With the rapid development of the electronic weighing industry, the weighing market is growing. The use of various floor scales and truck scales is becoming more and more widespread, but users often encounter inaccurate weighing, unstable digital display values, touching the weighing sensor, and jumping faults during use. After inspecting the problematic products, 60% are caused by poor insulation of the weighing sensor, so this article further analyzes the causes of poor insulation and the problems that need to be paid attention to in the production and manufacturing of weighing sensors.
2. Overview
Weighing instrument manufacturers often receive complaints from users, reflecting the large zero drift of the scale, the large full-scale drift of the scale, and the unstable indication. Some customers are normal after replacing one or more weighing sensors. After checking and testing the indicators of the replaced weighing sensors, it is found that many weighing sensors are not well insulated or sometimes large and sometimes small. This article analyzes the causes of insufficient insulation of weighing sensors, so as to control the quality of key points in production and improve the qualified rate of weighing sensors.
3. Consequences and causes of insulation damage
The insulation damage of weighing sensors in the weighing process mainly manifests in the following two ways:
1. The digital display shows unstable values. When the finger touches the sensor, the value of the instrument jumps. This is mainly due to the poor insulation between the bridge circuit and the elastic body, which destroys the balance of the bridge. The large current passes through the film to produce thermal drift. During the power-on process, due to moisture discharge, the zero point of the sensor changes, the zero point temperature drift increases, the nonlinearity of the bridge increases, and even cannot be adjusted.
2. The digital display shows unstable values. When the finger touches the sensor, the instrument display has no response. The insulation resistance between the bridge circuit and the sensor is qualified when the instrument is used. This is mainly caused by the destruction of the insulation inside the bridge. There is leakage between the strain gauge solder joints, the solder joints of the wiring board, and the cable terminals and the circuit board.
4. Phenomena and examples of zero point fluctuations caused by poor insulation
A weighing instrument customer installed a weighing sensor on the scale and found that the zero point of the scale was unstable after less than half a year of use. Therefore, all the weighing sensors of a scale were returned. After testing the basic performance indicators of the weighing sensor, it was found that the insulation impedance of the weighing sensor was unqualified, and other performance indicators were qualified. The main reason is that the sealant is not reliable, and the continuous rainy weather and humid environment cause moisture to enter the weighing sensor. After dissecting one of the weighing sensors and checking its interior, there is no obvious moisture on the surface of the sealant. The remaining weighing sensors that have not been disassembled were soaked in hot water and tested a week later. It was found that the insulation of the weighing sensors was constantly decreasing. Finally, it was found that the root cause of the insulation decline was that the sealing performance of the cable outlet hole during installation did not meet the requirements. Since the weighing sensor installed by the customer has been in an open-air and humid environment for a long time, the insulation performance of the weighing sensor gradually decreased, which eventually led to the instability of the zero point of the weighing sensor and made it impossible to use it normally.
Therefore, it is completely impossible to achieve the effect of sealing the wire installation hole by relying solely on the locking seal ring of the lock wire connector. The wire installation hole must be sealed on the premise of ensuring that the cable is firmly installed. Before sealing the glue, the hole must be cleaned, and then a suitable sealant must be selected for sealing, which not only ensures that the performance of the weighing sensor is not affected by the glue, but also ensures that a good sealing effect is achieved.
V. Analysis of details leading to poor insulation during production and use
1. When patching, the surface of the elastic patch is not cleaned thoroughly, and the residual sand and iron particles may scratch the base of the strain gauge, causing the sensitive grid of the resistance strain gauge to bulge or break. As a result, the zero point of the weighing sensor is large, the bridge is broken, and the insulation is insufficient.
2. When patching, cotton threads and dirt are left on the elastic patch, which leads to poor insulation and zero drift of the weighing sensor.
3. The short-circuit wires of the bridge assembly, the solder joints of the terminal board, and the core wires exposed by the sheath of the plastic bridge assembly wire are in contact with the elastic body, which will cause insulation to exceed the tolerance. If the strain gauge surface glue is not fully dried when it is applied, it is also very easy for the medium to conduct electricity, which reduces the insulation performance.
4. The solder joints must be cleaned thoroughly, otherwise the residual flux will corrode the solder joints. Since the span between the two solder joints of the strain gauge is generally only 0.5 mm, once the gap is filled with rosin, the insulation impedance of the weighing sensor will definitely decrease. When the solder joint surface contacts moisture, the dirt absorbs moisture and the conductivity is enhanced. As the working time of the weighing sensor increases, the solder joint is corroded and carbonized, and the insulation performance of the weighing sensor will be more seriously damaged.
5. The compensation wire exposes a redundant tail. If it is not discovered during cleaning, after the glue is sealed, the compensation wire touches the elastomer under the curing and shrinkage of the glue, causing the weighing sensor to short-circuit.
6. If the solder joint is not round, cotton thread is hung between the two solder joints without careful cleaning. Once the cotton thread absorbs moisture, it is equivalent to a resistance of several thousand ohms across the two solder joints, thereby destroying the performance of the weighing sensor.
7. During the test compensation process, the welding points of the compensation resistors on the terminal board are relatively large, and they are also the most easily overlooked places for cleaning. Some elastomers are not well machined, the threading holes are not polished, and the insulation layer is cut when the cables are welded. Therefore, after welding, it is necessary to carefully check whether there are cold welding points, cotton threads, and unwashed rosin residues. Everything must be intact before filling and sealing.
8. Weighing sensors generally work in a poor environment for a long time, so the exposed parts of the weighing sensor cables are cut off at the head and tail. In order to avoid conductivity after contacting water vapor, heat shrink tubes are used for heat shrink protection. When installing the weighing platform, the cable must be protected in a steel pipe, and the cable must also be protected during the pipe wearing process. Once the cable is scratched or pulled off, it will cause poor insulation after rain.
9. The unreasonable welding of cables leads to poor insulation of weighing sensors. If the copper washers and sealing rubber pads in the cable locking nuts are of poor quality, or if the locking wire connector is tightened endlessly in order to lock the cable, the copper washers in the locking wire connector are likely to squeeze or even break the outer skin of the cable, causing the cable shielding net to contact the elastomer, resulting in drift of the display value or signal jump.
10. Glue sealing is a major process in the production process of weighing sensors. Its object is a weighing sensor with qualified performance. Poor sealing will leave fatal quality risks and will completely destroy the results of all previous processes. The glue sealing process is the last process in the production of weighing sensors. The product production and circulation process has gone through many processes. A large number of fingerprints, sweat stains, rust, etc. are left in the patch holes and on the wiring board. Products with long turnover time have accumulated dust. If the operator does not thoroughly clean and inspect the product before glue sealing, the weighing sensor will leave many quality risks after sealing, resulting in poor insulation of the weighing sensor.
11. During thunderstorms or electric welding, large current backflow causes insulation damage to the weighing sensor. During thunderstorms, there are a large number of electric clouds in the sky. Once lightning strikes the cloud charges, the charges accumulated on the scale are quickly released, which will induce a large current on the weighing sensor bridge, which will inevitably break down the base of the resistance strain gauge on the elastic element of the weighing sensor, causing damage to the weighing sensor or insulation damage.
VI. Conclusion
This paper analyzes the reasons for inaccurate weighing of electronic scales and concludes that poor insulation resistance of weighing sensors during production and use is the main cause of such failures. On this basis, the details of poor insulation caused by the production, use and manufacturing process of weighing sensors are analyzed, and various factors leading to poor insulation are briefly listed.