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.
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.
Update: I bought several sizes of kevlar string to use to replace the PE fishing line particularly in the high tension areas that may face the highest durability challenges. I found it on amazon in many sizes. I just bought one of each size: 0.5mm diameter 50lb test, 0.8mm diameter 100lb test, 1.1mm diameter 200lb test and 1.3mm diameter 300lb test. This is probably a game changer for my project IMO.
After further consideration, I'm scrapping using the elastic cord for a bracelet idea (as a tension spring for the winch in place pulley). The point of that was to use as little space as possible but I just don't trust it. I am not sure what material it is made of and my experience with rubber bands has always been dry rot issues. I am going with 2mm OD tension spring instead. It has to stretch 12.5" and so I'm using a 12.7" strip of it to start. That feels like a snug stretch but does comfortably reach the 12.5" of stretch needed. This brings its stretched total length to 25.2". I bought 3mm ID 4mm OD TPFE tubing to be its guidance tube for this. That arrives tomorrow and then I can begin assembly.
This 4mm OD guidance tube is a bit bulky and long for the arm IMO so I will relocate it to the torso since if I use this method for other motors these 4mm OD tubes will add up in space taken up fast. The arm can't house them - it's just too much space taken at that point for these. But the torso can house them in the back or sides I think. For now, since the torso is not yet attached, I'm going to place this tube ON the string suspended from my ceiling and treat that string as though it were to torso for now.
Good news: I had mentioned before I was planning to use 2mm OD tension spring for the winch in place pulley tension solution but once I got the 3mm ID 4mm OD PTFE tubing to go over the spring, I saw that the 4mm was just WAY too big once you multiply that out to 300 motors. 300 of 4mm OD tubing starts to take up a massive area at that point and I struggled with that. I MUST be miserly on space taken up by parts to get all the Smile I need to fit in there to fit in there! Anyways, I fortunately discovered that you can buy tension spring down to 1mm in OD! I was unaware of this before now! You can find it if you search "0.2x1.5x1000mm tension spring" where 0.2mm is wire thickness, 1.5mm is OD, 1000mm is length. So I ordered 1mm OD tension spring and 1.5mm OD tension spring to test and see what seems best. If the 1mm OD spring seems reliable to me, I'll go with it. Anyways, since the spring is now smaller, I can use also a smaller PTFE tubing to house the spring so I ordered uxcell PTFE Tubing 1.8mm ID x 2.2mm OD off amazon. 2.2mm OD tubing compared to 4mm tubing is SHOCKINGLY smaller when you look at them. So it will be WAY more space efficient now.
Ok so I currently have an order for 0.2x1x1000mm tension springs stuck in customs for weeks and placed another order just today for the same in hopes it goes through faster. But at $9 for a single spring that is 3ft long, I am feeling RIPPED OFF on price. It is bullcrap. All relating to the tariff nonsense. So I decided today to pivot and just roll with the elastic band in place of tension spring. It's a jewelry making elastic band I bought some time ago in a roll. WAY cheaper at $0.03 for 3ft instead of $9 for 3ft. That's 99.7% off! Talk about a discount! The issue I had before when I looked into this option was the tie-off point. I would need a way to tie PE fishing line to the end of the elastic band without the tie point being bulky. Well I figured out a way to do it without any bulk at all! See I want this to fit into my 1.8mm ID PTFE tubing to keep size down. My solution was to just glue the fishing line lengthwise directly to the elastic band. No knot at all. No turns at all. Just literally lay it on top and glue it down flush. I figured about 6mm length of joint would be solid. And I did this on both sides with my PE fishing line. I used 0.08mm 6lb test braided PE fishing line for this. So now I have two fishing line segments coming off the end of it for double the strength of this connection. But I only wanted one piece of fishing line to go the distance to attach to the motor end so I twisted the pair of fishing line segments together and glued the twisted pair with 401 glue then cut one of the two away leaving just one of the pair to go the distance to the winch in place pulley that this is all supposed to tension for me.
I will use this string and elastic band method for now as I wait on springs and stick with this method for at least this first motor actuator setup for now. If the elastic bands don't last, we'll upgrade to the metal springs later on during maintenance or w/e.
