Discussion on the selection of weighing sensors based on the actual working environment of the sensors

Weighing sensor is actually a device that converts mass signal into measurable electrical signal output. When using sensors, the actual working environment of the sensor must be considered first. This is crucial to the correct selection of sensors. It is related to whether the sensor can work normally, its safety and service life, and even the reliability and safety of the entire weighing instrument . The impact of the environment on the sensor mainly includes the following aspects: (1) High temperature environment causes problems such as melting of coating material, melting of solder joints, and structural changes in stress in the elastic body. For sensors working in high temperature environments, high temperature resistant sensors are often used; in addition, heat insulation, water cooling or air cooling devices must be added. (2) Dust and moisture cause short circuits on the sensor. Under this environmental condition, a sensor with high airtightness should be selected. Different sensors have different sealing methods, and their airtightness varies greatly. Common sealing methods include sealant filling or coating; rubber pad mechanical fastening seal; welding (argon arc welding, plasma beam welding) and vacuum nitrogen filling seal. In terms of sealing effect, welding seal is the best, and filling and coating sealant is the worst. For sensors working in clean and dry indoor environments, sensors with glue sealing can be selected. For sensors working in humid and dusty environments, sensors with diaphragm heat-shrink sealing or diaphragm welding sealing and vacuum nitrogen filling should be selected. (3) In highly corrosive environments, such as humidity and acidity, which may damage the elastomer of the sensor or cause short circuits, sensors with plastic spraying or stainless steel covers on the outer surface, good corrosion resistance and good airtightness should be selected. (4) The influence of electromagnetic fields on the output of disordered signals by sensors. In this case, the shielding of the sensor should be strictly checked to see if it has good anti-electromagnetic ability. (5) Flammable and explosive materials not only cause thorough damage to the sensor, but also pose a great threat to other equipment and personal safety. 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 cover of such sensors should not only consider its airtightness, but also the explosion-proof strength, as well as the waterproof, moisture-proof and explosion-proof properties of the cable lead. Secondly, the number and range of sensors should be selected. The number of sensors is selected based on the purpose of the electronic scale and the number of points that the scale body needs to support (the number of support points should be determined based on the principle of coinciding the geometric center of gravity of the scale body with the actual center of gravity). Generally speaking, as many sensors as there are support points on the scale body, but for some special scale bodies such as electronic hook scales, only one sensor can be used, and some electromechanical combined scales should determine the number of sensors to be selected based on actual conditions. The selection of sensor range can be determined based on a comprehensive evaluation of factors such as the maximum weighing value of the scale, the number of selected sensors, the deadweight of the scale body, the maximum possible eccentric load and dynamic load. Generally speaking, the closer the sensor range is to the load assigned to each sensor, the higher the weighing accuracy. However, in actual use, since the load added to the sensor includes the deadweight of the scale body, tare weight, eccentric load, vibration and impact in addition to the object being weighed, many factors should be considered when selecting the sensor range 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 all factors affecting the scale body. The formula is as follows: C = K-0K-1K-2K-3 (Wmax + W) / N C - rated range of a single sensor; W - deadweight of the scale; Wmax - maximum net weight of the object being weighed; N - number of support points used by the scale; K-0 - insurance factor, generally between 1.2 and 1.3; K-1 - impact factor; K-2 - center of gravity offset factor of the scale; K-3 - wind pressure factor. For example: a 30t electronic truck scale, the maximum weighing is 30t, the deadweight of the scale is 1.9t, and four sensors are used. According to the actual situation at the time, select insurance factor K-0 = 1.25, impact factor K-1 = 1.18, center of gravity offset factor K-2 - = 1.03, wind pressure factor K-3 = 1.02, and try to determine the tonnage of the sensor. Solution: According to the sensor range calculation formula: C = K-0K-1K-2K-3 (Wmax + W) / N, we can know that: C = 1.25 × 1.18 × 1.03 × 1.02 × (30 + 1.9) / 4 = 12.36t. Therefore, a sensor with a range of 15t can be selected (the tonnage of the sensor is generally only 10T, 15T, 20t, 25t, 30t, 40t, 50t, etc., unless specially ordered). According to experience, the sensor should generally work within 30% to 70% of its range, but for some scales that have a large impact during use, such as dynamic track scales, dynamic truck scales, steel scales, etc., when selecting sensors, it is generally necessary to expand their 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. Thirdly, 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 that the installation is appropriate and the weighing is safe and reliable; on the other hand, the manufacturer's suggestions should be considered. Manufacturers generally specify the applicable scope of the sensor based on the force conditions, performance indicators, installation form, structural type, material of the elastomer and other characteristics of the sensor. For example, aluminum cantilever beam sensors are suitable for pricing scales, platform scales, case scales, etc.; steel cantilever beam sensors are suitable for hopper scales, electronic belt scales, sorting scales, etc.; steel bridge sensors are suitable for track scales, truck scales, overhead crane scales, etc.; column sensors are suitable for truck scales, dynamic track scales, large tonnage hopper scales, etc. Finally, the sensor accuracy level must be selected. The accuracy level of the sensor includes technical indicators such as nonlinearity, creep, creep recovery, hysteresis, repeatability, and sensitivity of the sensor. When selecting a sensor, do not simply pursue a high-level sensor, but consider both the accuracy requirements of the electronic scale and its cost. The selection of the sensor level must meet the following two conditions: 1. Meet the requirements of the instrument input. The weighing display instrument displays the weighing result after amplifying and A/D converting the output signal of the sensor. Therefore, the output signal of the sensor must be greater than or equal to the input signal required by the instrument, that is, the output sensitivity of the sensor is substituted into the matching formula of the sensor and the instrument, and the calculation result must be greater than or equal to the input sensitivity required by the instrument. The matching formula of the sensor and the instrument: sensor output sensitivity * excitation power supply voltage * maximum weighing of the scale scale division number * number of sensors * sensor range For example: a quantitative packaging scale with a weighing capacity of 25kg, the maximum division number is 1000 divisions; the scale body uses 3 L-BE-25 sensors, the range is 25kg, the sensitivity is 2.0±0.008mV/V, and the bridge voltage is 12V; the scale uses AD4325 instrument. Question: Can the sensor used match the instrument? Solution: After checking, the input sensitivity of the AD4325 instrument is 0.6μV/d, so according to the matching formula of 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／dTherefore

, the sensor used meets the requirements of the instrument input sensitivity and can match the selected instrument.

2. Meet the accuracy requirements of the entire electronic scale. An electronic scale is mainly composed of three parts: the scale body, the sensor, and the instrument. When selecting the accuracy of the sensor, the accuracy of the sensor should be slightly higher than the theoretical calculated value, because the theory is often limited by objective conditions, such as the strength of the scale body is a little worse, the performance of the instrument is not very good, the working environment of the scale is relatively bad, etc. These factors directly affect the accuracy requirements of the scale. Therefore, it is necessary to improve the requirements from all aspects and consider economic benefits to ensure that the purpose is achieved.