Tobacco stem pretreatment method and system based on stem diameter grading and ratio discharging

Abstract

This invention discloses a tobacco stem pretreatment method and system based on stem diameter grading and proportional discharge, relating to the field of tobacco processing technology. The method includes: first, separating mixed tobacco stems into coarse, medium, and fine grades according to stem diameter using a three-stage vibrating screen and storing them separately in parallel quantitative silos; second, at the beginning stage, calculating the natural stem diameter ratio of the current batch of tobacco stems by collecting the material level height in each silo, and setting the reference frequency of each unloading motor accordingly; third, during normal production, employing a dual closed-loop control strategy of inner-loop proportional synchronous tracking and outer-loop material level stability coordination to ensure that the three rotating unloading motors discharge material at a constant ratio and maintain stable silo material levels; finally, locking the process parameters of the moistening and pressing processes based on constant ratio linkage, and adaptively adjusting the discharge ratio and subsequent process parameters at the end stage. This invention ensures a constant stem diameter composition entering the pressing machine, enabling the pressing machine to operate stably with a fixed gap, significantly improving pressing quality and reducing breakage rate, while also having low modification costs and strong adaptability.

Description

Technical Field

[0001] This invention relates to the field of tobacco processing technology, and in particular to a method and system for graded and proportionally discharged tobacco stems from the storage tank in the pretreatment section to the stem pressing machine. Background Technology

[0002] In tobacco processing, stem pressing is a core step in stem pretreatment. This process uses pressure rollers to compress the stems into stem sheets, thereby loosening the stem fiber structure and providing a foundation for subsequent shredding and expansion processes. However, stems of different diameters exhibit significant differences in plasticity and compressive strength: coarse stems require a larger roller gap to ensure thorough pressing and prevent breakage, while fine stems require a smaller roller gap to ensure a good calendering effect. If a fixed gap is used to process stems of mixed diameters, problems often arise such as coarse stems not being fully pressed and fine stems being over-pressed and broken. This results in a stem sheet thickness deviation exceeding 0.2 mm, a breakage rate exceeding 8% in subsequent shredding, low whole stem yield, and poor filling value, severely impacting the quality of cigarette products.

[0003] Currently, there are two main solutions in the industry to address the above problems: one is to directly use tobacco stems of mixed diameters for pressing, frequently adjusting the gap between the pressing rollers to adapt to fluctuations in the incoming material. However, the pressing rollers of the pressing machine have large diameters and large inertia. Frequent gap adjustments not only lead to increased equipment wear and poor operational stability, but also have a significant lag in gap adjustment, making it impossible to adapt to the instantaneous fluctuations in the diameter of the incoming stems in real time, resulting in poor quality control. The other solution is to process the tobacco stems separately after screening, that is, coarse, medium, and fine stems are processed through independent stem moistening and pressing production lines. Although this solution can ensure the quality of pressed stems, it has problems such as a lengthy process, large equipment footprint, high investment costs, and complex control, making it difficult to adapt to the transformation and upgrading needs of existing compact tobacco processing lines.

[0004] Currently, there is no mature online stem diameter grading and fixed-ratio mixing discharge scheme, which makes it impossible to achieve a constant stem diameter composition entering the stem press and to fundamentally solve the core pain point of the need for frequent gap adjustments in the stem press. At the same time, the existing technology does not coordinate the entire process of storage tank discharge, stem diameter grading, fixed-ratio discharge, stem moistening and stem pressing, and cannot achieve stable matching of parameters throughout the process, resulting in poor consistency of tobacco stem pretreatment quality. Summary of the Invention

[0005] This invention aims to overcome the shortcomings of existing technologies and solve core problems such as poor pressing quality of mixed-diameter tobacco stems, frequent adjustment of the gap between pressing rollers, high cost of post-screening processing, and lack of online fixed-ratio mixing scheme. It provides a tobacco stem pretreatment method and system based on fixed-ratio discharge of graded stems to achieve a constant stem diameter composition entering the pressing machine, ensuring that the pressing machine can operate stably at a fixed gap, and significantly improving pressing quality and production efficiency.

