Load cell

I have a new saying on the weighing sensor

The load cell is actually a device that converts the mass signal into a measurable electrical signal output. Sensors should first consider the actual working environment in which the sensor is located. This is crucial for the correct selection of the load cell. It is related to whether the sensor can work properly and its safety and service life, and even the reliability and safety of the entire scale. Sex. In the basic concepts and evaluation methods of the main technical indicators of the load cell, there are qualitative differences between the old and new national standards. There are several types of S-type, cantilever type, spoke type, plate ring type, film box type, bridge type, and column type.

Chinese name Heavy sensor Foreign name The weighing sensor

Range 0-470 tons

1 basic introduction

The old national standard considers the use of two kinds of sensors, "weighing" and "measuring force," whose environmental conditions are completely different, to be considered together. No distinction is made between tests and evaluation methods. The old national standard has a total of 21 indicators, all of which are tested at room temperature; and the weight is determined by the maximum error among the six items of nonlinearity, hysteresis error, repeatability error, creep, zero-point temperature additional error, and rated output temperature additional error. The accuracy level of the sensor is represented by 0.02, 0.03, and 0.05 respectively.

A force sensor used on an instrument. It can convert the gravity acting on the measured object into a quantifiable output signal in a certain proportion. Taking into account the influence of gravity acceleration and air buoyancy on the conversion of different locations, the performance indicators of the load cell mainly include linear error, hysteresis error, repeatability error, creep, zero temperature characteristics and sensitivity temperature characteristics. In various weighing and mass measurement systems, integrated errors are usually used to bring about comprehensive control of the sensor accuracy, and the integrated error band is linked to the scale error band so that a weighing sensor corresponding to an accuracy scale can be selected. The OIML stipulates that the error band ?′ of the sensor accounts for 70% of the error band ?” of the scale, the linearity error of the load cell, the lag error, and the error due to the effect of temperature on the sensitivity within the specified temperature range. The sum cannot exceed the error band ?′. This allows the manufacturer to adjust the individual components that make up the total error in the measurement to obtain the desired accuracy.

Category 2

According to the conversion method, the load cell can be divided into 8 types: photoelectric type, hydraulic type, electromagnetic type, capacitive type, magnetic pole change type, vibration type, gyro ceremony, and resistance strain type. The most widely used is the strain resistance type.


Including grating type and code disc type.

The grating sensor uses the moire fringes formed by the grating to convert the angular displacement into photoelectric signals. There are two gratings, one is a fixed grating, and the other is a moving grating mounted on the dial axis. The measured object added on the bearing platform rotates the dial shaft through the force transmission lever system, and drives the moving grating to rotate so that the moire fringe also moves. With the use of photocells, conversion circuits and display instruments, the number of moving moire stripes can be calculated and the size of the grating rotation angle can be measured to determine and read out the quality of the measured object.

The encoder disc (code plate) of the encoder is a transparent glass mounted on the axis of the dial with a black and white code programmed according to a certain coding method. When the measured object added to the bearing platform rotates the dial shaft through the force transmission lever, the code wheel also rotates with a certain angle. The photocell will receive the optical signal through the encoder and convert it into an electrical signal. The circuit will then perform digital processing. Finally, the display will show the digits representing the measured mass. Photoelectric sensors have been mainly used in electromechanical combination scales.

Hydraulic type

When the subject's gravity P acts, the pressure of the hydraulic oil increases, and the degree of increase is proportional to P. Measuring the increase in pressure, you can determine the quality of the measured object. The hydraulic sensor has a simple and robust structure and a large measuring range, but the accuracy is generally not more than 1/100.


It uses the proportional relationship between the oscillation frequency f of the capacitor oscillation circuit and the plate spacing d. There are two plates, one fixed and the other movable. When the load-bearing table is loaded with the measured object, the leaf spring is deflected, the distance between the two plates changes, and the oscillation frequency of the circuit also changes. The change of frequency can be used to determine the quality of the measured object on the bearing platform. Capacitive sensors consume less power, have lower cost, and have an accuracy of 1/200 to 1/500.

The main advantage

Resistance, inductance and capacitance are three types of passive components in electronic technology. A capacitive sensor is a sensor that converts a measured change into a change in capacitance, which is essentially a capacitor with a variable parameter.

