Tuesday, 31 July 2012

Spider Walker Project

So I saw a video of a spider walker that someone made and was driving around at burning man, and I thought it would make a cool project to make a remote controlled miniature version.




The design will be somewhat based on the stick-figure animation shown on http://www.matrix.mechanicalspider.com/ as this gives a simple system which should be fairly easy to replicate.
This will be made on the laser cutter in order to machine precise "layers" and potentially can be driven using some micro gearmotors I have lying around, although these are small and potentially too weak to drive it.

In order to build it and get it laser cut I first have to build it in solidworks, but this also helps me prototype as I can set sizes and lengths and see whether these work in simulation without the cost and effort of building it first.

so far this is what I've got to, but if the parts for my honours project don't come back soon this could happen.



Once I've worked out how to drive both sides from the same wheel assembly, all that remains is finding a motor drive system that will work. Potentially this will involve a gearing-down to the main drive wheel, which will also look cool if I can get it done in acrylic or put cut-outs in the right places.

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UPDATE 1
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Parts still haven't come for honours project so I have finished designing the thing and have made some bin-packing sheets for the laser cutter. You can see the size it'll end up with, this sheet is 600mm x 450 mm (with the actual material being 600x400 so the bottom strip is empty on purpose).


The final system should look something like this:


And should operate something like this, although possibly the legs on one side should be swapped so it isn't inside/outside.


I'm still not 100% about how I'll set up the motors, as it possibly needs both sides in-sync to walk properly. If this is the case it wouldn't be able to turn corners without some linear actuation on the drive wheel, for changing the circle of travel for the legs on one side...

Laser cutter is booked for Monday, and I'm gonna do half the thing just to see if it works and test if the motor can drive it. That way, if it works I can go print the other half and if not I haven't wasted so many sheets of MDF.

stay tuned for more TRIUMPHS OF INDUSTRY

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UPDATE 2
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it's been a good week so far, no parts or boards came back for honours project till today (wednesday) so after cutting the beast out on monday I had lots of time to put it together.

One thing I have learnt is that sometimes it does work better to just make it up as you go along: there were a lot of potenital problems to nut out to get this thing working but all of the solutions were spur of the moment and it worked ok... didn't even leave tolerances on the sizings I just did it tight and assumed the like 2mm kerf on the laser cutter would do it for me.

Down at the design school on monday morning I was sightly taken aback by the size of the thing I was printing - originally this was intended to be a scaled-down version of the final thing. It took over an hour to cut out two sheets, so looking forward to ECS getting one in a few weeks and being able to do it while I'm doing other work.

Yes those gears are even proper involute, I modified the gears from the mech 1 gear assignment (so they had smaller teeth and would mesh smoother).


Once I got it to school I had to work out how all the bits would go together, and how the layers would be laid out to stop bits interfering. Stole some 5mm steel rod off Tim Sherry's desk, got some PVA and a file from the workshop and set to work.



I was planning on doing up the system with a load of electronics, using a maple to control it with a motor driver shield and a zigbee for wireless control... that fell by the wayside pretty quickly and I just resolved to get it working at all.


 After our experience with the laser cutter during the gears assignment, I wasn't even sure the thing would work out as I had completely neglected to organise shaft collars and we had major issues getting our gear system layers to spin past each other, and it only worked at the demo due to a solid 5 hours of filing done by me the night before.


To attach most of the bits together I just soldered washers to small segments of rod cut with a hacksaw. This worked surprisingly well, even the workshop were impressed... plus I think it gives it a bit more of a mechanical, siege engine look. Where things needed to be out of the way so other things could slide past, I countersunk for the washers using the milling tool for the mill in the workshop as the bit was flat on the top.



Also got my workshop on and used a thread cutting die to thread the rod so that everything didn't have to be soldered - using nuts makes it way easier to dismantle.



The three small gears drive the big wheel as well as holding the three parts together. The off-center crossbars betweent he big wheels mean if all three aren't turned at once, it swivels and everything doesn't work. In order to attach the drive wheel to the motor (with its tiny D section shaft), a plastic wheel was repurposed, with the extending bit put through the wheel to reduce space.


Luckily for me I printed out a load of spacer disks in the spare space on my laser cutting sheets, these helped me to lay out where everything was going to go and holds it in place.




The only real issue I had with the entire assembly was that the corner of the long vertical bar interfered with the horizontal leg bar, meaning that the walker could complete most of a cycle but at the extreme of extension it would jam. This was easily fixed by filing off the edge of the bars, although this leaves them weakened and potentially they could break when put under strain.


I also had to remove the spacers from the upper "beam" rods which the long verts hinge on, as even with the bars sanded the spacers would prevent it from walking if the leg was not perfectly aligned (cuz they stick out too much)...

In the end I did get it working (although the motor is running at like 6V and running over 1A through it... it got hot pretty quickly, and was still too weak to stop it jamming when held the wrong way. 

Remember this is only half the walker, I figured I'd do one side and then if it worked i'd do the other, or be able to fix it up before i wasted all that MDF. It also took an hour to cut just this lot, and that's $10 I could spend on food (beer).



Still haven't worked out how to run both sides together, as I don't think it'll walk properly if both sides are not synched, and then it won't turn corners... This is not myth busted, but I defo need to get some bigger motors before I try to make the full unit. I also need to do some work on honours project because iteration 3 of my boards are back now and I need to get those done before everything else is due...

if anyone wants them, the solidworks files and laser cutter drawings are available here http://www.mediafire.com/?hynpeq4lv9bngwr

enjoi
T

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