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. Considering the motor is $24, $2 to have a extra spring isn’t too bad. It could really make or break the reliability I think. Now the issue is the spring is supposed to provide tension for all 32" of travel of the winch. That is a long way. My mini tape measure surely has this type of spring in it and it has that tension spring the whole time and presumably uses the same type of spring I just bought. So it is possible for a tension spring to do this for this length of travel. So hopefully one of the ones I bought works for this. If not I may have to upgear it trading tension for more travel distance. I will have to fit these extra springs into the body which may be tough. Space is VERY constrained but hopefully we can pull it off without any issues. In any case, these are going to take a few weeks to arrive so I’ll be testing without it at first.
Another improvement is I made the diameter of the larger pulley of this turn in place winch bigger which means it will provide more downgearing. Not sure how much maybe an extra 5% or w/e but it’s something.
I have routed the final output string using the TPFE tubing around the side of the elbow this time and then across the forearm and over to where I have setup my Archimedes pulley system from before. This Archimedes pulley system is getting a total overhaul. Many improvements planned for it. So that is up next, overhauling that. And then I will be ready to connect the turn in place winch pulley and its 2.4:1 or so downgearing with the 16:1 Archimedes pulley downgearing to achieve around 38:1 downgearing in total.
I must say it is a bit demoralizing and annoying to rebuild the winch in place pulley system 3-4 times now but I guess it’s fine. That is part of the trial and error refinement process and learning process. That is how we improve on it and correct any false presuppositions we had. That is why experimentation and testing are important. So in some sense, even just having to build it just 3-4 times is a testament to the design process actually going pretty smoothly. Much better than having to do 20-30 rebuilds to get it right. So we were pretty accurate then on our overhauls and can limit rebuilds if we think it through well and do a great job. I hope this time we got it perfect and can move on to other things! Testing will tell.
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.
Here’s a better image of the grooved outer race with more refinement. To refine it I cut away excess glue with exacto knife and then sanded it with a nail file a bit to smooth it out.
Ok so I was struggling to plan out how the flat spiral coil constant force spring would maintain constant tension on my first winch in place pulley the past couple days and I was studying how tape measures use these springs. Then it hit me when a colleague was mentioning belt pulley based downgearing that a belt pulley based downgearing for this first pulley would remove all the issues of derailment and need for constant tension during whole duration of travel a winch style would require in this design. Also, since its just .4lb-.8lb of force for the first pulley downgear, as long as the belt is reasonably tensioned and has some decent grip to it, I should not deal with a ton of slippage issues and the motor’s output should be passed along well. So here is my beginning attempt at converting my first pulley to a belt based pulley instead of fishing line winch based pulley.
This is made just using adhesive transfer tape applied to one side of a nitrile glove and cut out into a 1.1mm wide strip and applied to the two pulleys directly. Built in place.
Early testing shows it needs more layers to have less stretchiness or needs to be reinforced internally with fishing line wraps between layers to prevent so much stretch to it which causes slippage. Also, the motor output shaft acting as the winch pulley is a combination of a bit too small in diameter and a bit too smooth to create a proper grip. So I’m thinking of thickening it up some and adding a grippy surface to it so that it grips the belt better with less slippage.
I am considering using silicone rubber to coat the motor output shaft or several wraps of nylon upholstery thread and super glue to thicken it then coating that with carpet anti-slip paint. Or silicone.
I’m considering making the belt from a cloth coated in silicone or carpet anti-slip paint and then sewn tightly into place over the pulleys - creating a sewn seam for a tight grip.
I’m considering a tensioner pulley but I think that’s overkill and should be avoided unless it proves absolutely necessary.
I have not explored purchasing options at this time but of course I’m open to look into this in the future. The thing about a premade is it would have to be a perfect fit in both length and width and I’m not sure if that will be easy to find or not. This is all a very new approach so I can investigate that later. For now I’m happy to just move quickly on the prototyping with materials on hand.
