"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.
This is such crative way of
This is such crative way of making a robot move, I am really looking forward to see the results of this project
never mind, the pictures on
never mind, the pictures on your comment “Musings on Paint Sticks” show just what i wanted to tell, sorry I saw them too late
Get The Cheap Fixed Casters !
Wow, how did you find that Doc BOA? What an excellent reference !
How about this though? If you went down to the local hardware store and found the smallest set of drawer handles with a round plastic covering - you could just buy a bunch and mount then around a cylindrical hub ! Or for that matter, the smallest fixed casters (without swivels)
My 45 degree orientation of the casters is not correct in the picture - but I imagine you get my drift - Large & Cheap (just the way I like em)
No special tools required. The hub could be a ABS plastic pipe sawed and capped. Boy, this could be fun.
Motorisin’
I’m thinking it’s time I went down to the car breaker to get a third Nissan Micra windscreen wiper motor.
My bad, completely, sorry 
My bad, completely, sorry
I like your thinking as
I like your thinking as always, Grog - however, A note; The tiny experience I did get was that discs will work… but you want cylenders for a controllable slide / an even result - specially if your floor is bumpy!
Back to Meccanum
Yea. Those mecanum wheels (the ones at $400 per set) have near-cylinrical wheels with sort of bullt-shaped ends so they roll really smooth. You know, when you look at them, you don’t see $400, but when you start dismantling it in your mind, you can see all the manufacturing costs start to mount up.
I see your point Frits.The
I see your point Frits.
The cross section of one of those casters would be a rectangle @ 45 degrees and rotated would put it on its edge.
I was looking around for drawer pulls with a cylindrical rotating bead but none seemed very applicable. I was thinking that maybe a sphere would be better, no matter what the angle the surface friction would be the same - probably a smoother ride too. Plastic beeds are already drilled in the center, but you would have to make the axel assembly etc. - I looked up "fixed ball caster" but did not find much. Ball casters are not usually of the fixed axel variety.
Large casters still seem like they would be fun...
Time to shine
OKay, I’m starting to think servos are not the way forward with this. I can’t seem to get much resolution on the speed of my continuous rotation servos.
I’ve observed that full speed one way is given by a pulse width of 1564us. Full speed the other way is 1436us. I just can get a decent range of speeds from my servos.
It takes a good 25us to perform the PWM comparison, so that makes for a resolutoin of about 5 forward and 5 reverse speeds. I was hoping for more like 30!!
Has anyone got a mechanism for finely controlling the speed of a continuous rotation servo?
The Seattle Robotics Society says here that the approach used to control the position of a standard servo (varying the “on” time of the pulse) cannot be done very finely in software and I’m in starting to agree. They go on to describe how more sucess can be obtained by varying the length of teh “off” pulse. this seems counter-intuitive, but the writer speaks authoritatively.
Has anyone seen this done? I’m thinking of moving to bigger rotational motors. Which means a horrible smelly messy speed controller. Yuck.
omnirobot --> line follower ( is it possible? )
guys, is it possible when i had an idea to make a line follower (line following robot) with omniwheeler as basic?
sure why not, however
it will not be easy though!
A robot with two motors and normal wheels are much easier to program then this setup.
Wheel Question
BaseOverApex-
Glad I stumbled upon this post. I have a quick question I hope you couled help me with. In regards to omni wheels, I have a setup (or close to completing) which is outlined in the below drawing (crude). The top 2 wheels are driven off the same motor and share a drive shaft.The lower wheel has it’s own motor. It is also worth mentioning these wheel are the Vex Omni Wheels.
The question is: what possible movements will the bot posses given all possible scenarios? Logic tells when I can achieve a “floating” motion, but I’m just not sure how this setup will respond. Any insight would be super helpful.
Could be educational
Hi.
Hmmm… Let’s give the bits names. The ones at the top are the “coaxial pair” and let’s call the bottom one the "transverse.
There are only two motors. Driving the coaxial motor alone should certainly cause it to go in a straight line.
If you drive the transverse keeping the coaxials steady, you would hope to spin (probably about a point close to the middle of the coaxials). However, since there’s nothing to stop the coaxials sliding sideways, my fear is that they will do so, possibly causing it to move in a straight line, or at least in an arc of a (nearly) unpredictabe radius.Let’s hope I’m wrong; if I’m right, it won’t be possible to correct the motion as the coaxial pair can only be driven in the same direction.
I think the correct way to control this robot woud be like a car. Use the coaxial motor for drive and teh transverse for steering. In fact, the transverse motor needn’t be a roataional motor at all: hae you considred just using a servo?
You have a fixed “back axle” but by using poly wheels, you have eliminated the need for a diferential drive.
I think it would work very well in this fashion, but I’m not confident you can achieve the desired “floating” motion.
By the way, those wheels are bloody brilliant looking! What a great find!! You should consider adding them as one of our LMR links!
Good luck!
Thanks
Base-
I hear what you are saying, make wonderful sense. I think I am going to just finish it out as planned and see what happens.
What I’d like to see happen is this “floating” effect as well as dead straight driving and the ability to rotate on a dime. I will keep you posted.
Wait and see…
I applaud your approach. Bang something to gether and see what happens is the accepted norm around here. My design will definitely "float." But, let us never forget that Penicillin was discovered by accident!
Fill your boots
That’s what it’s for. I wouldn’t use servos, though.
Stuck
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.