Types of load cells

According to the conversion method, the load cell is divided into 8 types: photoelectric type, hydraulic type, electromagnetic force type, capacitive type, magnetic pole change type, vibration type, gyro ceremony, resistance strain type, etc. The resistance strain type is the most widely used.

Photoelectric

Including grating type and code disc type.

The grating sensor converts the angular displacement into a photoelectric signal using the moire 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 measured object added on the load-bearing platform rotates the dial axis through the force transmission lever system, which drives the moving grating to rotate, so that the moire fringes also move. Using the photocell, conversion circuit and display instrument, 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 object to be measured.

The code disc (symbol plate) of the code disc sensor (Figure 3) is a piece of transparent glass mounted on the dial shaft, with a black and white code programmed according to a certain coding method. When the measured object added on the load-bearing platform rotates the dial shaft through the force transmission lever, the code disc also rotates by a certain angle. The photocell will receive the light signal through the code disc 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 were mainly used in electromechanical combined scales.

hydraulic

As shown in Figure 4, under the action of gravity P of the measured object, the pressure of hydraulic oil increases, and the degree of increase is proportional to P. The mass of the measured object can be determined by measuring the increase in pressure. The hydraulic sensor has a simple and firm structure and a large measurement range, but the accuracy is generally not more than 1/100.

electromagnetic force

It works on the principle that the load on the bearing platform is balanced with the electromagnetic force (Figure 5). When the object to be measured is placed on the load-bearing platform, one end of the lever is inclined upward; the photoelectric device detects the inclination signal, which flows into the coil after amplification, and generates an electromagnetic force to restore the lever to a balanced state. The mass of the measured object can be determined by digitally converting the current that produces the electromagnetic balance force. The electromagnetic force sensor has high accuracy, up to 1/2000 ~ 1/60000, but the weighing range is only between tens of milligrams to 10 kilograms.

Capacitive

It works with the proportional relationship between the oscillation frequency f of the capacitor oscillation circuit and the distance d between the plates (Fig. 6). There are two plates, one is fixed and the other is movable. When the load-bearing platform is loaded with the measured object, the leaf spring is deflected, the distance between the two polar plates changes, and the oscillation frequency of the circuit also changes. The mass of the measured object on the load-bearing platform can be obtained by measuring the change of frequency. Capacitive sensors consume less power, cost less, and have an accuracy of 1/200 to 1/500.

Magnetic pole variant

As shown in Figure 7, when the ferromagnetic element is mechanically deformed under the action of gravity of the measured object, internal stress is generated and the magnetic permeability changes, so that the induced voltage of the secondary coil wound on both sides of the ferromagnetic element (magnetic pole) also changes with the change. The force applied to the magnetic pole can be obtained by measuring the variation of the voltage, and then the mass of the measured object can be determined. The accuracy of the magnetic pole variant sensor is not high, generally 1/100. It is suitable for large tonnage weighing work, and the weighing range is tens to tens of thousands of kilograms.

vibrating

After the elastic element is stressed, its natural vibration frequency is proportional to the square root of the applied force. By measuring the change of the natural frequency, the force of the measured object acting on the elastic element can be obtained, and then its mass can be obtained. There are two types of vibration sensors: vibrating wire and tuning fork.

The elastic element of the vibrating wire sensor is the wire. When there is an object to be measured on the load-bearing platform, the intersection of the V-shaped string wires is pulled down, and the pulling force of the left string increases, and the pulling force of the right string decreases. The natural frequencies of the two strings change differently. Find the difference between the frequencies of the two strings to find the mass of the object to be measured. The vibrating wire sensor has a high accuracy, up to 1/1000 ~ 1/10000, and the weighing range is 100 grams to several hundred 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. A piezoelectric element is fixed at the end of the tuning fork, which oscillates at the natural frequency of the tuning fork, and the oscillation frequency can be measured. When an object to be measured is added to the load-bearing platform, the tuning fork is subjected to force in the stretching direction 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 the natural frequency, the force exerted by the weight on the tuning fork can be obtained, and then the weight of the weight can be obtained. The tuning fork sensor has low power consumption, the measurement accuracy is as high as 1/10000~1/200000, and the weighing range is 500g~10kg.

gyro ceremony

As shown in Fig. 10, the rotor is housed in the inner frame and rotates stably around the X-axis at an angular velocity ω. The inner frame is connected with the outer frame through bearings, and can tilt and rotate around the horizontal axis Y. The outer frame is connected with the machine base through a universal joint, and can rotate around the vertical axis Z. The rotor shaft (X-axis) remains horizontal when no external force is applied. When one end of the rotor shaft is subjected to an external force (P/2), it tilts and rotates (precesses) around the vertical axis Z. The precession angular velocity ω is proportional to the external force P/2. By measuring ω by detecting the frequency, the magnitude of the external force can be obtained, and then the mass of the measured object that generates the external force can be obtained.

The gyro sensor has a fast response time (5 seconds), no hysteresis, good temperature characteristics (3ppm), little vibration influence, and high frequency measurement accuracy, so high resolution (1/100000) and high measurement accuracy can be obtained. (1/30000～1/60000).

Resistance strain type

It works on the principle that when the resistance strain gauge is deformed, its resistance also changes (Figure 11). It is mainly composed of four parts: elastic element, resistance strain gauge, measurement circuit and transmission cable. The resistance strain gauge is attached to the elastic element. When the elastic element is deformed by force, the strain gauge on it is deformed and the resistance changes. The measurement circuit measures the change of the resistance of the strain gauge and converts it into an electrical signal output proportional to the magnitude of the external force. The electrical signal is processed to display the quality of the measured object in digital form.

The weighing range of the resistance strain sensor is from tens of grams to hundreds of tons, the measurement accuracy is 1/1000 ~ 1/10000, the structure is relatively simple, and the reliability is good. Most electronic scales use this sensor.