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 had an epiphany. So in the winch in place pulley system I was working on before, my concern was that when winching in the string things would be taught and reliable but when the motor reverses and releases string, that is when any snags in the system could cause the string to not be taken up rigidly and tension on the system is then lost and the motor is then unspooling string which isn’t being taken up which will result in a spaghetti mess of string spraying everywhere out of control and getting all tangled up. The solution I had was a constant tension spring attached to the turn in place pulley output that would ensure that always keeps the string in tension as the motor unwinds. However, that was a extra cost and complexity and volume taken up by yet another thing and when you multiply that out by 300+ motors that’s a LOT of springs added taking up a ton of extra space. That is why I moved to a belt based system instead of string and winch based for the first pulley. So the epiphany was this: it hit me that I can simply have the spring that does the extension of the final finger joint be what puts tension on the whole system and then if at any point in the system a snag were to happen, rather than tension being lost as the motor blindly unravels, not detecting the snag, I could have the motor NOT actively unwind anything at any point! So the motor, when unwinding is to occur, will simply turn OFF, rather than actively drive the unwinding electronically. It can pulse width turn off just acting as a brake to moderate speed of extension but at no point do any counter clockwise release or unwinding of the string. This way, the system only itself pulls string off the motor output shaft and if the system at any point snags, the extension stops and the string is all still under moderate tension but just no further advancement takes place and the motor does nothing further but blindly turning on and off but not actually spraying out thread everywhere at all. Eventually, the potentiometer measuring the joint angle of the finger joint would detect things are not moving and the system would KNOW it has a snag somewhere and at that point it would perhaps try to contract then attempt extension again hoping to dislodge the snag. If this did not work, the system would go into a troubleshooting routine like notifying the user (myself) to fix it or fixing it itself or w/e. But no damage would occur in this setup involving a unraveling mess or tangled mess. Simply the snag itself would be discovered and addressed but no catastrophic series of failures would result in theory under this new setup.
So with all of that said, and this solution in place, I am ready to return to the turn in place style winch style first pulley setup I had before and then the Archimedes pulley will do the rest. So the first pulley will be 2:1 downgearing and the Archimedes system will do 16:1 for a total of 32:1 downgearing. No constant tension spring needed anymore! Much simpler now. Everything I was concerned about is then solved now.
The belt based system fix ideas I was going for may have worked but as of right now I’m abandoning that course. I prefer the winch style and think belts would be higher maintenance and slippage would perhaps be an issue even with all the changes I had mentioned to improve on it. The fact is, belts only have so much surface area to grip onto so they don’t scale down too well to tiny pulleys IMO. Large pulleys are better due to large surface area and more for the belt to grip. So my super miniature belt idea was a bit doomed from the start even if it could have worked (and it may well have worked) it just isn’t ideal theoretically and I’d rather go with something I trust more intuitively for now.
Here’s my latest progress on the winch in place pulley setup. I opted for 10lb test 0.12mm diameter PE fishing line (orange color) as the output that will interface into the first pair of downgearing pulleys of my archimedes pulley downgearing system.
This turn in place pulley achieves 2.77:1 downgearing ratio now. The motor shaft reels in 32 inches of string that is 6lb test 0.08mm pe fishing line (black) and after the downgearing pulley, the final amount of orange fishing line reeled in is 11.55". That’s a much more manageable amount of runout for the archimedes pulley system to deal with to keep it more compact.
The archimedes pulley downgearing system will add an additional 16:1 downgearing to this which brings me to a total of 44:1 downgearing. The motor itself pulls at .5lb pulling force so after 44x that increases to 22lb of pulling power. After mechanical disadvantage is factored in, I estimate the finger can curl 5.5lb ideally which is about the same strength as my finger. So that’s perfect and VERY strong IMO.
Not the most substantial update but I wanted to share my top cap solution for the winch in place pulley. In this photo, you can see that I cut out a small piece of the clear plastic from strawberry container into a little square and poked a hole in it with sewing needle then pressed it onto the tack firmly till the tack jutted out a bit like 1mm. Then I glued the tack to the top cap with 401 glue. This keeps the pulley from coming off the winch when the motor is upside down which it is now.
Another small update is I just ordered some plastisol to experiment with for robot skin making or even other parts of the robot like the artificial lungs or even ligaments perhaps. I ordered the hard and the soft versions which you can mix together to get medium variants. This is the stuff used to make fishing lures but the harder formulations make pvc medical skeletons. It is a thermal plastic so its like TPU but unlike TPU, not so fussy since you can microwave it for 3 minutes and use it - much easier and lower fumes. You can reuse it too by just microwaving it again. So that’s a improvement over silicone. The worm fishing lures are quite durable. It comes in clear and you add pigment. I plan to add acrylic paint and may switch to dies or lacquer paints to see what works. I think using this as skin is being slept on. It seems like it could have huge potential. You can shoot it into a mold or apply it over a 3d model by spray or brush or knife application methods. Then peel off and use. I love that it can cure instantly in theory if you spray the hot surface of it with upside down compressed duster can - this is how I get hot glue to insta cure. A instant cure is amazing for fast results. I like super glue/401 glue because it insta cures with accelerator spray. Anything with no wait time for curing speeds up workflow and enables me to move quicker in getting steps done. This would make it superior to silicone due to no wait times. A power mesh backing fabric will give it the rip resistance it needs just like silicone mask makers use.
