Load cells are classified into eight categories based on their conversion methods: photoelectric, hydraulic, electromagnetic force, capacitive, magnetic pole deformation, vibrating, gyroscopic, and resistance strain types. Among these, the resistance strain type is the most widely used.
Photoelectric Type
This category includes two types: grating-based and code-disk-based.
1. Grating-based sensors utilize Moiré fringes formed by gratings to convert angular displacement into photoelectric signals. They employ two gratings: a fixed grating and a moving grating mounted on the dial shaft. When an object is placed on the load platform, a force-transmission lever system rotates the dial shaft, driving the moving grating and causing the Moiré fringes to shift. By using a phototube, conversion circuitry, and a display instrument, the system counts the shifted Moiré fringes and measures the rotation angle of the grating, thereby determining and displaying the mass of the object.
2. In code-disk sensors, the code disk (or symbol plate) consists of a transparent glass disk mounted on the dial shaft, featuring a pattern of alternating black and white codes arranged according to a specific encoding method. When the object on the load platform causes the dial shaft to rotate via the lever system, the code disk rotates by a corresponding angle. A photocell receives the light signal transmitted through the code disk and converts it into an electrical signal; this signal undergoes digital processing, and the mass of the object is finally displayed as a numerical value. Photoelectric sensors were historically used primarily in electromechanical hybrid scales.
Hydraulic Type
When subjected to the weight (force P) of the object being measured, the pressure of the hydraulic fluid increases in direct proportion to P. By measuring this increase in pressure, the mass of the object can be determined. Hydraulic sensors feature a simple, robust structure and a wide measurement range, though their accuracy generally does not exceed 1/100.
Capacitive Type
These sensors operate based on the direct proportionality between the oscillation frequency (f) of a capacitor oscillator circuit and the spacing (d) between the capacitor plates. They utilize two plates: one fixed and one movable. When the object is placed on the load platform, a leaf spring deflects, altering the distance between the two plates and consequently changing the oscillation frequency of the circuit. The mass of the object on the weighing platform can be determined by measuring the change in frequency. Capacitive sensors feature low power consumption and low manufacturing costs, with an accuracy ranging from 1/200 to 1/500.