The next step for my project is to add 3d position support by determining what angle and how fast the wheel is spinning at any given moment. The chip is in the wheel; I just need to interpret the data.
I initially bought the LSM303D 3D Compass and Accelerometer Carrier with Voltage Regulator from Pololu ($15).
However, I learned the LSM303D is just a compass, and gyro data is needed to determine pitch and yaw. So I bought a $20 MinIMU-9 v2 Gyro, Accelerometer and Compass from Pololu (well, two, in case I screw one up — and the price today is $30 per chip).
I am cloning their repository to play with it at https://github.com/corbinstreehouse/minimu-9-ahrs-arduino
Calibration may be tricky…so here are notes on it:
Notes on calibration:
1. AdaFruit learn resource: http://learn.adafruit.com/lsm303-accelerometer-slash-compass-breakout/calibration
India, Fruit delivery
This guy didn’t look too happy, but I took his picture as he was walking by on the road. India, February 2014.
Another slightly longer video from last weekend. I have about 3 or so hours on it, and it seems to be holding up well!
Some quick test footage from last week; I used the wheel for the first time last Friday for an event put on at the Exploratorium by Velocity Circus. It works AWESOME!
A lot of people email me asking what materials to use for building cyr wheels (also known as a roue cyr, rouè cyr, or simple wheel).
I like “thick” wheels (also known as a “fat boy”), with 1 3/4″ to 1 7/8″ outer diameter. Your preference may vary. Length of materials may vary depending on your wheel circumference.
* Main tubing: 20′ of 6061 Aluminum Tubing, 1 1/2″ Outer Diameter (OD), 1/8″ sidewall thickness. I buy pieces from a local supplier, Gorilla Metals which used to be Metal Supermarkets. You can buy shorter pieces online; factor in the fact that you what a 5 piece wheel, and will probably have 6″ to 1′ of an under-bend on the end of each piece (again, varies by technique and how you roll or bend the wheel).
* Inserts (solid): 5′ of 6061 solid Aluminum Tubing, 1 1/4″ Outer Diameter (OD). Turned down on the lathe to a bout 1.230″, or, use sanding discs on an angle grinder after you bend it, and sand till they fit (not ideal, time consuming, but it works). Same suppliers as the Main Tubing. Inserts are anywhere from 10″ – 12″ long.
* Inserts (alternative, tubing): 5′ of hollow steel tubing, 1 1/4″ OD and 3/16″ sidewall thickness. This is heavy, but works for my LED cyr wheels. Inserts are shorter (but I haven’t stress tested them), usually 8″ long. Turned down on the lathe to 1.230″ (or so). Same supplier as above.
* PVC Covering: 25′ of PVC tubing, 1.5″ outer diameter and 1/8″ or 3/16″ thickness. Bought online, pwmall.com, search for Tubing. This or this. I’ve tried both..and one is superior and lasts longer, but I forget which one.
I’m using a 6amp 120V on/off switch. I might be running up to 16 amps through it. We’ll see if this works..
The switch is fished through with wire and fishing line.
I learned some more things about casting the EpoxaCast 690. On one of the last pieces it was really opaque:
I called Smooth-On the next day and asked about it. They didn’t know, so I figured I needed some new stuff and ordered some. When I got home from work that day, I decided to microwave the Part A and this made it start to become clear again. So, apparently it is rather sensitive to temperature, and it being 50-55F in my garage wasn’t good for it. I warmed it up in front of my powder coating oven and the next casting was clear.
Here’s some notes on casting…maying for myself when I go back to do this again and try to remember what I was doing.
First, to get the curve, I would cut the LEDs every 6 and re-solder them together:
All us techie programmer know a curve is an approximation of straight lines; and mine are 6-pixel straight lines.
Next, casting: I had two materials to cast with (see Casting experiments). Smooth-on EpoxaCast 690, and some Polyester casting Resin. The resin is fairly cheap, and they sell it at my local Tap Plastics store (I discovered that after I ordered it on Amazon). I experimented with both, and they seemed about equal. However, the Resin was harder after it dried, while the smooth-on was sort of more plyable. I’m not sure if that is good or bad (yet).
My first official wheel piece didn’t go so well. I used the EpoxaCast, and tried taping over the groove horizontally (in line with the wheel curve), but this was a mess. I kept getting a lot of bubbles and it was just not going well.
In the above picture, you can see I bought a scale, as it is a two part epoxy measured by weight.
Eventually, I realized it was better to spread the tape perpendicular to the wheel…but even then, I got lots of air bubbles under it. Worse, the next day a bunch “drained” out, probably from capillary action:
Hmm..not so great.
So, I decided to try the Resin for the second casting. It went on really well, as I put the tape perpendicular to each piece and seemed good. But the next day, the resin (which dries MUCH faster than the Smooth-on Epoxy) was sort of “blotchy” under the tape. Grr. (Note: later I fixed this by putting some epoxy on top of it).
So, I tried the smooth-on epoxy again, and with the horizontal technique it was working pretty well, despite some spots getting air bubbles and dripping out.
One thing I would do is hot-glue fill in the edge to keep the epoxy running out:
After I taped it, I discovered I could “cotterize” it with more hot-glue to keep it from dripping:
The glue is fairly easy to cut off with an exacto knife.
Next thing I learned: tape makes a difference. I was using Heavy Duty Scotch shipping tape to “cast” the LEDs in; it pulls off pretty easily and cleanly. I ran out, and used the weaker/cheaper ACE tape.
Bad idea. It reacted funny with the epoxy:
Luckily most of the white opaque mess peeled off, but some I had to scrape off. Plus, this tape left a sticky residue. It was not the right tape to use.
Other notes: the temperature should be warm..ideally 70F or higher. The stuff takes forever to cure when thin; it says 24 hours, but it seems much longer — at least 36, but probably 48 hours. And I think it takes a week before it is fully cured. Outside temps have been 50-60F (or up to 70F during the day).
Each time I wire up a new piece I test it to make sure I didn’t mess something up. Here is a test of 4 pieces, rainbow pattern:
I did one section covered in the smooth on epoxy..it didn’t work so well. It was to liquidy and probably need more catalyst than what it says in order to cure a small thickness. The test piece sort of felt rubbery, and I forgot to add more. Rubbery might actually be good. The other resin I got was rock solid hard, so I’m trying it on a second piece (so far, it seems better, but it will need to dry overnight).
On the first test piece with smooth on, I would add the epoxy and then try to cover it with clear packing tape to mold it into a curve. This was a mess….and frequently captured air bubbles underneath. Using the other casting resin worked much better (no pictures yet) with the same process.
Worse, the first test piece sucked a bunch of stuff out via capillary action overnight:
Luckily it dries really slow (24+hours) so I could wipe off the overflow with acetone on a rag.