Indoors Navigation Robot ("Driving Drawers") Post 1

[Foreword: you may notice the fonts are mismatched in this post. Or maybe you noticed that the images in all the posts in this blog, when viewed on a larger screen (as opposed to a small mobile device screen), are placed haphazardly within the body of the post. This is because Blogger leaves much to be desired when it comes to the WYSIWYG post editor. In part because WYS ("What you see") is not always WYG ("what you get") in the context of Blogger's post editor, but also because there are very limited options for aesthetic image layouts in the body of a post. I am planning a Jekyll based webpage for this blog in the general future that will fix these problems. Until then, this is a cheap and easy way to make sure I keep a blog at all.]


I had an idea for a chest of drawers that would drive to my location in a space (assumes no stairs).

Here's the first part of my progress on that project.

Physical Build

I started by dissecting my collection of Roombas and taking the wheels from two different models and hot gluing them, along with some support material, to the base of a plastic chest of drawers.

I purchased a Raspberry Pi 3 and a PiCam, and while I waited I put together a 3D pan-tilt design from Thingiverse. I'll provide a link (link to thing), but keep in mind I do not recommend this design. Unless your 3D printer is very precise, you'll have to do quite a bit of filing and sanding to get it to go together. The pan servo will absorb all the impact whenever it hits something, and will fail (mine did). If you can find a ring of the right thickness and diameter to stick in between the orange disc and blue plate in the photo, the problem is mitigated (I used a bearing I found in a bin--way overkill as it isn't even acting as a bearing--but a quicker solution than cutting my own ring on the lathe).

Not entirely certain of the layout I wanted, I just taped everything in place with masking tape. The battery is stored underneath, in the topmost drawer of the robot. It's a 30,000 mAh battery I bought for use with my smart phone. It has a port that will source 2.5A, which is needed by the Raspberry Pi. I paid about $75 for this model; you should be able to find comparable batteries by other brands if that one is not available (sometimes, when an item is out of stock, a few vendors will offer it for an inflated price, so beware. The price on Amazon for this model was briefly $399 before dropping to $69.99 again). 

I pulled the Arduino Mega out of a project I'm not working on at the moment, though it is of course overkill for this application. I wasn't sure how many sensors and actuators I wanted on board, so this 54 I/O pin Arduino allows for quite a bit of room to grow the project. The Raspberry Pi 3 itself only has one PWM enabled pin available to me, so the Arduino is convenient for handling all the low level stuff. It talks to the Raspberry Pi over the USB. The micro servos are powered from the Arduino Mega which are in turned powered off the Raspberry Pi. The micro servos stall current is low enough for this to be possible with the Arduino Mega.

 The Roomba wheel motors are safe at 12 volts (the Roomba battery voltage), so I put another battery in the system just for them. The battery is a 3 cell Lithium Polymer battery, which measures in at roughly 11.1 volts when the battery needs to be recharged and 12.6V when the battery is fully charged. The motor drivers are L298 chips on those red-colored Sparkfun breakout boards, with the heatsinks mounted to them.

So at this point the robot was driving, but only in a straight line. Turns would drag at least one wheel and make a terrible noise. Only very slight turns worked. This was fairly predictable, but trying to make it work anyway was very much in keeping with my glue-and-tape, then iterate style of prototyping. Jesse helped me put together a steering mechanism in a very short amount of time. It worked, so that evening I took the robot out for an inaugural journey around Sector67, using it as a sort of telepresence robot as I controlled it from my desk.

Then I broke the steering mechanism gearmotor by switching it back and forth too fast when I got stuck in a corner. The gear before the final output shaft broke into a bunch of tiny pieces.

I replaced it with the gearmotor in the image above on the right that has the partly blue casing. Now that I had a working robot again, it was time to work on the high level path planning and code. I'll put that in the next post.

Giant Display and Home for my First Arduino

The extra large (6.5") seven segment display array pictured above has been at my desk for a long time now, but I never posted about it. Remembering how it came together is a fun walk through memory lane.

