brewpi/components/plant/TODO.md

components/plant design backlog

Findings from reviewing pot.py's water-bath model against real-plant behavior (sim/real control mismatch reported by user), refreshed against the current delay-line rewrite of Pot.

• Heat-loss term has wrong units. Fixed by the Pot rewrite: process() now computes p_loss = self.L * self.M * (self.temp - self.theta_amb) in Watts and applies `self.temp += (self.delay.get()

  • p_loss) / (self.M * self.C) * self.dt— the same/(M*C)normalization for both the gain and loss terms, instead of the old mismatchedleak = 1/self.M * self.L * (...)/ self.theta_amb`.

• Thermal lag modeled as single-pole lag, not transport delay. Fixed: Pot no longer uses HeatDiffusion's alpha=dt/Td exponential smoothing (that module was deleted). It now delays the input power through plant/delay.py's Delay (true dead time) before integrating it, matching what this item recommended.

• kn is loaded but never applied. Resolved differently than originally proposed: kn was removed from Pot entirely rather than wired up as plant-side measurement noise. Sensor noise is now modeled one layer up, in TempSensorSim (temp_offset/variance, wired from brewpi.py's TempSensorFactory.create(..., temp_offset= -0.15, variance=0.01)), not in the plant model. The stale Model.kn/Plant.kn keys this item flagged have since been deleted from config-sim.json.tpl/config.json.sim too.

• theta_amb is hardcoded, not configured. Fixed: added a top-level ambient_temperature key to config.json(.templ/.sim); brewpi.py reads it once and passes it to both the real Pot and (via a new theta_amb param) temp_controller_smith.py's internal models, so the Smith predictor's model matches the real plant's assumed ambient instead of silently defaulting to 20. Pot also dropped its separate params['theta'] initial-temperature field — it now always starts at theta_amb (so the same config value governs both the assumed ambient and the pot's starting temperature; there's no longer a way to start a sim at a temperature other than ambient). Real ambient temperature still isn't modeled as a drifting disturbance (it's a static config value, not a live sensor reading, though Pot.set_ambient_temperature() now exists as a hook for that), so robustness to ambient drift remains untested.

• No actuator/process realism. Partially improved (process() clamps self.temp to min(100, ...), won't simulate past boiling) and partially regressed: Pot also dropped its gain (heater efficiency) factor entirely — process() now applies 100% of commanded power with no loss, whereas before params['gain'] could model e.g. an 80% efficient transfer. Model.gain/Plant.gain were dead keys in config.json.sim (already absent from .templ) before that file was removed entirely — see components/pid/TODO.md's config-validation item. Still no heater power saturation enforced inside Pot itself (handled ad hoc in demo harnesses), no evaporation/lid heat loss, and no varying thermal mass M as volume changes (e.g. boil-off, adding water/grain) — M is a fixed constant for the whole simulation.

• Debug print()s left in Pot.process(). Fixed: the unconditional p_loss/theta_amb/temp prints (leftover from the heat-loss-units fix) were removed.