The performance indicators of weighing instruments used in industrial enterprises are usually characterized by accuracy (also known as precision), deterioration, and acuity. Indicators are usually calibrated for accuracy, deterioration and acuity.
Variation refers to the maximum difference between the indicated value of the instrument when the measured variable (which can be understood as the input signal) of the weighing instrument deviates from the difference to the same value multiple times. It is probably said that the instrument is measured under stable external conditions The change in paraindicators from small to large (forward characteristics) and the measured paraindicators change from large to small (reverse characteristics) are not uniform. The difference between the two is the instrument deterioration. Reliability The reliability of weighing control instruments is another important performance index sought by instrument workers in chemical enterprises. Reliability and instrument maintenance are complementary. High instrument reliability means that instrument maintenance is small. On the contrary, if instrument reliability is poor, instrument maintenance is large. To deal with the detection and process control instruments of chemical enterprises, most of them are installed on process pipelines, various towers, kettles, tanks, and vessels.
The stability of the weighing instrument in the load cell is within the specified working conditions, and the ability of the weighing instrument to maintain certain performance over time is called stability (degree). The stability of the instrument is the performance index of a chemical enterprise’s instrument workers in one day. Because chemical companies use instruments in a harsh environment, and the change in temperature and pressure of the medium to be measured is also relatively large. When instruments are used in this environment, the ability to use certain parts of the instrument to connect firmly over time will weaken. The stability will fall. There is no quantitative value for the stability or characterization of the indicator. Chemical companies usually use the zero drift of the indicator to measure the stability of the indicator. The stability of the weighing instrument is directly related to the scope of use of the instrument, and occasionally directly affects the chemical production. The impact of poor stability is greater than the impact of the accuracy drop of the double instrument on the chemical production. The stability of the instrument is not good, and the amount of instrument maintenance is large, which is the last thing that instrument workers want to appear.
The acuity of the weighing instrument is occasionally called the “magnification ratio”, which is also the slope of the static characteristics of the instrument at each point on the tangent line. Increasing the magnification factor can improve the acuity of the instrument. Simply increasing the acuity does not change the performance of the instrument, that is, the accuracy of the weighing instrument is not improved. On the contrary, there will be occasional oscillation characteristics, resulting in unstable output. The acuity of the indicator should be connected to an appropriate amount.
For most customers, although the accuracy of the instrument is an important indicator, in actual use, the stability and reliability of the instrument are more emphasized, because the detection and process control instruments of chemical enterprises are few, and a large number of instruments are used for measurement. It is used for detection. In addition, the stability and reliability of measuring instruments used in the process control system are more critical than accuracy.
With the upgrading of instruments, especially the introduction of microelectronics skills into the weighing instrument manufacturing industry, the reliability of the instruments is greatly improved. Instrument manufacturers are more and more cherish this performance index, usually using the average clear time MTBF to describe the reliability of the instrument. The MTBF of a fully intelligent weighing transmitter is about 10 times higher than that of an ordinary non-intelligent instrument such as an electric Ⅲ transmitter. The weighing instrument must be digitally calibrated with the load cell before use. Calibration is actually to calibrate the weighing instrument with standard weights. The calibration coefficient relative to this group of load cells is stored inside the calibrated instrument. With this coefficient, the indicator can convert the analog signal of the load cell into a digital weight display.
Plate ring type load cell uses and characteristics: compact structure, good protection performance. High precision and good long-term stability. It is suitable for the measurement of hook scales, electromechanical combined scales and other force values.
The digital load cell is a force-to-electric conversion device that can convert gravity into an electrical signal. It mainly refers to a set resistance strain gauge load cell, an electronic amplifier (abbreviated as AMC in English), and analog-to-digital conversion technology (abbreviated as ADC in English). , Microprocessor (referred to as MCU) in a new type of load cell.
Features and applications
The development of digital weighing load cell and digital indicatoring technology has gradually become the new favorite in the field of weighing technology. It is emerging in this field with its advantages of simple and efficient debugging and strong ability to adapt to the scene.
S-type load cell is the most common load cell in the load cell. It is mainly used to measure the tension and pressure between solids. It is also called a pull-pressure load cell for general use. Because its shape is like an S shape, it is also called habitually. S-type weighing load cell, this load cell is made of alloy steel material, rubber sealing protection treatment, easy to install, convenient to use, suitable for electronic force measuring and weighing systems such as crane scales, batching scales, and machine scales.
A component that directly feels the measured (quality) and outputs other quantities that have a certain relationship with the measured. For example, the elastic body of the resistance strain type load cell transforms the mass of the measured object into deformation; the elastic body of the capacitive load cell transforms the measured mass into displacement.
is also called a load cell element, which converts the output of the sensitive element into a signal that is convenient for measurement. For example, the resistance strain gauge of the resistance strain type load cell (or called the resistance strain gauge) converts the deformation of the elastic body into the change of resistance; the capacitor of the capacitive load cell converts the displacement of the elastic body into the change of the capacitance . Sometimes some components have both the functions of sensitive components and transforming components. Such as the piezoelectric material of the voltage type load cell, under the action of external load, it outputs electricity while deforming.
Transform the output of the transforming element into an electrical signal to provide convenience for further transmission, processing, display, recording or control. Such as the bridge circuit in the resistance strain load cell, the charge preamplifier of the piezoelectric load cell.
provides energy for the electrical signal output of the load cell. Generally, the load cell needs an external power supply to work. Therefore, as a product, the power supply requirements must be marked, but it is not a component of the load cell. Some load cells, such as magnetoelectric speed load cells, can work normally without auxiliary power supply due to their large output energy. So not all load cells need to have auxiliary power.
