hexapod-robot/2026-current/robot/README.md

# Hexapod robot firmware (Teensy 4.0)

This is the **robot-side** firmware. It is fully functional on its own
(driven by the serial test harness below) and receives commands from the
remote (`../remote/`) over nRF24L01. The packet format lives in
`../shared/HexapodProtocol/HexapodProtocol.h` — the single definition both
sketches compile against.

## Building

From the repo root (PlatformIO): `pio run -e robot`, flash with
`pio run -e robot -t upload`, monitor with `pio device monitor -e robot`.
RF24 is pulled in automatically via `lib_deps`; Servo ships with the Teensy
framework.

Arduino IDE also works if you copy `shared/HexapodProtocol/` into your
sketchbook `libraries/` folder and install **RF24** by TMRh20.

## Before you trust this on the floor

Everything in `Config.h` is a placeholder for a generic small hexapod. Measure
your own robot and edit:

- `LEG_MOUNT_ANGLE_DEG` / `LEG_MOUNT_X_MM` / `LEG_MOUNT_Y_MM` — where each
  coxa pivot actually sits on your chassis, and which way it points
- `COXA_LEN_MM` / `FEMUR_LEN_MM` / `TIBIA_LEN_MM` — your link lengths
- `COXA_MIN_DEG..TIBIA_MAX_DEG` — your servos' real safe range of motion
- `STANCE_RADIUS_MM` / `STANCE_HEIGHT_MM` — how far out/down you want the
  resting foot position
- `SERVO_PINS` — must be PWM-capable pins on your Teensy 4.0 wiring
- `SERVO_US_AT_0DEG` / `SERVO_US_PER_DEG` — calibrate per servo, they vary
  batch to batch
- `RADIO_CE_PIN` / `RADIO_CSN_PIN` — your nRF24 wiring. Address, channel,
  data rate, PA level and payload size are in `shared/HexapodProtocol` so
  they can't drift from the remote.
- **Pin conflict:** `SERVO_PINS` leg 3 uses pins 11 and 12, which are the
  Teensy 4.0's SPI MOSI/MISO that the nRF24 needs. Move those two servos
  before wiring the radio.

## Bench-check checklist (do this before the first stand-up)

These are sign conventions that depend on which way your hardware is
physically mounted, and the code has a best-guess default that may be wrong
for your build:

1. **IK elbow/tibia direction** (`Kinematics.cpp`) — command one leg to a
   few known foot positions by hand (e.g. via the serial harness) and watch
   whether the knee bends the way you expect. Flip the sign noted in the
   comment on `tibia_rad` if it bends backwards.
2. **BodyPose roll/pitch sign** (`BodyPose.cpp`) — command a small roll/pitch
   offset and verify the body leans the direction you asked for, not the
   opposite.
3. **RobotIMU register map** (`RobotIMU.cpp`) — written for an MPU6050-style
   chip (I2C addr `0x68`). If your gyro is a different part, its register
   addresses and scale factors will differ; swap them in.
4. **Sit-pose assumption** — the built-in sit pose in `robot.ino` assumes all
   six feet stay grounded and simply lowers the body (a 0.4x height blend).
   If your chassis is meant to rest its belly on the ground instead, this
   needs a different, sequenced approach (fold legs up rather than just
   lowering).
5. **Gait travel direction** (`GaitEngine.cpp`) — the protocol defines
   `+vx` = forward, `+vy` = left, `+yaw_rate` = turn left (CCW). As written,
   the stance phase sweeps each foot from `neutral - half_stride` to
   `neutral + half_stride`, i.e. the foot moves *with* the commanded
   direction, which pushes the body the opposite way. Verify with `w` on the
   bench; if the robot backs up, swap `start`/`end` in `GaitEngine::update`
   (fixes translation and yaw together). The `z`/`c` harness keys were
   written against the current behavior — recheck them after that fix.

## Testing without a remote yet

`robot.ino` has `#define TEST_SERIAL_INPUT` enabled by default. With that on,
open the Arduino Serial Monitor (115200 baud, no line ending needed — single
keystrokes) and use:

| Key | Action |
|-----|--------|
| `a` | toggle armed |
| `s` | toggle sit / stand |
| `w` / `x` | walk forward / backward (`vx` ±) |
| `q` / `e` | strafe left / right (`vy` ±) |
| `z` / `c` | yaw left / right |
| `g` | toggle gait: tripod ↔ wave |
| `m` | toggle walk mode: direct (A) ↔ heading-hold (B) |
| space | zero all velocity (stop walking, stay in current state) |

Each keystroke injects a `CmdPacket` through `Comms::injectPacket()` — the
exact same path a real remote's received radio packets go through, so the
rest of the pipeline (mapper → state machine → gait → pose → IK → servos)
runs identically to how it will with real hardware. Once you have a real
remote transmitting, just remove the `#define TEST_SERIAL_INPUT` line (or
leave it — injected packets simply won't happen if you don't type).

Arm (`a`), then stand (`s`), before trying to walk — the state machine
enforces `DISARMED → SIT → STAND → WALKING` in order and will otherwise
ignore walk commands.

## Link with the remote

- `CmdPacket` and the `FLAG_*` bits in `shared/HexapodProtocol` are the
  exact wire format. Both radios set a static payload size of
  `sizeof(CmdPacket)`, so a robot and remote built from different protocol
  versions won't receive each other at all (and the robot fails safe).
- Failsafe: if no packet arrives for `LINK_TIMEOUT_SEC`, `Comms::linkOk()`
  goes false and the main loop falls back to a zeroed `CmdPacket`, which
  disarms the robot.
- The right stick's roll/pitch/yaw (or X/Y/Z) always apply. The remote's
  handset tilt arrives in its own `remote_roll` / `remote_pitch` fields and
  is added on top only while `FLAG_REMOTE_GYRO` is set, capped at
  `MAX_TILT_DEG` total (`InputMapper`). `FLAG_ROBOT_GYRO` independently
  enables the robot's own IMU stabilization.
- `FLAG_RSTICK_XLATE` switches the right stick between rotation-axes and
  translation-axes interpretation, and `FLAG_MODE_B` switches between the
  two walk modes, all inside the existing `InputMapper`.

## File layout

```
robot/
├── robot.ino          - main sketch: setup/loop, state wiring, serial test harness
├── Config.h           - all placeholder constants (edit this first)
├── SmoothedValue.h     - generic rate-limited setpoint smoother
├── Kinematics.h/.cpp   - per-leg 3-DOF analytical IK
├── GaitEngine.h/.cpp   - tripod/wave gait, per-leg phase, dynamic gait swap
├── BodyPose.h/.cpp     - 6-axis body pose composition (feedforward + IMU correction)
├── RobotIMU.h/.cpp     - gyro/accel read + complementary filter
├── StateMachine.h/.cpp - DISARMED/SIT/STAND/WALKING with smoothed sit-stand blend
├── InputMapper.h/.cpp  - CmdPacket -> twist/pose/mode decoding
├── Limits.h/.cpp       - workspace clamp, joint rate limit, stance polygon check
├── Comms.h/.cpp        - nRF24L01 receive (packet definition is in shared/HexapodProtocol)
└── ServoDriver.h/.cpp  - servo attach/relax/write
```