Current Situation And Development Trend of Belt Balance Technology

1、 Overview

 As an automatic weighing instrument for continuous cumulative weighing, belt scale is used in belt conveyor system for conveying bulk materials. According to statistics, in 2011, China's annual output of coal was 3.5 billion tons, and imported coal was 200 million tons; The annual output of ore is 1.3 billion tons, and the imported ore is 690 million tons. Only the above two kinds of materials in the production and use of two links need to be measured by the belt scale, it can be seen that the amount of use is large. With the implementation of the national energy saving strategy, the measurement and monitoring management of coal as the main energy has been paid more attention. At the same time, with the rise of coal and ore prices, its measurement accuracy and reliability are becoming more and more important.

 Since the development of belt weigher, it has gone through the period of mechanical belt weigher and electronic belt weigher, and now it has entered the period of intelligent belt weigher. The first generation of belt weigher is pure mechanical, through the lever force transmission, displayed as a mechanical counter; With the development of electronic industry, the emergence and application of strain type weighing sensors and microprocessors, the belt scale has entered the second generation of electronic belt scale period; The new generation of intelligent belt scale is based on the electronic belt scale, which greatly improves the accuracy and reliability of the measurement system through digital, network and intelligent technology, and enhances the durability of the belt scale through the interaction of measurement information, real-time online monitoring, online calibration, weighing and calibration integration technology. At present, the national standards of belt weigher are mainly based on the international recommendations of OIML and R50 (1997 edition). The Secretariat of automatic weighing instrument of international organization of legal metrology has begun to study and discuss the revision of the international recommendations of belt weigher. The 2011 4CD version of the international proposal has been consulted, and the development of belt scale technology has promoted the development of the international proposal.

 2、 Belt scale device

 The belt scale device is mainly composed of three parts: weighing carrier (weighing scale frame), displacement sensor and weighing instrument. The following is described according to three main parts.

 （1） Belt scale weighing carrier (weighing frame)

 The weighing carrier (weighing frame) is mainly composed of a weighing sensor and a steel beam of a supporting roller. The supporting roller transmits the material weight on the belt conveyor to the weighing sensor through the steel beam, and the weighing sensor sends the material weight signal to the weighing instrument.

 Belt scale is divided into scale frame belt scale and conveyor belt scale according to weighing carrier. The former scale frame is a part of conveyor, while the latter scale frame includes a complete conveyor (also known as weighing feeder). This paper focuses on the former scale frame belt scale.

 According to OIML R50 "continuous accumulation automatic weighing instrument (electronic belt scale)" 4CD edition in September 2011, weighing frame is divided into three categories: full suspension frame, modular frame and lever frame.

 Full suspension scale frame: four load cells are mainly used, two longitudinal beams are supported by four points, and three to eight groups of idlers are installed on the longitudinal beams. The more the number of idlers, the higher the measurement accuracy.

 Modular scale frame (also known as array scale frame in China): it mainly uses one or two load cells together with the steel frame to support one or two groups of idlers to form a weighing unit. According to the accuracy requirements, the weighing unit can be composed of one group to 16 groups of idler scales.

 The main advantage of modular scale frame is that it is easy to combine on site. According to its accuracy, the number of idler is different. The error of single idler scale frame is between 0.5% ~ 2%, the error of double idler scale frame is between 0.5% ~ 1%, the error of three idler scale frame is between 0.5% ~ 0.25%, and the error of four idler scale frame is between 0.5% ~ 2% ± More than 0.25%. For the 0.2 belt scale, the number of heavy rollers is more than 6 or 8 groups, and the number of individual scale frame rollers reaches 16 groups, forming an array, also known as array belt scale.

 Lever type scale frame: mainly uses one side fulcrum, the other side is supported by load cell, this kind of scale frame is less and less used with the reduction of load cell price.

 （2） Displacement transducer

 The displacement sensor detects the displacement information of the corresponding belt length, and sends the material length information on the scale frame to the weighing instrument. The displacement sensors mainly include magnetoresistance pulse type and photoelectric pulse type. The displacement sensor has not changed much in recent years.

 （3） Weighing instrument

 The weighing instrument is to process the weight signal of the weighing sensor and the length signal of the displacement sensor on the weighing frame to get the accumulated weight of the transported materials. Today, the development of electronic technology platform has opened up a broad space for the design of belt scale instrument. CPU technology has developed from 8 bits to 32 bits, the main frequency of the system has also increased from tens of megabytes to tens of megabytes or even hundreds of megabytes, and AD converter has also increased from 10 bits to 24 bits. SOC (system on chip), DSP (digital signal processing), CPLD (programmable logic device) and other new technologies are gradually applied to the belt scale instrument.

