This project was done in association with the IEEE Student Branch on the UCLA campus. This is the first iteration of the project.
The goal of the project was to: 1. Design and assemble a custom PCB for a drone, 2. Write a program to hover the drone, then fly the drone. Because of the nature of the project, the PCB was required to be small and compact (meaning SMD soldering was needed) and the program needed to be quick and responsive.
MCU: The main chip is the model STM32F405. This was chosen for its ideal balance between size and functionality. It is able to support all the needed sensors and motor drivers for our drone and can be powered by a 3.3v supply which closely matches the 3.7v of standard drone batteries (3.7v is stepped down to 3.3 using a voltage regulator). Additionally, this chip was more ideal than other chips of similar style because it handles floating point calculations better which allows for simplicity and efficiency when writing the program.
Voltage Regulator: The voltage regulator used is the Richtek RT6150B-33GQW which was choosen for its size and the use of buck-boost as well as operating within the voltage supplied by the battery. The voltage regulator was simple to use and had a well documented spec sheet in comparison to other choices but used exposed pads instead of pins for soldering which proved to be the biggest challenge in assembly.
Wireless Module: The wireless module used is the nrf24l01 which is standard, easy to use, and familiar so we used it in this application since it fits well with the acclerometer module.
Accelerometer/Gyroscope: The module used is the mpu6050 which is standard, easy to use, and familiar so we used it in this application since it fits well with the wireless module.
Oscillator: The oscillator used is the Abracon ABLS-12.000MHZ-B2-T which is a crystal oscillator at 12 mHz which was suitable for the MCU. This specific oscillator was choosen because the documentation was clear on how to configure the module. A SMD mounted version was chosen.
Passive Components: For the resistors, capacitors, and inductors, we primarily used parts with the package size 0603 which would fit the board without being overly difficult to solder. Each piece was checked for its operating voltages as well as the operating temperatures to make sure no issues would arise.
General: The board is composed of two sides where each side has routes and vias to connect the different components. The negative spaces are occupied by polygons. On the front side, there is a seperate polygon for 3.7v and 3.3v that are connected by the voltage regulator that steps the voltage down. On the back, there are seperate polygons for ground and motor ground. Motor ground is seperated because the motor ground will have more noise and we dont want to include the added noise in other components.
Battery: The battery is a generic 3.7v drone battery that connects via a JST connector. A couple of decoupling capacitors are placed directly alongside the female header on the board. It is placed as close as possible to the JST headers that connect to the motors as we need to decrease the resistance by having a large polygon that gives a direct connection.
Voltage Regualtor: The voltage regulator is placed between the 3.7v and 3.3v planes such that the two are completely isolated. There is an inductor included directly alongside the voltage regulator as recommended by the spec sheet. A few decoupling capcitors are also included.
Voltage Divider: A couple resistors are used to form a voltage divider that gives an analog input to the MCU which can be used to read battery status. These resistors are placed such that they do not have signficant effect on the routing or the polygons.
Programmer: The programmer is a set of 4 male header pins that allow us to upload programs to the drone through a seperate board. It is placed in the top left corner for easy access and to make sure the sensors do not cover the pins.
Reset Button: The switch is placed such that there is easy access to it while not having the placement disrupt any polygons. A capacitor is included to stabalize the signal sent to the MCU.
Oscillator: The oscillator is placed as close to the MCU as possible to reduce unwanted affects on the traces. As indicated by the spec sheet, there are two resistors that ground both the input and output of the oscillator.
Motors: The motors connectorsare placed along the top of the board for easy access when plugging in the JST connectors. Each motor is paired with a motor driver circuit including a diode and a mosfet. These components are located as close as possible to each other and have the thickest possible traces to decrease resistance. Each motor driver is connected to a PWM capable pin located on the MCU.
MCU: The MCU is placed in a central position on the board as it needs to connect to most components on the board. It is turned such that the length of traces is optimized and components that are more essential to have short traces are accounted for. The decoupling capacitors for the MCU are located on the opposite side of it and are connected by vias.
Sensor Modules: Both modules are lifted above the board using headers and because they sit at similar heights above the board, their placement does not overlap. The decoupling capacitors are placed near the respective pins on the MCU.
Not Space Efficient: A few choices made the board more cluttered and more difficult to make routes and polygons. The motor driver circuits could have made better use of the two sided board as in the current configuration, they occupied a quarter of the board. The oscillator would have benefitted from being a through-hole component as the soldering pads and capacitors occupied a lot of space. In a different part of the board. There is a lot of open space meaning there are parts of the board where there are unnecessary bottlenecks.
Accelerometer Placement: The idea behind the accelerometer and wireless module not overlapping is that since they were both lifted off the main board at a similar height, they could not occupy the same space. However, the accelerometer lies slightly lower than the wireless module so it can be moved closer the the MCU, allowing for shorter traces.
Bottlenecks: There are some moderate bottlenecks in this design and also places where the polygons have rather indirect routes. For example, some components rely on power that comes gaps in the MCU which signficantly bottlenecks the polygons. The 3.3v needed by the wireless module comes from a couple vias and a route used as a bridge rather than having a direct connection as it is cut off from the 3.3v polygon.
Voltage Regulator: The voltage regulator was small and simple but due to the exposed pad design and its small size, was a pain to solder. This ended up being by far the biggest hiccup in the assembly of the board and required backup components.
Decoupling Capacitors: The decoupling capacitors are not optimally placed in every case but rather are placed to prevent bottlenecks. Capacitors for certain components are placed near the MCU rather than the component itself.