Ultra-high frequency partial discharge (UHF) sensors are typically connected to a Wheatstone bridge via leads. Normally, when no external pressure is applied to the sensing element, the bridge is in a balanced state (referred to as zero position). When pressure is applied to the sensor, the chip resistance changes, and the bridge becomes unbalanced. If a constant current or voltage power supply is applied to the bridge, it will output a voltage signal corresponding to the pressure. Thus, the sensor's resistance change is converted into a pressure signal output by the bridge. The bridge detects the resistance change, amplifies it, and then converts it into a corresponding current signal through voltage-to-current conversion. This current signal is compensated by a nonlinear correction loop, resulting in a standard output signal of 4–20 mA that is linearly related to the input voltage.
Seven common faults of UHF partial discharge sensors:
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1. If the electronic ruler has been used for a long time, and the seal has aged and contains many impurities, and the water mixture and oil will seriously affect the contact resistance of the brush, causing the displayed numbers to fluctuate continuously. In this case, the electronic ruler can be said to be damaged and needs to be replaced.
2. If the power supply capacity is too small, many problems will arise. Therefore, the power supply needs to have sufficient capacity. Insufficient capacity will cause the following issues: the movement of the molten plastic will cause fluctuations in the display of the mold closing electronic ruler, or the movement of the mold closing will cause fluctuations in the display of the injection electronic ruler, resulting in large measurement errors. These problems are more likely to occur if the solenoid valve's drive power supply and the ultra-high frequency partial discharge sensor's power supply are connected simultaneously. In severe cases, voltage fluctuations can even be measured with a multimeter in voltage mode. If the problem is not caused by high-frequency interference, electrostatic interference, or insufficient neutralization, then it is likely due to insufficient power supply capacity.
3. Both FM interference and electrostatic discharge (ESD) interference can cause the digital display on the sensor's electronic ruler to fluctuate. The signal cable of the electronic ruler must be kept in a separate cable tray from the equipment's power lines. The electronic ruler must be equipped with a grounded bracket, and the ruler's casing must have good contact with the ground. Shielded cables must be used for the signal cable, and one end of the cable should be grounded to the shield.
When there is high-frequency interference, a multimeter voltage measurement will usually show normal readings, but the displayed numbers will fluctuate continuously. The same situation occurs with electrostatic interference. To verify if it's electrostatic interference, you can short-circuit the cover screw of the electronic ruler with some metal parts on the machine using a power cord. The electrostatic interference will immediately disappear upon short-circuiting. However, eliminating high-frequency interference is difficult using the above method. Variable frequency drives and robotic arms frequently experience high-frequency interference, so you can try stopping the high-frequency drive or robotic arm to verify if it's high-frequency interference.
4. If the electronic ruler displays data that jumps regularly at a certain point during operation, or if no data is displayed, it is necessary to check whether the insulation of the connecting wire is damaged and whether it is making regular contact with the machine casing, resulting in a short circuit to ground.
5. The power supply voltage must be stable. Industrial voltages need to meet a stability requirement of ±0.1%. For example, if the reference voltage is 10V, a fluctuation of ±0.01V is permissible. Otherwise, it will cause fluctuations in the display. However, if the amplitude of the display fluctuation does not exceed the amplitude of the voltage fluctuation, then the electronic ruler is normal.
6. The installation requires excellent alignment, but a parallelism error of ±0.5mm and an angle error of ±12° are permissible. However, if both the parallelism and angle errors are too large, the displayed numbers may fluctuate. In such cases, adjustments to the parallelism and angle are necessary.
7. During the connection process, extra care must be taken. The three wires of the electronic ruler must not be connected incorrectly, and the power wire and output wire cannot be interchanged. If the above wires are connected incorrectly, a large linear error will occur, which will be difficult to control, the control accuracy will become very poor, and the display will easily exhibit fluctuations, etc.
