How to choose a load cell for weighing scale?

How to choose a load cell for weighing scale? 

The weighing sensor is regarded as the heart of the electronic scale. Its performance largely determines the accuracy and stability of the electronic scale. When designing electronic scales, the problem of how to select sensors is often encountered. The load cell is actually a device that converts the mass signal into a measurable electrical signal output. The first thing to consider when selecting a sensor is to consider the actual working environment in which the sensor is located. This is crucial for the correct choice of sensor. It is related to whether the sensor works properly, its safety and service life, and even the long-term reliability of the entire electronic scale. Here are some points to note when choosing:

First, the impact of the environment on the load cell mainly includes the following points:

1. The high temperature environment causes problems such as the sealing material and melting of solder joints, structural changes in the elastic body and Strain Gauge internal stress caused by the sensor. Sensors that work in high temperature environments often use high temperature sensors; in addition, in harsh environments, devices such as heat insulation, water cooling, or air cooling must be added.

2. Dust and damp heat have a great influence on the sensor. In this environment, the sensor with high airtightness should be selected. Different sensors have different sealing methods, and there are great differences in their tightness. Common seals are filled with sealant or coated; rubber gaskets are mechanically tightly sealed; welding (laser welding, argon arc welding, plasma welding, etc.) and vacuum nitrogen-filled seals.

From the point of view of the sealing effect, the welding seal is the best and the filling sealant is poor. For sensors that work in a clean, dry environment indoors, sensors that are sealed by a sealant can be selected. For sensors that operate in a humid, dusty environment, the diaphragm seal or diaphragm seal must be selected. Vacuum nitrogen sensor.

3, in a corrosive environment, such as moisture, acid damage to the sensor caused by elastomers and accessories, corrosion, etc., should be selected to spray the outer surface or stainless steel elastomer and stainless steel outer cover, corrosion resistance and good Confined sensor.

4. The influence of the electromagnetic field on the sensor output signal interference. In this case, the sensor's shielding performance should be strictly checked to see if it has a good anti-electromagnetic interference capability.

5, flammable, explosive not only cause thorough damage to the sensor, but also poses a great threat to other equipment and personal safety. Therefore, sensors that work in flammable and explosive environments pose higher requirements for explosion-proof performance. Therefore, explosion-proof sensors must be selected. The seal covers of such sensors must not only consider their tightness, but also take into account the explosion-proof strength, and Waterproof, moisture-proof, explosion-proof, etc.

Second, pay attention to the number of weighing sensors and range selection.

The selection of the number of sensors is based on the use of the electronic scale and the number of points the scale body needs to support (the number of support points should be determined based on the principle that the geometric center of gravity of the scale body is coincided with the actual center of gravity). In general, the scale body has several support points to select several sensors, but for some special scale bodies, such as the electronic crane scale, only one sensor can be used. Some electromechanical combination scales should be based on the actual situation to determine the selection of sensors The number of.

The selection of the sensor range can be determined based on the comprehensive evaluation of factors such as the maximum weighing value of the scale, the number of selected sensors, the weight of the scale body, the maximum partial load that may be generated, and the dynamic load. In theory, the closer the sensor's range is to the load assigned to each sensor, the higher the accuracy of its weighing. However, in actual use, due to the load imposed on the sensor, in addition to the object being weighed, there are loads such as the weight of the scale body, tare weight, eccentric load, and vibration impact. Therefore, when selecting the sensor range, many factors must be taken into account to ensure The safety and life of the sensor.

※ The empirical formula for the calculation of the load cell range is determined after extensive verification by taking full account of the various factors affecting the scale body. The formula is as follows:

C=K0K1K2K3(Wmax+W)/N

C—the rated range of a single sensor; W—the weight of the scale body; Wmax—the maximum value of the net weight of the object being called; N—the number of support points used by the scale body; K0—the insurance coefficient, generally ranging from 1.2 to 1.3; K1—impact coefficient; K1 value is determined according to the following conditions of use: a) When N sensors are uniformly loaded and operated without vibration and without impact, K1=1.1-1.3; b) When there is occasional impact vibration In the case of K1=1.3-1.5;

c) When dynamic weighing under repetitive shock and vibration conditions, K1=1.7; when N sensors are not loaded uniformly, then the maximum load that may be formed by partial load is multiplied by the above coefficient; K2— Scale center of gravity offset coefficient; K3 - wind pressure coefficient.

