Biped ankles, & The hand of Zychor - WIP

[Slugman]

Hi everyone. After converting my 22 DoF AX-12 hex to a biped, I have come up against the old "Ankle servos getting too hot" problem. I have worked out a simple hinge & lever design that would reduce my available foot tilt to + or - 20 degrees & increase the holding torque significantly, but that leaves the lever only connected to the servo-horn by one 2mm bolt. Basically, the servo pushes a hinged lever down (23mm centre to centre), which is connected to the foot, while the foot tilts on a hinge set 16mm off to one side, & approx 23mm below the centre of the servo-horn.

Apart from the increased 'slop' due to the tolerances in the lever & hinge connections, is there any point to me bothering to make it? I.E. Have any of you done this already & found the problems?

I have access to a lathe, so if the connection to the servo horn needs strengthening, then I should be able to make up a little plate to allow all four servo-horn bolts to be used, with the lever bolting seperately to the plate, but I guess my question is more about the basic mechanics of the system - Will it, or 'should it' work? - I am trying to play "Devils advocate" to make sure I have covered all potential issues.

The reduced range of movement should be more than sufficient for walking on flat ground, while the available 20kg/cm holding torque when the foot is level, & much higher when the foot is at 20 degrees tilt, should prevent the servo overheating. Disclaimer - My torque calc's may be wrong! The foot doesn't need to be tilted fast, so lack of speed shouldn't be an issue either, & it doesn't make it that much taller compared to the normal plastic brackets. The extra weight will be low down, & will only be a few grams.

PS DarkBack - Yes, I know that it really means that my robot is too fat......... I WANT to carry more uneccessary weight, while being able to maintain static stability in the entire stepping motion. I hate shuffling, so replacing the servo with a hinge & two opposing springs is not my preferred option.

[darkback2]

PS DarkBack - Yes, I know that it really means that my robot is too fat......... I WANT to carry more uneccessary weight, while being able to maintain static stability in the entire stepping motion. I hate shuffling, so replacing the servo with a hinge & two opposing springs is not my preferred option.

Who said your robot is fat?

Currently I have tried to take the load off of my bipeds ankles using springs...Levers are always an option (especially if they work!)

Seams like the only way you will know for sure is if you try. Who knows maybe we will all be using the "slugman levered ankle design" by this time next week.

Good luck.

DB

[Slugman]

Yeah, I looked at using a spring on the outside of the foot to take some of the weight, but thought the lever might be a better option with a similar weight penalty. I was just hoping someone had already done it & could give me a few pointers on what they have already learned. Dunno about next week - Given how long it took me to make my prototype mech warfare machine gun, it could take a while.... Not so sure about trying to claim the design either. I am pretty sure levers have been used once or twice before in the history of robotics.

FYI, the gun prototype came in at over 400 grams on its own, & I have been too worried that I might blow it or the Arbotix up, so I haven't tried to get it to work - Need to test controlling cheap micro-servos first, before I try with the high-amperage motor & speed controller. I ended up ordering three of the good gearboxes on 16th June instead - Hope they turn up soon.... Dang snail-mail.

[Kar]

Hi Slugman

I dont know much about bipeds but if you could post some pictures of you Ankle servo set up with changes and with out. I will look over them and see what we can come up with. Im prity good with this sort of thing. Not realy shaw what your trying to do but i hope some pics will clear this up then i can offer some thoughts to what could be done.

[Slugman]

Well, I'll try to attach a picture of what I was thinking of making - the sketchup drawing I made to work everything out. Currently the weight of the robot is supported by the join between the two servos, so I might have to change where the hinge plate connects to the ankle, so the weight is more evenly distributed.

Anyway, since I have been too slack to start cutting the parts out yet, comments, suggestions & critiques are more than welcome. At the moment, all I am sure of is that the max & min angle of the foot is enough for walking. I am going to cut the new parts out of 1mm stainless & bend & drill them. Yes, it's a little heavy, but I need the strength.

[Kar]

Ok thanks for the pictures. Now that I can see what you are try to do.


