Application of weighing technology in feed production
This article briefly describes the latest developments in weighing technology and several typical applications of modern weighing technology in feed production.
With the increasingly fierce market competition, feed manufacturers pay more and more attention to product quality. In order to adapt to changing market demands, the feed processing industry is characterized by small batches and multiple varieties. Therefore, the weighing technology represented by precise feeding, production process measurement, ingredients and quantitative packaging will be increasingly used in feed production.
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. Basic parameters of analog load cells:
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. Digital sensors have resolutions up to one millionth .
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. The analog weight transmitter converts the weight signal of the load cell into a unified standard signal, such as: 0/4 ～ 20mA, 0 ～ 5V (DC), 0 ～ 10V (DC).
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 Intermediate measurement during precise feeding and production
Accurate feeding refers to the use of high-precision weighing equipment to precisely control the feeding process in production, so that the single batch output of feed products meets the needs of the production plan, and avoids feeding errors caused by human factors.
Intermediate measurement in the production process is to precisely control the process of feed production to improve product quality and production efficiency, making the adjustment of the process of production more targeted.
2.1 Structure and working principle of accurate feeding scale
Accurate feeding scales can be divided into coarse, fine (large, small) feeding devices, weighing components and unloading components. Its structure is shown in Figure 1. When the system is working, the coarse and fine feeding devices add material to the weighing hopper (weighing component). After the stable weighing, the real weight of each scale is obtained and accumulated. After it has been emptied, the computer weighing controller controls the material in the weighing hopper to be discharged into the lower hopper to complete a weighing cycle.
Accurate feeding scale adopts special weighing control software, and feeds according to the preset target weight. When the accumulated feeding weight reaches the preset weight value, the system automatically stops working and prompts the operator.
2.2 Structure and working principle of intermediate measuring scale (flow scale)
The structure of the intermediate flow scale is basically the same as the structure of the precise feeding scale. Since the feeding accuracy is not considered, the intermediate flow scale only needs one level of feeding to meet the requirements of use. The structure of the intermediate flow scale is also a feeding device and weighing component. And cutting parts. Its structure is similar to that of precision feeding scale.
When the system is working, the feeding device adds material to the weighing hopper (weighing component). After the stable weighing, the true weight of each scale is obtained and accumulated. When the material level device on the feeding (hopper) component detects that the material in the hopper has been emptied After that, the computer weighing controller controls the material in the weighing hopper to be discharged into the lower hopper to complete a weighing cycle.
Both precision feeding scales and intermediate flow scales can transmit the accumulated weight and other information to the higher-level computer system through the RS485 interface of the international standard in order to centrally control the production process.
3 Realization method of weighing ingredients
3.1 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. 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 Figure 2, the discharge device of the reduction scale is connected to the weighing unit, eliminating the need for a measuring bucket, and the structure is relatively simple. 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.
Application of weighing technology in feed production
3.2 Incremental method to achieve ingredients
The basic structure of the additive batching system is shown in Figure 3. 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 feed production
3.3 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, which can complete automatic feeding and batching of various materials with different formulas.
3.3.1 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.
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.
3.3.2 Control system composed of digital transmitter, PLC and touch screen
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.
PLCOn 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.
3.3.3 Control system composed of digital transmitter, PLC and industrial 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 4. The configuration of large and medium-sized plastic automation control systems is more common with the configuration of "industrial computer + PLC". 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.
Application of weighing technology in feed production
4 Quantitative packaging of feed and additives
Due to differences in physical properties such as the specific gravity and fluidity of feed, additives and premixes, the corresponding packaging equipment structure used will also be different.
The computer quantitative scale can adopt the international recommendations of the International Organization for Legal Metrology (OIML R61) or the equivalent national metrological identification code JJG564-2002 "Gravity Automatic Loading Weighing Apparatus" standard, and its accuracy grade is expressed by X (x), for example: 0.2 And 0.5 grades are expressed as X (0.2) and X (0.5) , respectively.
The well-designed weighing control software has a good user interface, and can optimize system working parameters such as large feed points and small feed points. The optimized system parameters can be stored for ten years under power failure.
Computerized quantitative scales can transmit information such as the weight of each package to the upper computer system through the RS485 interface in order to calculate the accumulated weight and accumulated measurement errors, etc., and generate daily reports, weekly reports, monthly reports, etc., which is very convenient to manage.
4.1 Quantitative packaging of granular materials
The fluidity of granular materials (such as aquatic feed) is very good. Therefore, the computer quantitative scale basically uses the characteristics of good fluidity of the material, using the valve self-falling feeding method. For the occasions requiring high quantitative accuracy or small weighing range, the structure of vibration feeding can also be used.
4.1.1 Bulk quantitative packaging of granular materials
20-50Kg computer quantitative scale adopts three load cells to suspend the weighing bucket, and the cylinder drives the large and small valve to feed. It has the characteristics of good stability of the weighing bucket, fast quantitative speed and high precision.
4.1.1 Small bag quantitative packaging of granular materials
1.0-5.0Kg small particle computerized quantitative scale adopts dual-scale structure with two independent weighing units. Use a single cantilever load cell to connect the weighing bucket. The cylinder driven valve constitutes a large feeder and a vibrating feeder with adjustable amplitude. A small feeder constitutes a feeding system of a computer quantitative scale. Such a feeding system fully utilizes the characteristics of fast valve feeding and accurate vibration feeding. , The computer quantitative scale has fast packing speed and high precision.
4.2 Quantitative packaging of granules and powdery mixtures
Granular and powdery mixtures (feed for livestock and poultry) are mostly fed by belt conveyors, that is, small belt conveyors are used to feed materials. Frequency conversion technology can be used to change the belt conveyor belt speed, adjust the feeding speed, or A two-pole feeding mode is formed by using the insert plate and the cut-off gate control to control the material thickness.
The use of a series of technical measures such as cutting gates, material thickness adjustment baffles on belts, and frequency conversion motors ensures high-speed, high-precision, and stable work of the feeding computer quantitative scale.
4.3 Quantitative packaging of powdery materials
Powdered materials use spiral