Trossen Distance Sensor  Top Banner
Page 2 of 2 FirstFirst 12
Results 11 to 17 of 17

Thread: 4 DOF leg mechanics and torque requirements

  1. #11
    Join Date
    Sep 2010
    Posts
    177
    Images
    16
    Rep Power
    21

    Re: 4 DOF leg mechanics and torque requirements

    Quote Originally Posted by Th232 View Post
    That's pretty much what I'm doing Gertlex, slicing it and determining the shear stress and moments at the point I've sliced it.

    Results are coming out better than I expected, given that this whole thing weighs about 8.5 kg according to my model in Inventor. Still need to add sensors (or placeholders for them) though.

    Out of curiosity, has anyone found out how much of a radial load RX-24Fs can take? Robotis didn't know when I asked, and I can save a good 200 grams per leg if I use the thrust washer kit, but I'm still leaning towards making my own brackets for better stability.
    I'm not sure how you are loading your legs/servos, but I think the servo case will fail before the kit bearing fails, so there isn't too much of a need to design in a bigger more beefy bearing for the side opposite of the servo horn.

    @Gertlex, i suspect he is at 8.5kg because he has a 4dof hexapod, which is like 14 more joints then insanity wolf, so the number doesn't seem too high.

  2. #12
    Join Date
    Feb 2012
    Location
    Sydney, Australia
    Posts
    357
    Rep Power
    18

    Re: 4 DOF leg mechanics and torque requirements

    I figure a few pics would help. Here's the design so far:


    In that pose, it's 715 mm wide, 520 mm long and 335 mm tall.

    My 8.5 kg was off, I'd forgotten to get the correct motor weights. It's now a tad over 10 kgs. All these weights are off what Inventor is telling me (i.e. excluding wiring &c.):
    Skeleton: 1.48 kg. Aluminium L extrusion, 1"x1" with 1/8" wall thickness. Then covers to protect the servos.
    Battery pack: 2.64 kg. 14.8 V Lipo, 20 Ah. The battery is 1.64 kg, the rest comes from an enclosure that's there in case the battery decides to... play nasty. Will contain flames and direct any gases in a direction away from any important bits.
    Legs: 890 grams each. This includes brackets made by me to not present radial loads to the AX-24Fs. Load on them should be pure axial torque. I got a good source of CF rod for relatively cheap (by Australian standards), so I'll be using those for the femur and tibia "bones". Feet will contain pressure sensors fed into an AVR's ADC.
    Electronics: Incomplete, but I'm budgeting ~600 grams. On-board computing done by either a BeagleBoard or a FriendlyARM of some kind, plus a separate AVR board for sensors (IMU, the aforementioned pressure sensors &c.).

    Two leg options:


    Left (brackets made by me) weighs 890 grams, the version on the right using the Robotis brackets weighs 635 grams. Might need a spacer between the two RX-24Fs though, if I go with this option then mounting it will make the first EX-106+ collide with the skeleton if it's rotated far enough.

    My calcs say that when the robot is static, joint 3 will be under 2.19 N.m, joint 4 under 1.27 N.m. Double those for when it's moving using a tripod gait, and I'm leaning towards an EX-106+ for joint 3, maybe just stick with RX-64s for joint 4.

    To do list includes:
    * Lighten frame (maybe move to 0.75" x 0.75" L channel)
    * Lighten battery pack. I may just dump the enclosure and go for something like a Lipo sack, especially since I have doubts about cooling the pack inside the enclosure. In any case the enclosure is overkill, 1/8" thick Al for the panels. This is an area for serious weight reduction.
    * Sensor locations. IMU, rangefinder, camera.
    * Work out better feet.

    Yes, this will be a long-term project...
    Last edited by Th232; 02-29-2012 at 05:39 AM.

