insulation resistance and average life of piezoelectric ceramic hydrophone

Views: 45 Author: Site Editor Publish Time: 2019-10-16 Origin: Site

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Piezoelectric ceramic hydrophones have a wide range of applications in sonar due to their flatness, stable performance and simple structure. The main failure mode of piezoelectric ceramic hydrophones is the reduction in the insulation resistance. The average insulation resistance of the hydrophone is reduced to exceed the specified failure criteria Rfc, which is judged as a failure. Therefore, studying the variation of the insulation resistance of piezoelectric ceramic hydrophone, the influence of insulation resistance reduction on sensitivity and the determination of average life are of great significance for correctly understanding and mastering the performance and reliability index of hydrophone. The maintenance and protection also have certain reference value.

1 The average life of a hydrophone
According to the reliability theory , the number of piezoelectric disc transducer products was tested under the same conditions, and the total life data was measured.Then, the MTTF of the average pre-fault time is where r(t) is the cumulative number of failures of the product during the working hours from 0 to t. When the product's life distribution obeys the exponential distribution, its MTTF is the reciprocal of the failure rate λ (the probability of failure of the product to work at a certain time and the unit time after that time), that is, 1/λ. Hydrophones can use MTTF to represent their average life. In engineering, the average insulation of all the hydrophones in the batch is often used, and the resistance Rm is reduced to the time tav of the fault standard Rfc as the average life. In the Rfc value, some hydrophones have been faulty. Although other hydrophones can work normally, their sensitivity will drop sharply in the low frequency band, which will have a serious impact on the performance of the receiving array. Therefore, it should be replaced as soon as possible. .

2 hydrophone insulation resistance

2.1 Mechanism and law of the decline of insulation resistance

The core component of a piezoelectric ceramic hydrophone is a piezoelectric ceramic component. When the piezoelectric ceramic component is dried, the insulation resistance is high, and when water molecules infiltrate, the insulation resistance is lower. The more water molecules infiltrate, the more the insulation resistance drops. Piezoelectric ceramic hydrophones work in the water for many years. The piezoelectric ceramic components are encapsulated by water-tight materials (such as neoprene, polyurethane, etc.), but water molecules always pass through the surface of these materials or two materials. The bonding surface penetrates into the surface and inside of the piezoelectric ceramic component, so that the insulation resistance of the hydrophone is lower. The longer the hydrophone is used, the more water molecules accumulate on the surface and inside of the piezo ceramic component, and the greater the insulation resistance is, the more the hydrophone malfunctions. It can be seen from the above analysis that the drop in the insulation resistance of the hydrophone is inevitable and irreversible. We can't let the insulation resistance of the hydrophone not drop, all we can do is delay the speed of the drop. The slowing down of the insulation resistance means an increase in the life of the hydrophone. The following measurement can be used to slow down the resistance of the hydrophone insulation resistance (1) The use of piezoelectric ceramic components with low moisture absorption. Generally speaking, materials with higher density are less hygroscopic than materials with lower density. The commonly used PZT piezoelectric ceramic material has a density of 7.8 g/cm3, and the rarely used barium titanate piezoelectric ceramic material has a density of 5.7 g/cm3, and the latter has a much larger "hygroscopicity"; 2) Use water-tight materials with low water permeability; (3) Improving the manufacturing process of hydrophones, and often receive obvious effects. For a single piezoceramic hydrophone, we can't predict the law of its insulation resistance drop, nor can it predict when it will fail. But for a batch of hydrophones, and the number of these hydrophones is large, their insulation resistance will follow a certain law, the so-called statistical law. Looking at an example first, although it is not practical, it is abstracted from reality and has a practical basis. hydrophones are put into use, and the standard Rfc = 0.5 MΩ for determining the malfunction of the hydrophone is assumed. In the early days of use, a typical distribution of the insulation resistance of the hydrophone was measured. Most of the hydrophones have an insulation resistance Rm greater than or equal to 100 MΩ, while 50 <Rm < 100 MΩ has only a small portion, and 20 < Rm ≤ 50 MΩ is even less. In the middle of use, the insulation resistance distribution of hydrophones, such as the insulation resistance Rm of hydrophones,it is mostly between 2 and 10 MΩ, and is rarely in the range of 20<Rm≤50 MΩ. Many insulations of hydrophones have been Less than 0.5 MΩ, some of the hydrophones have failed. If no replacement measures are taken, the insulation resistance distribution of the hydrophone is as shown in the later stage of use. Most hydrophones have an insulation resistance of 0.5 MΩ or less. At this time, the average insulation resistance Rm of the batch of hydrophones is less than Rfc, indicating that the batch of hydrophones has exceeded their average life. It can be seen from the above three figures that the "center of gravity" of the distribution of the insulation resistance of the hydrophone gradually shifts to the left as the time increases, that is, the average insulation resistance decreases year by year. A large number of experimental results show that the average insulation resistance of piezo ceramic disc sensor is subject to exponential law. That is, at a certain temperature, the following formula holds.

