brewpi/docs/overshoot_hold_windup.md

HOLD-state overshoot from a disturbance: root cause and fix plan

Investigation of the overshoot visible in docs/overshoot.png: a cold-water disturbance during a steady HOLD at 30°C caused theta_ist to overshoot to ~30.5°C and stay there, rather than settling back at soll. This document records the diagnosis and the (not yet implemented) fix plan. Numbers below are pulled from logs/log_latest.json.

Incident summary

• Steady HOLD at 30.0°C. Cold water added → theta_ist (Smith-corrected) dipped to ~29.49°C (diff ≈ 0.51°C).
• diff never reached Thresholds.HoldHeat (1.0°C, temp_controller_fsm.py), so the FSM never left HOLD — the entire episode happened inside what the system still considered "holding".
• The outer loop (pid_hold, Hold: {kp:0.6, ki:0, kd:0, kt:0} — pure proportional) tracked 0.6 * max(0, diff) exactly, tick for tick, peaking at y ≈ 0.31 — nowhere near its y_max=1.0 ceiling. Confirmed directly against logged samples: rate_soll == 0.6*diff to the last digit throughout.
• The inner loop (pid_heat, Heat: {kp:0.08, ki:0.02, kd:0, kt:1.5}) then chased that heatrate_soll target, driving commanded power from a baseline ~500W burst-cycle up to ~2600W (y ≈ 0.74 of the 3500W max, see tasks/heater.py:58-59: power = get_power_max() * y).
• Plant dead time (Td=17s) and thermal mass (M=27.96 kg, C≈3403 J/(kg·K), from PlantParams in the log) meant heat from that burst kept arriving after theta_ist had already crossed back over soll. By the time rate_soll had fallen back to exactly 0 (outer loop asking for zero heat rate), power_set was still 1442 W, decaying only gradually over the next ~35s (1193 → 979 → 817 → 632 → 487 → 337 → 142 W) rather than snapping to 0.
• No active cooling actuator exists; the passive loss coefficient (L=0.2 W/(kg·K)) gives a time constant C/L ≈ 17,000s (~4.7h), so the resulting ~0.4°C plateau above soll does not visibly decay within any reasonable observation window.

Root cause: the inner loop's integral term (yi) accumulated during the ~130s the outer loop demanded a real (if modest) heat rate, and nothing bounded that accumulation — the existing anti-windup in Pid.process() (components/pid/pid.py:45-48) is back-calculation that only corrects yi when the combined output y is actually clamped at y_min/y_max. Since y peaked at ~0.74, well under the 1.0 ceiling, awu was 0 the entire time and the anti-windup mechanism never engaged. The integrator just had to unwind naturally against real (delayed) negative error, which took ~35+ seconds after the outer loop had already zeroed its target.

Rejected approach: a flat yi_max clamp on the inner loop

The first fix considered was bounding yi directly, with one constant for the inner loop regardless of state. Numeric check against the real plant model kills this as a standalone fix:

scenario	P needed	y needed
1.5 K/min ramp @ 30°C	2435 W	0.696
1.5 K/min ramp @ 50°C	2547 W	0.728
1.5 K/min ramp @ 66°C	2636 W	0.753
the actual HOLD disturbance (peak)	2600 W	0.74
steady HOLD loss compensation @ 66°C	~257 W	~0.073

At steady state (err=0), kp*err contributes nothing — the entire y≈0.7-0.75 needed to sustain a genuine 1.5 K/min ramp has to come from yi alone, for as long as the ramp lasts. That's the same range the disturbance transient itself peaked at, but far above what real steady-HOLD loss compensation ever needs (~0.07-0.15 at realistic brew temperatures). A single constant can't serve both: tight enough to matter for the disturbance (well under ~0.74) permanently starves a real ramp of the ~0.7 it needs; loose enough not to interfere with ramps (~0.75+) never engages during the disturbance at all. Rejected as a single global constant.

An FSM-gating alternative (freeze the inner loop's output to 0 while in HOLD unless explicitly engaged by command or a new threshold) was also sketched, but left an open question about how a new engage threshold should relate to the existing HoldHeat FSM threshold, and needed extra engage/disengage hysteresis to avoid chatter. Superseded by the plan below, which reaches the same effect with less new machinery.

