Add docs/temp_control_calibration.md

Covers Smith predictor parameter identification (C, M, L, Td) from log
data, model-plant replay verification, and controller performance metrics.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NSo8R6GjoBQdStB3j67zSs

docs/temp_control_calibration.md

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+# Temperature controller calibration
+
+## Controller overview
+
+`components/pid/temp_controller_smith.py` implements a Smith predictor. Each tick it computes:
+
+```
+theta_ist = theta_ist_model + (theta_ist_plant − theta_ist_model_delay)
+```
+
+`theta_ist_model` comes from a fast internal `Pot` (transport delay Td = 0);
+`theta_ist_model_delay` from a second `Pot` with the full configured Td.
+The correction term `(theta_ist_plant − theta_ist_model_delay)` removes the dead
+time from the feedback path.
+
+The plant model (`components/plant/pot.py`) integrates:
+
+```
+temp += (delayed_power_in − L·M·(temp − theta_amb)) / (M·C) · dt
+```
+
+Four parameters must be calibrated:
+
+| Parameter | Unit | Meaning |
+|---|---|---|
+| C | J/(kg·K) | Specific thermal capacity |
+| M | kg | Mass of pot + water + grain |
+| L | W/(kg·K) | Heat loss coefficient |
+| Td | s | Transport (heater-to-sensor) delay |
+
+
+## 1. Verify model-plant match: replay simulation
+
+Feed the **recorded `power_eff`** from a log file into a fresh `Pot` instance
+with the same parameters and compare the simulated temperature to `temp_ist`.
+
+```python
+from components.plant.pot import Pot
+
+pot = Pot(dt=1.0)
+pot.set_plant_params({'C': C, 'M': M, 'L': L, 'Td': Td})
+pot.set_ambient_temperature(theta_amb)
+pot.initial(samples[0]['temp_ist'])
+
+sim_temp = []
+for s in samples:
+    pot.set_power(s['power_eff'])
+    pot.process()
+    sim_temp.append(pot.get_temperature())
+```
+
+Plot `sim_temp` (model) against `temp_ist` (real) and the residual
+`temp_ist − sim_temp`. What divergence tells you:
+
+| Symptom | Likely cause |
+|---|---|
+| Ramp slopes differ | C·M wrong |
+| Phase shift between power step and temperature rise | Td wrong |
+| Wrong equilibrium temperature during HOLD | L wrong |
+| Residual grows over a long run | Model drift / L temperature-dependent |
+
+
+## 2. Parameter identification from log data
+
+### Td — transport delay
+
+Cross-correlate `power_eff` with `temp_ist`. The lag at peak correlation is the
+actual Td. Alternatively, find a sharp power step (start of a ramp) and measure
+the visible delay before `temp_ist` begins rising.
+
+### L — heat loss coefficient
+
+At steady-state HOLD, `d(temp)/dt ≈ 0`, so all input power compensates losses:
+
+```
+P_hold = L · M · (T − theta_amb)
+→  L = P_hold / (M · (T − theta_amb))
+```
+
+Repeat across hold phases at different temperatures and fit a line through
+`(T − theta_amb)` vs `P_hold` for a more robust estimate.
+
+### C — specific heat capacity
+
+During a ramp where heat loss is small relative to input power:
+
+```
+C ≈ power_eff / (M · rate_ist_K_per_s)
+```
+
+`rate_ist` in the log is K/min; divide by 60 to get K/s. Use a mid-ramp window
+where `power_eff` and `rate_ist` are both stable.
+
+
+## 3. Controller performance metrics
+
+These can be read directly from the log without re-simulation:
+
+| Metric | How to compute |
+|---|---|
+| Overshoot | `max(temp_ist) − temp_soll` after each setpoint step |
+| Settling time | First time `|temp_ist − temp_soll| < threshold` is sustained after a step |
+| Steady-state error | Mean of `temp_soll − temp_ist` during HOLD phase |
+| Rate tracking error | `rate_ist − rate_soll` during RAMP |
+
+### Smith predictor correction signal
+
+The term `theta_ist_plant − theta_ist_model_delay` is the Smith correction; a
+growing correction signal indicates model drift. This is not currently written to
+the log. To observe it, log `theta_ist_model` and `theta_ist_model_delay` from
+`temp_controller_smith.py` alongside the existing fields.
+
+
+## 4. Suggested addition to analyze_log.py
+
+A replay panel added to `utils/analyze_log.py` would show:
+- `temp_ist` (blue) vs `sim_temp` (red dashed) — model accuracy at a glance
+- Residual `temp_ist − sim_temp` in a separate subplot — reveals systematic
+  parameter errors vs noise floor