Capacitive sensors have the following advantages:

(1) High impedance, low power, requiring only low input energy.

(2) Larger variations can be obtained, resulting in higher signal-to-noise ratio and system stability.

(3) The dynamic response is fast, the operating frequency can reach several megahertz, the thick b contact measurement, and the measured object can be conductor or semiconductor.

(4) Simple structure. Adaptable, can work in harsh environments such as high and low temperature, strong radiation, and has a wide application.

With the development of electronic technology and computer technology, the disadvantages of capacitive sensors, such as being susceptible to interference and susceptible to distributed capacitance, have been continuously overcome, and capacitive displacement sensors and integrated capacitive sensors have also been developed: Widely used in measurement and automatic detection, can measure pressure, displacement, speed, acceleration, A degree, thickness, liquid level, humidity, vibration, component content and other parameters. Capacitive sensors have very good prospects for development.

The main drawback

Disadvantage 1: high output impedance, poor load capacity

Disadvantage 2: Non-linear output characteristics

Disadvantage 3: Large influence of parasitic capacitance

Electromagnetic type

It uses the principle of balancing the load and electromagnetic force on the load-bearing platform. When the object to be measured is placed on the bearing platform, one end of the lever is tilted upward; the photoelectric element detects the inclination signal, and after being amplified, flows into the coil to generate electromagnetic force, so that the lever is restored to an equilibrium state. Digital conversion of the current generating electromagnetic balance force can determine the quality of the measured object. Electromagnetic force sensors have a high accuracy of 1/2000 to 1/60000, but the weighing range is only a few tens of milligrams to 10 kilograms.

Magnetic pole variation

When the ferromagnetic element mechanically deforms under the influence of the measured object's gravity, internal stress is generated and the permeability is changed, so that the induced voltage of the secondary coil wound on both sides of the ferromagnetic element (magnetic pole) also changes. By measuring the change in voltage, the force applied to the magnetic pole can be determined to determine the quality of the measured object. The accuracy of the magnetic pole change sensor is not high, generally 1/100, suitable for large-tonnage weighing work. The weighing range is from tens to tens of thousands of kilograms.

Vibration type

After the elastic element is stressed, its natural vibration frequency is proportional to the square root of the force. By measuring the change in the natural frequency, the force acting on the elastic element of the measured object can be obtained, and the quality thereof can be determined. Vibrating sensors have two types of vibrating wire and tuning fork.

The vibrating wire sensor's elastic element is a string. When the object to be measured is loaded on the bearing platform, the intersection of the V-shaped string is pulled downward, and the pulling force of the left string increases, and the pulling force of the right string decreases. The natural frequency of the two strings changes differently. Calculate the difference between the frequencies of the two strings to determine the quality of the measured object. The accuracy of the vibrating wire sensor is as high as 1/1000 to 1/10000, and the weighing range is from 100 g to several hundred kilograms. However, the structure is complex, the processing is difficult, and the cost is high.

The tuning element of the tuning fork sensor is a tuning fork. At the tip of the tuning fork is a piezoelectric element that oscillates at the natural frequency of the tuning fork and measures the oscillation frequency. When the test object is loaded on the load-bearing table, the tuning fork pulls its direction and the natural frequency increases. The degree of increase is proportional to the square root of the applied force. By measuring the change in the natural frequency, the force exerted by the weight on the tuning fork can be found, and the mass of the weight can be obtained. The tuning fork sensor has low power consumption, the measurement accuracy is as high as 1/10000 to 1/200000, and the weighing range is 500g to 10kg.

Gyro Ceremony

The rotor is mounted in the inner frame and rotates stably around the X axis at an angular velocity ?‰. The inner frame is coupled with the outer frame via a bearing and is tiltable about a horizontal axis Y. The outer frame is connected to the machine base via a universal joint and can be rotated about the vertical axis Z. The rotor shaft (X-axis) remains horizontal when it is not subjected to external forces. When one end of the rotor shaft is subjected to an external force (P/2), it tilts and rotates around the vertical axis Z (precession). The precessional angular velocity ?‰ is proportional to the external force P/2. By measuring the frequency, ?‰ is obtained, and then the magnitude of the external force can be obtained, and the mass of the test object that generates the additional force can be obtained.