[0006] The technical solution adopted in this invention is as follows:

[0007] This invention provides a tobacco stem pretreatment method based on stem diameter grading and fixed-ratio output, comprising the following steps:

[0008] S1: Storage tank discharge and online stalk diameter screening and grading. This step follows the upstream stalk storage process. After the predetermined heat preservation and moisture retention time is completed for the top-mounted stalk storage tank, the system starts the discharge program. The PLC controller issues a command to drive the storage tank discharge conveyor belt to run at a precise, synchronized, and adjustable speed, continuously and evenly conveying the mixed stalks to the downstream screening equipment. Subsequently, the stalks enter a three-stage vibrating screen, which is designed with a multi-layer screen structure, arranged from top to bottom: the first-stage screen is used to intercept coarse stalks with a diameter greater than D1, and the oversize material, i.e., coarse stalks, falls into the first buffer bin; the second-stage screen is used to intercept medium stalks with a diameter between D2 and D1, and the oversize material, i.e., medium stalks, falls into the second buffer bin; the third-stage collection plate is used to collect fine stalks with a diameter less than D2, and the fine stalks fall into the third buffer bin.

[0009] S2: Determination of the natural distribution ratio of stem diameter and calculation of the initial discharge benchmark. In the initial stage before formal discharge, the system activates the three-stage screening unit and the bottom conveyor belt of the storage tank, while keeping the rotary unloading motors at the bottom of the three buffer hoppers off. The grating-type continuous level gauges on the side walls of each hopper monitor the changes in material level in real time. When the material level in any hopper first reaches the preset safe high level threshold, the PLC controller immediately collects and records the instantaneous material level height of each hopper. Since the cross-sectional area of ​​each hopper is the same or known, the ratio of these height values ​​directly reflects the volume ratio of the coarse, medium, and fine components in this batch of tobacco stems. The system uses this ratio as the target discharge ratio benchmark and calls the pre-calibrated characteristic curve of the unloading motors to convert this ratio into the drive frequency benchmark values ​​of the three unloading motors.

[0010] S3: During normal production, precise ratio control and stable material level control are achieved through a dual closed-loop system. Upon entering normal production, the system starts all rotating unloading motors and activates the core dual closed-loop control strategy. The inner loop is a ratio-synchronous tracking closed loop, aiming to maintain a constant ratio of material flow from the three unloading motors. The PLC calculates the adjustment based on the material level deviation at the press inlet and synchronously adjusts the drive frequency of each motor in a fixed ratio using a master-slave synchronization method. The outer loop is a material level stabilization and overflow / disruption prevention closed loop, aiming to maintain the material levels in the three buffer silos within a reasonable operating range. The PLC continuously monitors the material level data of each silo, stabilizes the material level by adjusting the discharge speed of the upstream storage tank, and executes a preset conflict decoupling strategy under complex operating conditions.

[0011] S4: The upstream and downstream processes operate in a coordinated manner with constant parameters based on a constant output ratio. Through the dual closed-loop control in step S3, the diameter composition of the tobacco stems entering the stem-lubricating machine is ensured to remain constant over a long period, thus creating optimal conditions for the stable operation of the upstream and downstream processes. The system uses the measured target output ratio benchmark as a key process parameter, automatically calculating and setting the optimal process parameters for the stem-lubricating machine and the stem-pressing machine, such as steam flow rate, water spray volume, roller gap, and rotation speed. These parameters remain locked throughout the production process and do not require adjustment based on fluctuations in incoming materials.

[0012] S5: Adaptive proportional switching and parameter transition in the tail stage. When the storage tank finishes discharging and a certain buffer hopper is emptied first, the PLC automatically executes the tail adaptive program, rereads the instantaneous material level of the remaining hoppers and calculates the new discharge ratio, and then reallocates the drive frequency of the remaining unloading motors; at the same time, the PLC sends parameter change instructions to the stem lubricator and the stem pressing machine to ensure that the tail material can also be processed in the best state until the system is completely emptied.

[0013] The present invention also provides a tobacco stem pretreatment system for performing the above method, including a host computer, a PLC controller, a storage tank discharge unit, a three-stage vibrating screening unit, a quantitative and proportional discharge unit, a stem moistening machine, a stem pressing machine, and a detection unit; the modules of the system cooperate with each other to jointly realize the above tobacco stem pretreatment method.

[0014] Compared to existing technologies, this invention fundamentally solves the industry pain point of frequent gap adjustments required for tobacco stem pressing machines. By combining stem diameter grading with a fixed-ratio discharge, the diameter composition of the tobacco stems entering the pressing machine is ensured to remain constant. The pressing machine can operate continuously and stably at a preset fixed gap without frequent adjustments. The thickness deviation of the pressed stem pieces becomes adjustable and controllable, significantly reducing the breakage rate and increasing the whole stem yield. Furthermore, based on a constant discharge ratio, the process parameters for the moistening and pressing processes are pre-set, and the entire process operates stably with fixed parameters, effectively reducing batch-to-batch deviations in tobacco stem pretreatment quality and significantly reducing the labor intensity of operators. In particular, dedicated control logic is designed for the three stages of material initiation, normal production, and material outflow. Through dual closed-loop control of inner loop fixed-ratio synchronization and outer loop material level stability, the discharge ratio deviation is guaranteed to be ≤±2%, effectively solving the problems of material interruption, overflow, and ratio fluctuation, and adapting to the needs of continuous production.

[0015] Furthermore, the three-stage screening and parallel quantitative silo structure of the present invention can be directly embedded between the existing tobacco processing line storage tank and the stem lubricator, without the need for large-scale modification of the production line. The equipment occupies a small area and the investment cost is significantly lower than the line processing solution, making it suitable for the new construction and renovation needs of various cigarette factory tobacco processing lines.

[0016] Furthermore, this invention can form a complete tobacco stem pretreatment process technology system with the layered feeding method of the top-type stem storage cabinet after washing based on water absorption grading and the gap control method of the stem pressing machine, etc., achieving full chain coverage from post-washing stem storage to stem diameter fixed-ratio discharge and then to the stem pressing process, with optimal process synergy effect. Attached Figure Description

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described below with reference to the accompanying drawings, wherein:

[0018] Figure 1 This is a schematic diagram of the overall architecture of the tobacco stem pretreatment system provided in an embodiment of the present invention;

[0019] Figure 2 This is a timeline diagram of the entire process of the tobacco stem pretreatment method provided in the embodiments of the present invention;

[0020] Figure 3 This is a block diagram of the dual closed-loop constant ratio discharge control logic provided in an embodiment of the present invention.

[0021] Explanation of reference numerals in the attached diagram: 1-Host computer, 2-PLC controller, 3-Top-type stalk storage tank, 4-Three-stage vibrating screening unit, 5-Coarse stalk silo, 6-Medium stalk silo, 7-Fine stalk silo, 8-Rotary unloading motor, 9-Stalk lubricator, 10-Stalk presser, 11-Grating continuous level gauge, 12-Stalk presser inlet level sensor, 13-Discharge drive frequency converter, 14-Discharge motor drive frequency converter. Detailed Implementation

[0022] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0023] This invention proposes an embodiment of a tobacco stem pretreatment system based on stem diameter grading and proportional discharge. Specifically, the system in this example can be applied to a 6000 kg / h tobacco stem pretreatment line in a cigarette factory. The system specifically includes a host computer 1, a PLC controller 2, a top-mounted stem storage cabinet 3, a three-stage vibrating screening unit 4, a quantitative proportional discharge unit, a stem moistening machine 9, a stem pressing machine 10, and a detection unit. Preferably, the host computer 1 uses an industrial touch screen and communicates with the PLC controller 2 via a Profinet industrial bus for setting process parameters, monitoring equipment status, and storing and analyzing production data. The PLC controller 2 can be a Siemens S7-1500 series PLC, with a built-in proportional calculation module, a PID dual closed-loop control module, a synchronous speed regulation module, a front-to-back process linkage module, and an anomaly handling module, which are electrically connected to each execution unit and detection unit. The stem storage cabinet discharge unit is a top-mounted stem storage cabinet 3, including discharge bottom belts for left and right storage cabinets and a discharge drive frequency converter 13, used to complete the storage and continuous discharge of the washed tobacco stems.

