1. Photoelectric
Including two kinds of raster type and code disc type.
The grating sensor uses the moiré fringe formed by the grating to convert the angular displacement into a photoelectric signal (Figure 2). There are two gratings, one is a fixed grating, and the other is a moving grating mounted on the dial shaft. The measured object added to the load-bearing platform rotates the dial shaft through the force transmission lever system, which drives the mobile grating to rotate, so that the moiré fringe also moves. With the use of photoelectric cells, conversion circuits and display instruments, the number of Moiré fringes that have been moved can be calculated, and the rotation angle of the grating can be measured, so as to determine and read the quality of the measured object.
The code disc (symbol plate) of the code disc sensor is a piece of transparent glass mounted on the dial shaft, with black and white codes coded according to a certain coding method on it. When the measured object added to the load-bearing platform rotates the dial shaft through the force transmission lever, the code disc also rotates through a certain angle. The photocell will receive the light signal through the code wheel and convert it into an electrical signal, which is then digitally processed by the circuit, and finally the number representing the measured quality is displayed on the display. Photoelectric sensors were mainly used in electromechanical scales.
2. Hydraulic
Under the action of the gravity P of the measured object, the pressure of the hydraulic oil increases, and the degree of increase is proportional to P. By measuring the increase in pressure, the quality of the measured object can be determined. The hydraulic sensor has a simple and firm structure and a large measuring range, but the accuracy is generally not more than 1/100.
3. Capacitive
It uses the proportional relationship between the oscillation frequency f of the capacitor oscillation circuit and the distance between the plates d to work. There are two plates, one is fixed and the other is movable. When the load-bearing platform loads the object under test, the leaf spring flexes, the distance between the two pole plates changes, and the oscillation frequency of the circuit also changes. The quality of the measured object on the load-bearing platform can be obtained by measuring the change in frequency. Capacitive sensors consume less power, are low in cost, and have an accuracy of 1/200 to 1/500.
4. Electromagnetic force type
It uses the principle of balancing the load on the load-bearing platform with the electromagnetic force. When the object to be measured is placed on the load-bearing platform, one end of the lever is tilted upward; the photoelectric element detects the inclination signal and flows into the coil after being amplified to generate electromagnetic force to restore the lever to a balanced state. The quality of the measured object can be determined by digital conversion of the current that produces the electromagnetic balance force. The electromagnetic force sensor has high accuracy, up to 1/2000 to 1/60000, but the weighing range is only between tens of milligrams and 10 kilograms.
5. Magnetic pole change form
When the ferromagnetic element is mechanically deformed under the action of the gravity of the measured object, internal stress is generated and the permeability changes, 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 obtained, and then the quality of the measured object can be determined. The accuracy of the magnetic pole variable form sensor is not high, generally 1/100, which is suitable for large tonnage weighing work, and the weighing range is tens to tens of thousands of kilograms.
6. Plate ring type
The structure of the plate-ring type load cell has the advantages of clear stress streamline distribution, high output sensitivity, elastic body as a whole, simple structure, stable stress state, and easy processing. At present, it still occupies a large proportion in the production of sensors, and the design formula for sensors of this structure is currently not perfect. Because the strain calculation of this kind of elastic body is more complicated, it is usually estimated as a ring-shaped elastic body in the design. Especially for plate-ring sensors with a range of 1t and below, the design calculation error is greater, and at the same time, larger nonlinear errors often occur.
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