Concept
To achieve the afore-mentioned temperature stability, a new controller will be implemented. For that purpose, new electronics and sensors will replace the original wiring. An overview of the modified machine is shown in the figure.
Figure: Overview of the espresso machine hardware with new control electronics.
The four temperature sensors (T in the figure) are used to identify the system behavior. The goal is to measure temperatures at points in the systems which are normally omitted due to implementation cost. In particular, will the water temperature inside the boiler and the brew temperature of the coffee puck be measured. The thermostat of the unmodified machine was based only on the temperature of the boiler outside. In addition to the temperature sensors, a flow meter (F) and manometer (P) will be added. These are used to provide feedback on the brew to the user during espresso shots.
A temperature controller of an espresso machine alone does not justify the use of an RTOS. Thus, a high background load is introduced in the form of a graphical user interface (UI). The touch screen will allow the user to configure the machine and display live measurements. Additionally, there will be an ethernet connection for data logging representing a uncertain load arising from connectivity to external devices.
All in all, the example application will feature a critical set of threads responsible for controlling the machines actuators, a background load updating the UI and dynamic load from connectivity.