Quadcopter

During the summer after my freshman year, I decided to build a quadcopter as one of my first independent projects. I chose it because it’s a popular beginner project and seemed like a great introduction to personal engineering builds.

Objectives
  • Remote controlled
  • 1:4 Thrust to weight ratio
  • Flight height of 200+ feet
  • Wrist rotation
  • Weight: 750-1000 grams
Materials
  • 4 XING 2306 Brushless Motors
  • SoloGood F722 FC and 4-in-1 ESC stack
  • 14.8V Lipo Battery
  • FS-iA6B Receiver
  • Flysky FS-i6X Controller
  • Propellers
Research

I watched several videos on custom designed quadcopters. Many used carbon fiber, cardboard or 3D printed frames. I chose the latter route, following Great Scott’s video.

I also did a deep dive into the specs of motors, propellers and batteries to find the ones that would produce sufficient thrust to carry the weight of the frame and the onboard electronics.

Motors

Motors are classified by a 4 digit number, the first two digits represent the width of the stator and the latter two represent the height of the rotor. All measurements are in mm. The wider the width, the more torque the motor has. The narrower the stator width, the faster the RPM. Motors with a wider stator are generally more efficient. The higher the stator volume, the more torque a motor can produce.

kV represents the RPMs per one volt. Motors with lower kV produce more torque, but spin at slower speeds.

Propellers

Propellers are classified by a four digit number; the first two digits represent the diameter of the propeller, while the latter two represent the pitch, the ability to move a substance (air). These measurements are in inches.

Batteries

Lipo batteries are characterized by the arrangement of cells, capacity, and C-rating. Ex. 4S represents four Lipo cells in series and since it possesses 4.2V at full charge, we can find the total voltage of the battery pack. Capacity is a measure of capacitance, and C-rating measures the current output. Definitions vary on whether C-rating refers to the maximum current in bursts or continuously.

  • Total Weight = ~ 760
  • Total Thrust = 626 * 4 = 2520
  • This satisfies 4:1 TW ratio
  • Voltage of motors:
    • Max continuous power (W) x 4 = ~700 x 4 = ~2800 W
  • Voltage of battery:
    • 2.2 Ah x 120C = 264 A
    • 4 x 3.7V = 14.8V
  • Power (W) = Amp x Volts
    • = 264 A x 14.8V = 2800W

Since 3907 > 3280, the battery is able to supply sufficient voltage to the motors. ESC must have a current rating that is 20-30% greater than 36.21A so 50A should be sufficient.

Build

I 3D-printed the frame provided by Scott using PLA and then soldered the electronics. I encountered several issues due to shorts, which required multiple attempts to troubleshoot and get everything working correctly. Through this process, I learned techniques such as changing soldering tips to provide wider thermal coverage for larger pads and covering one’s iron tip with solder between uses to protect against oxidation. By the end, I had significantly improved and gained more confidence in my soldering skills.

Electronics
ESC (Electronic Speed Controller)

A 4-in-1 ESC integrates four individual ESCs into a single circuit board, each controlling a separate motor. ESCs are characterized by their sizes: 30x30mm, 20x20mm, and 16x16mm. Typically, a drone requires four ESCs along with a power distribution board. When selecting an ESC, verify that it supports the voltage of your battery and has a greater amperage than that required by the motors by about 20-30%. An ESC’s burst limit is typically higher than its continuous current limit. The most popular ESC firmware include iterations of BLHeli and AM32.

ESC protocols are the method of communication of the motor signal between the FC and the ESC. The Common ones include PWM, Oneshot variations, Multishot and DShot variations. Dshot is the standard ESC protocol in FPV drones.

An important capability that DShot supports is Bidirectional DShot which allows the ESC to report RPM data back to the FC. This allows the FC to set notch filters at the right frequency to remove noise that appears as error percentages grow. Notch filters, also known as band stop filter or band reject filter, are filters that reject/attenuate signals in a specific frequency band called the stop band frequency range and pass the signals above and below the band.

The ESC consists of a microcontroller unit (MCU), gate driver, MOSFET, low dropout voltage regulator (LDO), current sensor, and filtering capacitors. An LDO is a voltage regulator that is used to convert battery voltage to an acceptable lower voltage to power the microcontroller and other components. The MCU, or processor, is the brain of an ESC where the firmware is stored. Gate drivers drive the MOSFETS and are connected to the gate of a MOSFET. Gate drivers improve active braking effectiveness.

