Let's Make Robots!

Mini Groucho

I've decided to make Groucho into a two-wheeled balancing bot. This would make him 5' tall at least and 2 wheelchair motors will give him enough torque.

However, because I don't want to make Groucho-sized holes in the wall or destroy Groucho by ramming him face first into the basement concrete floor, I'm going to do a test case first.

My plan is to create a 2' tall, 3" deep, and the width of a long breadboard plastic frame. If I can, I will get it laser cut from acrylic and if I can't I'll cut it out the hard way.

For motors I have a bunch of used Dynamixel MX-64 robot servos, I'm hoping that two of them are strong enough, otherwise I'll have to use other motors and I only have one set that is probably too long for this and I don't want to buy anything new for this.

If this pans out and doesn't self-destruct, I'll put the plans and software up as open source. And maybe make a kit if people are interested.

Have a nice day!

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I just got in a pair of Pololu gear motors that should work for Mini-Groucho. I'm hoping that the Robo-Claw will accept changes fast enough that the control loop is tight enough to keep it balancing.

What I'd like to know: where do people get high quality small 12 volt (or 24 volt) DC motors for robots? Seeing how responsive the ball robot (it balances on a single ball) is, I want to know where they get their motors. I'm sure that there are places to get decent quality high torque motors at reasonable prices, but I haven't found them yet.

Oh well, I now have two sets of motors I can try to see if I can get this working.

Most of the balancing robots I've seen here get the balancing part right, but they tend to wobble.

I suspect that this is because they don't use encoders to keep the position stable. Odometry should allow the balancer to change position more fluidly than just using the accelerometer/gyroscope.

It would also need a fast processor to be able to adjust the motor PWM fast enough.

I'm still debating between using a BBB with the PRU handling the interrupt loops or the Teensy 3.1. The BBB has floating point hardware which will help.

OK, I couldn't help myself. I did a quick test by joining the three sized beams that I needed to make the chassis together to see the size and get a feel for the size and how easily they go together.

The size is about right. On the other hand, I had forgotten how small M3 screws really are. I used three corner pieces from MakerBeam to join the three beams. The result is a strong join. The one minor problem is that my hands have gotten older or M3 bolts have gotten smaller, but they are a little tiny to get them all together. However, it did all work, so I won't kick them off the island.

Keep building!

So next week I get to start building Mini-Groucho. Delay because of Lee's medical tests to be done in a hospital a few miles away from home which means we stay for a few days in a hotel; Lee can't handle too many hours away from a bedroom/bathroom.

The beams look pretty much like a scaled down version of the 8020 beams, and I've had good luck with those. There is a small indentation so they can fit a circuit board in the T-slot, if you can find a board that doesn't have anything near the outside. You can also fit cardboard or something of roughly that thickness. I also ordered some plastic to make levels from, and if I'm very lucky it will fit in the slot to make it easier on me. I have to see how wide it needs to be based on my motors; I had all the dimensions worked out in inches, but the beams are precut in metric sizes so I'll remeasure everything again. My guess is that it will come to 1 meter tall, 300 mm wide, and 100 mm in depth.

If the motors I have around have enough torque to run this, all's fine. Otherwise I'll use wheelchair motors/tires and find a way to fix them to the frame.

I should have a bunch of OpenBeam stuff arriving Monday and Tuesday so I can put Mini-Groucho together. If only my wife didn't have medical tests at Hershey this week, I could get started making him fall down. :)

The BOM:

  1. 4 x 1m OpenBeam
  2. 4 x 300mm OpenBeam
  3. 4 x 100mm OpenBeam
  4. 2 motors (eBay)
  5. 2 motor mounts (will make from Polystyrene)
  6. 2 wheels (still need to get or make these)
  7. Teensy 3.1 for low level control
  8. IMU
  9. Normal sensors (IR Rangers, SF04s, etc)

I'm hoping to get him working by Halloween so I can put a big bowl of candy on him or a silly head with a bad laugh track.

I'm debating using OpenBeam instead of the 8020 extrusions. OpenBeam is lighter and cheaper, and I wouldn't have to cut it myself.

I really want a table saw! Maybe for the holidays I can get one.

The screw needed to hold the motor in place is a #10-32, the one screw my local good hardware store (well, farm supply store) didn't have. Luckily the local industrial supply had them in abundance, so I got two dozen of them.

I've got a motor hub and wheels on order that should arrive tomorrow.

So if I don't have too many other tasks, I should be able to get going on the motor mount this evening, maybe.. Then I have to cut my scrap 8020 extrusions to the sizes I need. I found, 10 years ago, some nice Quick Connect nylon pieces that were inexpensive and held the extrusions together. They don't have the types I need now, but I may be able to reuse the ones I have.

And when I get my 3d printer I can make my own.

TTFN

you might want to give a look see at how Dickel does some of his fantastic work.

http://letsmakerobots.com/node/32828

It may not be laser cut, but, damn if it isn't impressive. :)

Dickel's write ups of how he creates his plastic pieces is exactly what I was thinking of.

Unfortunately I don't have motors both fast and powerful enough to work. I have a set powerful enough, but not fast enough. And I think they use a metric screw somewhere between an M5 and an M6 and the only place that might have such a thing in town is closed on the weekend! Luckily tomorrow is Monday.

So I have to redesign my bot, get new motors, and new connectors from 8020 (aluminum extrusions and things to attach them together and to other things).