A load cell is actually a device that converts a mass signal into a measurable electrical output. When using a load cell, the actual working environment of the load cell should be considered first, which is crucial to the correct selection of the load cell. It is related to whether the load cell 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 load cell mainly includes the following aspects:
(1) The high temperature environment causes problems such as melting of coating materials, open welding of solder joints, and structural changes in the internal stress of the elastomer. For load cells working in high temperature environments, high temperature load cells are often used; in addition, devices such as heat insulation, water cooling or air cooling must be added.
(2) The influence of dust and humidity on the short circuit of the load cell. In this environmental condition, a load cell with high air-tightness should be selected. Different load cells have different sealing methods, and their air-tightness is very different.
Common seals include sealant filling or coating; rubber pads are mechanically fastened and sealed; welding (argon arc welding, plasma beam welding) and vacuum nitrogen filling seals.
From the perspective of sealing effect, welding sealing is the best, and filling and coating sealant is the poorest. For load cells that work in a clean and dry indoor environment, you can choose a glue-sealed load cell, and for some load cells that work in a humid and dusty environment, you should choose a diaphragm heat seal or diaphragm welding seal, pumping Vacuum nitrogen filled load cell.
(3) In a highly corrosive environment, such as moisture and acidity, which will damage the elastomer or cause a short circuit, the outer surface should be oversprayed or should be covered with stainless steel , which has good corrosion resistance and good air-tightness.
(4) The influence of electromagnetic field on load cell output disorder signal. In this case, the shielding of the load cell should be strictly checked to see if it has good electromagnetic resistance.
(5) Flammable and explosive not only cause complete damage to the load cell, but also pose a great threat to other equipment and personal safety. Therefore, load cells working in flammable and explosive environments put forward higher requirements for explosion-proof performance: explosion-proof load cells must be selected in flammable and explosive environments. The sealing cover of this load cell must not only consider its air-tightness, but also The explosion-proof strength, as well as the waterproof, moisture-proof and explosion-proof properties of the cable leads should be considered.
Secondly, the selection of the number and range of load cells.
The selection of the number of load cells is determined according to the purpose of the electronic weighing instrument and the number of points that the scale body needs to support (the number of supporting points should be determined according to the principle of making the geometric center of gravity of the scale body coincide with the actual center of gravity). Generally speaking, several load cells are used for the scale body with several supporting points. However, for some special scale bodies such as electronic hook scales, only one load cell can be used. For some electromagnetically combined scales, the selection of the load cell should be determined according to the actual situation. number.
The selection of the load cell range can be determined according to the comprehensive evaluation of factors such as the maximum weighing value of the scale, the number of selected load cells, the self-weight of the scale body, the possible maximum eccentric load and dynamic load. In general, the closer the range of the load cell is to the load assigned to each load cell, the more accurate its weighing will be. However, in actual use, since the load applied to the load cell includes the self-weight, tare weight, eccentric load and vibration impact of the scale in addition to the object to be weighed, many factors should be considered when selecting the load cell range to ensure that load cell safety and longevity.
The calculation formula of the load cell range is determined through a large number of experiments after fully considering various factors affecting the scale body.
The formula is as follows:
C=K-0K-1K-2K-3(Wmax+W)/N
C—the rated range of a single load cell; W—the self-weight of the scale body; Wmax—the maximum value of the net weight of the object being weighed; N—the number of support points used by the scale body; K-0—the insurance factor, generally between 1.2 and 1.3; K-1—impact coefficient; K-2—the center of gravity offset 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 load cells, according to the actual situation at that time, select the insurance factor K-0=1.25, the impact factor 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 load cell.
Solution: Calculate the formula according to the load cell range:
C=K-0K-1K-2K-3(Wmax+W)/N
It is known that:
C=1.25×1.18×1.03×1.02×(30+1.9)/4
=12.36t
Therefore, a load cell with a range of 15t can be selected (the tonnage of the load cell is generally only 10T, 15T, 20t, 25t, 30t, 40t, 50t, etc., unless it is specially ordered).
According to experience, the load cell should generally work within 30% to 70% of its range, but for some weighing instruments with large impact force during use, such as dynamic rail scales, dynamic truck scales, steel scales, etc., when selecting load cells, Generally, it is necessary to expand its range, so that the load cell works within 20% to 30% of its range, so that the weighing reserve of the load cell is increased to ensure the safety and life of the load cell.
Again, consider the applicability of each type of load cell.
The selection of the load cell type mainly depends on the type of weighing and the installation space to ensure proper installation and safe and reliable weighing; on the other hand, the manufacturer's recommendations should be considered. Manufacturers generally specify the scope of application of the load cell according to the force of the load cell, performance indicators, installation form, structural type, and elastomer material. For example, aluminum cantilever beam load cells are suitable for pricing scales, platform scales, case scales, etc.; steel Cantilever beam load cells are suitable for hopper scales, electronic belt scales, sorting scales, etc.; steel bridge load cells are suitable for rail scales, truck scales, crane scales, etc.; column load cells are suitable for truck scales, dynamic rail scales, and large-tonnage hopper scales. Wait.
Finally, there is a choice of load cell accuracy class.
The accuracy level of the load cell includes technical indicators such as the load cell's nonlinearity, creep, creep recovery, hysteresis, repeatability, and sensitivity. When selecting load cells, do not simply pursue high-level load cells, but consider meeting both the accuracy requirements of electronic scales and their cost.
The selection of the load cell class must satisfy the following two conditions:
1. Meet the requirements of instrument input. The weighing display instrument displays the weighing result after processing the output signal of the load cell through amplification and A/D conversion. Therefore, the output signal of the load cell must be greater than or equal to the input signal size required by the meter, that is, the output sensitivity of the load cell is replaced by the matching formula of the load cell and the meter, and the calculation result must be greater than or equal to the input sensitivity required by the meter.
The matching formula of load cell and meter:
Load cell output sensitivity * excitation power supply voltage * maximum weighing of the scale
The number of divisions of the scale * the number of load cells * the range of the load cell
For example: a quantitative packaging scale weighing 25kg, the maximum number of divisions is 1000. The scale body adopts 3 L-BE-25 type load cells, the range is 25kg, the sensitivity is 2.0±0.008mV/V, the arch bridge voltage pressure 12V. The scale uses an AD4325 meter. Ask whether the load cell used can match the meter.
Solution: After consulting, the input sensitivity of the AD4325 meter is 0.6μV/d, so according to the matching formula of the load cell and the 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 load cell used meets the requirements of the input sensitivity of the instrument 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: scale body, load cell and instrument. When selecting the accuracy of the load cell, the accuracy of the load cell 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.
Post time: Aug-11-2022