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!
Originally Posted by Andrew
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