Abstract: A load cell is actually a device that converts a mass signal into a measurable electrical signal output. The sensor must first consider the actual working environment in which the sensor is located. This is crucial for the correct selection of the sensor. It is related to whether the sensor can work properly, its safety and service life, and even the reliability and safety of the entire instrument. The impact of the environment on the sensor mainly includes the following aspects:
(1) The high temperature environment causes problems such as melting of the coating material, opening of the solder joint, and structural changes in the elastic body stress. Sensors that work in high temperature environments often use high temperature sensors; in addition, they must be insulated, water-cooled, or air-cooled.
(2) The effect of dust and humidity on the short circuit of the sensor. In this environment, the sensor with high airtightness should be selected. Different sensors have different sealing methods, and there are great differences in their tightness.
Common seals are filled or coated with sealant; mechanically sealed rubber gaskets; welded (argon arc welding, plasma beam welding) and vacuum nitrogen-filled seals.
From the point of view of the sealing effect, the welding seal is the best, and the amount of the filling sealant is poor. For sensors that work in a clean, dry environment indoors, sensors that are sealed by glue may be selected. For sensors that operate in a humid, dusty environment, the diaphragm seal or diaphragm seal must be selected. Vacuum nitrogen filling sensor.
(3) In the corrosive environment, such as moisture, acid damage to the sensor caused by elastomers or a short circuit, etc., should choose to spray the outer surface or stainless steel cover, corrosion resistance and good sealing sensor.
(4) The influence of electromagnetic field on the sensor output turbulence signal. In this case, the sensor's shielding performance should be strictly checked to see if it has a good anti-electromagnetic capability.
(5) Flammability and explosiveness not only cause complete damage to the sensor, but also pose great threat to other equipment and personal safety. Therefore, sensors that operate in flammable and explosive environments place higher requirements on explosion-proof performance. In flammable and explosive environments, explosion-proof sensors must be selected. The sealing covers of such sensors must not only consider their tightness, but also Take into consideration the explosion-proof strength and the waterproof, moisture-proof and explosion-proof properties of the cable lead-out.
Secondly, the choice of the number of sensors and the selection of the number of ranges depends on the use of the electronic scale and the number of points the scale body needs to support (the number of support points should be determined based on the principle that the geometric center of gravity of the scale body is coincided with the actual center of gravity). In general, the scale body has several support points to select several sensors, but for some special scale bodies such as electronic crane scales can only use a sensor, some mechanical and electrical combination scales should be based on the actual situation to determine the choice of sensors Number.
The selection of the sensor range can be determined based on the comprehensive evaluation of factors such as the maximum weighing value of the scale, the number of selected sensors, the weight of the scale body, the maximum partial load that may be generated, and the dynamic load. In general, the closer the sensor's range is to the load assigned to each sensor, the higher the accuracy of its weighing. However, in actual use, due to the load imposed on the sensor, in addition to the object being weighed, there are loads such as the weight of the scale body, tare weight, eccentric load, and vibration impact. Therefore, when selecting the sensor range, many factors must be taken into account to ensure The safety and life of the sensor.
The calculation formula of the sensor range is determined after a large number of experiments after fully considering the factors that affect the scale body.
The formula is as follows:
C=K-0K-1K-2K-3(Wmax+W)/N
C—the rated range of a single sensor; W—the weight of the scale body; Wmax—the maximum value of the net weight of the object being called; N—the number of support points used by the scale body; and K-0—the insurance factor, generally ranging from 1.2 to 1.3. Interpersonal; K-1—impact coefficient; K-2—center-of-gravity displacement coefficient of the scale body; K-3—wind pressure coefficient.
For example: a 30t electronic truck scale, the maximum weighing is 30t, the scale body weight is 1.9t, using four sensors, according to the actual situation at that time, select the insurance coefficient K-0 = 1.25, the impact coefficient K-1 = 1.18, Center-of-gravity displacement coefficient K-2—=1.03, wind pressure coefficient K-3=1.02, try to determine the tonnage of the sensor.
Solution: According to the calculation formula of the sensor range:
C=K-0K-1K-2K-3(Wmax+W)/N
Know that:
C=1.25×1.18×1.03×1.02×(30+1.9)/4=12.36t
Therefore, an optional sensor with a range of 15t (generally the tonnage of the sensor is only 10T, 15T, 20t, 25t, 30t, 40t, 50t, etc., unless specifically ordered).
According to experience, the sensor should generally be operated within its range of 30% to 70%, but for some weighing instruments that have a large impact during use, such as dynamic track scales, dynamic truck scales, steel scales, etc., when selecting sensors, Generally, it is necessary to expand the range so that the sensor works within 20% to 30% of its range, so that the weighing reserve of the sensor is increased to ensure the safety and life of the sensor. Again, consider the scope of application of various types of sensors.
