End-to-end implementation per docs/sprint-3-plan.md.
Closes the cascade: outer loop (heading control, 10 Hz on Core 1) drives
the inner loop (rudder position control, 50 Hz from Sprint 2). First real
mode other than STANDBY is now activable: HEADING_HOLD.
Builds: pio run -e esp32-dev SUCCESS, RAM 6.8%, Flash 27.1% (355 KB).
Tests: pytest 258/258 green (231 Sprint 2.5 + 27 Sprint 3 new).
Python (arautopilot/studio/simulator/):
- vessel_heading.py: first-order yaw model. ROT responds to
rudder*speed; damping returns ROT to zero when rudder is centred.
Defaults tuned so 5 deg rudder @ 10 kn -> ~3 dps steady-state ROT.
Includes heading_error_deg() shortest-arc helper.
- pid_outer.py: pure-Python outer heading PID. Anti-windup via back-
calculation, gain scheduling by SOG, deadband, derivative LPF,
output saturation, ROT feed-forward (brief sec. 6 -- the term that
distinguishes a premium autopilot from a basic one), rate limit on
produced rudder setpoint, shortest-arc heading wrap-around.
Firmware (firmware/ar_autopilot_v1/src/pid/):
- pid_outer.h: header-only C++17 port. Same algorithm, same variables,
same numerics. Fixed-capacity gain schedule (up to 8 points).
- pid_outer_task.{h,cpp}: 10 Hz FreeRTOS task on Core 1. Subscribes to
TWDT. Reads heading + ROT from the NMEA 2000 snapshot. Uses
operator-configurable SOG (default 15 kn until PGN 129026 wiring in
Sprint 5). Pushes rudder setpoint into the inner loop only when
current_mode == HEADING_HOLD.
Modes (firmware/ar_autopilot_v1/src/modes/standby.cpp):
- HEADING_HOLD activable via request_mode(). Pre-conditions:
* NMEA 2000 heading sensor valid (fresh PGN 127250)
* Rudder sensor valid (median filter filled)
On success, captures current heading as initial setpoint so the
operator doesn't get a sudden swing toward an old setpoint.
Modbus (regenerated from YAML):
- 7 new INPUTs (50-56): outer heading setpoint, produced rudder
setpoint, error, current SOG, live kp/ki/kd.
- 5 new HOLDINGs (24-28): writable heading setpoint, SOG override,
outer base gains. Writing any of kp/ki/kd disables the built-in
3-point gain schedule (operator override).
Tests:
- test_vessel_heading_simulator.py: 6 dynamics tests + 9 parameterised
heading_error_deg edge cases (wrap-around).
- test_pid_outer_python.py: 12 tests covering gain interpolation,
per-tick PID behaviour (deadband, sign, ROT feed-forward,
saturation, rate limit, allowed=false), and three end-to-end cascade
tests (positive step, negative step, wrap-around 360->10).
Cascade verification: outer + inner + rudder dynamics + vessel-heading
simulator settles a 30 deg step within +-2 deg in 60 s.
NOT in Sprint 3 (intentional):
- True Course / Track Keeping / Dodge -- Sprint 5
- Off-course alarms + auto-disengage on sensor loss -- Sprint 6
- COG / SOG / Position via N2K PGN 129025/9/6 -- Sprint 5
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
