Weighing sensor is a kind of force that can convert gravity into electrical signal - electrical conversion device, which is a key component of electronic weighing device.

There are many kinds of sensors that can realize force-electric conversion, common ones are resistance strain type, electromagnetic force type and capacitance type. The electromagnetic force type is mainly used for electronic balances, the capacitive type is used for some electronic hanging scales, and the vast majority of weighing products are still used by resistance strain type weighing sensors. The electropositive strain type weighing sensor has the advantages of simple structure, high accuracy, wide application range, and can be used in relatively poor environment. So the resistance strain gauge weighing sensor has been widely used in the weighing apparatus.

The resistance strain gauge weighing sensor is mainly composed of elastomer, resistance strain gauge and compensation circuit. Elastomer is the loading element of the load cell and is made of high quality alloy steel, stainless steel and high quality aluminum profiles. The resistance strain gauge is made of metal foil corrosion into a grid shape, four resistance strain gauge in the structure of the bridge glued to the elastic body, in the absence of force, the resistance value of the four resistors of the bridge is equal, the bridge is in a state of balance, the output is zero. When the elastomer deforms under stress, the resistance strain gauge deforms with it. In the process of elastic force buckling, two strain gauges are strained, the metal wire becomes longer, and the positive value increases. When the other two pieces are pressed, the resistance value decreases. This leads to the original balance of the bridge imbalance, the voltage difference generated at both ends of the bridge, the voltage difference is proportional to the size of the force of the elastomer, the detection of this voltage difference, you can get the size of the sensor by gravity, the voltage signal after the instrument detection but after calculation, you can get the corresponding weight value.

In order to apply to the installation needs of various weighing structures, the weighing sensor has been made into a variety of structural forms, and the name of the sensor is often called according to its shape. Such as bridge sensors (mainly used in truck scales), cantilever beam type (ground scale, hopper scale, truck scale), column type (truck scale, hopper scale), box type (platform scale), S type (hopper scale) and so on. A weighing carrier often has a variety of structural forms of sensors to choose from, if the sensor selection is appropriate, it is very helpful to improve the performance of the weighing device.

There are many specifications of resistance strain gauge load cells, ranging from a few hundred grams to several hundred tons. When choosing the range of the weighing sensor, it should be determined according to the maximum weighing weight used, and its empirical formula is:

Total sensor load (maximum allowable load of a single sensor X number of sensors)=1/2~2/3 of the maximum weighing weight.

The accuracy level of the weighing sensor is divided into four levels: A, B, C and D. Different levels have different margins of error. Class A sensors are the most demanding. The number after the grade indicates the calibration index value, and the larger the number, the better the sensor quality. For example, C2 indicates class C, 2000 calibration index values; C5 stands for Class C, 5000 calibration scale values. Obviously C5 is higher than C2. The commonly used level of sensors is C3 and C5, and these two levels of sensors can be used to make electronic weighing instruments with an accuracy level of Ⅲ.

The error of load cell is mainly caused by nonlinear error, hysteresis error, repeatability error, creep, zero point temperature additional error and rated output temperature additional error.

The emergence of digital sensors in recent years, the A/D conversion circuit and CPU circuit into the sensor inside, the sensor output is no longer an analog voltage signal, but a processed weight digital signal, which brings the following advantages:

1. The instrument can collect the signal of each digital sensor separately, and calibrate each sensor separately through linear equation calculation, which makes it possible to complete the four-corner error correction at one time. The most troublesome problem in the use of analog sensors is the four-angle error correction, which is often adjusted repeatedly to meet the requirements, and each adjustment is to move the heavy weights around, which is time-consuming and laborious.

2. Because the instrument can detect the signal of each sensor, the problem of any sensor can be observed from the instrument, which is convenient for repair and maintenance.

3. The digital sensor uses 485 or 232 interfaces to transmit digital signals, which can be transmitted far and free from interference. The problems of remote transmission difficulty and easy interference of analog signal are overcome.

4. Inside the digital sensor, the errors of the sensor can be corrected through the microprocessor, making the output sensor data more accurate.

Weighing sensor is regarded as the nervous system of electronic weighing device, and its performance determines the accuracy and stability of electronic weighing device to a large extent. When designing an electronic weighing device, the problem of how to select a sensor is often encountered. A load cell is actually a device that converts a mass signal into a measurable electrical output. With the sensor, we must first consider the actual working environment of the sensor, which is crucial to the correct selection of the sensor, which is related to whether the sensor can work normally and its safety and service life, and even the reliability and safety of the entire weighing device.

The ambient noise caused by the sensor mainly has the following aspects:

(1) The high temperature environment causes problems such as the library of coating materials, the development of solder joints, and the structural change of the internal stress of elastomers. For the sensor working in high temperature environment, high temperature sensor is often used; In addition, insulation, water cooling or air cooling devices must be added.

