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Thread: Mecanum Wheeled Robot with Machine Vision

  1. #21

    Default Re: Mecanum Wheeled Robot with Machine Vision

    Vex mecanum wheels received and are INFINITELY better than Sparkfun's junk. No comparison whatsoever. Rollers are still locked together through the shaft, but there's no play and the bearing is nearly frictionless. Big, heavy, & strong.

    Sparkfun, hang your head in shame!

  2. #22

    Default Re: Mecanum Wheeled Robot with Machine Vision

    Good to know!

  3. #23

    Default Re: Mecanum Wheeled Robot with Machine Vision

    This is great. I'll very likely purchase some now.

    Thanks erco!

  4. #24

    Default Re: Mecanum Wheeled Robot with Machine Vision

    I think it is funny that the pdf file (on the first post) was from a FRC team... I was apart of a FRC team back in high school! we are the byting bulldogs 3539

  5. #25

    Default Re: Mecanum Wheeled Robot with Machine Vision

    Quote Originally Posted by erco View Post
    Mecanum wheels, I'm guessing?
    Yes, and I followed your advice and got a set of the Vex Mecanum wheels. You're right; they are very nice. I haven't drilled a hole for the Rover 5's axles yet. I just purchased Ravenkallen's Rover 5 so I can compare the two different sets of mecanum wheels side by side.

  6. #26

    Default Re: Mecanum Wheeled Robot with Machine Vision

    My only complaint about the Vex mecanum wheels is that silly small square hole. Gotta drill it out to 1/4" and get a setscrew in there before it can carry a 30 pound toddler!

  7. #27

    Default Re: Mecanum Wheeled Robot with Machine Vision

    Since Kris over on the robot forum was asking about mecanum wheels, I thought I'd update this thread a bit.

    I recently purchased Ravenkallen's Rover 5 so I now have two chassis to work with.

    I'm thinking of using L298N motor controllers on the second bot. I think the L298N can control two motors so the Rover 5 will require two of them.

    I've been thinking about moving the LED array off the bot. It's a lot of fun and looks really cool (IMO) but it's just not very practical.

    I'll probably have the LED array as a separate project and start a different thread for it. I still haven't made a video of it in action. I hope to work on this project some today.

    Both the 298N and the SN754410 chips will need heat sinks and flyback diodes. I've read more than one place the internal diodes of the SN754410 aren't enough protection from the fly back voltage spikes.

  8. #28

    Default Re: Mecanum Wheeled Robot with Machine Vision

    I drilled out my Vex mecanum wheels. Here's one of the drilled out mecanum wheels next to an original Rover 5 wheel.




    This is the way the original Rover 5 wheel is secured to the axle.



    As you can see in the next picture, the Vex mecanum wheels are wider than the original wheels. (The wheel is is on the axle as far as it will go.)



    I'll need to figure out a way of securing them. Preferably non-permanent. I'll likely try using PolyMorph to hold them in place.
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  9. #29

    Default Re: Mecanum Wheeled Robot with Machine Vision

    I've made a couple of attempts at attaching the Vex mecanum wheels to the Rover 5 axle. So far, I've had limited success.

    My first try was to squish PolyMorph down the hole to fill the empty area left by the flat side of the axle. Here's a couple of pictures of the attempt.
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    While I could get the wheels secure enough that the axle and wheel would turn together, the wheel would easily come off the axle.

    I was hesitant to do this, but I finally decided drilling a hole for a set screw would be worth attempting. It was difficult to get a drill bit that was thin enough to reach as far as I needed it to go. I did manage to drill down to the center. Here's a picture of the hole.
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    I used yellow electical tape to protect the rollers. As you can see in these next pictures, I didn't hit dead center.
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    The drill was a little off to one side. I still think this should work if I can find a bolt with threads in the right place. I had three 3mm bolts that I thought look promising.
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    The long one was long enough, but it doesn't have threads where I'd need them. The two other bolts are just a little too short to reach to the center. I was hoping the bolt would cut its own thread into the center section of the wheel. I used a larger drill bit to make the outer hole large enough not to engage the threads of a bolt. While it seemed like the bolt was cutting threads into the center section, once I tried to attach the wheel to the axle the bolt just continued to turn as I tried to tighten it. I think the solution will be to have a nut in the spokes section of the wheel. The threads will engage the nut and use it to push against so the set screw can be tightened agaist the axle.

