Some projects are never really finished. They rather evolve and improve over their lifespan. For the Dot Bot project, it was time to upgrade the drawing gondola. The old gondola was more like an improvised experiment and really had its downsides. The biggest issues were the bad weight distribution, and the center of rotation not being the tip of the pen and an improvised and inflexible mechanism to hold pens. Another big downside was the fact that the battery was integrated in such a way that it was impractical to replace it, so the bot needed some major downtime during the charging of the gondola.
The weight distribution issue was most noticeable while doing fast horizontal changes of velocity. Since the gondola’s center of mass was way below the axis of rotation, an acceleration to the right would create an angular momentum turning the gondola clockwise, an acceleration to the left counterclockwise. This issue was made worse by the fact that the center of rotation was not exactly the tip of the pen, this meant that the swinging of the gondola would result in squiggly lines. To mitigate this problem somewhat, I added lots of weights to the top which resulted in a much heavier gondola as necessary.
The new gondola has a completely different design approach. It consists of 3 big bearings in the center. Two large ball bearings connected to the belts that are controlled by the stations, and one linear bearing in the center of the ball bearings that is used to guide the pen with precision towards the canvas. Also the heaviest component, the battery, is now above the center of rotation, and the electronics and some light counterweights below the center of rotation. This results in a gondola that rotates exactly around the tip of the pen, and also has almost no tendency to rotate when accelerating horizontally. The center of mass sits only slightly below the center of rotation to keep the battery and the electronics clear of the belts.
The next big improvement is the large inner diameter of the linear bearing, and the tube that slides through it. With minimal effort, custom adapters can be 3D printed that center pens with a wide range of diameters in the center of the bearings, and therefore also perfectly in the center of rotation of the Dot Bot.
Finally, the new battery is super easy and quick to exchange. It can be replaced by another battery in less than 5 seconds due to its magazine-like connection. And while one battery is in use, the second battery can be charged. This means almost no downtime due to battery charging anymore.
In the end, this upgrade was really worth it and time well spent. The pen holder and the quick battery connection are big quality of life improvements and the perfectly centered center of rotation as well as the optimal weight distribution allow for much faster drawing speeds. Also, this new gondola design results in a much cleaner drawing result which is what counts the most in my opinion.
If you have a robot or an automation that you would like to improve then please shoot me a message using this email address.
