How to choose a sensor

A load cell is actually a device that converts a mass signal into a measurable electrical output. When using a sensor, the actual working environment of the sensor should be considered first, which is crucial to the correct selection of the sensor. It is related to whether the sensor can work normally, its safety and service life, and even the reliability and safety of the entire weighing instrument.

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

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

(2) The influence of dust and humidity on the short circuit of the sensor. In this environmental condition, a sensor with high airtightness should be selected. Different sensors have different sealing methods, and their airtightness is very different.

Common seals include sealant filling or coating; rubber pads are mechanically fastened and sealed; welding (argon arc welding, plasma beam welding) and vacuum nitrogen filling seals.

From the perspective of sealing effect, welding sealing is the best, and filling and coating sealant is the poorest. For sensors that work in a clean and dry indoor environment, you can choose a glue-sealed sensor, and for some sensors that work in a humid and dusty environment, you should choose a diaphragm heat seal or diaphragm welding seal, pumping Vacuum nitrogen filled sensor.

(3) In a highly corrosive environment, such as moisture and acidity, which will damage the elastomer or cause a short circuit, the outer surface should be oversprayed or a stainless steel cover should be selected, which has good corrosion resistance and good airtightness. sensor.

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

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

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

The selection of the number of sensors is determined according to the purpose of the electronic weighing instrument and the number of points that the scale body needs to support (the number of supporting 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 for the scale body with several supporting points, but for some special scale bodies such as electronic hook scales, only one sensor can be used, and some electromechanical combined scales should be determined according to the actual situation. number.

The selection of the sensor range can be determined according to the comprehensive evaluation of factors such as the maximum weighing value of the scale, the number of selected sensors, the self-weight of the scale body, the possible maximum eccentric load and dynamic load. In general, the closer the range of the sensor is to the load assigned to each sensor, the more accurate its weighing will be. However, in actual use, since the load applied to the sensor includes the self-weight, tare weight, eccentric load and vibration impact of the scale in addition to the object to be weighed, many factors should be considered when selecting the sensor range to ensure that Sensor safety and longevity.

The calculation formula of the sensor range is determined through a large number of experiments after fully considering various factors affecting the scale body.

The formula is as follows:

C=K-0K-1K-2K-3(Wmax+W)/N

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

For example: a 30t electronic truck scale, the maximum weighing is 30t, the scale body weight is 1.9t, using four sensors, according to the actual situation at that time, select the insurance factor K-0=1.25, the impact factor K-1=1.18, The center of gravity offset coefficient K-2—=1.03, the wind pressure coefficient K-3=1.02, try to determine the tonnage of the sensor.

Solution: Calculate the formula according to the sensor range:

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

According to experience, the sensor should generally work within 30% to 70% of its range, but for some weighing instruments with large impact force during use, such as dynamic rail 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 30% of its range, so that the weighing reserve of the sensor is increased to ensure the safety and life of the sensor.

Again, consider the applicability of each type of sensor.

The selection of the sensor type mainly depends on the type of weighing and the installation space to ensure proper installation and safe and reliable weighing; on the other hand, the manufacturer's recommendations should be considered. Manufacturers generally specify the scope of application of the sensor according to the force of the sensor, performance indicators, installation form, structural type, and elastomer material. 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 rail scales, truck scales, crane scales, etc.; column sensors are suitable for truck scales, dynamic rail scales, and large-tonnage hopper scales. Wait.

Finally, there is a choice of sensor accuracy class.

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

The selection of the sensor class must satisfy the following two conditions:

1. Meet the requirements of instrument input. The weighing display instrument displays the weighing result after processing the output signal of the sensor through amplification and A/D conversion. Therefore, the output signal of the sensor must be greater than or equal to the input signal size required by the meter, that is, the output sensitivity of the sensor is replaced by the matching formula of the sensor and the meter, and the calculation result must be greater than or equal to the input sensitivity required by the meter.

The matching formula of sensor and meter:

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

The number of divisions of the scale * the number of sensors * the range of the sensor

For example: a quantitative packaging scale weighing 25kg, the maximum number of divisions is 1000 divisions; the scale body adopts 3 L-BE-25 type sensors, the range is 25kg, the sensitivity is 2.0±0.008mV/V, the arch bridge voltage pressure 12V; the scale uses an AD4325 meter. Ask whether the sensor used can match the meter.

Solution: After consulting, the input sensitivity of the AD4325 meter is 0.6μV/d, so according to the matching formula of the sensor and the meter, the actual input signal of the meter can be obtained as:

2×12×25/1000×3×25=8μV/d>0.6μv/d

Therefore, the sensor used meets the requirements of the input sensitivity of the instrument 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: scale body, sensor and instrument. When selecting the accuracy of the sensor, 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 scales. The strength of the body is a little bit worse, the performance of the instrument is not very good, the working environment of the scale is relatively bad and other factors directly affect the accuracy requirements of the scale.