MOSFETs are a field effect transistor and are often used as switches or to amplify electrical signals. Here, it’s used as a power switch to drive the motors. Bigger MOSFETS mean the ESC can handle higher voltage and current, making the ESC more robust.

The current sensor measures the current that goes through the ESC and sends that information to the FC.

A capacitor is often soldered to the power pads of the ESC to reduce noise and improve flight performance. 

FC (Flight Controller)

FC consists of a processor, or a central processing unit, which acts as the “brain” of the device. It also consists of an Inertial Measurement Unit (IMU) which contains an accelerometer and a gyroscope. The Gyroscope measures angular velocity while the accelerometer measures linear acceleration.

The Gyroscope and processor communicate through SPI or I2C. SPI is preferred between the IMU and processor, because it allows for a higher gyroscope refresh rate than the I2C.

A UART is a serial port on the flight controller through which a serial protocol is sent that is completely digital (composed of ones and zeroes). It is used to connect external components such as radio receivers, video transmitters, etc. Each UART has two pins: one for transmitting data (TX pin) and one for receiving data (RX pin).

A peripheral device is a nonessential hardware component that connects to a computer to enhance its functionality. TX on the peripheral device connects to the RX on the FC, and vice versa. The TX and RX of the same number must be connected for a device.

CAM is where you connect the FPV (first-person view) camera in an analog FPV system. VTX is where you connect the video transmitter if you run an analog FPV system. The I2C pads (SDA and SCL) are for connected devices like a GPS compass and a barometer. BZ+ and BZ- are buzzer pads for beepers. The LED pad is for connecting RGB lights. A BEC (battery eliminator circuit) are voltage regulators on a flight controller.

Receiver & Transmitter

The receiver is responsible for receiving the user’s commands from the transmitter and passing that data to the FC.

A protocol is the language spoken between devices within an FPV drone. TX protocols is the communication between radio transmitter and radio receiver. RX protocols is the communication between radio receiver and flight controller. ESC protocols is the communication between FC and ESC.

PPM and SBUS are often preferred over PWM when using an FC, which passes all the channels through a single wire. IBUS is Flysky’s serial protocol which is a two way communication, it can send and receive data.

Lower frequency systems tend to be better at long range. Radios controllers can support a certain number of channels. Each control or switch requires a separate channel to transmit data to the receiver. The two gimbals take up four channels because they have four inputs.

A radio transmitter is a handheld device that sends signals to the drone’s receiver which the FC translates into drone movement.

There are two main styles: full size and gamepad style. Two of the most commonly used frequencies are 2.4Ghz and 900MHz. 2.4GHz is the current standard for radio control, while 900MHz is used for long-range flying due to its better signal penetration and reliability.

I relied heavily on the posts by Oscar Liang to learn about the above details.

Firmware

Betaflight is one of the most common flight controller firmware options. Additionally, BLHeli is very common for ESCs. RPM filtering is a feature in Betaflight that uses RPM telemetry data to remove motor noise.

Control

The sticks of the remote are often referred to as “gimbals”. The left stick when pushed up or down controls the throttle, or how fast the motors spin. Moving this stick left or right, makes the drone rotate left or right, which refers to the yaw. Pushing the right stick forward makes the drone move forward, while pulling it back does the opposite, this is known as the pitch. Moving this stick to the left or right makes the drone lean and move in that direction, this refers to the roll. The configuration of the two control sticks is known as the radio mode. Above we have described mode 2, which is most commonly used.

Commands from the radio controller and set wirelessly to the radio receiver. The radio receiver sends these commands to the flight controller. The flight controller processes these commands along with data from the sensors onboard to calculate the speed at which each motor should move. The ESC receives a signal from the FC and uses it to adjust the motor speed to generate the desired thrust.

Software

I configured my ESC and FC stack using Betaflight. I ran into several configuration issues and referred to resources provided by Joshua Bardwell and RCwithAdam to resolve them.

Finally, I was able to arm the quad and control it with my controller. Now, it was time to fly!

Results

I had the first test flight in my backyard. The quadcopter struggled to lift off the ground and in the process broke one of the arms. Also, the antennas of the receiver got caught in the propeller. I quickly made some repairs and double-checked all the Betaflight configurations before giving it another go, but ran into similar issues. It seems like there was an unbalanced weight distribution on the frame or that the motors weren’t strong enough. In the future, I plan to look into reducing the weight of the drone and studying the drone’s aerodynamics so next time it’s able to get in the air!