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Author Topic: The Failed Base  (Read 6011 times)

kingart3

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The Failed Base
« on: March 30, 2017, 09:11:48 PM »
Once upon a time ago, I was working on Bert the Bling Bot (lots of flashy lights).

One of my goals was to make Bert with four powered wheels.  In order to test this out, I attached the motors to a test base and then just hard wired them into my battery pack.  By switching the wires around using terminal strips I could make it go forward or turn.

As it worked out going forward was GREAT, turning ...no so much...

It does turn but the big "dune buggy" style wheels actually pull up the carpet squares covering my living room floor ...not good.

I have see several other multi-wheel robots and am not sure what I did wrong here.

I realize that each pair of wheels wants to turn around its own center and thus each pair of wheels is trying to ...whip... the other pair around their own axis.  The "Robot Builder's Bonanza" book warns of this and recommends putting the wheels sets close together to try and counter this problem.

Below is a picture of the failed robot base, with some D-cells tossed in for scale.

The base is 10" by 12", the wheels are about 8" apart center to center.

I suspect this base would be fine outdoors where the grass and dirt would allow it to slip, but inside on carpet, it is a home wrecker ...literally.

My current plan is to go back to two driven wheels and just use two swivel wheels in the back, but I thought I would toss the picture of the failed base out there, to see if anyone has something "enlightening" to say about it.



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Impala

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Re: The Failed Base
« Reply #1 on: March 30, 2017, 11:40:48 PM »
The problem with this mobile base turning is the spikes on the tires. They dig in and increase the surface friction. If you want this to turn better, then you gotta run the motors in opposite directions to force the turns. I.E. right side forward, left side backwards. Or you gotta cut the nipples off the tires, so they can slip on the carpet.

Either way this mobile base should make for a good start to a indoor robot.


edit -

I would not consider this mobile base a failure. It just isn't finished.

mogul

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Re: The Failed Base
« Reply #2 on: March 31, 2017, 08:15:28 AM »
I think you understand the problem very well already. I'd say "congratulations" you have nailed one problem to reach another. Usually we struggle to get our wheels have enough traction. That's not your problem!

I see you have several options:
  • downgrade to two wheels
  • reduce traction
  • turn wheels according to speed and direction
  • use omni-wheels
All have some pros and cons
  • cheap, less stable
  • cheap, waste good power
  • expensive and complicated
  • expensive but easy
But before fixing the robot, you should shoot a video of it tearing up your carpet tiles...

kingart3

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Re: The Failed Base
« Reply #3 on: March 31, 2017, 05:38:59 PM »
You all have made some very good points.

Now that you point it out Impala, i am sure the little spikes on the tire contribute  greatly to pulling up the carpet.  I was driving all four wheels, two on a side in the same direction just like it was a two-wheel base.  I could turn perfectly in place but, the motion was jerky.

I was going to downgrade to two drive wheels but now that you have pointed out omni-wheels Mogul, I am hugely tempted.  I have seen them before, but didn't even consider them as a solution.  They clearly would smooth my 'turning issues' right out.

I had some tall, thin, spoke wheels on the base before using the dune buggy tires and the side pressure was enough turning on vinyl to peel the rubber band tires right off the wheels.

I am now twelve wheels in on this robot (I bought two swivel wheels and two socket/ball wheels when I decided to go with two-wheel drive), but I will see if I can find some omni-wheels I like since four-wheel drive would push a lot more robot across the floor.

Thanks for the advice,

            Hal
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Impala

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Re: The Failed Base
« Reply #4 on: April 01, 2017, 12:05:11 AM »
Vex has some nice mecanum (omni) wheels LINK

tinhead

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Re: The Failed Base
« Reply #5 on: April 01, 2017, 02:23:52 AM »
Hi Kingart,

It cannot turn because the distance on the sides between the wheels is too long. Try using the shorter sides of the rectangle instead to have the motors mounted.