Commonly used materials
The performance of the load cell depends largely on the choice of manufacturing materials. The load cell material includes the following parts: strain gauge material, elastomer material, patch adhesive material, sealant material, lead sealing material and lead material.
Strain gauge and resistance element materials
Strain gauge is the sensing part of the load cell. It converts the magnitude of external force into electrical output. It is the most important part of the load cell. The commonly used strain gauge substrate is made of polymer film material, and the strain material is usually high-purity constantan. The performance of the strain gauge is not only related to the purity of the substrate and constantan, but also related to the manufacturing process. Improving the level of process technology is also a very important aspect of improving load cell performance.
The function of the elastic body of the load cell is to transmit external force. It must have the same deformation when subjected to the same force, because the strain gauge is pasted on the elastic body, and the deformation of the elastic body is the deformation of the strain gauge; at the same time, it must It is resettable and can be reset automatically when the external force disappears. Elastomer materials usually choose various metals, mainly aluminum alloy, stainless steel, alloy steel and so on.
Patch adhesive material
Patch adhesive is to firmly fix the strain gauge and the elastic body together, so that the deformation produced by them is always consistent. It can be seen that the patch adhesive is also an important component. At the beginning of the 21st century, the most commonly used patch adhesive was a two-component polymer epoxy series adhesive. At the beginning of the 21st century, its performance has a lot to do with its own purity, mixing method, storage time, curing method, curing time, etc. Please read its detailed introduction carefully before using it.
Early load cell seals all used sealant. Later, due to the development of manufacturing technology, welding technology can improve the stability and service life of the great load cell. Although many welding techniques were adopted in the early 21st century, some important parts still need to be coated with some sealant. The sealant generally uses silica gel, which has the advantages of good stability, moisture resistance, corrosion resistance, and excellent insulation properties.
Lead seal and lead material
If the load cell output lead is not fixed, it will be damaged or loose, resulting in unstable signal or no output. At the beginning of the 21st century, the output of load cells all adopted the way of connectors, and the material and tightening force of the connectors will also affect the output. It is best to use a connector and sealant together. The inner lead also needs to be fixed to prevent it from moving around. The quality of the lead is also very important. The order of its material properties from high to low is silver-plated, copper wire and aluminum wire. If the surrounding high-frequency signal and radio wave interference are serious, shielded cables should be used; in corrosive environments and flammable and explosive occasions, anti-corrosion, flame-retardant and explosion-proof cables should be used for protection with additional sleeves.
The load cell should be handled with care, especially for the small-capacity load cell using aluminum alloy material as the elastic body. Any shock or drop caused by vibration is likely to cause a large output error. When designing the loading device and installing it, it is necessary to ensure that the loading force is applied to the load cell. The load cell’s force axis coincides so as to minimize the influence of the inclined load and the eccentric load.
In terms of level adjustment. If you are using a load cell, the installation plane of its base should be adjusted until it is level with a spirit level; if multiple load cells are measuring at the same time, then the installation surface of their base should be kept on a level surface as much as possible. The purpose of this is mainly It is to ensure that the strength of each load cell is basically the same.
Determine the rated load of the load cell used by selecting the range of the load cell in its description. To prevent chemical corrosion. It is advisable to coat the outer surface of the load cell with petroleum jelly during installation. Avoid direct sunlight and use on sites with drastic changes in ambient temperature. A bypass made of copper braided wire is added to both ends of the load cell loading device.
The cable should not be lengthened by itself. When it is really necessary to lengthen, the joint should be soldered and moisture-proof sealant should be added. It is best to use some baffles around the load cell to cover the load cell. The purpose of this is to prevent debris from falling into the moving part of the load cell and affect its measurement accuracy.
The cable of the load cell should be far away from the strong power line or the place with pulse wave. When it is impossible to avoid competition, the cable of the load cell should be inserted into the iron pipe alone, and the connection distance should be shortened as much as possible.
Determine the rated load of the load cell used according to the load cell range selection in its description. Although the load cell itself has a certain overload capacity, it should be avoided as much as possible during installation and use. Sometimes a short-term overload may cause permanent damage to the load cell.
In high-precision applications, the load cell and instrument should be used after preheating for 30 minutes.
Build a load detection branch road at the entrance of the expressway. When the truck passes the dynamic weighing bridge, the load cell and electronic scale will check and judge by themselves, and give signals to control the traffic lights. In this way, we can well know whether the vehicle is overweight, and then consider whether or not this vehicle should pass. This type of load cell used on highways requires a large range and not particularly high accuracy requirements, but long-term stability must be good. With the development of load cells and other electronic equipment, they will become more and more intelligent, so as to realize unmanned control. It can prevent overweight vehicles from passing, and can also charge vehicles based on their weight.
With the expansion of the market, there are more and more manufacturers of load cells. How to do better in the market is not difficult to analyze. Only by continuously improving load cell technology and services can we be at the top of the market. With the advent of the new technological revolution, China and the world have begun to enter a new information age. In the process of using information, the first thing to be solved is to obtain accurate and reliable information, and load cells are the most important way and means to obtain information in the fields of nature and production.
In modern industrial production and automated production processes, a variety of load cells are needed to monitor and control various parameters in the production process. The function of the load cell is to make the equipment work in a normal or optimal state and make it out of production. The products reach the best quality. It can be said that without numerous excellent load cells, modern production has lost its foundation. In this way, the load cell will have a bright future in this intelligent production industry.