多轴传感器
Your current location : home page >> Support >> The principle of resistance strain type load cell

The principle of resistance strain type load cell

Resistance strain gauge load cells are used for force measurement or weighing under static and dynamic conditions, and have been widely used in my country's industrial production process detection and control, automatic measurement and other fields. It is the core component of electronic weighing instruments. Its quality is the main factor affecting the measurement accuracy of electronic weighing instruments. In actual use, due to the influence of raw materials and manufacturing processes, installation methods, use conditions and external environment, it is easy to malfunction, which affects the accurate and stable operation of electronic weighing instrument measurement data. Therefore, understanding the basic principles and failure causes of the load cell, and mastering the analysis and judgment technology of the failure are the keys to quickly and accurately deal with the failure of the electronic weighing instrument and ensure its accurate and stable operation.


1.1 The components of the load cell


   The load cell is mainly composed of three parts: resistance strain gauge, elastomer and detection circuit. Strain gauge is a kind of sensing element, its function is to transform deformation into resistance change; elastic body is a structural part with special shape, its main function is to convert force into deformation; the main component of the detection circuit is Wheat Boarding the electric bridge, it can solve the compensation problem of the load cell more conveniently, and its function is to convert the resistance change of the resistance strain gauge into the corresponding electrical signal output.


1.2 Working principle


   The resistance strain sensor is composed of a resistance strain gauge pasted on an elastic element through a specific process. A sensor that uses the strain effect of resistive materials to convert the internal deformation of engineering structures into resistance changes. This type of sensor mainly converts the measurement into the deformation of the elastic element through a certain mechanical device, and then converts the deformation of the elastic element into a change in resistance by a resistance strain gauge, and then converts the change in resistance into a voltage or current change through a measuring circuit Signal output. It can be used to detect various non-electrical physical quantities that can be transformed into deformation, such as force, pressure, acceleration, torque, weight, etc., and is widely used in industries such as machining, measurement, and construction measurement.


 1.2.1 Resistance strain effect of strain gauge


   The so-called resistance strain effect means that a metal conductor or semiconductor material with a regular shape produces strain under the action of an external force and its resistance value changes accordingly. This physical phenomenon is called the "resistance strain effect". Take a cylindrical conductor as an example: when its length is L, its radius is r, and the resistivity of the material is ρ, it can be obtained according to the definition of resistance


            (1—1)

When the conductor is strained for some reason, the change of its length L, cross-sectional area A and resistivity ρ is dL, dA, dρ, and the corresponding resistance change is dR. Comparing the formula (1-1), the resistance change rate dR/R is obtained by the total differentiation:

            (1—2)

Where: dL/L is the axial strain εL of the conductor; dr/r is the transverse strain εr of the conductor


From the mechanics of materials: εL=-μεr (1—3)


In the formula: μ is the Poisson's ratio of the material, and the Poisson's ratio of most metal materials is about 0.3 to 0.5; the negative sign indicates that the two change directions are opposite. Substituting formula (1-3) into formula (1-2), we get:


         (1—4)

Equation (1-4) shows that the resistance strain effect mainly depends on its geometric strain (geometric effect) and its own unique conductivity (piezoresistive effect).


  1.2.2 Strain Sensitivity


     It refers to the relative change of resistance produced by the resistance strain gauge under the action of unit strain.


     (1) The strain sensitivity K of metal conductors: mainly depends on its geometric effects; it is advisable


          (1—5)

The sensitivity factor is:

K=                               

                            

The resistance of a metal conductor will change when it is subjected to strain. The resistance increases when stretched, and decreases when compressed, and it becomes proportional to its axial strain. The resistance strain sensitivity of metal conductors is generally around 2.


   (2) The strain sensitivity of semiconductors: mainly depends on its piezoresistive effect; dR/R<≈dρ⁄ρ. The reason why the semiconductor material has a larger rate of change in resistance is because it has a much more pronounced piezoresistive effect than that of metal conductors. When the semiconductor is deformed by force, it will temporarily change the symmetry of the crystal structure, thus changing the conductive mechanism of the semiconductor, causing its resistivity to change. This physical phenomenon is called the piezoresistive effect of the semiconductor. Different materials of semiconductor materials have different piezoresistive effects under different stress conditions, which can be positive (increasing resistance) or negative (decreasing resistance). In other words, the same stretching deformation, the semiconductor of different materials will get the completely opposite effect of resistance change.


