The feeding process is a crucial step in tobacco processing, and its accuracy directly affects the flavor and taste of the tobacco. For a long time, operators have relied on manually observing changes in the liquid level in the feeding tank to determine the correct feeding accuracy. If the flow meter malfunctions, the feeding accuracy cannot be guaranteed, affecting the flavor and taste of the tobacco. Furthermore, deviations in the workshop's level gauges, coupled with the inability to be used for comparison and control, significantly increase the likelihood of improper feeding due to human error and inaccurate flow meter readings. In workshop-level liquid measurement and control, the focus is primarily on the precise detection of the liquid level in the tank. This type of liquid level control requires high precision, involves high temperatures, and is subject to agitation, volatility, and foaming. Due to these reasons, the workshop has previously tried various level gauges, such as capacitive and ultrasonic level gauges, but these have consistently suffered from inaccurate detection and therefore have not been used in control systems. Furthermore, the pipelines for adding materials and incense in the workshop are directly connected to the drainage ditch. Valve malfunctions can lead to leaks; there have been previous instances where improper valve positioning caused liquid to enter the ditch or leak. However, because the workshop's level gauges were not previously used for control, these incidents went undetected, resulting in quality accidents. In light of the workshop's current situation, and after summarizing past experience with level detection equipment and analyzing the performance of various level detection instruments, we improved the WNA static magnetic grating level gauge and tested it at the stem and blade feeding points.
1. Improvements to the WNA static magnetic scale
The improved installation diagram of the WNA static magnetic scale is shown below:
Previously, WNA (Wireless Magnetic Grating) was used to detect the position of moving objects. The WNA source only needed to be fixed to the object being measured. The movement of the object was used to detect its position. Since we wanted to use it for liquid level detection, we installed a glass tube outside the liquid tank and connected it to the tank; this is the bypass glass tube. We modified the WNA source into a float-type magnetic grating source (which we'll call the float) and placed it inside the bypass glass tube. We designed the float to be thicker at both ends and thinner in the middle, with the thicker ends being irregularly shaped. Only three points contact the glass tube, reducing the float's resistance within the bypass glass tube. When the liquid level changes, the "magnetic grating source," i.e., the "float," can move freely within the bypass tube with the changing liquid level, reducing measurement error.
2. Applications of static magnetic level gauges:
The output signal of the static magnetic level gauge can be directly fed into the PLC on site, and can be used in the feeding and adding process. The tank is U-shaped, with 8KG of liquid at the bottom that is unmeasurable, and the top is a uniform cylinder. Within this range, if the scale on the static magnetic level gauge changes by 1mm, the liquid level in the tank changes by 1.1KG.
During the feeding process:
△L =LL-LC -K
L% = (△L / LL) * 100%
LL = Theoretically, the weight of the liquid to be fed into the tank / 1.1
K=(The unmeasurable material liquid weight at the bottom is 8KG + the material liquid weight in the feeding pipeline)/1.1
Note: For every 1 mm change in the reading of the static magnetic scale, the weight of the liquid in the tank changes by 1.1 kg.
Where LL is the theoretical liquid level, LC is the measured liquid level, i.e., the reading of the static magnetic scale, and K is the compensation coefficient (fixed value).
Because the feeding flow rate is relatively large at 1800 kg/h, and considering that the liquid level in the feeding pipeline and at the bottom of the tank cannot be measured, the PLC program starts reading, calculating, and comparing the liquid level in the tank every 5 seconds 30 seconds after feeding begins. If the value read into the PLC at a certain moment, after calculation and comparison, results in L% > ±0.5%, the system will issue a low accuracy alarm signal and display the alarm information on both the central control computer and the field computer. If L% > ±1%, the system will issue a low accuracy shutdown signal, stopping the equipment, and simultaneously displaying the alarm information on both the central control computer and the field computer, notifying maintenance personnel to check.
Data changes throughout the entire feeding process will also be stored in the centralized control database, which can be retrieved for data analysis at any time.
During the feeding process:
△L = Actual material usage - (LC0 - LC1 - K) * 1.1
L% = (△L / Actual material usage) * 100%
Note: 1. For every 1 mm change in the reading of the static magnetic scale, the weight of the liquid in the tank changes by 1.1 kg.
2. The actual amount of material used is the weight displayed by the mass flow meter.
Where LC0 is the reading of the static magnetic scale before feeding, LC1 is the reading of the static magnetic scale at a certain moment during feeding, and K is the compensation coefficient, i.e., the weight of the liquid in the feeding pipeline (a constant value).
Initially, the tobacco leaf flow rate is low, resulting in a small material consumption that doesn't meet the detection accuracy of the static magnetic grating level gauge, preventing readings and comparisons. Once the tobacco leaf flow rate reaches the set value, the PLC program begins reading, calculating, and comparing the liquid level in the tank every 60 seconds. If the calculated value read into the PLC at a certain moment is L% > ±1%, the system will issue a low-accuracy alarm signal and display the alarm information on both the central control computer and the field computer. If L% > ±2%, and after a 10-second delay, the system will issue a low-accuracy shutdown signal, stopping the equipment and displaying alarm information on both the central control computer and the field computer, notifying maintenance personnel for inspection.
Data changes throughout the entire feeding process will also be stored in the centralized control database, allowing for data analysis of each batch of feeding at any time to determine whether the feeding system is functioning properly.
3. Conclusion
Using a static magnetic level gauge allows for real-time monitoring and control of the liquid in the tank and the feeding equipment. In case of malfunction, an alarm and shutdown will be triggered promptly, reducing the occurrence of quality incidents and improving feeding quality.
In nearly three months of operation, the static magnetic grating level gauge has met the measurement and control requirements in production, achieving the purpose of our renovation. However, some details still need further improvement.