    So that's the current plan. Order some bolts that have the threads in the correct location. I needed to order some nuts and bolts anyway since I'd used up all my 2mm nuts and washers making the popsicle stick robot. So this robot will be in "waiting for parts to arrive" limbo for a bit. Actually I have lots of other things to work on with this robot. I should try to get the L298N motor controllers working with the Propeller.
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  10. #30

    Default Re: Mecanum Wheeled Robot with Machine Vision

    I've had some success with the Vex mecanum wheels. It turned out if I trimmed some of the plastic where the head of the bolts sits, I could get a 30mm 3mm bolt to engage the axle. You can see in the photos, the nut in the spokes area that allows the bolt to push against the axle.





    I hadn't tried these particular bolts earlier. I opened my box labeled "Nuts & Bolts 2mm - 3mm" (to see what other hardware I should order) and there was a bag of 30mm 3mm bolts that wasn't inside the bag of the other 3mm hardware (which I had been using).

    I've been using this wheel as my "experimental" wheel. It has been subjected to the most abuse of the four Vex mecanum wheels. I was going to have this beat up side inward but the set screw hole is drilled a little off center and my needle nose pliers wouldn't reach the nut to hold it secure if I had the wheel turned around.

    Here's an out of focus video of the mecanum wheel under power by the Rover 5. (Not an exciting video. I wont feel bad if no one watches it.)



    Three wheels to go.
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  11. #31

    Default Re: Mecanum Wheeled Robot with Machine Vision

    I watched it, Pal. Good job! Looks like you got it running pretty true, which is very critical for these wheels.

  12. #32

    Default Re: Mecanum Wheeled Robot with Machine Vision

    Quote Originally Posted by erco View Post
    I watched it, Pal. Good job! Looks like you got it running pretty true, which is very critical for these wheels.
    \

    Woot! It now has "1 views". Cool.

    I ought to switch to my real video camera for these YouTube videos. I'm using the video function on my still camera because it's so easy to transfer the SD card to the PC for uploading to YouTube. My video camera uses those MiniDV tapes which would make transferring the video to the PC more work. But I think the video camera would probably focus better so it might be worth the effort.

    I decided drilling holes for set screws isn't so bad. This video showed up as an option after I watch mine. Ouch! Making your own mecanum wheels looks like a lot more work than drilling a few holes.

    I'm hoping the Rover 5 has enough power to drive these larger mecanum wheels. After my popsicle stick hexapod, I'm not very confident in my ability to guess power requirements (though I did suspect the hexapod legs would be too long for the servos).

  13. #33

    Default Re: Mecanum Wheeled Robot with Machine Vision

    Update:

    All four wheels are drilled, have set screws and are installed on the Rover 5.

    I'll work on getting some motor controllers working today. I'll wait a bit on adding the encoders.

  14. #34

    Default Re: Mecanum Wheeled Robot with Machine Vision

    Major Update, It's alive!

    I wired up a couple L298N H-bridge chips to power my newest Rover 5 chassis (thanks Ravenkallen).

    Here's a video of the latest progress. It will hurt my feelings if you all don't watch this one (at least the first minute). (See edit.)

    Edit: I'm leaving this video here as record of my mistakes. See post #49 for the reason the robot wont rotate and to watch a much better video.



    It turns out the L298N chips can be used on a breadboard with a little persuasion.

    So far, so good. I'm not really sure how to add the encoders without using a bunch of cogs. I have some code I previously wrote but it needs some polishing (with a jackhammer).
    Last edited by Duane Degn; 03-01-2012 at 10:41 PM.

  15. #35

    Default Re: Mecanum Wheeled Robot with Machine Vision

    Nice work, Duane!

    agfa

  16. #36

    Default Re: Mecanum Wheeled Robot with Machine Vision

    Excellent Duane, the video strikes a blow to casters everywhere.


