As I said, I started to build a first prototype (with only the structure and mechanical parts for now).
The roof is ready, the system holding the balloon and the RGB LED strips works well, the results are convicing. I also tested the robot tolerance to wind, as the big balloon offers a huge surface area and the robot is quite small and light. For that I used a fan and powered the robots motors making it go towards the fan. The result is that the robot is able to move without problem with a light wind and should work properly outside most of the time.
I also prototyped the connectors to the power rail with aluminium strips, but it needs more work as I couldn’t make a reliable electrical connection last Thursday.
I won’t be able to do much next week as all the exams are coming. I will try to find some time to work more on the 3D model by the end of the week.
As planned, I worked on our robot mechanical structure since Monday. The goal is to create the best solution with the following constraints (most important first) :
- The robot should be able to move in various flat grounds
- The robot should be as stable as possible
- The robot should be able to reach 0.7m/s
- The robot should be as quick as possible to build (since we will need to build 20-25 of them)
- The robot should be as cheap as possible
So I needed to find the best balance between a stable and quick robot (that would be easy if it could be big) and a cheap and easy to make one. Indeed the 3D printed parts needs to be as small as possible to be printed in a reasonable amount of time and to use a minimal quantity of PLA to keep the cost down.
Here is the design I came up with for the first prototype. To give you a sense of proportion the balloon is 400mm in diameter.
The “antennas” we can see on the top are meant to hold LED strips in contact with a ballon (each of the 4 pieces of strip holds 2 RGB LEDs). The light will be then diffused by the balloon, as demonstrated a few weeks ago.
Some rectangular holes are also open underneath the robot for aluminium strips that will get in contact with a rail, allowing the robot to charge its battery automatically. The rail is yet to be designed and we’ll tell you more in the next few weeks.
By the end of the week I will make a first prototype to test this design.
In the last post I wrote I would work on some software for the Decawave modules in the last part of the week. But as the team had to split for the week-end, I gave the modules to Perceval and focused on the mechanical design of our robots. It’s hard to admit it but I think I like it, but of course what I do is heavily based on intuition rather than real knowledge.
So I started to design the robots wheels. I created the ball caster (in the left of the image), and after 3 attempts we had a quite convincing solution. For now the ball is made of 3D printed PLA, but it will be replaced by a PTFE one when we receive the order. Using PTFE should lower the friction, but it is quite low already, so we will be able to test our prototype as soon as it’s ready.
I also made a prototype of wheel. I first had to measure the angular velocity at the output of our motoreductor, which is about 6.7Hz under a reasonable load, to guarantee that our robot could reach the 0.7m/s required by our specifications. Then I designed the wheel. I tried to make it flexible to maximise contact area with the ground, hence the spiral and the thin parts. The wheel was printed and then dipped in acetone, this has two main effects :
- improved strength of joints in between layers
- PLA becomes more flexible
Apart from that, I created my first part with ExpeditionPCB (our controller, the STM32F303R6T6). Fortunately, the tougher part (the pad stack, i.e. the footprint of the part in the PCB) was already done, and all I had to do was to adapt the schematic from a similar STM32.
In the next few days I’ll keep on working on the mechanical design, creating the structure and the system holding the balloon above the robot.