Note: the total length of the elastic band I am using for this is 2ft and it stretches to 3ft snugly without too much force. I'm just going by feel and instinct for this measurement. If I were to go 1ft with 1ft of stretch, the stretch is more intense and the pull is harder. But I don't think I need much pull for just tensioning the winch in place pulley and I also think the more tension you place the more wear and tear on the elastic band which will shorten its lifespan. So playing it conservatively with the 2ft length selection for now.
Note: to apply the 401 glue I used an exacto knife handle with a sewing needle in place of the xacto knife blade and the tip of the sewing needle acts as my precision glue applicator.
Sometimes to get the braided PE fishing line threaded through the fine PTFE tubing can be tricky, so I came up with a neat device to assist in this. I will be making a threading tool based on a needle threader tool I've been using. It's basically a wire folded in half that you shove through a needle eye and then stick your string into its end and then pull it back through the needle eye. In my use case, I'm creating a custom one of these threading tools that will feed through my entire length of tubing till its folded end comes out the other side and I can thread my string through that end and then draw it back, bringing the string through the tubing with it.
I just ordered some 40ga copper and stainless steel wire to use to make this device in question. I'll see which metal is best. Gonna try the copper first I think.
Minor update: I have now carefully mounted the PTFE tubing that leads to the elastic string tensioner for the winch in place pulley. I mounted it snugly to the side of the PTFE tubing coming off the same winch in place pulley that leads to the Archimedes pulley system. I routed both of these using my CAD for reference in such a way that their routing will not interfere with the next motors that will be installed later. I mounted this PTFE tubing that leads to the elastic string tensioner using ONLY 401 glue which is something I've never tried before now. Usually I first wrap the tubing in adhesive transfer tape and spandex cloth wrap and coat the cloth in 401 glue but skipping that made it able to be more snugly mounted to the other tube by way of only glue. We'll see how that holds up without the other reinforcement the cloth provides etc. Seems to look so far so good though. They are in turn glued to paper soaked with 401 glue and to a little piece of stainless steel wire bent at a 90 and that wire in turn glued to the winch in place pulley mount baseplate which is itself made of paper and 401 glue. So basically everything is becoming 401 glue construction! I have some concerns about how this will hold up in the event of a fall or w/e but perhaps we can create some sort of protective cage around any delicate outcroppings like this in the future. For now I am just going for ease of construction and speed of construction to get things back on track and rolling again.
Note: The PTFE tubing that leads to the elastic string tensioner for the winch in place pulley is 0.66mm ID 1.16mm OD PTFE teflon tubing. The string coming off the winch in place pulley feeding into this tubing that will act as tensioner string tension carrier string is 6lb test 0.08mm PE braided fishing line. I was able to thread this fishing line into this TPFE tubing by hand with no issues at all very easily.
The next task will be to mount the end of this string to the 2 feet of elastic string for jewelry making and thread that into 1.8mm ID 2.2mm OD PTFE tubing and tie it off at the end of that tubing and then mount that tubing to the gray string hanging from my ceiling for now. That will conclude the tensioner mechanism for the winch in place pulley and this will usher in the next round of manual hand testing to see how much tension that is giving us. I also will be moving the tension spring mounted on the motor to align it better and shorten it more since it only moves like 4mm and so can be way shorter than it is now.
I finally finished making the tensioner mechanism for the winch in place pulley and I taped it off up the string descending from the ceiling and taped the far end of it onto the ceiling. I noticed I have to keep it as straight as possible since when curling with too much turning the elastic bracelet cord grips the sides of the PTFE tubing which could interfere with the amount of tension it brings to my winch in place pulley. So this will mean on the robot itself it will have to go from the shoulder all the way down the torso in a straight line and then down the leg to about the knee as well. It's 44" long in total. I ended up bumping up the elastic bracelet cord to 30" long to reduce the amount of tension it puts on the winch in place pulley more. The longer it is the less tension it brings and the shorter it is the more tension it brings. If it really can't fit into the leg I can cut the elastic bracelet cord in half and place braided PE fishing line in between the two halves and have that make a 180 degree turn around a pulley and thereby have the same length of elastic bracelet cord but separated into two halves mounted parallel to eachother that create a in series matching tension but taking up half the overall length. This way I could keep it out of the leg area if needed. However I think it might fit into the leg area fine perhaps. Not sure (once we get all the other motors and their tension strings that amount of 2.2mm OD PTFE tubing will start to add up.
