Project Overview
This robot uses 4 DC motors to drive mecanum wheels, which enable it to be driven in any direction. The Pixy2 camera is mounted on a pan & tilt mechanism, giving it plenty of freedom to track objects that are presented to it. Attached to the base of the camera is laser range finder, enabling it to measure distance in millimetres. The wheel drive shafts have optical slot encoders which allow the robot to measure how far it has travelled or turned. The ESP32 micro can be connected to a Wi-Fi transceiver, enabling it to be controlled remotely using a Wii Nunchuk controller, and to send data over the air to be displayed on a PC screen..

Click on the image to the right to see the robot being put through its paces. The OLED display contents can be sent to a Windows application, over WiFi, to give a large screen view..

This design was created using a 3-D modeller and printer, to ensure that the assembly goes together with ease. The released design benefits from a range of improvements that were determined from building the first prototype. You will learn from the documentation how this design was put together and wired up.

The body is essentially made in two halves, held together by small screws. This makes the overall assembly easier to build and offers a degree of maintainability. The pan mounting has graduations built into it to make the calibration process simpler to achieve. This element of the design could be used as a stand-alone pan & tilt solution for another project, albeit the Pixy2 makers can provide a kit for this if needed.

In order to accurately calibrate the Pan & Tilt servo, and to adjust the PWM values needed to drive the 4 DC motors, I have provided a custom Windows app, which makes this task much easier. It also allows you to play around with the motor drives, to gain a better understanding of how they work. The code also contains control algorithms, making it easy to drive and position the camera.

The nylon gears used within the wheel motors, combined with the inertia associated with the weight of the robot, means that a large starting torque is required to overcome the initial friction, but once the robot is moving, much less torque is required. This characteristic is addressed by superimposing a low frequency (30 Hz) dither signal on top of the motor demand value, which effectively vibrates the wheels into action. This presents a much lower starting torque, and makes it much easier to control.

The circuit diagram is shown here on the left, with the ESP32 connected to the 4 DC motors, with 3v3 power being derived from a DC-DC step down module, to power the RGB LEDs and most of the attached circuitry. Because of the number of PWM signals and slot encoders used in this design, we use most of the pins on the ESP32. I initially intended to use interrupts to capture changes in the wheel slot encoders, but as this proved not to be possible, I used the second core of the ESP32 to pole these sensors on a regular basis.; which works fine on slow speeds where automated movement requires some precision.

In construction I use wire wrap technology to make the process easier and more reliable. There is however an initial outlay in buying the wrapping tool and spools of wire. One benefit of this approach is that you can run and test your design before soldering the connections for improved reliability. Any errors can therefore be easily addressed during construction.

All of the 3-D models are provided as STL files, zipped together into one file. They can therefore be used with a slicing application of your choice, to create the g-code files for use with a 3-D printer. For my project I used PrusaSlicer, which is a free download from the internet. I’ve also included a simple stand, which clips onto the base of the PixyBot2 for testing or demonstration.

The project is meant to be built in two parts, one being the PixyBot2 robot and the other being the Wii wireless transceiver box (optional). You will need to download models and instructions for that project from this page here. Whilst the PixyBot2 does operate in several stand-alone modes, without doubt the use of the Wii Nunchuk and wireless link are great additions to this project. One feature is that you can send the contents of the OLED display over the air to a PC screen or large monitor. The wireless project is also used as a common component with several other projects, and is likely to be needed for others in the future. So it is well worth considering.