Design and Control of a Robust Automatic Coffee Washing Machine for improved efficiency, consistency, and product quality

Arabica coffee is at the core of Ethiopia’s agricultural economy, being the main driver of export earnings and a source of income for millions of small-scale farmers. The quality of coffee is the key to staying competitive on the world market and it largely depends on post-harvest techniques mainly on depulping and washing processes, which have a direct effect on flavor and aroma. The washing is generally done with manual or semi-mechanized systems, which are the most common traditional methods in Ethiopia. However, these methods are labor intensive, take a lot of time and produce quality that is not consistent, while also using a lot of water and energy. This research proposes the design and control of an automatic coffee washing machine capable of processing coffee beans up to 360 kg of coffee in a single batch within 30 minutes or the production efficiency is increased to 12 kg/min, making it more suitable for large-scale coffee processing applications. A Programmable Logic Controller (PLC) was put into operation and simulated by using Delta ISPSoft, while the Human Machine Interface was created in DOPSoft for real-time monitoring. A Proportional Integral Derivative controller was designed in MATLAB to control the coffee drying temperature, while a Variable Frequency Drive was used to regulate the coffee washing agitator speed. The results of the simulation indicate that the system has a rapid transient response characterized by a rise time of 375.04 Sec, settling time of 696.01 Sec, no overshoot and a steady-state error of 0.18°C. Coffee bean moisture decreased from 0.6 (dry basis) to 12%, confirming effective drying. Additionally, the system recorded a water efficiency of 0.01 kg/L, a time efficiency of 12 kg/min, an energy efficiency of 72 kg/kWh and a cleaning effectiveness improvement of 15%, while the recycling of wastewater greatly help for the reduction of the environmental impact.