Gyro ceremonial sensor response time is fast (5 seconds), no hysteresis, good temperature characteristics (3ppm), small vibration effects, accurate frequency measurement accuracy, it can get high resolution (1/100000) and high measurement accuracy (1/30000 to 1/60000).

Resistance strain type

When the strain gauge is deformed, its resistance also changes. Mainly consists of elastic components, resistance strain gauges, measuring circuits and transmission cables.

Plate ring

The structure of the plate-ring load cell has the advantages of a clear distribution of stress lines, high output sensitivity, an elastic body as a whole, a simple structure, a stable stress state, and easy processing. At present, the sensor production still occupies a large proportion, and the design formula of this structure sensor is not very perfect at present. Because the strain calculation of this kind of elastic body is more complicated, usually it is regarded as the annular elastic body to estimate while designing. In particular, platen sensor designs with a range of 1t and below are more computationally intensive and often exhibit large nonlinearities.

Plate ring weighing sensor uses and features: compact structure, good protection performance. High precision and long-term stability. Applicable to the measurement of crane scales, electromechanical combination scales and other force values


1. Definition

The digital load cell is a force-electricity conversion device that can convert gravity into an electrical signal. It mainly refers to a resistance strain-type load cell, an electronic amplifier (abbreviation AMC), and an analog-to-digital conversion (ADC for short). A new type of sensor, microprocessor (abbreviated as MCU).

2. Features and Applications

The development of digital load cell and digital metering technology has gradually become the new darling of the weighing technology field. It is emerging in this field with the advantages of simple and efficient commissioning and adaptability to the field.

3.S type definition

S type load cell

The S-type load cell is the most common sensor in the sensor as shown in the figure. It is mainly used to measure the tension and pressure between solids. It is also commonly known as pull pressure sensor because it looks like an S shape. Also known as S-type load cell, this sensor is made of alloy steel, rubber seal protection, easy installation, easy to use, suitable for hanging scales, batching scales, machine scales and other electronic measuring weighing system.

3 constitute 1, the sensitive element

Directly feels the measured (mass) and outputs other quantities of components that have a definite relationship with the measurement. For example, the elastic body of the resistance strain-type load cell transforms the mass of the measured object into deformation; the elastic body of the capacitive load cell transforms the measured mass into displacement.

2, the transformation element

Also known as a sensing element, it converts the output of a sensitive element into a signal that facilitates measurement. Such as resistance strain gauge load cell resistance strain gauge (or resistance strain gauge), the deformation of the elastomer is converted into a change in the resistance; capacitance of the load cell capacitor, the displacement of the elastomer into a change in capacitance . Sometimes some components combine the functions of both sensitive and transforming components. For example, the piezoelectric material of the voltage-type load cell under the effect of the external load will output the electricity while being deformed.

3, measuring elements

Transform the output of the transform element into an electrical signal to facilitate further transmission, processing, display, recording, or control. For example, the bridge circuit in the resistance strain load cell, the charge preamplifier for the piezoelectric load cell.

4, auxiliary power supply

Provides energy for the sensor's electrical signal output. General load cells require external power supply to work. Therefore, as a product must be marked the requirements of power supply, but not as an integral part of the load cell. Some sensors, such as magnetoelectric speed sensors, do not need an auxiliary power supply because they output a relatively large amount of energy. So not all sensors need auxiliary power.

Principle 4

The resistance strain load cell is based on the principle that the elastic body (elastic element, sensitive beam) is elastically deformed under the external force, and the strain gage (transducing element) attached to the surface of the elastic strainer is also deformed along with it. After deformation, its resistance will change (increase or decrease), and this resistance change will be converted into an electrical signal (voltage or current) by a corresponding measurement circuit, thereby completing the process of converting the external force into an electrical signal. .

From this, it can be seen that the resistance strain gauges, elastomers, and detection circuits are indispensable parts of the resistance strain gauge load cell. The following brief discussion of these three aspects.

First, the resistance strain gauge

Resistance strain gauges are mechanically distributed on a substrate made of an organic material, ie, a strain gauge. One of his important parameters is the sensitivity coefficient K. Let us introduce its significance.

