One of the main projects this year for the Hardware section was the development of the MiniDrone – a small-scale version of the autonomous car. Its main purpose is to facilitate quicker, safer and more manageable testing of autonomous features, as well as to serve as a means of training for new and existing team members. In future, it will also serve as a more affordable means of assembling and testing interactions between multiple autonomous vehicles. Hardware’s role in this exciting project was to design and manufacture a MiniDrone chassis, allowing the autonomous capabilities to interface with the required sensors and actuators to navigate its environment. The MiniDrone chassis was designed to be lightweight, cost-effective, and customizable, and its total cost to develop was just under AUD$800.
Frame and Rail
A metal frame was used to provide rigidity and strength to the chassis, and served as the foundation for which all proceeding parts would connect. The frame consisted of aluminium extrudes, which were chosen for their light weight and, more importantly, their shape, which allowed for fasteners to be placed anywhere along the bar and on any face.
The rail is a 3D printed extrusion consisting of two slots along the top face, and a series of evenly spaced holes along the sides. It is connected to the metal frame along the centre axis, and its purpose is to allow ‘The Box’ and camera to be connected, as well as any future components, such as a rear camera. These components can be secured anywhere along the length of the rail, due to the even spread of holes.
A 3D-printed chassis body houses the power source and electronics, and is secured to the metal frame. The chassis is exposed on top to allow unimpeded access to the electronics, and to allow connecting wires to and from the Box to be unimpeded. This open-top design allows for ventilation, so the electronics and battery do not get overheated. A rectangular extruded cut is included to accommodate an emergency off switch.
The suspension system involves two shock absorbers attached to the front plate, and two shock absorbers attached to the rear plate. All four shocks are securely connected to the metal frame. Both front and rear plates are connected to the main body of the chassis via separate hinges, allowing the front wheels to pivot independently from the rear wheels. This suspension system allows for an even distribution of weight to all four shock absorbers.
The MiniDrone is outfitted with a CyberTruck-style cover, in order to give it an aesthetic and eye-catching look. The cover is fitted to the MiniDrone in multiple pieces via the metal frame, and can be attached and detached as desired. The roof of the cover also includes a LIDAR mount, intended for future upgrades.
The MiniDrone is powered by a 3-cell Li-Po battery, due to its superior power delivery whilst also being smaller and lighter. The electronics bay was designed to be detachable from the underside to allow for easy access to the electrical components, including the battery. This allows for repairs and upgrades to the MiniDrone to be made easily, without the need to replace components.
A relay is used for the on/off powering of the MiniDrone, due to the potential for high current draw. This also allows the implementation of the low voltage cut-off circuit. The low voltage cut-off circuit was designed to cut power to the MiniDrone when the Li-Po battery voltage is low. This prevents the Li-Po from over discharging, thus preserving the battery life. This also reduces the chance of it catching fire, as Li-Pos are known to do so if misused. An emergency switch has also been added to the outside of the chassis to allow anyone to physically shut-down the MiniDrone. Also included in the electronics bay is a PWM controller, capacitor bank, Electronic Speed Controller (ESC), and a buck converter.
The MiniDrone utilises a box that contains a Nvidia Jetson Nano, which can receive data from connected sensors such as a LIDAR and/or camera. The Jetson performs the image processing and outputs the control signals via I2C to the motor and servo based on ROS messages from either the autonomous processes or controller. The Box is designed to be able to control any MiniDrone developed, as well as be able to gather sensory data in any standard vehicle.
The drive system is powered by a single sensorless DC brushless motor connected to a transmission gearbox via a spur and pinion gear. These components are securely fixed to the rear plate, with sufficient ventilation for when the motor is running for extended periods of time. The output of the gearbox is connected to two separate axle rods connected to the left and right rear wheels. Ball bearings are used to keep the rods fixed along the same axis and rotate in place. A rear-axle differential was originally intended to connect the gearbox and wheels, but due to COVID-related shipping delays, an appropriate differential was unable to be sourced. It was therefore decided that a differential could be excluded for this version of the MiniDrone, and the repercussions of its absence could be managed. Without a differential, both wheels rotate at the same speed when steering, resulting in the inner wheel skipping steps, as the outer wheel will always rotate quicker. This effect was minimised by reducing the speed at which the MiniDrone operated, as well as reducing the maximum turning circle.
The implemented steering system is characteristic of linkage steering driven by a servo motor. The front wheels are attached to each side of the front plate and are able to pivot in place. To ensure the wheels steer at the same angle, a track rod is linked to both wheels. Additionally, a drag link connects the left wheel to the servo arm, allowing the servo to control the steering of both wheels. The turning circle was minimised by using a high torque servo to allow a direct connection to the steering, resulting in a greater turning angle. The voltage supplied to the servo is in the upper region of the specifications to maximise torque.
During manufacturing and testing phases, the team has identified a number of improvements and changes that can be considered for future versions of the MiniDrone. The most notable improvement will be the implementation of a rear axle differential, allowing the wheels on the same axle to rotate at different speeds. Another improvement would be to upgrade the suspension system to better isolate the wheels from the rest of the chassis. In the current version, vibrations from the environment are transferred through the front and rear plates via the wheels, and dampened partially from the rest of the chassis through the shock absorbers and hinges. A more sophisticated design is required to better eliminate vibrational loads to protect the electronics over rougher terrain. Due to the smooth terrain and slower speeds experienced with this first version MiniDrone, the current suspension system was deemed sufficient. Cable management was also identified as a potential area for improvement. Connecting cables to/from the Box, actuators and power supply could be more neatly bundled and cable lengths minimised. Finally, upgrading to a sensored motor would provide information regarding the rotor position to the ESC. This allows for smooth acceleration from zero RPM, making its motion more predictable which is ideal for improved precision of an autonomous vehicle.