 Weighing instrument mainly includes special instrument and general instrument. General instrument is to purchase general hardware platform such as industrial control computer and programmable controller, through which the weighing and speed signals of belt scale are collected. The special instrument is mainly based on the special weighing instrument developed by microprocessor.

 Software is the core technology of the instrument. Due to the development of the instrument hardware platform, the real-time embedded operating system has been more and more applied to the belt scale instrument, which completely changes the past simple software system framework and greatly improves the function and reliability of the instrument system. The application of digital filtering technology eliminates the interference caused by the impact movement of the measured materials of the belt scale, and ensures the weighing accuracy of the belt scale.

 Seymour Electric Co., Ltd. (hereinafter referred to as "Seymour company") innovates and invents the "three groups of cumulative weighing technology" to complete different combinations of measurement methods through instrument software, so that the belt scale can still measure when a certain load cell of the belt scale fails or is abnormal, which improves the reliability of the belt scale measurement.

 3、 New technology of belt scale calibration

 The weighing principle based on the belt scale is that the material is on the belt, and the weight of the material is transferred to the scale frame through the belt to get the weight of the material. Due to the tension of the belt, the hardness of the belt and other factors, the weight of the material on the belt can not be truly transferred to the scale frame.

 For the belt scale installed on the belt conveyor, the length, width, speed, conveying capacity, installation position, vibration and operation conditions of each belt conveyor are different, which leads to different factors such as the tension of each belt, the hardness of the belt and so on. These factors can not be calculated, and can only be compensated by material calibration, that is to say, each belt scale installed on that belt conveyor must be personalized material calibration compensation, in order to get accurate measurement results.

 Limited by the use conditions of the belt conveyor, it is difficult to calibrate and compensate the belt scale with the material calibration method. The analog calibration methods such as hanging code, chain code and circular chain code are often used to calibrate and compensate the belt scale There are great differences in the compensation of the belt's soft and hard degree and other factors. Although the belt scale has been simulated and calibrated, the weighing result still has a big error with the actual weighing. In addition, the mechanical change of the belt conveyor in the process of operation (especially after a long time of operation) and the change of the initial calibration lead to the measurement error becoming larger, and the long-term stability of the belt scale is not good.

 In order to solve the problems of inconvenient and untimely material calibration, which lead to large measurement error and poor long-term stability of belt scale, Seymour company has developed material superposition self calibration technology.

 Material superposition self calibration technology adopts on-site automatic reclaiming. After weighing with less materials, the belt scale can be calibrated and compensated at any time during the normal conveying process of belt conveyor (the whole belt is covered with materials). The calibration accuracy is high, which solves the problem that the on-site physical calibration is not easy and can not be calibrated in time.

 The principle of material superposition self calibration technology. Seymour's three bridge material superposition self calibration technology is to install two groups of weighing scales on the belt conveyor, buffer idler group is installed between the two groups of weighing scales, and weighing hopper is installed above them.

 The scraper reclaimer installed on the belt conveyor, in the process of conveying materials by the belt conveyor, reclaims materials regularly according to the setting of the weighing instrument, and the materials are sent to the weighing hopper by the screw conveyor.

 When calibration is needed, the weighing instrument controls the weighing hopper to open the door for discharging. At this time, the first group of belt scales weighs the weight of the transported materials, and the second group of belt scales weighs the weight of the transported materials and the weight of the standard materials put down by the weighing hopper. Based on the same amount of transported materials (excluding the materials of the weighing hopper) passed by the first group of belt scales and the second group of belt scales, The weighing instrument subtracts the weight obtained by the second group of belt scales from the weight obtained by the first group of belt scales to obtain the standard material weight value discharged by the weighing hopper. The standard material weight value is compared with the material weight value displayed by the weighing instrument itself to obtain the correction coefficient. According to the correction coefficient, the measuring range of the weighing instrument is modified to display the standard material weight value.

 According to the needs of timing comparison, correction, so that the belt scale measurement accuracy to achieve high stability and accuracy. At this time, the whole length of the belt conveyor is covered with materials, and the calibration condition is close to the normal conveying condition. The accuracy of the calibration belt scale is the most real, so as to realize the low-cost calibration with less materials instead of multi material calibration.

 The main features of the belt weigher are as follows:

 (1) The self calibration technology of self-developed small amount of weighed materials and normal conveying materials is adopted to solve the problems of high cost and difficult calibration of current belt scale.