Same system as on this long gone little guy: https://youtu.be/XJN2shxXgcA

Have fun,
TH


kingart3

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Re: The Failed Base
« Reply #6 on: April 02, 2017, 07:59:43 PM »
Interesting video clip, thanks tinhead.

Unfortunately, I think I need a broader wheelbase because this robot is suppose to go up.  I am looking for him to be 2 or 3 feet tall and come in between 20 and 25 lbs.

The omni-wheels look like a good solution, the trick is finding the right set (and making sure I can mount them).

I have been looking at the following sets:

    Set #1 - VEX:  https://www.vexrobotics.com/omni-wheels.html  (thanks Impala).

    Set #2 - VEX:  https://www.hobbychimp.com/products/vex-robotics-4in-large-omni-directional-wheel-kit-2-pack?gclid=CPelktOBhNMCFUo7gQod65sD7A

    Set #3 - VEX:  http://www.robotmesh.com/vexiq-200mm-travel-omni-directional-wheel-2-pack

    Set #4 - MR:   http://www.modernroboticsinc.com/dual-omni-wheel-50-0011-2

Set #1 is nice, looks like I will need a special hub at the least.  By the time you add the aluminum hub about $31/ea.

Set #2 is nice and again around $30/ea.  Might be easier to fit to my motors, (something like 3 mm D-shaft) but maybe not.

Set #3 Shockingly affordable.  Four wheels  runs me $20.  You get what you pay for, so I am a bit concerned over these, but, the previous sets are rated to around 200 lbs.  These might be enough for my project.  Has anyone tried these?  The price is so low I am tempted to order a set just to see what they are really like.

Set #4 - The usual $30/ea, a bit smaller but the hub looks like an sure fit.  Problem is I have never heard of Modernrobotics Inc.  Do any of you know if they are reliable?

Watching clips of some of the other four-wheelers skid around the house make me want to do a test with reduced friction.  I am thinking of setting the test base back up again and then temporarily wrapping the tires in duct tape to see how it works.  I am not keen on this as a long term solution, (duct taped tires aren't very sexy) but it might provide a date point for how I might expect the omni-wheels to perform.

I was curious about the differences between the single row omni-wheels and the double row omni-wheels.  From what I read, it seems the single row are smoother while turning, while the double row are smoother in forward motion. 

The Mecanum wheels are even more  charming but are out of my price range for this project.

Here is a charming video I found on YouTube, with Mecanum wheel driven robots.  WOW....

https://www.youtube.com/watch?v=8sH1a511_q4





« Last Edit: April 02, 2017, 08:05:45 PM by kingart3 »
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Impala

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Re: The Failed Base
« Reply #7 on: April 03, 2017, 01:18:45 AM »
When I use Omni wheels, I always pair them with a set of regular wheels. This allows the regular wheels to swing the Omni wheel end of the robot chassis for steering.

When I use Mecanum wheels, I always use 4 so that I can drive the robot in any direction without conventional turning.


Here's another place that sells robot goodies that you might want
- King

On the Vex wheels, you can drill the centers for most small robot motor shaft sizes. I have them (Vex Mecanum) on my Dagu Rover 5 chassis. I drilled to a matching size, then added a dot of hot glue before assembly.





« Last Edit: April 03, 2017, 01:23:54 AM by Impala »

tinhead

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Re: The Failed Base
« Reply #8 on: April 03, 2017, 01:21:52 AM »
Interesting video clip, thanks tinhead.

Unfortunately, I think I need a broader wheelbase because this robot is suppose to go up.  I am looking for him to be 2 or 3 feet tall and come in between 20 and 25 lbs.

The omni-wheels look like a good solution, the trick is finding the right set (and making sure I can mount them).

I have been looking at the following sets:

    Set #1 - VEX:  https://www.vexrobotics.com/omni-wheels.html  (thanks Impala).

