My Advanced Realistic Humanoid Robot Project

darth0 thanks for the pointers! Chat gpt did mention some of that but I never heard about "Springs also arent meant to be stretched out for prolonged periods or they will loose tension strength." --- that is really important to know for me.

As far as springs wearing out over time, that's unfortunate but not in my control. Fortunately, I intend my robots to do their own self maintenance to hopefully that's won't be my problem. I wonder which will go out first, the string or the springs or the tpfe tubing.

darth0 I think the setup I posted a photo of is going to work. But thanks for the links and additional information. I am not of the opinion that I have a spring problem currently but believe I have that problem already solved to my satisfaction now.

Ok so a few minor updates:

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.

I think I've solved it! So first, I want real force working on the extension aspect, not some wimpy spring. I already said there's a lot of frictions that extension system has to bust through to work. And I'd hate to have a very strong spring anyways since when grasping, the motor would then be fighting against a strong spring for extension which is a huge inefficiency that works to weaken the grasping action significantly at that point which is bad design frankly. So we want IN DEMAND opposition for the extension rather than a constant opposition of a spring fighting against the grasp attempt of the motor. We also want the motor that does the grasping to actively rotate in reverse direction rather than freewheeling in order to not have to fight it's static friction caused by its magnets which is significant. This means we either have to go with a two motor system - one for grasp direction of the joint and one for extension direction of the same joint (HORRIBLE WORST CASE SCENARIO BUT POSSIBLE IN A PINCH) or we need to go BACK and refute the notion that the motor is unable to operate two separate pulley systems for extension and grasping functions coming from a single motor attached to two pulley downgearing systems. Which would entail the motor turning clockwise to create grasping and counter clockwise to create extension. The problem with such a proposed system is that in theory it was said to be impossible due to the inevitable derailment issues and tension issues that this would invite. I am proposing we tackle those issues it invites head on rather than avoiding them entirely like we were trying to do for quite a while now. It is a VERY tall order to get that to work but that would be the best possible scenario IMO. It is great if we can get it to work since we tap into the full power of a single motor to do both flexion and extension and we then kill two birds with one stone. All the friction issues with the tubing and pulleys is solved by the motor when it reverses directions and actuates the opposing pulley system. We just have to have slack in the line due to the different diameter mismatches of the two different winding directions we face and also have to have that slack pulled taught by some mechanism to prevent slop that causes derailments. I really want to press for that HARD now. But to do that I really have to scrap the winch in place pulley idea basically I think. Well not necessarily - even that I think can be worked out but is higher risk and harder than my current favorite new, novel solution. So we can reattempt winch in place stuff perhaps in the future but I want to set it aside for now. My newest idea is for that first large run-out downgear to be 2:1 and use regular Archimedes pulley system approach but to put that pulley into the torso and have a weight hang off the bottom of it or have a VERY tiny motor attach to the bottom of it that is to place tension onto it regularly to remove all slop. This can be a motor the size of my pinky fingernail perhaps (not sure though). OR a weight. I lean toward using a weight now since that would be easiest I think to pull off. I got the weight idea from studying the cable machine for triceps at the gym the other day. I can have the same type of weights or something similar to those used by gyms. But doesn't have to be adjustable like those but same concept.

Granted one downside to this approach is what if the robot is laying down or upside down wouldn't it not have weight able to pull down by gravity then? So to solve this I can have 3 weights perhaps, one for each possible direction: upright robot, upside down robot, laying down robot... actually 2 weights should be fine: laying down and upright. Hmm... well if he's laying on back or stomach the weight would have to pendulum or slide past a central point to the other side of robot on a track. Yeah that should work! So 2 weights I think can do it. If upside down he's screwed we'll say. He won't use fingers in any direction change way until he flips back around upright or sideways if doing a cartwheel or handstand for a bit. That is a fine tradeoff. Right now I'm thinking a straw with lead tube in it as the weight or something like that. Even considering just using a fishing sinker perhaps at the moment. Have to think on this more...