Refined plan: split the clamp by which state is driving the same loop,

not by freezing the loop

The insight above — real HOLD-time demand (~0.07-0.15) and real ramp demand (~0.7-0.75) occupy clearly different ranges — means a per-state parameter set on the same PID instance solves this more cleanly than gating the loop on/off: same accumulated yi/d state carried across HOLD↔HEAT transitions (bumpless transfer for free, no explicit reset, no engage/ disengage hysteresis to tune), just a different yi_max ceiling depending on which state is currently active.

Rename for honesty

The current names don't match what actually runs when:

• pid_hold already runs unconditionally every tick regardless of state (process_pid()'s first line, temp_controller_base.py:134) — it's the outer loop, not "the HOLD-state PID". Rename to pid_outer.
• pid_heat already runs in both HOLD and HEAT today (only IDLE/ COOL skip it, temp_controller_base.py:151-158) — it's the inner loop for the heating direction. Rename to pid_inner.
• pid_cool only ever runs in COOL — no HOLD-time ambiguity, since a disturbance that pushes temp above soll during HOLD is still handled by pid_inner (the outer loop's max(0.0, pid_hold_y) floor sends heatrate_soll to 0, and pid_inner reacts to the resulting negative heatrate_err — the state only escalates to real COOL past Thresholds.HoldCool). Rename to pid_inner_cool for symmetry, kept as its own instance — today's explicit reset() calls when crossing between heat-direction and cool-direction states (temp_controller_fsm.py:104,108,115,123) stay exactly as they are; there is no reason to share integrator state across a heater/chiller boundary.

Config: Hold/Heat/Cool → Outer / Inner.{Heat,Hold,Cool}

"TempCtrl": {
  "pid_type": "Smith",
  "beta": 0.9,
  "Outer": { "kp": 0.6, "ki": 0.0, "kd": 0.0, "kt": 0.0 },
  "Inner": {
    "Heat": { "kp": 0.08, "ki": 0.02, "kd": 0.0, "kt": 1.5 },
    "Hold": { "kp": 0.08, "ki": 0.02, "kd": 0.0, "kt": 1.5, "yi_max": 0.3 },
    "Cool": { "kp": 0.08, "ki": 0.02, "kd": 0.0, "kt": 1.5 }
  },
  "Thresholds": { "...": "unchanged" }
}

• Inner.Heat keeps today's Heat gains, no yi_max (or a very loose one) — a real ramp must be able to reach y≈0.75.
• Inner.Hold starts as a copy of the same gains, with yi_max≈0.2-0.3 added — comfortably above realistic steady-loss compensation (~0.07-0.15) but well below what turned a 0.5°C dip into a 2600W burst.
• Inner.Cool is Cool's existing gains, moved under Inner purely for structural consistency — introduced now, not because we've observed a cooling-side incident. pid_inner_cool never runs during HOLD, so it doesn't need its own Hold variant the way Heat does; one params block is enough.
• This is a breaking config change — no backward-compat shim for the old flat Hold/Heat/Cool keys (per the "no compat hacks" convention). Every deployed config.json needs migrating, not just the repo's config-real.json.tpl/config-sim.json.tpl templates.

Code changes (planned, not yet implemented)

1. components/pid/pid.py — add the symmetric yi_max clamp inside process() (already planned): self.yi = max(-yi_max, min(yi_max, self.yi)) when self.params.get('yi_max') is set, applied right after accumulating yi and before it's summed into y.
2. components/pid/temp_controller_fsm.py — rename self.pid_hold → self.pid_outer, self.pid_heat → self.pid_inner, self.pid_cool → self.pid_inner_cool (constructor at lines 33-34/40, all reset() call sites and comments at lines 11-12, 83, 87-89, 99, 104, 108, 111, 115, 118-119, 123).
3. components/pid/temp_controller_base.py:
   • set_params() (line 28-34): self.pid_outer.set_params(params['Outer']); store self._inner_heat_params = params['Inner']['Heat'] and self._inner_hold_params = params['Inner']['Hold'] for the per-tick lookup below; self.pid_inner_cool.set_params(params['Inner']['Cool']).
   • process_pid() (lines 133-160): rename pid_hold_y → pid_outer_y and the self.pid_hold.process(...) call. In the combined HOLD/HEAT branch (today's else, lines 156-158), select the active param set before processing:

