artbyrobot

[artbyrobot]

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! 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*.

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Grooved outer race of bearing for pulley improved

[artbyrobot]

Ok so I've been now working toward creating the latest iteration of the Archimedes 16:1 pulley based downgearing system and as part of that I decided to remake some of the pulleys with grooved outer races as I had discussed previously wanting to do - in order to prevent the fishing line from walking to a corner of the pulley and wedging itself in between the bearing and the plastic disc sandwiching in the bearing and becoming jammed that way. Previously, we had glued in a very tiny piece of clear thread to block this gap anywhere I found the tendency for jams to happen, however, to create a grooved outer race from the outset is going to prevent this issue all together! So to do it, I took my little 1x3x1mm ball bearing and pinned it down with my left thumb against wax paper on top of a stack of post it notes, so holding it all with my left hand in the air. I used my highest zoom on my visor magnifier to see what I'm doing. I then loaded very tiny amounts of super glue onto my xacto knife with sewing needle tip instead of blade tip and using this sewing needle tip, very carefully placed super glue into the joint where the ball bearing outer race meets the wax paper. I did this for about 1/3 of the bearing then carefully lifted off the pressure of my left thumbnail pinning it down and rotated the whole thing then repinned the bearing down again with my left thumbnail and repeated the process of adding glue little by little. When one side was done, I was able to carefully peel it all up from the wax paper, flip, and do the other side. This needs to be carefully trimmed down still but the idea was a massive success. The bearing still spins freely and the grooved outer race is done! That fishing line can't go ANYWHERE now to jam anything! The photo is just the ball bearing and the glue. So the plastic discs will now be able to go over this and the fishing line won't be able to walk across the outer bearing surface and jam itself between the bearing and the plastic discs anymore! Note: my concern about this procedure was that the glue could potentially walk underneath the bearing and glue the outer race to the inner race and thereby ruin the bearing - but this did not happen! The gap between the underside of the bearing and the wax paper which were both firmly pressed together by my thumbnail was too tight for the glue to travel into there and ruin anything. So the glue only went where I wanted it - which is on the outer race of the bearing forming the intended groove on that outer race. A huge success. I used 401 glue btw.

[artbyrobot]

Okay so here is my latest iteration of my motor mounted winch in place pulley downgearing setup completed. I ended up doing a total overhaul of everything since my last iteration failed. In this iteration I made many small improvements. One thing I noticed is that the thumbtack shafts have a little bulbous section near their tip and I reasoned that perhaps this can catch on the #2 fishing crimp sleeve and impede it at times. So I sanded it off with a nail file so the whole shaft is now a cylinder with no protrusions. When I tested the rotation with the fishing crimp sleeve after this modification, it spun more freely than ever before by a long shot. So I think I'll do this every time going forward. Another improvement is I added more height to the sections of the pulley, taking up all the available vertical space that used to be planned to be used for reverse direction actuation which is now being done by a tension spring instead. The added vertical space on each pulley means that contiguous loops of string wrapping have more space and so the diameter taken up by the string as it winches doesn't change nearly as much as before which means it will have more consistent downgearing through the whole duration of the winching cycle. I prefer this. It is also easier to work with for gluing on the discs and whatnot with them more spread out. Another improvement is I added an extra pulley set on the top of the main winch in place pulley which I will use to attach a string which will be tensioned on one end by a spring and the purpose of this will be to put tension on the system to prevent derailments and ensure tight wrapping every time the winch releases its string (finger extensions). Now I may not actually need this extra tensioner pulley if the spring on the finger doing the extension actuation provides enough tension to the Archimedes pulley system and this winch in place pulley to cause them all to remain snug and tensioned, however, I think I probably should tension this winch with an additional tension spring dedicated to it exclusively even if it is just a redundancy just to play it safe and doubly ensure we get no derailments even when some issue may come up with the Archimedes pulley system. Last thing we need is a cascading of failures like Archimedes system fails so also winch in place pulley then derails and tangles so then we are really set back in the event of some unexpected issue. So better to have this redundancy. To provide constant tension on the winch in place pulley, I have considered using elastic thread used for making DIY necklaces, using a tension spring, and using a clock spring. The latter seems like it could be the best and most reliable option due to its constant tension. I bought a few sizes to experiment with from Aliexpress. Search terms to purchase such an item were "flat spiral coil constant force spring". They are around $2 each.