Here's the official design drawing of this proposed single motor actuating both forward and reverse directions with two separate Archimedes pulley systems opposing one another. You'll also note that the left hand side of the drawing has a pair of Archimedes 2:1 pulley downgear systems, one for forward and one for reverse directions of motor and these two are going to be very long (16 inches long) and therefore are located in torso. The remaining 16:1 Archimedes pulley downgearing systems will be kept in the forearms near to the finger joints they are actuating as we had planned originally and already have in place.

You'll also note the weight that hangs off the bottom of both of the 16" long 2:1 pulley downgear systems that can keep them both taught at all times despite their varying lengths that will always be changing. The weight is able to slide since it has a fishing hook eye above it and on both attachment points to the 2:1 pulley downgear systems so it is always adjusting these 3 fishing hook eyes to always keep tension on both systems freely.

Also, I recently stumbled upon a VERY much simplified version of my miniature pulleys. So up to now I've been using 1x3x1mm ball bearings to make tiny pulleys and been variously perfecting this approach but it is still not THAT small and is a bit complex to make and we have to make literally THOUSANDS of these to do the whole robot. That presents a bit of an issue due to the large work that requires. At least until mass manufacture of them comes in one day perhaps. But while DIYing that, it's alot to deal with making SO MANY somewhat challenging to make things. That said, my proposed EVEN MORE miniature and WAY WAY WAY simplified to make pulley is to just use a single fishing hook eye. Literally, that's it. I can use a tiny fishing hook eye and use that as my very first pulley for the 2:1 16" long Archimedes downgearing systems in the torso. This will cut down on size taken dramatically and complexity of its build. It will make the pulley basically failure proof too. The way it will EVENTUALLY fail is by the rope rubbing it enough to cut it in half. But I think the rope would fail before the pulley would fail and so that doesn't matter then. You'd replace them both at once on routine maintenance. No need then to worry about that eventuality. And the ridiculous ease of manufacture of such a simple pulley makes replacing it trivial. I also think that using this just in low load, high speed, low force early pulley downgearing stages is a non-issue since the friction with such a low load on the first downgear or two will be so trivial that the string itself would fail WAY before it would slice through the metal (acting like a saw over time). I think it would take literally MANY years due to the super low friction at these low forces. Now I'll still use the ball bearing style for later stages of downgearing where the loads go way up, but for the first stage or two I think this will work just fine.

So the idea to move a portion of the pulley system stuff over to the torso is now out because I've been kind of talked out of it so I'm putting that aside for now. Going to actually try to do that stuff within the forearm. Also instead of a fishing sinker I'm going to try to use an elastic cord made for making bracelets for kids. I think that will be enough force just to keep tension on the line that is being unreeled. Doesn't have to be much I don't think.

I'm also considering just hand testing my pulley systems for now. So disconnecting them from the motor shaft entirely so I can just do testing to see how things feel and can observe things easier way quicker and with less hassle. And when I do go to test by way of motor, I'm just going to use a brushed motor and connect a lab power supply by hand with alligator clips so I can avoid messing around with microcontrollers and firmware and custom motor controllers entirely which is a bunch of rabbit holes I want to avoid as I just secure testing my pulley designs for now. I don't want to get hung up in a year or two of electronics stuff just so I can test my pulleys which would be so stupid and annoying. I need to get my testing iterations done as soon as possible without distractions and longer delays. Once I am happy with the pulley's performance and they pass all my tests and everything seems solid then we'll go ahead and connect it back up to the BLDC motor and then will worry about the custom microcontroller and custom motor controller and all the firmware or whatever at that time and will be doing that with the confidence of a big win with the pulley systems giving us momentum as we enter into those rabbit holes of electronics.

Ok so a quick couple updates.