For example: a 30t electronic truck scale, the maximum weighing Wmax is 30t, the scale body weight W is 1.9t, the design uses four QS-A sensors, according to the actual situation at that time, select the insurance factor K0 = 1.25, the impact coefficient K1 =1.2, center-of-gravity offset coefficient K2=1.03, wind pressure coefficient K3=1.02, try to determine the range of the sensor.

Solution: According to the formula of sensor range: C=K0K1K2K3(Wmax+W)/N

It can be seen that: C=1.25×1.2×1.03×1.02×(30+1.9)/4=12.57t

Therefore, an optional range of 15t sensors (QS-A type sensors generally only 10t, 15t, 20t, 25t, 30t, 40t, 50t, etc., unless specifically ordered).

According to experience, the sensor should generally be operated within its range of 30% to 70%, but for some weighing instruments that have a large impact during use, such as dynamic track scales, dynamic truck scales, steel scales, etc., when selecting sensors, Generally, it is necessary to expand its range so that the sensor works within 20% to 40% of its range, so that the sensor's weighing reserve is increased to ensure the safety and lifespan of the sensor and to avoid overloading.

Third, the load cell to consider the application of various types of sensors.

The choice of sensor type depends mainly on the type of weighing and installation space, to ensure proper installation, weighing safe and reliable; on the other hand, consider the sensor manufacturer's recommendations. Sensor manufacturers generally describe the scope of application of sensors based on the sensor's stress conditions, performance indicators, installation forms, structural types, elastomer materials, etc., such as aluminum single-point cantilever sensors for scales, platform scales, Case scales, etc.; steel cantilever sensors for hopper scales, electronic belt scales, sorting scales, etc.; steel bridge sensors for track scales, truck scales, crane scales, etc.; column sensors for truck scale, dynamic rail scales , Fixed dynamic axle weight scales, large tonnage hopper scales and so on. Therefore, weighing manufacturers should try their best to adopt recommendations from sensor manufacturers.

Fourth, we must choose the accuracy level of the load cell.

Sensor accuracy levels include sensor nonlinearity, temperature effects, creep, creep recovery, hysteresis, repeatability, and sensitivity. When selecting sensors, instead of simply pursuing high-level sensors, consider not only the accuracy requirements of electronic scales, but also their costs.

The selection of the load cell grade must meet the following two conditions:

1, to meet the input requirements of the instrument. Weighing display instrument displays the weighing result after the sensor's output signal is amplified, A/D converted, and other processing. Therefore, the output signal of the sensor must be greater than or equal to the required input sensitivity value of the instrument, that is, the output sensitivity of the sensor is substituted into the matching formula of the sensor and the instrument. The calculation result must be greater than or equal to the required input sensitivity of the instrument.

※ Matching formulas for sensors and meters:

Sensor output sensitivity × excitation power supply voltage × maximum weighing scale ÷ (division of the scale × number of sensors × sensor range) ≥ input sensitivity of the instrument

For example: a weighing weighing pack weighing 25kg, the maximum number of divisions of 2000 divisions; scale body using three models HSX-A20kg sensor, the sensitivity of 2.0 ± 0.002mV / V, for the bridge voltage of 12V; Scale uses URL3280 instrument

Q. Can the sensor be matched with the meter?

Solution: After consulting, the input sensitivity of the URL3280 meter is 0.6μV/d, so the actual input sensitivity of the meter according to the matching formula of the sensor and the meter is:

2000μV/V×12×25/(2000×3×20)=5μV/d>0.6μv/d (d indicates graduation value)

Therefore, the sensors used meet the input sensitivity requirements of the instrument and can match the selected instrument.

2, to meet the entire electronic scale accuracy requirements. An electronic scale is mainly composed of a scale body, a sensor, and an instrument. When the accuracy of the sensor is selected, the accuracy of the sensor should be slightly higher than the theoretical calculation value because the theory is often limited by objective conditions, such as a scale. The strength of the body is poor, the performance of the instrument is not very good, and the working environment of the scale is relatively poor and other factors have a direct impact on the accuracy of the scale. Therefore, it is necessary to improve the requirements in all aspects, and consider the economic benefits to ensure that the purpose is achieved. Sensor selection principle

Modern sensors vary greatly in principle and structure, depending on the specific purpose of the measurement, the measurement object, and the measurement

The rational selection of sensors in the environment is the first problem to be solved when carrying out a certain amount of measurements. When the sensor is determined, the matching measuring method and measuring equipment can be determined. The success or failure of measurement results depends to a large extent on the reasonableness of the choice of sensors.

