Measurement by the batching weighing system of the mixing plant

Each electronic scale in the batching weighing system of the mixing plant is composed of several parts, including the batching bin (scale body), weighing sensor, junction box, control instrument and connecting wire. During use, common faults such as instability, drift and linear deviation of the batching scale affect the normal operation of the mixing plant.

Line faults are manifested as the controller indicating value overflow, displaying error messages, and the signal suddenly interrupting or jumping randomly during weighing. There are two possible reasons for this.

One is that there is a problem with the quality of the sensor itself. It is manifested as changes in the input and output resistances of the sensor. Under normal circumstances, the input resistance of the sensor is 400Ω±10Ω, and the output resistance is 350Ω±3Ω. The main fault of the sensor is that the solder joints of the terminal blocks or lead-out wires fall off. The zero point temperature compensation or sensitivity temperature compensation resistor of the sensor is poorly soldered or de-soldered. The sensor is not well sealed and the internal chip is damp.

The second issue is that the connection line between the sensor and the control instrument is disconnected. This kind of external fault is mainly caused by improper installation and use. If the input and output resistances of the sensor are checked with a multimeter, the resistance values are both normal. The cause of the malfunction is: the connecting wire broke, causing a circuit break. The connection wire joints are loose or not soldered firmly with solder, etc.

Recently, when the author was providing on-site service at a mixing plant, it was found that the readings on the aggregate weighing control instrument were jumping randomly, making it impossible to calibrate. After the on-site debugging personnel conducted multiple checks but still failed to identify the cause of the fault, they suspected that there was a problem with the sensor. After arriving at the site, the author measured the input and output resistance values and voltages of the four sensors respectively, all of which were normal, thereby determining that the sensors were in good condition. After replacing the control instrument, the fault phenomenon has not been eliminated. It is initially judged that it is a fault in the connection line. After inspection, it was found that the excitation power line from the control instrument to the sensor was loose at the wiring strip, resulting in unstable power supply to the sensor.

Abnormal zero point of electronic scales is also a relatively common fault. One of the reasons is that the sensor has not undergone temperature compensation. The zero point and sensitivity of the sensor change with the variation of the ambient temperature, causing the zero point of the electronic scale to malfunction. According to international standards, there is a temperature compensation process in the sensor production process, which should be compensated by simulating the temperature changes in the sensor's operating environment through a high and low temperature chamber. However, many domestic sensor manufacturers do not have a high and low temperature chamber at all. Temperature compensation for the sensor cannot be performed. The second reason is that the sensor is severely overloaded, causing plastic deformation of the elastomer and damaging the sensor.

Recently, the author found at a mixing plant site that a orthopedic scale had its zero point become larger shortly after installation, and it was impossible to accurately measure and determine the cause of the fault on the spot. The author carefully observed four S-shaped 2t sensors and found that one of them had obvious impact marks on the iron chain connected to the scale body, thus determining that this sensor was overloaded. Measure the zero-point output of the four sensors respectively. The zero-point output of this sensor is four times that of the others. Replace the overloaded and damaged sensor and eliminate the fault.

The faults of control instruments can be judged by the substitution method. After initially determining that there are no issues with the sensor and connection lines and suspecting a fault in the control instrument, a good instrument can be used as a substitute to observe whether the fault phenomenon disappears. After replacing the new control instrument, the relevant parameters must also be set and calibrated.

The author dealt with a case of control instrument failure not long ago. When a certain company was installing and commissioning a medium-sized mixing plant for a client, it was found that neither the water scale nor the admixture scale could be calibrated normally. The fault is manifested as the water scale controller showing stable values, but the loading response is very sluggish. The control instrument of the admixture scale indicates a stable value, but there is no response during loading.

After a preliminary inspection of the sensor and connection lines showing no faults, it was determined that the problem was very likely to lie in the control instrument. This control instrument is an integrated type, and all the sensors of the sub-scales share one power supply. As there were no other control instruments available for replacement on site, the location of the fault could not be accurately determined. Both sub-scales have problems at the same time. The author suspects that it is caused by an unstable power supply. The sensor is powered by the carried voltage stabilizing power supply, and the output signal of the sensor is transmitted to the control instrument, and the fault phenomenon disappears.

In addition, water ingress, moisture absorption, oxidation or resistance breakdown of the junction box may all lead to major malfunctions.