Quadruped (Arduino + PCA9685)

A 2-DOF-per-leg quadruped controller for Arduino Uno using a PCA9685 16-channel servo driver.

This firmware uses:

• Inverse kinematics (IK) for each leg
• A phase-shifted walking gait
• Servo safety clamping + saturation reporting
• Startup posture initialization + emergency stop from Serial
• MPU-6050 IMU integration with Mahony AHRS orientation estimation

Hardware

• Arduino Uno
• PCA9685 servo driver (I2C)
• 8 servo motors (Hip + Knee for each leg)
• External 5V servo power supply (recommended)

Servo Channel Map

Leg order used in code:

• LEG_BL = 0 (Back Left)
• LEG_FL = 1 (Front Left)
• LEG_BR = 2 (Back Right)
• LEG_FR = 3 (Front Right)

PCA9685 channels (src/globals.cpp):

• Back Left: Hip 0, Knee 1
• Front Left: Hip 4, Knee 5
• Back Right: Hip 8, Knee 9
• Front Right: Hip 12, Knee 13

I2C / Board Setup

• PCA9685 default address in code: 0x40
• PWM frequency: 50 Hz
• Serial monitor baud: 115200

platformio.ini is configured for:

• platform = atmelavr
• board = uno
• framework = arduino

Build and Upload

Option 1: PlatformIO (VS Code)

1. Open this folder in VS Code.
2. Install PlatformIO IDE extension.
3. Build from PlatformIO panel.
4. Upload to the Uno.
5. Open Serial Monitor at 115200.

Option 2: PlatformIO CLI

pio run
pio run -t upload
pio device monitor -b 115200

Runtime Controls

Serial commands:

• Send s or S to trigger emergency stop.

Emergency stop behavior:

• Gait loop halts
• Legs move to neutral IK posture
• Saturation statistics are printed

Code Architecture

The codebase is split by responsibility:

• src/main.cpp
  • System startup and main control loop
  • I2C device check and gait scheduler
• src/globals.cpp
  • Global state and leg configuration table
• src/hardware.cpp
  • I2C detection + angle-to-PWM tick conversion
• src/ik.cpp
  • 2-link inverse kinematics + reachability checks
• src/servo_control.cpp
  • IK-to-servo mapping, mirroring, clamp, saturation tracking
• src/gait.cpp
  • Swing/stance trajectory generation for each leg
• src/safety.cpp
  • Servo initialization, emergency stop, saturation reports
• src/imu.cpp
  • MPU-6050 readout, startup gyro calibration, Mahony AHRS, Euler angle state API

Headers are under include/.

Gait and Kinematics Tuning

Primary tuning parameters are in include/config.h:

• Leg geometry
  • UPPER_LEG_LENGTH
  • LOWER_LEG_LENGTH
• Gait motion
  • STEP_LENGTH
  • STEP_HEIGHT
  • GAIT_CYCLE_DURATION (defined in src/globals.cpp)
• Control timing
  • CONTROL_DT
• Workspace target
  • X_OFFSET
  • Y_GROUND
• Mechanical mapping
  • HIP_FRAME_ROTATION
  • legs[] mechanical offsets (hipMechOffset, kneeMechOffset)

Safety Model

Safety mechanisms currently implemented:

• IK reachability test (with singularity margin)
• Servo angle clamp to [0, 180]
• Saturation event counters per leg/joint
• Initialization mode disables saturation counting during startup posture

Startup Sequence

1. Initialize Serial and I2C
2. Detect PCA9685 at 0x40
3. Set PWM frequency to 50 Hz
4. Move all legs to neutral standing posture
5. Start gait loop

Notes

• Power servos from a dedicated supply; do not power multiple servos from Uno 5V.
• Keep grounds common between Uno and servo power supply.
• If you see frequent saturation warnings, retune offsets and/or gait targets.
• IMU implementation expects MPU-6050 at I2C address 0x68 and performs startup gyro calibration over initial samples while the robot is held still.

Repository Layout

QUADRUPED/
  include/
    config.h
    robot_types.h
    globals.h
    hardware.h
    ik.h
    servo_control.h
    gait.h
    safety.h
    imu.h
  src/
    main.cpp
    globals.cpp
    hardware.cpp
    ik.cpp
    servo_control.cpp
    gait.cpp
    safety.cpp
    imu.cpp
  platformio.ini