    -Tommy

  17. #37

    Default Re: Mecanum Wheeled Robot with Machine Vision

    Duane: Fabulous, I watched it all! Looks especially great traversing sideways. It drives better on the carpet than your hard tile floor. From my experience with those Fingertech wheels, some of that is weight distribution. It sinks into the carpet which evens things out and all wheels get some traction. On a hard (non-compliant) floor, one wheel probably has less weight (chassis irregularities) and it skids before the others do, which messes things up. I made my chassis swivel in the middle, which improved things somewhat.

    I agree you do need more power (along with encoders) to really get that thing humming. And IMHO a stronger chassis would help. Maybe something in a nice aviation plywood ...

  18. #38

    Default Re: Mecanum Wheeled Robot with Machine Vision

    agfa, Tommy and erco,

    Thanks for the feedback. I'm pleased with how the bot is progressing. I'm busy working on those encoders. I'm trying to balance sample time against refresh rate. If I check the speed too frequently, there haven't been enough encoder ticks to accurately measure speed, if I sample too infrequently the controls are very sluggish.

    I'm pretty much back to where I left off with my other Rover 5 chassis (this one I bought off Ravenkallen). I think it will take some sort of PI or PID algorithm to get the wheels to respond well to the encoders. This time I'm not trying to get it all to fit in one cog. I'm using two cogs for the PWM motor control (I'm currently using an object by Kye. I will likely write my own object so the PWM will only take one cog) and one cog for the encoders (my PASM code is reading all four quadrature encoders in one cog successfully) and one for the main loop. I haven't decided if I'll run the PID algorithm (which I have yet to write) to run within the main loop or to use another cog for it. Oh, I'm also using a cog for Jason Dorrie's "RC_Receiver_6.spin" object that reads the RC receiver pulses. How many does that add up to now?

    1 (Main Loop)
    2 (PWM)
    1 (RC pulses)
    1 (Encoders)
    1 (Serial Driver for Debug)
    1 (Floating Point Math)

    I count seven. I could do without the serial driver once I'm done with debugging the program. I'm seeing again why I used two Propellers with the other Rover 5 chassis (that's a bit on hold while I work with this chassis and the Vex mecanum wheels). I still a have a cog left to run a PID algorithm if I need it. I have'nt started adding any sensors yet so I might need to reclaim some of the cogs. As I said earlier, I'm pretty sure I could drive all four PWM signals from one cog with some PASM code (the LED array code drives 120 PWM channels with one cog using 595 shift registers).

    I hadn't realized until just now, that if I use another cog for the PID algorithm, I might as well use Kye's PID, PWM with quadrature encoder object that requires a cog for each motor/encoder combination. I'm reluctant to do so since I could never get the PID to tune the way I'd like it too. My assumption is the encoders on this bot are too course for Kye's PID object (I'm not sure about this, I just might not know how to tune a PID).

    I think I will probably give Kye's PID object another try.

    I hadn't planned on posting code until I'm a bit further along. I'm sure I could be persuaded to post my "in progress" code if someone asks.

    Quote Originally Posted by erco View Post
    I agree you do need more power (along with encoders) to really get that thing humming.
    Tell me about it. I'm sure you know how much four powerful geared motors with encoders cost. Each one will probably cost as much as what I paid for this Rover 5 ($40). Even though they'd cost a bit, I'm seriously considering some more powerful motors with encoders.

    Quote Originally Posted by erco View Post
    And IMHO a stronger chassis would help. Maybe something in a nice aviation plywood ...
    This thing doesn't need to support a toddler. You probably recall, my other omni bot has an aviation plywood chassis.

  19. #39

    Default Re: Mecanum Wheeled Robot with Machine Vision

    Check this vid, they also say the weight distribution is critical: http://www.youtube.com/watch?v=7r18uN7M1mA

    And we've all seen this one: http://www.youtube.com/watch?v=YaTn-_354R4

    I think more weight means better traction & performance.

  20. #40

    Default Re: Mecanum Wheeled Robot with Machine Vision

    If you aren't using encoders, weight distribution is critical. If you are using encoders, it isn't.

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