Analysis of common faults of Strain Load Cell

 

[ Summary ] This paper discusses the structure principle, process flow and fault analysis of resistance strain sensor, summarizes and analyzes the common faults in the manufacturing process of the variable load cell, and explores the technical methods for improving the process quality of the sensor, so as to provide theoretical basis for standardizing the design and manufacturing process of the load cell.

[Key Word] Strain load cell;  process flow; fault analysis; , design and manufacture load cell

In modern industrial production, especially in the process of automatic production, the first thing to solve is to obtain accurate and reliable information, which requires the use of various sensors to monitor and control various parameters in the production process, so that the equipment can work in the normal state or the best state, and make the product reach the best quality. Therefore, it can be said that without many excellent sensors, the foundation of modern production will be lost. With the progress of technology, the electronic weighing instrument made by weighing sensor has been widely used in all walks of life, realizing the rapid and accurate weighing of materials. Especially with the emergence of microprocessor and the continuous improvement of automation in industrial production process, the weighing sensor has become a necessary device in process control. Because of its simple manufacturing process and low processing cost, strain sensor has been produced in large quantities, and has been widely used in industrial production process detection and control, automatic measurement and other fields in China.

 

1:Strain Load Cell

1.1:Overview

1.1.1Basic definition

The resistance strain sensor is a kind of functional element which converts force into electrical signal by metal elastomer. It is a kind of force sensitive sensor, which can realize the transformation of force strain resistance electrical signal under the excitation of external power through the resistance strain gauge pasted on the sensitive surface of elastic body and its bridge circuit. Because the resistance strain sensor must realize the force transmission through a certain form of mechanical components, through the resistance strain gauge to realize the transmission of electrical signals, so as to achieve the force measurement, therefore, the resistance strain sensor is called electromechanical integration device. In the 1930s, strain gauge was invented. The strain gauge was used to analyze the stress of materials to study the mechanical properties of materials. With the deepening of the research, people use the stability of metal materials to design a variety of resistance strain sensors to measure various physical quantities. In the 1950s, with the rapid development of electronic technology and the further maturity of strain gauge manufacturing technology, resistance strain sensors began to be widely used, and various structures and different uses of resistance strain sensors were designed accordingly.

1.1.2. Basic classification

There are various names for resistance strain sensors. According to their structural forms, strain sensors can be divided into flat film type, parallel beam type, column type, bridge type, cantilever beam type, double beam type, wheel amplitude type, pressure ring type, plate ring type, etc. It can be divided into sensor, speed sensor, tension sensor, etc.

1.1.3. Scope of application

The detection object of resistance strain sensor force, not only refers to the tension or pressure in the vertical direction, but also can be any single component (force or torque) among the six components in the force space. As long as the elastomer is specially designed, it can detect the six components fused on a component without interference, and output six electrical signals with linear relationship with the measured at the same time. The resistance strain sensor can also detect the pressure (unit: kg / m2), including gas pressure, liquid pressure, soil pressure, etc. Resistance strain sensor can also be used in displacement detection, strain detection, shape and position detection, density detection and liquid level detection. Because these geometric and physical quantities can be transformed into mechanical quantities through the structural changes of elastomers. Nowadays, resistance strain sensors have been widely used in aviation, aerospace, steel, oil, coal, transportation, measurement, machinery manufacturing and other fields.

 

1.2:Basic principle of sensor

1.2.1Circuit principle of sensor

The resistance strain sensor transfers the force through the elastic body, realizes the force electric signal conversion by changing the resistance value of the strain gauge pasted on the elastic body, and outputs the signal through the amplification effect of Wheatstone bridge.

Figure 1 basic circuit diagram of resistance strain load cell

 

1.2.2. Circuit calculation of  load cell

Figure 1 shows a typical Wheatstone bridge, and its calculation and analysis process are as follows:

U=U0 R3/（R2+R3）-U0R4/（R1+R4）

 =U0［R3（R1+R4）-R4（R2+R3）］/（R1+R4）（R2+R3）

 =U0（R1·R3-R2·R4）/（R1+R4）（R2+R3）

The differential equation is as follows:

△ U/U0 =（△ R1/R1-△R2/R2+△R3/R3-△R4/R4)/4

According to the principle of strain gauge: Kε= △ R/R

△ U/U0 =（Kε1-Kε2+Kε3-Kε4）/4=K（ε1+ε3-ε4-ε2)/4

In the above formula, u is the output voltage of the sensor; U0 sensor input voltage is the bridge supply voltage; R1, R2, R3, R4 are the nominal resistance values of each bridge arm strain gauge; K is the sensitivity coefficient of the resistance strain gauge; and ε is the strain of the elastomer pasted with the strain gauge. If R1=R2=R3= R4 = R, then ε1= ε 3 = -ε 4 = -ε2 = ε, then the sensor output △U/U0=kε.

