oracid

Quadruped kinematic workbench

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With Big Toy, https://www.eurobricks.com/forum/index.php?/forums/topic/167863-ev3-big-toy-quadruped-robot/, I realized very quickly that the kinematic was largely part related to the attachment points of the legs.

To progress in my quadruped project, I had to choose between the different existing kinematics, such as Jansen, Klann, Hoecken, Chebyshev etc. But how to choose without testing?
To make as many quadrupeds as there are linkages was unrealistic. In addition each linkage has a variation to infinity, as soon as you change any piece.

So I tried with a software like Algodoo, http://www.algodoo.com/, which is absolutely extraordinary, but after a few days, I considered that it was very tedious to modify the least element.
So I had the idea to make a Lego workbench with which I will use my brain and my hands. And even if my brain is not exceptional, I noticed that I was going much faster to change the structure of the pieces. When I say faster, I mean 50 times, or 100 times, faster.

In this video, I show a simple mechanism, based on the parallelogram. This parallelogram has a peculiarity, that it looks like the letter "d". The activation / rotation point being elevated relative to the fixed pivot point.

With reference to the parallelogram and the letter "d", I named this mechanism "d #".
As you can see, in a few seconds, I show that changing the location of the pivot point has a strong impact on the kinematics of the end of the leg.

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That's a great idea!

I'm a big fan of walking Lego contraptions and actually used Algodoo in the past, but this is much more convenient.

I always wondered if one could make a quadruped with walking pattern similar to star wars AT-AT e.g. moving only a single leg at a time, while standing on the remaining three. Potentially, that would be more stable than usual solutions.

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This solution for testing linkage patterns is excellent. I like the ease of adjusting the pivot point and the fact that you can draw several patterns on one piece of paper. 

Will you be using one of the the "classic" linkages for future walkers? The "d#" one doesn't have much of a flat line unless you start playing with the orientation of the whole mechanism. 

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6 hours ago, Davidz90 said:

that would be more stable than usual solutions.

I don't know much about walker, that is why I investigate, but I am not sure of that. 

As I can see, this one move 2 legs at the same time, http://www.algodoo.com/algobox/details.php?id=176820

May be you think about camel walk.

Can you show me an exemple ? I am very interested.

 

5 hours ago, BusterHaus said:

Will you be using one of the the "classic" linkages for future walkers? The "d#" one doesn't have much of a flat line unless you start playing with the orientation of the whole mechanism. 

I don't know yet which linkage I will use.

The Jansen linkage has bottom flat line of course, it has too a square in the middle and the pivot is in one of the square corner, the actuator is aside too. Somes have suppress the triangles hypotenuses.

Klann walker is flat too, even if it is not very flat in my video. Here an example, https://youtu.be/WsRVu8BoSN4

The both, Klann and Chebyshev linkages have less pieces than Jansen.

I would like very much to make my own bottom flat line. But is it sure that is it absolutely necessary ? Do you have any arguments about that ?

 

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5 hours ago, oracid said:

 

Can you show me an exemple ? I am very interested.

 

I mean something like this:

https://www.youtube.com/watch?v=GUsOouwjsL4

One leg is moving at a time. The tree legs form a stable base, and the centre of gravity (Cg) is moving to keep within this base. I've made schematics:

45354109252_eb000d559d_b.jpgwalk by David_Z1, on Flickr

 

Blue circle is the moving leg. Triangle is the current base. Note that Cg could simply move in a circle, either by rotating counterweight or by leaning whole quadruped on the standing legs.

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@Davidz90, thank you for your diagrams.

That make me look for a video and I found this, https://youtu.be/8AothYl4evU .

If you are accurate all the legs move, but only one gets up. At the end, I think it's the same walk as the horse.

Look at this very good video (at 2'16" for quick), 

<Your link is not good.>

 

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10 hours ago, oracid said:

I would like very much to make my own bottom flat line. But is it sure that is it absolutely necessary ? Do you have any arguments about that ?

I'm not sure if this applies to all walkers, but in bipeds the flat line helps keep the foot on the ground for a longer time, increasing the portion of the cycle that the body travels over that foot. You can slow down the cycle yet still travel a (relatively) big distance with each step, making the walker advance faster.  Having a slower cycle helps increase the stability.

If the bottom curve is not flat, the body is dropping or lifting during the step, adding instability to the walker.  Sometimes this instability is desirable (if you are counting on it to shift the center of gravity), but it can be quite difficult to control/predict at different speeds.

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Nice work.  So much can be done with some nice linkage mechanisms (walkers, GBC modules, factory machines, etc).  As you mentioned, each pivot can have almost infinite adjustments, leading to almost infinite paths that can be made.

This video from Disney Research Hub is also good to watch: 

 

 

For a good computer linkage simulator I can recommend this one: http://blog.rectorsquid.com/linkage-mechanism-designer-and-simulator/

It allows you to quickly and easily make simple or complex mechanisms, and allows you to trace the various points in the mechanism.  It is 2D, so trying to replicate things such as worm gears can be a bit challenging, and it doesn't have any other physics such as gravity or friction, but sometimes that is a good thing to keep things simple.  In addition to linkages, it also has gears, chains, and linear actuators.

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Thank you so much @Splat

I will test this software very closely. I need something like this. I knew this video, it is very good.

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A big thank to @Splat for recommending me its linkage software.


With this software, I made 3 videos of studies for the gait of my next quadruped.
The first two studies are not strictly for Lego.
Nevertheless the third study was specifically devoted to the adaptivity with Lego.

https://youtu.be/-FS4TKkBR8s

https://youtu.be/yp_R0U7BvwY

 

 

Edited by oracid

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@oracid - Those are some interesting walking mechanisms.  I like how the foot lifts up quickly and goes down quickly, leaving more time for the foot to be on the ground.

I'm glad that you found that Linkage Simulator software to be useful, and it looks like you have mastered it pretty quickly.

FYI: you can also export videos directly from the software (rather than using another screen capture program).  On my computer I did have to install x264VfW to get it to export videos properly.

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@Splat Glad you like my linkages.

Thank you for this new software. I will try it next time. But tell me, do you think my screen capture software is not good ?

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Just now, oracid said:

But tell me, do you think my screen capture software is not good ?

Your screen capture software is fine and does a good job.  I just wanted to let you know of Linkage Simulator's option to export to video in case you weren't aware of it.  Either way works well.  :classic:

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59 minutes ago, Splat said:

I just wanted to let you know of Linkage Simulator's option to export to video in case you weren't aware of it.  Either way works well.  :classic:

I will take care of this for the next video. Thank you.

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Wow, this is some interesting stuff! I once studied how AT-AT's walk, because I was interested in replicating the mechanism.

On 10/19/2018 at 3:26 AM, Splat said:

This video from Disney Research Hub is also good to watch:

That is a very nice video indeed :thumbup:

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