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Thread: Parallel Leg Mechanism Design

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    Parallel Leg Mechanism Design

    Alright, so after finally seeing this type of design in action at the Korean Robot Game Festival, I've started giving it some serious thought. I have an interesting email discussion forming with some colleagues of mine and I'd like to share that as well as open it up here for some Q&A. Consider this an open discussion on furthering humanoid leg designs, as I feel this will be beneficial to all humanoid builders and especially those of us building bipeds for Mech Warfare.

    What is a Parallel Leg Mechanism (PLM)? I'll get to this shortly, first let's lay down some ground for comparison.

    In a standard humanoid leg design, you generally have 3 pitch servos: ankle-pitch, knee pitch, and hip pitch. For the sake of clarity in this discussion, we're going to assume that hip-roll and ankle-roll also exist, giving us a minimum of 5dof (or 6dof if you have hip-yaw, but that's not important for this discussion) for a starting point as a 'standard leg' design.

    Here's an example of a standard (non-PLM) 6dof leg design:



    So with that said, let's ignore roll & yaw servos and focus on the pitch servos, as that is what is important here. Each one of these pitch servos gives flexibility to the leg, but the leg is only as strong in terms of lift/push/pull as a single servo joint. Each pitch axis also suffers from gear backlash, also known as 'gear slop'. These two problems end up hurting biped leg design a pretty decent amount, as your payload is always pretty limited, and the more weight you put on a robot, the more backlash hurts you.

    How about ways to prevent this and/or increase torque? You could very well double up each pitch axis by placing the servos back to back. This would effectively double your torque and could be used to eliminate backlash. Problem here is you now have 6 pitch servos and a very wide leg, not to mention the increased weight. The weight then starts to cause a problem for your ankle and hip roll servos. So while that's one option to increase your torque and help eliminate backlash, it's probably not ideal for more compact robots.

    This brings us to Parallel Leg Mechanism (PLM) Design. What is it? In short, it uses mechanically paired linkages to bind your rotation points together. This effectively creates a parallelogram mechanism for each the top and bottom leg halves. There are a couple different variations of it that use a different number of pitch servos. Here is a pretty standard example of it using 2 pitch servos, one in the ankle and one in the hip:



    Here's another variation that uses 2 in the knees for each top/bottom part of the PLM and then normal pitch servos in the ankle/hip:



    And some video of the bottom half of the leg:



    Another variation uses a total of 4 pitch servos, 2 paired per PLM in the upper and lower leg. Here is a very rough draft of a model I was playing with in Inventor:



    Another example of the 4 pitch servo PLM can be found at these links:

    http://bipedrobotnewsjapan.blogspot....parts-for.html

    http://www.rakuten.ne.jp/gold/grass-...sa/melissa.htm (scroll down a bit, you'll see it)

    And... http://www.rakuten.co.jp/grass-road/...852661/926782/

    And lastly, here's a great japanese blog of 'Ignus' - the red robot shown in the videos: http://snjrobots.dtiblog.com/

    So as you can see, a PLM design leg uses additional linkages which bind the rotation of the axis points together mechanically. The ankle, knee, and hip all remain parallel to the ground.

    Pros:
    • Ability to utilize 2, 4, 8, 16 pitch servos per leg.
    • Increased stability/reduced backlash.
    • Increased torque when servos are added to PLM, without widening leg.
    • Can be very compact.
    • Ability to only use 2 pitch servos instead of 3 while maintaining a functional leg, reduced weight.


    Cons:
    • Decreased range of motion. (can't kick, etc)
    • Standard IK models will not work without additional constraints.
    • More mechanically complex.
    • Potential to add stress to servos if not properly paired.


    So what does this mean for you (I'm looking at you Mech Warfare biped pilots)?

    If you utilize a 2 pitch servo PLM leg design, you've eliminated a 3rd pitch servo and thus lowered your weight while severely reducing your backlash. This means more payload and stability.

    If you utilize a 4 pitch servo PLM leg design, you've greatly increased the torque on each PLM and have the ability to virtually eliminate all backlash. This means more even more payload and stability.

    I'll dive into the design implementation a bit later, there are definitely some pitfalls and tricks you need to know to successfully pull it off. I'm working on upgrading Giger to a 4 servo-pitch PLM design as well as tinkering with a 2 servo-pitch and 4 servo-pitch AX-12 biped for mech warfare, so I'll have plenty to share in the near future.

