Interference Analysis And Anti-interference Measures of Weighing Display Instrument

In the production process of a company, there are a large number of intermediate tank farms. The storage tanks contain viscous materials, and the storage tanks are equipped with agitators. For the measurement of storage tank materials, we choose the weighing sensor and weighing display instrument. In the long-term use, we timely analyze and eliminate the faults in the system due to improper installation and use, and achieved obvious results. Due to more accurate measurement, the product quality is also very stable.
1. Influence of external electromagnetic interference on symmetrical display instrument
Due to the long-term use of the weighing display instrument, the sensor is connected to the signal transmission line of the display instrument, which may reduce the insulation resistance of the line between the signal line and the power line due to its proximity to the power line, resulting in leakage. In this way, the input end of the weighing display instrument receives the working signal and the leakage current interference signal, resulting in error in the reading of the instrument. In addition, there are some external strong magnetic interference signals, and when the signal strength reaches a certain order of magnitude, the amplification unit will work in the saturation area, which also leads to a significant decrease in the sensitivity of the whole instrument, resulting in weighing errors.
2 sources of external electromagnetic interference
When the weighing display instrument is used on site, the introduction of external electromagnetic interference is multifaceted, and the introduction point also changes with environmental temperature, humidity and other factors. It can be summarized in the following two aspects:
2.1 input interference at instrument input
The so-called input interference is that for some reason, there is a signal with frequency change between the input signal lines of the weighing display instrument, resulting in the input interference to the display instrument.
The source of input interference signal is mainly caused by alternating magnetic field induction. In general, transformers, AC motors, power supplies, high-power electromagnets, etc. will produce strong alternating magnetic field within a certain range. If the signal line of the weighing display instrument and the weighing display instrument itself are within the range of alternating magnetic field, they will be affected by the alternating magnetic field and induce interfering alternating electrical signals at the input end of the weighing display instrument.
2.2 interference to ground
The interference signal to ground refers to the interference signal with frequency change between one end (positive end or negative end) of the input working signal line of the weighing display instrument and the ground.
The sources of ground interference signals mainly include:
(1) Interference signal to the ground generated by the ground current. There are high-power electrical equipment near the load cell or weighing display instrument. When the insulation performance of these equipment is poor, there will be leakage current to the ground; or some electrical equipment can also generate ground current by using the earth as a transmission line and the imbalance of power distribution system, and there is potential difference at different points of the earth. If we ground the load cell and the metal structure frame of the said storage tank and the weighing display instrument, there will be a ground current because the load cell and the weighing instrument are not installed in the same place. When the current flows through the input circuit of the amplification unit, the generated voltage drop will become an interference signal to ground.
(2) Interference signal to ground caused by high-voltage electric field. If the weighing sensor of the storage tank is connected with the signal line of the weighing display instrument and laid parallel to the high-voltage power supply line, there will be interference current flowing through the distributed capacitance between the power supply line and the signal line under the action of high-voltage electric field, and the current will be grounded through the grounding capacitance at the input end of the weighing display instrument. In the input circuit The voltage drop of resistor R is converted into a ground interference signal.
(3) The ground interference signal caused by the DC interference signal. At the connection of the signal line, a chemical potential is formed at the connection due to the infiltration and oxidation of salt, alkali or acid, resulting in the DC ground interference signal. Like the AC interference signal, the DC interference signal will also appear in the amplification circuit of the amplification unit of the weighing display instrument, and the DC interference signal is right The influence of weighing display instrument will be greater than AC interference signal, which will significantly reduce the sensitivity.
3 anti interference measures
3.1 anti input interference
(1) Laying of signal line between weighing display instrument and weighing sensor. Electromagnetic induction is the main source of input interference of weighing display instrument, so when laying the working signal line of weighing display instrument, it shall be away from electrical equipment as far as possible (such as motor, high-voltage power line), etc. it should be noted that it is absolutely not allowed to lay the working signal line and power line of the weighing display instrument in the same metal pipe.
(2) The weighing display instrument is connected with the weighing sensor. The interference voltage introduced into the weighing display instrument is not only related to the strength of the AC magnetic field, but also related to the cross-sectional area passing through the working signal. Twisting the signal line can reduce the influence of the interference signal on the one hand, and on the other hand, make the two signal lines offset part of the interference signal, which effectively weakens the interference signal Intensity of external AC magnetic field.
(3) Shielding of the signal line between the weighing display instrument and the weighing sensor. For the connecting line between the weighing display instrument and the weighing sensor, not only the shielding line shall be selected, but also the signal line shall be sheathed with a metal tube, so that the magnetic flux caused by external interference will pass along the metal tube with small magnetic resistance, so as to reduce the impact of external interference magnetic field on the signal line.
3.2 anti ground interference
(1) Signal line bypass capacitance method: ground one end of the signal line of the load cell and a capacitor C large enough to make the interference signal bypass through capacitor C.

(2) The weighing display instrument and transmission line are grounded separately. During field installation and use, the grounding wire of the weighing display instrument is often connected to the public grounding wire of other electrical equipment, or to the nearby water pipe and heating pipeline; for the metal pipe laying signal line, there are multiple grounding points due to fixed support and other reasons, so that there are electrical signals in the metal pipe A potential difference is formed on the metal tube, which is coupled to the signal line through the distributed capacitance between the metal shielding tube and the signal transmission line, causing interference. Connect the grounding wire of the weighing display instrument and the grounding wire of the metal shielded pipe laying the signal line to the same grounding point.

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