Patinas and metallic paints

I've been experimenting with metallic paints on 3d printed objects and spray on patinas. Photos and then a description of technique and products below.

My friend purchased some Sculpt Nouveau Metal Coating (Brass, B) that she didn't need, and I applied it to all the pieces above. Tips for using that paint:

• Sand surfaces with 220 grit if smooth. Surface that are too smooth will be hard, though not impossible, to paint thinly.
• Brush a layer on as thinly as possibly. The print surface will show through. It will dry fairly quickly. If you can see brush strokes while the paint is wet, that is too thick (unless you like that look). Once dry, if you want to get rid of a brush stroke or drip you'll have to sand it down.
• Once dry, apply another layer. The deposited metal particles will roughen the surface so that subsequent layers of paint adhere better and get more coverage.
• The brass paint got brighter as it dried. It sometimes had a dull look when wet but that goes away.
• The patinas must be applied when the paint is wet to stand a chance of interacting with the metal particles, so apply a final thin wet coat and patina immediately. If too thick, the paint has a tendency to crumble up (cottage cheese is another description I've heard). Some patinas will also just deposit some color on the surface and so would be at least partially effective even on a dry coat of paint.

The patinas I used were the following products from Sculpt Nouveau:

• Tiffany Green
• Jade Green
• Deep Brown
• Mahogany
• Vista Rust (Bender head only)

I mixed and matched the for different effects on the different objects above. The Jade Green leaves obvious green deposits immediately on the surface.

Robot Arm

As part of a mentorship/tutoring thing, I built a robot arm with a student participating in a competition. The design is my own save for the gripper which is modified from this gripper from Thingiverse. This was built mainly during weekends over this January 2016.



The servos are two HS-53 at $8 a pop, a Hitec HS-311 at $8 also, a Tower Pro Metal Gear 995 and a Tower Pro Metal Gear 996 (about $10 each).

The rest is hot glue, wooden poles, zip ties, tape, a paper tower roll, 3D printed brackets. etc. The shoulder bracket was borrowed from a biped robot kit, but could be printed.

The arm is controlled by a smaller model that has potentiometers embedded in it. An Arduino takes care of mapping the values to angle and reflecting it to the servos.

The counterweight is very necessary. This arm reaches nearly 75 cm outstretched. The shoulder (highest up servo) output interface is the biggest ongoing problem. The horn strips out after hard collisions or just slow wear and tear. If the servo output had bigger teeth that would be nice, or if I could find a properly sized hardened steel servo horn that would also help. But all I have is a slightly too big aluminum horn and a set of slightly too small plastic horns. And epoxy, and superglue.

Battlebots Application

Sector67 submitted an application to Battlebots in December and heard back earlier this week. We were not selected (260 applicants, 56 slots). Here are our videos. I made the renders and bossed people around a lot to get the application out on time (and then became defacto team leader due to that). Credits to Jim for the design, Tim for the proof of concept build and 3d model, Kate for the video editing, and everybody else who was part of the Bad Penny team and help in any regard.

Museum Exhibits Preparation

I'm preparing a robotics exhibit for a local museum that is opening up soon. To that end, I learned how to weld just now (I'm not very good yet. Also now I'll smell like welding all day).

The exhibit is simple--drive a small remote controlled car around a map of the city, from a monitor that tracks the car and displays a birds-eye view camera image, auto-rotated to the car.

The biggest hurdles:

• Light weight and high capacity battery (but low max discharge rate--which is fine, I don't need a high discharge rate)
• Sleep mode for the wireless radio and microcontroller to conserve battery when nobody is using the exhibit (chose the Wixel here, for reasons I'll cover in another post)
• Safety against, stall, over-current, under-voltage, and other battery damaging conditions
• Making sure the parts the user touches (console, perimeter wall) stand up to the forces users will likely put on these

To adjust thresholds for my wake-up circuit, I finally installed drivers for the portable oscilloscope I have: http://www.gabotronics.com/development-boards/xmega-xprotolab.htm

First impressions: Quick-start was very easy. Installed drivers and software on Windows to get this screenshot, but you don't need to, as the scope has a tiny display.

In the image above I am bringing a flashlight closer and farther from the sensor. The green shows the phototransistor circuit output directly. The red is the output of the comparator circuit (LM358N, CNY70 if you want the parts. Just what I keep in stock, not chosen for any other reason. Don't turn on the CNY70 LED for this application, durr).

I was able to get it to robustly sense when I walked under the fluorescent light fixtures in this building, holding it at chest height.

More on this later.

EDIT: One more fun image.

My fluorescent bulb desk lamp apparently pulses on/off with a cycle taking 7ms or so. The other pattern (where the waveform is getting "pinched" and then expanded) is aliasing from the slow sampling frequency of the oscilloscope (or at least the points drawn in the software) at this large time scale and the 7ms lamp pulse.

Current Limiter Project Wrap-Up

Four years ago I made my first printed circuit board using EagleCAD and purchased a few units using BatchPCB. It was a version of this current limiting circuit with R5 as a potentiometer, which lets me modify the cutoff current. By the time the boards arrived I probably had exams or something, and so the project was forgotten.

Today I was trying to clean my desk, but I found the boards and got distracted. I scavenged around Sector67 for parts (making backwards progress on the desk-cleaning situation) and populated the board.

