The Control Stack: From Stick to Prop

Executive Summary

When you move the sticks on your radio, you aren't moving the servos directly. You are feeding a request into the top of a complex "Cascade Controller." The pilot (or autopilot) requests a Position or Angle. The controller converts this into a Rate. The Rate controller converts this into Motor Output.

Theory & Concepts

1. The Hierarchy of Constraints

In robotics, you cannot change position without first changing velocity, and you cannot change velocity without first changing acceleration.

• Mode Layer: Defines the Goal (e.g. "Stay at 10m").
• Position Layer: Defines the Velocity needed to reach the goal.
• Attitude Layer: Defines the Acceleration (Lean Angle) needed to reach the velocity.
• Rate Layer: Defines the Torque needed to reach the angle.

2. Time Constants & Loop Rates

The stack is organized by frequency.

• Outer Loops (Position): Slow and stable. If they run slightly late, the drone drifts a few centimeters.
• Inner Loops (Rate): Fast and critical. If they run late, the drone oscillates or flips.
• Why this matters: This is why ArduPilot prioritizes the Rate Controller as a FAST_TASK in the scheduler.

The Main Loop (400Hz)

The heartbeat of ArduCopter is the fast_loop.

• Frequency: Typically 400Hz (every 2.5 milliseconds).
• Execution: Every cycle, the scheduler runs the following critical tasks in order:

1. The High-Level Planner (update_flight_mode)

• Input: Pilot Sticks, Waypoints, Fence.
• Logic: "Where do I want to be?"
• Output: Target Lean Angles (Roll/Pitch) & Climb Rate.
• Libraries: AC_WPNav, AC_PosControl.

2. The Attitude Controller (run_rate_controller)

• Input: Target Lean Angles.
• Logic: "How fast do I need to rotate to get to that angle?"
• Output: Target Rotation Rates (Deg/s).
• Libraries: AC_AttitudeControl.

3. The Rate Controller (Inner Loop)

• Input: Target Rates vs Gyro Rates.
• Logic: "How much power do the motors need to achieve this rotation speed?"
• Output: Normalized Motor Throttle (0.0 to 1.0).
• Technique: PID + FeedForward.

4. The Mixer (motors_output)

• Input: Roll, Pitch, Yaw, Throttle (0-1).
• Logic: "Which motors need to spin to create this torque?"
• Output: PWM / DShot pulses to ESCs.
• Libraries: AP_Motors.

Data Types

• Position/Velocity: Vector3f (North, East, Down).
• Attitude: Quaternion (Rotation from Earth to Body frame).
• Rates: Vector3f (Radians/second).

Source Code Reference

• Scheduler Table: Copter::scheduler_tasks[]
• Loop Runner: AP_Vehicle::loop()