Design and control of a robust automatic coffee washing machine for improved efficiency, consistency, and product quality

Abstract

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.