Weighing sensor is actually a quality signal into electrical signal output device can be measured. With sensor should first consider the actual work environment, it is very important to correctly choose the weighing sensor, it is related to sensor can work normally and its safety and service life, and even the whole weighing apparatus of the reliability and security. In weighing transducer on the basic concept of the main technical indicators and evaluation methods, a new JiuGuo marked qualitative difference. S type, cantilever type, main hub and spokes, plate ring, bellows, bridge, column cylinder etc. Several kinds of style.

Weighing apparatus used by a force sensor. It can function in the gravity on the object to be tested in a certain proportion into quantifiable output signal. Considering the different locations of acceleration of gravity and buoyancy influence on transformation, the main performance index weighing sensors are linear, hysteresis error, repeatability error and creep error, zero point temperature characteristic and temperature characteristic sensitivity, etc. In all kinds of weighing apparatus and quality measurement system, usually with integrated comprehensive control accuracy of the sensor error, and take comprehensive error band and instruments (figure 1), in order to choose corresponding to a certain accuracy of weighing apparatus of weighing sensor. International legal metrology (OIML) regulation, the error of the sensor with the delta accounts for 70% of the error of weighing belt Δ, weighing sensor, the hysteresis error and linear error in the specified temperature range due to the influence of temperature on the sensitivity caused by the error, etc. The sum total of no more than the error with the delta. This allows manufacturers to measure to adjust the total error of each component, to obtain the desired accuracy.

Resistance strain type

When its use of resistance strain slice deformation resistance change the principle of work (figure 11). Mainly by the elastic element, resistance strain gauge, measurement circuit and transmission cables of four parts.

Resistance strain type weighing sensor is based on the principle that elastomer (elastic element, sensitive beam) elastic deformation under the action of external force, made in its surface resistance strain gages (cell) also along with the deformation and deformation resistance strain gauge, its value will change (increase/decrease), and then through the corresponding measurement circuit convert the resistance to electrical signals (voltage or current), so as to complete the process of external force transform into electrical signals.

Thus, resistance strain gauge, elastomer and detection circuit is indispensable in resistance strain type weighing transducer several main parts.

One, the resistance strain gauge

Resistance strain gauge is a root of the distribution of resistance wire machinery on a substrate made of organic materials, which become a strain gauge. He is an important parameter of sensitive coefficient K. We introduce the meaning of it.

Equipped with a metal wire, its length is L, cross section is the circular radius r, its area as S, the resistivity as rho, the poisson coefficient of the material is mu. When this wire resistance was not affected by external force, its resistance to R:

R = rho L/S (Ω) (2, 1)

When he at both ends of the force F, will be extended, namely deformation. Set its elongation Δ L, its cross-sectional area is narrow, namely reduce Δ it cross section of the round radius r. In addition, still can use experiments show that the metal wire after deformation, the resistivity will also change, as Δ rho.

The type (2, 1) the perfect differential is to find the resistance wire after elongation, his resistance to change. We have:

Δ R = Δ rho L/S + Δ rho L/S - Δ S rho L/S2 (2, 2)

Use type (2, 1) removing type (2, 2)

Δ R/R = Δ rho/rho + Δ L/L - Δ S/S (2, 3)

In addition, we know the cross-sectional area of the wire S = PI r2, the Δ Δ S = 2 PI r * r, so

Δ Δ S/S = 2 r/r (2, 4)

We know from mechanics of materials

Δ r/r = - mu Δ L/L (2, 5)

Among them, the minus sign said elongation, radius direction is narrow. Mu is the said material poisson coefficient of transverse effect. Fantasy (2, 4), (2-5) substitution (2, 3), there is

Δ R/R = Δ rho/rho + Δ L/L + 2 mu Δ L/L

= (1 + 2 mu (Δ rho/rho)/(Δ L/L)) * Δ L/L

= K * Δ L/L (2, 6)

Among them

K = 1 + 2 mu + (Δ rho/rho)/(Δ L/L) (2, 7)

Type (2-6) illustrates the resistance rate of change of resistance strain gauge relative changes (resistance) and the resistance wire elongation of the relationship between relative changes (length).

To be sure: the size of the sensitivity coefficient K value is determined by the production of the nature of the metal wire materials to a constant, it has nothing to do with strain gauge the shape, size, different materials of K value generally between 1.7 3.6; Secondly K value is a dimensionless quantity, namely, it has no dimensions.

