Regarding the selection of weighing and force measuring sensors

Regarding the selection of weighing and force measuring sensors

The weighing sensor is known as the heart of electronic scales, and its performance largely determines the accuracy and stability of electronic scales. When designing electronic scales, it is often necessary to encounter the problem of how to select sensors.

1、 How to choose sensors

A weighing sensor is actually a device that converts a mass signal into a measurable electrical signal output. When using sensors, it is important to consider the actual working environment in which the sensor is located, which is crucial for selecting the sensor correctly. It is related to whether the sensor can work properly, its safety and service life, and even the reliability and safety of the entire weighing apparatus.

The impact of the environment on sensors mainly includes the following aspects:

1. The high temperature environment causes problems such as melting of coating materials, cracking of solder joints, and structural changes in the stress inside the elastomer for sensors. High temperature resistant sensors are often used for sensors working in high-temperature environments; In addition, insulation, water cooling or air cooling devices must be added.

2. Dust and moisture can cause short circuits in sensors. Under these environmental conditions, sensors with high airtightness should be selected. Different sensors have different sealing methods, and there are significant differences in their sealing properties.

Common seals include sealant filling or coating; Rubber pad mechanical fastening seal; Welding (argon arc welding, plasma beam welding) and vacuum nitrogen sealing.

From the perspective of sealing effect, welding sealing is the best, while filling and applying sealant is the difference in quantity. For sensors working in clean and dry indoor environments, adhesive sealed sensors can be selected, while for sensors working in humid and dusty environments, membrane hot sleeve sealing or membrane welding sealing, vacuum filling and nitrogen filling sensors should be selected.

3. In highly corrosive environments, such as humidity and acidity, which can cause damage to the elastic body or short circuits in sensors, sensors with good corrosion resistance and good sealing properties should be selected by spraying or stainless steel covers on the outer surface.

4. The influence of electromagnetic field on the output disorder signal of sensors. In this case, the shielding of the sensor should be strictly checked to see if it has good electromagnetic resistance.

5. Flammable and explosive materials not only cause complete damage to sensors, but also pose a great threat to other equipment and personal safety. Therefore, sensors working in flammable and explosive environments have higher requirements for explosion-proof performance: explosion-proof sensors must be selected in flammable and explosive environments, and the sealing cover of such sensors should not only consider their airtightness, but also their explosion-proof strength, as well as the waterproof, moisture-proof, and explosion-proof properties of cable leads.

2、 Selection of the number and range of sensors.

The selection of the number of sensors is based on the purpose of the electronic scale and the number of points that the scale body needs to support (the number of support points should be determined according to the principle of making the geometric center of gravity of the scale body coincide with the actual center of gravity). Generally speaking, several sensors are selected based on the number of support points on the scale body. However, for some special scales such as electronic hook scales, only one sensor can be used. For some electromechanical combined scales, the number of sensors selected should be determined according to the actual situation.

The selection of sensor range can be determined based on a comprehensive evaluation of factors such as the large weighing value of the scale, the number of sensors selected, the weight of the scale body, possible large bias loads, and dynamic loads. Generally speaking, the closer the range of a sensor is to the load assigned to each sensor, the higher the accuracy of its weighing. However, in practical use, due to the presence of loads on the sensor, in addition to the object being weighed, there are also loads such as the weight of the scale body, tare weight, offset load, and vibration impact. Therefore, when selecting the sensor range, various factors should be considered to ensure the safety and lifespan of the sensor.

The calculation formula for the sensor range is determined through extensive experiments after fully considering various factors that affect the scale body.

The formula is as follows:

C＝K-0K-1K-2K-3(Wmax＋W)/N

C - rated range of a single sensor; W - self weight of the scale body; Wmax - the maximum net weight of the object being referred to; N - the number of support points used for the scale body; K-0- Insurance coefficient, generally ranging from 1.2 to 1.3; K-1- Impact coefficient; K-2- Center of gravity offset coefficient of the scale body; K-3- Wind pressure coefficient.

