DEVELOPMENT OF A PROTOTYPE OF A STATIC DRYER FOR SPECIALTY COFFEE WITH PI TEMPERATURE CONTROL USING ARDUINO AND PHASE-ANGLE TRIAC CONTROL

Abstract: It is well known that the coffee drying process can directly affect beverage quality, thus requiring precise thermal control to ensure stability and uniformity in order to preserve the sensory attributes of the beans. This work aimed to develop a prototype of a static experimental dryer for specialty coffee, featuring automatic temperature control based on a PI (proportional-integral) controller, using an Arduino microcontroller and phase-angle control with a TRIAC. Initially, commercial dryers were analyzed to understand their limitations, especially regarding temperature control, which is typically restricted to fixed settings. To overcome this, a manual power control system was developed using a dimmer, with voltage adjustment via a potentiometer. However, thermal instability caused by environmental temperature fluctuations throughout the day made this approach inaccurate, with temperature deviations of approximately 15% from the setpoint. As a solution, an automatic control system was implemented, consisting of three main components: (i) temperature reading via a DS18B20 digital sensor; (ii) power control using a BTA41-600B Triac through phase-angle firing synchronized with the AC zero-cross; and (iii) a user interface with an analog potentiometer and a 16x2 LCD display with I2C communication. Thus, the continuously monitored temperature is compared to the user-defined target value, and power delivery is dynamically adjusted. The hardware was divided into two circuit boards: (i) a power board, housing the Triac, MOC3021 optocoupler, and zero-cross detection circuit; and (ii) a control board, comprising the Arduino Uno, temperature sensor, and control logic. The embedded algorithm implements a PI controller with a gain constant Kp = 110 and reset time σI = 0.5. At each AC zero-crossing, the system calculates the appropriate delay before triggering the Triac, thereby regulating the energy supplied to the heater. Preliminary experiments with the prototype demonstrated that the system could maintain the target temperature between 20 °C and 76 °C with high stability and a precision of 1 °C, even under external disturbances. Furthermore, the user interface proved to be intuitive and easy to operate, while the automatic PI control ensured the thermal precision required for the process. The prototype proved to be a viable alternative for experimental applications in the drying of specialty coffee and other grains, with potential for expansion into industrial or academic contexts.