Experimental study on automatic weighing device of

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Experimental research on automatic weighing device of modified atmosphere packaging machine for rice (Part 2)

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 packaging materials with certain accuracy requirements or large measurement. Weighing type generally includes mechanical type, mechanical electronic type and electronic type. The 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 10kg mechanical platform scale is selected, the scale pan is changed into a weighing hopper (with a 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

see Fig. 2 for the working diagram of automatic two-stage loading of weights. As a mechanical scale with lever mechanism, the impact of blanking has a great impact on its working stability in 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, an X (e.g. 200g) in the weight is supported on the support seat (above figure in Figure 2) and is not loaded on the lever. In this way, the weight loaded on the lever is only set to weigh -x=y. when the material in the hopper + blanking impact force reaches y, the lever will lift up, and weight x will leave the support seat. Its weight is loaded on the lever, overcoming the inertia of lever lifting, The lever is rebalanced according to the set weight. Balance result if the lever can still be raised a little, the photoelectric switch will sense and close the large blanking port. The small blanking opening continues to blanking. The long carbon chain nylon has the characteristics of low water absorption, strong dimensional stability, good toughness and softness, wear resistance, excellent electrical performance, etc. until the lever is slowly lifted to the horizontal position, the photoelectric switch senses and closes the small blanking opening. See Figure 3 for the photo electric controller

the greatest advantage of this device is that it has simple structure, low cost and can meet the requirements of weighing accuracy. However, due to the difference of residual column weight and its impact force corresponding to different set weights, it is necessary to subtract different values from the set weights, which is very inconvenient in operation. In addition, the mechanical electronic automatic weighing device has a larger structural size, resulting in a larger overall size. Therefore, this scheme was abandoned after experiments

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 verified by the National Metrology Department, with an accuracy of 1/3000 (those with an accuracy 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 is selected. The automatic weighing of the electronic scale is controlled by the weighing controller with the single chip microcomputer as the core. There are two displays on the 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 signals, the weighing controller also inputs the allowed 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. 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 control. In this way, the interference controller composed of the peak current caused by the transient conduction and cut-off of the current load is greatly reduced. The software compiled by the controller according to a large number of experimental data controls the opening and closing of the large, medium and small material doors, and the weight value is displayed by the display on the panel

the electronic automatic weighing device has been tested and weighed 5 kg rice for 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 feeding and filling mechanism is an essential 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 contact the bucket bottom during weighing, and the control part can only contact the bucket bottom for work when filling. For this reason, several schemes were tested. Finally, the air cylinder pull-down type was selected, and the balance weight was added at the bottom of the bucket. The return spring was very soft and the impact force was small. The feeding and filling mechanism has the advantages of simple and compact structure, rapid response and reliable operation

4 conclusion

as one of the key components of the rice modified atmosphere fresh-keeping automatic packaging machine, the automatic weighing device adopts the self weight DC feeding mechanism with simple and compact structure. At the same time, the experimental comparison is carried out with the feeding devices with double feeding ports (double doors), three feeding ports (three doors) and four feeding ports (four doors for impact test at room temperature), The results show that the three port feeder is obviously superior to the double port feeder in solving the contradiction between high accuracy and high speed of weighing, and is close to the four port feeder. The mechanical electronic automatic weighing device has simple structure, low cost, and the weighing accuracy can meet the requirements; However, the size is large and the automation level is not high enough. The electronic automatic weighing device is directly added with a weighing controller on the basis of the commercial electronic scale, so there is no problem with the accuracy and verification of the electronic scale itself. The automatic weighing device of the rice modified atmosphere fresh-keeping automatic packaging machine developed in this way has the error of no more than 15 g when the maximum weight is 5 kg, and the average weighing time is 2.5s. It can not only fully meet the requirements for high speed and high precision of the rice modified atmosphere fresh-keeping packaging with PS as the yield point s, but also has simple structure, reliable operation and low cost

source: packaging and food machinery

Author: zhangcuiming, liujianwei, xurunqi, baoqingbin

Copyright © 2011 JIN SHI