What is a load cell?

What is a load cell?

The load cell is actually a device that converts the quality signal into a measurable electrical signal output. The actual working environment of the sensor should be considered first when using the sensor. This is very important for the correct selection of the load cell. It is related to the normal operation of the sensor, its safety and service life, and the reliability and safety of the entire weighing instrument. Sex. In the basic concepts and evaluation methods of the main technical indicators of the load cell, there are qualitative differences between the new and the old national standards. There are mainly S type load cell, cantilever type load cell, spoke type load cell, plate ring type load cell, bellows type load cell, bridge type load cell, column cylinder type  load cell and other styles load cell.

Basic Introduction of Load Cell:

The old national standard considers the two sensors of "weighing" and "force measurement" with completely different application objects and use environmental conditions into one, without distinguishing between test and evaluation methods. The old national standard has a total of 21 indicators, all of which are tested at room temperature; and the maximum error among the six indicators of non-linearity, hysteresis error, repeatability error, creep, additional error of zero temperature and additional error of rated output temperature is used to determine the weighing The accuracy level of the sensor is expressed as 0.02, 0.03, and 0.05 respectively.

A force sensor used on weighing instruments. It can convert the gravity acting on the measured object into a measurable output signal in a certain proportion.

Considering the influence of gravitational acceleration and air buoyancy in different places of use on conversion, the performance indicators of load cells mainly include linearity error, hysteresis error, repeatability error, creep, zero temperature characteristic and sensitivity temperature characteristic. In various weighing instruments and quality measurement systems, the integrated error is usually used to bring the accuracy of the integrated control load cell sensor, and the integrated error band is linked with the error band of the weighing instrument in order to select a weighing sensor corresponding to a certain accuracy of the weighing instrument weighing indicator. The International Organization for Legal Metrology (OIML) stipulates that the error band δ of the sensor accounts for 70% of the error band Δ of the weighing instrument weighing indicator. The linearity error, hysteresis error of the load cell, and the error caused by the influence of temperature on the sensitivity within the specified temperature range, etc. The sum cannot exceed the error band δ. This allows the manufacturer to adjust the various components that make up the total error of the measurement to obtain the desired accuracy.

Classification:

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

Photoelectric:

Including two kinds of raster type and code disc type.

The grating sensor load cell uses the moiré fringe formed by the grating to convert the angular displacement into an optical signal. 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 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 quality of the test is displayed on the display. Photoelectric sensors have been mainly used in electromechanical scales.

Hydraulic type Load cell:

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.

Capacitive Load cell:

It uses the proportional relationship between the oscillation frequency f of the capacitor oscillation circuit and the distance d between the plates to work. There are two plates, one is fixed and the other is movable. When the load-bearing platform loads the object to be measured, the leaf spring flexes, the distance between the two 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.

Main Advantage Load cell:

Resistance, inductance and capacitance are the three major types of passive components in electronic technology. A capacitive sensor Load cell is a sensor Load cell that converts the measured change into a change in capacitance. It is essentially a capacitor with variable parameters.

Capacitive sensors Load cell have the following advantages:

(1) High impedance, low power, only low input energy.

(2) A larger amount of change can be obtained, which has a higher signal-to-noise ratio and system stability.

(3) The dynamic response is fast, the working frequency can reach several megahertz, the dense b contact measurement, the measured object can be a conductor or a semiconductor.

(4) Simple structure. It has strong adaptability and can work in harsh environments such as high and low temperature and strong radiation, and has a wide range of applications.

With the development of electronic technology and computer technology, the shortcomings of capacitive sensors such as susceptibility to interference and susceptibility to distributed capacitance have been continuously overcome, and capacitive displacement sensors and integrated capacitive sensors have also been developed: It is widely used in measurement and automatic detection, and can measure parameters such as pressure, displacement, rotation speed, acceleration, A degree, thickness, liquid level, humidity, vibration, and component content. Capacitive sensors have very good development prospects.

Main disadvantages of load cell:

Disadvantage 1: High output impedance, poor load capacity;

Disadvantage 2: Non-linear output characteristics;

Disadvantage three: parasitic capacitance has a large impact;

Electromagnetic force type load cell:

It uses the principle that the load on the bearing platform is balanced with the electromagnetic force. When the object to be measured is placed on the load-bearing platform, one end of the lever tilts upward; the photoelectric element detects the inclination signal and flows into the coil after being amplified, generating 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.

Magnetic pole change form:

When the ferromagnetic element is mechanically deformed under 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. The force applied to the magnetic pole can be obtained by measuring the change of voltage, 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.

Vibration load cell:

After the elastic element is stressed, its natural vibration frequency is proportional to the square root of the force. By measuring the change in 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 load cell: vibrating wire type and tuning fork type.

The elastic element of the vibrating wire sensor is the string wire. When the test object is added to the load-bearing platform, the intersection of the V-shaped strings 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 between the frequencies of the two strings to find the mass of the measured object. The vibrating wire sensor has a high accuracy, which can reach 1/1000～1/10000, and the weighing range is 100 grams to several hundred kilograms, but the structure is complicated, the processing is difficult, and the cost is high.

The elastic element of the tuning fork sensor  load cell is a 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 the test object is added to the load-bearing platform, the tension direction of the tuning fork is forced and the natural frequency increases, and the degree of increase is proportional to the square root of the applied force. By measuring the change in natural frequency, the force exerted by the weight on the tuning fork can be obtained, and then the mass 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.