Design of online monitoring system for material moisture content in air-impingement drying process

Abstract: Moisture content is an important quality index of drying products, which can be obtained by automatic weighing and analyzing its mass in the drying process. Compared with other moisture content measurement technology, the automatic weighing method has good versatility, high accuracy, and simple architecture. Relative humidity of drying air has significant effect on drying kinetics and product quality, while, for interior weighing devices, the humidity of hot air can easily be kept stable during drying process. In order to mitigate the effects of air velocity, equipment operating vibration, drying temperature, and other factors on the interior automatic weighing system of air-impingement drying, a new drying monitoring system based on Modbus communication protocol and RS 485 bus was designed. This system consists of five parts: an HMI (human machine interface) as master, a weighing system, a drying temperature control system, humidity control system, and air velocity control system. This paper mainly focuses on the measurement of material mass and moisture content. For the hardware of weighing system, strain gauge load cell and 0-5 V analog voltage transmitting instrument XST were applied in this system, and in order to implement temperature compensation to weighing system, the Pt100 type temperature sensor was adopted to sample the temperature of the weight sensor. Besides, signal acquisition, processing circuit with 8-bit microprocessor chip PIC16F1829 as core was designed, among which the analog transmission signal of weighting instrument is collected and converted by using 12-bit A/D chip TLC2543, and signal is converted by level conversion chip MAX487 to complete communication between master (HMI) and microprocessor PIC16F1829. And the weighting measurement range of this system is 0-2000 g and the resolving power is 1 g. For software design, median filtering algorithm, "shutdown-maintain-weight-start" automatic weighting procedures, linear calibration at different temperature sections, and load ranges were designed to eliminate the effect of accidental errors on automatic weighing, to weaken the effect of air velocity and equipment operating vibration on automatic weighing without affecting the drying process, and to reduce the effect of higher drying temperature and change of the temperature in wider range on automatic weighing, respectively. Through the above measures, automatic weighing accuracy was ensured effectively. In addition, the status of the slave controllers was incorporated into the run logic judgment of the system to guarantee the system's operational safety. This monitoring system could also realize the functions such as isochronal and instant measurement of material mass and moisture content, real-time display, and storage of experiment data in the drying process. After the completion of the system's hardware and software design, monitoring system' detection accuracy of mass and moisture content were tested in air-impingement dryer. Results of the weighing test showed that the maximum relative error to the mass of the samples is less than 0.7%. Then a series of drying tests on carrots were conducted. The results indicated that the error of the moisture content is within ±1.0% in drying process, which achieved the requirement of the on-line monitoring moisture content. The online air-impingement drying monitoring system designed in the current work is of great significance for evaluating and regulating the drying process for various materials, ensuring the product quality, and reducing the energy consumption and the labor intensity of drying operators.