For example, a 30t electronic car scale with a large weighing capacity of 30t and a self weight of 1.9t is equipped with four sensors. Based on the actual situation at that time, the insurance coefficient K-0=1.25, impact coefficient K-1=1.18, center of gravity offset coefficient K-2-=1.03, and wind pressure coefficient K-3=1.02 are selected to determine the tonnage of the sensors.

Solution: According to the sensor range calculation formula:

C＝K-0K-1K-2K-3（Wmax＋W）/N

It is known that:

C＝1.25×1.18×1.03×1.02×（30＋1.9）/4＝12.36t

Therefore, sensors with a range of 15t can be selected (the tonnage of sensors is generally only 10T, 15T, 20t, 25t, 30t, 40t, 50t, etc., unless specially customized).

Based on experience, sensors should generally operate within 30% to 70% of their range. However, for some weighing instruments that have significant impact forces during use, such as dynamic track scales, dynamic truck scales, steel scales, etc., when selecting sensors, their range should generally be expanded to operate within 20% to 30% of their range, increasing the weighing reserve of the sensor to ensure its safety and lifespan.

3、 Consider the applicable range of various types of sensors

The selection of sensor types mainly depends on the type of weighing and installation space, ensuring appropriate installation and safe and reliable weighing; On the other hand, we need to consider the manufacturer's suggestions. Manufacturers generally specify the scope of application of sensors based on their stress conditions, performance indicators, installation forms, structural types, and the material of the elastic body. For example, aluminum cantilever beam sensors are suitable for pricing scales, platform scales, cutting scales, etc; Steel cantilever beam sensors are suitable for hopper scales, electronic belt scales, sorting scales, etc; Steel bridge sensors are suitable for railway scales, truck scales, overhead crane scales, etc; Column sensors are suitable for car scales, dynamic track scales, large tonnage hopper scales, etc.

4、 We also need to choose the accuracy level of the sensor

The accuracy level of sensors includes technical indicators such as nonlinearity, creep, creep recovery, hysteresis, repeatability, sensitivity, etc. When selecting sensors, do not simply pursue high-grade sensors, but consider both meeting the accuracy requirements of electronic scales and their cost.

The selection of sensor level must meet the following two conditions:

1. Meet the requirements for instrument input. The weighing display instrument displays the weighing result after amplifying and A/D converting the output signal of the sensor. Therefore, the output signal of the sensor must be greater than or equal to the input signal size required by the instrument, that is, the output sensitivity of the sensor must be represented by the matching formula between the sensor and the instrument, and the calculated result must be greater than or equal to the input sensitivity required by the instrument.

Matching formula for sensors and instruments:

Sensor output sensitivity x excitation power supply voltage x scale's large scale/scale's division number x number of sensors x sensor range

For example, a quantitative packaging scale weighing 25kg with a maximum division of 1000 divisions; The scale body adopts three L-BE-25 sensors with a range of 25kg, a sensitivity of 2.0 ± 0.008mV/V, and an arch bridge electric pressure of 12V. The scale adopts EVT-800A instrument. Can the sensors used be matched with the instrument.

Solution: After consulting, the input sensitivity of the EVT-800A instrument is 0.6 μ V/d. Therefore, according to the matching formula between the sensor and the instrument, the actual input signal of the instrument can be obtained as:

2×12×25/1000×3×25＝8μV/d＞0.6μv/d

So, the sensors used meet the requirements of instrument input sensitivity and can be matched with the selected instrument.

2. Meet the accuracy requirements of the entire electronic scale. An electronic scale is mainly composed of three parts: the scale body, sensors, and instruments. When selecting the accuracy of the sensors, the accuracy of the sensors should be slightly higher than the theoretical calculation value, because theory is often limited by objective conditions, such as the strength difference of the scale body, the poor performance of the instruments, and the harsh working environment of the scale, which directly affect the accuracy requirements of the scale. Therefore, it is necessary to improve the requirements from all aspects and consider economic benefits to ensure the achievement of the goal.