The choice of sensor type mainly depends on the type of weighing and installation space, to ensure proper installation, weighing safe and reliable; on the other hand, to consider the manufacturer's recommendations. Manufacturers generally specify the applicable range of the sensor according to the force conditions of the sensor, performance index, installation form, structure type, elastomer material, etc. For example, aluminum cantilever sensors are suitable for use in pricing scales, platform scales, case scales, etc.; Cantilever beam sensors are suitable for hopper scales, electronic belt scales, sorting scales, etc.; steel bridge sensors are suitable for track scales, truck scales, crane scales, etc.; column sensors are suitable for truck scales, dynamic rail scales, large-tonnage hopper scales Wait.
Finally, the sensor accuracy level should be selected to include the sensor's non-linear, creep, creep recovery, hysteresis, repeatability, and sensitivity. When selecting sensors, do not simply pursue high-grade sensors, and consider not only meeting the accuracy requirements of electronic scales, but also their costs.
The choice of sensor grade must meet the following two conditions:
1, to meet the input requirements of the instrument. Weighing display instrument displays the weighing result after the sensor's output signal is amplified, A/D converted, and other processing. Therefore, the output signal of the sensor must be greater than or equal to the size of the input signal required by the instrument. That is, the output sensitivity of the sensor is based on the matching formula of the sensor and the instrument. The calculation result must be greater than or equal to the required input sensitivity of the instrument.
Matching formulas for sensors and meters:
Sensor output sensitivity * Excitation power supply voltage * The scale's maximum weighing scale's indexing number * The number of sensors * The sensor's measuring range For example: A weighing scale with a weighing capacity of 25kg, the maximum indexing number is 1000 divisions; the scale body uses 3 L-BE-25 sensor only, the range is 25kg, the sensitivity is 2.0±0.008mV/V, the arch bridge voltage is 12V; the scale uses AD4325 instrument. Q. Can the sensor be matched with the meter?
Solution: After consulting, the input sensitivity of the AD4325 meter is 0.6μV/d. Therefore, according to the matching formula of the sensor and the meter, the actual input signal of the meter is:
2×12×25/1000×3×25=8μV/d>0.6μv/d
Therefore, the sensors used meet the input sensitivity requirements of the instrument and can match the selected instrument.
2, to meet the accuracy of the entire electronic scale requirements. An electronic scale is mainly composed of a scale body, a sensor, and an instrument. When the accuracy of the sensor is selected, the accuracy of the sensor should be slightly higher than the theoretical calculation value because the theory is often limited by objective conditions, such as a scale. The strength of the body is poor, the performance of the instrument is not very good, and the working environment of the scale is relatively poor and other factors have a direct impact on the accuracy of the scale. Therefore, it is necessary to improve the requirements in all aspects, and consider the economic benefits to ensure that the purpose is achieved.
(1) The high temperature environment causes problems such as melting of the coating material, opening of the solder joint, and structural changes in the elastic body stress. Sensors that work in high temperature environments often use high temperature sensors; in addition, they must be insulated, water-cooled, or air-cooled.
(2) The effect of dust and humidity on the short circuit of the sensor. In this environment, the sensor with high airtightness should be selected. Different sensors have different sealing methods, and there are great differences in their tightness.
Common seals are filled or coated with sealant; mechanically sealed rubber gaskets; welded (argon arc welding, plasma beam welding) and vacuum nitrogen-filled seals.
From the point of view of the sealing effect, the welding seal is the best, and the amount of the filling sealant is poor. For sensors that work in a clean, dry environment indoors, sensors that are sealed by glue may be selected. For sensors that operate in a humid, dusty environment, the diaphragm seal or diaphragm seal must be selected. Vacuum nitrogen filling sensor.
(3) In the corrosive environment, such as moisture, acid damage to the sensor caused by elastomers or a short circuit, etc., should choose to spray the outer surface or stainless steel cover, corrosion resistance and good sealing sensor.
(4) The influence of electromagnetic field on the sensor output turbulence signal. In this case, the sensor's shielding performance should be strictly checked to see if it has a good anti-electromagnetic capability.
(5) Flammability and explosiveness not only cause complete damage to the sensor, but also pose great threat to other equipment and personal safety. Therefore, sensors that operate in flammable and explosive environments place higher requirements on explosion-proof performance. In flammable and explosive environments, explosion-proof sensors must be selected. The sealing covers of such sensors must not only consider their tightness, but also Take into consideration the explosion-proof strength and the waterproof, moisture-proof and explosion-proof properties of the cable lead-out.
Secondly, the choice of the number of sensors and the selection of the number of ranges depends on the use of the electronic scale and the number of points the scale body needs to support (the number of support points should be determined based on the principle that the geometric center of gravity of the scale body is coincided with the actual center of gravity). In general, the scale body has several support points to select several sensors, but for some special scale bodies such as electronic crane scales can only use a sensor, some mechanical and electrical combination scales should be based on the actual situation to determine the choice of sensors Number.
