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Compact Pulley System Test Went Badly...

A project log for My Advanced Realistic Humanoid Robots Project

Building bio inspired realistic looking humanoid robots to do chores and sports and stuff.

larryLarry 11/08/2024 at 06:474 Comments

I just ran a test of the second turn in place winching pulley and ran into several problems. First I noticed my main lines turning the motor were not the full 27"+ which I thought they were but just remeasured and found they weren't. My bad. So I have to rewind those to fix that. Next, I noticed that just as we depart from the motor output shaft we experience mechanical advantage with each downgearing, so also when traveling from the downgeared area back to the motor output shaft we experience mechanical disadvantage. Up-gearing. Which means the bolt hanging as a load to place tension on the pulleys during a release cycle was not enough weight anymore (was barely enough before now clearly not enough). Now note that the bolt represents what a tension spring will normally be doing, tensing up the winch system to keep it all solid and tight. I don't want this to have to be much heavier than the bolt. I want the system to not need much pulling to remain good in tension. The friction of the teflon tubing plus mechanical disadvantage etc was causing the pulleys to not remain tense (and their not being lubed yet on the junction between fishing crimp sleeve and thumb tack. So my solution I'm now contemplating is either moving one of the turn in place winches down to the location of the Archimedes pulleys on the forearm area and putting the tensioner apparatus between it and the previous pulley mounted on the motor so that the tensioner apparatus does not suffer as much mechanical disadvantage due to up-gearing OR I get rid of the second pulley entirely and just have the winch in place be a single pulley 2:1 and the Archimedes system be 16:1. Which still works as we have then 32:1 which is great still. Under such a system, the original 27" winching would be reduced to 13.5" by the winch in place pulley attached to the motor. The Archimedes pulley system then needs to go down, around one pulley, back up, around another pulley, then down and around another pulley, then back up and tie off. The total travel for those one down, one up, one down, one up (4 trips) is 13.5/4 so 3.4". And we'd sit at 8:1 at that point. so adding two more pulleys beneath that first group would add another 4:1 for 32:1 total. And those two pulleys would add another half inch tops so that gives us around 4" total length of the Archimedes system and not too crazy many turns in that first system like we had in our first prototype. Still quite simplified comparatively speaking. So this is a very viable solution. And that 4" is around 10cm and we had 11cm already planned for this purpose in the CAD in the forearm from before. So we are still within that target and viable still without any change to the CAD at all which is great.

Anyways, back to the test's issues discovered. Oh yeah, also, the load (in this case a bolt hanging) struggled to keep the turn in place winches under tension while the motor was releasing the bolt (loosening or unwinching itself) not only because of the mechanical disadvantage from the pulley upgearing itself and from the friction in the TPFE guidance tubing but also from the friction of yet another pulley and its friction between its fishing crimp sleeve and its thumbtack. So I was having to manually pull down assisting the bolt, pulling down fairly hard just to get the system to stay taught and release without becoming a derailed tangled mess.

One other work around if I were insistent on going with more than one winch in place pulley would be to wind up extra line onto each turn in place winch pulley and have that directly attached to a tensioner spring placed wherever on the robot. This would always keep tension on just that winch in place pulley and be responsible for just that pulley and suffer no mechanical disadvantage beyond the TPFE guidance tubing it has to pass through to get there which shouldn't be too bad if the spring can be nearby. This is a valid solution but adds another layer of complexity to the winch in place pulleys and now more routing and string to deal with. It also means loads of extra springs to place. Attached is a drawing of the proposed tensioning mechanism for tensioning each pulley individually.

In any case, were I to add this type of tensioning apparatus to each pulley and the necessary extra plastic disc and vertical spacing to glue string to the fishing crimp sleeve and wrap it up, that takes up even more vertical space in the system and we were already really lacking sufficient space as is. So to gain the extra space needed to do that, we'd have to extend the height of the fishing crimp sleeve to accommodate this which would then remove the option to add the reverse direction set of pulleys to the same thumb tack. Although that is probably fine now that we were planning to achieve that with just a tension spring as the actuator for extension of fingers instead of motor actuated extension and coupling that with a n20 gear motor for extra oomph in demand on a rare as needed basis for extension action when the tension spring is not strong enough to do it for the task at hand (rare). So yeah, this apparatus would work to solve the issues I'm having with my current test setup I think. But just going 16:1 on the Archimedes instead of 8:1 on the Archimedes pulleys and simply deleting the second winch in place pulley on the motor seems like the best option to me right now. Doing so means the Archimedes pulleys bumps up from 27/4 = 6.75" for Archimedes pulleys to deal with 6.75/4 (for up and down passes around first group of pulleys) so 1.68" in length then another pulley brings it to 2.1" total length compared to 27/2 (only one winch in place pulley) = 13.5" for Archimedes pulleys to deal with 13.5/4 (for up and down passes around first group of pulleys) so 3.4" then add 2 more pulleys so 4.2". So 2.1" vs 4.2". If we keep the second winch in place pulley we shave off 2.1" in Archimedes pulley system total length and shave off one pulley from its system too. Well I think just going 16:1 on the Archimedes is my move here. The winch in place pulleys have been a finicky mess to me. I prefer the Archimedes style pulley more and prefer to have that do the lions share of the downgearing after that first winch in place pulley cuts our total run-out in half. It still is a very useful help to cut things in half like that and much appreciated. But any more winch in place action is asking for trouble. I am much less able to control it and prevent issues that I feel I can do with the Archimedes pulley system. And you all have not seen my Archimedes pulley system in action it is really beautiful and elegant to watch and totally silent. So I'll rely on it more and keep the winching turn in place pulleys to the minimum 2:1.

