Experimental study on weighing mechanism of modifi

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

1 preface

rice is the most important food for our people. According to statistics in recent years, China's per capita annual consumption of rice is about 100 kg. With the gradual development of market economy, the packaging of commercial rice has also gradually changed from 100 kg gunny bags leaving the factory and selling in bulk in the era of planned economy 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, Teyou rice and so on. These high-quality rice are packed in 5kg and 2.5kg. However, the modified atmosphere packaging technology of rice, which is widely recognized by technical experts in the industry and has a significant effect of mold prevention, insect prevention and delaying aging, has not been applied. To this end, we have developed an automatic packaging machine for modified atmosphere fresh-keeping of rice, and carried out a series of experimental studies on the automatic weighing device, one of its key components, as follows

2 experimental study of blanking mechanism

for the weighing of granular materials, blanking and then reset at startup, the commonly used structural forms of equipment mechanisms are electromagnetic vibration type, belt conveying type and dead weight DC type. Electromagnetic vibration type has good adaptability to various materials, less randomness of blanking weight, and easy to ensure weighing accuracy, but it has complex structure, large volume, high cost, and large vibration and noise. Belt conveyor type has certain adaptability to the same materials without continuous adjustment of experimental speed, and the weighing accuracy is also high. The disadvantages are also complex structure, large volume, high cost, and material leakage is a common problem. The self weight DC type is not very suitable for all kinds of materials, and the randomness of the blanking weight is large, so the control of weighing accuracy will be the key problem; However, the self weight DC type has the advantages of simple and compact structure, small volume and low cost. According to the working object and characteristics of the automatic small rice packaging machine, we should choose the self weight DC blanking mechanism. Therefore, in order to ensure the control of weighing accuracy, the research of blanking device will be one of its key problems

the usual self weight DC type blanking device generally adopts double ports (double doors) for blanking, that is, the large port is responsible for the main blanking task, and it can be underground as soon as possible to reach the set weight; The small feeding port only adds the difference to the set weight. After repeated tests, due to the randomness of the blanking weight and its impact force, and the weight of the residual material column, the contradiction between high accuracy and high speed of weighing is not easy to solve and cannot meet the higher requirements. For this reason, three feeding ports (three doors) and four feeding ports (four doors) are designed. Each material port of the four material port tripper is set as No. 1, 2, 3 and 4 from large to small. During blanking, each material port is simultaneously discharged to the value equal to or slightly greater than the first weight in the last 5-min period of the cooling section, and No. 1 material port is closed; 2. At the same time, No. 3 and No. 4 material openings are lowered to equal to or slightly greater than the second weight value, and No. 2 material opening is also closed; 3. At the same time, No. 4 material opening is unloaded to equal to or slightly greater than the third weight value, and No. 3 material opening is also closed; Only the No. 4 feeding port is left to drop the material to the set weight value, and the No. 4 feeding port is also closed, and the blanking is completed. The blanking areas of No. 1, 2, 3 and 4 outlets of the test device are 30, 18, 8 and 3.2 cm2 respectively. The average values of the blanking flow measured in the experiment (50 times) are 1977.1, 1072.5, 119.8 and 54.4 g/s respectively, and the standard deviations of the blanking flow 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 blanking areas have a great impact on the standard deviation of blanking flow

when reaching the first weight value, due to the randomness of the blanking weight and its impact force, and the weight of the residual material column, the error is very large. Only from the perspective of the standard deviation of the blanking flow, press the general 3 σ In principle, it can be considered that the error range is ± 249 g/s, and its reliability is 99.7%. However, from the perspective of packaging accuracy requirements, choose ± 5 σ， That is, the error range is ± 415 g/s, and its reliability is 99.99995%. The probability of falling outside this error range is only 5/10 million. The standard deviation of blanking flow is the timing test result (that is, the time from the controller issuing the door opening command to the controller issuing 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 blanking force and the weight of residual material column (for the standard weight, the influence of the former is negative, and the influence of the latter is positive). However, the non-uniformity of the blanking impact force and the weight of the residual material column is essentially caused by the non-uniformity of the blanking flow, so the standard deviation of the blanking flow can largely reflect the quantitative test results (that is, the test results of setting the standard weight and blanking separately by each material port), which is proved by the verification test

