【 Abstract 】The weighing load cell is known as the core component of electronic scales. This article mainly elaborates on 9 types of weighing load cells: resistance strain type, electromagnetic force type, photoelectric type, capacitive type, hydraulic type, vibration type, magnetic pole change type, gyroscope ceremony, and fiber optic grating type. The introduction and analysis of them separately are beneficial for weighing and force sensing technicians to further understand and master relevant knowledge.
【 Keywords 】Weighing load cell; Resistance strain gauge; Electromagnetic force; Capacitive load cell; Fiber Bragg Grating
The three foundations of modern information technology are information collection, transmission, and processing technologies, namely sensing technology, communication technology, and computer technology, which respectively constitute the “senses”, “nerves”, and “brain” of the information technology system. The primary component of an information collection system is the load cell, which is located at the forefront of the system. In a modern automatic detection system, without load cells, it is impossible to monitor and control various parameters characterizing various stages of the production process, and thus automatic control cannot be achieved. In modern technology, load cells are actually the foundation of modern testing and automation technologies.
The weighing load cell is referred to as the core component of electronic scales. With the rapid development of science and technology, electronic scales assembled from weighing load cells have also been widely applied in various industries, achieving fast and accurate weighing of materials. Especially with the emergence of microprocessors and the continuous improvement of automation in industrial production processes, weighing load cells have become a necessary component in certain process control. Currently, weighing load cells are almost applied in various industries.
According to the conversion method, weighing load cells are usually divided into 9 categories: resistance strain type, electromagnetic force type, photoelectric type, capacitive type, hydraulic type, vibration type, magnetic pole change type, gyroscope type, and fiber optic grating type, with resistance strain type being the most widely used. The following are the working principles, characteristics, and advantages and disadvantages of several commonly used weighing load cells.
1:Resistance strain type weighing load cell
The resistance strain gauge using metal materials as conversion components is based on the resistance strain effect of metal resistance wires. The so-called strain effect refers to a physical phenomenon where the resistance value of a metal conductor (resistance wire) changes with deformation (elongation or shortening) (Figure 1).
The resistance value of a metal conductor can be represented by the following equation: R= ρ In the formula △ L/△ S: ρ—— Electrical resistivity of metal conductors (Ω · cm2/m)
S – cross-sectional area of conductor (cm2)
L – Length of conductor (m)
The resistance strain gauge load cell is based on this principle
The load cell elastic body (elastic component) produces elastic deformation under the action of external force, so that the resistance Strain gauge (Figure 2) pasted on its surface also produces deformation. After the resistance Strain gauge is deformed, its resistance will change (increase or decrease), and then the resistance change will be converted into an electrical signal (voltage or current) through the Wheatstone bridge (Figure 3), thus completing the process of transforming the external force into an electrical signal.
The resistance strain type weighing load cell has a weighing range of tens of grams to thousands of tons, and a measurement accuracy of 1/1000~1/10000. With its simple structure, convenient installation, and reliable operation, it is widely used in most electronic scales. The advantages of resistance strain load cells are high accuracy, wide measurement range, long service life, simple structure, good frequency response characteristics, ability to work under harsh conditions, and easy to achieve miniaturization, integration, and variety. Its disadvantages include mechanical hysteresis, creep and zero drift, strain limit, significant nonlinearity for large strains, and weak output signals. However, certain measures can be taken to improve it. The resistance strain type weighing load cell is the most widely used type of weighing load cell, and its characteristics have been studied the most.
2:Electromagnetic force balance weighing load cell
The weighing process of the electromagnetic force balance load cell is based on the ampere force. The force acting on an energized wire in a magnetic field is shown in Figure 4. Add a scale to the wire, with the gravity direction of the scale and the wire itself facing downwards, while the energized wire is subjected to an upward electromagnetic force. When the current value passing through the wire reaches a certain value, these two forces balance each other, and the load cell is in a balanced state to determine the weight of the measured object. The electromagnetic force balance load cell relies on the electromagnetic force generated by the current carrying coil in a constant DC magnetic field to balance with the gravity of the measured mass, achieving the electrical signal conversion of the measured mass.
