Comet Drone — Dual-Core ESP32 Flight Controller

An open-source, high-performance micro-quadcopter flight controller firmware built for the dual-core ESP32-WROOM-32 (Xtensa LX6). It features hardware-accelerated 400 kHz I2C sensor fusion, 20 kHz PWM motor drive, and a dual-pipeline input arbiter multiplexing a high-speed STM32 nRF24 SBUS radio receiver and a zero-latency WebSocket Web Dashboard over local SoftAP.

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1. System Architecture & Dual-Core Topology

The firmware exploits the asymmetric dual-core architecture of the ESP32-WROOM-32 using FreeRTOS task pinning to ensure the real-time flight control loop is never interrupted by networking or TCP/IP jitter.

                  ┌─────────────────────────────────────────────────────────┐
                  │                 ESP32-WROOM-32 SILICON                  │
                  └────────────────────────────┬────────────────────────────┘
                                               │
               ┌───────────────────────────────┴───────────────────────────────┐
               │                                                               │
  ┌────────────▼────────────┐                                     ┌────────────▼────────────┐
  │         CORE 0          │                                     │         CORE 1          │
  │   (Networking Core)     │                                     │   (Flight Control)      │
  └────────────┬────────────┘                                     └────────────┬────────────┘
               │                                                               │
     ┌─────────┴─────────┐                                           ┌─────────┴─────────┐
     │  WiFi SoftAP      │                                           │  IMU (MPU6050)    │
     │  (SSID: "DRONE")  │                                           │  400kHz Fast I2C  │
     └─────────┬─────────┘                                           └─────────┬─────────┘
               │                                                               │
     ┌─────────┴─────────┐                                           ┌─────────┴─────────┐
     │  HTTP Web Server  │                                           │  SBUS Receiver    │
     │  (Port 80 HTML5)  │                                           │  UART2 100k 8E2   │
     └─────────┬─────────┘                                           └─────────┬─────────┘
               │                                                               │
     ┌─────────┴─────────┐                                           ┌─────────┴─────────┐
     │  WebSockets (81)  │                                           │  Input Arbiter    │
     │  100ms Telemetry  │                                           │  Setpoints/Safety │
     └─────────┬─────────┘                                           └─────────┬─────────┘
               │                                                               │
               │   Shared Telemetry Snapshot & Control Spinlock (Thread-Safe)  │
               └───►───────────────────────────────────────────────────────◄───┤
                                                                               │
                                                                     ┌─────────┴─────────┐
                                                                     │  Cascaded PID     │
                                                                     │  Angle & Rate     │
                                                                     └─────────┬─────────┘
                                                                               │
                                                                     ┌─────────┴─────────┐
                                                                     │  LEDC Motor Mixer │
                                                                     │  20kHz Coreless   │
                                                                     └───────────────────┘

Core Separation Details

• Core 0 (Networking & Dashboard Task):
  • Manages the 802.11 b/g/n SoftAP network (192.168.4.1).
  • Serves the integrated HTML5/JS Glassmorphism Ground Control Station over HTTP Port 80.
  • Handles bidirectional WebSocket communication (Port 81), parsing incoming command streams and broadcasting live attitude/telemetry packets at 10 Hz.
• Core 1 (Deterministic 250 Hz Flight Loop):
  • Executes a strictly timed 4000 µs loop.
  • Reads raw 6-DOF gyroscope and accelerometer registers over 400 kHz I2C with complementary attitude estimation.
  • Ingests inverted Futaba/FrSky SBUS frames at 100 kbps over hardware UART2.
  • Computes outer angle and inner rate PID loops.
  • Drives 4 coreless DC motors using 20 kHz ultrasonic PWM via ESP32 LEDC timers.
  • Samples battery voltage on dedicated ADC1 (GPIO 34) without interference from the WiFi radio.

