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Principle of resistance strain weighing sensor

resistance strain weighing sensor is based on the principle that the elastomer (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 with it. After the deformation of the resistance strain gauge, its resistance value will change (increase or decrease), This resistance change is then converted into an electrical signal (voltage or current) by the corresponding measuring circuit, thus completing the process of converting the external force into an electrical signal. The resistance strain type load cell is based on the principle that 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 with it. After the deformation of the resistance strain gauge, its resistance value will change (increase or decrease), and then this resistance change will be converted into an electrical signal (voltage or current) through the corresponding measuring circuit, Thus, the process of transforming the external force into an electrical signal if the internal components are seriously worn is completed

it can be seen that resistance strain gauge, elastomer and detection circuit are indispensable parts of resistance strain type load cell. The following is a brief discussion on these three aspects

I. resistance strain gauge

resistance strain gauge is a strain gauge that mechanically distributes a resistance wire on a substrate made of organic material. One of his important parameters is the sensitivity coefficient K. Let's introduce its significance

there is a metal resistance wire, whose length is l, the cross section is a circle with radius r, its area is recorded as s, its resistivity is recorded as, and the Poisson coefficient of this material is. When this resistance wire is not subjected to external force, its resistance value is r:

r = l/s () (21)

when its two ends are subjected to f force, it will stretch, that is to say, it will deform. Let its elongation L, its cross-sectional area is reduced, that is, its cross-sectional circle radius is reduced by R. 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

find the total differential of formula (2--1), that is, find out how much the resistance value changes after the resistance wire is extended. We have:

R Jinan testing machine factory high and low temperature impact testing machine = l/s + L/s s l/s2 (2 2)

use formula (2--1) to remove formula (2--2) to get

in addition, we know that the cross-sectional area of the conductor s = R2, then s = 2 R * r, so

~~ ~~

~~ from the mechanics of materials, we know ~~

r/r = - l/l (25)

where the minus sign indicates that the radius direction is reduced during elongation. Is the Poisson's coefficient representing the transverse effect of the material. Substituting formula (24) (25) into (2--3), there are

= (1 + 2 (/)/(l/l)) * L/L

= k * L/L (2--6)

where

k = 1 + 2 + (/)/(l/l) (2--7)

formula (2--6)) that explains the resistance change rate (relative change of resistance) and the elongation of resistance wire (relative length).Change)

it should be noted that the value of sensitivity coefficient K is a constant determined by the nature of the material making the metal resistance wire, which is independent of the shape and size of the strain gauge. The value of K of different materials is generally between 1.7 and 3.6; Secondly, the value of K is a dimensionless quantity, that is, it has no dimension

in mechanics of materials, l/l is called as strain and is recorded as. It is often too large to express elasticity with it, which is inconvenient.

often takes its one millionth as a unit and is recorded as. In this way, formula (2-6) is often written:

R/r = K (28)

II. Elastomer

elastomer is a structural member with special shape. It has two functions: first, it bears the external force on the weighing sensor, and generates reaction force against the external force to achieve relative static balance; Secondly, it should produce a high-quality strain field (area), so that the resistance strain gauge pasted in this area can ideally complete the task of strain and electrical signal conversion

explanation of some relevant terms of pressure testing machine automatic control: the whole experimental process is controlled by computer

take the elastomer of sb series load cell of Toledo company as an example to introduce the stress distribution

a rectangular cantilever beam with a blind hole is set

the center of the bottom of the blind hole is subject to pure shear stress, but tensile and compressive stresses will appear in its upper and lower parts. The principal stress direction is tension and compression. If the strain gauge is pasted here, the upper half of the strain gauge will be stretched and the resistance value will increase, while the lower half of the strain gauge will be compressed and the resistance value will decrease. The strain expression of the center point at the bottom of the blind hole is listed below without derivation

= (3Q (1+)/2eb) * (b (h2-h2) +bh2)/(b (h3-h3) +bh3) (2--9)

among them: Q - Section realizes the shear force on the industrialization of polyglutamic acid and polylysine; E -- Young's modulus: Poisson coefficient; B. B, h and H are the geometric dimensions of the beam

it should be noted that the stress states analyzed above are all local conditions, while the strain gauge actually feels the average state

III. detection circuit

the function of the detection circuit is to convert the resistance change of the resistance strain gauge into voltage output. Because Wheatstone bridge has many advantages, such as inhibiting the influence of temperature changes, suppressing lateral force interference, and solving the compensation problem of load cell conveniently, Wheatstone bridge has been widely used in load cell

because the full bridge equal arm bridge has the highest sensitivity, the parameters of each arm are consistent, and the effects of various interferences are easy to offset each other, the full bridge equal arm bridge is adopted for the load cell

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