Talking about the selection of load cell from the actual working environment of the sensor
The load cell is actually a device that converts the quality signal into a measurable electrical signal output. When using a sensor, we must first consider the actual working environment in which the sensor is located. This is essential for the correct selection of the sensor. 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 environmental impact on the sensor mainly has the following aspects:
(1) The high temperature environment causes problems such as melting of the coating material, opening of the solder joints, and structural changes in the stress in the elastomer. For sensors working in high temperature environments, high temperature resistant sensors are often used; in addition, heat insulation, water cooling or air cooling must be installed.
(2) The influence of dust and humidity on the sensor caused by short circuit. In this environmental condition, a sensor with high airtightness should be selected. Different sensors have different sealing methods, and their airtightness is very different.
Common seals include sealant filling or coating; rubber gasket mechanical tightening seal; welding (argon arc welding, plasma beam welding) and vacuum nitrogen filling seal.
From the perspective of the sealing effect, welding and sealing are good, and filling and coating sealant is poor. For sensors that work in a clean and dry indoor environment, you can choose a glue-sealed sensor. For some sensors that work in a humid and dusty environment, you should choose a diaphragm heat seal or a diaphragm welding seal, and Vacuum nitrogen filled sensor.
(3) In a highly corrosive environment, such as damp or acid causing damage to the elastic body or short circuit of the sensor, the outer surface should be sprayed or stainless steel cover, with good corrosion resistance and good airtightness sensor.
(4) The influence of electromagnetic field on sensor output disturbance signal. In this case, the shielding of the sensor should be strictly checked to see if it has good electromagnetic resistance.
(5) Flammable and explosive not only cause thorough damage to the sensor, but also pose a great threat to other equipment and personal safety. Therefore, sensors that work 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 housing of such sensors must not only consider its airtightness, but also Consider the 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 selection of the number of sensors is based on the purpose of the electronic weighing instrument and the number of points that the scale body needs to support (the number of support points should be determined according to the principle that the geometric center of gravity of the scale body coincides with the actual center of gravity). Generally speaking, if the scale has several supporting points, several sensors are used. However, for some special scales such as electronic hook scales, only one sensor can be used. For some electromechanical scales, the sensor should be selected according to the actual situation. Number.
The selection of the sensor range can be determined based on the comprehensive evaluation of the large weighing value of the scale, the number of selected sensors, the self-weight of the scale body, the possible large eccentric load and the dynamic load. Generally speaking, the closer the range of the sensor is to the load assigned to each sensor, the higher the accuracy of its weighing. However, in actual use, in addition to the object being weighed, the load on the sensor also contains the weight, tare, eccentric load, and vibration shock. Therefore, when selecting the sensor range, many factors must be considered to ensure The safety and longevity of the sensor.
The calculation formula of the sensor range is determined after a large number of experiments after fully considering the various factors affecting the scale body.
The formula is as follows: C=K-0K-1K-2K-3(Wmax＋W)/N
C—Rated measuring range of a single sensor; W—weight of the scale body; Wmax—maximum value of the net weight of the object to be weighed; N—number of support points used by the scale body; K-0—insurance factor, generally taken from 1.2 to 1.3 Between people; K-1—impact coefficient; K-2—gravity deviation coefficient of the scale body; K-3—wind pressure coefficient.
For example: a 30t electronic truck scale, the maximum weighing is 30t, the weight of the scale body is 1.9t, four sensors are used, according to the actual situation at the time, the insurance coefficient K-0 = 1.25, the impact coefficient K-1 = 1.18 , The center of gravity offset coefficient K-2-= 1.03, the 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
It can be seen 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 used (the tonnage of the sensor is generally only 10T, 15T, 20t, 25t, 30t, 40t, 50t, etc., unless it is specially customized).
According to experience, the sensor should generally work within 30% to 70% of its range. However, for some weighing instruments that have a large impact during use, such as dynamic railroad scales, dynamic truck scales, steel scales, etc., when selecting sensors, Generally, it is necessary to expand its range to make the sensor work within 20% to 30% of its range and increase the weighing reserve of the sensor to ensure the safety and life of the sensor.
Again, we must consider the scope of application of various types of sensors.
The choice of sensor type mainly depends on the weighing type and installation space to ensure proper installation and safe and reliable weighing; on the other hand, the manufacturer’s recommendations must be considered. Manufacturers generally stipulate the scope of application of the sensor according to the characteristics of the sensor's force, performance indicators, installation form, structural type, and elastic body material. For example, aluminum cantilever beam sensors are suitable for price calculation scales, platform scales, and case scales; steel; Cantilever beam sensor is suitable for hopper scales, electronic belt scales, sorting scales, etc.; steel bridge sensors are suitable for rail scales, truck scales, crane scales, etc.; column sensors are suitable for truck scales, dynamic rail scales, and large-tonnage hopper scales Wait.
After ^, the accuracy level of the sensor must be selected.
The accuracy level of the sensor includes the sensor's non-linear, creep, creep recovery, hysteresis, repeatability, sensitivity and other technical indicators. When selecting sensors, don't simply pursue high-level sensors, but consider meeting the accuracy requirements of electronic scales and the cost.
The choice of sensor grade must meet the following two conditions:
1. Meet the requirements of instrument input. The weighing display instrument displays the weighing result after the output signal of the sensor is amplified and A/D converted. 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 is substituted for 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 sensor and meter:
Sensor output sensitivity * Excitation power supply voltage * Large scale weighing
Scale division number * number of sensors * sensor range
For example: a quantitative packaging scale weighing 25kg, ^ the maximum division number is 1000 divisions; the scale body adopts 3 L-BE-25 type sensors, the range is 25kg, the sensitivity is 2.0±0.008mV/V, the arch bridge voltage force 12V; Scale adopts AD4325 meter. Ask whether the sensor used can match the meter.
Solution: After consulting, the input sensitivity of AD4325 meter is 0.6μV/d, so according to the matching formula of sensor and meter, the actual input signal of the meter can be obtained as:
2×12×25/1000×3×25=8μV/d＞0.6μv/d
Therefore, the adopted sensor meets the requirement of 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 a scale body, a sensor, and an instrument. When selecting the accuracy of the sensor, 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 scales. The strength of the body is a little bit worse, the performance of the instrument is not very good, the working environment of the scale is relatively bad and other factors directly affect the accuracy requirements of the scale. Therefore, the requirements must be improved from all aspects, and economic benefits must be considered to ensure that the purpose is achieved.
