Backstory
After 4 Years, the day had finally come, the DOT-BOT project, that actually started its journey under a different name (Spraybot) was allowed to do what it was designed to do, and what its logo was still showing, painting with a can of spray paint!
But how did we get here? A professional artist from Germany called me up, and asked me about my drawing robot project, knowing I had done some large scale (pen) painting in the past. He explained that he also worked on a drawing machine in the past, that was able to use a spraycan to plot vector images onto a wall. However, his gantry approach did not scale up very well and was intrigued by my polargraph, which was already fitted with 10 meters of GT2 belts on each side.
He asked me if I was interested in revisiting my drawing robot and making it ready to draw with spray paint. I was more than happy to accept his offer and he then got in touch with old friends in Berlin that had the perfect spot for testing.
Although the robot was working fine in my clean and tidy room scale setup using pens, I knew I had to redesign several parts to make it work reliably in a potentially much larger and less friendly space. Also, due to limited time constraints, I had to make sure that on site assembly would be minimal.
Most notably, the changes that were done for the first day of spray action were:
Developing a Spray Gondola
Until very recently, every gondola design cycle revolved about the ability to push a pen against the wall. For this iteration however, spraycans were the name of the game. Since I was already quite happy with the last gondola design, I reused several parts like the battery holder and the electronics compartment, but redesigned the mount for the spraycans and mounted everything on a wooden board, which is more economical than to 3D print a large slab of plastic. A noticeable upgrade is that I preserved the USB cables data lines, which means I can now reprogram this new gondola by attaching a USB extension to the battery compartment and my PC without the need of disassembling the electronics every time (Yes, I learn from my mistakes).
Splitting up the Motor and the final pulley
On my wall setup, the “stations” contained the motor and were at the same time also the pulley that the belts were finally redirected from. This meant, making a larger canvas available, would require the entire station with electronics and motor to be mounted at the maximum height. With the new approach, only the free rolling final pulley needs to be mounted at the max height. The motor can now be inline in the belt in the downwards path. This uses more belt, but allows for an easier installation of the redirection pulley e.g. on a wooden post, without the need to worry about power cables, wifi strength, weight and space constraints etc.
The upgrade from ESP8266 to ESP32
The old setup used two ESP8266 for each motor station. Each pair shared their work with one managing wifi communication, reporting and buffering and the other one only driving the motor using the AccelStepper Library. This worked fine, but you cannot use AccelStepper for commercial projects for free, which this project inadvertently has become since the artist paid me to develop all the required modifications.
Also since I always wanted to open source all of this project at some point, I really wanted to replace AccelStepper with a library that had a more open license. AccelStepper uses GPL V2 which would have meant that I would have needed to release all of the code under this license as well. Lots of projects stay away from GPL code for this exact reason and to increase the chance someone can reuse this code, I really wanted to use “MIT” licensed libraries only.
Luckily, I found FastAccelStepper, a different library that can be used as a drop in replacement with the desired open source license. The downside of FastAccelStepper is, however, that it does not run on ESP8266. Hence the switch
Getting rid of the birds nest
One major constraint is the sturdiness of the entire construction and therefore also ease of use and transportation. My old clean lab setting was very gentle, and therefore the electronics would also work even when all the wired connections were made basically with jumper cables. However, for any serious use in the field, or during transportation, these jumper cables could be a potential source of trouble. Therefore, and not because I acquired the skill of PCB designing and want to use it on every project now, I opted to design a simple but clean PCB that was capable of housing two microcontrollers, a motor driver and a voltage regulator. After only 10 days (thanks to priority shipping), I held the manufactured PCBs in my hands and they worked beautifully after assembly and powering them on. Now, even the biggest vibrations are unable to mess with any connection and will therefore facilitate transport and setup by a lot.
Protecting the electronics from the elements
Since we want to do outdoor testing sometime in the future, I did not want to have the electronics exposed to the elements. I therefore opted to design a neat case that can house the PCB, and has only cutouts for ventilation and cables. This should keep light rain, dust and occasional bird droppings out and further increase reliability.
Preparations
The second time the artist and I met in person, we met at the location we could use for our tests. A rough wall in a basement, nothing fancy but 5 meters high and perfect for our use case.
Since the wall was rough, and I knew we would only have very limited time during our test run, I decided to premount all of the station components onto wooden boards such that we could attach each station to the walls by only using two more screws. I then 3D printed standoffs, to test these completed assemblies in my lab environment to make sure we would not have to deal with tiny screws or belt alignment issues at the location.
First Test Run
After only a few weeks after the first contact, we met up for the third time. This time, we had a clear goal in mind. We really wanted to see if this robot can actually spray! First, we got to the hardware store and bought protection to not spray directly onto the walls, but onto some white wallpaper. We then headed to the location and set everything up which went surprisingly smoothly. When we put a can into the gondola, we were both full of anticipation of what to expect. Would the servo be strong enough to actuate the cap, would the speed of the motors be sufficient to create a nice line, would the wifi connection hold up and so on.
Surprisingly the test was a blast. After I changed so many variables in this project, without the ability to test everything in full in my lab, I did not expect the Dot-Bot to perform as well as it did. Sure the painting we produce does not look like much, but given that our goal was to draw a single line, we managed to exceed both of our expectations by a ton.
What comes next
When one thinks about the differences between pens and spraycans, one big challenge becomes immediately obvious. Spraycans will apply a lot of paint in one spot when not being moved sufficiently. Pens suffer from the same problem, but much less severe. Some pens like ballpoint pens don’t suffer from this at all. This means spray cans should really be used with a constant speed. During the next weeks, the software will get an additional feature that allows the gondola to move over different waypoints, without stopping at each coordinate and accelerating again. This will mean that the dripping will be much less, and images with more coordinates can easily be used without wasting a ton of time for accelerating and decelerating. Also, real artwork will be prepared such that the next canvases will become true masterpieces for sure!