“I’ve planned the brains/software alot as well. I’m going to have a laptop or powerful tablet or two in the chest of the robot running the main logic engines” i would say go with something like raspberry pi jetson nano. you would need something with gpio pins to control servos such Arduino or raspberry pi pico could controll the the servos
Well actually a full blown mini itx gaming pc is the latest plan. And it interfaces by usb to arduino mega custom microcontroller which then interfaces with motor controllers that are also custom. You don’t have to have I/O on the main logic pc that is GPIO just use USB of the main PC then the GPIO of the microcontroller from there. Plus you want downstream microcontrollers doing some of the lower level processing to give the main pc some help so it can delegate out stuff and not have too much work to do by itself.
I had a eureka moment recently that I wanted to share. So basically I was thinking that I may not need to read back emf from a BLDC motor in my custom motor controller. Instead, I can have it just mindlessly advance the motor at a fairly low power mode by default and a default speed of advancement of the rotating electromagnetic field. Without feedback, it may overshoot, rotating faster than the output shaft and thereby skipping some turns. That is the reason why people want to read the back emf to avoid that issue and instead only advance the electromagnetic field forward at just the right moment - the zero point crossing moment. But I was thinking about it and realized that is not really necessary. For this application, if skips start happening, it doesn’t really matter. To the degree that skips are happening, the motor will stop advancing the load with its winch system and this will show up when readings are taken by the potentiometer measuring the final joint angle. If alot of skips were taking place, the advancement of the potentiometer would not match the angle it thought it would be at were no skips involved and this would tell the motor controller that it has been having skips and give it an idea of how many skips as well based on the divergence of projected joint angle by now and actual joint angle by now. So then it would turn down the speed a bit or turn up the amount of on time of its pwm and thereby put more force into the rotating magnetic field to give a bit more oomph to the motor. It would then track progress by way of the potentiometer again and see if that solved it. If it still is skipping a fair amount that could indicate the load is more than expected or there is a jam in the system or it just needs more power and it could turn up the power more and slow the speed down more on its rotating magnetic field overall speed and try again. Rinse and repeat until it finds the sweet spot or finds out it simply cannot lift the load because its too heavy or there’s a jam in the pulleys or w/e. So in a way then this would give it collision detection as well as the ability to have an idea of how heavy loads are based on how much it had to slow down and add forces to get the joint to move. I then see no real need to implement ANY back emf reading NOR any need for hall effect sensors etc to monitor rotation progress. The potentiometer on the final joint the motor is actuating is enough clues to tweak the rotating magnetic field to our satisfaction. By eliminating the back emf circuitry we greatly simplify the schematic of the motor controller, suffer negligible performance hit, and eliminate a lot of processing for the microcontroller chip handling the logic of many bldc motors simultaneously which means it can handle more bldc motors by itself. It doesn’t get bogged down so much by having to read in all the zero point crossings as part of its routine. This saves on processing demands and processing speed demands. Getting this all to work in real time and perfecting it will require a fair bit of trial and error but this is how I’m seeing it working out and my proposed solution for simplifying things. I think it should work great! I’m excited to have much more dumbed down circuitry like this and to get to working on this soon. Just have to finish making my pulleys and then this electronics development can get underway again. That’s why I’ve been thinking ahead about it a fair bit since it seems I’m likely nearing the end of solving the pulleys situation soon.
I realized the 1x3x1mm ball bearings are really the perfect size being so small which is ideal to keep things compact but the only disadvantage is they only support I think 10lb weight put on them before they’d break. So I was going to use them for the first couple pulleys in the archimedes pulley system then switch to a plain bearing I made for when the forces get too high for the 1x3x1mm ball bearing to handle in the last couple pulleys. But recently it hit me that I can stack two of the 1x3x1mm bearings on top of eachother and use two fishing lines for that section of pulley to go around these double stacked pulleys in order to double the load capacity. If that is not enough I can add another single or double pulley below it and they would all come up together acting as a single pulley as far as the downgearing goes distributed across more than one bearing. With this approach I can use this type of ball bearing exclusively for everything since I can just add more and more of them for higher load situations in theory. I mean maybe for leg motor downgearing I could bump up to a beefier pulley but we’ll see. So that is yet another nice breakthrough idea I had recently.
I’m currently wrapping up my 2nd archimedes pulley system prototype and will be posting an update on that soon.
