In order to provide new technical support for the variable temperature drying process and realize the adaptive control of drying temperature based on the change of moisture content, the study designed a temperature adaptive control system with the function of online detection of material moisture content. A moisture content online detection model with weight detection value, air velocity, the temperature of load sensor elastic substrate, airflow impingement distance as inputs, and the real material weight as outputs was established by using a convolutional neural network. A validation test of the moisture content online detection model was carried out. The results showed that the model meets the accuracy requirements of online moisture content detection in the variable temperature drying process, and the coefficient of determination R² and root mean square error (RMSE) of the five groups of model validation tests were 0.9934 and 1.20% in that order. In this reasearch, an improved neural network-PID (INN-PID) controller was designed to realize temperature control in the variable temperature drying process. The dynamic performance of PID, neural network-PID (NN-PID), and INN-PID controllers was simulated in MATLAB software with unit step signal as input. The three controllers were tested for drying temperature control at 50-55 ℃. The results showed that the control stability and regulation time of the INN-PID controller were significantly better than the other two controllers in the simulation test, the drying temperature control results had approximately the same law with the simulation results, and the peak time of the INN-PID controller was 208.00 s, the regulation time was 120.59 s, and the maximum overshooting was 4.87%, which meets the requirements of temperature control in the variable-temperature drying process. In this paper, a temperature adaptive control system was built in the air-impingement dryer, and the temperature adaptive control test based on the moisture content change was carried out. The results showed that the system could quickly and effectively regulate the drying temperature in the variable temperature drying process based on the change in moisture content. This research is of great significance for improving the automation level of drying equipment, developing new variable-temperature drying processes, and providing reference for multi-information fusion detection and control strategy research in other fields.

This study aims to accurately reveal the heat and mass transfer of yam slices during infrared and hot air drying (IR-HAD). A multi-field IR-HAD model was established to couple the temperature and humidity fields in the yam slices using finite element (FE) software COMSOL 6.1, considering the shrinkage and deformation. The heat and mass transfer was simulated at different temperatures (50℃, 60℃ and 70℃) and then verified by a series of experiments. A systematic investigation was implemented to clarify the effects of different temperatures on the quality (color difference, rehydration ratio, polysaccharide content and allantoin content) of yam slices. The results showed that (1) the volume ratios of yam slices increased with the increase in drying temperature, which were 34.55%, 37.23%, and 39.04% at the drying temperatures of 50°C, 60°C, and 70°C, respectively. (2) The R² values of the simulated temperature and moisture content considering shrinkage and deformation were 0.973, 0.976, 0.981, and 0.983, 0.976, and 0.974, respectively, indicating better agreement with the test ones. (3) The temperature field of yam slices presented consistent simulation and experimental drying. The surface temperature of the yam slice was higher than the internal temperature at the early stage of drying (0-15 min). The peripheral temperature was slightly higher than the center, leading to the forced hot-air heating. The infrared radiation further heated the inside of the yam slices, as the drying proceeded. As such, the internal and center temperatures were gradually higher than the external and peripheral ones. The moisture vapor diffusion of yam slices also showed a tendency to increase and then decrease during drying. There was an increase in the temperature of the yam slice and the rate of water evaporation in the pre-drying period (0-60 min); The temperature of the yam slice was stable in the mid-drying period (60-120 min). The infrared hot air was mainly used to evaporate the latent heat, and the amount of water evaporation and diffusion increased gradually to the maximum; The surface of the yam slice formed a hard layer in the post-drying period (120-195 min), leading to the less water evaporation and diffusion. At the same time, there was an uneven moisture concentration of yam slices during the whole drying. The internal moisture concentration was higher than the surface ones, where the center concentration spread to the outside. The reason was that the heating occurred mainly on the surface and periphery of the yam slices, leading to the vaporization of moisture in the formation of an internal humidity gradient. (4) The quality of dried yam slices was evaluated by the coefficient of variation. The highest performance was achieved at 60 ℃, with a color difference of 7.49, a rehydration ratio of 2.65 kg/kg, a polysaccharide content of 24.17 mg/g, and an allantoin content of 2.66 μg/g. This finding can provide a strong reference for the simulation of various materials in infrared and hot-air drying.