Someone trying to do their own robot may appreciate that I'm leaving these options for further exploration open for future devs. I'm going the way I am most comfortable but if you think the winching method is more comfy for you, downgear more with those than I chose. I am not ruling that out here - just preferring the Archimedes more based on my experiences so far.

Discussions

FulanoDetail wrote 5 days ago point

Hello, just saw your project and I think it is interesting.

(I just took a superficial look at your project logs, so I don't know if what I'm speaking here is something that you are already aware)

I'm also trying to make humanoid robots to a certain extent, but my little nugget of knowledge that I would like to suggest to you is looking into the anatomy and performance of the human body on top of biomimicry articles.

Human muscles can contract at a speed of more or less 40 cm/s and since they are normally levers connected to 1:10 to the limbs, each limb would have a tip speed of 4 m/s more or less. 
If you are really intending on making it run and jump just as fast as real humans, this means that you will need high performance actuators, with several kilowatts.

And just like airplanes and rockets, you will be fighting for every gram of weight that will go into the humanoid robot. Either as the weight of the structure, weight of actuators, reduction systems, joints, power source etc.

If you really don't care about efficiency, only performance, I would advise you to check filament McKibben air muscles. You could actuate them with a centrifugal compressor, evaporative cooling (using a coolant that evaporates at room temperature) or a reversible fuel cell that separates and combines oxygen and hydrogen. This last one is kinda dangerous due to the hydrogen, but if you are willing risk, it would be a really compact and light system.

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Larry wrote 5 days ago point

thanks for the input.  To my knowledge mckibben air muscles are not a established commonly employed and proven method of robotics.  It would be lab settings only theoretical use type item that may not work in practice and may come with many downsides.  People tend to find such things and recommend them without having any proven robotics projects that have successfully implemented them.  I am not interested in deadends or highly untested never before used options.  Fuel cells is another example of this.  Nobody has to my knowledge ever used fuel cells in a robot on youtube for example.  If it has NEVER been used on a working robot on youtube, then it is not a viable option for me.  I need to see other roboticists using it or else it is just a theoretical option and possibly a deadend.  I am innovating SOMEWHAT on existing PROVEN solutions but have ZERO interest in experimenting with something that is not proven.  Case in point:  spider web is supposed to be stronger than steel in theory yes?  So it would make the best robotic cable actuator yes?  In theory sure.  Well I saw a video recently of a guy who milked spiders for days to make a single guitar string and gather 7km of spider silk and braided it into a string.  It was the same size as his other guitar strings in diameter.  It should be the strongest guitar string on earth yes?  Well it was weak as hell and broke before he could even tension it enough to make a E note.  It was not as strong as steel at all.  It sucked.  This type of thing could just as easily apply to fuel cells or mckibben muscles IMO.  In theory could work but its not being used in robots for a reason.  Why isn't either in the tesla optimus bot?  Why are zero humanoid robot companies on earth using either?  Probably because they are not proven technologies.   You mentioned fuel cells could be dangerous.  Well I don't want anything dangerous in my home aorund my family which is what my robot will be doing.  A household butler robot should not be a significant explosion risk.  Also I've seen a hydrogen extractor from water device and it was HUGE.  I don't have room in my house for such a massive contraption.  Better to use proven solution like high quality lithium batteries.

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FulanoDetail wrote 4 days ago point

Now that you said it, these youtubers out there that did in fact use McKibben muscles in their projects, did it with limited sucess.

Just like you said, these videos/projects don't make use of McKibben muscles in a dexterous, delicated and precise way, like you need. They only use open/close pneumatic valvues without position control systems.

And when I tried to search on the subject, I could only find James Bruton making a video about pneumatic cylinders with Arduino position control and the Festo Air Arm, which is a decade old project that uses really expensive pneumatic valves.

Anyway, sorry for taking your time and I hope your project is successful :)

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Larry wrote 4 days ago point

well I appreciate the ideas anyways and appreciate you taking your time to try to help.  

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