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5Bar Quadruped - Mojo4


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#41 TNERA

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Posté 18 décembre 2021 - 12:02

Hi All, Oracid,

 

I hope to get some time for more trials this weekend. :)

 

The measurements!  yes, I made the robot wider with some additional parts. The current ratio is currently approximately square 20/19  = ~1.0

I measured from the top pivot points (top of the diamond) for Length, and approximate 'outside' of the leg. 

This ratio has provided more ballance.  i am still looking for improvements.

 

Here are some more details of this robot.  NOTE:  This is a FRANKENSTEIN Prototype. ;)   it is built mostly with reused 3D printed parts. All of the Dark Green Leg parts were printed so that I could experiment with the 5-Bar design.  Everything else is reused or scrap from previous projects.  This includes the black frames used to mount the servo motors.  My plan is that once this is walking, I will start printing a new chassis with my new-found experiences, also to add the 12DoF.  But first this one must walk.

 

Here are the pictures:

 

Length - measured from the pivot points:

mojo4-length-18dEC.JPG

 

Width - measured approximately outside of the legs

mojo4-width-18dec.JPG

 

Top - Yes, this collection of PLA is a mess.  Arduino Pro Mini clone, PCA9685 servo driver, and battery box. The 4 servo boxes are on the sides all held together by random parts that fit, bolted together by M3 screws and nuts, some super glue, and hot glue.  somewhere in there is a 9V battery for the microcontroller, there is a small power switch on the mounting board.

mojo4-top-18Dec.jpeg

 

Battery Box:  3D printed, for 2x 18650 batteries. I reused the spring contacts from small toys.  This is a problem since the spring that I have highlighted with an arrow can not handle the current of the batteries and heats up (to the point of melting the hot glue holding it in place).  something else to fix. ;)

mojo4-battery-box.JPG

 

Here is a picture of the leg configuration.  3D printed, minimal material.  It has been strengthen (i made it thicker) to prevent bending. The pivot points are small common nails placed into tubular channels in the leg components. a very cheap easy way of making a hinge. The paw needs a new design.  the current version is 3D printed, has rubber on the bottom, and part of a wine cork to hold it in place.  reuse!  ;)

mojo4-leg-18Dec.jpeg

 

Any any all recommendations welcomed!  if I am missing something please let me know!  :)

 

 

 



#42 Oracid

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Posté 18 décembre 2021 - 02:13

But first this one must walk.

Good goal !

 

Thank you for sharing all this pictures.



#43 TNERA

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Posté 06 août 2022 - 10:43

Hi All, Just an Update on Mojo4.  (it has been awhile... life and all sometimes gets in the way of projects)

 

I was able to repair my battery problem. At least it does not smoke and smell anymore. 

 

When Mojo4 walks, it still has a limp leg.  I am guessing that perhaps it is due to the PCA9685 board (servo distribution).  I think that it is not distributing the power evenly, although I know this makes no sense.

 

Unfortunately, the little robot has taken too many falls.  Part of the chassis is damaged.  Instead of printing out old parts, I am thinking a redesign would be best. (I am sure you all have had similar decisions to make)

 

The only results I wanted to know before a redesign is if the servos combined, in the 5 bar configuration, will be able to continuously lift the robot.  I know this has been a problem with Oracid's quadrupeds.  Plus you all warned me that my robot is very heavy (1.2kg) for these MG995 servos.  

 

As a result, I may first breakdown the robot to a single leg and see if I can lift 500g reliably.



#44 Sandro

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Posté 06 août 2022 - 12:43

Hi,

If your problem is simply to lift the weight of the robot itself, there is a simple solution : just add a pull spring or some elastic rubber rings across the big diagonal off the parallelogram. If you choose carefully the strength of the spring (or the number of rubber rings), then you can make it so that you need zero torque to keep the robot standing. Then the servos will need positive torque to make the leg longer, and negative torque to make it shorter.

That way, you need far less powerfull motors in order to be able to walk.

 

The basic idea behind that is that whenever you need a non zero average force, then, if you can, it is best to remove it by adding some static force requiring no energy (spings, counterweight, ...). Nb : depending what exactly you want to optimize (minimize maximal force needed, minimize energy consumption, maximize speed, ...) it can make sense to add a bit more or less static force. But needing to always provide torque or force (from motors) in the same direction is certainly sub-optimal.


Aidez-nous à vous aider : partagez toutes les informations pertinentes : description précise du problème, contexte, schéma de câblage, liens vers la documentation des composants, votre code (ou encore mieux un code minimal reproduisant le bug), ...

Vous recevrez ainsi plus de réponses, et elles seront plus pertinentes.


#45 Oracid

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Posté 06 août 2022 - 01:33

 you can make it so that you need zero torque to keep the robot standing. 

Hi Doug.

Sandro's idea is excellent, and today I always put a very light rubber band between the 2 sides of the parallelogram. Just enough for the quadruped to stand without being powered. No need for a big rubber band, on the contrary, it would be harmful.
 
I have already tried to make a quadruped with your type of servos, but I never succeeded. Sorry.


#46 TNERA

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Posté 09 août 2022 - 04:52

Thanks Sandro, Oracid,

 

I am working on just that.  A new configuration with some sort of elastic support.  

 

I need to get this dog walking, the Internet tells me there is already a Line Following Quadruped ready race! (and that robot needs some VERY FRIENDLY competition) :)






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