Society of Robots - Robot Forum

I wound up my 6lb test Hercules PE braided fishing line onto the previous pulley system setup only to find out that the pulley could only handle about 21 inches of fishing line wound onto it before it started to come dangerously close to overfilling the pulley. The aim is to have plenty of the plastic disc overlapping the fishing line even when it is wound up fully to one side because that plastic disc acts as the guide to keep the line in its proper channel. I want at least 32:1 mechanical advantage out of this downgearing so if I want my final output to be 1" then the first pulley has to be able to wind 32" of fishing line onto it comfortably. So I realized at least the first pulley has to be a few more millimeters increased in diameter. So I had to rebuild the thumb tacks arrangement to accommodate these changes and make that first pulley bigger.





With this increased size first pulley, I realized I'm getting what looks to be 7:1 mechanical advantage from just the first pulley alone! At least initially when it starts. As the fully wound up pulley gets winched in by the motor, the relative size differential gets smaller which means it will speed up and the torque will be less than the starting torque and increasingly so as the size differential decreases. This will create a natural sort of acceleration effect and high initial power and gradually less power. I think these side effects of this system seem to be quite good but I'll know for sure in testing. The next steps will be to wind up the reverse direction of the first pulley and start connecting the first pulley to the second pulley and so on. I may not even need all 5 pulleys but we'll see. With the first pulley being already 7:1, if the remaining 4 are 2:1 say, then we'd have 7:1, 14:1, 28:1, 56:1, 112:1 so 112:1 would be the final output. That seems quite overkill and perhaps will be too slow. Although very strong. The motor outputs about 0.42 lb on average so .42*112= 47lb! Now the lever of the joint itself makes you lose mechanical advantage due to the fulcrum location etc so it would drop down to say 15lb but my finger individual joint flexion power is only like 5-7lb so that's double mine. So a bit overkill. So I might skip using one of the 5 pulleys. Having it there is nice though just in case we wanted to trade speed for power for some of them we'd then use that one as an optional strength boost we can tap into in the future if we want to trade speed for strength so I might just leave it in the design even if I don't use it just yet. In testing I may find I prefer to use it afterall. Nice to have that option if needed.

Here is a progress update on the silent pulley downgearing system I came up with using thumb tacks and a #2 fishing crimp sleeve and little plastic discs. It is some tiny fine precision necessary work but I'm getting it done and things seem to be looking pretty good so far.





For now, I ended up just using 401 glue to glue the thumb tacks down onto post it note paper. I then put another coat of the glue over the tops of the thumb tack heads to secure it further. I am planning to use nylon upholstery thread lashings to lash all the tacks down onto the top of the 2430 bldc motor tightly and glue the lashings down as well in order to make the thumbtacks even more solidly set into place. Now I'll grant welding them down would be ideal, however, not having a micro tig welder made yet (future project), I just wanted to get going fast and I thought with enough care, it is possible these can be constructed solidly enough with composite material techniques to function reliably. I'm crossing my fingers. We'll see.

I'm currently working to sew all the finger and wrist bones together for the Dinah robot and mount them to the arm. I wanted to show how I'm doing this process.


Here I tape the bone with adhesive transfer tape 3M 300 LSE. Note that I leave space on either end of the bone to allow some free fabric which is necessary to allow for elasticity as the bones need to rotate after all. Have to have enough free fabric to stretch as the joint rotates, allowing the rotation. But not so much free fabric that the joint is loose either. Has to be just right and snug.




Next I wrap the compression workout shirt fabric onto the tape and cut it to size. Here is just a rough wrap that hasn't been cut down to size yet.




Then here is a bone finished on the sewing and ready to attach to a neighboring bone. The sewing is done with nylon upholstery thread and a curved suturing needle and a surgical pliers using a suturing technique.

In these photos you can see the progression of going from the stock wrist to an axial rotating wrist assembly acting as a plain bearing. Pardon the Orgrimmar welding - it's a cheapo welder.








The process involved cutting the bolt head off then grinding smooth the threads and then sliding on a stack of washers and welding the last couple washers into a mushroom head end stop then welding the other washers to eachother and these ones are to spin freely. They will do the pronation and supination. This replaces the need for a ulna and radius for that purpose, simplifying the skeleton some.

The metal outcroppings I left on the washers were meant to jut out significantly to give the fiberglass something to bite onto well for a dependable attachment.

Note: The stock skeleton does rotate axially already at a spot just near the elbow but that rotation is stiff and requires significant force to get it to move and loosening it is something I don't know how to do. I don't even know how it works at all. Advice on that for future reference would be helpful.

Note: There is too much clearance on the stack of washers so they can slide distally or proximally a good 8mm which is not okay - too much play. I need to fill that gap and lube it all with white lithium grease.

Note: I'm planning to probably just go fiberglass wraps over and over onto the stack of washers to grip it tightly and build outward from it and then go out and around the welded mushroom cap and then wrap onto the ABS wrist ulna/radius fused section.

Also note that I have reconsidered adding a dual hinge to the elbow joint and lean now toward just leaving it stock. I think the double hinge would add complication to the bicep attachment and cause some issues I'd rather avoid. A single hinge is easier to deal with IMO.

The Dinah robot is coming along well. I modeled the full steel skeleton in CAD to match the dimensions of the Dinah base mesh and created a human bones variation as well to compare that to the steel simplified skeleton and make sure all the joint pivots matched the locations of the human skeleton joints pivot points. With this CAD, I will be able to modify the proportions of the steel skeleton I have on hand. I also added several key additional joints using reference photos of a skeleton I found online. For example I now have 2 pivot points for the knee joint instead of one which gives more clearance when knee bends back. I also gave a few more degrees of freedom to the neck and shoulder area. Here's the CAD:


Note also that I am well on my way to finishing up printing ABS solid infill fingers and wrist bones which I will retrofit onto the steel skeleton so that I can have full 27 degrees of freedom robot hands and wrists to match perfectly the dexterity of the human hand, which is a must.

I have decided that once I finish the arm and head, I will not go on to complete the building of the rest of the robot's body but instead will switch my focus to the AI entirely from there forward. I will code the AI to cause that arm and head to build the rest of its own body.