Load cell selection

The load cell is actually a device that converts the quality signal into a measurable electrical signal output. The actual working environment of the sensor should be considered first when using the sensor. 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 cylinder type and so on.

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.

The S-type sensor is the most common type of sensor, mainly used to measure the tension and pressure between solids. It is also called a pull-pressure sensor in general. Because its shape is like an S shape, it is also called S-type weighing. Sensor, this sensor is made of alloy steel material, rubber sealing protection treatment, easy to install, convenient to use, suitable for electronic force measurement and weighing systems such as crane scales, batching scales, and machine scales.

The resistance strain type load cell [3] is based on the 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. After the resistance strain gauge 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 electricity. Signal process.
Load cell
Load cell
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.
1. Resistance strain gauge
The resistance strain gauge is to mechanically distribute a resistance wire on a substrate made of organic material, which becomes a 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 recorded as S, and its resistivity is recorded as ρ. 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, it can also be proved by experiments that the resistivity of this metal resistance wire will also change after deformation, which is recorded as Δρ.
Calculate the total differential of the formula (2--1), that is, find out how much the resistance value of the resistance wire changes 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 the mechanics of materials
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)

in
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 properties 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 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)
Second, the elastomer
The elastomer is a structural member with a special shape. It has two functions. The first is that it bears the external force of the load cell, and the external force produces a reaction force to achieve a relatively static balance; second, it must generate a high-quality strain field (zone) to stick in this area The resistance strain gauge is ideal to complete the conversion task of strain jujube electrical signals.
Take the elastic body of the load cell as an example to introduce the stress distribution.
It 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 will increase, while the lower part of the strain gauge will be compressed and the resistance 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" situations, while the strain gauges actually feel the "average" state.

Three, 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.