Classification of weighing sensors: Weighing sensors are divided into eight categories based on their conversion methods, including photoelectric, hydraulic, electromagnetic force, capacitive, magnetic pole variation, vibrational, gyroscopic, and resistance strain gauge types. Among them, the resistance strain gauge type is widely used.

photoelectric

 Including grating type and code wheel type.

 The grating sensor converts angular displacement into photoelectric signals using the Moiré fringes formed by the grating (Figure 2). There are two gratings: one is a fixed grating, and the other is a moving grating mounted on the dial shaft. The object being measured, which is placed on the load-bearing platform, rotates the dial shaft through the force transmission lever system, driving the moving grating to rotate and causing the Moiré fringes to move accordingly. Using a phototube, conversion circuit, and display instrument, the number of Moiré fringes that have moved can be calculated, and the size of the grating rotation angle can be measured, thereby determining and reading the mass of the object being measured.

 The code disc (symbol plate) of the code disc sensor (Figure 3) is a transparent glass mounted on the dial shaft, with black and white codes arranged according to a certain coding method. When the object being measured placed on the load-bearing platform rotates the dial shaft through the force transmission lever, the code disc also rotates through a certain angle. The photocell receives the light signal transmitted through the code disc and converts it into an electrical signal, which is then digitally processed by the circuit. Finally, the display shows the number representing the measured mass. Photoelectric sensors were mainly used in electromechanical combination scales.

hydraulic

As shown in Figure 4, when subjected to the gravitational force P of the object being measured, the pressure of the hydraulic oil increases, and the degree of increase is proportional to P. By measuring the increase in pressure, the mass of the object being measured can be determined. The hydraulic sensor has a simple and robust structure and a wide measurement range, but its accuracy generally does not exceed 1/100.

 Electromagnetic force type

 It operates based on the principle of balancing the load on the load-bearing platform with electromagnetic force (Figure 5). When an object to be measured is placed on the load-bearing platform, one end of the lever tilts upward; the photoelectric component detects the inclination signal, which is amplified and flows into the coil, generating electromagnetic force to restore the lever to a balanced state. By digitally converting the current that generates the electromagnetic balancing force, the mass of the object to be measured can be determined. Electromagnetic force sensors have high accuracy, reaching up to 1/2000 to 1/60000, but their weighing range is only between tens of milligrams and 10 kilograms.

capacitive

It operates based on the direct proportional relationship between the oscillation frequency f of the capacitor oscillation circuit and the plate spacing d (Figure 6). There are two plates, one fixed and the other movable. When a load is applied to the load-bearing platform, the leaf spring deflects, causing the distance between the two plates to change, and the oscillation frequency of the circuit also changes accordingly. By measuring the change in frequency, the mass of the object being tested on the load-bearing platform can be determined. Capacitive sensors consume little power, are low in cost, and have an accuracy of 1/200 to 1/500.

 Magnetic pole variation form

As shown in Figure 7, when the ferromagnetic element undergoes mechanical deformation under the gravitational force of the object being measured, internal stress is generated and causes changes in magnetic permeability, which in turn leads to variations in the induced voltage of the secondary coil wound around the two sides of the ferromagnetic element (magnetic pole). By measuring the change in voltage, the force applied to the magnetic pole can be determined, and thus the mass of the object being measured can be ascertained. The accuracy of the magnetic pole deformation sensor is not high, typically 1/100, and it is suitable for large-tonnage weighing work, with a weighing range from tens to tens of thousands of kilograms.

 Vibratory type

 After the elastic element is stressed, its natural vibration frequency is proportional to the square root of the applied force. By measuring the change in natural frequency, the force exerted by the object being measured on the elastic element can be determined, and subsequently, its mass can be calculated. There are two types of vibrating sensors: vibrating wire sensors and tuning fork sensors.

 The elastic element of a vibrating wire sensor is the wire. When an object to be measured is placed on the load-bearing platform, the intersection point of the V-shaped wire is pulled downwards, and the tension on the left wire increases while the tension on the right wire decreases. The natural frequencies of the two wires change differently. By calculating the difference in frequency between the two wires, the mass of the object to be measured can be determined. Vibrating wire sensors have high accuracy, reaching up to 1/1000 to 1/10000, with a weighing range from 100 grams to several hundred kilograms. However, they have a complex structure, are difficult to process, and are expensive.

 The elastic element of the tuning fork sensor is the tuning fork. A piezoelectric element is fixed at the end of the tuning fork, which oscillates at the natural frequency of the tuning fork and its oscillation frequency can be measured. When an object to be measured is placed on the load-bearing platform, the tuning fork is subjected to a tensile force in its direction, causing an increase in its natural frequency. The degree of increase is proportional to the square root of the applied force. By measuring the change in natural frequency, the force applied to the tuning fork by the heavy object can be determined, and subsequently, the mass of the heavy object can be calculated. The tuning fork sensor has low power consumption, high measurement accuracy of up to 1/10000 to 1/200000, and a weighing range of 500g to 10kg.

 Gyroscope ritual

 As shown in Figure 10, the rotor is installed in the inner frame and rotates stably around the X-axis at an angular velocity ω. The inner frame is connected to the outer frame via bearings and can tilt around the horizontal Y-axis. The outer frame is connected to the base via a universal joint and can rotate around the vertical Z-axis. The rotor shaft (X-axis) remains horizontal when 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 Z-axis (precession). The angular velocity ω of precession is proportional to the external force P/2. By measuring the frequency, ω can be determined, and thus the magnitude of the external force can be calculated. Subsequently, the mass of the object under test that generates this external force can be determined.

 The gyroscope sensor features a fast response time (5 seconds), no hysteresis, good temperature characteristics (3ppm), minimal vibration impact, and accurate frequency measurement with high precision. Therefore, it can achieve high resolution (1/100000) and high metrological accuracy (1/30000 to 1/60000).

Resistance strain gauge

 It operates based on the principle that the resistance of a resistance strain gauge changes when it deforms (Figure 11). It mainly consists of four parts: the elastic element, the resistance strain gauge, the measurement circuit, and the transmission cable. The resistance strain gauge is attached to the elastic element. When the elastic element deforms under stress, the strain gauge on it deforms accordingly, leading to a change in resistance. The measurement circuit measures the change in resistance of the strain gauge and converts it into an electrical signal output proportional to the magnitude of the external force. After processing, the electrical signal is displayed in digital form as the mass of the object being measured.

 The weighing range of the resistance strain gauge sensor spans from tens of grams to hundreds of tons, with a measurement accuracy of 1/1000 to 1/10000. It boasts a relatively simple structure and good reliability. Most electronic weighing instruments utilize this sensor.