  3. #13
    Join Date
    Sep 2010
    Posts
    177
    Images
    16
    Rep Power
    21

    Re: 4 DOF leg mechanics and torque requirements

    I see what you mean about the bearing now, I have been in the same issue before when wanting to mount servo's like that. I ended up putting a thrust washer underneath the servo horn to help distribute the bending moment on the horn onto the case. Your bearing setup would be even better then that. That being said, I wouldn't recommend using RX24's on either of those joint's now that I have seen them and know how much the robot weighs. RX24's are more for extremely quick movements, but overheat quickly if held under load for a duration. You do not need super fast servo's at those joints, because your legs are so long. You would be surprised how fast even the slowest dynamixels rotate when it comes to an extended leg/arm like that.

    Also one thing to keep in mind is the voltages of the servo's, RX24's are limited to 12v, while RX-28's, 64's, and E106 can be run at higher voltages, and should be if you want to get your money's worth on performance. Of course you can run multiple voltages on one robot, but you don't get to take full advantage of daisy chaining them all together if you mix in a lower voltage servo.

    Also, if you plan on buying EX106's, Tyberius is trusted (I bought all my RX servo's from him) and you won't find a better deal.

  4. #14
    Join Date
    Feb 2012
    Location
    Sydney, Australia
    Posts
    357
    Rep Power
    18

    Re: 4 DOF leg mechanics and torque requirements

    Thanks for the observation on the -24s, will be altering my plans soon. Should also simplify my power issues, I was planning on putting a couple of buck regulators in to power the -24s, but wasn't iffy about how things would go if Rx, Tx and GND were all common but power wasn't. Would've probably just required careful planning of the wiring to avoid ground loops and all, but then again I've seen such assumptions come back to bite whoever made them.

    Thanks for the heads up on Tyberius' EX sale, I've already been talking to him about that.

  5. #15
    Join Date
    Dec 2007
    Location
    Trossen Robotics
    Posts
    3,117
    Images
    933
    Rep Power
    110

    Re: 4 DOF leg mechanics and torque requirements

    Do not under any circumstance use the RX-24s on a robot of this size. I would say RX-64 is the absolute minimum.
    Andrew Alter
    Trossen Robotics
    Join the Robotic Combat Revolution @ Mech Warfare!

  6. #16
    Join Date
    May 2008
    Posts
    2,230
    Images
    155
    Rep Power
    110

    Re: 4 DOF leg mechanics and torque requirements

    Gonna have to concur -- I had a big pile of RX-24 servos that I eventually sold off because they just aren't right for added torque (on a light, fast bot, they would be great).

    -Fergs

  7. #17
    Join Date
    Feb 2012
    Location
    Sydney, Australia
    Posts
    357
    Rep Power
    18

    Re: 4 DOF leg mechanics and torque requirements

    Thanks for the confirmation on that guys. Revised plan is to use EX-106s on joint 1, with RX-64s on joint 2. If the -64s prove to be insufficient then I'll just swap them for more -106s. Fortunately their location and the stand-off design I've got will make that easy to do.

    In that case, at least the -64s can be recycled for use in the function modules I aim to add later (you can see the mounting for the front one in my render above).

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Question(s) Best Torque to weight servo
    By rebel in forum Mechanics / Construction
    Replies: 4
    Last Post: 12-16-2011, 09:42 PM
  2. Servo's, Torque and gearing
    By mrbencowell in forum Mechanics / Construction
    Replies: 5
    Last Post: 11-13-2011, 10:51 PM
  3. Servo requirements
    By Stobs in forum Mech Warfare
    Replies: 10
    Last Post: 10-15-2010, 01:42 AM
  4. Reverse Knee and Humanoid Mechanics
    By lnxfergy in forum Humanoids, Walkers & Crawlers
    Replies: 29
    Last Post: 10-25-2009, 11:24 AM
  5. CrabFu Named Popular Mechanics Backyard Genius
    By Alex in forum Robotics General Discussion
    Replies: 4
    Last Post: 09-09-2009, 04:41 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •