Experimental study on weighing mechanism of rice modified atmosphere fresh-keeping packaging machine

1 Introduction

Rice is the main food for the Chinese people. According to statistics in recent years, the average annual per capita rice consumption in China is about 100 kg. With the gradual development of the market economy, the packaging of commercial rice has also gradually transitioned from the 100 kg hemp bags shipped and sold in bulk in the planned economy era to 10 kg and 15 kg plastic woven bags. In recent years, in order to adapt to the continuous improvement of the living standards of urban people, many factories have developed new varieties of processed rice: non-washing rice, fortified rice, germ rice, special rice, etc. These fine rice are packaged in 5 kg and 2.5 kg. However, the rice modified atmosphere fresh-keeping packaging technology, which is generally recognized by technical experts in the industry and has significant effects of mildew prevention, insect prevention and delayed aging, has not yet been applied. For this reason, we have developed a rice modified atmosphere fresh-keeping automatic packaging machine and conducted a series of experimental studies on the automatic weighing device, one of its key components, as briefly described below.

2 Experimental study of the feeding mechanism

For the weighing of bulk materials, the feeding mechanism commonly used has electromagnetic vibration type, belt conveyor type and self-weight direct current type and other structural forms. The electromagnetic vibration type has good adaptability to various materials, the randomness of the falling weight is small, and the weighing accuracy is easy to ensure, but the structure is complex, the volume is large, the cost is high, and the vibration and noise are large. The belt conveyor type also has a certain adaptability to different materials, and the weighing accuracy is also high. The disadvantages are also complex structure, large volume, high cost, and leakage is a common problem. The self-weight DC type is not very adaptable to various materials, the randomness of the falling weight is large, and the control of weighing accuracy will be its key issue; but the self-weight DC type has a simple and compact structure, small volume, and low cost. According to the working object and characteristics of the automatic small rice packaging machine, it is appropriate to select the self-weight DC type unloading mechanism. Therefore, in order to ensure the control of weighing accuracy, the research of the unloading device will be one of its key issues. The

usual self-weight DC type unloading device generally adopts double-material port (double door) unloading, that is, the large material port is responsible for the main unloading task, and can unload the material to close to the set weight as soon as possible; the small material port only supplements the difference to the set weight. After repeated tests, it was found that the weight of the material dropped, its impact force, and the weight of the residual material column were highly random when the large material port was used for material discharge, which made it difficult to resolve the contradiction between high-precision weighing and high-speed weighing, and failed to meet higher requirements. For this reason, three-port (three-door) and four-port (four-door) feeders were designed. The four-port feeders are set to No. 1, 2, 3, and 4 from large to small. When discharging, the materials are discharged from each port at the same time to a value equal to or slightly greater than the first weight value, and the No. 1 feeder port is closed; the materials are discharged from No. 2, 3, and 4 ports at the same time to a value equal to or slightly greater than the second weight value, and the No. 2 feeder port is also closed; the materials are discharged from No. 3 and No. 4 ports at the same time to a value equal to or slightly greater than the third weight value, and the No. 3 feeder port is also closed; only the No. 4 feeder port is left to discharge the material to the set weight value, and the No. 4 feeder port is also closed, and the material discharge is completed. The drop areas of the No. 1, 2, 3, and 4 material ports of the test device are 30, 18, 8, and 3.2 cm2, respectively. The experimentally measured average drop flow rates (50 times) are 1977.1, 1072.5, 119.8, and 54.4 g/s, respectively. The drop flow rate standard deviations S1, S2, S3, and S4 are 44.61, 31.22, 5.16, and 2.01 g/s, respectively. It can be clearly seen from the experimental results that different drop areas have a great influence on the drop flow rate standard deviation.

When the first weight value is reached, the error is large due to the large randomness of the drop weight, its impact force, and the weight of the residual material column. From the perspective of the drop flow rate standard deviation alone, according to the general 3σ principle, it can be considered that its error range is between ±249 g/s, and its credibility is 99.7%. However, from the perspective of packaging accuracy, ±5σ is selected, that is, the error range is between ±415 g/s, and its credibility is 99.99995%. The probability of falling outside this error range is only 5 in 10 million. The standard deviation of the blanking flow is the result of a timing test (that is, the time from the controller issuing the door opening command to the door closing command is 1 second, and the blanking test result within this set time). On the surface, it does not reflect the impact of the blanking impact force and the residual material column weight (for the standard weighing weight, the former has a negative impact and the latter has a positive impact). However, the unevenness of the blanking impact force and the residual material column weight is actually caused by the unevenness of the blanking flow, so the standard deviation of the blanking flow can largely reflect the quantitative test results (that is, the standard weighing weight is set, and the test results of blanking from each material port are separately dropped), and the verification test proves this point.