[0024] Refer again Figure 1 In actual operation, the three-stage vibrating screening unit 4 includes an upper screen surface, a middle screen surface, and a lower collection base plate arranged sequentially from top to bottom. In one specific embodiment, the upper screen surface is a circular perforated screen plate with a screen aperture of 2.38 mm; the middle screen surface has a screen aperture of 1.7 mm; and the lower layer is a non-perforated stainless steel plate used to form a fine stem collection trough. This screening unit is used to separate the mixed tobacco stems into three components according to their diameter: coarse stems (diameter ≥ 2.38 mm), medium stems (diameter between 1.7 mm and 2.38 mm), and fine stems (diameter ≤ 1.7 mm). The discharge ports of the three screen surfaces are arranged vertically along the longitudinal direction, corresponding to the coarse stem, medium stem, and fine stem outlets, respectively. The quantitative and proportional discharge unit includes three vertically installed cuboid quantitative silos (i.e., coarse stem silo 5, medium stem silo 6, and fine stem silo 7) and rotary discharge motors 8 installed at the bottom of each silo. A stem moistening machine 9 and a stem pressing machine 10 can be sequentially arranged downstream of the quantitative and proportional discharge unit. Continuing from the previous description, the three silos are installed vertically, with their upper openings corresponding to the three discharge ports of the screening unit, and their lower openings each housing a rotary discharge motor 8. Each rotary discharge motor 8 is driven by a corresponding discharge motor drive frequency converter 14, with variable frequency speed regulation, and its discharge rate is linearly positively correlated with the drive frequency.

[0025] The detection unit in this embodiment includes three sets of grating-type continuous level gauges 11, a feed level sensor 12 at the inlet of the press machine, a speed sensor, and a vibration sensor. The grating-type continuous level gauges 11 are vertically installed on the side walls of the three hoppers to detect the material level in real time. The feed level sensor 12 at the inlet of the press machine 10 is installed at the feed hopper to detect the feed level in real time. The speed sensors are installed at the discharge belt of the storage tank, the drive end of the rotary unloading motor, and the pressure roller to detect the operating speed in real time.

[0026] Corresponding to the above preprocessing system, such as Figure 2 As shown, the present invention also provides a tobacco stem pretreatment method based on stem diameter grading and fixed-ratio output, the specific implementation steps of which include:

[0027] Step S1: Storage tank discharge and online screening and grading by stem diameter;

[0028] After 2 hours of heat preservation and moisture retention storage in the top-mounted tobacco stem storage tank 3, the PLC controller 2 issues a command to drive the bottom conveyor belt of the storage tank to start at a speed of 0.5m / s, continuously and evenly conveying the mixed tobacco stems to the three-stage vibrating screening unit 4; the vibration frequency of the three-stage vibrating screening unit 4 is set to 15Hz and the amplitude is set to 3mm.

[0029] After the tobacco stems enter the screening unit, coarse stems with a diameter greater than 2.38 mm are intercepted by the upper screen and conveyed to the coarse stem outlet, falling vertically into the coarse stem bin 5; medium stems with a diameter between 1.7 mm and 2.38 mm pass through the upper screen and are intercepted by the middle screen and conveyed to the middle stem outlet, falling vertically into the middle stem bin 6; fine stems with a diameter less than or equal to 1.7 mm pass through the upper and middle screens and are intercepted by the lower plate and conveyed to the fine stem outlet, falling vertically into the fine stem bin 7, thus completing the continuous stem diameter grading.