The electromagnetic force balance weighing load cell is a kind of high-precision load cell, with an accuracy of 1/2000~1/60000, but the weighing range is only between tens of milligrams to ten kilograms. It is widely used in electronic Analytical balance, and is a common load cell for precision mass weighing. Due to its high resolution, sensitivity, reliability, and affordability, it has been increasingly applied in fields such as precision quality weighing, chemical reaction monitoring, acceleration measurement, and moisture detection. To reduce the volume of the load cell, simplify the manufacturing process of the load cell, and reduce manufacturing costs, permanent magnets are used for the constant DC magnetic field of the electromagnetic force balance load cell. As a permanent magnet for electromagnetic force balance load cells, the stability of its magnetic performance directly affects the accuracy of the electromagnetic force balance load cell.
The electromagnetic force balance load cell is a relatively mature patented technology that requires high production conditions for the load cell. Currently, only a few large factories in the world have mastered this technology. This high-precision electronic weighing device, due to its unique working principle and technological level, has very high accuracy and is mainly used in the high-end market. Due to the lack of internal connecting screws, its reliability is greatly improved, while its efficient mechanical transmission performance and torque protection extend its service life.
3:Capacitive load cell
Capacitive weighing load cell is a load cell that converts measured mechanical quantities such as displacement, pressure, etc. into changes in capacitance. Its sensitive part is a capacitor with variable parameters, and its most commonly used form is a capacitor composed of two parallel electrodes with air as the medium between the electrodes (Figure 5). It operates using a positive proportional relationship between the oscillation frequency f of the capacitor oscillation circuit and the spacing between the plates 6. If the Edge effects is ignored, the capacitance of the plate capacitor is C=εA/6
In the equation ε Is the dielectric constant of the interpolar medium, A is the effective area of the two electrodes covering each other, and 6 is the distance between the two electrodes. 6. A ε Any change in any of the three parameters will cause a change in capacitance and can be used for measurement. Therefore, capacitive load cells can be divided into three types: pole distance variation type, area variation type, and dielectric variation type. The pole distance variation type is generally used to measure small linear displacements or pole distance changes caused by forces, pressures, vibrations, etc.
The advantages of capacitive weighing load cells are simple structure, low price, high sensitivity, strong overload capacity, low power consumption, and low cost,
The accuracy is generally 1/200-1/500, with good dynamic response characteristics and strong adaptability to harsh conditions such as high temperature, radiation, and strong vibration. The disadvantage is that the output has nonlinearity, and parasitic capacitance and distributed capacitance have a significant impact on sensitivity and measurement accuracy. Due to the inability of resistance strain type weighing load cells to be made into ultra-high temperatures or the high failure rate in ultra-high temperature environments, the use of capacitive weighing load cells usually has certain advantages in many ultra-high temperature environments where the weighing accuracy is not high or only for safety overload protection, such as some high-temperature hook scales use;
4:Photoelectric weighing load cell
Photoelectric load cells include two types: grating type and encoder type.
The working principle of a grating type load cell is to convert the angular displacement of Moire fringes formed by the grating into an optoelectronic signal. There are two gratings, one mounted on the dial shaft and movable, and the other fixed. After loading, the force lever drives the dial shaft to rotate, thereby driving the moving grating to rotate, and the Moire fringes also move accordingly. The Phototube, conversion circuit and display instrument are used to calculate the number of moire fringes moved, measure the rotation angle of the grating, and determine and read the quality of the measured object.
The code plate (symbol board) of the code plate type load cell is a transparent glass installed on the dial shaft, with black and white codes programmed according to a certain coding method. After loading, use a lever to rotate the dial shaft, and the code plate also rotates at a certain angle. Photocells receive optical signals through a code disk and convert them into electrical signals, which are processed by the circuit to obtain measurement results.
