TOW - TriOmniWheeler

Let's Make Robots Robots
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Here's the latest from BOA-Labs: My first omni-wheeler.

It draws very little from the suggestion here. Most roving robots must turn to face their direction of travel. The plan with this one is that it should be able to move around in 2D space without having to turn to face "forward." It's called "holonomic" motion. It's annoying that so many variations on this have already made it to market. Other omni wheelers have a perceived flaw where they might slide off the sloped roof of your house. Not this one.

Programming it is going to be the entertaining bit. Have barely contemplated it. All ideas welcome.

I have reused the the controller from Harmenszoon. This controller was also used in my first biped. In fact, in modifyig the software to operate continuous rotation servos, I found a glitch in the biped code which may well re-ignite the project.

I'm seriously considering my old Psion Series 5 as a controller because it's got an RS-232 port and its own built-in high-level programming language (OPL). It looks like some clever bugger has already thought of that, although they use a Palm Pilot.

Video

The second video show the first signs of life. You're thinking "Cool, all he has to do it flip it over!" Not just yet. First I have to do the next bit....

The first video is the last video. It "does stuff."

Closeup Pictures

Some pics of the chassis. Here's the underside, showing the arrangement of the three continuous rotation servos.

TOW02_Bottom.jpg500×499

Detail of the servo mounting.

The Maths

All the maths (discussed later in the threads) boils down to this:

d = SQRT(3)/2 (this also happens to be COS(30). Useful because 30+90 is the relative angle of each wheel to the chassis.

N1 = w - x

N2 = x / 2 - y * d + w

N3 = x / 2 + y * d + w

N1, N2 and N3 are the wheel speeds. w is the angular velocity (it can rotate as it travels).

The Conclusion

It's time to take this one apart for the motors. I wanted to get it to do "something" first, though, so here it is moving around in all the prescribed ways on the floor.

The wheels weren't good - they were too slippy.

The servos in continuous rotation mode were extremely difficult to control accurately for speed.

A pity, really. I will try coating them with liquid latex or something at some point to make 'em more sticky.

Case closed.

Holonomic Rover

  • Actuators / output devices: Continuous rotation servos
  • Target environment: indoor, Smooth floor
This is a companion discussion topic for the original entry at https://community.robotshop.com/robots/show/tow-triomniwheeler
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Clean start BOA,Is the

Clean start BOA,
Is the general idea 2 wheels rotating will move the bot in one of 3 linear directions - pushing or pulling depending on direction.

3 wheels will rotate the bot.

Looks great, umm, where’s the paint sticks?

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programming

As soon as I read the word triangular, there was a bit of a brain haze right behind my eyes. Then my visual cortex kicked in and now I have a migraine!

Please let us know… Forget video. We want code! Or in your case: high level pseudo code.

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perfect pivot rotations,

perfect pivot rotations, and side strafe capability eh…

not bad!

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very clean mounting
very clean mounting technic!! good start!
want to see what you will do with ti :wink:

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nice! i like those wheels
nice! i like those wheels and base.

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But! It has 3 wheels in a
But! It has 3 wheels in a triangle, how can it drive forward?

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Who cares about forward.

You can be such a techy sometimes :wink:

Apearantly, Wowwee built a similar base for one of their’s, proving it can be done.

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Here’s some formulas that

Here’s some formulas that can be used to calculate the direction and spin of a Killough platform robot like this.

And one robot similar, made for the Trinity Fire Fighting contest.

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Musings on Paint Sticks

TOW04_Fwd.jpg

Paint sticks are for artists. I’m a scientist and an engineer. That’s why nothing I start is ever really "finished."

The idea is that one or more wheels, potentially rotating an various speeds and directions relative to one another will move the “bogie” in a given direction regardless of what direction it’s facing.

TOW05_Right.jpg

Yeah, 3 wheels rotating the same direction and speed should rotate it about its centre. Any two wheels rotating in opposite directions will send it off in a straight line in the direction perpendicular to the third (non-rotating) wheel.

But look at the scribbles here. It’s clear that the wheels should be rotating in the direction of teh blue arrows in order to produce the overall motion indicated by the red arrows. The question is: what should the speed be?

To get it to go “forward” (bear in mind it has no “front”) as fast as possible, we drive the “front” wheels as fast as possible, leaving the “back” wheel fixed. To get it to go to the “right,” the “back” wheel rotates to the right with the frontleft wheel rotatingforward and the frontright wheel rotating backward. (Even though it relies on <I>Starfleet Universal Up</I> as it has no “front,” I’m now going to stop putting quotes round every reference.)

Hands up who doesn’t see the problem? Clearly, when arrempting tomove to the right, if I make the back wheel go full speed, it will move in a curve towards the top right.

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Voodoo

You call it “voodoo.” I call it "science."

It doesn’t have “forward”. I thought you’d enjoy that as an artist. The simplest way to make it go in a straight line is to drive two wheels in opposite directions while keeping the other one still. It’s a real brain bender at first!

Edit: actually “opposite directions” is even a relative concept.It might be more accurate to call them “opposite directions in terms of rotational symmetry.”

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The ups and downs

Heh! I knew one of you would come up with the difficult maths. I’m a bit pissed that it was so easy to find that someone has already done it. I had thought it was quite novel.

Bugger. I’m all demotivated now.

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Bits and stuff
I found a piece of soft plastic rod (dunno what it is, but it’s about as soft as a stick of hotmelt glue). I found that a 5.5mm hole would press-fit quite nicely onto the splined head of a servo motor. I turned the rod down on the lathe to the right diameter. The wheels have a keyway in them. I cut a V groove up the length of the rod, inserted it in the middle of the wheel in line with the keyway and filled the gap up with hotmelt glue.

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PPRK

This kind of platform is unique enough that a kit was made of it, the PPRK - Palm Pilot Robot Kit. Lots of good experimentation. Killough was the original who developed this type at MIT I think.

Killough platforms are a subset of omnidirectional robots, sort of like how mechanum wheels can be considered a subset of omniwheels. They are unique and a lot can belearned from building and experimenting with one. A little vector and matrix math anyways.

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I loving the design of the
I loving the design of the robot. I can’t wait to see what it looks when you add a microcontroller to it.

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2 + 2 = 0.866025

This is a LabVIEW program which calculates the wheel velocities given the desired robot velocity in the co-ordinate frame and the angular velocity. I know not many of you read LabVIEW, but this is easy enough to follow.

TOW06_Software.jpg

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For inspiration not

For inspiration not de-motivation

http://gizmodo.com/377498/segway-rmp-is-badass-war-machine+worthy

See BOA, your holding the torch of DIY possibilities !

That is if you can complete your project for under $50,000 :smiley:

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Sorry, bad joke :wink:

I quite fully grasp the concept :slight_smile:

It was only a (bad) joke, refering to this thread question that you asked me :wink:

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How about making them turn

How about making them turn the same way, dragging the third wheel behindit (or pushing it foreawrd).

If you do it your way, it would go in a straight line, but from my perspective this would be sideways, not forward

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Don’t be sorry
Ah. You were practising your irony. I can take it. Might need a bit of notice next time…