Note: I'm also considering taking the elastic bracelet cord out of the tubing and lubing it then putting it back in since lube on the grippy elastic bracelet cord would take away it gripping the sides of the PTFE tubing some I think. Silicone lube is best for this according to chat gpt.
Note: to secure the far end of the elastic bracelet cord I used 401 glue to glue on PE fishing line onto its end the same way as we discussed before and then took the far end of this PE fishing line and came out the end of the PTFE tube with it and taped it off onto the outside of the tube. We'll see how that holds up it might need to be glued down if it gradually is pulled through the tape over time which would be no good.
The tension spring mounted to the motor setup upon further testing seems like a somewhat bad method. The issue I'm having is too much play in the tubing running between that spring and the finger joint. When tension is applied to that spring by way of the tubing, the tubing recoils and moves quite alot and allows alot of slack out to the joint so that the spring has very little involvement in the joint and doesn't really get used much period. So the full range of motion of the joint is just absorbed by tubing slack. When I tried to pretension the tubing so that the joint movement translates to the spring, the total tension placed on the joint by this became too high.
Fortunately, I came up with a much more elegant and simple solution for all of this. Basically, my plan now is to just use the bracelet cord tied point to point across the joint directly on the joint and that will be my spring for extension that counters the motor. This eliminates the need for metal springs at all which cuts costs, it also eliminates the PTFE tubing run, saving some space, and should be easier to install and easier to give precise amount of elasticity/resistance to taste. If I want more springback on the joint I can just add more bracelet cord in parallel to the first. This way I can add more resistance pretty easily.
So in recent testing a fresh issue I ran into was the TPFE tubing would start gradually pulling through its tightly wrapped tape sleeve to my surprise. Its low friction surface gradually pulls free of the tape over time. To resolve this, I decided to thread through the tip of the tubing to create a mechanical bond for the tip and once threaded through I just 401 glued down the ends of each thread onto the sleeve that was originally supposed to hold it in place to begin with. This seems to work great so far in the little bit of testing I've done since.
Comments
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.
Update: I bought several sizes of kevlar string to use to replace the PE fishing line particularly in the high tension areas that may face the highest durability challenges. I found it on amazon in many sizes. I just bought one of each size: 0.5mm diameter 50lb test, 0.8mm diameter 100lb test, 1.1mm diameter 200lb test and 1.3mm diameter 300lb test. This is probably a game changer for my project IMO.
After further consideration, I'm scrapping using the elastic cord for a bracelet idea (as a tension spring for the winch in place pulley). The point of that was to use as little space as possible but I just don't trust it. I am not sure what material it is made of and my experience with rubber bands has always been dry rot issues. I am going with 2mm OD tension spring instead. It has to stretch 12.5" and so I'm using a 12.7" strip of it to start. That feels like a snug stretch but does comfortably reach the 12.5" of stretch needed. This brings its stretched total length to 25.2". I bought 3mm ID 4mm OD TPFE tubing to be its guidance tube for this. That arrives tomorrow and then I can begin assembly.
This 4mm OD guidance tube is a bit bulky and long for the arm IMO so I will relocate it to the torso since if I use this method for other motors these 4mm OD tubes will add up in space taken up fast. The arm can't house them - it's just too much space taken at that point for these. But the torso can house them in the back or sides I think. For now, since the torso is not yet attached, I'm going to place this tube ON the string suspended from my ceiling and treat that string as though it were to torso for now.
Here is the tension spring in question from the previous post. I want this spring inside the tubing though which is not shown in the drawing of it.
Good news: I had mentioned before I was planning to use 2mm OD tension spring for the winch in place pulley tension solution but once I got the 3mm ID 4mm OD PTFE tubing to go over the spring, I saw that the 4mm was just WAY too big once you multiply that out to 300 motors. 300 of 4mm OD tubing starts to take up a massive area at that point and I struggled with that. I MUST be miserly on space taken up by parts to get all the Smile I need to fit in there to fit in there! Anyways, I fortunately discovered that you can buy tension spring down to 1mm in OD! I was unaware of this before now! You can find it if you search "0.2x1.5x1000mm tension spring" where 0.2mm is wire thickness, 1.5mm is OD, 1000mm is length. So I ordered 1mm OD tension spring and 1.5mm OD tension spring to test and see what seems best. If the 1mm OD spring seems reliable to me, I'll go with it. Anyways, since the spring is now smaller, I can use also a smaller PTFE tubing to house the spring so I ordered uxcell PTFE Tubing 1.8mm ID x 2.2mm OD off amazon. 2.2mm OD tubing compared to 4mm tubing is SHOCKINGLY smaller when you look at them. So it will be WAY more space efficient now.
