The load cell is actually a device that converts the quality signal into a measurable electrical signal output. The actual working environment in which the sensor is used should first be considered. This is very important for the correct selection of the load cell. It is related to the normal operation of the sensor, its safety and service life, and the reliability and safety of the entire weighing instrument. sex. In the basic concepts and evaluation methods of the main technical indicators of the load cell, there are qualitative differences between the new and the old national standards. There are several styles such as S type, cantilever type, spoke type, plate ring type, bellows type, bridge type, column type and so on.
- Basic introduction
The old national standard considers the combination of “weighing” and “force measurement” sensors with completely different application objects and environmental conditions, and does not distinguish between test and evaluation methods. The old national standard has a total of 21 indicators, all of which are tested at room temperature; and the maximum error among the 6 indicators of non-linearity, hysteresis error, repeatability error, creep, additional error of zero temperature and additional error of rated output temperature is used to determine the weighing The accuracy level of the sensor is expressed as 0.02, 0.03, and 0.05 respectively. A force sensor used on weighing instruments. It can convert the gravity acting on the measured object into a measurable output signal in a certain proportion.
Sensor structure diagram
Taking into account the influence of gravitational acceleration and air buoyancy at different locations of use on the conversion, the performance indicators of the load cell mainly include linearity error, hysteresis error, repeatability error, creep, zero temperature characteristics and sensitivity temperature characteristics. In various weighing instruments and quality measurement systems, the integrated error is usually used to bring the accuracy of the integrated control sensor, and the integrated error band is linked with the error band of the weighing instrument in order to select a weighing sensor corresponding to a certain accuracy of the weighing instrument. The International Organization for Legal Metrology (OIML) stipulates that the error band δ of the sensor accounts for 70% of the error band Δ of the weighing instrument. The linearity error, hysteresis error of the load cell, and the error caused by the influence of temperature on the sensitivity within the specified temperature range, etc. The sum cannot exceed the error band δ. This allows the manufacturer to adjust the various components that make up the total measurement error to obtain the desired accuracy.
According to the conversion method, the load cell is divided into 8 types: photoelectric, hydraulic, electromagnetic, capacitive, magnetic pole change form, vibration type, gyro ceremony, resistance strain type, etc. The resistance strain type is the most widely used. It works based on the principle that the resistance of the resistance strain gauge changes when it deforms. It is mainly composed of 4 parts: elastic element, resistance strain gauge, measuring circuit and transmission cable.
1, sensitive components
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.
2, conversion element
is also called a sensor element, which converts the output of a sensitive element into a signal that is convenient for measurement. For example, the resistance strain gauge (or resistance strain gauge) of the resistance strain type load cell 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 load cell, under the action of external load, it outputs electricity while deforming.
3, measuring element
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.
4, auxiliary power supply
provides energy for the electrical signal output of the sensor. 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 sensors, such as magnetoelectric speed sensors, can work normally without auxiliary power supply due to their large output energy. So not all sensors need to have auxiliary power.
Resistance strain type weighing sensor is based on such a principle: the elastic body (elastic element, sensitive beam) produces elastic deformation under the action of external force, so that the resistance strain gauge (conversion element) pasted on its surface also deforms, resistance strain After the sheet is deformed, its resistance value will change (increase or decrease), and then the resistance change will be converted into an electrical signal (voltage or current) by the corresponding measuring circuit, thus completing the transformation of external force into an electrical signal process.
It can be seen that resistance strain gauges, elastomers and detection circuits are indispensable parts of resistance strain load cells. The following briefly discusses these three aspects.
- Resistance strain gauge
Resistance strain gauge is a piece of resistance wire mechanically distributed on a substrate made of organic material, that is, a piece of strain gauge. One of his important parameters is the sensitivity coefficient K. Let’s introduce its meaning.
There is a metal resistance wire with a length of L and a circular cross section with a radius of r. Its area is denoted as S, and its resistivity is denoted as ρ, and the Poisson coefficient of this material is μ. When this resistance wire is not subjected to external force, its resistance value is R:
R = ρL/S (Ω) (2—1)
When the two ends of him are subjected to F force, they will stretch, that is to say, they will deform. Assuming its extension ΔL, its cross-sectional area is reduced, that is, its cross-sectional circle radius decreases by Δr. In addition, experiments can also be used to prove that the resistivity of this metal resistance wire will also change after it is deformed, which is recorded as Δρ.
