Introduction to load cells

According to Xingke Hardware Network, the device or component that people usually convert the measured physical quantity or chemical quantity into electricity is called a sensor (also called a converter). Among them, the physical quantity, humidity, mass, weight, force, pressure, speed, acceleration, length, angle, liquid level, flow rate, density, etc. are usually contacted by the physical quantity, and the temperature sensor is needed in production and life. , humidity sensor, weighing force sensor, pressure sensor, etc.

1. Definition of load cell:

A sensor that has been used to measure mass using local gravitational acceleration and air buoyancy. The load cell converts the measured mass into a voltage signal. There are a variety of load cells, such as capacitive load cells; electromagnetic balance sensors, piezoelectric load cells and so on.

2. Foil type resistance strain gauge

A sensitive component based on the strain-resistance effect, using a metal foil as a sensitive gate, which can convert the strain of the tested component into a resistance change is called a foil-type strain gauge.

3. Strain gauge load cell

A load cell manufactured by using a strain gage as a sensitive component is called a strain gauge load cell.

4. Strain gauge load cell

A sensor that can be used to produce various types of mechanical quantities by using strain gauges as sensitive components is called a load cell. Examples of tension, pressure, pressure, twist rejection, acceleration and other sensors.

5. The relationship between strain gauge weighing force sensor and load cell

In theory, a property of a quality characterization entity is measured in kilograms, while the mechanical quantity is a vector, and the unit of measure is Newton and other derived quantities, which have nothing to do with each other. However, since the mass cannot be directly measured, the mass is measured by the force effect (weight) of the mass in the earth's gravitational field, so they are homogeneous from each other in terms of measurement technology.

Load characteristics

Rated range: The rated range of a sensor is calculated by how much force the sensor is designed to design. However, in actual use, it is generally only 2/3~1/3 of the rated range or even 1/6. (See the analysis below for reasons).

Allowable use of load (or safety overload): Allows overload work within a certain range. Generally 120%~150%;

Extreme load (or extreme overload): means that when the work exceeds this value, the sensor will be damaged. This value is generally 200% FS~500% FS

When the range of the electronic scale is determined, the range of the sensor should be selected to be larger than the calculated value. For example, the 6-ton 30-ton scale is calculated according to the average score. The range of the sensor should be 5 tons, but in fact, we only need 15 for the range. ~20 tons, which is 3~5 times of the calculated value. This is the same: because of the eccentric load in actual work, overload and shock are inevitable.

Another point to note is that the range is closely related to the sensitivity factor. When the design sensitivity is 1mv/v, it is obviously three times larger than the sensor with a design sensitivity of 3mv/v under the same structural conditions.

Therefore, the choice of sensor range has certain maneuverability and regularity. Generally, the user is required to provide the total weight, net weight and working condition. Before ordering, it is necessary to know what the user will buy for this sensor. This is very important to ensure the user to use. success. Will have the company's success and reputation.

Technical Parameters

Sensitivity factor: The sensitivity coefficient of the sensor has national specifications: 1mv/v, 2mv/v, 3mv/v. This parameter must be specified when ordering. Because when the company's products are matched with other company's products, the sensitivity coefficient must be the same. Otherwise, it will not match, and the highest precision will not be used. This parameter characterizes the output characteristics of the sensor.

Nonlinear error: This is a parameter that characterizes the accuracy of the correspondence between the voltage signal output from this sensor and the load. For a sensor with a maximum range of 5 tons, under extremely ideal conditions, the air output is zero. When the full load output is 5000, the output should be 1000 at 1 ton, 2000 at 2 ton, and output at 3 tons. 3000, 4 tons should be output. But in fact, such a sensor is impossible, it must have errors, and this one-to-one corresponding error is called nonlinear error. National standard expression: No less than 5 levels of load are applied sequentially on the test sensor, usually 0%, 20%, 40%, 60%, 80%, 100% of the rated load. The value of the output signal on the sensor at each test point is measured after each load is applied. The above two strokes are one measurement cycle, usually a total of 3 cycles. Then the nonlinearity (L) of the sensor is calculated as follows: L = ΔθL / θn × 100% ΔθL - the maximum difference between the arithmetic mean of the actual output signal values ​​of the three upstrokes at the same test point and the average end point straight line (mv)

Repeatability error: The repetitive error characterizes whether the output value of the sensor can be repeated when the same load is repeatedly applied under the same conditions. This feature is more important and better reflects the quality of the sensor. The national standard indicates the error of repeatability: the repeatability error can be measured simultaneously with the nonlinearity. The repeatability error (R) of the sensor is calculated as follows: R = ΔθR / θn × 100%.