Firstly the hinged idear. I realy dont think this is going to work. The reson why is beause you are making your servos work like a crank pushing a con rod. From what I under stand about bipeds is, that you are a lot better of keeping to the center's of your servos. Because you are no longer using the center line as a tiping point you will start to have other problems I think down the track. I think one of the bad sernarios could be rather then the leg tilting to the side the foot might lift up on the out side.
It also gives you to many hinged points. The more hinged points you have the more problems your going to have. Specily when there not central hinged points.


Ok I recon the best thing to do and you have probly been told this by other's is use bigger servos.
Sadly I have just change my desktop from window to linux and I dont have any good software to ezely draw up some iders for you to see what els you could try.


Ok I hope that makes sence for you. I do suffer from dyslexia and if your not shaw what that is google it. LOL


Thanks
Garry

[Slugman]

I wouldn't worry about the spelling too much - It's certainly not as bad as some that post on here, & yes, I understand. It's one thing that got drummed into me as a kid - Listen to the message, not the messenger.
Good point about the foot tilting rather than the leg, although I was thinking of making the hinge on the outside of the foot, not the inside, so there should usually be more weight over the crank side of the foot. I might have to make the foot wider, so the hinge is closer to the centre line of the foot surface.

I understand your point about moving the hinge away from the center line of the leg, but I think I will try to make this anyway, just to try it out. I should be able to adjust the walking sequence to account for the change in the way the leg tilts, as I am using poses, not Inverse Kinimatics.
I am currently working out how to draw up a file to send away to a CNC / Water Jet company to cut out small bits of steel for another project, so I think I will add these in since the extra cost for the ankle parts would be quite small compared to the total cost to get the other items cut out.

[Slugman]

Hi everyone. As you might remember, I am working on a changed ankle & hip design to try to ‘fix’ the overloading of the servos for bipeds. Unfortunately I have run into a problem I didn’t foresee, & was hoping for a little assistance – I need a .dwg file format for the laser-cutter business before they can cut my design out of 0.8mm stainless. I wanted to share the file on Trossen, as I have nested the ankle parts with all the parts required for my experimental hand – It uses the same basic principle as the MechaTE hand, but in AX12 scale with moving thumbs… I have been calling it “The Hand of Zychor”, since that’s the name of my ‘bot & I have delusions of grandeur. >:P It should work in any configuration, from two to five fingers.

I drew everything up in Google Sketchup & used a free CAD program called DoubleCAD to open it, then saved it to a .dwg file. Unfortunately the CAD program has cut lines out of the original – According to the Laser cutting company down here it is “Heaps of them” & although I only found two large ones, they could be right, as the CAD program is really abrupt with zooming in – One click on the mouse wheel & you are close, & one more click & you are past where you wanted to be. I am afraid I am not good enough with CAD to work out what the %&^&* I am doing with it! Unfortunately the program was free, & I seem to be getting my moneys worth. *grumble grumble, whinge* It’s not their fault, but definitely mine. *sigh*

Could I please beg, grovel & generally plead with the many CAD experts here to have a quick look at it & let me know just how bad it is? & if they could fix it, I am more than willing to make whatever they want in Sketchup as payment, as I am not too bad with it....

[elaughlin]

Ill take a look at it. It does look pretty strange to me. It seems to have one continuous line through all of the parts... and like you said, some lines are missing. I use autodesk inventor so I can make it into a new dwg or dxf file if you want for the cutting service if you want to send me the CAD file. Can you convert it into a STEP file or anything like that?

This is what it looked like in the Autodesk DWG Viewer program.

[Slugman]

Oh, I thought that since it was going to be cut out on sheet metal that they would need it to be in one piece, so I removed the lines of each individual part while it was in sketchup. I wasn't sure if the lines would confuse the laser cutter or not - I.E. Didn't know if the cutter would try to cut along the line, so I removed them. Unfortunately the conversion between sketchup & .dwg removed some lines as well.
I'm at work now, so I will have a play this evening. The image you posted looks practically identical to what I get to see with DoubleCAD. I'll put the lines back in sketchup at lunchtime so the individual bits are complete, then tonight I will see if I can change it to a STEP file. Thanks for your time. I really hope this works - Both the ankle & the hand!

OK, just had a few quiet minutes at work, so the sketchup file is now fixed - All the seperating lines are back in.