Calculate the total differential of formula (2–1), that is, find out how much the resistance value of the resistance wire has changed after it is stretched. We have:
ΔR = ΔρL/S + ΔLρ/S –ΔSρL/S2 (2-2)
Use formula (2–1) to remove formula (2–2) to get
ΔR/R = Δρ/ρ + ΔL/L – ΔS/S (2—3)
In addition, we know that the cross-sectional area of the wire S = πr2, then Δs = 2πr*Δr, so
ΔS/S = 2Δr/r (2—4)
We know from material mechanics
Δr/r = -μΔL/L (2-5)
Among them, the negative sign indicates that the radial direction is reduced when it is stretched. μ is the Poisson coefficient representing the lateral effect of the material. Substituting formula (2-4) (2-5) into (2–3), we have
ΔR/R = Δρ/ρ + ΔL/L + 2μΔL/L
=(1 + 2μ(Δρ/ρ)/(ΔL/L))*ΔL/L
= K *ΔL/L (2–6)
K = 1 + 2μ +(Δρ/ρ)/(ΔL/L) (2–7)
Equation (2–6)) illustrates the relationship between the resistance change rate (relative change in resistance) of the resistance strain gauge and the elongation rate (relative change in length) of the resistance wire.
It should be noted that the sensitivity coefficient K value is a constant determined by the nature of the metal resistance wire material. It has nothing to do with the shape and size of the strain gauge. The K value of different materials is generally between 1.7 and 3.6; Secondly, the K value is a dimensionless quantity, that is, it has no dimensions.
In material mechanics, ΔL/L is called strain, and it is recorded as ε. It is often too large to express elasticity, which is very inconvenient.
It is often used as a unit of one millionth and recorded as με. In this way, the formula (2–6) is often written as:
ΔR/R = Kε (2-8)
Elastomer is a structural member with a special shape. It has two functions. The first is that it bears the external force received by the load cell, and the external force produces a reaction force to achieve a relative static balance; second, it has to generate a high-quality strain field (zone) so that it can be pasted in this area. The resistance strain gauge is ideal to complete the conversion task of strain and electrical signal.
Take the elastic body of the load cell as an example to introduce the stress distribution.
is provided with a rectangular cantilever beam with a hole.
The center of the bottom of the hole is subjected to pure shear stress, but tensile and compressive stresses will appear in the upper and lower parts. The main stress direction is tension and compression. If the strain gauge is attached here, the upper part of the strain gauge will be stretched and the resistance value will increase, while the lower part of the strain gauge will be compressed and the resistance value will decrease. The strain expression at the center point of the bottom of the hole is listed below, and will not be deduced.
ε = (3Q(1+μ)/2Eb)*(B(H2-h2)+bh2)/(B(H3-h3)+bh3) (2–9)
Among them: Q-shear force on the section; E-Young’s modulus: μ-Poisson coefficient; B, b, H, h-the geometric dimensions of the beam.
It should be noted that the stress states analyzed above are all “local” conditions, while the strain gauges actually feel the “average” state.
- Detection circuit
The function of the detection circuit is to convert the resistance change of the resistance strain gauge into a voltage output. Because the Wheatstone bridge has many advantages, such as the ability to suppress the influence of temperature changes, the side force interference, and the more convenient solution to the compensation problem of the load cell, the Wheatstone bridge is used in the load cell. It has a wide range of applications.
Because the full-bridge equal-arm bridge has the highest sensitivity, the parameters of each arm are the same, and the influence of various interferences is easy to cancel each other, so the load cell adopts the full-bridge equal-arm bridge.
Four, 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.
1, resistance element material
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 sensor. 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 sensor performance.
2, elastomer material
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.
3. Adhesive material
SMD 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 SMD adhesive called Duo is 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.
4, sealant material
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 sensor. Although many welding techniques were used 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.
5, lead seal
If the sensor 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 sensors 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 wire 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 situations, anti-corrosion, flame-retardant and explosion-proof cables should be used, and sleeves should be used for protection.
In addition, the sensitivity of the load cell, the maximum number of divisions, and the minimum calibration division value are also indicators that must be considered in the selection of the sensor.
