Temperature Control
The goal set at the beginning of the espresso machine application was to implement a controller that provides repeatable conditions for every coffee preparation. The evaluation is based on the measurement and control states of the machine logs in the database. Additionally, the coffee temperature was measured using an external multimeter (Keysight U1242C).
Figure: Temperature measurement and power input.
The measurement in the figure shows a stabilization phase for the first 17 min. During which no fresh water was pumped to the boiler allowing for the machine to heat up. The water (red) and boiler (yellow) reach the target temperature within three minutes. However, it takes more than 15 min for the group head to heat up. A slight overshoot in water temperature (red curve) indicates some discrepancy between the simulation and the real world. The heating element is constantly turned on for the first few minutes, as indicated by the power input in the bottom plot. Thus, the controller tries reaching the set point with the maximum speed. When the controller has settled the temperature remains stable.
Following the heat up phase are three espresso shots. The temperature curve during the first two shots is plotted in the figure.
Figure: Measured temperatures during espresso preparation.
The coffee temperature (blue) curve is the most important indicator of the controller performance. The temperature profile from espresso shots should be within a 2°C margin. Deterministic water temperature reduces the uncertainty of one out of many parameters of coffee preparation. At the beginning of the espresso shot, the temperature within the coffee puck (blue) first drops to 70°C for while the remaining cold water between the boiler and group head is pumped through. Then, hot water from the boiler enters the group head and quickly reaches an equilibrium at 90°C. The short peak before the 20 min mark is due to flushing the group head after the portafilter was removed. Comparing the two espresso shots at the 19 min and 21 min mark proves that the temperature profiles are in fact equal. The raw data from all three espresso preparations revealed that the brew temperature of the coffee puck stayed within 89.3°C and 90.6°C.
The water temperature (red) indicates a stark change in temperature when fresh water enters the boiler. Even more so is the effect on the boiler temperature (yellow). The change in temperature of the boiler might be due to the fresh water supply being screwed onto the boiler. The brass boiler conducts thermal energy quickly. Comparing water (blue) and water (red) temperatures indicates that a change inside the boiler does not immediately affect the coffee temperature. Hence, the thermal model of the espresso machine is forgiving.
Looking at the power input in the bottom plot, it becomes evident that the controller turns the heating element on as soon as the fresh water enters the boiler. The heating element stays on until the temperatures reach a sensible level. Thus, the hardware capabilities limit the temperature control.
Overall the measurements demonstrate that the controller keeps temperatures within the limits set for the application.