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Application of weighing technology in ingredients of plastic industry

Release time:
2013/08/28 10:50
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[Abstract]:
This article briefly explains the latest developments in weighing technology and several architectures that use modern weighing technology to form the batching system for the plastics and rubber industries.

Summary:

This article briefly explains the latest developments in weighing technology and several architectures that use modern weighing technology to form the batching system for the plastics and rubber industries.

Introduction

With the increasingly fierce market competition, production enterprises pay more and more attention to the quality of products. In the production industries such as plastics and rubber, whether the proportion of each raw material of the product is accurate has become a key factor affecting product quality. Therefore, weighing and batching technology are increasingly used in the plastics industry.

1 Recent developments in weighing technology

1.1 Analog load cell and A / D conversion technology

The resistance strain gage load cell (that is, an analog load cell) is based on the principle that an elastic body deforms elastically under the action of an external force, and the resistance strain gage (conversion element) attached to its surface also deforms along with it. After the resistance strain gauge is deformed, its resistance value will increase or decrease, and then this resistance change will be converted into an electrical signal by the corresponding measurement circuit, thereby completing the process of converting external force into an electrical signal. The internal circuit diagram of the resistance strain gauge load cell is shown in Figure 1-1.

Application of weighing technology in ingredients of plastic industry

Basic parameters of analog load cell:

Output sensitivity: Under the condition of full load (load), each time the DC excitation voltage of the load cell increases by one volt, the output increase is defined as the output sensitivity of the load cell. The unit of output sensitivity of the load cell is millivolt. / Volt.

Comprehensive error: The error of the load cell is composed of non-linearity, hysteresis, creep, temperature drift, etc. The comprehensive error of the load cell is generally 0.03% (FS), which is about three ten thousandths of the full scale of the sensor.

Impedance: The input impedance of the load cell is generally around 450Ω, and the output impedance is generally around 350Ω.

With the rapid development of microelectronics technology, the A / D conversion technology in weighing technology has undergone rapid changes, from double integration to successive approximation, from parallel port output to serial port output, and from pre-amplification of discrete components to height. Integrated digital amplification and advances in microelectronic technology have laid a solid foundation for the rapid development of weighing technology.

At present, high-precision A / D conversion devices using Sigma-Delta (Sigma-Delta) technology are widely used in the field of weighing control. This device combines digital programmable gain amplifier (PGA), gain and zero correction circuits, digital filter circuits and A The / D conversion circuit is packaged in a high-performance integrated circuit. Due to the serial I2C bus output, the size of the device becomes very small. A / D conversion can be up to 24 bits, while its sampling speed can also reach 4000 times / second, A / D data output speed can reach 100 times / second or higher, and linear error can reach 0.0007% FS.

1.2 Integrated Digital Sensor

Integrated digital sensor adds digital conversion and even digital correction, digital filtering and digital compensation functions on the basis of the original analog sensor. Because the digital signal output replaces the original analog signal output, it overcomes many shortcomings such as the analog signal of the traditional sensor is easily affected by interference and attenuation, and greatly improves the reliability of the weighing system. The resolution of the digital transmission circuit in the digital sensor can achieve 1 million internal codes, the sampling rate can be 100 times / second, and the temperature drift can be 10PPM / ℃.

The digital load cell is easy to use and easy to constitute a weighing control system. But the high cost of digital sensors has become its biggest drawback.

1.3 Split Digital Sensor

In recent years, split-type digital sensors have also developed rapidly. The split digital sensor is to move the A / D conversion circuit originally inside the digital sensor into the junction box. Generally, the junction box with the A / D conversion module is called the digital junction box, and the digital signals output by the digital junction box are passed on. For the display controller, compared with the integrated digital load cell, the split digital sensor has the obvious advantages of low cost, easy to construct a weighing unit with multiple sensors, and easy maintenance.

1.4 Weight Transmitter

Weight transmitters are commonly used in analog and digital. Analog weight transmitter converts the weight signal of the load cell into a unified standard signal, such as: 0/4 ~ 20mA, 0 ~ 5VDC, 0 ~ 10VDC.

The digital weight transmitter adds the functions of calibration and tare processing on the basis of the split digital sensor. The output of the digital weight transmitter can be the net weight of the weighing system, and the digital weight transmitter also has a certain remote control function.

2 Realization method of weighing ingredients

2.1 Weight reduction (quantity) method to achieve ingredients

Weight loss (or Schenck) scales were named after they were first invented by German engineers. The structure of the weight reduction batching system is shown in Figure 2-1. Because the weight-reduction batching scale is "an automatic charging scale that determines the quality of the charging by controlling the weight output in the weighing hopper". Relevant standards should be implemented in accordance with QB / T2501-2000 "gravity-type automatic charging instrument" industry standards and national appraisal regulations JJG564-2002 "gravity-type automatic charging instrument" or the international legal metrology organization OIML R61.

