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Thread: MX-Phoenix hexapod and some more

  1. #11

    Re: MX-Phoenix hexapod and some more

    I like it!

    Looks more life like. Almost makes me want to get a bunch of MX servos...

  2. #12
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    Re: MX-Phoenix hexapod and some more

    Quote Originally Posted by KurtEck View Post
    I like it!

    Looks more life like. Almost makes me want to get a bunch of MX servos...
    Glad you did! Just let me know if you want the .stl files. If anyone is interested I'll probably make them puplic after I've posted a video.
    Kåre Halvorsen aka Zenta
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  3. #13
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    Re: MX-Phoenix hexapod and some more

    Sure looks nice! The smoothed parts photo really nicely. Can't wait to see it moving around and towering over stuff
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  4. #14
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    Re: MX-Phoenix hexapod and some more

    Thanks R3n33!

    I'm also looking forward to get it moving. Just need to be extra careful. I wish I had a better tool for testing the positions of all servos combined with IK control similar to what I did using RC servos/SSC32 using my PEP sheet or SSC sequencer..
    Kåre Halvorsen aka Zenta
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  5. #15

    Re: MX-Phoenix hexapod and some more

    My suggestion is to set the "max torque" to about 200/1024 while developing.
    That way, when your limbs smash into each other, they aren't going to immediately self-destruct.
    Yet, 200 is enough to show the gait and move the legs at intended speed, at least while the bot is resting with its body on a stand and feet in the air.
    Also, writing software to turn off torque, poll all the servos, and send positions to PC 10 times a second (plus the PC program to read that data and plot it on the screen,) as well as software to go the other way (sliders on PC screen -> servos on bot) is well worth it.
    It doesn't take that long to build, and really helps with figuring out things like joint limits, direction of movement, etc.

  6. #16
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    Re: MX-Phoenix hexapod and some more

    Thanks Jwatte, I'll definitely set the torque to low at first. I'll also have all servos running free before mounting the brackets just to see they move as expected. I might try to write a simple test routine. Perhaps check out Kurt's work, I know he made a servo-tester program. Maybe there are some updates?

    I've not had time to do anything lately though. I hope to be able soon.
    Kåre Halvorsen aka Zenta
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    Re: MX-Phoenix hexapod and some more

    Probably more than you want to take on right now but I use ROS rViz to test out any code changes before I turn on the bot. Maybe in the future you'd like to give it a go.

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    Re: MX-Phoenix hexapod and some more

    Quote Originally Posted by KevinO View Post
    Probably more than you want to take on right..
    You are correct Kevin!

    I thought it was about time to post a little update.
    First, a couple of days ago MX-Phoenix made her first steps! Not to brag but it look awesome, lol! I just love the fluid motion of the servos and the very quite sound of the MX's in action.

    But there was some bumps in the road before I managed to make her walk..
    At my first power-up I used a lab DC powersupply with current limited to 3,15 amps @12v. Very soon I realized there was something very wrong. The current drawn was at max and the voltage dropped to 2v or so. There had to be a short-cut. After some minutes of searching I found one faulty servo-wire!
    Click image for larger version. 

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    As you can see, the GND and Power was crossed! I'm not sure how much reverse current the servos can handle, but I'm really glad I didn't loose a MX-106. Under normal use I'm using a LiPo batt with a 25 amp fuse, not sure if a servo handle 25 amps in revers?

    After finding the faulty wire I thought everything was fine, but it wasn't.. For some unknown reason I didn't get any respond from the servos at all. I had tried only one leg and all coxa servos before, so I knew the code was ok. Connecting one leg at the time I managed to locate the "faulty leg". At first I thought one of the servos had an ID set to 1. Actually, it was another faulty servo-wire! This one:
    Click image for larger version. 

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    It's a bit hard to see on the picture, but the signal and GND is crossed.
    From now on, I'll always check the servo-wires to see if they are properly wired..

    After the wire-issues everything worked fine and I was able to work on the code to make it move and walk properly. I hope to post a video soon, but there is one issue I want to improve first: GEAR-BACKLASH, OMG . On the MX-64 coxa servos this is a bit annoying. I'm going to use some long coil-springs to add a constant load for reducing the backlash. I made a very fast setup and it helped a little. The spring add about 1 kg/cm of load and should be ok. At first I'll design a 3D printed holder for the spring to make sure the servos get a constant load. I'm still note sure if I'll go this route, but its worth trying.

    A question: What's your experience of the MX-28's when it comes to gear-backlash?
    My experience so far, is that the MX-106's is slightly better than the 64's.
    Kåre Halvorsen aka Zenta
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  9. #19

    Re: MX-Phoenix hexapod and some more

    Yes, the backlash is noticeable. It's actually no worse than you'll get from any other geared actuator with typical planetary or spur gear boxes.
    To get lower backlash, you need fancy gearboxes like strain wave or cycloidal gearing.
    I would suggest loading the servos in the direction of gravity -- as in, help the servos out holding the body up!
    That way, you will consume less current and build up less heat in the motors.
    And, when you do this, you can add a fair bit of pre-loading, as the servos will never really work hard trying to lift the legs up anyway.

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    Re: MX-Phoenix hexapod and some more

    Thanks Jwatte,
    The backlash isn't a problem for the servos with more or less constant load like the femur and tibia. The problem is very noticeable on the coxa's since they move horizontally with very low to no load during the swing phase.

    So you are saying the MX28 also have much the same gear backlash as the 64?
    Kåre Halvorsen aka Zenta
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