Go Down

Topic: Wheel Placement on Robot (Read 16023 times) previous topic - next topic

drewdavis

Quote
If you mean 4WD with the drive wheels close together, and 2 casters on the ends, that sounds doable, but I don't see a situation where that design will be at all useful. IE, why even bother with 4WD?


Yes that is what I mean. The only reason I'm doing it that way is that I have 4 of the same motors. If I understand torque correctly using  4 drive wheels will double the torque the robot would have if It was using only 2 drive wheels. To push the robot up the needed incline (I don't remember what it was off the top of my head) two of the motors I have will not have enough torque. That is why I need four drive wheels.

Quote
I think a lot depends upon where this robot will operate most of the time. If outdoors on earthen terrain, I don't think having casters is a good idea at all, as they will probably dig into the dirt a lot. For that, I would go with a basic design with 4 drive wheels on the corners of the robot, and relie on a very robust motor/wheel system to keep it from breaking down.


The caster wheels will be fine (link below). Also, I was going to put the drive wheels towards the center to improve the turning. I don't believe the motors will break. I need the turns to be exact when its driving autonomously and I think this design will give me the needed torque and turning capability.


http://www.trossenrobotics.com/store/p/5881-Parallax-Caster-Wheel-Kit.aspx

oric_dan

This is more what I had in mind for idler wheels, although smaller than 10" :-),
http://www.harborfreight.com/10-inch-pneumatic-swivel-caster-38944.html

drewdavis

Something like that would be fine. The price is also much cheaper :) Like I said I don't have the caster wheels.

This one might work. http://www.amazon.com/Swivel-Caster-Pneumatic-Wheel-Brake/dp/B005UZ4FQA/ref=sr_1_14?ie=UTF8&qid=1379185406&sr=8-14&keywords=Pneumatic+Wheels+Casters


Before I buy them I need to know if  this design work. Am I right that when calculating torque you add up the torque of all the motors thus 4 drive wheels will have 2x the torque of a 2 drive wheels?

Thanks!

oric_dan

For identical motors, that sounds about right. However, one thing you probably haven't totally conceptualized is that having a single idler wheel front and back is really back to the "tripod" geometry that I mentioned, and on both front and back. If the idler takes any substantial amount of weight, say in turning while going downhill, don't be surprised with a possible tumble.

drewdavis

95% of the weight of the robot will be in the center so I don't think it will tip. Thanks for your help. I will post photos once the robot is built as well as the results.




oric_dan

Build it and see how it works, and refine as necessay. That's how this stuff works. Bootstrapping and iterative development. What you're doing is basically a Bobcat loader with outlier wheels. You can also do a search on "shrimp wheeled robot", but that is for really rough terrain.

keeper63


...and they certainly work best on surfaces where the wheels can, in fact, "skid" easily (eg, not carpets).


Well - they'd work ok on carpet - the carpet just might not survive all that well...

Quote from: oric_dan

If outdoors on earthen terrain, I don't think having casters is a good idea at all, as they will probably dig into the dirt a lot.


Depending on the size of the robot, you could go with a couple of larger "off road" casters; they make these larger casters that have knobby tires for such purposes. Not ideal, but they do exist...
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

keeper63


This is more what I had in mind for idler wheels, although smaller than 10" :-),
http://www.harborfreight.com/10-inch-pneumatic-swivel-caster-38944.html


Doh! Just saw this comment - you already know about 'em! Heh!  :P
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

zoomkat

Quote
Before I buy them I need to know if  this design work.


You probably need to be specific with the size and use of the bot. About all I've seen is it is to be 2'x3' in size, goes up an incline, and maybe make zero turns. Kind of vague description for anybody to tell you if anything will actually work.
Google forum search: Use Google Search box in upper right side of this page.
Why I like my 2005 Rio Yellow Honda S2000  https://www.youtube.com/watch?v=pWjMvrkUqX0

oric_dan

#39
Sep 17, 2013, 07:28 pm Last Edit: Sep 17, 2013, 07:36 pm by oric_dan Reason: 1
If only life were so simple that we would know ahead of time that our designs would always work. Rod Brooks, head of the MIT Robotics Lab, was fond of saying "the problem with simulations is, they're always doomed to succeed". IOW, things usually tend to work in unexpected ways in the 'real' world. A priori designs and simulations are necessarily always incomplete.

Go Up