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2. Hardware Pinout & Wiring Specification

Component / Subsystem	Pin Name	ESP32 GPIO	Peripheral Channel	Wiring & Hardware Notes
STM32 SBUS Receiver	RX2	GPIO 16	UART2_RX	Connect to STM32 UART2 TX (Inverted 100k 8E2).
MPU6050 IMU SDA	I2C_SDA	GPIO 21	I2C0_SDA	400 kHz I2C Data (Requires 4.7kΩ pull-up to 3.3V).
MPU6050 IMU SCL	I2C_SCL	GPIO 22	I2C0_SCL	400 kHz I2C Clock (Requires 4.7kΩ pull-up to 3.3V).
Motor 1: Front-Right	M1_FR	GPIO 19	LEDC_CH0	CCW Rotation. Driven via N-MOSFET (20 kHz PWM).
Motor 2: Rear-Left	M2_RL	GPIO 18	LEDC_CH1	CCW Rotation. Driven via N-MOSFET (20 kHz PWM).
Motor 3: Front-Left	M3_FL	GPIO 23	LEDC_CH2	CW Rotation. Driven via N-MOSFET (20 kHz PWM).
Motor 4: Rear-Right	M4_RR	GPIO 25	LEDC_CH3	CW Rotation. Driven via N-MOSFET (20 kHz PWM).
Battery Sense (ADC1)	BATT_ADC	GPIO 34	ADC1_CH6	Input-only. $2:1$ divider ($R_1 = R_2 = 10\text{k}\Omega$). Safe with WiFi.
Status LED	STATUS_LED	GPIO 2	GPIO	Active-HIGH onboard LED on ESP32 DevKit.

                       X-QUAD MOTOR ORIENTATION

                            ▲ FRONT (Heading)
                            │
               M3 (FL, CW)  │    M1 (FR, CCW)
               [GPIO 23]    │    [GPIO 19]
                     \      │      /
                      \     │     /
                       \────┴────/
                       │  ESP32  │
                       │ WROOM32 │
                       /────┬────\
                      /     │     \
                     /      │      \
               M2 (RL, CCW) │    M4 (RR, CW)
               [GPIO 18]    │    [GPIO 25]
                            │
                          REAR

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3. Input Arbitration & Failsafe Logic

The Input Arbiter (input_arbiter.h) provides seamless multiplexing between the physical 2.4 GHz nRF24/STM32 radio receiver and the emergency/auxiliary WiFi Web Dashboard.

                           INPUT ARBITRATION MATRIX
  ┌───────────────────────────────────────────────────────────────────────────┐
  │ RADIO ACTIVE?  FAILSAFE BIT?  AUX1 DELEGATION?  WIFI ACTIVE?  │  ACTIVE   │
  ├───────────────────────────────────────────────────────────────┼───────────┤
  │     YES             NO              NO             ANY        │   RADIO   │
  │     YES             NO             YES             YES        │   WIFI    │
  │     YES             NO             YES              NO        │   RADIO   │
  │      NO             --              --             YES        │   WIFI    │
  │      NO             --              --              NO        │ DISARM/FS │
  │     YES            YES              --              NO        │ DISARM/FS │
  └───────────────────────────────────────────────────────────────────────────┘

Channel Mapping & Normalization

• Channel 1 (Roll / Aileron): $1000 \dots 2000,\mu\text{s} \longrightarrow -30.0^\circ \dots +30.0^\circ$ (Angle Mode).
• Channel 2 (Pitch / Elevator): $1000 \dots 2000,\mu\text{s} \longrightarrow -30.0^\circ \dots +30.0^\circ$ (Angle Mode).
• Channel 3 (Throttle): $1000 \dots 2000,\mu\text{s} \longrightarrow 0 \dots 255$ PWM (Headroom reserved for PID).
• Channel 4 (Yaw / Rudder): $1000 \dots 2000,\mu\text{s} \longrightarrow -180^\circ/\text{s} \dots +180^\circ/\text{s}$ (Rate Mode).
• Channel 5 (AUX1 — Control Delegation Switch):
  • $< 1300,\mu\text{s}$: Physical Radio Control.
  • $> 1700,\mu\text{s}$: Delegate control authority to WiFi WebSocket dashboard.
• Channel 6 (AUX2 — Arm Switch):
  • $> 1500,\mu\text{s}$: Armed (Enforces throttle $< 5%$ safety interlock before spin-up).
  • $< 1500,\mu\text{s}$: Disarmed.
• Channel 7 (AUX3 — Flight Mode):
  • $< 1500,\mu\text{s}$: Stabilized Angle Mode (Self-leveling cascaded PID).
  • $> 1500,\mu\text{s}$: Acro / Rate Mode (Direct rate gyro tracking).

