What is a load cell? Common faults and handling methods

Weight sensors are a type of device that converts quality signals into measurable electrical signal output, mainly including S-type, suspended wall type, spoke type, plate ring type, capsule type, bridge type, column type, etc. According to the conversion method, weight sensors are divided into 8 types: photoelectric type, oil pressure type, magnetic force type, capacitive type, magnetic deformation type, vibration type, gyroscope type, and resistance strain type.

The old national standard combined two types of sensors, "scale" and "force measurement", with completely different application objects and natural conditions, without distinguishing between experimental and evaluation methods. The old national standard had a total of 21 indicators, all of which were tested at room temperature; The accuracy level of the weight sensor can be determined by 0.02, using the maximum error, 0.03, 0.05... 1.0, among the six indicators of nonlinearity, hysteresis error, repetition error, creep, zero temperature additional error, and rated output temperature additional error.

The working principle of a weight sensor is to convert the gravity on the measured object into a measurable output signal in a certain proportion. Considering the impact of gravity acceleration and air buoyancy on conversion in different application locations, the performance parameters of weighing sensors mainly include linear error, hysteresis error, repetition error, creep, zero temperature characteristics, and sensitive temperature characteristics.

In various weighing instruments and quality measurement systems, the accuracy of sensors is generally controlled by comprehensive error. The comprehensive error band is combined with the weighing instrument deviation band to correctly use the weight sensor of the corresponding precision weighing instrument.

According to the requirements of the International Organization of Legal Metrology (OIML), the sensor deviation band accounts for 70% of the weighing instrument deviation band. The deviation caused by the linear error, hysteresis error, and temperature sensitivity to the required environmental temperature of the weight sensor application knowledge shall not exceed the deviation band. This allows sensor manufacturers to adjust each component of the total measurement error to achieve the expected accuracy.

The principle of the cutting beam weight sensor is to measure not only the normal stress but also the shear stress caused by the shear force under the action of the elastic body. However, the shear stress itself cannot be measured, and the vertical principal stress generated only at 45 ° with the central axis of the I-beam is the tensile stress and compressive stress caused by the shear stress. Therefore, four strain gauges are pasted on both sides of the I-beam end and are perpendicular to the central axis by 45 degrees.

Common problems and solutions include:

After power on, the display only shows a string of "8" and continues to shine. When this fault occurs, the first thing to check is whether the position of the scale is correct. Secondly, check if there are any moving objects blocking the weight of the sensing scale, and then check if the analog switch integration block is damaged. When adding a heavy load of the same scale to the electronic pricing scale. The representation value of each scale is different. Generally speaking, this type of fault occurs when the machinery falls behind or repeatedly exceeds the standard value. The reasons for mechanical backwardness deviation are as follows: the characteristics of the resistance Strain gauge are not good; Poor elastic material and geometric structure;

The adhesive is deteriorated, and the Strain gauge and elastomer are not well bonded. When weighing, there is no net weight mark on the display screen of the load pricing scale. If this fault occurs, you can check the power supply first. If the power supply is normal, check the amplification circuit and observe if there is a weighing signal input in the amplification circuit. If there is no weighing signal input for the load effect, the possible fault phenomenon and handling method is to check the bridge power supply circuit. If there is no bridge supply voltage in the bridge supply power circuit, measure the voltage of the bridge supply power to see if it is normal. In general, the current of the bridge power supply is accurate and its stability is 5 times better than that of the weight sensor.

Check if the cable socket connected to the weight sensor has poor contact; Or check if the weight sensor cable itself is broken, causing the weighing signal to be unable to input into the amplifier. Check if there are empty or open welds at each welding point of the weighing sensing bridge, which may cause the weighing sensing bridge to not work.

Check the weight sensor itself to see if there is any foreign matter or adhered dirt on it, which limits the elastic deformation of the sensor, causing the load effect on the sensor. The elastomer cannot produce elastic deformation normally, making the resistance of the resistance Strain gauge unchanged, resulting in no output of the scale signal from the scale sensor bridge. When no scale load is applied to the electronic pricing scale, the displayed value of the scale is not zero, and it indicates that the value is unstable and constantly changing. This type of fault is commonly referred to as zero drift, non return to zero, and skip word. The reason for this fault is that the bridge arm of the weighing sensing bridge has virtual connections and welding, or the welding points have "grounding"; Check if the power supply current is stable. If the power supply current stability is not good, it is also easy to skip words.

The scale weight range is not accurate. The reasons for this phenomenon are:

The elastic body is impacted, causing stress in the elastic body; Or the elastic body partially ruptures, causing the strain generated by the elastic body when the scale load acts on the elastic body to be non-linear, resulting in inaccurate data;

The bridge supply voltage is too high, resulting in overheating of the resistance Strain gauge, damaging the adhesive of the Strain gauge, or changing the resistance value of the Strain gauge, resulting in inaccurate reading;

The impact of environmental factors. Electronic pricing scales have been applied in high-temperature or humid environments, resulting in a decrease in insulation resistance between strain bridges and elastomers; Excessive fatigue of the elastic body leads to the loss of its expected stress transformation range.

When adding a stable weight load to an electronic pricing scale, the weight representation value of the scale changes over time. The larger the load, the greater the change. Usually, this situation is called creep. The reason for creep is usually due to the adhesive layer. Improper use of adhesive, or aging and deterioration of adhesive; When pasting, the Strain gauge is wet and cold, the pasting layer is too thick, and the pasting layer is not dry.

In short, to check the weight sensor failure of the electronic pricing scale, first check the appearance of the weight sensor, and then gradually detect it. On the basis of understanding its working principle, careful observation, analysis, and detection will quickly eliminate the fault.

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