Photoelectric load cells were mainly used in electromechanical combined scales, which have the advantages of stable operation and strong anti-interference ability. They can be used in harsh industrial environments such as coal mines, but their operating speed is low and cannot meet the requirements of high-speed weighing. In recent years, due to the increasingly mature technology of resistance strain type weighing load cells, such load cells have not been widely used. As a transitional product, they have completed their mission and have almost withdrawn from the market.
5:Hydraulic weighing load cell
Hydraulic weighing load cell refers to the hydraulic oil pressure that increases when subjected to the gravity of the measured object, and the degree of increase is proportional to the gravity of the object. By measuring the increase in pressure, the mass of the measured object can be obtained. Hydraulic load cells have a simple and sturdy structure with a large measurement range, but their accuracy is generally less than 1/1000, and sometimes even less than 1/100. At present, it is mainly used in weighing and force measurement occasions with low precision requirements for weighing or safety overload protection, such as loader scales.
6:Magnetic pole variable form weighing load cell
When the ferromagnetic component of the magnetic pole variable form weighing load cell undergoes mechanical deformation under the gravity of the measured object, internal stress is generated and changes in permeability are caused, causing the induced voltage of the secondary coils on both sides of the ferromagnetic component (magnetic pole) to also change. Measure the change in voltage
The force applied to the magnetic pole can be calculated to determine the mass of the measured object. The accuracy of magnetic pole variant load cells is not high, usually 1/100, and is suitable for large tonnage weighing work. There is almost no practical application in the entire weighing load cell market.
7:Vibration type weighing load cell
After the elastic component is subjected to force, its natural vibration frequency is directly proportional to the square root of the applied force. By measuring the change in natural frequency, we can determine
The force exerted by the tested object on the elastic component is used to determine its mass. There are two types of vibration load cells: vibrating wire type and tuning fork type.
The elastic component of a vibrating wire load cell is a string wire. When a test object is added to the load-bearing platform, the intersection of the V-shaped string wires is pulled downwards, and the tension of the left string increases, while the tension of the right string decreases. The natural frequencies of two strings undergo different changes. Find the difference in frequency between two strings, i.e
The mass of the tested object can be determined. The accuracy of vibrating wire load cells is high, reaching 1/1000~1/10000, and the weighing range is 100 grams to
Several hundred kilograms, but with complex structure, high processing difficulty, and high cost.
The elastic component of a tuning fork load cell is a tuning fork. The end of the tuning fork is fixed with a piezoelectric component, which oscillates at the natural frequency of the tuning fork and can measure the oscillation frequency. When a test object is added to the load-bearing platform, the tuning fork is subjected to force in the stretching direction and its natural frequency increases, with the degree of increase proportional to the square root of the applied force. By measuring the change in natural frequency, the force exerted by a heavy object on the tuning fork can be determined, and then the mass of the heavy object can be determined. The tuning fork load cell has low power consumption, measuring accuracy of up to 1/10000~1/200000, and weighing range of 500g~10kg.
8:Gyro ritual weighing load cell
When analyzing the motion of a gyroscope from a mechanical perspective, it can be regarded as a rigid body with a directional pivot, and the gyroscope can rotate three degrees of freedom around this pivot. Therefore, the motion of the gyroscope belongs to the rotational motion of the rigid body around a fixed point. More precisely, a flywheel rotor that rotates at high speed around the axis of symmetry is called a gyroscope. Install the gyroscope on a frame device, so that the rotation axis of the gyroscope has angular degrees of freedom, and the overall structure of this device is called a gyroscope.
The basic components of a gyroscope include:
(1) Gyro rotor (often driven by synchronous motor, hysteresis motor, three-phase AC motor, etc. to rotate the gyro rotor at high speed around the autorotation axis, with its speed approximately constant);
(2) Inner and outer frames (also known as inner and outer rings, which are structures that enable the gyroscope’s rotation axis to obtain the required angular rotational degrees of freedom);
(3) Accessories (refer to Torque motor, signal load cell, etc.).
