Resistance strain gauge load cell is based on a principle: elastomers (elastic components, sensitive beam) elastic deformation under external force, so that he pasted on the surface of the resistance strain gauge (conversion element) also accompanied deformation, resistance strain gauge deformed, its resistance will change (increase or decrease), and then by a corresponding change in resistance of the measuring circuit is converted to an electrical signal (voltage or current), thereby completing the process of force is converted into an electrical signal . 
Thus, the resistance strain gauges, elastomers, and detection circuit resistance strain gauge load cell is indispensable main parts. Here's a brief discussion of these three aspects. 
First, the resistance strain gauge resistance of the film is the distribution of a mechanical resistance wire made ??of organic materials on a substrate, it becomes a strain gauge. He is an important parameter is the sensitivity coefficient K. We have to introduce its meaning. 
There is a metal resistance wire, the length L, cross-section is circular with radius r, the area denoted by S, the resistivity denoted by ρ, this material Poisson coefficient μ. When the root of the resistance wire were not the external force, and its resistance value R: 
R = ρL / S (Ω) (2-1) 
When his ends by the force F, it will stretch, ie deformation. Let elongation ΔL, the cross-sectional area is reduced, which reduces its cross-sectional radius Δr. In addition, also proved by experiment, this metal resistance wire after deformation, resistance there will be changes, denoted by Δρ. 
The formula (2 - 1) perfection differential, ie calculated resistance wire elongation after his resistance to change much. We have: 
ΔR = ΔρL / S + ΔLρ / S -ΔSρL/S2 (2-2) 
By the formula (2 - 1) removal of formula (2 - 2) to give 
ΔR / R = Δρ / ρ + ΔL / L - ΔS / S (2-3) 
In addition, we know that the wire cross-sectional area 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) 
Wherein said negative elongation, the radial direction is reduced. μ is the Poisson coefficient of lateral effect materials. Substituting (2-4) (2-5) into (2 - 3), a 
ΔR / R = Δρ / ρ + ΔL / L + 2μΔL / L 
= (1 + 2μ (Δρ / ρ) / (ΔL / L)) * ΔL / L 
= K * ΔL / L (2 - 6) 
Among 
K = 1 + 2μ + (Δρ / ρ) / (ΔL / L) (2 - 7) 
(2 - 6)) describes the resistance strain gauge resistance change rate (relative change in resistance) and a resistance wire elongation (relative change in length) between. 
It should be noted that: the sensitivity value of the size coefficient K is determined by making metal wire material of the nature of resistance is a constant, shape it and the strain gauge, size regardless of the value of K different materials in general - between; followed by K- value is a dimensionless quantity, that is, it does not dimensionless. 
In material mechanics ΔL / L referred to as strain, denoted by ε, it ??is often too large to represent elastic, very convenient 
Often it's one millionth as a unit, denoted by με. Thus, the formula (2 - 6) Chang Writing: 
ΔR / R = Kε (2-8) 
Second, the elastic member 
Elastomer is a special shape structure. It has two functions, first it withstand the load cell force suffered and external force from, to achieve a relatively static equilibrium; Secondly, it is to produce a high quality strain field (area), the paste in this area resistance strain gages ideal strain jujube electrical signal conversion completed tasks. 
In TOLEDO SB series load cell elastomer, for example, to introduce one of the stress distribution. 
Has a rectangular cantilever with a blind hole. 
The bottom center of the blind hole under pure shear stress, but its upper and lower portions of the tensile and compressive stress will appear. Principal stress direction is pulling a god, one for compression, if the strain gauges posted here, you will be subject to the upper half of the strain gauge tensile resistance increases, while the lower half of the strain gauge is compressed, the resistance decreases. Listed below are the center of the bottom of the blind hole expression strain, rather derived. 
ε = (3Q (1 + μ) / 2Eb) * (B (H2-h2) + bh2) / (B (H3-h3) + bh3) (2 - 9) 
Wherein: Q - shear force on the section; E - Young's modulus: μ-Poisson coefficient; B, b, H, h-geometry of the beam. 
It should be noted that the above analysis of the stress state are "local" situation, and actually feel the strain gauge is the "average" state. 
Third, the detection circuit 
Detection circuit function is to change the resistance of strain gauge resistors into a voltage output. Because of the Wheatstone bridge has many advantages, such as the effect of temperature variation can be suppressed, can inhibit the lateral force interference, can be more convenient to solve the issue of compensation load cell, etc. So Wheatstone bridge to get the load cell a wide range of applications. 
Because full-bridge arm bridge highest sensitivity, the same parameters of each arm, a variety of easy to offset the impact of interference, so the full-bridge load cells are used such as arm bridge. 
Resistance strain gauge load cell working process during the measurement, the weight loaded into the load cell elastomer can cause plastic deformation. 
Strain (positive and negative) on the elastic body by mounting the strain gauge is converted to electrical signals. The easiest bending beam load cell is only a strain gauge. Typically, the elastic body and the strain gauge to combine a variety of ways, for example, the housing, the sealing member such as to protect the strain gauges.