(2) Dust, moisture on the sensor caused by short circuit. Sensors with high tightness should be selected under this environmental condition. The sealing methods of different transmitters are different, and there are great differences in their airtightness.

Common seals are filled or coated with sealant; Rubber pad mechanically tight seal; Welding (ammonia arc welding, ion beam welding) and vacuum nitrogen seal.

From the sealing effect, welding seal is the best, filling coated sealant is the worst. For sensors that work in clean and dry coal environments indoors, you can choose to use the glue sealed sensor, and for some sensors that work in damp and dusty environments, you should choose the diaphragm hot sleeve seal or diaphragm welding seal, vacuum nitrogen filled sensor.

(3) In a highly corrosive environment, such as moisture, acidity to the sensor caused by elastomer damage or short circuit, should choose the outer surface of the spray or stainless steel cover, good corrosion resistance and good airtight sensor.

(4) The influence of electromagnetic field on the sensor output disturbance signal. In this case, the shielding of the sensor should be strictly checked to see whether 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 working in flammable and explosive environments put forward higher requirements for explosion-proof performance: explosion-proof sensors must be selected in flammable and explosive environments, and the sealed outer cover of this sensor should 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 leadout head. Secondly, the selection of the number and range of sensors:

The selection of the number of sensors is based on the use of the electronic weighing device, the number of points that the scale body needs to support (the support number should be determined according to the principle of making the geometric center of gravity of the scale body and the actual center of gravity). In general, the body has several support points to choose a few sensors, but for some special scales such as electronic hook scales can only use a sensor, some electromechanical combined scales should be based on the actual situation to determine the number of selected sensors.

The selection of sensor range can be determined according to the comprehensive evaluation of the maximum weighing value of the scale, the number of selected sensors, the weight of the scale body, the maximum possible partial load and dynamic load. In general, the closer the sensor's range is to the load assigned to each sensor, the higher the accuracy of its weighing. However, in actual use, due to the load added to the sensor in addition to the object, there are also loads such as body weight, tare, partial load and vibration impact, so when selecting the sensor range, many factors should be considered to ensure the safety and life of the sensor.

The calculation formula of the sensor is determined by a large number of experiments after fully considering various factors affecting the scale body.

The formula is as follows: C=K×(W max+W)/NC- the rated range of a single sensor; K- insurance factor (generally between 1.3 and 1.5);

W max- the maximum value of the net weight of the said object; W- body weight; N- The number of support points used by the scale body

According to experience, the weighing sensor should generally work in its 30%~70% range, but for some weighing instruments with large impact in the use process, such as dynamic rail scales, dynamic truck scales, steel scales, etc., when selecting sensors, it is generally necessary to expand its range, so that the sensor works within 20%~30% of its range. The weighing reserve of the sensor is increased to ensure the safety and life of the weighing sensor.

Again, the scope of application of various types of sensors should be considered.

The choice of sensor type mainly depends on the type of weighing and installation space, to ensure proper installation, safe and reliable weighing; On the other hand, consider the manufacturer's suggestion. Manufacturers generally specify the scope of application of the sensor according to the stress of the sensor, performance indicators, installation form, structural type, material of the elastomer and other characteristics, such as aluminum cantilever beam sensor is suitable for pricing scales, platform scales, case scales, etc. Steel cantilever beam sensor is suitable for hopper scale, electronic belt scale, sorting scale, etc. Steel bridge sensor is suitable for rail scale, automobile scale, crane scale, etc. Column sensor is suitable for automobile scale, dynamic track scale, large tonnage hopper scale, etc.

Finally, the sensor accuracy level should 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, do not simply pursue high-grade sensors, but consider both the accuracy requirements of the electronic scale and its cost.

The selection of the sensor grade must meet the following two conditions:

1. Meet the input requirements of the instrument. The weighing display instrument displays the weighing result after the output signal of the weighing sensor is amplified and processed by A/D conversion. Therefore, the output signal of the weighing sensor must be greater than or equal to the input signal size required by the instrument, that is, the output sensitivity of the weighing sensor is substituted into the matching formula of the sensor and the instrument, and the calculation junction must be greater than or equal to the input sensitivity required by the instrument. Matching formula of weighing sensor and instrument:

Weighing sensor output sensitivity x excitation power supply voltage x maximum weighing/scale number x number of sensors x sensor range

2. Meet the accuracy requirements of the whole 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 weighing sensor, the accuracy of the weighing sensor should be slightly higher than the theoretical calculation value, because the theory is often limited by objective conditions, such as the strength of the scale body. 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, so it is necessary to improve the requirements from all aspects, and consider the economic benefits to ensure that the purpose is achieved.