ΔθR--the maximum difference (mv) between the actual output signal values ​​measured 3 times at the same test point.

Hysteresis error: The common meaning of hysteresis error is: when the load is applied step by step and then the load is sequentially removed, the corresponding reading should be made corresponding to each level of load, but in fact the same is true, the degree of inconsistency is the same as the hysteresis error. Indicators to indicate. In the national standard, the hysteresis error is calculated as follows: the hysteresis error (H) of the sensor is calculated as follows: H = ΔθH / θn × 100%.

ΔθH--the maximum difference (mv) between the arithmetic mean of the actual output signal values ​​of the three strokes at the same test point and the arithmetic mean of the actual output signal values ​​of the three upstrokes.

Creep and Creep Error: It is required to test the creep error of the sensor from two aspects: one amount of creep: the rated load is added without impact in 5-10 seconds, and the reading is 5~10 seconds after loading, then Record the output values ​​in sequence at a certain time interval within 30 minutes. The sensor creep (CP) is calculated as follows: CP = θ2 - θ3 / θn × 100%.

The second is creep recovery: remove the rated load as soon as possible (in 5~10 seconds), immediately read the load within 5~10 seconds after unloading, and then record the output value at a certain time interval within 30 minutes. The creep recovery (CR) of the sensor is calculated as follows:

CR = θ5 - θ6 / θn × 100%.

Temperature characteristics: Permissible temperature: specifies where this sensor is suitable. The normal temperature sensor is generally labeled as: -20 ° C - + 70 ° C. The high temperature sensor is labeled: -40 ° C - 250 ° C.

Temperature compensation range: This sensor has been compensated for in this temperature range at the time of production. For example, the normal temperature sensor is generally marked as -10 ° C - + 55 ° C.

Zero temperature effect (commonly known as zero temperature drift): Characterizes the stability of this sensor's zero point as the ambient temperature changes. Generally, the drift generated per ° C is a unit of measurement.

The temperature effect of the output sensitivity coefficient (commonly known as coefficient temperature drift): This parameter characterizes the stability of the output sensitivity of this sensor when the ambient temperature changes. Generally, the drift generated per ° C is a unit of measurement.

Electrical characteristics: Output impedance: When the company's sensors are used in parallel with other manufacturers' sensors, the output impedance of the company's products must be clarified. This value must be consistent with it. Otherwise, it will directly affect the output characteristics of the electronic scale and the debugging of the four-angle error. The output impedance of strain gauge sensors at home and abroad has certain specifications: generally 350 ohms, 700 ohms, and also 600 ohms (for example, Phillips sensors).

Input impedance: The input resistance of the sensor is greater than the output resistance due to the sensor's input compensation module and the sensitivity coefficient adjustment resistor, but it can be changed by the parallel resistance method. The input impedance of each sensor is required to be the same, if it matches the sensor of other manufacturers. The input impedance should be consistent with it, otherwise the man-hour will be increased when debugging the four-corner error, because the input impedance of the sensor is a load for the regulated power supply, and the same regulated power supply will provide the same power supply voltage only for the same load.

Insulation resistance: This is an important performance parameter of the sensor. The insulation resistance is equivalent to a resistor with a resistance value between the bridge and the ground. Calculating such a circuit network can prove that the insulation resistance affects the performance of the sensor. When the insulation resistance is lower than a certain value, the bridge will not work properly.

Recommended excitation voltage: generally 5~10 volts. Because the regulated power supply in the general weighing instrument is 5 or 10 volts.

Allowable maximum excitation voltage: In order to increase the output signal, in some cases (such as large tare weight), it is required to obtain a larger signal by using the condition of increasing the excitation voltage. In this case, the excitation voltage can be increased; the strain gauge is allowed to pass. The maximum current is 25 mA, so when the bridge impedance is 350 ohms, the maximum allowable voltage is 17 volts. Due to the high quality strain gage, we set the maximum excitation voltage to 20 volts. For sensors with a bridge impedance of 700 ohms, the maximum excitation voltage is allowed to double.

Other features: cable length, which is related to the site layout. You must see the regular cable length of the company's products before ordering. In addition, pay attention to whether the environment is corrosive, whether there is impact, whether it is high temperature or low temperature.

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