As can be seen from the figure, the discharge device of the weight reduction scale is connected to the weighing unit, which eliminates the measuring bucket and has a relatively simple structure. Particularly suitable for powdery and special viscous ingredients.

Due to the structural characteristics of the weight reduction scale, the range of the weighing unit is much larger (usually five times or higher) relative to each loading of the weighing unit, so the quantitative accuracy of the weight reduction scale is relatively low.

2.2 Incremental method to achieve ingredients

The basic structure of the additive batching system is shown in Figure 2-2. Multiple feeding devices are set to feed the same weighing unit, and multiple weighing cycles constitute a batching process.

As it meets the definition of “gravity automatic charging scale with one weighing unit that controls the loading quality of each output through multiple weighing cycles”, the cumulative batching system belongs to the category of gravity automatic charging scales The relevant standards should be implemented in accordance with QB / T2501-2000 "Gravity Automatic Loading Weighing Instrument" industry standard and national appraisal regulations JJG564-2002 "Gravity Automatic Loading Weighing Instrument" or the international legal metrology organization OIML R61.

In order to ensure the accuracy of the system's ingredients, the proportion of the ingredients in the system is generally limited. The higher the weight ratio between the ingredients, the lower the accuracy of the ingredients in the corresponding proportion.

Application of weighing technology in ingredients of plastic industry

Application of weighing technology in ingredients of plastic industry

3 Control method of batching system and human-computer interaction technology

Weighing and batching control system is mainly composed of storage bin, feeding device, weighing device, conveying device, electric control cabinet, and microcomputer control system. It can complete automatic feeding and batching of various materials with different formulas. It is widely used in chemical industry, Weighing control of a variety of materials in building materials, medicine, food, feed processing and other industries. System features: storage, quantitative feeding, weighing, sealing and conveying in one, microcomputer control of the whole process, high degree of automation; each single scale has advanced technology, excellent performance, compact system structure; a variety of users in a multi-material multi-formulation environment The input and output of data can be completed through the operation of terminal equipment; the setting data is conveniently output or displayed on the terminal screen. It is a centralized monitoring system for the purpose of achieving automatic control. It uses PLC, man-machine, and configuration software to implement automatic control. On-site monitoring, fault alarm, data acquisition, recording, statistics, query, printing and integrated automated processes. The system's long-term work performance is stable and reliable, eliminating artificial batching errors, improving batching efficiency, and improving operating environment and labor conditions.

According to the batching method, it can be divided into continuous and non-continuous (or batch-type, batch-type) batching. From the perspective of increasing the production capacity of powder / powder production enterprises and reducing investment in fixed assets, a continuous batching production line is an ideal process choice. However, due to the continuous measurement of various granular materials (and powders), the automatic batching device (electronic batching scale) for weighing accuracy, range, stability, etc., or some aspect cannot meet most production enterprises, especially weighing The accuracy is required for the specific ingredient production field, so the application is limited. Many continuous measuring instruments can only be applied to the device of powder flow measurement and control system. Due to its low accuracy of continuous weighing, it is generally used for internal measurement and material cost calculation in enterprises, and some are also used for real-time material flow measurement and control in the production process of production enterprises.

Due to the rapid development and popularization of electronics and automation, discontinuous accumulation scales (commonly known as cumulative hopper scales) for ingredient weighing have been applied in almost all the bulk / powder industry sectors that require ingredients. It has the characteristics of high weighing accuracy, wide adaptability, fast batching speed, large variable range of batching weighing range, and high production efficiency. Therefore, this article describes the object is a discontinuous automatic batching scale.

At present, the most common batching process has several forms such as multi-bin number scale, multi-bin one scale and one bin one scale. The more commonly used batching technology is the process form of multi-bin double scale and multi-bin triple scale. The advantage is that large scales use large scales and small scales use small scales, so the accuracy of the batching can be improved; meanwhile, the variety and quantity of raw materials that can be directly batched are also increased. In addition, the large and small scales are batched at the same time, which can shorten the batching cycle.

3.1 Control system composed of analog sensors and dedicated batching controller

In addition to the functions of general weighing display instruments, the batching controller also has functions such as recipe input, recipe storage, feeding sequence setting, discharging control, automatic compensation, and so on.

示意图 The schematic diagram of the batching control system composed of analog sensors and special batching controller is shown in Figure 3-1. As long as the process formula is input on the special batching controller during work, the batching controller will control the feeding sequence, feeding method, weighing, and discharging of each bin. This system is suitable for applications where there are few types of materials (generally less than 4 types), the number of commonly used formulas is not large, and there is no need to manage the formula data. Its human-computer interaction is poor, but its simple structure, low system cost and easy maintenance.