    Here's a video that demonstrates the difference between a standard and PLM leg design. The first 20 seconds are using 4 pitch servos in a standard, non PLM, configuration (so double knee servos). After that, you see the same robot with a redesigned 2 pitch servo PLM configuration. Look at how much smoother and stable it is. This doesn't even demonstrate the increased torque capabilities of 4+ pitch servos on a PLM design.



    So that's about it for now. Thoughts, questions, comments?

    I'm going to include the emails of us kind of thinking through all of this (we're all still learning) thus far for those of you interested in reading more of the discussion, but for those of you who've had enough of my yapping, feel free to stop talking here!

    Quote Originally Posted by Andrew
    I have two designs in my head right now.

    First Design:

    Uses 2 pitch servos per PLM, for 4 total in the leg dedicated towards pitch.
    1 hip-pitch servo which is mechanically tied to a 1 upper-knee-pitch servo
    via the PLM, and then 1 ankle-pitch servo which is mechanically tied to 1
    lower-knee-pitch servo via the PLM. The drawback to this is that you limit
    your range of motion significantly as you can't really 'kick'. The hip,
    knee, and ankle more or less stay parallel to the ground. The bonus to it is
    that it could theoretically double your pitch torque while nearly
    eliminating backlash in the pitch axis's of the leg. I think without the PLM
    tying the servos together (such as a normal leg design but with double knee
    servos), you'd just get double speed. Am I correct here?



    The first picture demonstrates this using only 2 servos, my proposed design
    would replace the idle hubs in the knee with servos on the previously
    un-powered linkage from each PLM, so that each linkage of the PLM has a
    powered servo on one side of it (ankle-knee, knee-hip). The 2nd pic is my 4
    pitch servo version.

    http://img214.imageshack.us/img214/8765/dscn0049z.jpg (2 pitch servo
    version)

    http://forums.trossenrobotics.com/ga...e_original.jpg
    (4 pitch servo version, poorly designed rough draft)



    Second Design:

    Now the second design idea utilizes a normal pitch servo in the ankle and
    hip, and then two servos in the knees- one for each PLM (upper and lower)..
    So instead of having two servos per PLM, you only have one (each located in
    the knee). This doesn't sacrifice your range of motion, but you lose your
    backlash reduction in the ankle/hip. I think you end up with less backlash
    overall though, because the knees have the PLM supporting them to the
    ankle/hip. You don't get the (questionable?) added torque benefit along all
    pitch axis though.

    This design is demonstrated well here:

    http://24.dtiblog.com/s/snjrobots/file/CIMG3338.jpg
    Quote Originally Posted by billyzelsnack
    The four advantages I see for a PLM design are..

    It's just easier to make a stiff frame.
    Lets you move an ankle servo to the knee.
    Lets you double up your servos in a convenient and compact way.
    You can get away with 4 servo legs if you're fine with your foot
    always parallel.

    The first design with the 2 servos only benefits from the stiffer PLM
    frame and less required servos.

    I think the second design with 4 servos only gives you a stiffer PLM
    frame. The geometry you have setup does not allow for doubled torque
    since paired servos are not in parallel and it also does not allow for
    doubled speed since the paired servos are not in series either. It
    would allow for backlash tweaking though.

    The Ignis design puts the PLM servos in the knee so there are more
    options for the ankles. I guess he does not like the parallel foot
    issue and that's the point of the extra ankle servo. I have no clue of
    the point of the extra hip servo.

    I think the biggest bang for the buck comes from putting all the PLM
    servos in the knee and doubling them up there too. This gives you
    doubled up torque and lots of room in the ankle and hip. My current
    MCAD stuff is experimenting with 4 servos in the knee ( two for each
    half ) and pushrod ankles and hips.
    Quote Originally Posted by Andrew
    A 4 pitch-servo PLM Leg, if done correctly, should increase torque. It's never going to exactly double it, due to friction and magical fairies, but it's certainly a load distribution method.

    Comparison between the two types of leg designs, PLM and standard:

    Lets say we have a standard, non parallel leg design with double knee servos, so 4 pitch-servos total but without any mechanical link between them. If two of the servos move in a counter rotation of 30 degrees for X seconds, they'll have covered 60 degrees in X seconds. This can create double speed, but not any additional torque as each pitch axis is only as strong as a single servo. You also still have backlash to contend with.