Central to this project is my very first Arduino. My boyfriend at the time bought it, spent an afternoon on it with a friend, and then decided he didn't want to do anything else with it and asked me if I wanted it. It is an Arduino Diecimila, and this must have been around 2008-2009, because I remember one of my first projects with Arduino was interfacing with an ultrasonic rangefinder and an optical encoder that year. It later become Swervy's brain. Memories of my freshman dorm remind me why students need hackerspaces:

I was a really bad roommate...I shared this room with my unfortunate roommate. The dividing line is supposed to be roughly in between the black backpack and black storage cube.

My 'hackerspace' at the time was a $99 refurbished Ryobi drill+circle saw, a $30 set of Ryobi drill bits,  a Radioshack soldering iron, and misc. things taken out of the dumpster from USC, along with some sensors I got from my FIRST robotics team and the aforementioned Arduino Diecimila.

Sophomore year of college, beginning of the spring semester, Sparkfun has a big promotional event/server load test. They were giving away one hundred $100 free orders (Sparkfun Free Day). I skipped that morning's Calculus lecture and hit refresh on the page until I won my free order. The four 6.5" red seven segment displays were in my cart for that order, along with a few other things including a Simon Says kit that taught me how to do surface mount soldering.

At some point in 2015 I dug up these parts and put it all together. The circuit is just a bunch of BJT transistors and some buttons to set the input. I never figured out what I wanted it to do, so I put it up on the wall and update it as a calendar, manually, whenever I come in to my office. The Arduino doesn't have enough output ports to handle all the digits at the same time so it enables/sets one at a time, repeatedly and very quickly. Very common solution to a common problem, but it was a fun afternoon project anyway. The wiring harness took the most time. I think I glued the whole thing to the wall with no consideration for whether I'd want to move it in the future. I'll have to tear it off the wall eventually, since Sector67 is getting a new building. Chris jokes sometimes that it looks like what the TSA might think is a bomb; this clock slightly predates the recent clock incident. Which also reminds me of this page which left a huge impression on me when I first read it in college and turned me into a fan.

Telegraph Sounder Player

Yesterday I put the finishing touches on my latest project, the telegraph sounder player.

Video link first if you want to see it in action before you read: https://www.youtube.com/watch?v=vKzZMfzuw68

The device on the left is the telegraph sounder player, and the device on the right is the telegraph sounder. The telegraph sounder itself is a vintage telegraph module. The player allows for up to four sounders to be plugged in to it, and plays back random snippets from text files loaded up on an SD card. It uses American Morse Code (as opposed to international). I loaded the SD card with excerpts from my favorite novels as well as full novels from Project Gutenberg.

The project was made for the owner of the telegraph sounder, Don. Don used to work as a telegrapher for a railroad company (US land based telegraph operations used American Morse code). He had a first version of the player built for him during the most recent Build Madison event and came back to Sector67 with notes for some features he would like for the next version.

Features I included in this telegraph player include: SD card playback, 4 sounder support, knob for speed of playback, separate knob for inter-sentence interval length, status led and ON/OFF toggle, CNC milled hardwood box with brass corners, battery power (est. 10 hours of playback), battery charging circuit onboard.

Design process in brief:

I had four 5V relays I found in a bin here at Sector, so I chose to support up to 4 sounders and to use a 5V Arduino for the brains. The relays are each switched by a BJT transistor circuit, and when switched they connect the sounder port to the battery pack.

The ports are barrel jack connectors with a 10uF capacitor across the leads and a high power rating. low resistance resistor. The resistor is mostly just in case the leads going to the telegraph are placed so that they are touching each other.