A metal wire having a length of L and a cross-section of a circle with a radius of r is denoted by S. Its resistivity is denoted by ?? and the Poisson coefficient of this material is ??. When this resistance wire is not affected by an external force, its resistance value is R:

R = ??L/S(??) (2a€”1)

When his ends are subjected to F forces, they will stretch, that is, they will be deformed. Assuming its elongation ?”L, its cross-sectional area is reduced, that is, its sectional circle radius is reduced by ?”r. In addition, experiments can also be used to prove that after the metal resistance wire is deformed, the resistivity will also change and be denoted as ?”??.

The formula (2--1) is fully differentiated, that is, after the resistance wire is stretched, how much the resistance value changes. We have:

?”R = ?”??L/S + ?”L??/S a€“ ?”S??L/S2 (2-2)

Removal of Formula (2--2) by Formula (2--1)

?”R/R = ?”??/?? + ?”L/L a€“ ?”S/S (2-3)

In addition, we know that the cross-sectional area of a€?a€?the conductor S = ?€r2, then ?”s = 2?€r*?”r, so

?”S/S = 2?”r/r (2a€”4)

From material mechanics we know

?”r/r = -???”L/L (2-5)

Among them, the negative sign indicates that the radius is reduced when it is stretched. ?? is the Poisson coefficient that represents the lateral effect of the material. Substituting formula (2-4) (2-5) into (2--3),

?”R/R = ?”??/?? + ?”L/L + 2???”L/L

= (1 + 2??(?”??/??)/(?”L/L))*?”L/L

= K *?”L/L (2--6)

among them

K = 1 + 2?? + (?”??/??)/(?”L/L) (2--7)

The formula (2--6)) describes the relationship between the rate of change in resistance of the electrical resistance strain gauge (relative resistance change) and the elongation rate of the electrical resistance (relative length change).

It should be noted that the sensitivity factor K value is a constant determined by the properties of the metal wire material. It is independent of the shape and size of the strain gauge. The K value 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 material mechanics, ?”L/L is referred to as strain, denoted as ?μ, and it is often used to indicate that the elasticity is too large and it is inconvenient.

One millionth of its unit is often referred to as ???μ. In this way, formula (2--6) is often written as:

?”R/R = K?μ (2-8)

Second, the elastomer

The elastomer is a specially shaped structure. It has two functions. The first is that it withstands the external force applied to the load cell, generates a reaction force to the external force, and achieves relative static balance; secondly, it produces a high-quality strain field (zone) so that it can be pasted in this area. The resistance strain gauge is ideal for completing the conversion task of the strained jujube signal.

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

There is a cuboid cantilever beam with a boring hole.

The center of the bottom of the borehole is subjected to pure shear stress, but tensile stress and compressive stress will appear in the upper and lower parts. The main stress direction is a pull God, a compression, if the strain gauge is affixed here, the upper half of the strain gauge will be stretched and resistance increases, and the lower half of the strain gauge will be compressed, reducing the resistance. The following is a list of strain expressions for the center point of the bottom of the boring hole. It is not deduced.

?μ = (3Q(1+??)/2Eb)*(B(H2-h2)+bh2)/(B(H3-h3)+bh3) (2--9)

Among them: the shear force on the Q-section; E--Young's modulus: ??-Poisson coefficient; B, b, H, h- are the geometric dimensions of the beam.

It should be noted that the above analysis of the stress state is a "local" situation, and the strain gauge actually feels the "average" state.

Third, the detection circuit

The function of the detection circuit is to convert the resistance change of the resistance strain gauge into a voltage output. Because the Wheatstone bridge has many advantages, such as suppressing the influence of temperature changes, suppressing the lateral force interference, and more easily solving the compensation problem of the load cell, the Wheatstone bridge is obtained in the load cell. A wide range of applications.

Because the full-bridge equal-arm bridge has the highest sensitivity, the parameters of each arm are the same, and the influence of various interferences is easy to cancel each other out. Therefore, the full-bridge equal-arm bridge is used for the load cell.

5 commonly used materials

The performance of a load cell depends largely on the choice of materials of manufacture. The load cell material includes the following components: strain gauge material, elastomer material, patch adhesive material, sealant material, lead sealing material, and lead material.

Strain gauges and resistance element materials

The strain gauge is the sensing part of the load cell, which converts the size of the external force into the electrical output. It is the most important component of the sensor. The commonly used strain gauge substrate is made of polymer film material, and the strain material is usually high-purity Constantan. The performance of strain gauges is not only related to the purity of the substrate and Constantan, but also related to the manufacturing process. Improving the level of process technology is also an important aspect of improving sensor performance.