The selection of the sensor range can be determined based on the comprehensive evaluation of factors such as the maximum weighing value of the scale, the number of selected sensors, the weight of the scale body, the maximum partial load that may be generated, and the dynamic load. In general, the closer the sensor's range is to the load assigned to each sensor, the higher the accuracy of its weighing. However, in actual use, due to the load imposed on the sensor, in addition to the object being weighed, there are loads such as the weight of the scale body, tare weight, eccentric load, and vibration impact. Therefore, when selecting the sensor range, many factors must be taken into account to ensure The safety and life of the sensor.
The calculation formula of the sensor range is determined after a large number of experiments after fully considering the factors that affect the scale body.
The formula is as follows:
C=K-0K-1K-2K-3(Wmax+W)/N
C—the rated range of a single sensor; W—the weight of the scale body; Wmax—the maximum value of the net weight of the object being called; N—the number of support points used by the scale body; and K-0—the insurance factor, generally ranging from 1.2 to 1.3. Interpersonal; K-1—impact coefficient; K-2—center-of-gravity displacement coefficient of the scale body; K-3—wind pressure coefficient.
For example: a 30t electronic truck scale, the maximum weighing is 30t, the scale body weight is 1.9t, using four sensors, according to the actual situation at that time, select the insurance coefficient K-0 = 1.25, the impact coefficient K-1 = 1.18, Center-of-gravity displacement coefficient K-2—=1.03, wind pressure coefficient K-3=1.02, try to determine the tonnage of the sensor.
Solution: According to the calculation formula of the sensor range:
C=K-0K-1K-2K-3(Wmax+W)/N
Know that:
C=1.25×1.18×1.03×1.02×(30+1.9)/4=12.36t
Therefore, an optional sensor with a range of 15t (generally the tonnage of the sensor is only 10T, 15T, 20t, 25t, 30t, 40t, 50t, etc., unless specifically ordered).
According to experience, the sensor should generally be operated within its range of 30% to 70%, but for some weighing instruments that have a large impact during use, such as dynamic track scales, dynamic truck scales, steel scales, etc., when selecting sensors, Generally, it is necessary to expand the range so that the sensor works within 20% to 30% of its range, so that the weighing reserve of the sensor is increased to ensure the safety and life of the sensor. Again, consider the scope of application of various types of sensors.
The choice of sensor type mainly depends on the type of weighing and installation space, to ensure proper installation, weighing safe and reliable; on the other hand, to consider the manufacturer's recommendations. Manufacturers generally specify the applicable range of the sensor according to the force conditions of the sensor, performance index, installation form, structure type, elastomer material, etc. For example, aluminum cantilever sensors are suitable for use in pricing scales, platform scales, case scales, etc.; Cantilever beam sensors are suitable for hopper scales, electronic belt scales, sorting scales, etc.; steel bridge sensors are suitable for track scales, truck scales, crane scales, etc.; column sensors are suitable for truck scales, dynamic rail scales, large-tonnage hopper scales Wait.
Finally, the sensor accuracy level should be selected to include the sensor's non-linear, creep, creep recovery, hysteresis, repeatability, and sensitivity. When selecting sensors, do not simply pursue high-grade sensors, and consider not only meeting the accuracy requirements of electronic scales, but also their costs.
The choice of sensor grade must meet the following two conditions:
1, to meet the input requirements of the instrument. Weighing display instrument displays the weighing result after the sensor's output signal is amplified, A/D converted, and other processing. Therefore, the output signal of the sensor must be greater than or equal to the size of the input signal required by the instrument. That is, the output sensitivity of the sensor is based on the matching formula of the sensor and the instrument. The calculation result must be greater than or equal to the required input sensitivity of the instrument.
Matching formulas for sensors and meters:
Sensor output sensitivity * Excitation power supply voltage * The scale's maximum weighing scale's indexing number * The number of sensors * The sensor's measuring range For example: A weighing scale with a weighing capacity of 25kg, the maximum indexing number is 1000 divisions; the scale body uses 3 L-BE-25 sensor only, the range is 25kg, the sensitivity is 2.0±0.008mV/V, the arch bridge voltage is 12V; the scale uses AD4325 instrument. Q. Can the sensor be matched with the meter?
Solution: After consulting, the input sensitivity of the AD4325 meter is 0.6μV/d. Therefore, according to the matching formula of the sensor and the meter, the actual input signal of the meter is:
2×12×25/1000×3×25=8μV/d>0.6μv/d
Therefore, the sensors used meet the input sensitivity requirements of the instrument and can match the selected instrument.
2, to meet the accuracy of the entire electronic scale requirements. An electronic scale is mainly composed of a scale body, a sensor, and an instrument. When the accuracy of the sensor is selected, the accuracy of the sensor should be slightly higher than the theoretical calculation value because the theory is often limited by objective conditions, such as a scale. The strength of the body is poor, the performance of the instrument is not very good, and the working environment of the scale is relatively poor and other factors have a direct impact on the accuracy of the scale. Therefore, it is necessary to improve the requirements in all aspects, and consider the economic benefits to ensure that the purpose is achieved.