As shown in the figure, the rotor is installed in the inner frame at angular velocity ω Stable rotation around the X-axis. The inner frame is connected to the outer frame through bearings and can tilt and rotate around the horizontal axis Y. The outer frame is connected to the base through a universal coupling and can rotate around the vertical axis Z. The rotor shaft (X-axis) remains horizontal when not subjected to external forces. One end of the rotor shaft tilts and sags when subjected to external force (P/2)
Vertical axis Z rotation (precession). Precession angular velocity ω It is directly proportional to the external force P/2 and is measured by detecting frequency ω， The magnitude of the external force can be calculated, and then the mass of the tested object that produces this external force can be determined.
The gyroscope ritual load cell has a fast response speed (5 seconds), no hysteresis phenomenon, good temperature characteristics (3ppm), small vibration impact, and high frequency measurement accuracy. Therefore, it can achieve high resolution (1/100000) and high measurement accuracy (1/30000~1/60000).
9:Fiber Bragg Grating Weighing load cell
Fiber Grating load cell is a type of fiber optic load cell, and the sensing process based on fiber grating is,The sensing information is obtained by modulating the wavelength of fiber Bragg grating (Bragg) through external physical parameters. It is a wavelength modulated fiber optic load cell that can directly measure physical quantities such as temperature and strain. Due to the sensitivity of fiber Bragg grating wavelength to both temperature and strain, which causes the coupling wavelength of the fiber Bragg grating to shift simultaneously, it is impossible to distinguish between temperature and strain by measuring the coupling wavelength shift of the fiber Bragg grating. Therefore, solving the problem of cross sensitivity and achieving differential measurement of temperature and stress is a prerequisite for the practicality of load cells. By using certain techniques to measure stress and temperature changes, temperature and stress can be distinguished and measured. The basic principle of these technologies is to use two or two sections of fiber gratings with different temperature and strain response sensitivities to form a double grating temperature and strain load cell. By determining the temperature and strain response sensitivity coefficients of two fiber gratings, the temperature and strain are solved using two binary Linear equation.
The fiber Bragg gratings used in the experiment are all Bragg fiber gratings, and their refractive index modulation period is uniform, so they meet the requirements of Bragg strips
Pieces, i.e λ B=2 n eff Λ. In the equation: λ B is the Bragg wavelength; N eff is the effective refractive index of the grating (the average effect of refractive index modulation amplitude); Λ is the grating period (spatial period of refractive index modulation).
When the measured physical quantities (such as temperature, stress, etc.) used for fiber Bragg gratings change, it will cause corresponding changes in n eff and Λ, resulting in λ Drift of B. Conversely, by detecting λ The drift of B can reveal the information of the measured physical quantity. The research work on fiber Bragg grating load cells mainly focuses on quasi distributed measurement of temperature and stress. Caused by changes in temperature and stress λ The drift of B can be expressed as:
In the formula, t is the stress, and Pij is the light pressure coefficient; υ Is the transverse deformation coefficient (Poisson’s ratio), a is the thermal expansion coefficient, Δ T is the change in temperature; From the above equation, it can be seen that when excluding or ignoring the influence of temperature changes, the change in Bragg wavelength of fiber Bragg grating is only related to the change in stress and is proportional. Based on this principle, fiber Bragg grating can be used to measure the change in force.
Fiber optic grating type weighing load cells have the following advantages:
(1) Strong anti electromagnetic interference ability, generally the frequency of electromagnetic radiation is much lower than that of light waves, so the optical signal transmitted in optical fibers is not affected by electromagnetic interference.
(2) The electrical insulation performance is good, safe and reliable. The optical fiber itself is composed of dielectric and does not require power drive, making it suitable for use in flammable and explosive oil, gas, and chemical production.
(3) Corrosion resistance and chemical stability. Due to the extremely high chemical stability of the material used to make optical fibers, quartz, optical fiber load cells are suitable for use in harsh environments.
(4) Small size, light weight, and plastic geometric shape.
(5) Low transmission loss, enabling remote monitoring.
(6) Large transmission capacity, enabling multi-point distributed measurement.
(7) Wide measurement range, capable of measuring temperature, pressure, strain, stress, flow rate, flow rate, current, voltage, liquid level, etc.
At present, most fiber grating type weighing load cells are still in the research and experimental stage.