Elastomer material

The function of the load cell elastic body is to transfer the external force, it must have the same deformation when the same force, because the strain gauge is stuck on the elastic body, the deformation of the elastic body is the deformation of the strain gauge; With reset, when the external force disappears, it can be automatically reset. Elastomer materials are usually selected from a variety of metals, including aluminum alloys, stainless steels, and alloy steels.

Patch adhesive material

Patch adhesives firmly hold the strain gauges and elastomers together so that they will always have the same deformation. This shows that patch adhesives are also an important component. At the beginning of the 21st century, the use of a multi-patch adhesive is a two-component polymer epoxy adhesive. At the beginning of the 21st century, its performance was very much related to its own purity, mixing mode, storage time, curing mode, curing time, etc., and its details should be carefully reviewed before use.

Sealant material

Early load cell seals were made of sealant. Later, due to the development of manufacturing technology, the use of welding technology can improve the stability and service life of extremely large sensors. Although many welding techniques were used in the early 21st century, some important parts need to be coated with some sealant. Silicone sealants are generally used, silica gel has the advantages of good stability, can prevent moisture, corrosion, insulation is also very good.

Lead sealing and lead material

If the sensor output leads are not fixed, they may be damaged or loose, resulting in unstable signals or no output. At the beginning of the 21st century, the sensor output was used as a connector, and the material and fastening strength of the connector also affected the output. It is best to use connectors and sealants. The inner leads also need to be fixed to prevent them from moving around. The quality of the leads is also important. The order of their material properties from high to low is silver, copper, and aluminum. If the surrounding high-frequency signals and radio wave interference are serious, shielded cables must be used; in corrosive environments and flammable and explosive environments, anti-corrosion, flame-retardant and explosion-proof cables are required, and a sleeve is used for protection.

6 choice

In addition, the sensitivity of the load cell, the maximum number of divisions, the minimum value of the test division, etc. are also indicators that must be considered in the selection of the sensor.

The number and range of sensors

The selection of the number of sensors is based on the use of the electronic scale and the number of points the scale body needs to support (the number of support points should be determined according to the principle of the geometric center of gravity of the scale body and the coincidence of the actual center of gravity). In general, there are several supporting points on the scale body to select several sensors.

The range selection of the sensor can be determined based on the maximum weighing value of the scale, the number of sensors selected, the weight of the scale body, the maximum partial load that can be generated, and the comprehensive evaluation of the dynamic load factor. The following gives an empirical formula that has been validated by a large number of experiments.

The formula is as follows:


Where C is the rated range of a single sensor

W scale body weight

Wmax is the maximum value of the net weight of an object

The number of supporting points used by N-scale body

K0 insurance factor, generally between 1.2 and 1.3

K1 impact factor

K2 scale center offset coefficient

K3 pressure coefficient

Use environment

The load cell is actually an electrical signal output device that converts the mass signal into a measurable electrical signal. The sensor must first consider the actual working environment in which the sensor is located. This is crucial for the correct selection of the sensor. It is related to whether the sensor can work properly, its safety and service life, and even the reliability and safety of the entire scale. Under normal circumstances, the high temperature environment causes problems such as melted coating materials, open solder joints, and structural changes in the elastic body within the sensor. Dust and moisture may cause short-circuit effects on the sensor. In a highly corrosive environment, the sensor elastomer may be affected. Damage or short-circuiting; Electromagnetic fields can interfere with sensor output. Under the corresponding environmental factors we must select the corresponding load cell to meet the necessary weighing requirements.

Accuracy level selection

The accuracy level of the load cell includes the sensor's nonlinearity, creep, repeatability, hysteresis, and sensitivity.

Application range and use

For example, aluminum alloy cantilever sensors are suitable for electronic pricing scales, platform scales, case scales, etc.; steel cantilever sensors are suitable for electronic belt weighers, sorting scales, etc.; steel bridge sensors are suitable for track scales, truck scales, etc.; The sensor is suitable for truck scales, dynamic rail scales, large-tonnage hopper scales and the like. The load cell is mainly used in various electronic weighing instruments, industrial control fields, online control, safety overload alarm, material testing machine and other fields. Such as electronic truck scales, electronic platform scales, electronic forklifts, dynamic axle weight scales, electronic crane scales, electronic pricing scales, electronic steel scales, electronic rail scales, hopper scales, batching scales, canned scales and so on.

