Selection and application of weighing sensors in the steel industry

Abstract: This article briefly introduces the application of electronic scales in the steel industry, and conducts an in-depth analysis of the selection of sensors during use, for peer communication and reference.

keywordLoad CellResistance strain gauge, calibration

Weighing detection plays a very important role in modern steel industry production, as it is widely used in weighing measurement, ingredient control, logistics transportation, level (liquid level) detection, and other aspects. Improving the stability and measurement accuracy of the weighing system is of great significance for controlling enterprise costs and improving product quality.

In the steel industry, the most widely used electronic scales on site usually include electronic platform scales (such as alloy scales in steelmaking systems), hopper (warehouse) scales, car scales, track scales (static and dynamic), coal tower scales, weighing type molten iron level gauges, batching scales, belt scales, spiral scales, nuclear scales, etc.

Generally speaking, a weighing system consists of a scale bodyThe weighing sensor and weight transmitter are composed of three parts. The scale body is a device used to withstand the gravity of the weighed object and transmit this force to the weighing sensor. It can be the container (bin, tank, hopper) itself; The weighing sensor converts the weight signal into a proportional electrical signal; The weight transmitter amplifies, converts and processes weak current signals from sensors, displays and calibrates them, and communicates and controls them with other devices through various interfaces such as serial port/analog quantity/switch quantity/fieldbus. The weighing sensor is the most core equipment in the system. From the current development of weighing sensors, the resistance strain gauge sensor has become the mainstream choice in on-site applications due to its high accuracy, good stability, mature technology, and high cost-effectiveness. In the weighing system, the selection of sensors is the most crucial and should be comprehensively considered from the following aspects in order to make a reasonable choice.

1、 Selection of the number and range of weighing sensors

The determination of the number of weighing sensors is generally based on the number of support points of the scale body, with the principle of making the center of gravity of the scale body coincide with the geometric centers of multiple sensors (support points) as much as possible. However, in actual use, due to the irregular shape of the hopper, the center of gravity of the scale body changes with the amount of material. For this situation, the author suggests using three sensors and using the principle of determining a plane with three points to ensure that each sensor is subjected to force and avoid& Ldquo; Seesaw& Rdquo; Phenomena.

When selecting sensors on site, some people prefer to choose the range as large as possible for safety reasons, which is very inappropriate. In fact, the overload capacity of general sensors can reach 150% F& Middot; S (up to 300% F& middot; S in the short term). And we know that for sensors with the same accuracy, if the range is too large, the error caused by normal measurement will also increase. This is extremely unfavorable for some ingredient processes that require high precision in addition.

The selection of sensor range should be based on a comprehensive evaluation of factors such as the maximum weighing value of the scale, the number of sensors selected, the self weight of the scale body, the possible maximum bias load, vibration, impact, etc., to ensure the safety and lifespan of the sensor.

The above figure shows the requirements for selecting the range of symmetric measurement systems recommended by the International Organization for Legal Metrology (OIML) R60 "Load Cells". In daily applications, the author suggests considering the following formula:

C=K0& Times; K1& Times; K2& Times; K3& Times; (Wmax+W)/N

In the formula: C& Flash; The rated range of a single sensor; W& Flash; The self weight of the scale body; Wmax& Flash; The maximum net weight of the object being weighed; N& Flash; The number of support points used for the scale body; K0& Flash; The insurance coefficient is generally between 1.2 and 1.3; K1& Flash; Impact coefficient; K2& Flash; The center of gravity deviation coefficient of the scale body; K3& Flash; Wind pressure coefficient.

IISelection of weighing sensor characteristics

The production environment of the steel industry is relatively complex, so the installation environment of weighing devices varies. We must choose special weighing sensors with corresponding characteristics and meet the needs based on the actual situation on site.

For weighing devices such as billet scales and roller scales used in high-temperature environments, in order to avoid problems such as melting of solder joints, structural changes in elastic body stress, and insufficient temperature compensation range caused by high temperature, it is necessary to use high-temperature resistant sensors and add insulation, water cooling, or air cooling devices.

For high dust and humid environments such as raw material yards and lime scales, in order to avoid zero drift or even short circuit damage to sensors caused by insulation degradation, sensors should be selected with membrane hot sleeve sealing or membrane welding sealing, vacuum pumping and nitrogen filling. Sensors with filled and coated sealant should never be used.

For weighing sensors in chemical production areas, considering the damage caused by corrosive environments, stainless steel materials, good corrosion resistance and sealing performance should be selected as sensors. For injection tank scales, powder silo scales, coal tower scales, etc. in coal injection and coking areas, sensors that meet the corresponding explosion-proof level must be selected due to the flammable and explosive environment in which they are located.