I had been playing around with trying to minimize the currents induced by the sounder turning off (the clack, as opposed to the click). A large ferrite choke close to the telegraph with many windings of the connector cable worked, but was a little unwieldy. The capacitor doesn't entirely solve the problem, but helps make the clack sharper. Too large a value and the clack actually bounces presumably as the capacitor charges on the induced current, and then discharges back into the sounder and re-enables it somewhat. The 'problem' is in any case limited to the Arduino resetting only when connected to a computer during the disconnection of the sounder, so it was possible to ignore. 

The SD card is connected through a bi-directional level converter. The Arduino has limited memory, so the code handles things by grabbing small chunks of the files into four small buffers for the four ports. The Morse codes are stored as 2-byte unsigned int objects, with the exception of a few special 4-byte unsigned long objects for some of the punctuation. The code can be found here: Arduino Code.

Access to the Arduino is through the bottom of the box, which is attached with screws to corners that are glued inside the box.

The project uses two battery cells that each have overcurrent and undervoltage protection built in. The charging circuit is as described here.

You can find more photos of the project and an additional write up here: https://imgur.com/gallery/fIBFZ

And a video here:
https://www.youtube.com/watch?v=vKzZMfzuw68

Painting Robot Compilation

A while back somebody donated an old robot arm kit to Sector67. I built a small model of it with glue, tape, foamboard, and potentiometers, and hooked it up to an Arduino. The project is very simple, but it has been very popular at science fairs (and has survived many hours of use requiring minimal fixes only). Here's a compilation video.

Simple Educational Robot Arm with Inverse Kinematics

The point of this project is to test a lesson that covers the theory and construction of a simple, two degree of freedom robot arm that moves in a plane and is controlled by specifying the X and Y position of the end of the arm (the 'hand'), rather than by specifying the angles of the two servos directly.

I ran most of this lesson last year at a summer camp, but due to limited time/high variance in math background of the students, we weren't able to reach the point I wanted to reach. (Really what happened is that I volunteered to have another class join my class for three hours for an activity, which meant a lot of my prep time ended up being soldering leads on more potentiometers...)

This activity seemed promising so I went through it again, at a less hectic pace this time, to try and make a list of the tricky 'gotchas' that would be useful to provide as hints if the goal was to have most of the students complete the project in the class time allotted.

Students who have completed a class covering trigonometry should have sufficient mathematical background to cover the math, which is described nicely on this page: http://www.learnaboutrobots.com/inverseKinematics.htm

A partial list of the tricky parts students may get stuck on (beginners tend to try to build the whole thing at once; a few compound problems can make it very difficult to debug):

• Constructing the arm so it is sturdy enough to last until demonstration time.
• Including math.h and finding documentation for C trig functions
• Keeping track of radians vs degrees
• Measuring link lengths accurately and setting up the physical arm to match the model (the arm should physically be perfectly straight  line if both servos are at their max range
• Mapping the analog inputs (reading from the potentiometers) to a reasonable range of the X and Y axis, in accordance with the units chosen for specifying the link length in the code
• Dealing with floating point error--floating point operations are not the optimal solution here but the goal here is to avoid adding more complex C programming concepts onto an already dense lesson. See the code for more detail. A good summary of the issues can be found here: http://www.engblaze.com/faster-code-fridays-avoid-floating-point-math/
• Using a separate power supply or filtering the Arduino power supply. The arm can otherwise get stuck in a cycle where servo moves, drawing a lot of power momentarily, which causes the analog inputs to jitter (and the onboard LEDs to flicker), which causes a new reading which causes the servo to move... Most students will use the separate power supply if it is provided, but it is also common for students to forget and tie everything to a single rail.

Here's an illustration of an issue that will happen if the student mounts the second link so that it cannot fold inwards. The green area on the left shows where the end of the second link (the 'hand') can reach. The shape is awkward to navigate. The right hand side shows the area reached by a better choice of angle limits for the second link. If it can fold inward, similar to how a human forearm/elbow/upper arm works, then it is easier to use. (This image assumes links are equal length).

I did mount the second servo flipped downwards, to reduce overall height of the assembly. That was a mistake in hindsight, because it means the arm can collide with itself.