7 wiring method

There are four types of weighing sensor outlet line and six lines, the module or weighing transmitter wiring is also four-wire and six-wire two, to connect the four-wire or six-wire first depends on your hardware requirements are what The principle is: The sensor can receive 6 lines without 4 lines, and must be connected with 4 lines for short connection.
The general load cell is a six-wire system. When connected to a four-wire system, the power lines (EXC-, EXC+) and feedback lines (SEN-, SEN+) are shorted separately. SEN+ and SEN- are used to compensate for line resistance. SEN+ and EXC+ are pathways, SEN- and EXC- are pathways.
EXC+ and EXC-power the load cell, but due to line losses between the weigh module and the sensor, the actual voltage received by the sensor will be less than the supply voltage. Each load cell has a mV/V characteristic. Its output mV signal is closely related to the received voltage. SENS+ and SENS- are actually a high-impedance loop in the load cell, which can actually weigh the module. The received voltage is fed back to the weigh module. Assuming EXC+ and EXC- are 10V, line loss, and sensor 2mV/V, the actual sensor output signal is ()*2=19mV instead of 20mV. At this moment, the internal load cell will take 19mV as the maximum range, provided that the sensor must feedback the actual voltage to the weighing module through the feedback loop. Connect the EXC+ to the SENS+ short-circuit and EXC- to the SENS- to the load cell. Only connect the sensor to the weigh module. If the voltage loss is very small, there will be errors in the measurement.

8 Installation Precautions

1. The load cell should be handled lightly, especially for small-capacity sensors that use alloy aluminum as the elastomer. Any shock or drop caused by vibration is likely to cause a large output error.

2. Design loading device and the role of loading force should be ensured during installation The force axes of the load cell are coincident, so that the influence of the tilt load and the eccentric load is minimized.

3, in the level of adjustment. If a load cell is used, the mounting plane of the base must be adjusted to the level using a level gauge; if it is a case where multiple sensors are measuring simultaneously, then the mounting surface of their base should be kept on a horizontal plane as much as possible. The main purpose of this is to do so. It is to ensure that the strength of each sensor is basically the same.

4. Determine the rated load of the sensor used according to the range selection of the load cell in its description.

5, to prevent chemical corrosion. The Vaseline coating should be used to coat the outer surface of the load cell. Avoid direct sunlight and changes in the ambient temperature of the use of the station.

6. The bypass device made of copper braided wire is connected at both ends of the load cell loading device.

7, the cable should not be extended by itself, in the real need to lengthen the joint should be soldered, and add moisture-proof sealant.

8. It is best to use some baffles around the load cell to cover the sensor. The purpose of doing so is to prevent debris from falling into the moving part of the sensor and affecting its measurement accuracy.

9, the sensor cable should be away from the strong power supply line or pulsed wave place, can not avoid the competition should separate the load sensor cable into the iron pipe, and try to shorten the connection distance.

10. Determine the rated load of the used sensors according to the selection of the load cell range in the description. Although the load cell has its own overload capacity, it should be avoided during the installation and use. Sometimes short-term overloading may also cause permanent damage to the sensor.

11. In the case of high precision applications, the load cell/meter should be used after 30 minutes of warm-up.

9 working process

During the measurement process, weight loading on the elastomer of the load cell causes plastic deformation.

Resistance strain-type load cell working process

Strain (positive and negative) is converted to electrical signals through strain gauges mounted on the elastomer.

10 Instrument Applications

The weighing instrument is also called the weighing display control instrument. It is an electronic device that converts the weight sensor signal (or through the weight transmitter) into a weight digital display and stores, counts, and prints the weight data. Automated ingredients in agricultural production are weighed to increase production efficiency.

The performance indicators of weighing instruments applied in industrial enterprises are usually characterized by precision (also called accuracy), variation, and acuity. Meter calibration instruments are also often calibrated for accuracy, variation, and sensitivity.