Taking weighing 5 kg as an example, considering the work of No. 2, 3, and 4 feed ports in the future, the above error range is expanded by about one time, that is, the first weight value is set to 4200 g. Since the difference between the first weight value and the standard weighing value is 800 g, the error will not affect the final weighing result. The weighing time to reach the first weight value is 4200/(1977.1+1072.5+119.8+54.4)=1.303 S. The error range of the second weight value is between ±191.95 g/s. The second weight value is set to 4700 g. The difference between the second weight value and the standard weighing value is 300 g. The weighing time to reach the second weight value is (4700-4200)/(1072.5+119.8+54.4)=0.401 S. The error range of the third weight value is between ±35.85 g/s. The third weight value is set to 4900 g. The difference between the third weight value and the standard weighing value is left at 100 g. The weighing time to reach the third weight value is (4900-4700)/(119.8+54.4)=1.148 S. The error range of the No. 4 feeder is between ±10.05 g/s, which is the final packaging accuracy. In view of the fact that the weight of the residual material column is about 50 g greater than the impact force of the falling material (system error), the weight value is set to 4950 g, and the weighing time to reach the set weight value is (4950-4900)/54.4=0.919 S. The total weighing time is 3.77 S. This ensures that the final weighing result has sufficient high precision and at the same time ensures high weighing speed.

It can be seen that compared with the double-port feeder, the superiority of the four-port feeder is very obvious. However, the four-port feeder used in the above experiment is not very reasonable in the distribution of the area size of each feeder port. Experiments have shown that compared with the three-port feeder, the superiority in terms of accuracy and speed is not very obvious. The feeding process of the three-port feeder is basically the same as that of the four-port feeder, but the structure is much simpler. Therefore, the three-port feeder was selected for the test results. The feeding areas of the large feeder port (big door), the middle feeder port (middle door), and the small feeder port (small door) of the three-port feeder are 110×70㎜, 70×20㎜, and 20×20㎜, respectively. The average values ​​of the feeding flow are 4721.45, 827.17, and 64.21 g/s, respectively. The experimentally measured standard deviations of the feeding flow are 130.46, 39.67, and 2.67 g/s, respectively. The error ranges of each feeder port are between ±652.3, ±198.35, and ±13.35 g/s, respectively. Taking weighing 5 kg as an example, the first weight value is set to 4300 g, the second weight value is set to 4890 g, and the set weight value is 4955 g (eliminating the system error that the weight of the residual material column is greater than the impact force of the falling material). The time to reach each weight value is 0.766 S, 0.662 S, and 1.012 S, respectively, and the total weighing time is 2.44 S. The whole machine test proves that the weighing error range is ±15 g, and the weighing speed can reach 2.5 S, which is basically consistent with the theoretical value of the experimental design. It also solves the contradiction between high weighing precision and high weighing speed.

In terms of the form of the material door, the swing-type arc material door, the vertical reciprocating straight-insertion material door, and the horizontal reciprocating straight-insertion material door were designed and trial-produced for comparative tests. Considering the problems of the arrangement and installation of the pneumatic control components and the complexity of the structure of the swing-type arc material door, it was not adopted. The vertical reciprocating straight-insertion material door is more convenient in terms of the arrangement and installation of the pneumatic control components, but it is easier to wedge the material and get stuck. The test results showed that the horizontal reciprocating straight-insertion material door was selected.

Figure 1 Mechanical and electronic automatic weighing device

3 Experimental research on metering mechanism

The automatic metering devices for bulk materials at home and abroad can be divided into two categories: volumetric and weighing. The volumetric structure is relatively simple, but the metering accuracy is not high and the structure is not compact, so it is not suitable for material packaging with certain accuracy requirements or large measurements. There are three types of weighing types: mechanical, mechatronic, and electronic. Mechanical automatic metering devices are large, heavy, low in accuracy, and slow, and are rarely used in modern times. The first stage of design and trial production is a mechatronic automatic weighing device, as shown in Figure 1. The mechanical part uses a commercially available 10kg mechanical platform scale, and the scale pan is changed to a weighing hopper (with a filling door). The weight hook is modified to realize automatic two-stage loading of weights. Coupled with a photoelectric controller, the feeder can be controlled to realize automatic weighing. [page]