Here's a video of the test arm I built, demonstrating that it more or less works correctly.

Finally here's the sketch. It isn't clean or commented...but I've decided it is better to share these things than to never get around to it at all.

Arduino Sketch for this project

Talking Calipers 1

At a recent Sector67 monthly meeting, a member who teaches at a school for the blind and visually impaired mentioned a need for talking calipers. They are hard to find for sale and they typically cost in the realm of $250, though if you have a link to something significantly cheaper please let me know. My overall goal is to design a cheap little add-on that slots on to standard calipers that have a serial output already. This post is about the first step: a quick talking calipers proof of concept. Here's a picture of the working setup. See bottom of post for a video of the setup in action.

Here's what's in the photo. Everything came from either my collection of prototyping stuff, or from the collection of a fellow Sector member who took interest in this project.

• Arduino Uno and Wave shield https://www.adafruit.com/product/94
• Standard cheap calipers
• Adjustable breadboard power supply https://www.adafruit.com/product/184
• Logic Level Converter https://www.sparkfun.com/products/12009
• Earbuds
• A push button switch

If you add up the total cost of the items listed above, it comes out to about $85...so already cheaper than the $250 price point. (I'm aiming to get the total cost down a lot further than that though).

Here's how it all works together. The calipers are bumped up from their usual 1.5 volt voltage from the coin cell, to a 1.9 volt supply from the breadboard power supply. This makes it so that the logic level converter works with the signal from the calipers (because the minimum signal it will support is 1.8 volts, so 1.5 is too low). The signal from the calipers is easy to access; there's a port on the calipers accessible by a removable plastic cover. On the other end of the logic level converter we have the Arduino at 5 volts.

The wave shield comes with it's own example code. The calipers communication protocol has already been reversed engineering by a number of folks online; I got my code snippet from http://nut-bolt.nl/2012/reading-digital-calipers-with-an-arduino/.

The main challenge here is that the Wave shield uses all the external interrupt pins on the Arduino. I needed to use a different kind of interrupt. After a brief read through the forums and the datasheet for the Atmega328P, I decided to use the analog comparator. Comparing against some code snippets made it a fairly easy job to set all the registers I wanted to the values I needed. I used the internal bandgap reference as the positive input to the comparator and my calipers CLOCK signal as the negative input. Then I set the interrupt to trigger on rising output edges. [Noting that the positive input to the comparator, Atmega pin AIN0, refers to Arduino pin D6, and the negative input to the comparator, Atmega pin AIN7, refers to Arduino pin D7.]

The code for this project is available here.

Here's the video:

Next up I'll tackle building a prototype of a cheap and portable version.

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.

July Robotics Class Weeks 2 and 3

A bunch of motors from my late night eBay hunts

3D printed parts held together with popsicle sticks and hot glue; worked very well.

Neater gearbox designed by the TA. Very nice design. Short print times.

Robots coming together.

We ended up spending more of our time focusing on programming, which was important to cover.

Soon I finally begin my journey home. I plan to hibernate and cuddle with my cat for at least a week. Below is said cat.

July Robotics Class Week 1

Photo essay for week 1 of the July robotics class I'm teaching.

A simple H-bridge with switches and LEDs.

Omni-wheel proof of concept; 3d printed plate that fits on a motor shaft.

Printrbot has been printing lots of stuff.

Stickers, buttons, and temporary tattoos for prizes.

Robots under construction.

Another partially finished robot.

Robotics Course Picture Essay

I'm too exhausted to do a write up about my Robotics course. I'm also about to move to another location and teach it again to new students. So for now, here's a bunch of photos, a few videos, and a quick description.

We ended up using Vex kit parts, hot glue, foam board, assorted motors, and other stuff for building materials. The brains were Arduino Leonardos, and the wireless connection was accomplished with a Wixels. We used April Tags with an overhead camera to track the robots and send them position data wirelessly.