2.2 Insulation resistance and hydrophone sensitivity

From the equivalent circuit analysis, the hydrophone insulation resistance can be considered as parallel to the two ends of the hydrophone . As the number of water molecules permeating the surface of the piezo ceramic element and the internal water through the water-tight covering material and the bonding layer increases, the insulation resistance Rm of the hydrophone will continuously decrease. Lowering the Rm to a certain level will reduce the hydrophone sensitivity . The lower the operating frequency, the greater the reduction in M. The equivalent circuit diagram of the piezoelectric ceramic hydrophone can be given in the form of a constant current source, and can also be given in the form of a constant voltage source. The constant voltage source is equivalent circuit diagram shows the simulation and actual measurement results of the sensitivity reduction of a hydrophone under the different insulation resistances. Both theoretical calculations and actual measurements prove that the smaller the static capacitance of the hydrophone, the greater the impact of the decrease in Rm on M. Since the static capacitance of the hydrophone is being tested is very large, up to 100000 pF, the reduction in insulation resistance Rm has a relatively small effect on its sensitivity . When Rm ≥ 10 kΩ, the effect on M is negligible; when Rm < 10 kΩ, it will have a large effect on M, and the hydrophone is judged as a fault. We call the insulation resistance value that determines the fault of the hydrophone as the fault value Rf. In the above example, Rf = 10 kΩ.

Obviously, if the static capacitance of the hydrophone is 10000 pF, the sensitivity will be significantly affected when the insulation resistance is less than 100 kΩ. At this time, Rf=100 kΩ. Based on the above results and the value of the sensitivity of the hydrophone that the sonar machine allows, the failure criterion Rfc can be determined. Rfc should be more than 10 times larger than Rf, so as to ensure that the average insulation resistance of all hydrophones on the array is close to Rfc, the number of hydrophones whose insulation resistance Rm is less than Rf, that is, the number of fault hydrophones is hoarse. Within the range allowed by the whole machine. In addition, the hydrophone is installed below the waterline of the ship. Once the hydrophone is found to be faulty, it is generally necessary to wait until the ship is docked to implement the hydrophone replacement, so there is a delay. During this delay time, the insulation resistance of the hydrophone will continue to drop. Therefore, the fault standard Rfc must be set higher to ensure that the hydrophone can be used normally before replacement. In addition, the insulation resistance of the hydrophone has a great relationship with the ambient temperature, and it must be fully considered when determining the fault standard Rfc of the hydrophone.

2.3 Relationship between insulation resistance and ambient temperature

The insulation resistance of piezoceramic disc transducer is closely related to the ambient temperature: the ambient temperature rises, the insulation resistance decreases, Both theory and a large number of practices have proved that the relationship between the insulation resistance Rm of the piezoelectric ceramic hydrophone. the ambient temperature is similar to the relationship between the use time and the exponential law. In the formula, Rmo is the insulation resistance measured at the reference temperature t0; k3 is the I-type temperature coefficient. Similarly, the above formula can also be written in a more convenient and intuitive form, k4 = exp(−k3), which is a type II temperature coefficient, then mo R ≈ R k , modified barium titanate piezoelectric ceramic hydrophone .Simulation results and measured results of the relationship between insulation resistance and ambient temperature. The measuring results are close to the simulation results, k4 = 0.94 ~ 0.95 / 1 °C. The simulation results and measuring results of the relationship between the insulation resistance of PZT piezoelectric ceramic hydrophone and the ambient temperature are shown. The test results are also close to the simulation results, k4=0.90~0.94/1°C. The relationship between the insulation resistance of the piezoelectric ceramic hydrophone and the use time is irreversible; otherwise, the relationship between the insulation resistance of the piezoelectric ceramic hydrophone and the ambient temperature is reversible, that is, when the ambient temperature returns to its original value, its insulation .

The resistance will also return to its original value. The insulation resistance of the hydrophone varies greatly with the ambient temperature. For every 11 °C increase in ambient temperature, the insulation resistance is reduced by about half. Compared with the modified barium titanate piezoelectric ceramic hydrophone, the insulation resistance of the PZT piezoelectric ceramic hydrophone will change more with the ambient temperature. The above-mentioned variation rules are different for different types and different specifications of piezoelectric ceramic materials of different structures and different watertight coating materials, which need to be determined by experiments. When is determining the failure standard Rfc of a piezoelectric ceramic hydrophone, the relationship between the insulation resistance of the hydrophone and the ambient temperature should be fully considered. For equipment with high reliability requirements, the Rfc of the supporting hydrophone should be determined at the highest ambient temperature (for example, 30 °C). Thus, when the ambient temperature drops, the insulation resistance of the hydrophone will only increase, and will not affect normal use.

The main mode of piezoelectric ceramic hydrophones is the reduction of insulation resistance. The mechanism is that water molecules penetrate into the surface and inside of piezo ceramic components through the watertight coating material and the bonding layer. The insulation resistance decreases with the increasing use time and satisfies the exponential law. The insulation resistance decreases as the ambient temperature increases, and also meets the exponential law. When the insulation resistance is reduced to a certain level, it will have a significant impact on the sensitivity of the hydrophone, and even worse in the low frequency range. Practically, it is convenient to define the average time of dropping the average insulation resistance of a batch of hydrophones to the specified failure standard. In the determining the fault standard, based on the relationship between the insulation resistance of the hydrophone and the sensitivity, the relationship between the insulation resistance of the hydrophone and the ambient temperature is fully considered, and the delay between the failure of the hydrophone and the implementation of the replacement is found, and the fault is appropriately increased.