Here’s a little update on my version 2 Archimedes pulley system. It’s cleaner than v1 version and you’ll note that rather than tying off ends into the 1000 denier nylon fabric sleeve of the bone, which chafed the attachment point and caused premature failure on version 1, I’m now tying off onto the eye of a fishing hook that I get by snapping the hook’s eye off with wire cutter and sanding smooth with nail file. Also I’m using a fisherman’s knot rather than square knots as that handles higher loads without snapping or stress concentrating too much locally. What you see in this photo is 4:1 downgearing. Add this to my 2.77:1 downgearing with the winch in place pulley on the motor by its output shaft and you have nearly 11:1 downgearing so far. I need to add just two more pulleys to get to our 44:1 downgearing final output. Note that I have two yellow lines coming off the bottom pulley pair since I plan to load distribute across two lines instead of just one so I can use my load capacity limited 1x3x1mm ball bearing based pulleys and not overload them. This divides the load by two. I’ll be using double stacked pulleys for the next couple downgears to share the load across double pulleys instead of single pulleys. I’m getting so close to electronics phase for final testing of all this downgearing madness!
#1- I noticed it was about impossible to pull from the bottom of the Archimedes pulley system and get the motor to unwind. After discussing the issue and potential causes with chatgpt for a while we figured out that the culprit is the tensioned string I put onto the output shaft of the motor to allow for snug unwinding and winding of the opposing string pair that I installed for manual turning of the motor shaft during testing. This tensioned string wrapped around the motor shaft only requires about 1lb of force to pull the motor enough to turn the motor output shaft. However, after the downgearing, to fight past that 1lb resistance to turning the motor output shaft would require 12lb of force since you have to divide the force applied at the output end by the number of downgear ratio you are at! And so after all points of friction in the pulleys and teflon tubing and the motor output shaft’s magnetic cogging even while freewheeling we might be more like at 13-14lb of force required. And that is a TON of force to apply by just hand gripping fishing line. So I figured my system was just way too resistive somewhere or collectively and completely non-viable until we solved this issue! The 1lb at the motor might not seem big but it’s HUGE to overcome when pulling from the backside after all downgearing. Wow. So we solved that big scare. I was very concerned and exploring alternative plans thinking we might have failed with pulleys approach before this was finally solved today. I’m so relieved. So once we remove those strings which are impeding the motor shaft from turning, we should only need a reasonable say 3lb of force on the back end of the pulley system, exerted by springs, to get the motor to unreel for joint extension back to default stance.
2 - While exploring the aforementioned issues with trying to unwind the pulley system from the downgeared end, I began to realize the tension spring on the far side that unreels the motor and unwinds the pulley system has to be significant. I was exploring my options when an idea hit me: what if I used straight wires lashed onto the finger like a splint on the finger joint. I could put several fine spring steel straight wires parallel to eachother say .3mm in diameter wires and have them distributed as needed around the finger parallel to the finger. Then when the motor is done actively reeling in the finger to get the finger to flex, these resistive wires will be placing significant force to straighten the finger back out because they want to return to their straight state ASAP. By doing the return spring in this manner I save a TON of space since I’m putting it snugly around the joint itself and then don’t have to put tension wires (a ton of them) into the forearm somewhere or w/e. I’m using space hugging so tightly to the finger that its space that seems unuseful until this idea came to me! So I pretty much deleted the volume taken by all the otherwise necessary tension spring wires if this idea works! I bought a large assortment of 40cm length spring steel wire off amazon to experiment and try out my idea. This could be epic! As a side benefit, these can act as additional support for the joint itself preventing sprains and dislocations of the bones and keeping everything snug and compact in a way that really helps support and aid the artificial ligaments I already have in place.
Here’s my completed V2 archimedes pulley system finally done! It is 16:1 downgearing and this pairs with my 2.77:1 downgearing on the turn in place pulley on the motor for a total of 44:1 downgearing. It is fully rigged then from motor to finger and ready to go into testing soon.
I just need to do a couple reinforcements here and there on some stuff but overall we are more or less ready to move onto setting up the return springs that my last post mentioned. So that is next. Then electronics to actuate it and test it finally! Exciting times!
Also, I have come to the realization that these straight spring wires may be perfect for forming the exoskeleton mesh shapes that create the framework scaffolding over which the artificial silicone skin will overlay. The fact it has memory and wants to return to its prior shape after impacts is perfect for this application. I’d be simply forming a grid in the shape of the muscles over the bones using this stuff and then onto this grid I would overlay the silicone skin suit. The grid can be configured to even move under the skin emulating muscle contractions to simulate real muscles moving under the skin in terms of its appearance during movement. I was originally leaning toward zip ties to make this part or nylon 3d printer filament but this spring wire may be even better due to being strong, resistive to breaking even more durability wise, holding its shape perhaps a bit better, etc. The other options I mentioned aren’t bad but I just think I might like working with spring wire a bit more intuitively. We’ll see.
