Calculation check + motor question

[electrofan]

Hello all. I'm trying to find out what motor to use in my robot. It will have 4 powered motors, and a weight of about 1 kg. It needs to climb up a 30 degree incline, the acceleration below was approximated using the equation on the calculator page.



1. Are my calculations correct? Here is the motor I was considering, which doesn't seem to have the torque I need?
http://www.solarbotics.com/products/gm3/


Also, I haven't finalized the motor type I'll be using. The robot needs to follow a given path stored in its microcontroller, to a fair degree of accuracy (let's say within 20 cm of final theoretical destination after following a 5m long curvy path). I also need the entire motor driving + sensors (if I go with DC motors) to cost <$40. So the choice is either continuous servos, or DC + encoder (i'm guessing a custom-built IR). Keep in mind there must be 4 total motors for this price, and they must handle the above torque.

2. Which is the better choice, DC or servo?
3. What advantage would I have with a continuous servo as opposed to DC, unless I can buy a continuous servo that has position control (not just PWM speed - same as DC)?

Thanks guys.

[waltr]

The calcs are correct. The motors you linked to will not meet your specs and I doubt you will find motors that do for that price.
You need motors with over 576 in-oz torque.

There are large servos that have that torque but they are not cheap.
Do you really need a 1kg bot climbing a 30° incline?

You may be able to find gear motors in a junk yard

[electrofan]

It definitely will need to go over a 30 degree incline. Right now, I have another robot frame of similar size (in aluminum) in hand. However, maybe I can replace the final model with HDPE, which has about 1/3rd the density of aluminum. So let's say 0.5kg to be safe. That gives me 0.921 RMF.

Here is a robot of 350grams doing a 40 degree incline (video near bottom of page)
http://www.surveyor.com/SRV_info.html

It uses 4 of these:
http://www.solarbotics.com/products/gm12a/

So something like this must be possible...

Thanks

[waltr]

350gm is about one third the weight you desire and requires about one third the torque to climb an incline.
That bot also goes slower than the 1fps you spec'ed.

Try playing with the numbers in the motor calculator to see what is possible with the motors you picked.
Weight, speed and wheel diameter have large effects. Also set the acceleration very low for climbing an incline.

[electrofan]

Alright, I reduced my constraints on acceleration (I was using that equation on the tutorial page,  "acceleration for inclines = 32 ft/s^2 * sin((angle_of_incline * pi) / 180) - You must add this acceleration to what you already require for movement on flat terrain.")

I also found a stronger motor, a counterpart to mine:
http://www.solarbotics.com/products/gm9/specs/

So here are the new results:


Looks like it should be fine now. I also gave the company a call and they said it'll be fine as well.

However, do you have any recommendations for continuous servo vs. DC in my case? That could change everything...

Thanks again!

[waltr]

I have always used a DC gear motor and a processor controlled H-bridge. So no experience with using a modified servo. Many other people have so hope they can help you.

Also do a search on this forum and read through the many threads on this subject.