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I have added the final pulley and rigged the guidance TPFE tube up to that pulley and routed that to the general vicinity of the Archimedes pulley downgearing system. As seen in the photo, I used super glue and post it note paper to form a TPFE guidance tube support structure to hold it in place as well as wrapped it in fabric tape and soaked that tape in super glue. I applied the super glue with the tip of a sewing needle as a precision application method. The next step will be to test the pulley system as is and make sure everything is working really well. If all testing passes, we will then modify the Archimedes pulley system on the forearm that we were using before to simplify it some since it now deals with only 7" or so of string compared to 27" of string it dealt with when we did not have the turn in place pulley system in place. So it will now be much more compact and fewer pulleys needed in it. So a bit of redesign and part recycling and we'll be good to go on that. Also, before, it was a 16:1 Archimedes pulley system whereas now it will just be a 8:1 system.

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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.

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This is a slow, careful hand test of the pulley. Everything looks good. Also, I did fast tests but didn't capture a nice shot of those with good hd closeup like this. In any case, this can show you some idea of how it all looks in action so far. The motor shaft is not turning electronically but is being turned by me pulling string wrapped around it to screen left is my hand pulling. To screen bottom is a hanging bolt that is being winched (not shown its cropped out of the image). I wanted to avoid working on the electronic actuation which is a rabbit hole in itself until I have the pulley system fully done and tested. THEN I will make it all work electronically as the next phase.

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Testing Setup for Compact Pulley System - uses tension spring on left to act as pinchers to smooth manual feed into topmost pulleys. Uses an eyelet on right to guide that exist string which is attached to a bolt to add weight to tension the whole system.

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I started some testing on just the first pulley and lots of things went wrong: I had many derailments where the string came off the pulleys and started wrapping up on the axle off all the pulleys and getting things quite stuck that way. However, after taking a step back, it occurred to me that the tangling issues were largely due to forgetting to put the final outlet of the system under load to tension the whole system which would keep every pulley winding nice and tight and aligned well. So this was user error and oversight, not the fault of the basic concept of the system then. To create the eye that centers the string and forces it to neatly stay on the bobbin and not derail so easily, I plan to use 28 ga tinned copper bus wire. I will cut out a small section of that wire and glue it to the base platform the thumb tacks are glued to and then run it vertically and then form the eye shape that acts as a guide and derailment preventer. The eye will just be a oval with a couple legs glued down with 401 glue to hold that oval into position. For the tension maker, I'm planning to use just a couple windings of tension spring with two small square pieces of plastic which will sandwich together and be pinched together snugly by the tension spring and the fishing line will be fed through this. I will use the same produce container plastic I'm using for the pulley discs. The fishing line will not be abraded/damaged by this in theory but only some pinching force applied to it to give it some tension and cause its feeding action of winching onto the pulley to be tight and snug to help prevent derailments and tangles and loose wraps. This system is meant to emulate and replace holding the string snugly between thumb and index finger as it's fed into the pulley tightly.