[0030] Step S2: Determination of the natural distribution ratio of stem diameter and calculation of the initial discharge standard;

[0031] During the material head stage, the rotary unloading motor 8 remains off, and only the three-stage vibrating screening unit 4 and the storage tank are started to discharge material. The material level height of the three silos is collected in real time by the grating continuous level gauge 11.

[0032] When the material level in the fine stem hopper 7 reaches 80% of the total hopper height for the first time, the PLC controller 2 collects and records the material level in the coarse stem hopper 5 as 400mm, the material level in the medium stem hopper 6 as 600mm, and the material level in the fine stem hopper 7 as 800mm.

[0033] Since the three hoppers have the same cross-sectional area, the PLC calculates the fixed discharge ratio baseline value for this batch of tobacco stems: Kc:Km:Kf = 400:600:800 = 2:3:4; subsequently, the system calls the pre-calibrated linear curve of the discharge rate-speed of the rotary unloading motor (linear fitting degree R). 2 =0.995), based on the target discharge ratio of 2:3:4, the drive frequency ratio of the three rotary unloading motors 8 is also set to 2:3:4, and the reference frequency of the main unloading motor corresponding to the fine stem hopper is set to 40Hz, medium stem to 30Hz, and coarse stem to 20Hz.

[0034] Step S3: During normal production, precise ratio setting and stable material level control are achieved through internal and external dual closed-loop systems.

[0035] After entering the normal production stage, the three rotary unloading motors 8 are started and run at a set frequency ratio, while the dual closed-loop control logic is combined. Figure 3 As shown, the details are as follows:

[0036] (1) Inner ring fixed ratio synchronous closed loop: The rotary unloading motor of the fine stalk hopper 7 is the main regulating motor. The controlled variable of the PID controller of PLC controller 2 is the height of the material level at the inlet of the stalk press, and the set value is 600mm. The PID parameters are set as proportional band 30%, integral time 20s, and derivative time 2s.

[0037] When the material level at the inlet of the pressing machine is higher than 600mm, the PLC reduces the drive frequency of the main regulating motor and simultaneously reduces the drive frequency of the unloading motors for medium and coarse stems according to a fixed ratio of 2:3:4. When the material level is lower than 600mm, the drive frequency of all unloading motors is increased simultaneously to ensure that the discharge ratio remains constant at 2:3:4.

[0038] (2) Outer ring material level stabilization closed loop: PLC collects the material level height of the three silos in real time. When the material level of any silo is higher than 90%, it is determined that the feeding speed of the screening is greater than the fixed discharge speed. PLC prioritizes reducing the driving frequency of the bottom belt 13 of the storage tank discharge, and the step size is set to 5Hz. When the material level of all silos is lower than 20%, it is determined that the feeding speed is less than the discharge speed. PLC increases the discharge frequency of the storage tank, and the step size is set to 5Hz.

[0039] When complex operating conditions occur (e.g., the coarse stalk silo 5 level is above 90% while the fine stalk silo 7 level is below 10%), the system implements a contradiction decoupling strategy that prioritizes overflow. First, the discharge speed of the storage tank is reduced to alleviate the risk of overflow in the coarse stalk silo. At the same time, the frequency of the coarse stalk unloading motor is slightly increased by 5% in the inner loop proportional control to accelerate its emptying, thereby correcting the serious level deviation.

[0040] Step S4: The upstream and downstream processes operate in a fully coordinated manner with constant parameters based on a constant output ratio;

[0041] Because the dual closed-loop control in step S3 ensures that the stem diameter composition of the tobacco stems entering the stem-lubricating machine 9 remains constant at 2:3:4, the system pre-sets the steam spray rate of the stem-lubricating machine 9 to 100 kg / min based on this constant ratio, and sets the fixed roller gap of the stem-pressing machine 10 to 0.8 mm and the roller speed to 25 r / min. Both the stem-lubricating machine 9 and the stem-pressing machine 10 operate stably according to the above-mentioned preset parameters throughout the entire production process. The roller gap does not require frequent adjustments, achieving continuous and stable production and effectively avoiding equipment wear and control lag caused by gap adjustments.

