--two motors per timing tread sprocket)! Does this sound reasonable?

[Admin]

it will run on treads. does this make things difference than normal wheels?

yes! a huge difference! search the forum for treads, the issue has been discussed several times . . . basically dont use treads unless you have rough terrain, as all they do is waste a lot of energy and torque otherwise . . . even then, probably better to just use really large wheels, instead.

is there some kind of motor that can carry such torque at the right speed and be able to reverse instantly? Some kind of modified servo, perhaps?

Define 'instantly'. Also, search this forum for 'motor driver' for ways to control a motor.

When you do the math on that motor calculation tutorial, you will find that servos wont be able to handle what you need. Just plug in the numbers and it will tell you how well your robot will perform with your motor choice

[SeagullOne]

Thanks for the input on treads, Admin. I'll definitely take that into my design consideration.

What I mean by instant reverseable is being able to reverse and carry the rated load at a velocity of under one second without a voltage spike. Is it the gear train that makes this happen in gearmotors? is there some kind of other motor that would handle this application: something that can run on 12V doesn't go too high over 5Amps carrying a rated load of about 20 lbs, and is able to reverse and carry the load (say with a differencial drive turning manuever: the left and right motors spin in opposite directions). Once more, I'm make a safe guess that my finished robot will weigh about twenty-five pounds, but hopefully less than that.

A couple questions on the side. I'm been surfing the web on different kinds of motors, but I want to make sure I understand the specs:

What is the stall torque? I take it thats the amount of torque that is carried when the motor fails. Can the motor carry loads just under the stall torque?

What does Kt mean? I found it with a gear motor with a stall torque of a 1500 or so oz-in. The "Kt?" was 151 oz-in. Kv?

[Admin]

What I mean by instant reverseable is being able to reverse and carry the rated load at a velocity of under one second without a voltage spike.

Impossible
A voltage spike will exist as much as gravity does, its just physics. This is due to the inductance.

There are three ways to reduce the spike:
Decrease physical contraints - robot mass, required torque, acceleration, etc.
Get high efficiency motor - expensive!
get a buffed up motor driver with a huge cap across ground and power

carrying a rated load of about 20 lbs

This doesnt make sense - you need torque not load to select a motor.
http://en.wikipedia.org/wiki/Torque

And for your other torque questions . . .
http://www.societyofrobots.com/actuators_dcmotors.shtml
Again, use that excel sheet to calculate it. I cant guess if your motors will work, its all in the math.

As for kt and kv, just google them up. It has to do with the intrinsic motor properties - plotting voltage, current, and torque relationships.

[SeagullOne]

Those are great tutorial, Admin. I'll have to see what else there is...

I think I get it. Kt is the relationship between how much torque is applied and how much current the motor will draw. For instance, if the Kt of a motor is 150 oz-in/A, that means that the motor will draw 1 Amp at 150 oz-in, 2 amps at 300 oz in, and so on. Am I doing the math right on this?

[Admin]

Yeap, thats about right.

In reality the scaling is not linear like that, but it will get you a good estimate. The only way to get the exact value is if they publish a graph - something rarely done with motors.