According to the conversion method, weighing sensors are divided into eight categories, such as photoelectric, hydraulic, electromagnetic, capacitive, magnetic electrode transformation, vibration, gyroscope ritual, yin strain, etc. The resistance strain is the most widely used.
It includes two types: grating type and code disk type.
The grating load cell uses the Mohr fringe formed by the grating to convert the angular displacement into a photoelectric signal. 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 to the load-bearing platform rotates the dial axis through the force transmission lever system, driving the moving grating to rotate, so that the Mohr stripe also moves with it. Using photoelectric tubes, conversion circuits and display instruments, the number of removed Mohr stripes can be calculated, and the size of the grating rotation angle can be measured, so as to determine and read out the quality of the measured object.
The code disk (symbol board) of the code disk sensor is a transparent glass mounted on the dial axis, with a black and white code arranged according to a certain coding method. When the measured object on the load-bearing platform rotates the dial axis through the force transmission lever, the code disk also rotates at a certain angle. The photovoltaic cell will receive the optical signal through the code disk and convert it into an electrical signal, and then digitally process it by the circuit, and finally display the number representing the measured quality on the display. Photoelectric sensors have been mainly used in electromechanical combined scales.
When the measured gravity P is affected, the pressure of the hydraulic oil increases, and the degree of increase is proportional to P. The increase in pressure can determine the quality of the measured object. The structure of the hydraulic sensor is simple and solid, with a large measurement range, but the accuracy generally does not exceed 1/100.
Electromagnetic force sensor
It works on the principle of balancing the load on the load-bearing platform with the electromagnetic force. When the measured object is placed on the load-bearing platform, one end of the lever tilts upward; the photoelectric part detects the tilt signal, which flows into the coil after amplification, generating electromagnetic force, and restoring the lever to a balanced state. The digital conversion of the current that generates the electromagnetic equilibrium force can determine the mass of the object being measured. The electromagnetic force sensor has high accuracy, up to 1/2000 to 1 /60000, but the weighing range is only between dozens of milligrams and 10 kilograms.
It uses the positive relationship between the oscillation frequency f of the capacitor oscillation circuit and the polar plate spacing d. There are two plates, one is fixed and the other is movable. When the load-bearing platform loads the measured object, the plate spring deflects, the distance between the two poles changes, and the oscillation frequency of the circuit also changes. The change in frequency can be measured to find the quality of the measured object on the load-bearing platform. The capacitive sensor consumes less power, the cost is low, and the accuracy is 1/200 to 1/500.
Magnetic pole variable form sensor
When the ferromagnetic element is mechanically deformed under the action of the gravity of the measured object, the internal stress is generated and the magnetic conductivity changes, so that the induction voltage of the secondary coils around the ferromagnetic element (magnetic pole) also changes accordingly. The change of the voltage can be measured to find the force added to the magnetic pole, and then determine the mass of the measured object. The accuracy of the magnetic variable form sensor is not high, generally 1/100, which is suitable for large tonnage weighing work, with a weighing range of tens to tens of thousands of kilograms.
After the elastic element is under force, its natural vibration frequency is directly proportional to the square root of the force. When the change of natural frequency is measured, the force of the measured object on the elastic element can be found, and then its quality can be found. There are two kinds of vibration sensors: vibrating string type and tuning fork type.
The elastic element of the vibrating string sensor is the string wire. When the measured object is added to the load-bearing table, the intersection of the V-shaped string is pulled down, and the tension of the left string increases, and the tension of the right string decreases. The natural frequencies of the two strings change differently. Find the difference in frequency between the two strings, and you can find the quality of the object being measured. The accuracy of the vibrating string sensor is high, up to 1/1000 to 1/10000, and the weighing range is 100 grams to hundreds of kilograms, but the structure is complex, the processing is difficult and the cost is high.
The elastic element of the tuning fork sensor is the tuning fork. The end of the tuning fork is fixed with a piezoelectric element, which oscillates at the natural frequency of the tuning fork and can measure the oscillation frequency. When the measured object is added to the load-bearing platform, the tension direction of the tuning fork increases and the natural frequency increases, and the degree of increase is proportional to the square root of the applied force. By measuring the change of natural frequency, the force applied by the heavy object on the tuning fork can be found, and then the mass of the heavy object can be found. The power consumption of the tuning fork sensor is small, the measurement accuracy is as high as 1/10000 to 1/200000, and the weighing range is 500g to 10kg.
The rotor is installed in the inner frame and rotates steadily around the X-axis at an angular speed ω. The inner frame is connected to the outer frame by the bearing and can be tilted around the horizontal axis Y. The outer frame is connected to the base through a universal coupling and can be rotated around the vertical axis Z. The rotor shaft (X-axis) remains horizontal when not affected by external forces. When one end of the rotor shaft is affected by an external force (P/2), it tilts and rotates around the vertical axis Z (precede). The progressive angle velocity ω is directly proportional to the external force P/2. By measuring ω by detecting the frequency, the size of the external force can be obtained, and then the mass of the object that produces the additional force can be obtained.
The gyro ritual sensor has a fast response time (5 seconds), no hysteresis, good temperature characteristics (3ppm), low vibration impact, and high accuracy of frequency measurement, so high resolution (1/100000) and high measurement accuracy (1/30000 to 1/60000) can be obtained.
The principle of using the resistance strain piece to change when it is deformed (Figure 11). It is mainly composed of four parts: elastic element, resistance strain gauge, measurement circuit and transmission cable. The resistance strain pad is attached to the elastic element. When the elastic element is deformed by force, the strain plate on it will deform and cause the resistance to change. The measurement circuit measures the change of strain gauge resistance and converts it into an electrical signal output proportional to the size of the external force. After the electrical signal is processed, it shows the quality of the measured object in digital form.
The weighing range of the resistance-strain sensor is 300g to thousands ofkg, and the measurement accuracy is 1/1000~1/10000. The structure is simple and the reliability is good. Most electronic scales use this sensor.