 

It can be seen from the above that the output sensitivity △U/U0 of the strain sensor is independent of the input voltage, only related to the strain of the elastomer, with the unit of Mv/v.

 

1.2.3. Composition of strain load cell

The resistance strain sensor is mainly composed of elastomer, strain gauge, compensation circuit, cable, sealing material, etc

(1) Elastomer: force collection element, which is the basic component of strain sensor;

(2) The strain gauge is composed of a strain gauge and a strain gauge;

(3) Compensation circuit: it is used to modify the basic parameters of the sensor, such as zero point, output sensitivity, etc., to meet the product standards and user requirements;

(4) Cable: the carrier used to output the signal collected by strain gauge, which is a necessary part of the sensor;

(5) Sealing material: in order to protect the performance of the sensor, meet the design requirements, and adapt to the changes of the external environment, brush, paste or seal some materials on the sensor sensitive grid. The main purpose is to prevent the external dust, moisture and harmful gas from eroding the sensor and causing influence and damage to the product performance.

 

2: Load Cell Manufacturing Process

2.1: Main process steps

Patch → bridge assembly → compensation → test → protection

 

2.2: A brief introduction to the process of mounting

Patch is the process that the strain gauge is pasted on the elastomer by adhesive, and the elastomer is closely combined with the strain gauge through certain conditions (temperature rising and pressure increasing), so that the stress and strain of the elastomer can be transferred by the strain gauge. The chip is the key process in the whole sensor manufacturing process (except the machining part). The quality of the chip directly determines the performance of a sensor, and directly affects the key indicators of the sensor, such as zero point, zero compensation, hysteresis, creep, repeatability, stability and so on.

 

2.3: Brief introduction of bridge assembly process

Bridge building is the process of connecting strain gauge to Wheatstone bridge according to certain requirements. Bridge assembly is composed of solder joint grinding and hanging tin, preparation of welding wire, bridge circuit connection and other steps.

 

2.4: Brief introduction of compensation process

Compensation refers to the correction of zero point output and full range output of the sensor connected to the bridge.

2.4.1Correction of zero output

When the sensor is used in the weighing system, it is necessary to correct its zero position to a certain range, otherwise the error of the whole test system will be increased. At the same time, due to the change of the ambient temperature of the sensor in use, the zero point output of the sensor will change with the change of time and temperature, so it is necessary to compensate the sensor zero point in production, and the change caused by the environmental impact after it is applied to the system can be reduced to the error range that meets the requirements of use.

 

2.4.2 Correction of full  Capacity

When the sensor is in use, because the material is affected by the environmental temperature change, its elastic modulus decreases when the temperature rises, which makes the full range output larger; when the temperature decreases, the elastic modulus increases, making the full range smaller. However, the change of full scale with the change of output temperature often exceeds the allowable error range of sensor, so it is necessary to correct this phenomenon, that is, the correction of full scale, also known as temperature sensitivity compensation. In general, the compensation of temperature sensitivity has been modified according to certain parameters in the design of products and added to the bridge circuit.

 

2.5 brief description of test process

2.5.1 test principle and calculation

Theoretically, the sensor input and output should be linear, and when the input is zero, the output should also be zero. The theoretical calculation formula is y = ax (X-Force, y-output voltage, a-sensitivity coefficient), but the actual sensor will not achieve the above ideal effect, and the nonlinear and other properties will be affected by various factors such as environment, time, materials, etc.

 

2.5.2 sensor test performance requirements

The linearity between the measured force and the output is good, the hysteresis is small, the repeatability is small, and the creep is small.

 

2.5.3 input output relationship of sensor

The static performance is the input-output relationship when the measured value is in a stable state, and the dynamic performance is the influence relationship between the output and the input varying with time. The purpose of the test is to detect the static performance (linearity, hysteresis, repeatability, creep, impedance, zero position, output sensitivity, temperature sensitivity, etc.) of the sensor.

 

2.6 brief description of protection process

The qualified products need necessary protective treatment to enhance the moisture-proof performance and stability of the sensor and extend the service life of the sensor.