The number and range of sensors
The number of sensors is selected according to 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 overlaps the actual center of gravity). Generally speaking, if the scale body has several supporting points, several sensors are used.
The range of the sensor can be selected based on the comprehensive evaluation of the maximum weighing value of the scale, the number of sensors selected, the self-weight of the scale, the maximum eccentric load that can be generated, and the comprehensive evaluation of dynamic load factors. An empirical formula verified by a large number of experiments is given below.
The formula is as follows:
Where C is the rated range of a single sensor
W-weight of the scale
Wmax-the maximum value of the net weight of the object being weighed
N-the number of support points used by the scale body
K0-insurance coefficient, generally between 1.2 and 1.3
K2-the center of gravity offset coefficient of the weighing body
K3-wind pressure coefficient
The load cell is actually an output device that converts the quality signal into a measurable electrical signal. When using a sensor, we must first consider the actual working environment in which the sensor is located. This is very important 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. Under normal circumstances, the high temperature environment will cause problems such as melting of the coating material, opening of the solder joints, and structural changes in the stress in the elastomer; dust and humidity will cause short-circuit effects on the sensor; in a highly corrosive environment, it will cause the sensor elastomer Damage or short circuit; electromagnetic field will interfere with sensor output. Under the corresponding environmental factors, we must choose the corresponding load cell to meet the necessary weighing requirements.
Accuracy level selection
The accuracy level of the load cell includes technical indicators such as non-linearity, creep, repeatability, hysteresis, and sensitivity of the sensor.
Application range and purpose
For example, aluminum alloy cantilever beam sensors are suitable for electronic price calculation scales, platform scales, case scales, etc.; steel cantilever beam sensors are suitable for electronic belt scales, sorting scales, etc.; steel bridge sensors are suitable for railroad scales, truck scales, etc.; column type The sensor is suitable for truck scales, dynamic railroad scales, large-tonnage hopper scales, etc. Load cells are mainly used in various electronic weighing instruments, industrial control fields, online control, safety overload alarms, material testing machines and other fields. Such as electronic truck scales, electronic platform scales, electronic forklifts, dynamic axle load scales, electronic hook scales, electronic pricing scales, electronic steel scales, electronic rail scales, hopper scales, batching scales, canned scales, etc.
Six, development trend challenges
With the advent of the new technological revolution, the world has begun to enter the information age. In the process of using information, the first thing to be solved is to obtain accurate and reliable information. Sensors are the main way and means to obtain information in the fields of nature and production. In modern industrial production, especially in automated production processes, various types of information must be used. The load cell monitors and controls the various parameters in the production process, makes the equipment work in the normal state or the best state, and makes the product reach the best quality. Therefore, it can be said that without many excellent sensors, modern production will lose its foundation.
In the research of basic disciplines, sensors have a more prominent position. The development of modern science and technology has entered many new fields: for example, it is necessary to observe the vast universe of thousands of light years at the macro level, and the world of particles as small as cm at the micro level. Vertically, it is necessary to observe the evolution of celestial bodies for hundreds of thousands of years, and the instantaneous response as short as s. In addition, various extreme technology researches that play an important role in deepening the understanding of matter, opening up new energy and new materials, such as ultra High temperature, ultra-low temperature, ultra-high pressure, ultra-high vacuum, ultra-strong magnetic field, ultra-weak magnetic field, etc.
Obviously, to obtain a large amount of information that cannot be directly obtained by human senses, it is impossible without a suitable weighing sensor. The obstacles to many basic scientific researches are the difficulty in obtaining object information, and some new mechanisms and high sensitivity The emergence of detection sensors often leads to breakthroughs in this field. The development of some sensors is often the pioneer in the development of edge disciplines.
The load cell has already penetrated into such a wide range of fields as industrial production, space development, ocean exploration, environmental protection, resource investigation, medical diagnosis, bioengineering, and even cultural relic protection. It is no exaggeration to say that from the vast space To the vast ocean, to all kinds of complex engineering systems, almost every modern project is inseparable from a variety of sensors.
China’s sensor industry is at a critical stage of its development from traditional to new sensors. It embodies the general trend of new sensors to miniaturization, multi-function, digitization, intelligence, systemization and networking.