[0042] Step S5: Adaptive ratio switching and parameter transition in the material tail stage.

[0043] When the discharge signal from the storage tank is triggered and no new material enters the screening unit for 30 seconds, the system determines that it has entered the tail stage and locks the discharge speed of the storage tank at 0. The system continues to operate at a frequency ratio of 2:3:4 until the first coarse stem bin 5 is emptied (the material level drops to 0%). At this time, the PLC records that the remaining material level in the medium stem bin 6 is 300mm and the remaining material level in the fine stem bin 7 is 400mm. The new discharge ratio (i.e., the ratio of remaining material) is calculated to be 3:4. The PLC then adjusts the frequency ratio of the unloading motors for the medium and fine stems to 3:4, while adjusting the steam spray rate of the stem lubricator 9 to 75kg / min and smoothly adjusting the gap between the pressure rollers of the stem presser 10 to 0.75mm, until the medium stem bin 6 and the fine stem bin 7 are completely emptied, completing the entire process.

[0044] The above embodiments have been proven in practice to effectively reduce the deviation in the diameter composition of tobacco stems entering the stem presser. The stem presser operates at a fixed gap throughout the process without adjustment, which significantly reduces the thickness deviation of the pressed stem pieces and the subsequent shredding and breakage rate, and improves the whole stem shredding rate.

[0045] Finally, as a supplementary explanation, the present invention provides specific structural details of a tobacco stem pretreatment system for performing the above-described pretreatment method. See also... Figure 1 As shown, the vibration frequency and amplitude of the three-stage vibrating screening unit 4 can be adjusted according to the material characteristics and processing volume to achieve optimal screening efficiency. The characteristic curves of the rotary unloading motors 8 at the bottom of each buffer hopper are pre-calibrated and stored in the PLC controller 2 to ensure the accuracy of the output calculation. The system integrates a complete closed-loop control link from the upstream storage tank discharge to the downstream stem pressing machine inlet. It features a compact structure, clear control logic, and rapid response, and can well adapt to the new construction and renovation needs of existing cigarette factory tobacco processing lines. The host computer 1 and the PLC controller 2 communicate via an industrial bus, which can display key parameters such as the material level of each hopper, motor frequency, and stem pressing machine inlet material level in real time, facilitating monitoring and intervention by operators.

[0046] In this invention, when directional terms are mentioned, they are relative concepts based on the embodiments. Furthermore, "at least one" refers to one or more, and "more than one" refers to two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent the existence of A alone, A and B simultaneously, or B alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship. "At least one of the following" and similar expressions refer to any combination of these items, including any combination of single or plural items. For example, at least one of a, b, and c can represent: a, b, c, a and b, a and c, b and c, or a and b and c, where a, b, and c can be single or multiple.

[0047] The above description of the structure, features, and effects of the present invention is based on the embodiments shown in the figures. However, the above are only preferred embodiments of the present invention. It should be noted that the technical features involved in the above embodiments and their preferred methods can be reasonably combined and matched by those skilled in the art to form a variety of equivalent solutions without departing from or changing the design concept and technical effects of the present invention. Therefore, the present invention is not limited to the scope of implementation shown in the figures. Any changes made in accordance with the concept of the present invention, or modifications to equivalent embodiments, that do not exceed the spirit covered by the specification and figures, should be within the protection scope of the present invention.