Here's my updated tension spring concept drawing:
Ok so I currently have an order for 0.2x1x1000mm tension springs stuck in customs for weeks and placed another order just today for the same in hopes it goes through faster. But at $9 for a single spring that is 3ft long, I am feeling RIPPED OFF on price. It is bullcrap. All relating to the tariff nonsense. So I decided today to pivot and just roll with the elastic band in place of tension spring. It's a jewelry making elastic band I bought some time ago in a roll. WAY cheaper at $0.03 for 3ft instead of $9 for 3ft. That's 99.7% off! Talk about a discount! The issue I had before when I looked into this option was the tie-off point. I would need a way to tie PE fishing line to the end of the elastic band without the tie point being bulky. Well I figured out a way to do it without any bulk at all! See I want this to fit into my 1.8mm ID PTFE tubing to keep size down. My solution was to just glue the fishing line lengthwise directly to the elastic band. No knot at all. No turns at all. Just literally lay it on top and glue it down flush. I figured about 6mm length of joint would be solid. And I did this on both sides with my PE fishing line. I used 0.08mm 6lb test braided PE fishing line for this. So now I have two fishing line segments coming off the end of it for double the strength of this connection. But I only wanted one piece of fishing line to go the distance to attach to the motor end so I twisted the pair of fishing line segments together and glued the twisted pair with 401 glue then cut one of the two away leaving just one of the pair to go the distance to the winch in place pulley that this is all supposed to tension for me.
I will use this string and elastic band method for now as I wait on springs and stick with this method for at least this first motor actuator setup for now. If the elastic bands don't last, we'll upgrade to the metal springs later on during maintenance or w/e.
Note: the total length of the elastic band I am using for this is 2ft and it stretches to 3ft snugly without too much force. I'm just going by feel and instinct for this measurement. If I were to go 1ft with 1ft of stretch, the stretch is more intense and the pull is harder. But I don't think I need much pull for just tensioning the winch in place pulley and I also think the more tension you place the more wear and tear on the elastic band which will shorten its lifespan. So playing it conservatively with the 2ft length selection for now.
Note: to apply the 401 glue I used an exacto knife handle with a sewing needle in place of the xacto knife blade and the tip of the sewing needle acts as my precision glue applicator.
Sometimes to get the braided PE fishing line threaded through the fine PTFE tubing can be tricky, so I came up with a neat device to assist in this. I will be making a threading tool based on a needle threader tool I've been using. It's basically a wire folded in half that you shove through a needle eye and then stick your string into its end and then pull it back through the needle eye. In my use case, I'm creating a custom one of these threading tools that will feed through my entire length of tubing till its folded end comes out the other side and I can thread my string through that end and then draw it back, bringing the string through the tubing with it.
I just ordered some 40ga copper and stainless steel wire to use to make this device in question. I'll see which metal is best. Gonna try the copper first I think.
Minor update: I have now carefully mounted the PTFE tubing that leads to the elastic string tensioner for the winch in place pulley. I mounted it snugly to the side of the PTFE tubing coming off the same winch in place pulley that leads to the Archimedes pulley system. I routed both of these using my CAD for reference in such a way that their routing will not interfere with the next motors that will be installed later. I mounted this PTFE tubing that leads to the elastic string tensioner using ONLY 401 glue which is something I've never tried before now. Usually I first wrap the tubing in adhesive transfer tape and spandex cloth wrap and coat the cloth in 401 glue but skipping that made it able to be more snugly mounted to the other tube by way of only glue. We'll see how that holds up without the other reinforcement the cloth provides etc. Seems to look so far so good though. They are in turn glued to paper soaked with 401 glue and to a little piece of stainless steel wire bent at a 90 and that wire in turn glued to the winch in place pulley mount baseplate which is itself made of paper and 401 glue. So basically everything is becoming 401 glue construction! I have some concerns about how this will hold up in the event of a fall or w/e but perhaps we can create some sort of protective cage around any delicate outcroppings like this in the future. For now I am just going for ease of construction and speed of construction to get things back on track and rolling again.