1. The variation refers to the maximum difference between the indicator values a€?a€?when the measured variable of the weighing instrument (which can be regarded as the input signal) reaches the same value from the deviation multiple times. It is probably the instrument under the stable external environment. Measured parameters change from small to large (positive characteristics) and measured parameters change from large to small (reverse characteristics) to a degree that is not uniform, and the difference between the two is the instrument deterioration. Reliability Reliability of weighing control instruments is another important performance indicator sought by the instrumentation industry of chemical companies. The reliability and maintenance of the instrument are inversely related. The high reliability of the instrument clarifies the maintenance of the instrument. On the contrary, the reliability of the instrument is poor and the instrument maintenance is large. To deal with the detection and process control instruments of chemical companies, most of them are placed in process pipelines, various types of towers, kettles, tanks and vessels.

2. The stability of the weighing instrument in the load cell In determining the conditions of work, the ability of the weighing instrument to connect with certain properties over time is called stability (degrees). Instrument stability is a very considerate day performance indicator for chemical industry instrumentation workers. Because the chemical industry uses the environment of the instrument to compare badly, the temperature and pressure of the medium to be measured are also greatly changed. In this environment, the instrument is put into use, and certain components of the instrument will be connected with the stability of the time. The stability will land.??? ??? è?¨??? è?¨??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??–?·¥ ??–?·¥ ??–?·¥. The advantages and disadvantages of the stability of the weighing instrument are directly related to the scope of use of the instrument, and it directly affects the chemical production. The impact caused by the poor stability causes the accuracy of the double meter to fall even more to the chemical production. Poor stability The instrument maintenance is also large, and it is the most undesirable thing for the instrumentation worker.

3. The acuity of the weighing instrument is occasionally referred to as the "magnification ratio", which is also the slope of each point on the sticking line of the static characteristics of the instrument. Increasing the magnification can improve the sensitivity of the instrument. Simply increasing the sensitivity does not change the basic performance of the instrument. That is, the accuracy of the weighing instrument does not improve, but on the contrary, it will occasionally appear oscillation signs, causing the output to be unstable. Instrument sensitivity should be linked to the proper amount.

For most customers, although instrument accuracy is a stress indicator, in actual use, it always emphasizes the stability and reliability of the instrument, because the number of chemical industry inspection and process control instruments used for metering is not large, but a large number of Is for testing. In addition, the use of measurement instruments in the process control system is more tense than the accuracy and reliability.

With the upgrading of the instrumentation, especially the introduction of microelectronics skills to the weighing instrument manufacturing industry, the instrument's reportability has been greatly improved. Instrument manufacturers are also increasingly cherishing this day's performance metrics, usually using an average MTBF without barriers to the reliability of topographies. The MTBF of an all-intelligent weighing transmitter is about 10 times higher than usual non-intelligent instruments such as the electric III transmitter. The weighing instrument must be digitally calibrated with the load cell before use. Calibration is actually using standard weights to calibrate the instrument. The calibrated meter internally holds calibration coefficients relative to this group of sensors. With this factor, the instrument can convert the analog signal from the load cell into a weighted digital display.

11 product defects

Disadvantage 1: high output impedance, poor load capacity

The capacity of a capacitive load cell is not easily limited by the geometry of its electrodes, typically tens to hundreds of microfarads, or even only a few microfarads. Therefore, the capacitive load cell has a high output impedance, so the load capacity is poor, and it is vulnerable to external interference, resulting in instability, and even when it is severe, it cannot work. Proper shielding measures must be taken to bring inconvenience to design and use. The large capacitive reactance also requires that the resistance value of the insulated part of the sensor is extremely high. Otherwise, the insulating part will act as a bypass resistance and affect the performance of the instrument. Therefore, special attention must be paid to the surrounding environment such as temperature and cleanliness. If high-frequency power supply is used, the output resistance of the capacitive load cell can be reduced, but the high-frequency amplification and the sensor are far more complicated than the low frequency, and the parasitic capacitance has a large influence, and it is difficult to ensure the stability of the work.

Disadvantage 2: Non-linear output characteristics

The output characteristics of the capacitive load cell are non-linear. Although the differential type is used to improve, it cannot be completely eliminated. For other types of capacitive sensors, the output characteristic is linear only when the edge effect of the electric field is ignored. Otherwise, the additional capacitance generated by the edge effect will be directly superimposed on the sensor capacitor, making the output characteristic non-linear.