taking weighing 5 kg as an example, considering the future work of No. 2, 3 and 4 material openings, the above error range is doubled, that is, the first weight value is set to 4200 g. since the difference between the first weight value and the standard weight 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 ± 191.95 g/s, the second weight value is set to 4700 g, the difference between the second weight value and the standard weight value is left as 300 g, and the weighing time to reach the second weight value is ()/(1072.5+119.8+54.4) =0.401 s. The error range of the third weight value is ± 35.85 g/s, the third weight value is set to 4900 g, the difference between the third weight value and the standard weight value is left as 100 g, and the weighing time to reach the third weight value is ()/(119.8+54.4) =1.148 s. The error range of No. 4 feeding port is ± 10.05 g/s, which is the final packaging accuracy. Since the test shows that the weight of the residual material column is about 50 g greater than the blanking impact force (systematic error), the set weight value is 4950 g, and the weighing time to reach the set weight value is ()/54.4=0.919 s. The total weighing time is 3.77 s. This not only ensures that the final weighing result has enough high accuracy, but also ensures the high speed of weighing

it can be seen that compared with the double port tripper, the four port tripper has obvious advantages. However, the four port feeder used in the above experiment is not very reasonable in the distribution of the area of each port. Experiments show that compared with the three port tripper, it has no obvious advantages in accuracy and speed. The blanking process of the three port tripper is basically the same as that of the four port tripper, but the structure is much simpler. Therefore, the three port tripper was selected as the test result. The blanking areas of the large blanking port (gate), the middle blanking port (middle gate) and the small blanking port (small gate) of the three port tripper are 110 respectively × 70㎜、70 × 20㎜、20 × 20 mm, the average value of the discharge is 4721.45, 827.17 and 64.21 g/s respectively, and the standard deviation of the discharge measured in the experiment is 130.46, 39.67 and 2.67 g/s respectively. The error range of each feeding port is ± 652.3, ± 198.35, ± 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 (eliminate the systematic error that the weight of the residual material column is greater than the blanking impact force). The time to reach each weight value is 0.766 s, 0.662 s, 1.012 s respectively, and the total weighing time is 2.44 s. The whole machine test shows 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 test design. It also solves the contradiction between the high accuracy of weighing and the high speed of weighing

in the form of material door, pendulum arc material door, vertical reciprocating direct plug material door and horizontal reciprocating direct plug material door are designed and manufactured, and comparative tests are carried out. Considering the layout and installation of pneumatic control elements and structural complexity of tilting arc-shaped material door, it is not adopted. The vertical reciprocating straight plug door is more convenient in the layout and installation of pneumatic control elements, but it is easier to wedge materials and get stuck. The horizontal reciprocating straight plug door was selected as the test result

Fig. 1 Mechanical and electronic automatic weighing device

3 experimental research on measuring mechanism

automatic weighing devices for granular materials at home and abroad can be divided into two categories: volumetric and weighing. The volumetric structure is relatively simple, but the measurement accuracy is not high, and the structure is not compact, so it is not suitable for material packaging with certain accuracy requirements or large measurement. Weighing type generally includes mechanical type, mechanical electronic type and electronic type. Mechanical automatic metering device is bulky, cumbersome, low precision and slow speed, which is rarely used in modern times. The first stage of design and trial production is the mechanical electronic automatic weighing device, as shown in Figure 1. For the mechanical part, the commercial 10 kg mechanical platform scale is selected, the scale pan is changed into a weighing hopper (with charging door), and the weight hook is modified to realize the automatic two-stage loading of the weight. With the photoelectric controller, the feeder can be controlled to realize the automatic weighing

the biggest advantage of this device is that it has simple structure, low cost, and the weighing accuracy can meet the requirements. However, due to the different residual column weights and their impact forces corresponding to different set weights, it is necessary to subtract different values from the set weight values to correct, which is very inconvenient in use and operation. In addition, the structural size of the mechanical electronic automatic weighing device is larger, resulting in the larger size of the whole machine. Therefore, after the experiment, this scheme was abandoned

the prototype officially adopts an electronic automatic weighing device, which is directly installed on the commercial electronic scale with a weighing hopper with a charging door. The electronic scales sold on the market are standard products that have been verified by the National Metrology Department. Their accuracy is 1/3000 (those whose accuracy is higher than 1/3000 are electronic scales), and the maximum weighing is 3 kg, 6 kg, 15 kg, etc. Since the maximum weight of the packaging machine is 5 kg, plus the weight of the weighing hopper, it is already greater than 6 kg, so only a 15 kg electronic scale with a resolution of 5 g can be selected. The automatic weighing of the electronic scale is controlled by the weighing controller with single chip microcomputer as the core. There are two displays on its panel, one displays the weight value, the other displays various set values or the number of bags packed, and there are corresponding various setting keys. In addition to the weighing signal, the weighing controller also inputs the allowable weighing, stop weighing, dumping status, etc., while the weighing controller outputs the signals of dumping, inflation, large, medium and small material door control, etc., and directly outputs 220V to control the solenoid valve switches, as shown in Figure 4. Take out the weighing of the electronic scale from the serial port of the single chip microcomputer in the electronic scale