A load cell is actually a device that converts a mass signal into a measurable electrical signal output. The load cell has already leaked
It penetrates into extremely broad fields such as industrial production, space development, ocean exploration, environmental protection, resource survey, medical diagnosis, bioengineering, and even cultural relic protection. It is no exaggeration to say that from the vast space to the vast From the ocean to various complex engineering systems, almost every modern project is inseparable from various sensors. Therefore, how to correctly select a load cell has become particularly important. Below I will describe how to select a load cell from several aspects.
2 Structural type of load cell
Load cells are divided into resistance strain type, electromagnetic force type and capacitive type according to the force-to-electricity conversion method. The electromagnetic force type is mainly used in electronic balances, the capacitive type is used in some electronic crane scales, and the vast majority of weighing instrument products use resistance strain type load cells. The resistance strain gauge load cell has a simple structure, high accuracy, wide application, and can be used in relatively poor environments. Therefore, resistance strain gauge load cells are widely used in weighing instruments. Resistance strain gauge load cells are mainly composed of elastomers, resistance strain gauges and compensation circuits. The elastomer is the force-bearing component of the load cell and is made of high-quality alloy steel or high-quality aluminum profiles. The resistance strain gauge is made of metal foil etched into a grid shape, and four resistance strain gauges are bonded to the elastomer in a bridge structure. When there is no force, the resistance values of the four resistors of the bridge are equal, the bridge is in a balanced state, and the output is zero. When the elastic body deforms under force, the resistance strain gauge also deforms along with it. During the bending process of the elastic body, two strain gauges are stretched, the metal wire becomes longer, and the resistance value increases; the other two strain gauges are compressed, and the resistance value increases.
decrease. This causes the originally balanced bridge to become unbalanced, resulting in a voltage difference at both ends of the bridge. This voltage difference is proportional to the force on the elastomer. By detecting this voltage difference, the magnitude of the gravity on the sensor can be obtained. This After the voltage signal is detected by the instrument and then calculated, the corresponding weight value can be obtained. In order to meet the installation needs of various weighing instrument structures, load cells are made into various structural forms, and the names of the sensors are often called according to their appearance. Such as bridge type sensors (mainly used in truck scales), cantilever beam type (ground scales, hopper scales, truck scales), column type (truck scales, hopper scales), box type (bench scales), S-type (hopper scales), etc. A weighing instrument carrier often has a variety of structural types of sensors to choose from. If the sensor is properly selected, it is very helpful to improve the weighing performance. Resistive strain gauge load cells come in many specifications, ranging from a few hundred grams to several hundred tons. When selecting the weighing sensor range, it should be determined based on the maximum weighing capacity of the weighing instrument used. The empirical formula is: total load of the sensor (maximum allowable load of a single sensor Weigh. The accuracy level of load cells is divided into four levels: A, B, C, and D. Different levels have different error ranges. Class A sensors have the highest requirements. The number after the grade indicates the calibration graduation value. The larger the number, the better the sensor quality. For example: C2 means C level, 2000 calibration graduation values; C5 means C level, 5000
Verify the graduation value. Obviously C5 is higher than C2. The commonly used levels of sensors are C3 and C5. These two levels of sensors can be used to make electronic weighing instruments with an accuracy level of Level III.
3 Error of load cell
The errors of the load cell are mainly caused by nonlinear error, hysteresis error, repeatability error, creep, and zero temperature additional error.
And caused by additional errors in rated output temperature, etc. The digital sensors that have appeared in recent years put the A/D conversion circuit and CPU circuit into the sensor. The output of the sensor is no longer an analog voltage signal, but a processed weight digital signal. This brings the following advantages: :
3.1 The instrument can collect the signal of each digital sensor separately, and calibrate each sensor individually through linear equation operation, which makes it possible to complete the four-corner error correction at one time. The most troublesome problem in scales using analog sensors is the correction of the four-corner error, which often requires repeated adjustments to meet the requirements. Each adjustment requires moving heavy weights around, which is time-consuming and Effortless.
3.2 Since the instrument can detect the signal of each sensor, any problem with any sensor can be detected from the instrument.
Observed above, it facilitates inspection and maintenance work.
3.3 The digital sensor uses the 485 interface to transmit digital signals, which has a long transmission distance and is immune to interference. It overcomes the problems of difficulty in remote transmission of analog signals and susceptibility to interference.
3.4 The various errors of the sensor can be corrected by the microprocessor inside the digital sensor, so that the output sensor number
It is more accurate. The load cell is known as the nervous system of electronic scales and weighing systems. Its performance determines the accuracy and stability of electronic scales to a large extent. When designing electronic weighing instruments, we often encounter the problem of how to choose sensors.
4 The main effects of the environment on the load cell are as follows:
several aspects
4.1 The high-temperature environment causes problems such as melting of the coating material, cracking of the solder joints, and structural changes in the internal stress of the elastomer on the sensor.
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.
4.2 Dust and moisture may cause short circuit to the sensor.
Under this environmental condition, a sensor with high sealing performance should be selected. Different sensors have different sealing methods, and their sealing properties vary greatly. The sealing protection of load cells commonly includes sealant filling or coating, rubber pad mechanical fastening seal, welding (argon arc welding, plasma beam welding) and vacuum nitrogen filling sealing. From the perspective of sealing effect, welding sealing is the best, and filling and coating sealant is the difference. For sensors that work in a clean and dry indoor environment, you can choose glue-sealed sensors. For some sensors that work in humid and dusty environments, you should choose diaphragm heat-seal seals.
Or a sensor with a diaphragm welded and sealed, vacuumed and filled with nitrogen.
4.3 In a highly corrosive environment, such as moisture or acidity that may damage the elastomer or cause a short circuit on the sensor, the sensor should be
Choose a sensor with plastic spraying or stainless steel cover on the outer surface, which has good corrosion resistance and good airtightness.
4.4 The influence of electromagnetic field on the disordered signal output by the sensor.
In this case, the shielding properties of the sensor should be strictly checked to see if it has good anti-electromagnetic capabilities.
4.5 Flammable and explosive materials not only cause complete damage to the sensor, but also cause great harm to other equipment and personal safety.
threaten. Therefore, sensors working in flammable and explosive environments have higher requirements for explosion-proof performance: explosion-proof sensors must be used in flammable and explosive environments. The sealed outer cover of this sensor must not only consider its airtightness, but also It is necessary to consider the explosion-proof strength, as well as the waterproof, moisture-proof and explosion-proof properties of the cable lead. Secondly, select the number and range of load cells:
The selection of the number of sensors is based on the purpose of the electronic scale and the number of points that the scale needs to support (the number of support points should be determined based on the principle of making the geometric center of gravity of the scale coincide with the actual center of gravity). Generally speaking, if the scale body has several support points, several sensors will be selected. However, for some special scale bodies such as electronic hook scales, only one sensor can be used. For some electromechanical scales, the sensor selection should be determined based on the actual situation. number. The selection of the sensor range can be determined based on a comprehensive evaluation of factors such as the maximum weighing value of the scale, the number of sensors selected, the weight of the scale body, the maximum possible eccentric load and dynamic load, etc.