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Topics => Robotics => Topic started by: drewdavis on Aug 29, 2013, 02:03 am

Title: Wheel Placement on Robot
Post by: drewdavis on Aug 29, 2013, 02:03 am
Hello,

I'm building a 4WD robot, but I'm a little confused about how to optimize the robot for turning. I need to turn the robot 90 degrees, and I need the center of the robot to stay in the same spot as it turns. Anyway, were should I place the wheels? Should I put them far from each other, or as close a possible? The Robot will be 2ft wide and 3.5ft long.


Thanks!
Title: Re: Wheel Placement on Robot
Post by: MillerJLee98579 on Aug 29, 2013, 02:31 am
are you using a servo or motor on each wheel?
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Aug 29, 2013, 02:58 am
One 12v, 3 amp (no load) motor with 6in wheels.
Title: Re: Wheel Placement on Robot
Post by: MillerJLee98579 on Aug 29, 2013, 03:05 am
in my opinion i would say have the wheels near the corners with the transmission in the dead center of the robot if you want to keep the turning radius in the center
Title: Re: Wheel Placement on Robot
Post by: MillerJLee98579 on Aug 29, 2013, 03:06 am
but in that case you might just need a motor for each side of the robot
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Aug 29, 2013, 03:21 am
Sorry I wrote that wrong...
Quote
One 12v, 3 amp (no load) motor with 6in wheels.



One Motor for each 6in wheel.
Title: Re: Wheel Placement on Robot
Post by: MillerJLee98579 on Aug 29, 2013, 03:29 am
than possibly not sure if you have the access to scrap wood or not but you might want to make a couple of wood platforms with those dimensions and test different mounting positions for the wheels that way when you found a place that works with the radius you want you will have the measurements and everything (yes i learn a lot by trial and error)
Title: Re: Wheel Placement on Robot
Post by: zoomkat on Aug 29, 2013, 05:07 am
Bots like below operate like skid steer equipment, which operate similar to tanks. The wheels on the same side operate together.

(http://www.lynxmotion.com/images/product/large/a4wd2.jpg)
Title: Re: Wheel Placement on Robot
Post by: keeper63 on Aug 29, 2013, 07:00 am

I'm building a 4WD robot, but I'm a little confused about how to optimize the robot for turning. I need to turn the robot 90 degrees, and I need the center of the robot to stay in the same spot as it turns. Anyway, were should I place the wheels? Should I put them far from each other, or as close a possible? The Robot will be 2ft wide and 3.5ft long.


Hope those motors are fairly powerful. Hope it won't be run on carpet (unless you don't care about your carpet long-term).

What this is called is "skid-steering" (actually differential steering) - meaning the wheels skid sideways as the vehicle turns:

http://en.wikipedia.org/wiki/Skid-steer_loader

In fact, your dimensions seem really close (scaled) to that of many skid-steer loaders like depicted in that link above. Regardless, it really tears up the ground, and if used on hard surfaces (like concrete and such) that have no "give" - it can mess up bearings, transmission components, and tires in short order. Keep that in mind for your robot. If you plan on this being used on dirt or grassy surfaces only, all the better. But if being used on hard surfaces (especially with grippy tires and lots of weight), expect the components to fail quicker than normal.

The best orientation (as far as wheels are concerned - tracks are a whole 'nother matter) for the wheels are at the extreme corners of a square base (that is, at the points where, if a circle was circumscribed around the square, the vertices of the square would meet the line of the circle). Ideally, you would also be able to angle the wheels such that they tracked the circle as the vehicle turned in-place. This of course would be more complex to implement.

Title: Re: Wheel Placement on Robot
Post by: drewdavis on Aug 29, 2013, 03:51 pm
Thanks for the replies.


Quote
Ideally, you would also be able to angle the wheels such that they tracked the circle as the vehicle turned in-place

If your talking about what is in the image below,  it would be to hard and cost to much to add that into the robot I'm working on.


Quote
than possibly not sure if you have the access to scrap wood or not but you might want to make a couple of wood platforms with those dimensions and test different mounting positions for the wheels that way when you found a place that works with the radius you want you will have the measurements and everything


I will give that a try.

Quote
The best orientation… wheels are at the extreme corners of a square base


I will try that out on the wooden prototype.