 (2) The real-time online material self calibration technology can automatically carry out the material self calibration when the belt conveyor is running normally, which effectively ensures the accuracy and durability of the long-term operation of the belt scale.

 (3) The two belt scales are compared in real time and are standby for each other. When one of the belt scales fails, the system will give an alarm automatically, and use the other belt scale as the main measuring scale to continue to run, which improves the reliability of the system.

 4、 Features of new OIML R50 international recommendation (4CD - September 2011)

 （1） Accuracy level of products

 As for the classification of accuracy level, the new version of international recommendations is divided into four levels, namely 0.2, 0.5, 1.0 and 2.0, while there are only three levels in the current national standards and regulations, which is 0.2 less than the new version of international recommendations.

 In the new version of the international recommendation, the maximum allowable error of the first verification and subsequent verification of grade 0.2 is 0.1%, the maximum allowable error of the in-service inspection is 0.2%, the maximum allowable error of the influence factor test is 0.07%, and the cumulative graduation value of the minimum cumulative load is 2000.

 （2） Belt correction device

 The new version of the international proposal adds a belt shape correction device, which can correct the load changing on the carrier during the belt (no-load) running for one circle. The device uses the stored data of belt (no-load) running for one circle to correct. The device is used to correct the zero point of belt weigher. It can be combined with automatic or semi-automatic zero adjustment device, and can also operate independently of automatic or semi-automatic zero adjustment device.

 （3） Interference test

 The new version of international recommendations is more severe than the current national standards and regulations in the anti-interference test. The AC power supply voltage sag and short-term interruption immunity test, surge (impact) immunity test and RF field induced conduction disturbance immunity test are added. The requirements for the original electrical fast transient burst immunity test and RF electromagnetic field radiation immunity test are also improved.

 （4） Durability

 The new international proposal does not propose specific methods for durability test of belt weigher. It encourages countries to take some measures to ensure durability under the constraints of national laws and regulations. These measures include type evaluation, first verification, follow-up verification, in-service inspection and other tests. Because there are many factors affecting the durability of belt weigher, there is no consensus test method in the world.

 5、 Overview

 With the rapid development of sensor technology and electronic technology, and the reduction of component cost, the belt scale has benefited a lot. The use of advanced electronic technology improves the reliability and stability of the belt scale, expands the use function, and further meets the needs of users. Nowadays, the belt scale mainly faces three technical difficulties: accuracy, durability and calibration method. The development trend of the new generation of intelligent belt scale is mainly to solve these three problems. The application effect of belt weigher used in port, mining, electric power and other industries is not ideal, which is mainly related to the application conditions of belt weigher

 （1） Large flow

 Generally, the coal transportation flow reaches 3000t / H ~ 6000t / h, the ore transportation flow is larger, even more than 10000t / h, and the flow changes greatly, uneven.

 （2） Fast

 The speed of belt conveyor can reach 3m / S ~ 6m / s.

（3） The belt is wide and long

 The width of the conveyor belt is 1400m ~ 2200m, and the perimeter of the belt is more than 2000m, some even up to 10000m. It is because of the above three characteristics of these belt conveyors and the lack of ideal material calibration conditions that the accuracy of the belt scale installed on them is difficult to reach 0.5 and 1. Even if the accuracy is reached, the durability of the belt scale is also very poor, which can not be maintained for a long time, and some can only be maintained for a few days, Therefore, an international survey that the belt scale accuracy will not be better than ± 2% 。

 Due to the large amount of belt conveying, the belt scale on large belt conveyor can not be installed with physical calibration device, which makes the calibration of the belt scale difficult to trace the source, which is also the main reason why the belt scale is not used as trade settlement. At present, the international trade settlement of bulk materials is mainly based on the water gauge method, and the accuracy of the water gauge method depends on the service time of the ship and the accuracy of the original volume measurement. The fluctuation of water surface, water temperature, water quality density, ballast water, fuel and water consumption or supplement vary from 0.5% to 2.0%, The error of water gauge weight is as high as 5%.

China is a big country of resource trade. Inaccurate measurement will lead to huge economic losses. The market urgently needs a belt scale with high accuracy, strong durability and reliability, and traceable calibration. As a new generation of intelligent belt scale, belt scale based on material superposition self calibration technology (weighing and calibration integration) effectively solves the above three problems, It lays a solid foundation for the belt weigher to be widely used in large bulk material conveyor.

 With the continuous development and application of new technology of belt weigher, China is moving from a big production country of belt weigher to a powerful technology country of belt weigher. As long as continuous innovation is made, belt weigher will be able to replace water gauge and become the main measuring instrument for bulk trade settlement.

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