 

The protection of the sensor can be set according to the use environment of the sensor and the requirements of the sensor user. These products can be used in the general environmental protection of our products such as IP65, which can be used in the general environmental protection. Most truck scales are used outdoors, and the environment is bad, and the protection level of the sensor is high. We usually design the chip part of the sensor in the counterbore. When protecting, we encapsulate the silicone resin in the counterbore, and then add a protective seat cover. Such protection can generally meet the requirements of IP66.

 

With the development of welding technology, in order to improve the sealing performance of sensors, some sensors weld bellows, cover plates or bellows in some electrical components such as strain gauge patch holes and wiring points. The protection grade of this kind of sensor is the highest IP68 can not only achieve moisture-proof and waterproof, but also protect general corrosive gas and corrosive medium.

 

At present, laser welding and argon arc welding are common welding methods of sensors. However, for strain sensors, the deformation of the capsule will affect the stress collection of the elastomer by the strain gauge after welding the capsule at the stress collection point, especially for the small range sensor, which will affect the linear hysteresis performance of the sensor. Therefore, the thickness of the capsule used by each sensor must be considered Thickness, through the specific test to get the size of the influence, get the appropriate welding parameters, so that the performance of the sensor to achieve the best.

 

The main purpose of welding is to seal. Therefore, the welding seam must be inspected for the sensor after welding. The inspection method can be seen by eyes, observed by amplifier or monitor, air pressure test, damp heat test, etc. There must be no gap, bubble and air leakage in the weld. The insulation performance of the heat and moisture test must meet the requirements. In order to ensure the appearance quality, the weld should be smooth and even.

 

3:Failure analysis of strain Load Cell

3.1 common faults and analysis of mounting process

3.1.1 fault classification

The chip plays a very important role in the whole production process of the sensor. If the quality of the chip can not be guaranteed, the quality of the whole sensor is difficult to guarantee. The common quality problems in the process of mounting mainly include the following aspects: (1) strain gauge bulge; (2) strain gauge displacement; (3) insufficient strain gauge pressure.

 

3.1.2 Analysis of strain gauge bulge

The bulge of strain gauge refers to that the surface of strain gauge is uneven after sticking the piece. As is known to all, the strain gauge is attached to the surface of the elastomer to transmit the strain. Due to the characteristics of the elastomer material, the maximum strain of the sensor is 1500-2000 microstrain. After being converted by the strain gauge, the output signal is generally only 20-30 MV, and the design accuracy of the sensor is generally 0.02-0.03 (0.02-0.03) (0.02-0.03) ）The allowable deviation is only 4-10 MV. If the surface of the strain gauge is bulging, the strain will be distorted and the real strain of the elastomer can not be transferred, which will lead to the poor performance of the sensor. At the same time, because the wire grid of the strain gauge is very thin, if the surface of the strain gauge is bulged, the wire grid at the bulge will be thinner, and the resistance value will be uneven (the output resistance value will be increased), in the process of using the sensor, the thermal output at this point will be increased, or the sensor zero point drift will be caused, and the sensor can not be used normally; in serious cases, the sensor wire grid will be broken and the sensor will be scrapped. The causes of strain gauge bulging are analyzed as follows:

(1) The surface finish of elastomer is not enough. Because the surface finish of elastomer is not enough during machining, the whole surface itself is uneven, which results in the bulge of strain gauge. When the strain gauge is observed under strong light, it can be seen that the surface color of the strain gauge is consistent, but uneven. After removing the strain gauge, it can be seen that the surface of the elastomer is uneven and the bulge is bright. If the strain gauge remains intact, the metal particles on the back of the strain gauge can be observed with a magnifying glass or a projector.

(2) The elastomer is not cleaned clean, there are other foreign matters in the patch, and there are other impurities between the strain gauge and the elastomer. It mainly consists of dust, sand, rubber particles and suspended solids in the rubber. Observing the strain gauge under strong light, we can see that the color of the strain gauge at the bulge is different from that in other places, but darker than that in other places. After the strain gauge is completely removed, the surface of the elastomer is smooth and smooth, while the back of the strain gauge can be observed under the magnifying glass or projector.

