Expertise & Technical documentation

Stability Analysis of Electronic Belt Scale

Electronic belt scale is an important equipment for continuous weighing of materials in the process of conveying solid bulk materials by belt conveyor. There are many problems involving sensors in the process of use. After we have a thorough understanding of the working principle, signal category and relationship with display instruments of various sensors, it is possible to apply flexibly according to the site conditions to meet various requirements of production site. In order to achieve the accuracy of weighing and batching, there are high requirements for the accuracy and stability of electronic belt scale. This paper introduces the working principle and function of weighing sensor and speed sensor from the process of electronic belt scale, and puts forward the treatment methods of common problems. So as to ensure the stability of the electronic belt scale.
1 working principle of electronic belt scale
The principle of quantitative feeding of electronic belt scale is that when the material passes through the belt scale platform, the weighing sensor converts the weight signal into electrical signal and transmits it to the weighing display controller. The weighing display controller continuously detects the flow of materials on the scale frame and compares it with the set value in the instrument to obtain the deviation value between the actual flow and the set value. After a series of complex calculations by the software in the instrument, the weighing display controller outputs the PID current signal to the speed regulation execution equipment (such as frequency converter). Adjust the speed of the belt or feeding equipment and change the blanking or discharging amount to make it consistent with the set value, so as to achieve the purpose of quantitative feeding.
The information transmission process of measuring material flow by electronic belt scale is as follows: the weight of moving materials on the belt is measured by the weighing sensor and outputs the corresponding electrical signal; The running speed of the belt is measured by the speed sensor and outputs the corresponding electrical signal; The weighing display control collects the above two kinds of information, calculates and outputs the corresponding electrical signal. The common operation methods are integral method and cumulative addition. The so-called integration method is simply the integration of the product of the instantaneous measured material mass P (T) per unit length of the belt and the measured belt speed value V (T) over the time length t. Cumulative addition means that when the belt conveyor is running, the material quality is measured every fixed length. According to the weighing principle, the data acquisition of the instantaneous flow of materials measured by the electronic belt scale mainly adopts two kinds of sensors: weighing sensor and speed sensor. The weighing sensor measures the load value p (T) / (kg / M) of the material on the carrier, and the speed sensor measures the belt speed value V (T) / (M / s).
2 weighing sensor
The materials on the belt (including the weight of the belt) are transmitted to the scale frame through the weighing idler, and then the force is transmitted to the weighing sensor. The structural forms of weighing sensors include resistance strain type, piezomagnetic type, differential transformer type, capacitive type, piezoelectric type, etc. Because the resistance strain type weighing sensor has the advantages of simple manufacture, mature process and high accuracy (the highest accuracy can achieve nonlinearity, repeatability and lag index better than 0.01%), the resistance strain type weighing sensor is adopted for the 70 ton electric furnace steelmaking electronic belt scale of Bagang. The strain element of the resistance strain sensor is actually a force measuring strain cylinder. The measured pressure is converted into a corresponding force through the diaphragm, and then transmitted to the strain cylinder. When the strain tube is compressed and deformed, the compressive resistance of the strain gauge pasted along the axial direction becomes smaller, and the tensile resistance of the strain gauge pasted along the circumferential direction increases. The output voltage value can be obtained by forming a strain bridge, so as to measure the pressure value.
3 speed sensor
At present, the speed measuring sensors used on the electronic belt scale mainly include magnetoresistive pulse type and photoelectric pulse type, while the 70 ton electric furnace steelmaking belt scale of Bagang adopts magnetoresistive pulse type speed measuring sensor. Magnetoresistive pulse speed sensor: in the magnetoresistive pulse speed sensor, the coil and magnet are stationary and connected with the measured part, while the moving part is made of magnetic conductive material. When the rotating part rotates, the magnetic resistance of the magnetic circuit is changed, so the magnetic flux through the coil is changed, and the induced potential is generated in the coil. In terms of structure, there are two kinds of magnetoresistive pulse velocity sensor: open magnetic circuit and closed magnetic circuit. In a П Two induction coils connected in series are installed on the type a permanent magnet. The roller directly rubs with the belt and drives the bisector gear to rotate. When the convex part of the bisector gear is opposite to the magnetic pole, the loop magnetic flux is the largest. When the concave part of the bisector gear is opposite to the magnetic pole, the loop magnetic flux is the smallest, and the induced voltage varying with the magnetic flux is induced on the induction coil. The frequency f of the induced voltage change is proportional to the belt speed v. The structure of the speed sensor is simple, but the output signal amplitude is small. When the belt is running, the roller rotates through friction and drives the rotor magnetic cup to rotate. The rotor magnetic cup and stator magnetic cup are installed relatively, and a plurality of tooth grooves are evenly milled on the circumferential end face. When the convex teeth of the two magnetic cups are opposite, the magnetic flux is the largest, and when the convex and concave teeth of the two magnetic cups are opposite, the magnetic flux is the smallest, so the induced voltage varying with the magnetic flux is induced in the coil. The structure of magnetoresistive velocity sensor is complex, but it has good sealing performance and large output signal amplitude. When the magnetoresistive pulse speed sensor is used for high speed measurement, the induced voltage in the coil is too small and difficult to measure due to the influence of magnetic hysteresis.