1) Determine the type of sensor based on the measurement object and the measurement environment

To carry out a specific measurement work, we must first consider the principle of the use of sensors, which need to analyze many factors before you can determine. Because, even if it is to measure the same physical quantity, there are many kinds of principle sensors available for selection. Which kind of principle sensor is more suitable, we need to consider the following specific problems according to the characteristics of the measured and the use conditions of the sensor: the size of the range; The measured position of the sensor volume requirements; measurement method for contact or non-contact; signal extraction method, wired or non-contact measurement; sensor source, domestic or imported, the price can withstand, or self-developed.

After considering the above issues, it is possible to determine which type of sensor to use, and then consider the specific performance indicators of the sensor.

2) The choice of sensitivity

Generally, in the linear range of the sensor, it is desirable that the sensitivity of the sensor be as high as possible. Because only the sensitivity is high, the value of the output signal corresponding to the measured change is relatively large, which is advantageous for signal processing. However, it should be noted that the sensitivity of the sensor is high, and external noise that is not related to the measurement is easily mixed in, and it is also amplified by the amplification system, which affects the measurement accuracy. Therefore, it is required that the sensor itself should have a high signal-to-noise ratio, and the dedication should reduce the plant disturbance signal introduced from outside.

The sensitivity of the sensor is directional. When it is measured as a single vector and its directionality is higher, it should be

Choose sensors with low sensitivity in other directions; if the measurement is a multidimensional vector, the sensor's cross sensitivity is required to be as small as possible.

3) Frequency response characteristics

The frequency response characteristic of the sensor determines the frequency range to be measured. The measurement condition must be kept within the allowable frequency range without distortion. Actually, the response of the sensor always has a certain delay. The shorter the delay time, the better.

The frequency response of the sensor is high, the measurable signal frequency range is wide, and due to the influence of structural characteristics, the mechanical

The inertia of the system is relatively large, because the frequency of the sensor with low frequency can be measured is lower.

In the dynamic measurement, the characteristics of the signal (steady-state, transient, random, etc.) response characteristics should be used in order to avoid overheating errors.

4) Linear range

The linear range of the sensor refers to the range of the output proportional to the input. In theory, in this range, the sensitivity remains constant. The wider the linear range of the sensor, the larger the range, and can guarantee a certain measurement accuracy. When selecting a sensor, when the type of the sensor is determined, it first depends on whether or not the range meets the requirements.

But in fact, any sensor cannot guarantee absolute linearity, and its linearity is also relative. When the required measurement accuracy is relatively low, within a certain range, the sensor with smaller nonlinear error can be approximated as linear, which will bring great convenience to the measurement.

5) Stability

The ability of a sensor to remain unchanged after a period of use is called stability. Affects the long-term stability of the sensor

In addition to the structure of the sensor itself, sexual factors are mainly the environment in which the sensor is used. Therefore, to make the sensor have a good

Stability, the sensor must have a strong environmental adaptability.

Before selecting the sensor, it is necessary to investigate its use environment, select the appropriate sensor according to the specific use environment, or take appropriate measures to reduce the impact of the environment.

There are quantitative indicators of the stability of the sensor. After the period of use is exceeded, calibration should be performed again before use to determine if the performance of the sensor has changed.

In some applications where the sensor can be used for a long period of time and cannot be easily replaced or calibrated, the selected sensor requires more stringent stability and can withstand long-term tests.

6) Accuracy

Accuracy is an important performance index of the sensor, and it is an important link related to the measurement accuracy of the entire measurement system. The higher the precision of the sensor, the more expensive it is. Therefore, the accuracy of the sensor only needs to satisfy the entire measurement system.

Accuracy requirements can be, do not have to choose too high. This makes it possible to select relatively inexpensive and simple sensors among the many sensors that meet the same measurement purpose.

If the purpose of the measurement is qualitative analysis, the sensor with high repeatability can be selected, and high absolute value accuracy should not be used; if it is for quantitative analysis, accurate measurement values must be obtained, and sensors with an accuracy class that meets the requirements must be used.

For some special occasions, you cannot choose the right sensor, you need to design and manufacture the sensor yourself. Self-made sensor performance should meet the requirements