Society of Robots - Robot Forum

Ok so my belt drive system from my last update just is not quite up to par in terms of grip and anti-slippage. So my new series of changes are planned out and underway now. First, I will be bumping up the height of each pulley to 2mm up from 1.1mm. This will double the surface contact area for way more belt grip in and of itself. So then I can use a 2mm wide belt. Next, I'll be increasing the drive pulley diameter to 1.5-2mm additional diameter. This will also greatly increase surface contact with the belt for more grip. Then finally, I'll be using a commercial belt that is said to have the highest grip of all belts - its called a polyurethane belt. It is a flat belt with 2mm width and .9mm thickness. It should be a huge upgrade to my current setup! Here's some photos of it:


The best part is you can customize the diameter of the belt by melting the two ends together! This was a key thing I did not know! So I can create just the right size and it should be perfect! I can also double these up by melting two belts layer by laer for a 1.8mm thick square shaped belt that is even less stretchy and so can be even more able to tightly grip my pulleys. I'm very excited about this and think it will take us to where we need to be *crossing fingers*.

I've been wanting to put a robo together for years now,   but I'm really sick from smoking cigarettes so I still haven't got it finished.

I've got a really cool innovation I want to share with you guys about getting rid of the motor encoders and how it shouldn't matter, robots dont actually need them!!!  I'll bring it up sometime here in the near future.


This is work over the last couple of years->


Heres a virtual sim of my bot going (I paid a bit of attention to the look of the graphics.)  But the main thing is the physics being correct, and its a bit dodgy still, the robot is just moving with random motions.
The end product would be the two robots having a fight on screen, in a virtual environment.

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


Heres another one with the graphics taken off, of the two robots upside randomly moving.

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



(Computer vision)
heres a stereo rgb -> depth map on the gpu.  (ran pretty bad, theres alot of matches to do!)

https://www.youtube.com/watch?v=deRUU9j-uTU




Heres a little robot kit i put together and got some random movements out of it->

https://www.youtube.com/watch?v=LJfyyzP7ec8
https://www.youtube.com/watch?v=Hw6Q9GOrRnk
https://www.youtube.com/watch?v=wmVr-beVAJY
https://www.youtube.com/watch?v=GJaMAWIcH-c

I see there are many local competitions. I hope you find a suitable group soon.

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.

After further deliberation, I have concluded that I should put 4:1 downgearing on the motor's top with the turn in place pulleys and put 8:1 further downgearing located nearer to the joint being actuated - in this case the distal forearm.  My reasoning for this is as follows:  the routing from motor to near the joint is facing turns and friction etc and these become smaller factors when under lesser loads.  So leaving things more high speed low torque initially during this phase of the routing is advantageous to lower friction and issues relating to deformation and compaction on the guidance tubing.  This means less wear and tear and lower maintenance as well.  Next, the turn in place pulleys are quite difficult to work with being very small and compact and lots of winding and whatnot is hard to deal with and tedious.  Further, the turn in place style, when fully winched in has a much lesser downgear ratio compared to when fully extended due to the relative diameter size ratios of the pulley pairs involved changing in size during the winching.  Whereas in the Archimedes pulley downgearing system the mechanical advantage is fixed and doesn't change during the entire flexion nor extension process.  This makes it more reliable and limits our losses during the near end of the winching phase that are incurred in the turn in place technique.  This ensures we retain adequate mechanical advantage during all times. 

Another important update is I have added axial rotation to the proximal finger joint in CAD.  My index finger has a little bit of this type of control to it so I think it will be a nice boost to control and dexterity for the robot.  Really maxing out the ability of the robot to finely manipulate its finger positions and improve performance of the fingers at all tasks.  I added the necessary 4 additional motors to achieve this into the CAD as well.  You can see the highlighted pair of axial rotation red indicator arrows which show the angle and location of the tendons from where they terminate to where they will exit the guidance tubing - the range of motion if you will.







Yet another important update is I now plan to just use a spring for the extension actuation force rather than the reverse direction turning of the motor.  This is admittedly going to give the extension less strength and the flexion less strength.  The flexion will have less strength because it is now fighting against the extension spring to get the finger to flex.  The extension will have less strength because a spring alone is making it happen rather than a strong motor making it happen.  I don't mind either of these trade-offs though because it will greatly simplify the routing - cutting it in half, simplify the motor mounted pulleys, cutting it in half, and simplify the Archimedes pulley systems, cutting the amount of them we have to make in half.  That is just a massive amount of time and effort saved.  I just am not convinced that spending that level of time and effort just to have a stronger extension of the finger joints is worth it.  Relatively passive spring powered extension of fingers is very common in hobby humanoid robot hands from what I've seen and although I've always viewed it as a lazy solution, I do see some merit in embracing more simplicity at times.  Especially if you cannot JUSTIFY the added work of the alternative.  The more I think about when I have needed finger extension to be very strong, the more I find that it seems to be a relatively rare occurrence.  It just doesn't seem to happen often.  Now as the robot grows more able with its AI and more sophisticated, and gets into more and more types of work, the occasional scenario where fully powered extension of fingers will start to crop up more and more as a need.  So at that time, I am thinking we can revisit this and get the extension actuation installed.  So I still plan to reserve space for it on the CAD and ensure it can be done without any major problems or redesigns needed.  It should be a smooth and straightforward upgrade option.  But for a minimum viable product that can meet all of my goals, it is not necessary to implement in this stage of development.  In fact, it is also possible to just have the robot install these on himself once he's building the rest of his own body.  Which means me doing it would be a waste of time if the robot could do it later instead of me.  So in any case, this acts as a MAJOR shortcut and time-saver for me and will be a big game changer IMO.  I'm excited about it.  These types of big shortcuts really move the project forward in development very rapidly in large leaps saving countless hours and I love them.  As long as they aren't shortcuts that will come back to bite us later, I'm okay with them.  I don't think this one will bite us later so I say let's go with it!

Note: it also just occurred to me that the robot could potentially have the extension actuation be in the form of geared n20 motors instead of reverse direction of the main 2430 bldc motors with pulley based downgearing.  This would save alot of work but introduce noisy metal gearing to the robot.  The reason I think this is okay to do is that these geared n20 motors would be slack lined and not interfere with fingers AT ALL nor be used on any way at all UNTIL the fingers need strong extension actuation - which as I said is incredibly rare.  In this rare event, it tapping into these geared n20 motors for some extra oomph to get the extension to actuate harder would solve the problem and the little noise it created would be a rare occurrence type of noise.  It would hardly be noticeable then and 99.999% of the time you'd never encounter this noise.  The bigger issue would be noise in a common feature like blinking.  Now THAT is annoying to hear gears EVERY TIME the robot blinks.