Disadvantage 3: Large influence of parasitic capacitance

The initial capacitance of the capacitive load cell is small, and the so-called parasitic capacitance such as the lead capacitance connecting the sensor and the electronic circuit, the stray capacitance of the electronic circuit, and the capacitance formed between the plate and the surrounding conductor of the sensor is insufficient, which not only reduces the sensor The sensitivity, and these capacitors are often randomly changing, will make the instrument work very unstable, affecting the measurement accuracy. Therefore, there are strict requirements for the selection, installation and connection of cables. For example, high-frequency wires with good shielding properties and small self-distribution capacitance are used as the leads. The leads are thick and short, and the stray capacitance of the instrument must be small and stable. Otherwise, high measurement accuracy cannot be guaranteed.

It should be pointed out that with the rapid development of materials, processes, electronic technologies, and especially integrated technologies, the advantages of capacitive load cells have been developed and disadvantages have been overcome. Capacitance sensors are gradually becoming a highly sensitive, high-precision sensor with promising development in dynamic, low voltage and some special measurements.

12 Error Analysis

1. Errors in the use of load cells are caused by the operator. This also means that there are many reasons for the occurrence, for example, errors occurring when the temperature is different, including improper placement of the probe or improper insulation between the probe and the measurement address. Some application errors include faults that occur during the purification of air or other gases, and application errors also affect the placement of the transmitter's faults, so that positive or negative pressure will affect the correct reading.

2. The characteristic error is intrinsic to the equipment itself, which is the difference between the equipment's and the recognized transfer function characteristics and the actual characteristics. Such errors include the DC drift value, the incorrect slope, or the non-linear shape of the slope.

3. Dynamic errors Many sensor characteristics and calibrations are applicable under static conditions, which means that the input parameters used are static or static-like, and many sensors have strong damping so they do not change the input parameters. Fast response, for example, the thermistor requires a few seconds to respond to step changes in temperature.

4. The thermistor does not jump immediately to a new impedance or changes abruptly. Instead, it is slowly changed to a new value. Then, if the load cell with a deferred characteristic echoes the rapid change of temperature. ???è?“??o????3¢??¢?°??¤±???????”±?o??…?é—′??…????o???¨?€??·?é”??€???‘?”???¨?€??·?é”????è|??′???‰?‘??o”?·¥?¤??€???ˉ?1…?¤±????’????????¤±????€?




Model: STC-100Kg ????????·è§??????‰

Cap: 100Kg ???é???¨?è????′??‰

Date: 2005/01/14 ????”??o§?—¥?????‰

S/N: X02274 ?????o?????–??·??‰

FSO: 2.9981 mV/V ?????μ????o|??‰

Recommended Excitation: 10V AC/DC ?????¨è????€??±?”μ?????‰

Maximum Excitation: 15V AC/DC ?????€?¤§??€??±?”μ?????‰

Output at Rated Load: 2.9981 mV/V ???é¢????è′?è?·è?“??o??‰

Non Linearity: <0.020% ???é???o??€§??‰

Hysteresis: <0.020% ???????????‰

Creep???30 minutes??‰: 0.029% ???30???é’?è????¨??‰

Non Repeatability: <0.01% ???é??é???¤??€§??‰

Zero Retum???30 minutes??‰: 0.030% ???30???é’?é????1????§???‰

Temp. Effect/a?? on Span: <0.0015% ???????o|?????–1a???ˉ1é???¨??????±?“???‰

Temp. Effect/a?? on Zero: <0.0026% ???????o|?????–1a???ˉ1é????1?????±?“???‰

Compensated Temp.Range: -10 to 40a?? ???????o|è?¥???è????′??‰

Operating Temp.Range: -20 to 60a?? ????·¥???????o|è????′??‰

Zero Balance: ?±1% ???é????1?13è????‰

Input Resistance: 380?±5?? ???è?“?…¥é????—??‰

Output Resistance: 350?±3?? ???è?“??oé????—??‰

Insulation Resistance???50VDC??‰: >5000M?? ??????????”μé????‰

Deflecion at Rated Load: Nil ???é????‰ ???é¢????è′?è?·???????€??–??o|??‰

Safe Overload: 150% ????…?è??è?…è????‰

Ultimate Overload: 300% ?????€???è?…è????‰












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