The RoboCup Junior is a robotics competition aimed at secondary schools in Australia and around the world. Last year, I competed in it as part of a team of five other students from my school (Melbourne High School). The gist of the soccer competition is there are two teams in each soccer match, and each team has two robots on the field, which is about the size of a large tabletop and is covered in green felt with lines marked on it. The aim is to shoot goals. The constraints: the robots have to be autonomous - they can communicate with each other but cannot be remote controlled in any way, they can't damage other robots, and they have to fit within a certain size. How your team carries out tactics, choice of motors, wheels, sensors, etc. etc. is up to themselves.
A lot of work went into our team's robot and it is of the technical and engineering nature of this blog, so I have finally got around to writing up a blog post about my involvement in the project. At this stage, I think I'll separate the posts into the mechanical and software side of things. This part will go over the mechanical design. The GitHub repo for all the code/CAD models is at: https://github.com/BillyWoods/RCJ_Soccer_Robot
That model above is our solution to the problem. The robot has three omni-wheels set 120 degrees apart. This lets the robot turn on the spot and move in any direction regardless of where the front is pointing. All the red parts on the model are electronics and the green parts are 3D printed. I modeled/coded the whole robot in OpenSCAD, it's got a pretty hefty code base to it considering how simple it may look (about 4000 lines of code at the moment). This is due to the detail and how parametric it is, most of this turned out to be overkill. Still a good learning experience however, and having the model killed a few problems before the robot was even built and allowed us to change the electronics layout very quickly when we tested a few different boards.
One advantage to modeling out every little detail can be seen above: drilling/cutting templates can be produced for each of those layers of the robot automatically. Simply export as a .dxf file, add the cross hairs for the holes manually and then print out at 1:1 scale. If I ever need to make a large batch of these robots, the plates can also be made on a CNC with the .dxf files.
Seen above - the tricks of the trade. Stick the template onto whatever sheet material you're making the robot out of, centre punch all the holes, then cut out the circular plates with a jigsaw. Since the aluminium is so thin, the jigsaw was extremely rough on the 1.2mm aluminium sheet with even a small length sticking over the side of the bench. Using the "custom jig" on the right was my solution; having all that thick MDF around supporting the aluminium as it was cut made cutting less dangerous, less noisy and much more fun.
Here's a bottom plate, it has a curved section at the front cut out to trap the soccer ball. The DiBond it is made of looks much nicer than aluminium.
Next step, print out the motor mounts on the 3D printer:
It is very satisfying watching what started as a design on a screen get translated into something tangible.
Skip a month or so and a few missing photos and here's an operational robot:
The robot in these photos is controlled by an Odroid U3 and an Arduino on top of it as a shield. The Odroid is a single board computer based on the same ARM chip as the Samsung Galaxy S3. It is like a more powerful Raspberry Pi. We used it so I could code a computer vision program to make use of the camera on the robot. The Arduino shield on top of the Odroid is an Arduino Uno in all but shape. It uses an Atmega328, the Arduino bootloader and has all the same pins available. It communicates with the Odroid via serial/UART. The Arduino IDE runs on the Odroid and sketches are uploaded via it.
The wheels used on the robot were not straightforward to attach to the motors. They have a 9mm bore which only goes halfway through the wheel hub. To attach them, I machined couplings out of steel on my lathe and tapped a hole for an M3 grub screw in them. These couplings would then press fit into the wheel hub and the grub screw would secure them on the motor's shaft.
This worked, however the drill bit I used to bore out the hole for the motor's shaft was slightly bent so The hole it drilled was slightly too large and as a result the wheels were wobbly. Not wanting to have to spend time making more, and keeping in mind that I had to make multiples of all these parts for other groups in the school's robotics club, I looked for a 3D printed alternative. The first thing I tried was almost exactly a copy of the press fit metal couplings, though in plastic. These didn't work so well because the printed parts did not have a good enough tolerance for a snug press fit and the plastic couldn't take a thread well enough for the grub screw that secures the motor shaft.
The second attempt was much less naïve. The design made use of the spokes in the wheel to allow more torque to be transferred. There was also room for a captive M3 nut for the grub screw. This design worked very well once it was glued and pressed into the wheel.
Here's a bonus shot of the soccer ball and the fenders I designed and printed for the robot. These were an afterthought when we realized the bottom plate was too low and thin and would just chip the ball into the air.
In terms of improvements to the robot's design, I do have concerns about the motors wearing down, in particular the gearboxes. Though they have metal gears inside, we've noticed an increase in backlash after some use. The motors' output shafts only use bushings instead of ball bearings and the motors' shafts are carrying the entire weight of the robot. Stepper motors would be the best alternative. NEMA 17s are cheap and powerful, the same goes for their drivers. NEMA 17s are also very tough in terms of wear. Stepper motors are also designed for much more precise control of position compared with DC motors and the omnidirectional drive system needs precise control of the relative speeds of the wheels of the robot. Despite these concerns, in the end the mechanical side of the robot turned out to be the most reliable aspect of the robot. The success of the robot, or lack of it, was as far as we can tell, because we ran out of time to get the sensors, electronics and software working well. The good part is that all this work can be used and built upon in this year's coming competition.
