At present, foreign liquid filling technology has been more mature China's filling technology level compared with the advanced level of foreign countries, there is a large gap between the domestic enterprises rely mainly on the introduction and imitation of foreign advanced products, independent research and development capabilities are poor, resulting in low degree of automation of the domestic filling equipment, filling precision is low. Liquid filling system according to the filling principle can be divided into atmospheric pressure, equal pressure, negative pressure and pressurized class; according to the measurement method can be divided into liquid surface type, flow meter type, weighing type and metering pump type. Among them, the pressurized metering pump type liquid filling system is not only suitable for low, medium and high viscosity liquid filling, but also has high metering precision and high filling efficiency, so it is widely used. Piston type liquid filling system is a typical liquid filling machine in practical application. In the face of the market on the liquid filling accuracy requirements of the status quo, through their own construction of the piston liquid filling test equipment, the filling accuracy of the relevant experimental research on the piston high-precision liquid filling machine R & D and debugging has - a certain reference significance.

Piston filling accuracy factors

At present, the general filling accuracy of automatic filling machine is ≤ earth 1%, according to the weighing filling machine filling principle and structural composition analysis, the source of error is mainly piston pump gas volume, filling parameters, system inherent characteristics. Due to the different speeds of the piston reciprocating movement in the process of liquid absorption and filling, resulting in liquid absorption and filling process in the piston pump pressure is different, so that liquid absorption and filling in the piston pump gas accounted for the volume of different, resulting in the amount of liquid absorption and filling volume is not equal, resulting in loading error. Filling parameters are mainly liquid and filling speed, the viscosity of different liquids, gas content and other attributes are not the same, resulting in different liquid flow and gas volume in the filling process, affecting the filling accuracy. Different filling speed program on the piston pump pressure and liquid flow state will have an impact, thus affecting the filling accuracy. Because of the system mechanical structure of the processing and assembly errors and control system control deviation, resulting in piston liquid filling system there is a systematic inherent error, easy to cause loading error.

Piston filling construction

In order to facilitate the experimental research, piston filling equipment using modular design ideas, based on the design and assembly principles, the work is divided into suction and filling module, liquid level tracking module, automatic control module, and detection module part, and suction and filling module contains metering module, suction and filling module drive module, flow distribution module and nozzle module 4 modules.

In the liquid absorption module, the metering module adopts piston metering pump, the drive module adopts servo motor driven ball screw, the flow distribution module adopts cone valve check valve; the liquid level tracking module is realized by the stepping motor driven ball screw; the automatic control module adopts the widely-used PLC as the control center; and the detection module adopts the method of automatic weighing. In order to improve the operability of the test equipment, the human-computer interaction interface is designed so that the filling parameters can be easily selected and called, and the filling results can be obtained intuitively.

The filling equipment is designed for each module. The filling process of the test equipment is as follows: servo motor rotation generates driving force, under the transmission of the ball screw, driving the piston rod up and down movement, the screw and piston rod connected through the slider in order to ensure the smoothness of the slider movement, the installation of linear guides on the left side of the slider; 2 in the service motor drive, the piston rod of the piston type metering pump up and down reciprocating movement, the metering pump is a cylindrical structure, so the distance of movement of the piston rod corresponding to the The volume of liquid suction and filling of the metering pump; check valve is selected as the flow distribution mechanism, in the liquid suction, the inlet check valve opens, the outlet check valve closes; in the liquid filling, the outlet check valve opens, the inlet check valve closes, thus completing the suction and filling process; the filling nozzle is connected to the distribution structure through the steel wire hose, and the filling containers are placed in the tray, and the liquid level tracking module adopts the stepping motor and the ball screw, and the ball screw turns the stepping motor into the filling container, and the level tracking module adopts the stepping motor and the ball screw to turn the stepping motor into the filling container, and the level tracking module adopts the stepping motor into the filling container. The level tracking module adopts stepping motor and ball screw, and the ball screw converts the rotation of the stepping motor into the up and down movement of the pallet to ensure the distance from the nozzle to the liquid surface of the container is constant, and to reduce the splashing and foaming in the filling process. In the human-machine interface to change the filling volume, filling speed and other filling parameters PLC can be based on the input parameters to the servo motor, stepping motor to send the appropriate control signals, to achieve different filling parameters of the test.

Piston Filling Accuracy Test Study

Test principle and test method

According to the requirements of high-precision filling, the accuracy index of the test equipment is set as ≤+0.3% of the filling error under the filling volume of 150~550ml. First of all, the initial inherent accuracy test is conducted on the completed test equipment to derive the total filling error under the influence of various factors; then the accuracy test is conducted on the amount of gas in the piston pump, the filling liquid and the filling speed in turn to derive the relationship between the influence of various factors on the filling accuracy, and analyze and obtain the method of improving the accuracy; finally, the accuracy compensation method is adopted to compensate for the system's stabilizing error to achieve the accuracy of the test equipment set. Precision.

Test conditions

Test equipment for piston liquid filling test equipment; filling materials for water (viscosity value 1.14mPa-s), saturated sugar solution (viscosity value 1.54mPa-s) Nutrient Express (viscosity value 2.25mPa-s); test filling object for the 600ml ALKAOUA high-quality drinking natural water bottles; the test temperature of the room temperature (18C); the filling volume change range of 150 to 550ml; filling speed is 50~500ml/s.

Piston filling accuracy test and result analysis

1 Inherent accuracy test

As the first inherent accuracy test after the completion of the construction of the test equipment, the filling volume can be set to a certain value to determine the inherent accuracy of the test equipment. By investigating the capacity of common mineral water bottles and beverage bottles in the market, the fixed value is set to 550 ml, and the speed is set to 275 ml/s at a constant speed, and 50 inherent accuracy tests are done. Processing test data, the average filling accuracy error of 15%, compared with the set target ≤ earth 0.3% has a big gap.

2 living Zhai pump gas on the accuracy of the impact of the test

Considering the piston pump gas volume is large, reducing the filling accuracy of the problem, according to the piston downward movement from the bottom of the cylinder minimum distance, the calculation of the inherent accuracy of the test in the compression state of the gas volume of 57 ml. Now by reducing the lower limit position of the piston to reduce the amount of gas when the filling of the liquid to reduce the volume of gas to do 3 groups of tests, each time to reduce 17 ml. In order to make a comparison with the intrinsic accuracy of the test, the volume of liquid filling is set at 550 ml. For comparison with the intrinsic accuracy test, the filling volume was set at 550 ml, the speed was set at 275 ml/s hook speed, and each group of tests was done 20 times. The average filling accuracy of the three groups was obtained by processing the test data.

It can be seen that the average filling accuracy error of the first group of tests is 2.3%, the second group is 2.0%, the third group is 1.9%. By the filling accuracy of the trend of improvement can be seen, after the 2nd exhaust filling accuracy has been relatively stable, it can be seen that the less gas in the piston pump, the higher the accuracy. Therefore, for the living cottage type liquid filling system to strictly control the amount of gas in the piston pump to ensure filling accuracy.

3 filling liquid on the precision impact test

Taking into account the different viscosity of the liquid in the filling process by the liquid flow state is not the same, low-viscosity liquid and high viscosity liquid compared to better mobility, the flow state is biased in favor of the pro-state. And when the suction filling process pressure changes in different liquids with different air solubility, resulting in different amounts of foaming. Therefore, the common beverage water, saturated sucrose solution, and Nutrifresh were selected as the test liquids to do three groups of tests, in which the viscosity of saturated sucrose solution and Nutrifresh was higher than that of water, and the air solubility of Nutrifresh was larger than that of water and saturated sucrose solution. Set the volume of infusion as 550 ml, the speed of 275 ml / s speed, each group of tests to do 20 times. The test data were processed to obtain the average filling accuracy of the three groups.

The precision of filling water was lower than that of filling saturated sucrose solution, while the precision of filling nutrient fastener was lower than that of filling water. The reason for this is that saturated sucrose solution has a higher viscosity than water, which makes the flow pattern more stable during the filling process and produces fewer phenomena such as eddy currents, resulting in higher filling accuracy. Although the viscosity of Nutrifresh is higher than that of water, due to the fact that more gas is dissolved in Nutrifresh, the reduction of pressure in the pump when suctioning leads to the precipitation of more gas, which reduces the amount of suction and lowers the precision. Viscosity and gas solubility are the basic attributes of liquid, the viscosity value and gas solubility of different liquids are not equal, so the filling of different materials corresponds to different error values, and thus different liquids can be given a corresponding amount of compensation in the filling volume to compensate for the error and improve the filling accuracy.

4 Accuracy compensation test

Filling error caused by different filling liquid is mainly determined by the properties of the liquid, and for the entire piston filling system, the inherent error is determined by the system's unchangeable mechanical structure and control system, and therefore can be used as a fixed value, using the method of precision compensation to compensate for the error effectively. For the input filling volume of 550ml, filling speed of 275m/s speed, filling liquid for water, saturated sucrose solution, nutrient fast food test data for compensation, so that the actual average filling volume is equal to the input filling volume of 550ml, that is, the filling error is 0. For the filling liquid for the compensation of water, in order to change the program, the human-computer interface of the input filling volume plus the accuracy of the compensation volume of 10ml after the filling volume as the actual filling volume. In order to change the program, the filling quantity entered in the man-machine interface plus the accuracy compensation quantity of 10 ml is taken as the actual filling quantity to be input into the algorithm to control the motor work. In order to determine whether the test equipment in the range of test conditions to achieve the expected accuracy, the filling volume is set to 100 ~ 550ml, every 50ml set a group of tests each group of tests to do 20 times. Processing test data, the average filling accuracy of each group.

In the filling range of 150~550ml, the filling accuracy is within 0.2%, reaching the set accuracy index +0.3%. For the saturated sucrose solution, nutritional fast food using the same method, respectively, the input filling volume plus precision compensation volume 9ml and precision compensation volume 12ml after the filling volume as the actual infusion filling volume, after the same test found that the filling accuracy have reached the expected goal.

According to the test results, it can be seen that the method of precision compensation can effectively compensate for the filling error caused by the inherent error of the system and the liquid material, and improve the filling accuracy. Therefore, for different piston filling systems and different filling liquids, the corresponding accuracy compensation amount is obtained through the test, and the accuracy compensation method can effectively improve the filling accuracy by compensating the error.

5 filling speed program on the filling accuracy test

In the same filling time requirements, different filling speed program will also have an impact on the filling accuracy. To determine the filling time of 2s, filling volume of 550ml, designed three kinds of filling speed program. Program 1 is the first high-speed filling and then low-speed filling Program 2 is the hook speed filling, Program 3 is the first low-speed filling and then high-speed filling, each program to do 20 tests. Processing test data, the average filling accuracy of each program.

It can be seen that the accuracy of program 1 is the highest, and the accuracy of program 3 is the lowest. Combined with the test phenomenon, analyze the reasons, it can be concluded that when the filling liquid surface is close to the mouth of the bottle and the speed is high, it will lead to liquid splashing to the outside of the bottle, reducing the filling accuracy. The use of high-speed filling followed by low-speed filling speed program can ensure the filling efficiency and avoid the liquid surface close to the mouth of the bottle when the liquid splash, to ensure the filling accuracy.

Conclusion

Through the above experimental study on the accuracy of the piston liquid filling system, the following conclusions can be drawn:

(1) For piston liquid filling system, the amount of gas in the piston pump has a big impact on the filling accuracy. In the development and commissioning of high-precision piston filling machine, the amount of gas in the pump should be strictly controlled to improve the filling accuracy.

(2) Different piston liquid filling system inherent error is different, different filling materials corresponding to the error value is also different, but can be used as a fixed value, through the test to derive the corresponding amount of precision compensation, the use of precision compensation to reduce the error and improve the filling accuracy.

(3) For the piston liquid filling system, different filling speed program will affect the filling accuracy, and the first high-speed filling and then low-speed filling of the two-stage filling speed program can not only ensure the filling efficiency and avoid liquid splashing, to ensure the filling accuracy.