The load cell is often referred to as the heart of an electronic weighing instrument, and its performance largely determines the accuracy and stability of the instrument. When designing electronic weighing instruments, the question of how to select the right load cell is frequently encountered.
How to select a sensor
A load cell is essentially a device that converts a mass signal into a measurable electrical signal output. When using a load cell, the actual working environment must be considered first. This is crucial for the correct selection of a load cell, as it relates to the load cell's proper functioning, safety, lifespan, and even the overall reliability and safety of the weighing instrument.
The environmental impact on sensors mainly includes the following aspects:
(1) High-temperature environments cause problems such as melting of coating materials, opening of solder joints, and structural changes in stress within elastic bodies in sensors . High-temperature resistant sensors are often used for sensors that operate in high-temperature environments; in addition, heat insulation, water cooling or air cooling devices must be added.
(2) The effect of dust and moisture on short circuits in sensors. Under these environmental conditions, sensors with high sealing performance should be selected. Different sensors have different sealing methods, and their sealing performance varies greatly.
Common sealing methods include filling or coating with sealant; mechanical fastening with rubber gaskets; welding (argon arc welding, plasma beam welding); and vacuum sealing with nitrogen filling.
In terms of sealing performance, welding is the best, while filling and coating with sealant is the least effective. For sensors operating in clean, dry indoor environments, adhesive-sealed sensors can be selected. However, for sensors operating in humid or dusty environments, sensors with diaphragm heat-shrink sealing or diaphragm welding sealing, and vacuum purging with nitrogen filling, should be selected.
(3) In highly corrosive environments, such as humidity and acidity, which may cause damage to the elastomer of the sensor or cause short circuits, sensors with good corrosion resistance and sealing performance should be selected, with the outer surface coated with powder coating or stainless steel cover.
(4) The effect of electromagnetic fields on the disordered output signal of the sensor. In this case, the shielding of the sensor should be strictly checked to see if it has good electromagnetic resistance.
(5) Flammability and explosiveness not only cause complete damage to sensors, but also pose a great threat to the safety of other equipment and personnel. Therefore, sensors working in flammable and explosive environments have higher requirements for explosion-proof performance: explosion-proof sensors must be selected in flammable and explosive environments. The sealed outer casing of such sensors must not only consider its airtightness, but also its explosion-proof strength, as well as the waterproof, moisture-proof, and explosion-proof properties of the cable leads.
Secondly, the selection of the number and range of sensors.
The number of sensors is determined by the purpose of the electronic weighing instrument and the number of support points required for the scale body (the number of support points should be determined based on the principle of aligning the geometric center of gravity of the scale body with its actual center of gravity). Generally, the number of sensors should correspond to the number of support points on the scale body. However, for some special scale bodies, such as electronic crane scales, only one sensor can be used. For some electromechanical scales, the number of sensors should be determined based on the actual situation.
The selection of sensor range can be determined by comprehensively evaluating factors such as the scale's maximum weighing capacity, the number of sensors used, the scale's own weight, the maximum potential off-center load, and dynamic load. Generally speaking, the closer the sensor's range is to the load distributed to each sensor, the higher the weighing accuracy. However, in actual use, the load applied to the sensors includes not only the object being weighed but also the scale's own weight, tare weight, off-center load, and vibration and impact loads. Therefore, when selecting the sensor range, many factors must be considered to ensure the sensor's safety and lifespan.
The formula for calculating the sensor range was determined through extensive experiments after fully considering all factors affecting the scale.
The formula is as follows:
C = K0K1K2K3(Wmax+W)/N
C—Rated range of a single sensor; W—Weight of the scale body; Wmax—Maximum net weight of the object being weighed; N—Number of support points used by the scale body; K-0—Safety factor, generally between 1.2 and 1.3; K-1—Impact factor; K-2—Weight offset factor of the scale body; K-3—Wind pressure factor.
Based on experience, sensors should generally be operated within 30% to 70% of their weighing range. However, for some weighing instruments that experience significant impact during use, such as dynamic rail scales, dynamic truck scales, and steel scales, the weighing range of the sensor should generally be expanded to 20% to 30% of its range. This increases the sensor's weighing capacity reserve and ensures its safety and lifespan.
Secondly, the applicable scope of various types of sensors should be considered.
The choice of sensor type mainly depends on the type of weighing and the installation space, ensuring proper installation and safe and reliable weighing. On the other hand, the manufacturer's recommendations should also be considered. Manufacturers typically specify the applicable range of sensors based on their stress conditions, performance indicators, installation methods, structural types, and the material of the elastic element. For example, aluminum cantilever beam sensors are suitable for price-computing scales, platform scales, and bench scales; steel cantilever beam sensors are suitable for hopper scales, electronic belt scales, and sorting scales; steel bridge sensors are suitable for rail scales, truck scales, and overhead crane scales; and column sensors are suitable for truck scales, dynamic rail scales, and large-tonnage hopper scales.
Finally, the accuracy class of the sensor must be selected.
The accuracy class of a sensor includes technical indicators such as nonlinearity, creep, creep recovery, hysteresis, repeatability, and sensitivity. When selecting a sensor, do not simply pursue a high-grade sensor; instead, consider both meeting the accuracy requirements of the electronic scale and its cost.
The selection of sensor level must meet the following two conditions:
1. Meet the instrument input requirements. Weighing display instruments display the weighing results after amplifying and processing the sensor's output signal through A/D conversion. Therefore, the sensor's output signal must be greater than or equal to the instrument's required input signal. This means that the sensor's output sensitivity must be substituted into the matching formula between the sensor and the instrument, and the calculated result must be greater than or equal to the instrument's required input sensitivity.
Matching formula for sensors and instruments:
Sensor output sensitivity * excitation power supply voltage * maximum weighing capacity of the scale
The scale's graduations * the number of sensors * the sensor's range
For example: A quantitative packaging scale with a weighing capacity of 25kg and a maximum division of 1000; the scale body uses 3 L-BE-25 type sensors with a weighing range of 25kg, a sensitivity of 2.0±0.008mV/V, and a bridge voltage of 12V; the scale uses an AD4325 instrument. The question is whether the sensors used are compatible with the instrument.
Solution: After consulting the database, the input sensitivity of the AD4325 instrument is 0.6 μV/d. Therefore, according to the matching formula between the sensor and the instrument, the actual input signal of the instrument can be obtained as follows:
2×12×25/1000×3×25=8μV/d>0.6μv/d
Therefore, the sensor used meets the instrument's input sensitivity requirements and can be matched with the selected instrument.
2. Meet the accuracy requirements of the entire electronic scale. An electronic scale mainly consists of three parts: the scale body, the sensor, and the instrument. When selecting the accuracy of the sensor, the accuracy should be slightly higher than the theoretical calculation value. This is because the theoretical value is often limited by objective conditions. For example, factors such as slightly weaker scale body strength, less-than-ideal instrument performance, and harsh working environment directly affect the accuracy requirements of the scale. Therefore, it is necessary to improve the requirements in all aspects while considering economic benefits to ensure that the goal is achieved.