    Set #2 - VEX:  https://www.hobbychimp.com/products/vex-robotics-4in-large-omni-directional-wheel-kit-2-pack?gclid=CPelktOBhNMCFUo7gQod65sD7A

    Set #3 - VEX:  http://www.robotmesh.com/vexiq-200mm-travel-omni-directional-wheel-2-pack

    Set #4 - MR:   http://www.modernroboticsinc.com/dual-omni-wheel-50-0011-2

Set #1 is nice, looks like I will need a special hub at the least.  By the time you add the aluminum hub about $31/ea.

Set #2 is nice and again around $30/ea.  Might be easier to fit to my motors, (something like 3 mm D-shaft) but maybe not.

Set #3 Shockingly affordable.  Four wheels  runs me $20.  You get what you pay for, so I am a bit concerned over these, but, the previous sets are rated to around 200 lbs.  These might be enough for my project.  Has anyone tried these?  The price is so low I am tempted to order a set just to see what they are really like.

Set #4 - The usual $30/ea, a bit smaller but the hub looks like an sure fit.  Problem is I have never heard of Modernrobotics Inc.  Do any of you know if they are reliable?

Watching clips of some of the other four-wheelers skid around the house make me want to do a test with reduced friction.  I am thinking of setting the test base back up again and then temporarily wrapping the tires in duct tape to see how it works.  I am not keen on this as a long term solution, (duct taped tires aren't very sexy) but it might provide a date point for how I might expect the omni-wheels to perform.

I was curious about the differences between the single row omni-wheels and the double row omni-wheels.  From what I read, it seems the single row are smoother while turning, while the double row are smoother in forward motion. 

The Mecanum wheels are even more  charming but are out of my price range for this project.

Here is a charming video I found on YouTube, with Mecanum wheel driven robots.  WOW....

https://www.youtube.com/watch?v=8sH1a511_q4

There is always threads .... Tank threads :)

kingart3

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Re: The Failed Base
« Reply #9 on: April 03, 2017, 11:02:54 PM »
I ordered the 6" wheels from set #1 (and the hex hubs).  The price was breath taking, but I think this is The Solution to my base problems.  Thanks for all the help.
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kingart3

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Re: The Failed Base
« Reply #10 on: April 08, 2017, 05:11:02 PM »
The omni-wheels have arrived.  Time to make a robot.
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kingart3

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Re: The Failed Base
« Reply #11 on: April 14, 2017, 10:27:40 PM »
The test base gets omni-wheels...
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Impala

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Re: The Failed Base
« Reply #12 on: April 17, 2017, 03:29:23 AM »
This is where we need a "LIKE" button. Your base is coming along nicely. Now you need to throw a controller on it and toss some code in it, then post a video for us to ponder!   :o

ossipee

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Re: The Failed Base
« Reply #13 on: April 17, 2017, 06:18:35 PM »
Have you picked a controller and brain for it yet, love the new wheels.

kingart3

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Re: The Failed Base
« Reply #14 on: April 17, 2017, 08:29:45 PM »
Yes I love how the new wheels worked out.  I can literally ...push... the base side to side with one finger.

I have decided to widen and length the base 5 centimeters (2 inches) in both length and width.  The tall wheels will allow me to attached the battery compartment to the underside of the base (6 D-cells in a row) and probably the motors will end up down under as well. 

I have several PICAXE 28X1s laying around, so I will use one in the base.  The PICAXE will control the four motors using a Sabertooth 2x5, and four servos driving a 'pan and tilt' arrangement for two IR range sensors on the front.  In a sense the base will be a robot by itself.

Later, I will add a pedestal, body and head.  When I get that done, another PICAXE 28x1 will be the 'master' and control the base through I2C.

At least that is the plan.  First I and the table saw have to spend some quality time together to cut out the parts.

Much thanks for the help and suggestions, the omni-wheels have kicked started this task into motion once again.

Once the base is moving, I will start a "Rolling Robot" entry for Bert and of course there will be video!

             -Hal


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