Unified Failsafe & Disarm Triggers

1. Radio Signal Loss: If the STM32 receiver loses nRF24 link, it asserts SBUS Byte 23 Flag Bit 3 (0x08).
2. Dual Signal Loss Failsafe: If SBUS failsafe is asserted AND WiFi heartbeat times out ($> 500\text{ ms}$), motors_disarm() is invoked instantaneously ($0\text{ ms}$ delay), shutting off all 4 PWM channels and zeroing PID integrators.
3. Critical Battery Disarm: If battery voltage drops below $3.00\text{ V}$ ($1\text{S LiPo}$ cutoff), the flight controller performs an emergency auto-disarm.

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4. Setup, Building & Flashing

Prerequisites

• PlatformIO Core (CLI) or PlatformIO IDE (VS Code extension).
• USB-to-UART drivers (CP2102 / CH340) installed.

Build and Flash

# Clone the repository
git clone https://github.com/iD024/ESP32FlightController.git
cd ESP32FlightController

# Compile firmware for ESP32-WROOM-32
pio run -e esp32dev

# Upload to ESP32 (specify port if not auto-detected)
pio run -e esp32dev --target upload

# Launch serial telemetry monitor (115200 baud)
pio device monitor -b 115200

Sensor Calibration Workflow

1. Place the drone on a flat, level, stationary surface.
2. Power on the board. The status LED will turn solid ON during calibration.
3. The MPU6050 samples 2000 gyro/accelerometer readings to compute precise zero-rate offsets and level gravity vectors.
4. When calibration completes, the LED issues 3 rapid blinks, indicating the drone is ready for arming.

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5. Web Control Dashboard & Telemetry Guide

The flight controller hosts an onboard HTML5/JS Ground Station stored in flash memory.

 ┌─────────────────────────────────────────────────────────────┐
 │  COMET DRONE GROUND CONTROL STATION                         │
 │                                                             │
 │  Attitude: Roll: +2.14°  Pitch: -0.85°  Yaw: 142.30°        │
 │  Battery : [██████████] 4.12 V (92%)  Loop: 250 Hz          │
 │  Link    : Active (RADIO SBUS)        Arm Status: DISARMED  │
 │                                                             │
 │  [Motor FR: 0] [Motor RL: 0] [Motor FL: 0] [Motor RR: 0]    │
 └─────────────────────────────────────────────────────────────┘

1. Connect to SoftAP:
   • SSID: DRONE
   • Password: drone1234
2. Launch Dashboard:
   • Open any browser and navigate to http://192.168.4.1.
3. Real-Time Telemetry:
   • Displays 3D artificial horizon / attitude angles ($\text{Roll}, \text{Pitch}, \text{Yaw}$).
   • Individual real-time motor PWM outputs ($M_1 \dots M_4$).
   • Battery voltage and percentage calculation with visual alerts.
   • Active control source indicator (RADIO (SBUS) vs WIFI (WebSocket)).

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6. Serial Tuning CLI (115200 Baud)

Connect over USB Serial to tune PID gains live without recompiling:

Command	Description	Example
RP:<val>	Set Roll Rate $K_p$	RP:0.55
RI:<val>	Set Roll Rate $K_i$	RI:0.12
RD:<val>	Set Roll Rate $K_d$	RD:0.035
PP:<val>	Set Pitch Rate $K_p$	PP:0.55
PI:<val>	Set Pitch Rate $K_i$	PI:0.12
PD:<val>	Set Pitch Rate $K_d$	PD:0.035
AP:<val>	Set Roll Angle $K_p$	AP:2.20
BP:<val>	Set Pitch Angle $K_p$	BP:2.20
MT:<0-3>	Test spin individual motor at low speed	MT:0
MS	Emergency stop all motors	MS
ST	Print comprehensive system status report	ST

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7. License

Distributed under the MIT License. See LICENSE for details.