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Introduction to load cell (serial one)
Time:2016-10-31    Read:638 Time
The load cell is actually a device that converts the quality signal into a measurable electrical signal output. The sensor should be considered in the actual working environment of the sensor, which is essential for the correct selection of the load cell, it relates to the sensor can work properly and its safety and service life, and even the entire instrument reliability and safety Sex. In the load cell main technical indicators of the basic concepts and evalsuation methods, the old and new GB quality differences. There are several types of S-type, cantilever type, spoke type, plate ring type, membrane box type, bridge type, cylinder type and so on.

A force sensor used on the instrument. It can be applied to the measured object gravity by a certain percentage into a measurable output signal. Considering the influence of gravity acceleration and air buoyancy on the conversion, the performance indexes of the load cell are linear error, hysteresis error, repeatability error, creep, zero temperature characteristic and sensitivity temperature characteristic. In a variety of weighing and quality metering systems, the integrated error is often used to comprehensively control the sensor accuracy and to associate the error band with the instrument error band (Figure 1) in order to select the weight corresponding to an accuracy meter sensor. The International Organization of Legal Metrology (OIML) stipulates that the error of the sensor δ is 70% of the error zone Δ of the instrument, the linearity error of the load cell, the hysteresis error, and the error caused by the influence of temperature on the sensitivity in the specified temperature range The sum can not exceed the error zone δ. This allows the manufacturer to adjust the components that make up the total measurement error to achieve the desired accuracy.

Resistance strain type

The use of resistance strain gauge deformation of the resistance also changes with the principle of work (Figure 11). Mainly by the elastic components, resistance strain gauge, measuring circuit and transmission cable 4 parts.

The resistance strain gauge load cell is based on the principle that the elastomer (elastic element, sensitive beam) is elastically deformed under external force so that the resistance strain gauge (switching element) pasted on it is also deformed with the resistance strain gauge After deformation, its resistance will change (increase or decrease), and then the corresponding measurement circuit to convert this resistance into electrical signals (voltage or current), thus completing the external force into the process of electrical signals The

It can be seen that the resistance strain gauge, the elastomer and the detection circuit are several indispensable parts of the resistance strain type load cell.

First, the resistance strain gauge

Resistive strain gauge is a resistance wire mechanical distribution in an organic material made of the substrate, that is, a strain gauge. One of his important parameters is the sensitivity coefficient. Let's talk about its meaning.

There is a metal resistance wire whose length is L and the cross section is a circle of radius r whose area is denoted by S and its resistivity is denoted as ρ. The Poisson coefficient of this material is μ. When the resistance wire is not subjected to external force, its resistance value is R:

R = ρL / S (Ω) (2-1)

When both ends of the force by the F force, it will be elongated, that is to say deformation. Let the extension of ΔL, the cross-sectional area is reduced, that is, its radius of the cross-section reduction Δr. In addition, experiments can also be used to prove that the metal resistance wire deformation, the resistivity will change, recorded as Δρ.

Equation (2--1) for the whole differential, that is, find the resistance wire elongation, his resistance value changed how much. We have:

ΔR = ΔρL / S + ΔLρ / S - ΔSρL / S2 (2-2)

Use equation (2--1) to remove formula (2--2)

ΔR / R = Δρ / ρ + ΔL / L - ΔS / S (2-3)

In addition, we know that the cross-sectional area of ​​the conductor S = πr2, then Δs = 2πr * Δr, so

ΔS / S = 2Δr / r (2-4)

We know from material mechanics

Δr / r = -μΔL / L (2-5)

Where the negative sign indicates elongation and the radial direction is reduced. μ is the material effect of the Poisson coefficient. Substituting (2-4) (2-5) into (2--3), yes

ΔR / R = Δρ / ρ + ΔL / L + 2μΔL / L

            = (1 + 2μ (Δρ / ρ) / (ΔL / L)) * ΔL / L

  = K * ΔL / L (2 - 6)

among them

K = 1 + 2μ + (Δρ / ρ) / (ΔL / L) (2 - 7)

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

It should be noted that the size of the sensitivity coefficient K is determined by the nature of the metal wire material, which is independent of the shape and size of the strain gauge, and the K value of the different materials is generally between 1.7 and 3.6; Followed by the K value is a dimensionless dimension, that is, it is not dimension.