3、 Selection of sensor structure form

The choice of sensor form mainly depends on the type of load (whether there is impact), accuracy requirements, range size, installation space, and maintenance needs. Column type sensors are suitable for truck scales, large tonnage hopper scales, etc; The double shear beam sensor is suitable for large tonnage truck scales and track scales; The cantilever beam sensor is suitable for ground scale, platform scale, etc; The S-type pull-type sensor is suitable for electronic belt scales, hopper scales, etc; Curved beam type sensors are suitable for small tonnage belt scales, ingredient scales, etc; The pivot pin type sensor is suitable for areas such as track scales, coal tower scales, etc. where operations are frequent and maintenance times are tight.

4、 Sensor accuracy level selection

The selection of sensor accuracy level must meet the accuracy requirements of the entire electronic scale, because an electronic scale is mainly composed of three parts: scale body, sensor, and instrument. Factors such as scale body strength, transmitter characteristics, and usage environment can directly affect the accuracy of the scale. Therefore, when selecting sensor accuracy, the accuracy of the sensor should be slightly higher than the theoretical calculation value (30% higher than the recommended value).

On the other hand, if we choose sensors with high accuracy (such as C3 level), the investment cost will significantly increase. For weighing instruments used for measurement and settlement, when selecting sensors, we should prioritize their accuracy level to ensure that the overall accuracy of the system is better than level 0.5 after weighing. For some batching systems that require precise weighing and automatic control, we must also choose sensors with high accuracy levels. For ordinary weighing systems, the required weighing accuracy may not be very high, so we can choose sensors with lower accuracy levels to reduce investment costs. Similarly, for most systems that use the weighing method to detect material levels or liquid levels, it is not necessary to select the accuracy level of the sensor too high.

5、 Sensor sensitivity selection

The selection of sensor sensitivity must meet the requirements of the weighing instrument input: The weighing display instrument displays the weighing result after amplifying, A/D conversion, and other processing of the sensor output signal. Therefore, the output signal of the sensor must be greater than or equal to the input signal size required by the instrument, that is, the output sensitivity of the sensor must be substituted into the matching formula between the sensor and the instrument, and the calculation result must be greater than or equal to the input sensitivity required by the instrument, that is, the weight corresponding to each graduation value. The sensor output signal:

&Amp; Ge; Instrument input sensitivity

In the formula, S: sensor output sensitivity; U: Excitation power supply voltage; Wmax: The maximum weight of the scale; Nmax: the maximum number of divisions of the scale; N: Number of sensors; R: Sensor range.

The electronic scales used on site, except for the hook scale, generally use multi sensor group scales, which are divided into series and parallel group scales. The series connection method has low requirements for sensor consistency, but the circuit is complex and the investment is large, and it is now basically no longer used. The most common way to group scales is to use sensors in parallel. This method involves the input terminals of each sensor being connected in parallel, using a common power supply bridge, and the output also operates in parallel. For the sake of simplicity, the parallel operation of two sensors is introduced as follows:

Set the sensitivity of the two sensors to S1 and S2, the bridge arm resistance to R1 and R2, and the supply bridge voltage to U1 and U2, respectively. Through circuit analysis, it can be seen that the condition for parallel operation of these two sensors is S1R1=S2R2. Similarly, when n sensors are working in parallel, it can be obtained that: S1/R1=S2/R2=& Bdquo= Sn/Rn. Obviously, the parallel working state requires relatively high parameters for the sensor itself.

In the steel industry, some batching scales and hoppers in production sites have irregular shapes. During the loading and unloading process, the center of gravity of the hopper changes with the change of material quantity. Therefore, the load distribution of each sensor varies. If the sensitivity of each sensor is not consistent, its output characteristics will obviously be different from the calibration, resulting in significant measurement errors. Therefore, when selecting sensors, we must follow the previous instructions and choose sensors with better parameter consistency as much as possible to meet the requirements of S1/R1=S2/R2=& Bdquo= Sn/Rn.

6、 Special needs

In addition, when selecting sensors, we should also consider some other requirements in the field: for example, in certain situations where space or load-bearing structure and capacity limitations do not meet the conditions for calibration, it is recommended to consider using digital sensors; Because during the use of digital sensors, the weight transmitter (digital scale) will automatically address and address them, read their parameter information into the system, and determine the total output of the system based on their characteristics. In theory, without any changes in the scale structure, there is basically no need to recalibrate after replacing the sensor.

For example, some weighing systems are subjected to significant horizontal lateral impact forces during use. When they exceed the lateral force resistance or self resetting ability of the loading connection device, it is necessary to consider designing or adding specialized limit devices in the loading connection device, or adding overload protection structures in the system.

In summary, in order to ensure the reliable operation of the weighing system and provide high measurement accuracy, the primary issue that should be solved is the selection of sensors. As long as we master the working principle of sensors, fully understand the functional characteristics of various sensors, and fully consider the on-site process requirements, we can definitely solve this problem.

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