The working diagram of automatic two-stage loading of weights is shown in Figure 2. As a mechanical scale with a lever mechanism, the impact force of falling materials has a great influence on its working stability during the weighing process. In order to solve this problem, the method of automatic two-stage loading of weights is adopted. At the beginning of weighing, one of the weights X (for example, 200 grams) is supported on the support seat (the upper figure of Figure 2) and is not loaded on the lever. In this way, the weight loaded on the lever is only the set weighing weight - X = Y. When the material in the hopper + the impact force of falling materials reaches Y, the lever is lifted up, and the weight X leaves the support seat. Its weight is loaded on the lever, overcoming the inertia of the lever lifting up, and the lever is rebalanced according to the set weighing weight. If the lever can still be lifted a little bit after the balance result, the lower photoelectric switch senses it and closes the large discharge port. The small discharge port continues to drop materials until the lever is slowly lifted to a horizontal position, and the upper photoelectric switch senses it and closes the small discharge port. The photoelectric controller is shown in Figure 3.

The biggest advantage of this device is that it has a simple structure, low cost, and the weighing accuracy can meet the requirements. However, due to the different weights of the residual material column and its impact force corresponding to different set weighing weights, it is necessary to subtract different values ​​from the set weighing weight to make corrections, which is very inconvenient in operation. In addition, the mechanical electronic automatic weighing device has a larger structural size, resulting in a larger size of the whole machine. Therefore, after the test, this plan was abandoned.

The prototype officially uses an electronic automatic weighing device, which is directly installed on the commercially available electronic scale with a weighing hopper with a filling door. The commercially available electronic scale is a standard product verified by the national metrology department, with an accuracy of 1/3000 (those with an accuracy higher than 1/3000 are electronic balances), and the maximum weighing is 3kg, 6kg, 15kg, etc. Since the maximum weighing of the packaging machine is 5kg, plus the weight of the weighing hopper, it is already greater than 6kg, so only a 15kg electronic scale with a resolution of 5g can be selected. The automatic weighing of the electronic scale is controlled by a weighing controller with a single-chip microcomputer as the core. There are two displays on the panel, one for weight value and one for various set values ​​or the number of bags packed, and there are corresponding setting keys. In addition to the weighing signal, the weighing controller also inputs the status of allowing weighing, stopping weighing, and unloading, and the output signals of the weighing controller include unloading, inflation, large, medium, and small material door control, etc. It directly outputs 220V to control the switches of various solenoid valves, as shown in Figure 4. The weighing signal of the electronic scale is taken out from the serial port of the single-chip microcomputer in the electronic scale and directly input into the weighing controller. The software compiled by the controller based on a large amount of experimental data controls the opening and closing of the large, medium, and small material doors, and the weight value is displayed on the display on the panel.

The electronic automatic weighing device has been tested and weighed 5 kg of rice 100 times. The average weight is 4991.9 g (i.e. the average error is 8.1 g), the standard deviation is 4.9 g, the maximum error is 15 g, and the probability of occurrence is 2%. The average weighing time is 2.5S. The

automatic pouring and filling mechanism is an indispensable part of the electronic automatic weighing device. It is actually the controllable bottom of the weighing bucket. However, in order to ensure the weighing accuracy, the control part cannot touch the bottom of the bucket during weighing. Only when pouring and filling can the control part touch the bottom of the bucket to work. For this reason, several schemes were tested, and finally the cylinder pull-down type was selected. A balance weight block was added to the bottom of the bucket. The return spring is very soft and the impact force is small. This pouring and filling mechanism has a simple and compact structure, quick response, and reliable operation.

4 Conclusion

As one of the key components of the automatic packaging machine for rice atmosphere preservation, the automatic weighing device adopts a self-weight DC type feeding mechanism with a simple and compact structure. At the same time, the double-mouth (double-door), three-mouth (three-door), and four-mouth (four-door) feeders were used for experimental comparison. The results show that the three-mouth feeder is significantly better than the double-mouth feeder in solving the contradiction between high weighing precision and high weighing speed, and is closer to the four-mouth feeder. The mechanical electronic automatic weighing device has a simple structure, low cost, and can meet the requirements of weighing accuracy; but it is large in size and the automation level is not high enough. The electronic automatic weighing device is directly added to the weighing controller on the basis of the commercially available electronic scale, so there is no problem with the accuracy and verification of the electronic scale itself. The automatic weighing device of the rice controlled atmosphere fresh-keeping automatic packaging machine developed in this way has an error of no more than 15 g when weighing 5 kg at the maximum, and an average weighing time of 2.5S. It can not only fully meet the requirements of rice controlled atmosphere fresh-keeping packaging for high speed and high precision, but also has a simple structure, reliable operation and low cost. (end)