In material mechanics, ΔL / L is called strain, which is denoted by ε, and it is often used to indicate that elasticity tends to be too large and very convenient. It is often used as unit in parts. In this way, type (2 - 6) often write:

ΔR / R = Kε (2-8)


Second, the elastic body

The elastomer is a specially shaped structure. It has two functions, first of all, it is subjected to the external force of the load cell, the reaction force against the external force, to achieve a relatively static balance; Secondly, it produces a high-quality strain field (area), so paste in this area Of the resistance strain gauge is ideal to complete the conversion date of the signal conversion task.

Take the elastomer of the load cell as an example to introduce the stress distribution.

There is a rectangular cages with a blind hole.

The bottom of the blind hole is subjected to pure shear stress, but the upper and lower parts will exhibit tensile and compressive stresses. If the strain gauge is attached here, the upper part of the strain gauge will be stretched and the resistance will increase, and the lower part of the strain gauge will be compressed and the resistance will be reduced. The following lists the strain expressions at the center of the bottom of the blind hole without derivation.

(B (H3-h3) + bh3) (2 - 9)

Where: Q - shear force on section; E - Young's modulus: μ- Poisson coefficient; B, b, H, h- is the geometry of the beam.

It should be noted that the above analysis of the stress state are "local" situation, and the strain piece actually feel the "average" state.



Third, the detection circuit

The function of the detection circuit is to change the resistance of the resistance strain gauge into a voltage output. Because the Wheatstone bridge has many advantages, such as can inhibit the impact of temperature changes, can inhibit the lateral force interference, can be more convenient to solve the load cell compensation problems, so the Wheatstone bridge in the load cell A wide range of applications.

Because the full-bridge bridge with the highest sensitivity, the same arm parameters, the impact of a variety of interference easily offset each other, so the load cell are used full bridge and other arm bridge.

Common materials

The performance of the load cell depends largely on the choice of manufacturing material. The load cell material includes the following parts: strain gauge material, elastomeric material, patch adhesive material, sealant material, lead sealing material and lead material.
Strain gauge and resistance element material
The strain gauge is the induction part of the load cell. It transforms the external force into the electrical quantity output, which is the most important part of the sensor. The commonly used strain gauge substrate adopts the polymer film material. The strain material is usually high purity. The performance of strain gauges is not only related to the purity of the substrate and the carbon, but also to the manufacturing process. Increasing the level of process technology is also a very important aspect of improving sensor performance.
Elastomer material
The role of the load cell elastomer is to transmit the external force, it must have the same force in the size of the time, resulting in deformation, because the strain gauge is attached to the elastic body above the deformation of the elastic body is the deformation of the strain gauge; With a reset, when the external force disappears, you can automatically reset. Elastomer materials usually choose a variety of metals, mainly aluminum, stainless steel and alloy steel and so on.
Patch adhesive material
Patch adhesive is the strain gauge and elastomer firmly fixed together, so that they produce the deformation is always consistent. It can be seen, patch adhesive is also an important component. At the beginning of the 21st century, the use of multi-component adhesive adhesive is a two-component polymer epoxy series adhesive. At the beginning of the 21st century, its performance and its own purity, mixed mode, storage time, curing mode, curing time and other great relationship, before using it to carefully look at its details.
Sealant material
Early weighing sensor seals are made of sealant, and later due to the development of manufacturing technology, welding technology can improve the stability of the great sensor and service life. Although the early 21st century, many of the use of welding technology, but some important parts need to apply some sealant. Sealants are generally used silica gel, silicone has the advantages of good stability, can be moisture, corrosion resistance, insulation performance is also very good.
Lead seals and lead materials
If the sensor output lead is not fixed, it will be damaged or loose, resulting in signal instability or no output. At the beginning of the 21st century, the sensor output adopts the way of the connector, and the material of the connector and the fastening force will also affect the output. It is best to use a connector with a sealant. The internal leads also need to be fixed to prevent them from moving around. Lead quality is also very important, the material properties from high to low order of order silver, copper and aluminum. If the surrounding high-frequency signals, radio interference is serious, then the need to use shielded cable; corrosive environment and flammable and explosive occasions you need to use anti-corrosion and flame retardant cable, plus casing to protect.


To be continued

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