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So it turns out that when the forward and reverse directions portions of the thumb tack pulley downgearing system are doing their thing, they won't always have the same mechanical advantage and so will be moving at different speeds. Therefore, I have to treat the forward and reverse pulley systems as entirely separate systems that have to be completely decoupled and handled independently, each pretending like the other one doesn't even exist. They can share the same thumb tack, but have to be decoupled. So I cut the #2 fishing crimp sleeve in half using my miter saw and have to redo the plastic discs phase. Each half #2 fishing crimp sleeve will have 3 plastic discs, one for outside of the larger diameter pulley and one for the outside of the smaller diameter pulley and one to split the two. Three total. And so with 3 plastic discs on each half crimp sleeve we have 6 total discs per thumb tack. We only had to deal with 5 before so things will be even tighter but it's fine. We have enough room. Next, since both sets of pulleys have different speeds that vary over time, the one that is not being actively winched in at any given time will be randomly releasing slack in a chaotic way. This can lead to tangling and all sorts of problems. To resolve this, we need a automatic slack tensioner system to aid the pulley system by keeping this releasing group of pulleys in a state of good tension at all times. This I will resolve by the pictured method. So basically a tension spring connected to a metal eyelet will at all times be trying to pull the fishing line out of alignment and draw slack out of it. So as looseness is detected, it will immediately draw that in removing it from the system maintaining taughtness everywhere at all times. This will prevent the pulleys from getting tangled or anything like that. This setup can be placed anywhere in the path between the pulley system and the joint the pulley system is to actuate.

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I wound up my 6lb test Hercules PE braided fishing line onto the previous pulley system setup only to find out that the pulley could only handle about 21 inches of fishing line wound onto it before it started to come dangerously close to overfilling the pulley. The aim is to have plenty of the plastic disc overlapping the fishing line even when it is wound up fully to one side because that plastic disc acts as the guide to keep the line in its proper channel. I want at least 32:1 mechanical advantage out of this downgearing so if I want my final output to be 1" then the first pulley has to be able to wind 32" of fishing line onto it comfortably. So I realized at least the first pulley has to be a few more millimeters increased in diameter. So I had to rebuild the thumb tacks arrangement to accommodate these changes and make that first pulley bigger. With this increased size first pulley, I realized I'm getting what looks to be 7:1 mechanical advantage from just the first pulley alone! At least initially when it starts. As the fully wound up pulley gets winched in by the motor, the relative size differential gets smaller which means it will speed up and the torque will be less than the starting torque and increasingly so as the size differential decreases. This will create a natural sort of acceleration effect and high initial power and gradually less power. I think these side effects of this system seem to be quite good but I'll know for sure in testing. The next steps will be to wind up the reverse direction of the first pulley and start connecting the first pulley to the second pulley and so on. I may not even need all 5 pulleys but we'll see. With the first pulley being already 7:1, if the remaining 4 are 2:1 say, then we'd have 7:1, 14:1, 28:1, 56:1, 112:1 so 112:1 would be the final output. That seems quite overkill and perhaps will be too slow. Although very strong. The motor outputs about 0.42 lb on average so .42*112= 47lb! Now the lever of the joint itself makes you lose mechanical advantage due to the fulcrum location etc so it would drop down to say 15lb but my finger individual joint flexion power is only like 5-7lb so that's double mine. So a bit overkill. So I might skip using one of the 5 pulleys. Having it there is nice though just in case we wanted to trade speed for power for some of them we'd then use that one as an optional strength boost we can tap into in the future if we want to trade speed for strength so I might just leave it in the design even if I don't use it just yet. In testing I may find I prefer to use it afterall. Nice to have that option if needed.

[artbyrobot]

I got done cutting out the pulley discs and drilling them and mounting them to the thumb tacks and gluing them in place with 401 glue using a sewing needle tip as the applicator. They all are reasonably square and solidly in place I think. Everything is moving freely. Everything seems lined up okay. I then mounted them all to the 2430 bldc motor. These thumb tack based pulleys still need to be lashed down well and the lashings (upholstery thread) need to be coated in 401 glue to make them stiff and solid. I also need to add pulley discs to the 2430 bldc motor that are to line up well with the pulley disc slots the string is to go to. I then need to wind up the string sections themselves, loading up the system in preparation for actual testing.