When the output draws under some current limit (can be modified by the potentiometer), the green LED is on. When the output would draw more than that limit, the red light turns on, and the output current is limited to what flows through the red LED and resistor in series with it. The concept and operation is described in detail at the Instructables page of the source for this project: http://www.instructables.com/id/PC-Power-12-V-Current-Limiter/

I made the traces thinner than I would have liked. It was my first time getting a PCB made and I didn't realize that I had forgotten to set the trace width until it was too late. The default trace width is probably fine here anyway. It feels good to finally wrap up this project. I could go further with it (compute some values, measure some values, stress test the board, refine the design) but there are hundreds of other projects that will probably take priority.

Polyhedra Magnetic Tiles

My first submissions to Thingiverse:

Pentagonal Hexecontahedron Magnetic Tile

Pentagonal Icositetrahedron Magnetic Tile

Also on github @ https://github.com/eshira/polyhedra

Happy printing!

MOARbots Path Visualizations

A MOARbots volunteer (Scott) made some python visualizations for the data stored from the multiple waypoint navigation competition.

Each path comes from a different (completely or slightly) piece of code running on the robot. The waypoints are the large gray circles, and are always in these same pattern, though sometimes the board was rotated (the tags were physically taped to it).

You can see the curve of the path segments in robots that did not account for the differences in output wheel speed relative to PWM (pulse-width modulation) value between the left and right motors. The actual physical robots were not the same between all these runs (the number at the top is just the robot ID tag which is removable). They all curve left because the right hand wheel motor is in the bias direction when moving forward, but the left hand wheel motor is in the anti-bias direction.

The target is considered reached if the robot's tag center point is within 20 units (pixels, but our camera never moves) of the goal point tag center. The score is computed as such:

Score = 120 - t + 25*n
t: time elapsed in seconds since trial started
n: number of waypoints visited
Total number of waypoints: 5
Trial ends if the score reaches zero

The maximum score is therefore 245, for a theoretical 'teleporting' robot.

The competition can be summed up by the two key challenges: (1) Figuring out how to drive quickly towards a single waypoint without overshooting the goal, and (2) Computing the shortest path given the locations of the goals and the robot start position.

 The robot in the above visualization did a little too much zig zagging. It also didn't compute the best path.

The robot in the above visualization correctly figured out the minimum length path given the tag locations and robot start position.

Bubinga Magnetic Box

Another CNC project -- some bubinga scrap turned into a box with a magnetic closure, rounded interior pocket, and painted turquoise inlay.


Clamped down to the CNC spoilboard.

One half already cut out and inlayed with magnets.

The box half directly from the machine after the first pass.

 Jig for milling out the other side. This is because the reach of the end mill / bit is limited, and also in order to mill out the hole for the inlay.

Pale red look before applying walnut oil.

The wood absorbs the walnut oil quite well so I keep reapplying, especially to rough spots like the cracks.

The magnet side.

Spring Break CNC Adventures

While the UW students were off on spring break, I took some time to work on old projects from "The Big Project List."

I found a scrap of plywood which was just the right size to mill out Curio Shelf Revision 1.

The pieces before assembly:

Due to a mistake in the lengths of the dividers, the shelf curves outward in the middle. Not a bad look, but not a look I'd keep in Revision 2.

Another project is a bamboo magnetic board with a french cleat in a V shape for the wall mount. First I did a quick mockup in OpenSCAD to visualize it. The small piece here would be mounted on the board. The large piece would be mounted on the wall. In the first image, the wall would be on the positive x-axis side, and the board on the -x axis side.

Here's the finished cleat. It doesn't look very neat however it is not visible when the board is mounted. Now it is easy to remove or replace the board on the wall. This means you can use it as a screw organizer that you store on the wall.

Here it is with some things attached to it.

MOARbots update

MOARbots is now on github: https://github.com/MOARbots/

Here's a video of one of the waypoint navigation routines that have come out of the UW independent study.

What's next? Yet another revision to the parts list. The new MOARbots will be based around the Teensy and the ESP8266. Using the ESP8266 by itself (with something like this board) is still on the table but will have to wait a bit. Using the Teensy has the advantage of being able to make them into Teensyduinos. The Arduino programming language/environment is easy to use compared to writing in C. And, if I'm willing to wait a bit, the hobbyist community will develop the tools I need for the ESP8266. Take a look at this project, for example: https://github.com/nodemcu/nodemcu-firmware/

One of the most important things that has happened to MOARbots recently is the expansion of the team of volunteers working on it. Thanks to the Sector 67 community and the University of Wisconsin community, MOARbots is now more than just my own personal project.

Lastly, here's a teaser of one more cool direction MOARbots might take:

This $50 quadcopter...
http://www.amazon.com/Hero-RC-Matrix-Quadcopter-Battery/dp/9269802574

...plus this $40 3-axis gyroscope, 3-axis accelerometer, and on-board processor...
https://www.sparkfun.com/products/11028

...plus this github project to help free that device from the small-mindedness of the company that made it and then decided to make their code closed-source...
https://github.com/jrowberg/i2cdevlib/tree/master/Arduino/MPU6050

...plus April Tags and MOARbots.

A swarm of Wi-Fi connected autonomous self-stabilizing quadcopter at under $150 per quadcopter? I'm hoping it's possible, anyway!