     else:
         inner_params = self._inner_heat_params if self.state == States.HEAT else self._inner_hold_params
         self.pid_inner.set_params(inner_params)
         self.pid_inner.process(heatrate_err, -self.heatrate_ist)
         self.y = self.pid_inner.get_y()
     

     set_params() is a cheap dict-reference assignment (pid.py:22-23), so calling it every tick has no meaningful cost. Because kp/ki/kd/ kt are identical between Inner.Heat and Inner.Hold in the starting config, switching the active set at a HOLD↔HEAT transition changes no term of y at that instant — only the yi_max ceiling going forward. No bump at the transition, unlike the freeze/thaw approach.
4. Config files — config.json, config-real.json.tpl, config-sim.json.tpl: restructure into Outer/Inner.{Heat,Hold,Cool} as above. Inline "Cool": {...} dicts in scripts/demos/pid/ demo_temp_controller_smith.py:21, demo_temp_controller.py:21, and scripts/demos/sud/demo_sud.py:32 need the same restructure.
5. utils/replay_sim.py — _apply_gain_overrides() and the CLI flag loop (lines 50, 214-222) iterate ('Hold','hold'), ('Heat','heat'), ('Cool','cool'); becomes ('Outer','outer'), ('Inner.Heat','inner-heat'), ('Inner.Hold','inner-hold'), ('Inner.Cool','inner-cool') (nested dict access needed since these aren't flat top-level keys any more). The for section in ('Hold','Heat','Cool') print loop at line 284 and the _infer_heatrate_soll_set() docstring's "hold PID" reference (line 66) need the same rename.
6. components/pid/TODO.md — update the cross-link entry added for the previous (superseded) plan to point at this rewritten section instead.

Test plan (not yet implemented)

Stdlib unittest, no pytest — tests/components/pid/test_pid.py, discoverable via python -m unittest discover -t . -s tests/components/pid. Simpler than the FSM-gating plan's test plan: no engage/disengage hysteresis or threshold-relationship behavior to cover, since the loop is never turned off — only its yi_max ceiling changes with state.

A. Unit-level, isolated Pid — no plant involved. Feed a synthetic err sequence shaped like the incident (positive heatrate_err ≈ 0.3 held for ~130 ticks, then decaying/negative tail, matching the real rate_soll - rate_ist pulled from the log) into two Pid instances with identical gains, one with yi_max set (the Inner.Hold case) and one without (Inner.Heat). Assert: recovery time (ticks after error goes negative until y drops back under a small threshold) is measurably shorter when clamped; yi never exceeds the configured bound.

B. Closed-loop, self-contained synthetic scenario — no dependency on the multi-MB log file. Real numbers from PlantParams/config.json: Pot(dt) with M=27.96, C=3403.43, L=0.2, Td=17, ambient from config.json; TempController via PidFactory.create('Smith', dt) with the real Outer/Inner.* gains. Run closed-loop (controller's own y drives the plant, unlike utils/replay_sim.py's open-loop observe-only mode):

• Disturbance case: hold at 30°C until settled, knock plant.temp down ~0.5°C to emulate the cold-water event, keep ticking for several minutes. Assert peak overshoot above 30.0°C is measurably smaller with Inner.Hold's yi_max set than without.
• Ramp case: command a genuine 1.5 K/min ramp (state reaches HEAT) and assert the sustained heat rate actually reaches ~1.5 K/min — guards against reintroducing the flat-clamp regression from the rejected approach above.
• Transition case: drive a HOLD→HEAT→HOLD sequence and assert y has no discontinuity at either transition beyond what the changing heatrate_err itself would explain — confirms the per-state param swap is truly bumpless.

Status

Plan only — nothing in this document has been implemented yet.