Quote
Regardless, it really tears up the ground, and if used on hard surfaces



It will be running on hard surfaces (the road) but I'm not to worried about messing it up as it looks strong...


http://www.parallax.com/product/27971



Thanks!
Title: Re: Wheel Placement on Robot
Post by: Jantje on Aug 29, 2013, 04:41 pm
My robot is very similar in size, has similar sized wheels and runs on grass.
My experience:
I'm still looking for a motor that lasts longer than a couple of weeks :-( I have killed lots of motors of several types in the last years  :(
I have upgraded the rods to the wheels from 6mm steel to 6mm rust free steel to 10mm rust free steel; because they broke ( sometimes in a couple of hours) due to the turning.
I ended up using 2 ball bearings in a aluminium case (the plastic ones broke) to support each wheel rod.
The motor is tightly attached to the wheel rod but it is flexibly attached to the alu frame. This means that the motor can not turn around its axe (and as such can deliver power) but it can move when the wheel is forced up/down/left/right (so it does not take any axial or radial force except for its own weight induced force).
So from experience I'm fully with Cr0sh here.

I'd be interested to see the wheels you are planning to use as I'm not fully happy with the wheels I'm currently use and I hardly find 6 inch wheels.

Best regards
Jantje
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Aug 29, 2013, 10:26 pm
I posted the link to them above, but I will post again.


http://www.parallax.com/product/27971
Title: Re: Wheel Placement on Robot
Post by: Jantje on Aug 30, 2013, 02:46 am

I posted the link to them above, but I will post again.


http://www.parallax.com/product/27971


Och I wasn't aware it was a motor wheel assembly.
This looks good on paper but the device seems to be under revision which sounds bad to me.
Best regards
Jantje
Title: Re: Wheel Placement on Robot
Post by: zoomkat on Aug 30, 2013, 05:19 am
If turning is difficult, the bot might be hinged in the middle and rotated left/right to help with turning. Another setup below.

http://www.youtube.com/watch?v=0FGWKwIIKVM
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Aug 30, 2013, 01:37 pm
Quote
This looks good on paper but the device seems to be under revision which sounds bad to me.


I have been in contact with the company and they have had the kit for years, however, their supplier for the motors just stopped making that model. They found comparable motors, but the mounting is slightly different so they are working on a new mounting block.

Quote
If turning is difficult, the bot might be hinged in the middle and rotated left/right to help with turning. Another setup below.


I think making the robot swivel like that would just be to hard and time consuming for this bot, but that could be fun to do in the future with a smaller robot!
Title: Re: Wheel Placement on Robot
Post by: zoomkat on Aug 30, 2013, 06:22 pm
A lazy=susan like below could be used in the center of the bot to allow the flexing of the front and rear sections for turning. For skid steering keep the wheels close together for easier pivoting and turning.

http://www.homedepot.com/p/Shepherd-6-in-Lazy-Susan-Turntable-9548/100180572#.UiDGFynD_ix
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Aug 30, 2013, 08:23 pm
Quote
The best orientation… wheels are at the extreme corners of a square base


and

Quote
keep the wheels close together for easier pivoting and turning



Now I'm confused... keep the wheels as close as possible or as close to the corners as possible? I have a feeling there is only one right answer for this. Personally I would think as close together would be best, but I really don't know.

Also, were should I put most of the load (12lb of batteries) over the wheels?


As far as having the robot pivot it would just be to much for this robot. I know it is possible, but time wise I don't have time to redesign the frame of the robot which is probably already assembled. (i'm having somebody weld the frame together). However, that Lazy Susan Idea looks cool so I will add that to my "when I get some extra time and money" folder.
Title: Re: Wheel Placement on Robot
Post by: MillerJLee98579 on Aug 30, 2013, 08:35 pm
weight wise best placement of the load is in the center
Title: Re: Wheel Placement on Robot
Post by: zoomkat on Aug 31, 2013, 06:27 am
Quote
Now I'm confused... keep the wheels as close as possible or as close to the corners as possible? I have a feeling there is only one right answer for this. Personally I would think as close together would be best, but I really don't know.


A lot may depend on the size of the wheels compared to the size of the platform. Might be worthwhile to start by attaching the motors/wheels to a square piece of plywood and test the turning performance. Larger bots might benefit with some type of suspension if the bot will travel over uneven ground.
Title: Re: Wheel Placement on Robot
Post by: keeper63 on Aug 31, 2013, 08:25 am

Now I'm confused... keep the wheels as close as possible or as close to the corners as possible?


Think about it a bit: We are talking 4 wheels here. If you moved the wheels in the front back, and the ones in back forward, until they were near the center (but still spaced opposite each other), then yes, it would be easier to turn. Heck - if the wheels "merged", then you have the classic differential steered robot (add some casters on the front and back to balance).