Claims

1. A tobacco stem pretreatment method based on stem diameter grading and fixed ratio discharging, characterized in that, Includes the following steps: S1: Storage tank discharge and stem diameter online screening and grading: The mixed tobacco stems output from the upstream storage tank are continuously fed into the screening equipment and divided into three components according to the preset stem diameter range: coarse stems, medium stems and fine stems, which are then stored in the corresponding first buffer bin, second buffer bin and third buffer bin respectively. S2: Determination of the natural distribution ratio of stem diameter and calculation of the initial discharge benchmark. At the material head stage, start the storage tank discharge and screening equipment, but keep the unloading motor at the bottom of each buffer hopper closed. When the material level of any buffer hopper reaches the preset threshold, collect the material level height of the remaining buffer hoppers, calculate the volume or mass ratio of the three components of coarse, medium and fine, set the ratio as the target discharge ratio benchmark, and convert the target discharge ratio benchmark into the drive frequency benchmark value of each unloading motor according to the pre-calibrated unloading motor characteristic curve. S3: During normal production, the internal and external dual closed-loop collaborative control starts all unloading motors and activates the inner loop constant ratio synchronous tracking closed loop and the outer loop material level stabilization closed loop. The inner loop adjusts the frequency of the main regulating motor according to the real-time material level at the downstream press inlet, and simultaneously calculates and adjusts the frequency of each driven motor according to the target discharge ratio benchmark to ensure a constant discharge ratio. The outer loop monitors the material level of each buffer silo in real time, and adjusts the discharge speed of the upstream storage tank when the material level exceeds the limit to maintain the stability of the silo material level. S4: Based on the constant output ratio, the upstream and downstream processes operate in a fully coordinated manner with constant parameters. According to the target output ratio benchmark determined in step S2, the process parameters of the downstream stem lubricator and stem press are preset and locked so that the stem lubricator and stem press processes operate under constant parameters. S5: Adaptive ratio switching and parameter transition in the tail stage. When the upstream storage tank has finished discharging and the buffer hopper has been emptied first, the discharge ratio of the remaining material is recalculated, and the frequency of the still running unloading motor is adjusted accordingly. At the same time, the process parameters of the stem lubricator and the stem press are adjusted in linkage until the system is completely emptied.

2. The tobacco stem pretreatment method based on stem diameter grading and proportional output according to claim 1, characterized in that, In step S2, the step of collecting the material level height of the remaining buffer silos and calculating the volume or mass ratio of the coarse, medium, and fine components is specifically as follows: When the material level of the smallest or most easily filled material bin among the three buffer bins reaches the preset safe high material level threshold for the first time, the PLC controller collects and records the height Hc of the coarse stem bin, the height Hm of the medium stem bin, and the height Hf of the fine stem bin at this time. Since the cross-sectional area of ​​each bin is the same or known, the ratio of Hc:Hm:Hf is used as the natural stem diameter distribution ratio of this batch of tobacco stems, that is, the target output ratio benchmark Kc:Km:Kf.

3. The tobacco stem pretreatment method based on stem diameter grading and proportional output according to claim 1, characterized in that, In step S3, the inner loop constant ratio synchronous tracking closed loop adopts master-slave synchronous control logic, specifically including: The unloading motor of the fine stem hopper is defined as the main regulating motor, and the other two are driven motors. The PLC reads the signal from the inlet material level sensor of the stem press and compares it with the set value. When the inlet material level of the stem press deviates from the set value, the PID controller calculates the adjustment amount and applies it to the main regulating motor, so that its drive frequency increases or decreases proportionally. At the same time, the PLC calculates the frequency value that the other driven motors should be adjusted to according to the fixed target discharge ratio benchmark and outputs it to the corresponding frequency converter.

4. The tobacco stem pretreatment method based on stem diameter grading and proportional output according to claim 1 or 3, characterized in that, In step S3, the outer ring material level stabilization closed loop specifically comprises: The PLC continuously monitors the real-time material level of each buffer silo; When the material level of any buffer silo is higher than the set high-high threshold, it is determined that the speed of incoming screened material is greater than the constant proportion discharging speed, and the PLC gradually reduces the driving frequency of the discharging bottom belt of the storage cabinet by a preset step size to reduce the upstream incoming material; When the material level of any buffer silo is lower than the set low-low threshold, it is determined that the incoming material speed is less than the discharging speed, and the PLC gradually increases the driving frequency of the discharging bottom belt of the storage cabinet by a preset step size to increase the upstream incoming material; When the complex working condition that the material level of the first buffer silo is too high and the material level of the third buffer silo is too low occurs simultaneously, the system starts a contradiction decoupling strategy, preferentially implements the anti-spillage strategy and allows fine adjustment of the inner ring proportion within a certain safety range to correct the material level deviation.

