Practical Application of Electronic Belt Scale
Electronic belt scale is a measuring equipment for continuous automatic weighing of materials in the process of conveying solid bulk materials by belt conveyor. It can continuously measure the instantaneous flow and cumulative flow of materials passing through the belt conveyor. Electronic belt weighers are widely used in the production system of metallurgical industry for material measurement between branches and batching control in the production process. This paper introduces the working principle of electronic belt scale, material flow measurement, zero calibration and physical calibration of electronic belt scale.
2. Structure, performance and working principle of electronic belt scale
2.1 basic composition, structure and performance characteristics of electronic belt scale
The electronic belt scale is mainly composed of load-bearing device, load cell, speed sensor and weighing display. The scale frame is a load bearing and transmission device that vertically transmits the weight of materials on the belt to the load cell according to a specific proportion. It is mainly composed of motor, belt head and tail pulley, belt idler, transportation belt, auxiliary scraping, etc, It is the first link in the material weighing process. Although it seems to have a simple structure, it is an important basic part of the electronic belt scale. Its design strength, rationality of materials and processing accuracy directly determine the accuracy level of the whole equipment, and is very important for the stability in the measurement process. For example, the rigidity of the scale frame requires that the deflection of the scale frame is less than the set value under the rated load, and there is a certain margin; The long-term stability of the bearing quality used on the idler under rated load also directly affects the reliable operation of the electronic belt scale. The display controller is the core component to ensure the normal operation of the electronic belt scale. The collected weight signal and speed signal are integrated to generate instantaneous flow, and the PID output is adjusted according to the difference after being compared with the set flow value, By adjusting the frequency output of the frequency converter to control the motor speed, the purpose of regulating the flow is achieved, which is equivalent to the "brain" part of the whole belt scale. The calculation speed and accuracy, thermal stability, anti electromagnetic interference, dust and shock resistance of its electronic components
2.2 working principle of electronic belt scale
The weighing bridge of the electronic belt scale is installed on the conveying frame. When the materials pass by, the measuring idler detects the weight of the materials on the belt conveyor and acts on the weighing sensor through the lever to generate a voltage signal proportional to the belt load. The speed sensor is directly connected to the large-diameter speed measuring drum to provide a series of pulses. Each pulse represents a belt motion unit, and the frequency of the pulse is directly proportional to the belt speed. The weighing instrument receives signals from the weighing sensor and speed sensor, obtains an instantaneous flow value and cumulative weight value through integral operation, and displays them respectively.
3. Material flow calculation
According to the working principle of electronic belt scale, the calculation formula of instantaneous flow of materials is: F = kvq
Where, f is the flow, kg / S; K weighing coefficient; V is the belt speed, M / S; Q is the load of weighing section, kg / m. K is obtained by physical calibration, and V and Q can be obtained by processing the signals transmitted on site.
The belt speed can be measured by using the speed sensor or calculated according to the technical parameters of the electronic belt scale.
Load calculation formula of weighing section:
Among them, M1 is the real-time load signal transmitted by the field load sensor, M0 is the zero calibration of the electronic belt scale, and l is the length of the effective weighing section of the electronic belt scale.
To sum up, the calculation formula of instantaneous material flow is as follows:
The total material flow can be calculated by integrating the instantaneous material flow.
4. Zero calibration of electronic belt scale
The substances measured by the weighing sensor of the electronic belt scale include the weight of materials and the weight of the belt. The zero calibration of the electronic belt scale is actually to determine the weight of the belt itself. In practical use, the weight of the belt of the electronic belt scale is constantly changing. We use the average of the belt weight.
During zero calibration, pay attention to the change of load signal of electronic belt scale. Generally, we require the tare value to be stable in a small range (about 10 code values). If it exceeds this range, relevant personnel are required to go to the site to check whether the electronic belt scale operates normally.
Before the first zero calibration, the electronic belt scale shall be given enough running time to fully run in all parts of the mechanical part of the equipment. After the operation of the electronic belt scale is stable, the zero point calibration shall be carried out. After determining the tare weight, in theory, when the electronic belt runs without load, the cumulative total amount of materials should be equal to zero; If the difference is large, continue to modify the tare value until it meets the standard required by the specification. After determining the tare, if the tare value exceeds 30% of the scale of the load cell, You should go to the site to check whether the electronic belt scale is abnormal (for example, whether the belt will rub against the scale frame, whether the belt has serious adhesion, whether the speed sensor assembly is always in good contact with the belt, whether the special frame of the weighing sensor is blocked by foreign matters, whether all kinds of sensors are intact, etc.); If there is no abnormality, we should consider whether the selection of load cell is appropriate, and we can consider selecting a load cell with a larger range.