Regardless, though, when the robot pivots about its center, those wheels will describe a circle. The closer the wheels are to the line that runs thru the center of the circle, the less drag/skid they will exhibit (and in the case of the classic design, little drag at all - but there will always be some drag, as any wheel that is following a curve, the inner edge /must/ move slower than the outer - and so you have wear; which is why when you car doesn't have proper alignment, your tires are messed up in all sorts of different ways - which is why, even with perfect alignment, you must rotate your tires, to even the wear).

Therefore - if the wheels are going to describe a circle anyhow - with a 4-wheel arrangement, the configuration with the least amount of drag (but more drag than the classic differential two-wheel design, of course) will be where those wheels contact the circle they are describing - which has to be a square (and, again, ideally the wheels could pivot - but as you noted, this would drive up complexity and cost, which I also realized when I posted that originally; I was merely suggesting the ideal way to minimize drag on such a 4-wheel arrangement, irrespective of complexity or cost).

Now - you may wonder why the various skid-steer equipment makers don't make their machines square? Well, it mainly has to do with the purpose of those machines and where they are operated. They are meant to be operated on surfaces with "give" (indeed, the manuals for these machines generally explicitly say not to operate them on concrete, asphalt, or other hard surfaces), so that they can easily slide without taxing the drive and other mechanical components. They are also generally designed to be small, while still carrying a good sized load, and be maneuverable, which limits making them square. Engine layout and other design needs probably also factor into the reason why they aren't square.
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Sep 01, 2013, 05:05 am
Thank you for the useful insight. I will post some photos of the robot once the frame is finished.

Also, what would happen if you had 2 motors in the back that were higher torque but slower and 2 motors in the front that will lower torque but faster. Would the robot have better torque then if I just used 4 of the slower ones? (the wheel size would be the same)

Title: Re: Wheel Placement on Robot
Post by: Jantje on Sep 01, 2013, 07:37 pm

Thank you for the useful insight. I will post some photos of the robot once the frame is finished.

Also, what would happen if you had 2 motors in the back that were higher torque but slower and 2 motors in the front that will lower torque but faster. Would the robot have better torque then if I just used 4 of the slower ones? (the wheel size would be the same)



I would assume you would burn the higher speed motors as they will try to drag the lower speed motors forward.
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Sep 14, 2013, 01:05 am
Quote
I would assume you would burn the higher speed motors as they will try to drag the lower speed motors forward.



I was able to get 4 of the same motors. It took some time as the motors are not officially for sale, but parallax worked with me and sold me 4 new ones. I had 2 older ones, but they are slightly slower and you said I could not use two fast motors and two slow motors. So it took some time for me to convince them that I need 4 sets as they don't have many ready. 

Quote
Think about it a bit: We are talking 4 wheels here. If you moved the wheels in the front back, and the ones in back forward, until they were near the center (but still spaced opposite each other), then yes, it would be easier to turn. Heck - if the wheels "merged", then you have the classic differential steered robot (add some casters on the front and back to balance).



I was thinking about putting one wheel/motor on each corner of the robot but you explained how that would screw up the turning.
What if I slid the wheels from the corner down towards the middle of the robot so that they were almost touching and then put a caster wheel on each end of the robot. It would be like putting two, three wheel robots back to back.  That way the driver wheels are like .5in apart making the turning easy yet the caster wheels will help support the front/end.

http://www.geology.smu.edu/dpa-www/robo/trux/n06.jpg

Would that work? I think it would yet I have never seen a robot like that.
Title: Re: Wheel Placement on Robot
Post by: keeper63 on Sep 14, 2013, 05:46 am

I was thinking about putting one wheel/motor on each corner of the robot but you explained how that would screw up the turning.


It wouldn't screw up the turning, you would just have some (maybe a lot) of wheel drag; I also don't know how well the center would remain "centered". Is there a reason to keep the center from moving much?


Would that work? I think it would yet I have never seen a robot like that.


You could try it; it might work - but I fail to see how that would be better than just a single pair of wheels in the center, and a caster on the front and back?
Title: Re: Wheel Placement on Robot
Post by: oric_dan on Sep 14, 2013, 06:21 am
A little belated response, but for a smallish robot, like shown in reply #7, I would imagine that motor/wheel arrangement will be fairly reliable. After all, Lynxmotion has been selling those things for years.

OTOH, if you're talking about 6" wheels and a 2' x 3' or so base, that's pretty heavy-duty, and you really need to go with strong motors and probably wheels mounted on shafts riding on bearings, and connected to the motors via pulleys or chains.
http://damencnc.com/en/components/mechanical-parts/ballbearings/279

Skid steering works, but as mentioned, the carriage needs to be extremely robust for something that size and weight, and they certainly work best on surfaces where the wheels can, in fact, "skid" easily (eg, not carpets).

