Stepper-driven tank platform for remotely operated vehicle
by tjhowse, published
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First: Some history.
I found this cheap wifi pan-tilt camera on ebay. It's a Foscam knockoff, and I bought it for about AUD$60 delivered. I strapped it to the top of a cheap RC car chassis, compiled an application for the camera, wrote it into the ROM, banged out some python, stuck on an arduino and had a bit of fun:
The car was fun, but unusable over high-latency links due to its high speed and fairly unpredictable trajectory. It also had a terrible turning circle.
I've just finished designing a printable tank chassis for the camera. I re-jigged the control system to be wholly contained within the browser, using poorly-documented cleverness to send data straight to the camera's serial port. The result is a remote-controlled tank that can point a camera around independently of the body, that works anywhere there's wifi reception and behaves well enough to control over a 500ms internet connection:
I have successfully driven it through medium-length grass, over, under and around my house. It's a bit top-heavy at present, I need to move the battery lower to lower the centre of gravity. I predict about half an hour of usage from a 2200mAh 3 cell lipo battery, but I can't say for sure since I've never flattened one.
Print the parts described in assembly.scad, starting with the tread bids.
Please note that the gears in this design are made for my steppers, and probably won't work for you right away. My steppers are nema17, and have a belt drive sprocket on them with a 12.5mm diameter. If you are in this boat, great! If not you will have to modify the bore of the drive gear, and perhaps other parts of the design. The relevant variables are described in assembly.scad. It's not as scary as it sounds.
Assemble the treads by joining 20 pairs of the tread pieces together by threading some 3mm filament through the holes. The curved top of the linker piece should be nestled into the tread piece. If you have problems with the pins wiggling loose over time, give them a quick zap with a soldering iron to fuse the tread piece to the filament. Print these parts first and assemble them while you're printing the other parts. Once they're pinned together, run them through your hands whilst waiting for everything else to print in order to wear them in. The more handling, the better. I put a little zig-zag of hot glue on the base of each segment to give extra traction.
Attach the driven gear to one of the hub pieces by pinning them together with two pieces of filament. To get the orientation correct, lie the driven gear flat on the table, toothed-side down, then put the hub piece on top of it with the prongs on the bottom.
Clip the main beams onto the axles, then clip on the stepper holders, ensuring the countersinking holes are on the correct side. Next, clip the spacer pieces onto the front axles, then the hub pieces. The prongs on the hub pieces should be on the outsides of the assembled hub. There is no need to pin the idler hubs together, though you can if it makes you feel better.
Do not be tempted to lubricate anything! This will impede the natural process of the plastic wearing itself smooth. Only lubricate once the tank operates without lubrication, to improve performance. Everything will be very stiff to begin with, but will wear in quickly. Be patient!
Mount your stepper onto the stepper mounting plate and attach the drive gear. Clip your driven gear assembly onto the rear axle and ensure the gears mesh smoothly. If it is difficult to turn (it will be, initially) check that the teeth aren't being pushed against eachother, there should be some clearance (0.5mm) between the tips of the teeth and their corresponding valleys on the other gear. Manually spin the hubs around for a while to wear the axle in a bit.
Slip the tread assembly over the front hub, then over the rear. Clip the last remaining hub piece over the rear axle to secure the tread in place.
That's it for the printed assembly.
As for the rest, you're going to have to work it out for yourself a little bit. I can help, though:
Two stepper motors, nema17 for preference,
Foscam-knockoff wifi camera (Easyn FS 613A M136, Tomtop, etcetera),
2x Pololu stepper drivers,
Battery, 3S 2200mAh recommended,
A 5-watt 5V regulator (do not run camera off arduino's vreg),
A bit of cardboard,
Custom code for webpage built into camera: http://pastebin.com/9KBE2Kss
Code for the arduino between the camera and the pololus: http://pastebin.com/iFcs1EJ5
Script I used to decompile and recompile the webui for the camera, foscam_pkmgr: http://pastebin.com/8ayWk5fc
Compiled webUI replacement: www.tjhowse.com/hackerspace/camcar/myrom.7z
(You can write the webUI in using the existing webUI on the FS 613A M136. Beware: Might brick other cameras.)
Initially the steppers probably won't be able to turn the treads without being run scaldingly hot, but it will quickly wear in and you'll be able to turn the power down.
Probable future improvements:
Integrate the stepper mount and spacer pieces into the main beam, removing the need for the unique axle pieces and reducing part count.
A printable platform, to replace the cardboard.
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Stepper-driven tank platform for remotely operated vehicle by tjhowse is licensed under the GNU - GPL license.
What does this mean?
- Remixing or Changing this Thing is allowed.
- Commercial use is allowed.
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