Last week I blogged that there wasn’t enough room in the robot hull for the control board. Well, I finally made it fit. I reduced the circuit a bit (I decided some of it was redundant), I used some smaller components (mainly the DC-DC converter, but also some capacitors), and I realised I could extend the board by 10mm within the existing design. I have finished the board design and sent it to JCLPCB to have it manufactured. All available digits are crossed for a positive outcome!

The board was developed in KiCAD. This is new to me, but I found it pretty easy to get the hang of. I found this video tutorial to be particularly useful.

The circuit has the following features:

• The main controller is a Teensy 4.1 Arduino compatible. It is mounted so that it’s USB port will be exposed towards the back of the vehicle; this will make it possible to re-program it in-situ.
• There is a JST socket which will connect to the Pi’s header. It connects the UART on the Pi to a UART in the Teensy. There are two further GPIO connections, one to indicate that the Pi is running, one to request that the Pi closes down. Power for the Pi is also provided by the connection. In operation, the Teensy combines all the robot functions which the Pi accesses via a serial protocol.
• A LiPo protection circuit. If the LiPo voltage goes too low then this circuit can close the Pi down, then chop the power to the whole robot. The circuit expects to be connected to a 3S LiPo (which was chosen to match the AX-12 servos used).
• Four independent 11A motor controllers for the robot drive wheels. The motors have encoders fitted, and this circuit will maintain PID control for each. This system can also be used to calibrate distances travelled.
• An IMU (Intertial Measurement Unit). The robot uses this to understand it’s orientation in the World. It is mainly used to calibrate turns.
• A serial connection (using the SBUS protocol) to a radio control receiver.
• An ”implement” connector. Implements are added to the robot for some of the games. This connector presents a 3v3 serial connection and power supplies for the implements.
• A pair of AX-12 servo bus connectors. These servos provide more torque and more control options than regular radio control servos. They are accessed via a proprietary serial protocol which the Teensy can provide via an interface on the board. The base robot uses a pair of AX-12 servos to lift the front bulldozer blade, and further servos will be used in the Eco-Disaster barrel lift.
• An 3v3 serial interface to a further board that is mounted in the ”chin” of the robot. This board interfaces to three ToF (Time of Flight) sensors.
• A 3v3 serial and power interface to the robot’s decorative lid. The lid will have lights and a sound system (to play the Thunderbirds theme tune). Also, the little Doofus pilot has a head turn servo.
• A start button and a stop button interface.
• A status LED connection.

We’ve been accepted into PiWars 2024! So, I think it’s time for an update.

Since our last post, Firefly has jumped from the Fusion 360 CAD world into the real world. (In other words, it’s been 3D printed and sprayed). It has been fitted with a radio control system and two motors and it can now be driven.

It spent last Saturday being driven around an obstacle course (sometimes rather too enthusiastically) by people of all ages at the Sidmouth Science Festival. During the day, the tracks unclipped a few times, and the driving sprockets came undone a few times too, but basically the robot stayed in one piece for the day. The only real problem is that the paint flaked off of the cab. In my view, if it can stand being driven by random five-year-olds, then it is robust enough for PiWars, where either our expert driver, or a Raspberry Pi will be at the helm.

I was also pleased to note that the batteries (it has three 18650 LiPos wired in series giving a total capacity of 3Ah at a nominal 11.1V) were nowhere near exhausted by the end of the day.

However, there are some problems.

• I am rather disappointed by the finish of the cab. The “glass” has picked up the layers from the 3D printed mould. Also, the paint edges are not particularly tidy, even though they were done with masking tape. Some paint mist has got inside the glass, and now (i.e. following the Science Festival) much of the paint has flaked off of the cab anyway.
• It’s also not possible to mount the speaker in the cab, as the glass prevents access to the nuts that were meant to be used with bolts to mount the speaker.
• But most significantly, there is not enough room in the hull for the electronics. When I did the design I planned for a robot control PCB that was roughly the same size as last year’s. I knew there would need to be extra electronics, (two extra motor drivers), but I figured I could save space by building a PCB. So far, I have not been able to get the necessary components on the PCB (without even trying to lay the board out completely).

I’m wondering if I should re-work the design to include a bigger space in the hull for a bigger PCB. And perhaps re-work the cab to make fitting the speaker easier and to remove the need to spray the vacuum formed cab. For now, no decision on the way forward has been taken.

You can find more information about Firefly here.