 

3.1.3 analysis of strain gauge displacement

Strain gauge displacement refers to the displacement of the strain gauge from its location line. Because the strain gauge collects the maximum strain (determined at the design time), if the strain gauge deviates from the preset position, the strain collected by the strain gauge will be too small, which will affect the full range of the sensor, the output sensitivity is too small, and the linearity and hysteresis of the sensor will be affected. The main feature of strain gauge displacement is that the strain gauge positioning icon deviates from the positioning line. Generally, if the offset distance is greater than 0.5mm, the strain gauge should be reworked and re pasted. There are two main reasons for strain gauge displacement

 

(1) When the patch is mounted, it is out of alignment and deviates from the positioning line, resulting in the displacement of the strain gauge. Most of them are caused by too many adhesive brushes or the wrong proportion of adhesive, which causes the strain gauge to deviate from the position of the patch during the extrusion process, and the inspection is not carried out after the extrusion.

 

(2) The design of pressure tooling is unreasonable, resulting in the displacement of strain gauge. It mainly focuses on the steel sensors. Due to the different forms of the patch parts, unreasonable tooling design and easy to cause strain gauge displacement.

 

3.1.4 analysis of insufficient strain gauge pressurization

The main reason for insufficient pressure of strain gauge is unreasonable design of pressure tooling, or the tooling is not placed properly during pressurization, and there are other hard objects between the elastomer and the tooling, which affect the pressurization effect, resulting in insufficient pressurization of the strain gauge. The influence of insufficient pressure on the variometer is recessive, slight and does not affect the normal use of the sensor in the initial stage. After using for a period of time, the sensor zero point instability, full range offset and other phenomena will appear. Serious problems can be reflected in the initial stage, such as sensor zero point instability, temperature drift can not be compensated, full range drift, etc., more serious sensors can not be used at all.

 

The main characteristics of the strain gauge under insufficient pressure are: when the strain gauge is observed under strong light, it can be seen that the surface color of the strain gauge is not consistent with that of other strain gauges, and even the color of the whole strain gauge is not consistent with that of other strain gauges. If the strain gauge is slightly touched, that is, it is separated from the sticking surface. If the strain gauge is slightly removed, the force will be obviously smaller

 

3.2 common faults and analysis of bridge assembly process

3.2.1 fault classification

The common faults in bridge assembly process are as follows:

(1) The results show that the bridge circuit is wrong; (2) the zero point is unstable; (3) the insulation impedance is out of tolerance; (4) the bridge impedance is out of tolerance

3.2.2 analysis of Bridge Road assembly errors

The strain gauges are connected by wires according to certain requirements to form Wheatstone bridge. In the process of connecting the bridge, it should be noted that when welding the lead wire, the electric soldering iron should not contact the strain gauge solder joint for too long, otherwise the solder joint may fall off and float the grid. For the wire with diameter larger than Φ 0.5mm, a terminal block shall be pasted in front of the strain gauge, and the strain gauge shall be connected with the wiring terminal with silver plated wire or enameled wire with wire diameter less than Φ 0.2mm, and then the wire with diameter greater than Φ 0.5mm shall be welded on the terminal block. For high-precision sensors (the accuracy is higher than 0.02% FS), this method can eliminate the additional stress caused by the bridge wire, and make the sensor more stable and consistent. For the sensor connected to the bridge, the welding spot should be cleaned with acetone or alcohol cotton ball in time. If the cleaning is not timely, the sensor will be unstable. At the same time, we should pay attention to the nature of the strain gauge, tension or compression. If the bridge is connected wrongly, the sensor will have no output or output is negative. The correct bridge connection method should be that the input (+) and the output (+) are connected by pressing sheet, the input (+) and the output (-) are connected by the pulling piece, the output (+) and the input (-) are connected by the tension piece, and the output (-) and the input (-) are connected by the pressing sheet (Note: the strain gauge attached to the tensile deformation part of the elastomer is referred to as the tension piece; the elastic body is produced after the stress The strain gauge attached to the compression deformation part is referred to as the compression sheet.

After the bridge is connected, the sensor shall be tested for impedance first. After the impedance meets the process requirements, the insulation impedance test shall be carried out with an insulation impedance tester. Only when the insulation impedance is greater than 5000 megohm (100V DC power supply or 50V AC power supply) is qualified, the next step can be carried out.

 

3.2.3 analysis of zero point instability

The main reasons for the instability of zero point in bridge assembly are false welding, false welding and unclean cleaning of welding spot. When the sensor is calibrated, the zero point flickers indefinitely and swings 5 μ V up and down a certain value. In general, the bridge should be completely cleaned and the fault should be eliminated.