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Here is a progress update on the silent pulley downgearing system I came up with using thumb tacks and a #2 fishing crimp sleeve and little plastic discs. It is some tiny fine precision necessary work but I'm getting it done and things seem to be looking pretty good so far. For now, I ended up just using 401 glue to glue the thumb tacks down onto post it note paper. I then put another coat of the glue over the tops of the thumb tack heads to secure it further. I am planning to use nylon upholstery thread lashings to lash all the tacks down onto the top of the 2430 bldc motor tightly and glue the lashings down as well in order to make the thumbtacks even more solidly set into place. Now I'll grant welding them down would be ideal, however, not having a micro tig welder made yet (future project), I just wanted to get going fast and I thought with enough care, it is possible these can be constructed solidly enough with composite material techniques to function reliably. I'm crossing my fingers. We'll see.

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Okay, so I finally got the Dinah robot hand sewn in and it is looking pretty good. The fingers could use some tweaking but overall I'm quite happy with how it came out. It's solid and fully articulated. Now that out of the way, I want to announce I'm officially rerouting the Dinah project as far as its current goals and here's why: so basically I was thinking it would be nice to just crank out a working robot using some shortcuts and just do something quick and dirty as a learning experience side quest to get something going. It seemed reasonable at the time. Plus I could pace myself to match the build pace of a fellow roboticist and loosely follow his project's designs. But some things I missed in this decision: #1) I'd be lowering my commitment to excellent quality with no shortcuts - ignoring the adage "do it right the first time" #2) by cutting down on workmanship maxing, I'd be inviting harsh criticism on the new lowered bar of build quality which is the last thing I need when already inviting heavy criticism for a extremely ambitious set of goals to begin with #3) I'd be going against my outspoken commitment to campaign against loud metal gear noise based robots that are completely impractical for home use due to sounding like a construction site #4) it would take away from the focus on my "real" robot projects by creating a "ghetto" side quest robot that could have just been skipped altogether. #5) this would in turn delay me truly solving downgearing by pulleys and actuating the robot arm silently once and for all, proving it can be done and proving that achieving a fully human level DOF human body while maintaining a human form factor and making all of this silent can and should be done for humanoids. So is Dinah robot just trash now? No. I still plan to have this project be done, but only while using the best methods I have including silent BLDC motors with silent pulley based downgearing.

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I'm currently working to sew all the finger and wrist bones together for the Dinah robot and mount them to the arm. I wanted to show how I'm doing this process. First, I tape the bone with adhesive transfer tape 3M 300 LSE. Note that I leave space on either end of the bone to allow some free fabric which is necessary to allow for elasticity as the bones need to rotate after all. Have to have enough free fabric to stretch as the joint rotates, allowing the rotation. But not so much free fabric that the joint is loose either. Has to be just right and snug. Next I wrap the compression workout shirt fabric onto the tape and cut it to size. The sewing is done with nylon upholstery thread and a curved suturing needle and a surgical pliers using a suturing technique.

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So I finally got the wrist done. And aside from grinding off welds on bolts and backing off the bolts to allow for free movement at joints, I'm mostly going to try to keep this skeleton stock for the most part. So I may be attaching the hand and going immediately into electronics rather than fiddling with the skeleton adding more range of motion here and there. I can always add that later anyways. And in fact the poseable joints that are fairly stiff I'm finding is actually pretty convenient while working on it so I may only free joints on an as needed basis for testing electronic actuation of that joint. Until then I'll leave them alone.

Also note: I was planning to have the wrist rotate axially around the location of the wrist for the pronation and supination. However, I realized this will not look right since you can visibly see the forearms move and the muscles there moving when you pronate and supinate your arm. So I have to have the pronation and supination be where the skeleton was originally doing this near the elbow. This will allow for much more natural looking pronation and supination. So the wrist location will not rotate AT ALL after all. This made it all the easier to make the ulna and radius distal wrist joint where the little wrist bones and hand will attach to and rotate on. I sculpted it all in fiberglass and super glue with some nails and some ABS plastic pieces and epoxy to build up the shape. I used my ABS 3d print of this part as reference only. This thing needed to be very strong as it's likely going to the point of failure as the rest of the arm is steel. So I wanted to make sure it was maximally solid and didn't fully trust just going with a 3d print there.

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Adding Axial Rotation to Wrist of Dinah Robot