Society of Robots - Robot Forum

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...

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.

I recently dug an old PackBot 510 out of storage and want to get it running. Ultimately, I'll probably replace the guts and build out a custom system, I thought it would be fun to get it running "stock". I have the original controller laptop with Windows 10 on it, but no software. I believe these things used "Aware 2" from iRobot. Anyone know where I could get this?

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.

Here's just a couple of my latest design drawings for my archimedes pulley system and a double stacked pulley setup.





And here are assorted parts progress for the archimedes pulley system: