Brake Mode (Copter)

Executive Summary

Brake Mode is a dedicated safety mode designed to stop the vehicle as quickly as possible and hold its position. It is often assigned to a "Panic Switch" or triggered by the "Pause" button on GCS controllers (like the old 3DR Solo controller). Once engaged, it ignores all pilot inputs until switched out or a timeout occurs.

Theory & Concepts

1. Kinetic Energy Dissipation

Stopping a drone moving at 20 m/s requires dissipating a massive amount of kinetic energy.

• The Problem: If you just "level" the drone, it will coast for 50 meters due to momentum.
• The Solution: The Brake controller calculates the Deceleration Vector. It commands a negative pitch/roll to produce a horizontal thrust component that actively fights the drone's current velocity.

2. Open-Loop vs. Closed-Loop Stop

• Manual (Open-Loop): You tilt the drone back, but you have to guess when to level it.
• Brake (Closed-Loop): The EKF monitors the GPS velocity. As the speed reaches zero, the controller smoothly reduces the "back-angle" to exactly 0, preventing the drone from flying backwards after it stops.

Hardware Dependency Matrix

Since Brake mode relies on stopping and holding a 3D position, it has similar requirements to Loiter.

Sensor	Requirement	Code Implementation Notes
GPS	CRITICAL	Requires a valid position estimate to calculate velocity errors and hold position after stopping.
Compass	CRITICAL	Required for the inertial navigation system to function correctly.
Barometer	REQUIRED	Used for altitude hold (Z-axis stop).

Control Architecture (Engineer's View)

Brake Mode is essentially a simplified PosHold mode with hard-coded targets.

1. Input Lockout:
   • Unlike almost every other mode, ModeBrake::run() does not read pilot sticks.
   • Throttle, Roll, Pitch, and Yaw are completely ignored.
   • Code Path: ModeBrake::run() calls pos_control->input_vel_accel_xy with zero vectors.
2. Deceleration Logic:
   • The controller sets a Target Velocity of (0,0,0) cm/s.
   • The AC_PosControl library uses its internal PID loops to drive the vehicle to this velocity using BRAKE_MODE_DECEL_RATE (Default: 750 cm/s/s).
   • This is typically much more aggressive than a standard Loiter stop.
3. Position Hold:
   • Once velocity reaches zero, the underlying PosControl logic automatically latches the current position and fights drift.

Timeout Behavior

Brake Mode has a unique feature: it can automatically exit itself.

• Trigger: If timeout_to_loiter_ms() is called (usually by GCS logic), a timer starts.
• Action: When the timer expires, the vehicle attempts to switch to Loiter mode.
• Fallback: If Loiter fails (e.g., GPS glitch), it falls back to AltHold to at least maintain altitude.
• Code Path: timeout_to_loiter_ms.

Failsafe Logic

• GPS Loss: If GPS is lost while braking, the vehicle cannot determine its velocity or position. It will likely trigger a standard EKF Failsafe (switching to Land or AltHold), overriding the Brake mode.

Key Parameters

Parameter	Default	Description
BRAKE_MODE_DECEL_RATE	750	(cm/s/s) The deceleration aggression. Higher values stop faster but may cause pitch-up oscillations.
BRAKE_MODE_SPEED_Z	250	(cm/s) Max vertical speed allowed during the brake maneuver.

Source Code Reference

• Mode Logic: ardupilot/ArduCopter/mode_brake.cpp

Practical Guide: The Panic Switch

You lose video. You lose orientation. The drone is flying towards a tree. You need to stop NOW.

Configuration

1. RC Setup: Assign a momentary switch (like the "Trainer" switch) on your radio to a spare channel (e.g., Channel 8).
2. ArduPilot: Set RC8_OPTION = 17 (Brake).
   • Note: Do not assign this to a flight mode slot. An RC Option overrides any flight mode.

Usage

• Press and Hold: The drone slams on the brakes. It ignores all stick inputs.
• Regain Composure: Take a breath. Look at the drone. Orient yourself.
• Release: The drone stays in Loiter (or whatever mode you were in, usually).
• Warning: The braking is aggressive (0.75G). Ensure your battery is secure, or it might eject through the front.