artbyrobot

[artbyrobot]

Testing Multiple Downgear Methods at Once Decision

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.

[artbyrobot]

Pulley System Testing Shortcuts and Design Tweaks

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.

[artbyrobot]

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.

[artbyrobot]

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.

[artbyrobot]

Dual Direction Motor Use Proposal

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...

[artbyrobot]

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.

[artbyrobot]

Tension Spring Install for Robot Finger Extension

[artbyrobot]

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.

[artbyrobot]

A couple discoveries were made today.

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.

[artbyrobot]

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!

[artbyrobot]

Archimedes pulley system update

[artbyrobot]

Double Stacking 1x3x1mm Bearings for More Load Capacity

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.

[artbyrobot]

Avoiding Back EMF Feedback Need for BLDC Motor Controller

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.

[artbyrobot]

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.

[artbyrobot]

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.

[artbyrobot]

Ok so my belt drive system from my last update just is not quite up to par in terms of grip and anti-slippage. So my new series of changes are planned out and underway now. First, I will be bumping up the height of each pulley to 2mm up from 1.1mm. This will double the surface contact area for way more belt grip in and of itself. So then I can use a 2mm wide belt. Next, I'll be increasing the drive pulley diameter to 1.5-2mm additional diameter. This will also greatly increase surface contact with the belt for more grip. Then finally, I'll be using a commercial belt that is said to have the highest grip of all belts - its called a polyurethane belt. It is a flat belt with 2mm width and .9mm thickness. It should be a huge upgrade to my current setup! The best part is you can customize the diameter of the belt by melting the two ends together! This was a key thing I did not know! So I can create just the right size and it should be perfect! I can also double these up by melting two belts layer by laer for a 1.8mm thick square shaped belt that is even less stretchy and so can be even more able to tightly grip my pulleys. I'm very excited about this and think it will take us to where we need to be *crossing fingers*.

[artbyrobot]

Grooved outer race of bearing for pulley improved