Abstract:The change of material mass is an important parameter of material state evaluation during the drying process. Pulsed vacuum drying process is a sealed state, herein, the device of mass measurement must be installed inside the drying chamber, which leads to the measurement processing vulnerable to interference, including the mechanical vibration caused by equipment operation, temperature change caused by drying room heating, airflow interferences which with pulsating state. The multiple effects of these disturbances may cause low measurement accuracy. In order to solve this problem, a measurement plan for separating the heating plate fixed material rack and the material measuring rack was put forward. Firstly, the vibration characteristics of the rack were effectively analyzed. The minimum resonance vibration frequency was about 8Hz, and the main resonance frequency was about 25Hz. According to the vibration characteristic, twoorder IIR digital filter were designed, which effectively solved the vibration problem. Then the drift characteristics of the sensor were measured for zero drift of the sensor caused by temperature. The correction temperature range was 20~45℃, and it was found that it can be corrected by five or six order polynomials. However, the higher calculation of higher order polynomials was not conducive to the realtime control of the whole machine. Therefore, the piecewise polynomial fitting method was adopted. The temperature stage of the piecewise fitting formula of zero drift werey=647062-60410t+1403t2 (20℃≤t<215℃), y=21235-2172t+62t2-065t3 (215℃≤t＜43℃), y=-499406+23206t-2749t2(43℃≤t≤45℃). The air turbulence in the process of inlet and outlet was not regular, which had little effect on the whole drying process. Therefore, the measured data of intake and exhaust stages were removed and replaced with linear interpolation data. In order to get the final mass data, a linear relationship between filtered and corrected sensor output and standard mass was established, and the final mass conversion formula was mo=0.00044021(x-y)-1.4553. In the formula, through the static test, the measurement system of full scale maximum reference error was 0.06%. The results of material drying test showed that the maximum reference error of the drying end was 0.1%. Through the data recording during drying process, it can clearly reflect the quality change and drying rate change in the whole drying process. The research result provided technical support for studying the changing rule of material status and realizing automatic drying process control in the drying process of agricultural products.
