Talk about how to choose weighing sensor

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 an electronic scale, we often encounter the problem of how to select the sensor.

How to choose a sensor

A weighing sensor is actually a device that converts mass signals into measurable electrical signals. When using a sensor, you must first consider the actual working environment of the sensor. This is crucial to the correct selection of the sensor, as 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 instrument.

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

(1) High temperature environment can cause problems such as melting of coating materials, melting of solder joints, and structural changes in stress in the elastomer. For sensors working in high temperature environments, high temperature resistant sensors are often used; in addition, heat insulation, water cooling or air cooling devices must be added.

(2) Dust and moisture can cause short circuits on sensors. Under these environmental conditions, a sensor with high airtightness should be selected. Different sensors have different sealing methods, and their airtightness varies greatly.

Common seals include sealant filling or coating; rubber gasket mechanical fastening seal; welding (argon arc welding, plasma beam welding) and vacuum nitrogen filling seal.

From the perspective of sealing effect, welding sealing is the best, and filling and coating sealant is the least effective. For sensors working in clean and dry indoor environments, sensors with glue sealing can be selected, while for sensors working in humid and dusty environments, sensors with diaphragm heat sleeve sealing or diaphragm welding sealing and vacuum nitrogen filling should be selected.

(3) In highly corrosive environments, such as humidity and acidity, which may damage the elastomer of the sensor or cause a short circuit, you should choose a sensor with a plastic-sprayed outer surface or a stainless steel cover, good corrosion resistance and good sealing.

(4) The influence of electromagnetic field on the disordered signal output by the sensor. In this case, the shielding of the sensor should be strictly checked to see whether it has good anti-electromagnetic ability.

(5) Inflammable and explosive materials not only cause complete damage to the sensor, but also pose a great threat to other equipment and personal safety. Therefore, sensors working in inflammable and explosive environments have higher requirements for explosion-proof performance: explosion-proof sensors must be selected in inflammable and explosive environments. The sealed outer cover of such sensors must not only consider its airtightness, but also its explosion-proof strength, as well as the waterproof, moisture-proof and explosion-proof properties of the cable lead-out head.

Secondly, the selection of the number and range of sensors.

The number of sensors is determined by the purpose of the electronic scale and the number of points that the scale needs to support (the number of support points should be determined based on the principle of coinciding the geometric center of gravity of the scale with the actual center of gravity). Generally speaking, there are as many sensors as there are support points on the scale, but for some special scales such as electronic hook scales, only one sensor can be used. For some electromechanical combined scales, the number of sensors should be determined based on actual conditions.

The selection of sensor range can be determined based on a comprehensive evaluation of factors such as the maximum weighing value of the scale, the number of sensors selected, the deadweight of the scale, the maximum possible eccentric load and dynamic load, etc. Generally speaking, the closer the sensor range is to the load assigned to each sensor, the higher the weighing accuracy. However, in actual use, since the load added to the sensor includes not only the deadweight of the scale, the tare weight, eccentric load, vibration and impact, etc., in addition to the object being weighed, many factors must be considered when selecting the sensor range to ensure the safety and life of the sensor.

The calculation formula of the sensor range is determined after a large number of experiments after fully considering all factors that affect the scale.

The formula is as follows:

C＝K-0K-1K-2K-3(Wmax＋W)/N

C—rated range of a single sensor; W—weight of the scale; Wmax—maximum net weight of the object being weighed; N—number of supporting points used by the scale; K-0—safety factor, generally between 1.2 and 1.3; K-1—impact coefficient; K-2—center of gravity offset coefficient of the scale; K-3—wind pressure coefficient.

For example: a 30t electronic truck scale has a maximum weighing capacity of 30t and a deadweight of 1.9t. It uses four sensors. According to the actual situation at the time, select the insurance coefficient K-0=1.25, the impact coefficient K-1=1.18, the center of gravity offset coefficient K-2—=1.03, and the wind pressure coefficient K-3=1.02. Try to determine the tonnage of the sensor.

Solution: According to the sensor range calculation formula:

C＝K-0K-1K-2K-3（Wmax＋W）/N

It can be seen that:

C＝1.25×1.18×1.03×1.02×（30＋1.9）／4

=12.36t

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

According to experience, the sensor should generally operate within 30% to 70% of its range. However, for some scales that are subject to greater impact during use, such as dynamic track scales, dynamic truck scales, steel scales, etc., when selecting sensors, it is generally necessary to expand their range so that the sensor operates within 20% to 30% of its range, thereby increasing the sensor's weighing reserve and ensuring the sensor's safety and lifespan.

Again, consider the applicability of various types of sensors.

The choice of sensor type mainly depends on the type of weighing and the installation space, ensuring that the installation is appropriate and the weighing is safe and reliable; on the other hand, the manufacturer's recommendations should be considered. The manufacturer generally specifies the applicable scope of the sensor based on the force conditions, performance indicators, installation form, structural type, elastic material and other characteristics of the sensor. For example, aluminum cantilever beam sensors are suitable for pricing scales, platform scales, case scales, etc.; steel cantilever beam sensors are suitable for hopper scales, electronic belt scales, sorting scales, etc.; steel bridge sensors are suitable for track scales, truck scales, overhead crane scales, etc.; column sensors are suitable for truck scales, dynamic track scales, large-tonnage hopper scales, etc.

Finally, the sensor accuracy level must be selected.

The accuracy level of the sensor includes technical indicators such as nonlinearity, creep, creep recovery, hysteresis, repeatability, sensitivity, etc. When selecting a sensor, do not simply pursue a high-level sensor, but consider both the accuracy requirements of the electronic scale and its cost.

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

1. Meet the requirements of 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 required by the instrument, that is, the output sensitivity of the sensor is substituted into the matching formula of the sensor and the instrument, and the calculation result must be greater than or equal to the input sensitivity required by the instrument.

Matching formula for sensors and instruments:

Sensor output sensitivity * excitation power supply voltage * maximum weighing capacity of the scale

Scale division number * number of sensors * sensor range

For example: a quantitative packaging scale with a weighing capacity of 25kg, with a maximum division number of 1000 divisions;
the scale body uses 3 L-BE-25 sensors, with a range of 25kg, a sensitivity of 2.0±0.008mV/V, and a bridge voltage of 12V; the scale uses an AD4325 instrument. Question: Can the sensor used match the instrument?
Answer: After checking, the input sensitivity of the AD4325 instrument is 0.6μV/d, so according to the matching formula of 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

Therefore, the sensor used meets the instrument input sensitivity requirements and can match 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, the sensor, and the instrument. When selecting the accuracy of the sensor, the accuracy of the sensor should be slightly higher than the theoretical calculated value, because the theory is often limited by objective conditions, such as the strength of the scale body is a little poor, the performance of the instrument is not very good, the working environment of the scale is relatively bad, etc. These factors 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 that the purpose is achieved.