Well the straight spring wire acting as a finger joint spring idea was a bust. Turned out when it bent to 90 degrees it would not return to straight again. I thought spring wire would but this stuff didn’t. This is not what chatgpt said would happen so chatgpt failed me this time. Anyways, still glad for its help when its right which is most of the time I think.
That said, I fell back to my original spring solution which was to use a 3mm diameter tension spring as the return spring. I experimented with different lengths till I got one as short as possible that would stretch out the necessary .75" roughly to accommodate the finger joint’s reverse direction counter tension needs. The shorter the spring the more it resists being pulled and also the thicker the spring the more it resists being pulled. I used default thickness from my premade tension spring order and it seemed fine and the length of the spring I cut and tested trial and error till I found a good length for my need. For my .75" draw length I went with one 1cm long spring which stretches itself out to .75" + 1cm in total without ruining itself. It seems like it pulls around 2lb of pulling force but I haven’t measured it with a scale. I fed it through bowden tubing from the place I mounted it on the motor all the way to the joint being actuated - the backside of the index finger. It’s job is to keep the archimedes pulley system and winch in place pulley taught at all times and to return the finger to full extension when the motor is not actively pulling it into a grasp position. I have not yet tested if it is strong enough to do this job but assume I’ll need two of them to be strong enough. I’ll test with just one for now and add another spring to double it’s strength if needed later.
I deliberated alot on where to mount this spring and last minute decided to just mount it on the motor it is counter tensioning since I have enough space for it there and I can just follow the same bowden tube routing the motor is using generally. This seemed easiest for me given my massive space constraints and the need for a ton of these springs to handle all the finger joints. Seems like it should work well so far.
I have decided that since I am employing tension springs to actively work against the motors in a constant tug-of-war while the motors try to grasp, I’m losing grip strength based on that. To make up for that, I’m going to use a separate motor for the distal-most fingertip joint and the second to distal-most fingertip joint rather than have a single motor do both of these joints. I made these adjustments in my CAD. I will have to change the tubing setup for the grasping tubing of the index finger to reflect this change too. This will also give the fingers even more precision and dexterity in the end - not to mention a massive boost in strength - so it’s well worth it.
I also decided to use n20 gear motors for the axial rotation of the base of the fingers instead of BLDC motors like everything else since these will only be used when doing the tiniest of micro adjustments and rarely employed - so a little gear noise once in a blue moon for this precision work on a tiny scale should not be that bad. So that’s 4 N20 gearmotors going in. These are being used just to save on space taken and pulleys needed a bit. I’m putting these 4 into the forearm in location pictured.
Next, when the spring is pulling, I noticed the TPFE guidance tubing goes from straight and relaxed to wavy under the tension. It is trying to compress under the friction which is what causes this. In the worst cases, Will Cogley’s robot hand project had this same issue and the tubing literally compacted so much near the ends that it developed wrinkles/folds where it was crushing the tubing and destroying itself under the pressure. Mine is not to that extreme but this is WHY people put metal coils around the tubing for bike brakes to prevent crushing forces onto the tubing. I don’t think I will need this but I might put it in certain places as a last ditch effort if needed later. That said, to prevent some of this compaction stuff on the spring’s tubing, I’m going to be using TWO tubes which will divide up these forces causing this by 2. Sharing the load between them evenly. So the tension spring will have two fishing lines coming off of it and two tubes to guide that line to the finger joint where it does it’s thing.
Disaster has struck:
In testing recently, I had some VERY bad news: I don’t think the spring extension idea is going to work. The amount of force required to unravel the Archimedes pulley system when working against all the friction in that system, the friction in the winch in place pulley, all the friction in the teflon tubing runs, and the magnetic friction of the motor itself while working against the downgearing (since when working in reverse direction it acts as up-gearing) is all working against the spring and I think it’s too much to ask of that spring. I can’t even really pull by hand - pulling pretty hard like 3-4lb of force it wasn’t budging. So this is tragic for my whole approach so far and we have to go back to the drawing board. A proposed massive overhaul solution in next post.
Note: The name of the resistance to turning a BLDC motor has while freewheeling (no electric applied to it presently) is called cogging torque, which is caused by the interaction between the permanent magnets and the stator’s iron core. This force may seem insignificant but due to my downgearing system, the spring has to deal with it after it has been multiplied 44 times due to the downgearing the spring would be fighting through from reverse direction at the bottom of the pulleys and traveling through what then acts as upgearing when going in reverse direction from spring’s end.