    A PLM leg design with 4 servos ties each top and bottom pair of servos together mechanically, so that the servos cannot move in a counter rotation and must move at the same speed. If say, the two top PLM servos are paired via software or the EX-106+ link cable, they effectively move as one servo and the mechanism helps distribute the load between the two. If each servo moves in a synchronous rotation of 30 degrees in X seconds, they'll have covered 30 degrees in X seconds.

    A single PLM assembly (top or bottom half of a leg) would have 8 rotational pivots, all mechanically tied together. At this point, it doesn't really matter which of the pivots are powered, does it?
    Last edited by Tyberius; 06-02-2010 at 02:02 AM.
    Andrew Alter
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  2. Re: Parallel Leg Mechanism Design

    I would not consider what you have posted to be parallel manipulators. A parallel manipulator allows a single joint to have multiple degrees of freedom. Each of the joints in your sketch, and the robots you have linked to, are all serial. They have one DoF per joint but multiple actuators powering that joint.

    There are some great examples of parallel manipulators by Laval Uni. IRT built a biped, LISA, which uses one of Laval's joints. There is also a thread here where kiwibrisa has posted a render of a bot with parallel joints at the hips and ankles.

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    Re: Parallel Leg Mechanism Design

    Quote Originally Posted by devdsp View Post
    I would not consider what you have posted to be parallel manipulators. A parallel manipulator allows a single joint to have multiple degrees of freedom. Each of the joints in your sketch, and the robots you have linked to, are all serial. They have one DoF per joint but multiple actuators powering that joint.

    There are some great examples of parallel manipulators by Laval Uni. IRT built a biped, LISA, which uses one of Laval's joints. There is also a thread here where kiwibrisa has posted a render of a bot with parallel joints at the hips and ankles.
    Well then I guess it's a good thing that I didn't refer to it as a parallel manipulator. There isn't really a defined term for this type of design in humanoid nomenclature and between my description and the term 'Parallel Leg Mechanism', it seems fairly fitting and descriptive to me. Regardless, this is a discussion about the design, not the semantics.
    Last edited by Tyberius; 06-02-2010 at 08:49 AM.
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    Re: Parallel Leg Mechanism Design

    Another solid example of PLM implementation:

    http://www.vt.edu/spotlight/innovati...eo-charli.html

    And another video of Ignus that shows the mechanism a bit more clear:

    Last edited by Tyberius; 06-02-2010 at 10:22 AM.
    Andrew Alter
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    Re: Parallel Leg Mechanism Design

    Great post. The design seems entirely appropriate for biped mechs.

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    Re: Parallel Leg Mechanism Design

    Somebody was up late!

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    Re: Parallel Leg Mechanism Design

    Quote Originally Posted by billyzelsnack View Post
    Somebody was up late!
    I blame you entirely for my lack of sleep. Terrible thing to do to someone with an engineering mindset.
    Andrew Alter
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    Re: Parallel Leg Mechanism Design

    Quote Originally Posted by devdsp View Post
    There is also a thread here where kiwibrisa has posted a render of a bot with parallel joints at the hips and ankles.
    So the correct name for the mechanism with the cardan joint kiwibrisa has in that rendering is called a "parallel joint"? I keep calling it a pushrod ankle and would really like to know the proper name.

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    Re: Parallel Leg Mechanism Design

    Quote Originally Posted by Tyberius View Post
    I blame you entirely for my lack of sleep. Terrible thing to do to someone with an engineering mindset.
    Maybe now I'm done being a flip-flopper and really believe that putting a second servo anywhere in the parallelogram really does double your torque. Though probably not. I'm a proud flip-flopper.

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    Re: Parallel Leg Mechanism Design

    Hi,

    Very interesting topic Tyberius.

    I do see the con and pro's by using the PLM design. Like you mentioned the reduction of backlash and increased stability is the obvious pros. The backlash and the mechanical slope from the gear train do really irritate me for both the pitch and roll servos.

    The biggest con with the PLM is that you can't do proper rotation kinematics of the hip/body part without adding an ankle and hip pitch servo, like you can do with a standard 3 pitch servo design. I did demonstrate the rotation part on a simple biped design in my latest youtube video.

    Yesterday I read a little article in the servo magazine (June 2010 page 26) about the Charli robot made by Dr. Dennis Hong and his legion of robotics students at Virginia Tech's Robotics:



    It looks like they are also using the PLM design.

    -Zenta
    Last edited by Zenta; 06-02-2010 at 11:22 AM.

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