Note: The PTFE tubing that leads to the elastic string tensioner for the winch in place pulley is 0.66mm ID 1.16mm OD PTFE teflon tubing. The string coming off the winch in place pulley feeding into this tubing that will act as tensioner string tension carrier string is 6lb test 0.08mm PE braided fishing line. I was able to thread this fishing line into this TPFE tubing by hand with no issues at all very easily.
The next task will be to mount the end of this string to the 2 feet of elastic string for jewelry making and thread that into 1.8mm ID 2.2mm OD PTFE tubing and tie it off at the end of that tubing and then mount that tubing to the gray string hanging from my ceiling for now. That will conclude the tensioner mechanism for the winch in place pulley and this will usher in the next round of manual hand testing to see how much tension that is giving us. I also will be moving the tension spring mounted on the motor to align it better and shorten it more since it only moves like 4mm and so can be way shorter than it is now.
I finally finished making the tensioner mechanism for the winch in place pulley and I taped it off up the string descending from the ceiling and taped the far end of it onto the ceiling. I noticed I have to keep it as straight as possible since when curling with too much turning the elastic bracelet cord grips the sides of the PTFE tubing which could interfere with the amount of tension it brings to my winch in place pulley. So this will mean on the robot itself it will have to go from the shoulder all the way down the torso in a straight line and then down the leg to about the knee as well. It's 44" long in total. I ended up bumping up the elastic bracelet cord to 30" long to reduce the amount of tension it puts on the winch in place pulley more. The longer it is the less tension it brings and the shorter it is the more tension it brings. If it really can't fit into the leg I can cut the elastic bracelet cord in half and place braided PE fishing line in between the two halves and have that make a 180 degree turn around a pulley and thereby have the same length of elastic bracelet cord but separated into two halves mounted parallel to eachother that create a in series matching tension but taking up half the overall length. This way I could keep it out of the leg area if needed. However I think it might fit into the leg area fine perhaps. Not sure (once we get all the other motors and their tension strings that amount of 2.2mm OD PTFE tubing will start to add up.
Note: I'm also considering taking the elastic bracelet cord out of the tubing and lubing it then putting it back in since lube on the grippy elastic bracelet cord would take away it gripping the sides of the PTFE tubing some I think. Silicone lube is best for this according to chat gpt.
Note: to secure the far end of the elastic bracelet cord I used 401 glue to glue on PE fishing line onto its end the same way as we discussed before and then took the far end of this PE fishing line and came out the end of the PTFE tube with it and taped it off onto the outside of the tube. We'll see how that holds up it might need to be glued down if it gradually is pulled through the tape over time which would be no good.
The tension spring mounted to the motor setup upon further testing seems like a somewhat bad method. The issue I'm having is too much play in the tubing running between that spring and the finger joint. When tension is applied to that spring by way of the tubing, the tubing recoils and moves quite alot and allows alot of slack out to the joint so that the spring has very little involvement in the joint and doesn't really get used much period. So the full range of motion of the joint is just absorbed by tubing slack. When I tried to pretension the tubing so that the joint movement translates to the spring, the total tension placed on the joint by this became too high.
Fortunately, I came up with a much more elegant and simple solution for all of this. Basically, my plan now is to just use the bracelet cord tied point to point across the joint directly on the joint and that will be my spring for extension that counters the motor. This eliminates the need for metal springs at all which cuts costs, it also eliminates the PTFE tubing run, saving some space, and should be easier to install and easier to give precise amount of elasticity/resistance to taste. If I want more springback on the joint I can just add more bracelet cord in parallel to the first. This way I can add more resistance pretty easily.
So in recent testing a fresh issue I ran into was the TPFE tubing would start gradually pulling through its tightly wrapped tape sleeve to my surprise. Its low friction surface gradually pulls free of the tape over time. To resolve this, I decided to thread through the tip of the tubing to create a mechanical bond for the tip and once threaded through I just 401 glued down the ends of each thread onto the sleeve that was originally supposed to hold it in place to begin with. This seems to work great so far in the little bit of testing I've done since.