perfect pivot rotations,
perfect pivot rotations, and side strafe capability eh…
not bad!
very clean mounting
very clean mounting technic!! good start!
want to see what you will do with ti 
nice! i like those wheels
nice! i like those wheels and base.
But! It has 3 wheels in a
But! It has 3 wheels in a triangle, how can it drive forward?
Who cares about forward.
You can be such a techy sometimes
Apearantly, Wowwee built a similar base for one of their’s, proving it can be done.
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.
Musings on Paint Sticks
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.
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.
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.”
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.
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.
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.
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.
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.
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 
Sorry, bad joke
I quite fully grasp the concept 
It was only a (bad) joke, refering to this thread question that you asked me
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
Don’t be sorry
Ah. You were practising your irony. I can take it. Might need a bit of notice next time…
"opposites"
When you say “the same way” what do you mean? If I turn all the motors thew same way, it will spin. I know it was confusing of me to say “the top two motors in opposite directions”. Of course they’re only “opposite” if considered to be on opposite sides. Of course they’d be going the same direction, but as viewed from the top would appear to be going opposite directions.
Fristywheels
I have to say, I like Fritsy’s approach, but How hard could it be to copy this design? I’m thinking a bit of U-section aluminium and a cylidrical shaft. Turn down a couple of rollers onthe lathe… Boom. My target would be to knock them out for about $40 for a set of four.
Easy Peasy
OKay, this took a couple of days, because I had to go to work to pay the bills and stuff, but, what all this crap boils down to is that if we want to demand a movement from the current location (0,0) to a location (x,y), the three motor speeds are:
N1 = (0 - x) / r
N2 = (x / 2 - y * 0.866) / r
N3 = (x / 2 + y * 0.866) / r
Where 0.866 = SQRT(3)/2 = COS(30) and r is the radius of the wheels.
Haven’t quite worked out how far a distance of 1 is or what a speed of 1 equates to, but I don’t think it matters as the components are all in the same terms.
There’s an additional bit where if we want to robot to rotate, we can supply n angular velocity (w). If we wantt o spin it we also ned to supply the “wheel baseline” (the distance from the centre of teh chassis to the middle of the wheel).This is defined as ‘b’.
If we want to add this, the equations for each wheel become:
N1 = (b * w - x) / r
N2 = (x / 2 - y * 0.866 + b * w) / r
N3 = (x / 2 + y * 0.866 + b * w) / r
I’m gonna translate this into integer maths and see if it will work.