4. Measurement accuracy analysis of belt scale
4.1 analysis on installation and operation environment of belt scale and influencing factors of equipment
4.1.1 when the electronic belt scale is installed on the belt frame and operates, the vibration value exceeds the standard, resulting in the fluctuation of the weight signal of the material. In case of serious vibration, the sensor is loose and the wiring terminal is in poor contact, resulting in inaccurate measurement.
4.1.2 the belt scale frame is deformed by external force, which affects the accuracy of measurement.
4.1.3 the signals of the load cell and speed sensor of the electronic belt scale are subject to electromagnetic interference, which will cause inaccurate measurement.
4.1.4 the installation of electronic belt scale is unreasonable, and the inclination angle is too large, which does not meet the installation standard.
4.1.5 the speed sensor slips during operation, or the installation is too tight and the rotation is not smooth, which does not reflect the real belt running speed, resulting in inaccurate measurement and poor stability.
4.1.6 when the real object or chain code is used for belt scale calibration, the accuracy of standard instruments cannot be guaranteed, resulting in inaccurate calibration of belt scale.
4.1.7 during the operation of Bagang belt scale, due to rain and snow or wind and dust prevention and watering measures in summer, the belt will stick and freeze materials, which will also cause inaccurate measurement of the belt scale.
4.2 impact analysis of human factors
4.2.1 the belt length measurement of the belt conveyor is inaccurate. After the belt is repaired and replaced, the belt length must be re measured and the cycle must be re established. The length parameter of the belt scale has a great impact on the zero point and interval value of the belt scale. If it is not updated in time, the measurement will be inaccurate.
4.2.2 the time required for the whole cycle of belt operation of the belt conveyor is not measured accurately, and it must be revised again after the belt is repaired and replaced, otherwise the zero point of the belt scale will be inaccurate.
4.2.3 there are large errors in the calibration process of the belt scale. For example, the chain code calibration does not completely cover the weighing section, the blanking opening of the physical calibration is not cleaned, the transport vehicle is thrown on the way of weighing, or there are major hidden dangers in its own equipment, and the calibration is completed in a hurry without going to the site to check the scale body, sensors, speed sensors, etc., which not only fails to achieve the purpose of calibrating the belt scale, On the contrary, it causes great human error.
4.2.4 the production operators shall take the materials as smoothly as possible to avoid scattering and deviation caused by excessive flow. At the same time, cooperate with the calibration personnel to complete the calibration of the belt scale.
5 methods to improve the measurement accuracy of electronic belt scale through the analysis of the influence on the measurement accuracy and stability of electronic belt scale, under the current maintenance technology and belt scale technical conditions of Bagang, certain methods and measures can be taken to ensure the reliability and stability of belt scale and improve the measurement accuracy.
5.1 eliminate and reduce the influence of installation and operation environment on the measurement accuracy of belt scale
5.1.1 adjust the tensioning force of the belt tensioner of the belt conveyor installed on the electronic belt scale to the zero point of the belt scale, which is stable and meets the calibration standard.
5.1.2 take measures to eliminate electromagnetic interference. The signal cables of sensors and speed sensors must be shielded cables, and the grounding of shielded cables shall be done well.
5.1.3 the local terminal box of the belt scale shall be sealed, dustproof and moisture-proof, and the wiring terminal shall be crimped to prevent loosening.
5.1.4 during the installation process, the manufacturer must be tracked and supervised in the whole process, which must be completed according to the installation specification of belt scale. The selection of measurement points must be reasonable and meet the specification requirements.
5.2 methods to avoid factors of belt scale equipment
5.2.1 the load cell of the electronic belt scale shall not be impacted by external loads beyond the design range, otherwise it will cause inaccurate weighing. It is forbidden to conduct electric welding on the scale body when the sensor is not disconnected from the scale body.
5.2.2 the soft connection shaft of the speed sensor must be concentric with the shaft of the drum to prevent the soft connection of the bullet disk from being damaged due to the installation dislocation, and the bullet disk shall be subject to lubrication spot inspection from time to time.
5.2.3 conduct regular patrol inspection and spot inspection on the belt scale according to the plan, check the scale body, sensor, speed measurement, and other components, and deal with any problems found.
5.3 methods to reduce the impact of human factors on measurement accuracy
5.3.1 do a good job in measuring the length of the belt scale and the operation time of one week, so as to ensure accurate data, especially when bonding and replacing the belt during maintenance, it must be re measured, and the measured data must be averaged in multiple groups.
5.3.2 when the belt deviates, it shall be adjusted in time.
5.3.3 the chain code used to calibrate the belt scale shall be inspected from time to time to ensure the accuracy of the standard.
5.3.4 the belt scale that can be calibrated in kind must be calibrated in kind, because the physical calibration is the calibration method that can fully simulate all factors, and the calibration is the most accurate.

5.3.5 establish a database of belt scale calibration results, make statistical analysis on the changes of zero point, interval value and calibration results of all measuring belt scales, and find out the laws and characteristics, so as to better understand the operation of belt scale.

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