In material mechanics Δ L/L said as strain, as epsilon, use it to show flexibility often seems too big, is not very convenient, often put it as a unit, one over one million of as mu epsilon. In this way, (2-6) often writing:

Epsilon Δ R/R = K (2, 8)

Second, the elastomer

Elastomer is a structure with special shape. It features two, first, is it inherit the weighing sensor by external force, produces reaction to external forces, achieve relatively static balance; Second, it will produce a high quality strain field (area), make the paste resistance strain gauge is ideal in this area for strain jujube electrical signal conversion tasks.

In the weighing sensor elastomer, for example, to introduce the stress distribution.

Equipped with a rectangular cantilever beam with underground hole.

Being the center is under the pure shear stress at the bottom of the hole, but the upper and lower part will appear tensile and compressive stress. Principal stress direction for god, and one for compression, if stick strain gauge, here is a strain gauge half will stretch and resistance increase, and the lower co-produced by the compression of the strain gauge, resistance decrease. Huang listed below at the bottom of the hole center strain expression, and no longer is derived.

Epsilon = (3 q + mu (1) / 2 eb) * (B (H2 - H2) + bh2)/(B (H3 - H3) + bh3) (9) 2 -

Among them: Q - section on shear; E - the young's modulus: mu - poisson coefficient; B, B, H, H - beam geometry.

To be sure, the stress state of the above analysis is "local" situation, and the strain gauge actual feeling is "average" condition.

Three, the detection circuit

The function of the detection circuit is to the resistance of the resistance strain gauge change into voltage output. Because wheatstone bridge has many advantages, such as can restrain the influence of temperature change, can inhibit the side force, can be more convenient for solving the problem of compensation of weighing sensor, etc., so the wheatstone bridge has been widely used in the weighing sensor.

For the whole bridge arm bridge has the highest sensitivity, such as the arm parameters, the influence of various interference easily offset each other, so the weighing sensor adopts full bridge arm bridge, etc.

Commonly used material

The stand or fall of weighing sensor performance depends largely on the choice of materials. Weighing sensor material includes the following several parts: strain gauge material, elastomer material, patch adhesive material, sealant material, sealing material lead and lead materials.

Strain gauge and resistance element material

Strain gauge is induction part of weighing sensor, it will be the size of the external force into electrical signal output, is the most important part of the sensor, the commonly used strain gauge base material with polymer thin film materials, strain material usually for high purity constantan. The performance of the strain gauge not only related to substrate and constantan purity, also related to the manufacturing process. Improve the level of technology and improve the sensor performance is a very important aspect.

Elastomer materials

Weighing sensor elastomer is the function of external force, it must be under the same force, deformation, produced because of the strain gauge is pasted on the elastomer, the deformations of the elastomer is strain gauge deformation; At the same time it must be reset, disappeared in the external force, can be reset automatically. Elastomer material usually choose all kinds of metal, mainly of aluminum alloy, stainless steel, alloy steel and so on.

The patch adhesive materials

Patch adhesive is to make the strain gauge and elastomer firmly fixed together, make them produce deformation always consistent. Thus, patch adhesive is also an important part. At the beginning of the 21st century, the use of call more patch adhesive is a two-component epoxy polymer adhesive series. At the beginning of the 21st century, its performance and its purity, mixed mode, storage methods, cure time, cure time relationship is very big, watch it carefully before use according to the detailed introduction.

Sealant material

Early weighing sensor sealed with sealant, later, thanks to the development of manufacturing technology in welding technology can improve greatly the sensor's stability and service life. The early 21st century many welding technique is adopted, but some still need to apply some important parts of the sealant. Sealant commonly used silica gel, silica gel has advantages of good stability, can be moistureproof, corrosion resistance, insulation performance is also very good.

Lead sealing and lead materials

Sensor output pin if it is not fixed, happen damage or loose, lead to unstable signal or no output. At the beginning of the 21st century sensor output with the method of the connector, the connector of material and fastening strength will also affect the output. Had better use the connector with sealant. Internal lead also needs to be fixed to prevent its moved around. Lead quality is also very important, its material performance from high to low order followed by silver, copper and aluminum wire. If around the high frequency signal and radio interference is serious, still need to use shielded cable; In the corrosive environment and flammable and explosive occasions need to adopt anti-corrosion flame retardant and explosion-proof cable, plus casing to protect.