3.2 Control system composed of digital transmitter, PLC and touch screen

The schematic diagram of the ingredient control system composed of digital transmitter, PLC and touch screen is shown in Figure 3-2. Since the use of PLC control in the batching control system, the performance indicators of the system have been significantly improved, which fully reflects the reliable, fast and flexible control characteristics, which meets the more complex process requirements and achieves the previously difficult to do. A variety of complex control and fault protection have made the system with the characteristics of miniaturization, simple operation and maintenance and intelligent control. In the PLC hardware design, only the input and output interfaces that are directly oriented to people and equipment are retained, eliminating all intermediate links and greatly saving system investment. In the software design of the PLC, the flexible programming method is adopted, which makes the program more concise, readable, and easy to modify, and also makes the system have greater flexibility and adaptability in control functions.

The touch screen is highly reliable for monitoring and easy to implement. The communication problem between the touch screen and the PLC has been handled by the manufacturer. The user does not need to consider the communication problem, which can greatly shorten the engineering cycle. But the general flexibility, limited functions, can not meet the monitoring requirements of complex control systems, and the price is high. This method can be used in the case of high system reliability requirements and short construction periods.

Application of weighing technology in ingredients of plastic industry

3.3 Control system composed of digital transmitter, PLC and industrial control computer (configuration software)

The control system composed of PLC generally has three methods for monitoring: touch screen monitoring, configuration software monitoring, and monitoring software monitoring compiled by third-party software.

Using monitoring software compiled by third-party software to achieve monitoring, good flexibility, low system investment, and can be applied to various systems. However, the development system has a large workload and is difficult to guarantee reliability. It requires high technical personnel's experience and technical level, and must also purchase communication protocol software. This method can be used when the system capital investment is limited and the level of technical staff is high.

Using configuration software to achieve monitoring, powerful functions, good flexibility and high reliability. However, the software price is high, and the communication between the configuration software and the PLC needs to be solved. This method can be used in complex control systems. The configuration software is a general-purpose tool software running on the PC platform. It can also form an HMI product together with a PC or an industrial computer. The general configuration software supports more types of equipment, and because of the powerful performance of its hardware platform (in terms of speed and storage capacity), the function of the general configuration software is also much stronger. The schematic diagram of the batching control system composed of digital transmitter, PLC and industrial control computer (configuration software) is shown in Figure 3-3. The configuration of large and medium-sized plastic automation control systems is more common with the configuration of "industrial computer + PLC".

Application of weighing technology in ingredients of plastic industry

PLC is the field controller of the plastic ingredient measurement and control system, and the industrial computer is used as the upper computer. It can communicate with the PLC through a variety of communication methods, receive data information transmitted by the PLC, and use configuration software to conveniently carry out real-time and historical data. Storage, query, statistics, calculation, display, printing and other functions, complete the historical data and real-time data such as raw material types, quantities, prices.

4 Common structure of batching system

4.1 Weighing bucket type weight reduction batching system

As shown in Figure 2-1, there are multiple weighing units in the scale-type weight reduction batching system, and each weighing unit corresponds to only one kind of material and one load (within a certain period). The maximum weighing capacity, minimum weighing capacity, technical parameters and structure of the weighing bin may be the same or may be different. During operation, each weighing unit weighs the load in its carrier at the same time. The material weighed by each weighing unit may be the same or different. The load of each weighing unit is combined and adjusted by an industrial control computer, and then it is discharged to form a charge close to a preset value.

4.2 Scale-type incremental batching system

As shown in Figure 2-2, the scale bucket type batching system has only one weighing unit. The industrial control computer weighs the loads of different materials at different time periods to form a single charge.

4.3 Multi-station batching system

The multi-station batching system is usually divided into single weighing unit and multiple weighing unit, as shown in Figure 4-1 (a). The system has a weighing unit, which is mounted on a vehicle that can run straight along the track, and storage bins for various materials are equidistantly constructed on both sides of the track. The industrial control computer symmetrical weight unit picks up materials with different loads at different locations according to the process requirements, combines them, and then unloads them to the designated location to form a single loading. Electric vehicles equipped with weighing hoppers use infrared, logic coding and other technologies for positioning to achieve the purpose of accurately receiving and unloading materials. Figure 4-1 (b) is also a multi-station batching system commonly used in the plastics industry. This system has multiple weighing units, multiple weighing unit carriers (weighing conveyors), and other technical parameters. And structure is different. The weighing unit is installed on a belt conveyor line that conveys objects horizontally, and the storage bins for various materials are set above the center of the conveyor line. Calculations by industrial control.

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