Let's Make Robots!

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.

TOW02_Bottom.jpg

Detail of the servo mounting.

TOW03_Mounting.jpg

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.

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That's what it's for. I wouldn't use servos, though.
I think we've pretty firmly established that the speed of continuous rotation servos can't be controlled with sufficient resolution to make this work.

I'm going to use regular DC motors for the Mark II. Take a look for info on alternative options for driving steppers. I read somewhere that some guy gets really good control by chanhing the length of the LOW part of the PWM instead of the (more typical) cahnging of the "high."

 

  I too have gave omniwheels a shot but put it on the shelf for a while. I have looked into 3 and 4 wheel versions and will probally stay with 4. I like your wheels. They actually appear to work. I have seen them from another post in the parallax forum and u tube videos. I guess I will have to wait until I can get some wheels. My omniwheel growbot will have to collect some more dust until then. You are on the right path. A video speaks 1,000 words. Stay wit Dc motors. In my opinion, stepper motors are not worth the battery weight they have to carry. Very inefficient. I have 4 laying around with driver chips if anyone interestted. Anything that gets hot just sitting there is not good on low power batterys. Adding relays and junk is just going into reverse technoligy.

Could you represent each wheel speed as a vector?

So, three vectors (wheel speeds) added up to give a resultant vector. 

Resultant vector is vehicle direction. (maybe)

 

Awesome robot. 

 Robot1_0.jpg

Nik

This is correct, and you can also do the same thing with the moments to get the resultant rotation.

Just be aware that the magnitude of the vector 1+2+3 will not be equal to the magnitudes of 1, 2 and 3 added together.

This is what I have done.
I still think the 4 wheel system is much easier but I stopped until I can get some better wheels. Here is link that got me started. It has all been somewhat engineered before. Notice the wheels. http://www.societyofrobots.com/robot_omni_wheel.shtml