     The resistance strain effect of semiconductor materials is mainly reflected in the piezoresistive effect, and its sensitivity coefficient is relatively large, generally around 100 to 200.




1.2.3 Application of SMD Strain Gauge


Metal foil strain gauges are commonly used in SMD process sensors. SMD semiconductor strain gauges (temperature drift, stability, linearity is not good and easy to damage) are rarely used. Generally, semiconductor strain uses N-type single crystal silicon as the elastic element of the sensor, and a semiconductor resistive strain film is directly evaporated and diffused on it (the sensitive grid is diffused) to make a diffused piezoresistive (piezoresistive effect) sensor.


*This experiment uses metal foil strain gauges as the research object.


 1.2.4 The basic structure of foil strain gauges


The metal foil strain gauge is a substrate made of insulating materials such as phenol and epoxy resin, and the paste diameter is about 0.025mm


Made of metal wire or metal foil, as shown in Figure 1-1.


 

精密称重传感器,拉压力传感器,多轴传感器

(a) Wire strain gauge (b) Foil strain gauge


Figure 1-1 Strain gauge structure diagram


Metal foil strain gauges are strain-sensitive components made by processes such as photolithography and corrosion. The working principle is the same as that of wire strain gauges. When the resistance wire undergoes mechanical deformation under the action of external force, its resistance value changes. This is the resistance strain effect. The relational formula describing the resistance strain effect is: ΔR/R=Kε where: ΔR/R is the relative change of resistance wire resistance, K is the strain sensitivity coefficient, ε=ΔL/L is the relative change of the resistance wire length.


 1.2.5 Measuring circuit


    In order to convert the resistance change of the resistance strain sensor into a voltage or current signal, a bridge circuit is generally used as its measurement circuit in the application. The bridge circuit has the advantages of simple structure, high sensitivity, wide measurement range, good linearity and easy temperature compensation. It can better meet various strain measurement requirements, so it has been widely used in strain measurement.


The bridge circuit is divided into three types: single-arm, double-arm and full-bridge according to its working mode. The output signal of single-arm is the smallest, linear, and the stability is poor; the output of double-arm is twice that of single-arm, and its performance is better than that of single-arm. Improve; the output of the full bridge is four times that of the single arm, and the performance is good.


   The basic circuit of the load cell is shown in Figure 1


精密称重传感器,拉压力传感器,多轴传感器

Can launch:


U0=(R2 R4-R1 R3)Ui/(R1+R2)(R3+R4)


Where R1 R2 R3 R4 is the resistance of the strain gauge; is the input signal of the sensor; is the output signal of the sensor.


When R2 R4 = R1 R3, we call it the bridge balance. At this time


Figure 1 Basic circuit diagram


The output voltage of the load cell U0 = 0mV.


    The weight of the material acts on the load cell through the scale body or hopper of the electronic weighing instrument. The elastic body of the load cell deforms elastically under the action of external force, so that the resistance strain gage pasted on its surface is also deformed. After the resistance strain gage is deformed, Its resistance will change (increase or decrease). Then through the corresponding detection circuit, this resistance change is converted into an electrical signal (voltage or current) for output, thereby completing the process of transforming the external force into an electrical signal.


Let R1 = R2 = R3 = R4 = R


When subjected to gravity, the strain gauge resistance of the sensor changes. Assuming that the resistance of each bridge arm changes the same, the variable is ΔR, that is: R1 and R3 decrease by ΔR respectively, and R2 and R4 increase by ΔR respectively


It can be deduced that the output voltage of the sensor is: U0=ΔRUi/R


     The load cell is the application of the foil strain gauge and its full bridge measurement circuit. The experimental principle of the digital electronic scale is shown in Figure 1-1. This experiment only does the amplifier output Vo experiment. Through the calibration of the circuit, the voltage value of the circuit output is the value corresponding to the weight. The voltage dimension (V) is changed to the weight dimension (g) to become an original electronic scale.


 

精密称重传感器,拉压力传感器,多轴传感器

Figure 1-1 Principle block diagram of digital electronic scale


Consultation hotline

+86-0570-8769916

Fax:+86-0570-8769930

Mailbox:nanhua@nhelectronic.com    

Address:Floor 1, building 1, No. 10, yincang Road, Zhizao new town, Quzhou City, Zhejiang Province


  • 1659087716131899.png

    Enterprise official account

  • 图层 918.png

    Tiktok QR code

  • 浙ICP备19035189号 technical support:华企网络