add fanchars

README.md

@@ -82,7 +82,22 @@ Verify access: `test -w /dev/ttyUSB0 && echo ok`
 
 1. Flash firmware and open the serial monitor at 115200 baud.
 2. Confirm `TCA9548A detected` and four valid sensor channels (`ch2`–`ch5`).
-3. Tune **heat PI** (stored in EEPROM on autotune complete):
+3. Find **stir fan** speed (chamber ~35°C, idle):
+
+   ```
+   target 0
+   fanchars
+   ```
+
+   Heats to max **60°C** at **30%, 100%, 60%, 80%** fan (cool to **40°C** avg between each). If the best is not at 30% or 100%, runs once at the midpoint of the two lowest spreads (~1–3 h). Or: `python3 scripts/fan_characterize.py`. When done:
+
+   ```
+   fanchars save
+   ```
+
+   Stored PWM is the minimum-stir floor while regulating (≥40°C avg).
+
+4. Tune **heat PI** (stored in EEPROM on autotune complete):
 
    ```
    target 0
@@ -92,25 +107,9 @@ Verify access: `test -w /dev/ttyUSB0 && echo ok`
 
    Emergency cutoff is fixed at **70°C** — you can autotune at 50–55°C with ABS in the chamber while hot corners stay below that.
 
-4. Tune **mix PI** (spread → fan) while drying:
+5. Optional: adjust **mix PI** at target if spread still drifts (`mixpi`, `pid save`). Stir fan from step 3 is usually enough to start drying.
 
-   ```
-   target 55
-   log on
-   ```
-
-   Watch `spread` in status. Adjust mix gains without reflash:
-
-   ```
-   mixpi 40 2
-   pid save
-   ```
-
-   Defaults: heat Kp/Ki from autotune; mix Kp=40 Ki=2 in firmware until tuned.
-
-5. Optional: `stepresp 45 35` logs open-loop fan steps if you want to estimate mix gain before live tuning.
-
-Send `help` over serial for all commands (`target`, `mixpi`, `fan test`, `log on/off`, `status`, `pid`, etc.).
+Send `help` over serial for all commands (`target`, `fanchars`, `mixpi`, `fan test`, `log on/off`, `status`, `pid`, etc.).
 
 ## Raspberry Pi control