5. Physical calibration of electronic belt scale
The essence of the physical calibration of the electronic belt scale is to reproduce the actual working process of the electronic belt scale with the physical object of known quality. The more real the process is, the more accurate the calibration effect is. The physical object with the total mass of 10 minutes under the rated flow is better. The physical calibration has the advantage of accurate calibration, but it takes time and effort.
After the zero calibration of the electronic belt scale is completed, we will find that sometimes the instantaneous flow value of materials and the cumulative total amount of materials calculated by the display controller are quite different from the actual value. At this time, we can modify the weighing coefficient of the electronic belt scale through physical calibration, so as to reduce this error. Calculation formula of weighing coefficient:
K=（k0 × M）/MO
Where k is the new weighing factor; K0 represents the old weighing factor; M0 represents the cumulative total amount of materials calculated by the weighing display controller; M represents the actual material weight.
During calibration, the material is weighed in the static weighing device to obtain the actual material weight. When the electronic belt scale calibrates the weighing coefficient for the first time, it can be considered to calibrate with weight first. The weighing coefficient calibrated by the weight has a certain reference function (unnecessary work can be avoided and manpower and material resources can be saved), but there is still a large gap with the weighing coefficient finally obtained by physical calibration, and the gap of each scale is different. Therefore, in order to achieve better results in the use of electronic belt scale, it is necessary to carry out repeated physical calibration for many times.
The first physical calibration of weighing coefficient of electronic belt scale generally takes five or six times to complete. The data weighed by the display controller to be weighed is consistent with the actual data value, and it also needs to be calibrated several times. The calibration of electronic belt scale is not once and for all. In the production of, we will transform the scale body, or replace a new belt, or encounter some other abnormal conditions. In these cases, we will recalibrate the tare weight and weighing coefficient. If there is still a large difference between the instantaneous flow value and the cumulative total amount of materials calculated by the weighing display controller and the actual value after multiple physical calibration, it is recommended to replace the electronic belt scale.
6. Precautions for type selection, installation and commissioning of electronic belt scale
The actual use accuracy of the electronic belt scale is not only related to the quality of the electronic belt scale itself, but also depends on the selection of the installation position of the belt scale, the condition and installation quality of the belt conveyor. No matter how good the quality of the electronic belt scale is, the selection of the installation position of the belt scale is wrong or inappropriate, the condition of the belt conveyor is very poor and the installation quality is poor, the actual use accuracy of this electronic belt scale is still very low.
The selection of installation location includes two problems: one is which of several belt conveyors conveying the same material to install the electronic belt scale, and the other is where to install on a selected belt conveyor. The following points shall be considered when selecting which belt conveyor to install the electronic belt scale: feeding uniformity of different belt conveyors, belt speed, length of belt conveyor, section shape of belt conveyor, inclination angle of belt conveyor, groove angle of idler, length of straight section, belt tensioning device, blanking point, unloading equipment, scale frame support, etc.
The following points shall be considered when selecting the position where the scale frame is installed on the belt conveyor: where the belt tension value is small and the belt tension change value is small, the materials shall be stable on the belt before entering the weighing length, and there shall be no sliding phenomenon, in the straight section of the belt conveyor, away from the unloading plow and a certain distance from the unloading point.
The most important adjustment in the installation process is the alignment calibration and idler spacing calibration. The alignment calibration requirements are: there is no height difference between the weighing idler on the scale frame and its adjacent idler, and the height of its adjacent 2-3 groups of idler is required to be the same; It is required that the height of the idler in the weighing influence area is higher than that outside the weighing influence area, and this height can be selected as 3 ~ 5mm; The allowable error of collimation calibration can be 0.5mm for high-precision belt scale, usually 1mm; During the adjustment, the error value at the weighing idler shall be slightly positive, that is, slightly higher than the adjacent idler (e.g. 0.5mm). The requirements for idler spacing calibration are: the spacing between the weighing idler on the scale frame and 2 ~ 3 groups of adjacent front and rear idler is equal.
7. Verification and test
As a dynamic continuous measuring instrument, the accuracy of electronic belt scale is not only related to product quality, installation position and installation quality, but also closely related to periodic verification and test system. The verification and test system with reasonable verification and test methods and short verification and test cycle is conducive to improve the accuracy of electronic belt scale.
In the latest national metrological verification regulation jjg195-2002, in addition to material test, as an operation inspection device, weight, hanging weight and standard electrical signal can be used to simulate the effect of constant load per unit length, and load simulation device (circulating chain weight, chain weight and trolley weight) can be used to simulate the effect of materials passing through belt scale.
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