5. The tobacco stem pretreatment method based on stem diameter grading and proportional output according to claim 1, characterized in that, In step S4, the process parameters of the downstream stem conditioner and stem press at least comprise the steam flow, water injection amount and drum rotating speed of the stem conditioner, as well as the roller gap and roller rotating speed of the stem press, wherein the roller gap remains locked during production and is not adjusted frequently.

6. The tobacco stem pretreatment method based on stem diameter grading and proportional output according to claim 1, characterized in that, In step S5, recalculating the discharging proportion of the remaining materials and adjusting the motor frequency specifically comprises: When the grating continuous level gauge detects that the material level of a buffer silo drops to 0%, the PLC re-reads the instantaneous material levels of each remaining buffer silo at this time, calculates a new discharging proportion, and reallocates and sets the driving frequencies of the discharging motors still in operation according to the new proportion; meanwhile, the PLC sends parameter change instructions to the stem conditioner and the stem press, so that the process parameters of the subsequent process adapt to the stem diameter composition of the current remaining materials.

7. A tobacco stem pretreatment system for performing the tobacco stem pretreatment method based on stem diameter grading and proportional output as described in any one of claims 1 to 6, characterized in that, comprising: a storage cabinet discharging unit, configured to store and continuously output mixed tobacco stems; a three-stage vibrating screening unit arranged downstream of the storage cabinet discharging unit, configured to classify the mixed tobacco stems into coarse, medium and fine three grades according to stem diameters and output the classified tobacco stems respectively through corresponding outlets; a constant-quantity constant-proportion discharging unit comprising a first buffer silo, a second buffer silo and a third buffer silo which are respectively communicated with three outlets of the three-stage vibrating screening unit, and rotary discharging motors respectively installed at the bottom of each buffer silo, configured to buffer the classified tobacco stems and realize constant-proportion discharging; a stem conditioner and a stem press which are sequentially arranged downstream of the constant-quantity constant-proportion discharging unit, configured to perform stem moistening and stem pressing treatment on the mixed tobacco stems; a detection unit at least comprising level gauges respectively arranged on each buffer silo and a material level sensor arranged at the inlet of the stem press; a PLC controller electrically connected to the storage cabinet discharging unit, the three-stage vibrating screening unit, the rotary discharging motors, the stem conditioner, the stem press and the detection unit respectively, configured to execute the control logic in the pretreatment method.

8. The tobacco stem pretreatment system according to claim 7, characterized in that, The three-stage vibrating screening unit comprises an upper screen surface, a middle screen surface and a lower collecting bottom plate which are sequentially arranged from top to bottom; The upper screen surface has a screen pore diameter of D1 and is configured to intercept coarse stems with a diameter larger than D1; The middle screen surface has a screen pore diameter of D2, and D2 < D1, and is configured to intercept medium stems with a diameter between D2 and D1; The lower collecting bottom plate is of a non-porous or microporous flat plate structure, and is configured to collect fine stems with a diameter smaller than D2.

9. The tobacco stem pretreatment system according to claim 7, characterized in that, The detection unit also includes speed sensors installed on the bottom belt of the storage tank, the rotary unloading motor and the drive end of the pressure roller, respectively, for real-time detection of the operating speed of each actuator.

10. The tobacco stem pretreatment system according to any one of claims 7 to 9, characterized in that, The PLC controller has a built-in proportional calculation module, a PID dual closed-loop control module, a synchronous speed regulation module, a front-to-back process linkage module, and an anomaly handling module. The host computer communicates with the PLC controller through an industrial bus and is used for process parameter setting, equipment status monitoring, and data storage and analysis.

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