# TOW - TriOmniWheeler

Holonomic Rover

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.

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.

## Comment viewing options

Good Luck and will be interested how you finish the tophalf

I cant believe it - i just ordered parts with same idea in mind - fantastic............. cant wait to see it in motion.

I agree with Rik - the maths to calculate Tri-Motion is a bit right/left brain "fuzzy" .

if you want a bit of inspiration check out "TRIBOT"

What "Fuzzy Brain" are you going to drive it with? .(i hope Arduino)

I think I can make it move in ANY direction. The only unknown being the effect of friction. It's a pretty big factor, actually.Does teh tribot have rubberised wheels? That would make it a good deal more predictable.

The controller is reused from a different project, so it's PIC based, but I'm thinking I should just simplify it a bit so that it purely provides a serial interface to the motors. That way I can control it using a higher-level computer. The most likley candidate at the moment is my old PSION series 5 organiser as it has an RS-232 port and its own built in programming language.

DAMN! I hate that someone has already done this commercially. When I think of the number of things I've invented, just to be outdone by someone with cash. I invented freeze-dried instant tea when I was about 6, you know. Someone else beat me to market, though.

Are you saying its possible to use a Psion - i have  a series II "sulking" in my Archive box .......

...... Now where is my screwdriver....... i am curious whats inside....

Not all Palms have RS232. Some have the 12V version, some have the 3V version and some have asynchronous comms, but it's not usually provided on the stock cable you get with it.

Also check the Rovio (from wowwee as well). It's possibly even more fun, as the wheels don't seem to be laid out in an equilateral triangle (I haven't actually measured though).

The wheels of both the tribot and rovio are kind of a semi-hard foam rubber material.

FWIW, I've had my rovio apart several times, each wheel has an optical encoder setup. I don't know how much they're actually using the encoders for sideways driving. A few people have reported encoder failure as ruining the "rotate x degrees" functions, but it will still strafe just fine. I haven't had issue with mine yet.

If you are interested, I have a few photos of the innards of my rovio here.

They might be using the encoders quite a bit. I imagine that the relative speeds of teh wheels are absolutely critical for going in a straight line.

Hmmm... I wonder how an optical mouse works. If I had one of those sensors pointing at the ground, I could do a little bit of AI and have it "learn" how to go in a straight line.

Stepper motors would be ideal for getting consistant speeds
Good point. Although to date I've not heard of a rovio that can go in a straight line... LOL Maybe they should use the encoders more :)
the tribot seems a cool toy to hack  :D