This lab project is about making a quadcopter deliver a payload on its own. We built a quadcopter around a Pixhawk 4 flight controller running PX4, planned a mission in QGroundControl, and used a servo release mechanism to drop the payload at a chosen waypoint. The aircraft takes off, flies the planned route, releases the load at the drop point and flies back to land, with no one flying it. Along the way we learned how to set up a flight controller from scratch, calibrate its sensors, configure failsafes and integrate a servo into a mission.

Quadcopters are unmanned aerial vehicles with four rotors arranged in a square, each driven by its own motor. They have become popular for both hobby and professional work because they are maneuverable and stable and can hover in place. Unlike a traditional helicopter, with its complicated mechanical linkages, a quadcopter uses electronic speed controllers (ESCs) to change the speed of each motor independently, which gives very precise control of movement.

The design gives six degrees of freedom: three translations (forward and back, left and right, up and down) and three rotations (roll, pitch and yaw). That makes it a good platform for any task that needs precise positioning in three-dimensional space.

A quadcopter works on differential thrust. The four rotors are arranged so that diagonally opposite pairs spin the same way, one pair clockwise and the other counterclockwise. This balances the torque of the spinning propellers, so the aircraft keeps its orientation. Flight control comes from four basic actions:

• Throttle: when all the motors speed up or slow down equally, the quadcopter climbs or descends
• Roll: speeding up the rotors on one side and slowing the opposite side makes it roll, which moves it sideways
• Pitch: the same idea with the front and rear motors, which moves it forward or backward
• Yaw: speeding up the clockwise motors while slowing the counterclockwise ones (or the other way round) creates a net torque that turns the aircraft about its vertical axis

An autonomous dropping system lets a quadcopter deliver a payload to a specific place without a pilot stepping in. The quadcopter is programmed to release its load, which could be a package, a sensor or emergency supplies, at a set location. This is done by connecting a release mechanism to the flight controller and using mission planning software to define where the drop happens. It is useful for precision agriculture (dropping seeds or fertilizer), delivery services and emergency supply delivery.

• Get familiar with the QGroundControl interface
• Set up a flight controller such as the Pixhawk according to our requirements
• Achieve fully autonomous operation from takeoff to landing
• Deliver the payload accurately to the target location
• Optimize the flight path to get the most out of battery life and delivery range

The quadcopter is made of several parts, and we chose each one for the mission.

• Frame: an X configuration S500 frame with a PCB central plate, picked for its balance of maneuverability and stability
• Motors: four DJI 2212 920 KV brushless motors. The KV rating is the RPM per volt, and lower KV motors give more torque for larger propellers while higher KV ones give more speed with smaller propellers
• Propellers: chosen according to the motor rating and the total thrust the mission needs, using the motor data sheet below. Propellers have to be balanced, because vibration affects flight stability and sensor readings
• ESCs: four 30 A speed controllers, which turn the battery's DC power into the three-phase power the brushless motors need and control motor speed precisely
• Flight controller: a Pixhawk 4 running PX4, which does position hold, return to home and autonomous missions
• Power distribution board: shares battery power between the ESCs and the other electronics, with voltage regulators for the different parts
• Battery: an HRB 4500 mAh 60C lithium polymer battery. Capacity and discharge rate decide flight time and performance
• Radio control: a Futaba T6K 8-channel transmitter and receiver
• Sensors: the Pixhawk 4 has an accelerometer, gyroscope and magnetometer on board, and an M8N GPS module is connected externally
• Servo mechanism: an MG90S servo controlled by the Pixhawk 4 operates the release mechanism, chosen for reliability and precise control

We put the quadcopter together in this order:

• Attach the frame parts securely with screws
• Mount the motors on their arms
• Install the ESCs near each motor and connect them
• Secure the power module and connect it to the battery and flight controller
• Fix the Pixhawk 4 at the center of the frame
• Connect the GPS module to the flight controller
• Mount the transmitter and receiver for remote control
• Connect the telemetry system for data monitoring
• Attach the propellers, checking the orientation
• Secure the LiPo battery and connect it to the power module
• Connect the external servo to the flight controller
• Calibrate the flight controller and check every connection

QGroundControl (QGC) is an open-source ground control station for managing and operating drones and other unmanned vehicles. It works with several autopilots, including PX4 and ArduPilot, and has a friendly interface. The Pixhawk 4 was set up with PX4 in these steps:

• Install QGroundControl on the computer and connect the Pixhawk 4 by USB
• Install the firmware: in Vehicle Setup, open Firmware, choose the right PX4 version and follow the instructions to flash it
• Airframe: choose the airframe type (S500 generic), apply, and restart the controller
• Sensor calibration: compass (following the on-screen rotations), gyroscope (keeping the vehicle still), accelerometer (placing the vehicle in several orientations) and level horizon
• Radio setup: bind the transmitter and receiver, connect the receiver to the Pixhawk, calibrate the control channels in QGC, and configure and test the failsafe settings
• Flight modes: assign modes such as stabilized, mission, manual and return to launch to the transmitter switches, set their parameters and test the switching
• Power setup: set the battery parameters (cells, capacity, warning thresholds) and calibrate the voltage and current sensors if fitted
• Failsafes: in Vehicle Setup, open Safety and set all the failsafe modes and the action for each
• Servo output: in Parameters, set an unused servo output to the manual function
• PWM range: set the minimum and maximum to define the servo's range of motion, typically 1000 and 2000, and adjust them for the particular servo and mechanism
• Control channel: assign a transmitter channel to the servo for manual control, make sure it is calibrated in Radio setup, and test the servo from the transmitter

The mission itself is built in the Mission Planning view of QGC. We created a mission with waypoints, starting with takeoff, and added a DO_SET_SERVO command at the waypoint where the payload should be released, specifying the servo number and the PWM value that triggers the release. A geofence can be added for safe operation, and then the mission is uploaded to the Pixhawk.

Before flying, the mechanism was tested on the ground. We set COM_PREARM_MODE to always, tested the servo with the quadcopter disarmed, checked that the mechanism releases smoothly and reliably, and ran ground tests with the payload attached.

The flight log from the mission shows the quadcopter following its planned route closely. On the left, the estimated path, the setpoint and the GPS track lie almost on top of each other as the aircraft flies around the waypoints and returns. On the right, the actuator controls over the whole flight show a steady thrust command with roll, pitch and yaw corrections working continuously to hold the path, from the stabilized takeoff in the mission to the return and landing.

Autonomous dropping with a Pixhawk 4 quadcopter is a sophisticated use of UAV technology with many practical applications. By understanding the main components and working principles of a quadcopter, setting up the hardware and software properly through QGroundControl, and testing the system carefully, an operator can get a reliable autonomous dropping capability.

Flight control software like PX4 keeps improving, and so do sensors and power systems, which will make autonomous drops more precise, reliable and safe. As regulations change to allow more advanced UAV operations, this technology is likely to see wider use across many industries.