First, since the ideas for downgearing with pulleys have been coming in fast and furious, ways to do it easier or ways to fit it here or there or what have you, it's getting a bit scattered and I'm now starting to tear down my work too much for my comfort. It's like I'm chasing the next shiny new approach a bit overly now. So I decided to stick to the current approach as long as it is viable enough to be "good enough" so as to not waste my hard work anymore as I was starting to do. For example, the pulley system I was testing with a 10lb dumbbell did not need to be torn down and rebuilt I don't think. Stuff like that is starting to cripple progress in some sense. So my new approach is when I come up with a idea for a possibly better downgear implementation, I will just write it down and put it in a queue. Then on the next joint actuation I will use these. This way I can have like 10 different downgearing approaches over 10 joints and I can compare and contrast them, note the pros and cons of each, and over long term testing I can find the clear winners. This will also give me a greater understanding and experience and take more out of so much guesswork and into more concrete and tested territory on this stuff.

A side benefit is that people tend to think I've progressed zero with pulleys since I keep building them then taking them apart and starting over. At least under this new approach, I get joints done and over with and working before building the next downgear iteration so the progress feels more tangible and the robot gets done rather than just being in iteration and tear-down cycle hell where it appears from the outside like I am not actually accomplishing anything. So that part will be nice.

Another cool development is that I realized I can put a pulley downgear inside a tube. Normally up to now I was exiting the guide tubing to do a downgear and then afterward the string goes back into tubing to go to wherever. But I realized particularly if doing a fishing hook eye downgear that the entire downgear phase of that can fit into a tiny tube and that has some nice perks. For example, if the 2:1 downgear is the first downgear right off the motor, and the motor is reeling in 32" of string, that 2:1 will be 16" long. Well now that I can do my first 2:1 downgear all within tubes, I can run the downgear from the shoulder to the wrist, giving me PLENTY of room to deal with that amount of runout. This is quite exciting and just gives me more freedom and flexibility. I might do something with this for the first couple downgears so a 2:1 downgear pulley #1 and a 4:1 downgear pulley #2 but then do the rest in the forearm as initially planned and most likely using ball bearing based pulleys for the more heavily downgeared higher force phases of the downgearing process.

That all said, I have the downgear system of 44:1 downgear now done and attached to the finger fully and the extension spring attached to the extension side of that joint fully. So I am ready to begin testing and see how much that spring fails to extend the joint due to friction and motor magnetic cogging issues. I will then add more and more springs until it works. That is my solution. Yes, those springs collectively are fighting the motor when the motor goes to actuate grasping, however, that is just a concession we have to make with this design. Other downgearing designs that don't involve springs for that aspect but involve bidirectional motor actuation with pulley systems for either motor direction are coming next. But I'm finishing the spring based design I was talking about for some time now rather than scrapping it as I was planning of late. It is not THAT bad and it deserves to be at least tested and shown the light of day. It would be a shame to waste that work. It was good work. Also, I realize it MIGHT be the best solution. My theory says no but I can be wrong. Testing is the only way to know 100%. So it's worth keeping it as one of the downgearing methods I'll be testing out.

Ok so I did a big refactor of my pulleys and ran a test again and it still is not working. The first set of archimedes pulleys tops out and can't move anymore while the finger still hasn't moved. This is because of slack in the lines. I did not calculate slack in the lines into my calculations at all and am shocked by how much there is... Rather than do a major new overhaul with new math and new draw distances on every pulley AGAIN, I'm going to just drop the final pulley of the system so instead of 44:1 it will be 22:1 now. While we cut half the grip strength with this move, this might be okay after all. It still gives us 20lb of of burst grip strength I believe and 11lb of casual easy sustainable grip strength. Most common tasks should only require 8lb of grip strength anyways for a single joint tops. Because remember, I'm not doing a single motor for all 3 finger joints but one motor per joint which helps alot in the strength department and control department. So anyways, this hack I think is okay also because it hit me lately that I highly doubt I'd use the full beast mode burst strength of a 44:1 downgear anyways. I'd be too worried about the wear and tear on the fishing lines and pulleys and maintenance times getting too short between maintenance overhauls if the robot is using that level of grip strength for tasks. In reality, I am now imagining I will only let the robot do VERY minimal strength stuff to reduce the maintenance to a minimum. Like sewing, cutting, and delicately picking up small loads. I will treat it like it has the strength of my 4 year old just to baby it and make it last longer between repairs. Kind of like having a old beater car you don't trust and never throttling the engine hard but just gradually easing on the gas pedal to avoid blowing a gasket so to speak and avoid a trip to the mechanic. So that said I think 22:1 might actually be okay. And with that final pulley out of the way, I'll have WAY more than enough draw distance to bend the finger 90 degrees and account for string slack AND account for string stretch over time without any issues at all. Much better. Not to mention we do pick up speed this way and that might be a VERY nice feature when all is said and done. A faster moving finger can speed up its work I think. Like notice how 3d printers go way faster and that speeds up prints. So speed might be king over grip strength in the end perhaps. It's a tradeoff.