 

3.2.4 analysis of insulation impedance out of tolerance

Insulation impedance out of tolerance means that the impedance between the sensor bridge and the elastomer can not meet the requirements. There are two main forms of expression

(1) The insulation impedance between the elastomer and the bridge is less than 2000 megohm, but greater than 1 * 10 Ω, which is generally said that the insulation is not enough. This situation is generally due to the welding is not clean, the use of acid flux, etc. The corresponding treatment method is as follows: re weld the welding points of the sensor bridge and clean them thoroughly. In addition, due to the contact time between the electric soldering iron and strain gauge, flexible circuit board and terminal block is too long, the insulation strength of these electrical components is reduced. At this time, these electrical components should be replaced.

(2) The insulation impedance between the elastomer and the bridge is less than 1 * 10 Ω, that is, the insulation is on, which is shown by the insulation resistance tester

When measuring, the pointer swings violently. The reason for this is that some part of the bridge is in contact with the elastomer, and the insulation impedance is zero; or it seems that the connection is not connected. In this case, the insulation group impedance is extremely unstable during the test, which is generally caused by the outer layer damage of the enameled wire or plastic wire used for bridge assembly. In this case, the damaged components should be replaced.

 

Insufficient insulation impedance has always been a problem for many sensor manufacturers, because a little improper operation in the process of bridge assembly may lead to sensor insulation out of tolerance. The bridge circuit of the sensor has at least four strain gauges, most of which are composed of eight strain gauges, or even more. When insulation out of tolerance occurs, it is very difficult to find out and determine the fault. Therefore, in order to solve these problems, summarize the experience of field production in recent years, and find the fault point according to the following steps: ① disconnect the two ends of the output line of the sensor bridge, make it into two parts, measure the insulation impedance of these two parts respectively, and divide the bridge arm whose insulation impedance is out of tolerance into two parts and measure them in turn until the fault point is found. This method is the most effective method for multi strain gauge bridge circuit, which can quickly and effectively determine the fault point; ② weld the determined fault point again, and then connect the bridge circuit after the insulation impedance test is qualified; ③ after the bridge circuit connection is completed, check the bridge circuit again according to the drawing, and measure the input, output impedance and insulation impedance, which can be submitted after passing the test. If it is still unqualified, repeat steps ①, ② and ③ until it is qualified.

 

3.2.5 analysis of bridge impedance out of tolerance

Bridge impedance out of tolerance means that the input impedance, output impedance and zero bridge impedance (i.e. the impedance between adjacent bridge arms) of the sensor do not conform to the nominal value. As shown in Fig. 2, the circuit diagram of the sensor bridge shows the input impedance and output impedance of the sensor respectively.

Figure 2 impedance diagram of resistance strain Load Cell

 

3.3 common faults and analysis of compensation process

3.3.1 temperature drift

When the temperature of the sensor changes, the elastomer, patch adhesive and strain gauge will have different degrees of thermal expansion and cold contraction with the change of temperature, which will cause the change of strain gauge wire grid, so as to change the impedance and output value. In general, the zero point output value of the sensor will be proportional to the change of temperature. In this way, a resistance R with large resistance temperature coefficient in series in the sensor bridge circuit can counteract the zero temperature drift. This process is called temperature compensation.

Fig.3 temperature drift curve of load cell

 

3.3.2 Analysis of initial instability

Due to the solder joint false soldering and other contacts of the circuit are not reliable, the zero point output will be unstable.

 

3.3.3 Analysis of bridge break

In huisitong bridge, if one or more bridge arms are short circuited, the impedance will increase.

 

3.3.4 Common faults and analysis of testing process

The common faults in the test process are as follows:

(1) Initial out of tolerance: the output value of zero point is larger than the required range;

(2) No output of loading: due to tension, incorrect connection of pressure strain gauge or wrong bridge circuit in huisitong bridge;

(3) Linear out of tolerance: the linear error exceeds the accuracy required by the sensor;

(4) Lag out of tolerance: the lag error exceeds the accuracy required by the sensor;

(5) Creep out of tolerance: creep exceeds the accuracy required by the sensor;

 

4:Conclusion

In a word, there are many reasons for the failure of the strain gauge sensor in the actual manufacturing process, and sometimes several faults may occur at the same time. It is better to make a sensor checklist based on the working principle of the sensor, so as to improve the pertinence and efficiency of the analysis, so as to quickly and accurately find the fault. When checking the fault of strain sensor, we should first check the appearance of the sensor, check and measure the quality of the previous process in the next process, and then analyze and test it step by step according to the checklist, so that the fault can be found and quickly eliminated. Through the fault analysis of sensor manufacturing process, we can better optimize the design and standardize the process, so as to create high-precision, high-level and world-class electrical measurement products.