It's very common for people to use casters as a 3rd wheel [tripod arrangement] for differential-steered bots, but you'll notice this thing is "tiny" compared to what's being talked about,
http://www.geology.smu.edu/dpa-www/robo/trux/n06.jpg
A large, heavy frame will tip over if not properly balanced.
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Sep 14, 2013, 02:57 pm
Quote
Is there a reason to keep the center from moving much?


I wold like to keep the turning as accurate as possible.


Quote
you would just have some (maybe a lot) of wheel drag


that would make my turned less exact and decrease the life of the motors. (I'm mostly concerned with the turning) Right?

Quote
I fail to see how that would be better than just a single pair of wheels in the center, and a caster on the front and back?


That was my original plan, but two motors do not have enough torque to push the robot. It was cheeper to buy another two wheels/motors  then to buy better ones. I assume that by adding two more motors/wheels I doubled my torque. Right?

Quote
wheels mounted on shafts riding on bearings


They are.


Quote
A large, heavy frame will tip over if not properly balanced.


If the wheels are  towards the middle of the robot It will tip over, but the caster wheels will help balance it. Maybe tip is not the right word. Think of a seesaw. If the wheels are in the middle of the robot it will pivot and the back & front could hit the ground.


Thanks! Would my idea work? I can't test it out because I don't have the two caster wheels.
Title: Re: Wheel Placement on Robot
Post by: oric_dan on Sep 14, 2013, 05:31 pm
What I was referring to is, a heavy robot with tripod arrangement and only 1 caster can easily tip over.
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Sep 14, 2013, 06:22 pm
Quote
What I was referring to is, a heavy robot with tripod arrangement and only 1 caster can easily tip over.


I agree! That's why I thought that a caster on each end would prevent it form tipping. Would my idea for a 6 wheel robot work?
Title: Re: Wheel Placement on Robot
Post by: Jantje on Sep 14, 2013, 08:03 pm
Quote
That was my original plan, but two motors do not have enough torque to push the robot. It was cheeper to buy another two wheels/motors  then to buy better ones. I assume that by adding two more motors/wheels I doubled my torque. Right?

You can add the torque indeed. But you are probably not so much interested in torque but more in "speed" and "acceleration speed"
You speed will certainly not double.
The acceleration speed will increase but it will not be double.
There are many things at play in a 4WD and engine loads in general. To be honest try and see is the best way to find out.

cr0sh his explanation about the square and powers for skit-steering is 100% correct if you only turn on the spot. If you also turn while moving forward (or backward) the square shape no longer has a benefit to the rectangle because in this case the centre of the turning circle is outside the ground plane of the robot.
The width of the contact surface of the wheel is very important. The wider the more friction the more axial forces the stronger the wheel bearing must be. In case of on the spot turning a square will make that the centre of the wheel is friction less and you have half of the friction on both sides of the wheel.

Best regards
Jantje
Title: Re: Wheel Placement on Robot
Post by: oric_dan on Sep 14, 2013, 08:08 pm

Quote
What I was referring to is, a heavy robot with tripod arrangement and only 1 caster can easily tip over.


I agree! That's why I thought that a caster on each end would prevent it form tipping. Would my idea for a 6 wheel robot work?

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?

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.

Or else, use 2WD with 2 extra "large size" idler wheels [much larger than casters] on the opposite end from the drives. This guy has some ideas,
http://www.amazon.com/Build-All-Terrain-Robot-Robotics-ebook/dp/B001UQ5HW4

For indoor use on smooth surfaces, a caster system will work, You might check this out,
http://davidbuckley.net/FR/Cycler/Stability+Loads/Stability&LoadTest.htm
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Sep 14, 2013, 08:35 pm
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
Title: Re: Wheel Placement on Robot
Post by: oric_dan on Sep 14, 2013, 08:53 pm
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
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Sep 14, 2013, 09:05 pm
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!
Title: Re: Wheel Placement on Robot
Post by: oric_dan on Sep 14, 2013, 10:36 pm
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.
Title: Re: Wheel Placement on Robot
Post by: drewdavis on Sep 14, 2013, 10:43 pm
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.



Title: Re: Wheel Placement on Robot
Post by: oric_dan on Sep 14, 2013, 11:40 pm
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.
Title: Re: Wheel Placement on Robot
Post by: keeper63 on Sep 15, 2013, 09:37 am

...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...
Title: Re: Wheel Placement on Robot
Post by: keeper63 on Sep 15, 2013, 09:38 am

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
Title: Re: Wheel Placement on Robot
Post by: zoomkat on Sep 16, 2013, 06:48 pm
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.
Title: Re: Wheel Placement on Robot
Post by: oric_dan on Sep 17, 2013, 07:28 pm
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.