Another update is I realized I can wind a second very fine 0.08mm fishing line on the output portion of the winch in place pulley mounted next to the motor and this second line coming off that pulley will be attached to a tension spring consisting of a bracelet jewelery making cord for jewelry for kids. This line will maintain tension on that winch in place pulley and the motor output shaft at all times to prevent derailments. The metal tension spring that extends the finger will then have the help it needs to keep the whole system taught. That's the plan anyways. The runout of this line will need to be 12.48" of tensioned draw. To achieve that I need a length of this cord of about 15" I think which stretches to 27.48" at full extension. This would occur each time the motor causes the grasping actuation and it would be playing tug of war with the motor so that when motor relaxes or reverses direction, the winch in place is remaining tautly in opposition.

A note on fishing line durability. I think this was mentioned in passing quite a few times but at some point, a mention of durability concerns becomes that final mention that makes you really start to question it more and so I finally did some research on chatgpt and found out a human finger joint probably will actuate like 1-2k times per day which blew my mind since that would add up to like 10-30k times per month and so basically, once a month the main finger fishing lines will likely need replacement. I looked into alternative materials but didn't have much luck. So what to do?

Well after thinking about this a fair bit, my conclusion is to just shrug and move forward as it stands with the fishing line approach. I'll treat them as a consumable. My plan now is to just expect 1 hour of maintenance for every 20 hours of runtime. Or maybe to be more conservative, lets bump that to 1 hour of maintenance to every 10 hours of runtime? Maintenance will involve redoing pulley systems with fresh fishing line, or swapping in full new pulley systems to replace older ones every so often. It can have a pre-emptive maintenance schedule. My intention is that one robot will maintain his neighbor and the two will have a buddy system of maintaining eachother. Once I have expected time to failure of fishing lines established, they can swap in new ones automatically to prevent failures from happening during work times.

I don't think this is too bad of a deal or a deal breaker. Yes, hopefully materials advances will give me a better string one day, but for now, I'm okay with this maintenance scheduling thing. As long as its all automated, I think this is fine.

After all, our bodies muscles constantly need repairs and they grow from the repair process. So what do you expect for artificial muscles that can't self heal? Maintenance has to be a regular thing IMO.

Quote from darth0

1 hour of maintenance per 10 hour of work ( if they manage to repair their pulley system in 1 hour) is an awful "efficiency" rate in my humble opinion.

Have you thought about making a custom rope for your pulley system. Using a strong, durable material that doesnt strech. Something like Kevlar fiber rope comes to mind.

Well first they wouldn't have to do a particular repair in under an hour. It could be 8 hours of repair work in a single weekly session or w/e. The point is a 1 hr repair time to 10 hour of work time ratio, not schedule 1 hour sessions after every 10 hr work session necessarily. That said, it is not ideal I agree, but I'd be HAPPY with it honestly. I'm still getting 90% uptime.

That said, yes, the kevlar rope idea I think is a great one. I now plan to incorporate that. Someone else had the same idea yesterday and I ordered a bunch of different size kevlar thread. Didn't know you could buy kevlar in thread form on amazon. I was not able to find the finer thread sizes for all of it though. Like 0.08mm size kevlar I didn't find. So I might still be using the braided PE fishing line for those sections.

Anyways, very excellent suggestion and feedback darth0