Saturday, February 9, 2013

Differential Steer Robot: Overview

     Greetings. Its been a while. For that I apologize. Today I decided to take a break from Calculus to type up a new post. So here it is! Anyway, you may have been wondering, "What could he have done with that Arduino?" Well I did something no other person in the world has ever done. Guinness is on his way. I made a differential steer robot. I know. One of a kind. No one else has ever thought of that. And there it is!

     Actually, that is not what it currently looks like. That is what it looked like a month or so ago. It now is sporting 5 more sensors and a real time clock module, but that will come later.

     So what are we looking at? Well its pretty simple really. In the middle we have the Arduino Mega previously mentioned. It is wearing the v4 sensor shield.

     To the left of the Arduino is a solderless breadboard. I got mine off of ebay and I haven't seen one cheaper anywhere else. Mine was called "Mini Solderless Breadboard Bread Board 400 Contacts Available Test Develop DIY." Really, just make it has the rails on the side. This one was $3.22. Radio Shack is more than that, but China takes a while. I just order in advance

     Above and below the breadboard we have two ultra sonic range finders (pictured on the left). Now you can buy these for $30ish at places like robotshop, but these two came from ebay as well. HC-SR04, they cost about $2 each. Mine worked flawlessly. They are a great introduction to thinking like a computer. They work like sonar and are pretty good. They don't like angles all that much, but this is a learning  process. We'll move on to the $2000 LIDAR module later (as soon as a reader buys me one).

     To the right of the Arduino we have the batteries. Now these batteries are pretty up for grabs. I used what I had. They are pictured to the right. The big one is a 7 cell NiMh battery made to go in a Traxxas Stampede RC Truck. It is powering the Arduino. Needless to say, it will power it for a long time. It provides 8.4V to the Arduino voltage regulator. I also put a switch in the circuit because that makes things much more convenient. I ordered these switches as well as some limit switches(and LEDs, etc) I used later from I think they were 50 cents or something. Since then I have discovered which also has cheap stuff, but I have never ordered from there and am getting off topic.
     The one below the large battery is a 5 cell NiMh that powers the servo motors. Long story short, the Arduino can't provide enough power through a digital IO pin to power them so we wire in external power through the breadboard. Hobby servos like 4.8v or 6.0v (nominal) so this is perfect. The battery is one I got from Its a China importer based in California( I think). Shipping is faster than China. The connectors are Deans (T-connectors) and Traxxas connectors from Hobbyking. Connectors like that are one of the things you have to buy from there. I've found that even if I only use 2 or 3 of them, I'll break even with the hobby shop. Find Deans HERE, Traxxas HERE. If you are using dedicated batteries for this project feel free to use whatever connector you want. Tamiya, EC3, XT60, etc. But I will say that Deans are easy to solder. If you want, you could even use rechargeable AA batteries. I already had these and the charger from my previous rc plane endeavors so this is what I used. I kept the Traxxas connectors so the big battery will still fit in the truck.

     Below deck is a set of fps-148 Futaba servos modified for continuous rotation. How to do this? I used instructions HERE. Its a 60 second procedure. Google search it. I also had these servos sitting around. If I had to buy everything I would probably go with servos from HobbyKing. I've used some of the HXT servos before for planes, and they work well. I'd go with THESE most likely, but do what you wish. I haven't researched it all that extensively. You want something with a futaba spline if you are going to use the wheels I did.

     Speaking of which.. The wheels came from Pololu as well. I got the ones found HERE. They screw on just like a servo horn. Easy as can be. While I'm at it, the front ball caster also came from there. I got the .75" version found HERE. Alternatively you could search for ball transfers on a site like McMaster-Carr.

      The white thing on the side is just part of an old computer I put on there for its switches and LEDs. We'll get into that later.

     Assembly is quite straight forward. Wood glue, hot glue, and some machine screws. My total dimensions are 8" x 11". The ultra sonic sensors are mounted on a little block that cradles it (mine is balsa because it was easy to shape) and a tab that rotates.
    The Arduino is held on with some little dowel rods that I made to fit the holes and a few metal standoffs. If you use metal standoffs, be sure to put some sort of insulator between the metal and the board. I used homemade washers made from a cereal box.

Other things you'll want:

  • female-->female jumpers. Get these on ebay. Search for dupont connectors. 40pcs for $3.
  • jumper wire: any 22 awg solid wire will do.
  • Resistors: again, I like ebay. I got 400 metal film resistors in assorted sizes for < $5.
  • *optional* THESE. They make connecting the servos more tidy.
  • Other stuff: Electrical tape, heat shrink, etc. 
  • Basic Tools: Soldering Iron, Wire Strippers, Digital Multimeter (Must have some variation of all these things. You just do.)
  • X-Acto Knife: These are dirt cheap at hobbyking. 50 cents. I have many of THESE. And extra blades HERE.

     Ok. Well this post is getting pretty long so I better wrap it up. I hope this has inspired you to greatness. If you have any questions, comment or send me an email. I would encourage you to check out Jeremy Blum's YouTube channel too(HERE). He has some great tutorials for Arduino and knows much more about electronics than I do. I have learned a lot from his videos.

     If you want to go ahead and stock up for the future, order some IR LEDs, IR photodiodes, and 2.54mm male headers from Ebay and some of THESE too. Next post I 'll try to get into more details of wiring this thing up. I'll start throwing out some free code soon too. See you later!

Have Fun!

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