Acoustic Sensor Application Considerations for Metallic Modules

In a series of tests, data was collected on hypervelocity (5 km/s) particles

impacting aluminum plates (Figure 7-8). The plates were type 6061aluminum, 25 cm (10 inches) square, and 1.59 mm (1/16 inch) thick. The 0.8 mm particles penetrated the sample leaving a hole approximately 3 mm in diameter. The large acoustic signal developed at each of these sensors was found to have a voltage level of approximately 0.8 volts p/p. Similar signal voltage levels were found for both normal

(perpendicular) and 45° particle impacts. Smaller 0.3 mm diameter particles did not penetrate, and generated acoustic signals on the order of 0.3 volts p/p. Hence, from this limited data the signal levels (in volts) are approximately the same as the particle diameter (mm). For a 100 micron particle, the initial signal level should then be approximately 100 mV p/p.

The loss factor measured for these samples was about 0.13% for these 25 cm square plates. While much of this loss is due to reflection losses at the plate edges, this loss factor sets a maximum allowable signal reduction with propagation distance. Using this and the lowest speed estimate (for a plate wave in aluminum) of 1350 m/sec, the initial signal decreases to 10% of its initial level in 8.8 m (6.5 msec), 1% in 18 m (13.1 msec), and 0.1% in 27 m (19.6 msec).

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Figure 7-8. Typical signal from impact with aluminum plate (with envelope and exponential fit)

A second factor is signal loss due to spreading. In this test, the sensors were placed approximately 10 cm from the point of impact. Signal amplitude in the plate will decrease approximately as the square root of the distance (due to “cylindrical spreading”). Hence, it will be reduced to 10% of its original magnitude at a distance of 10 meters.

Considering the two sources of signal loss, for a 100 micron particle striking at a distance of 10 meters from the sensor, we expect 10% reduction from spreading and no more than an additional 99% reduction due to damping. Hence, the final signal level will be 0.1 mV p/p. This is approximately the anticipated noise floor of a reasonably quiet facility or platform. (The actual background noise level at the facility used in these tests was 1.3 mV; however, the system has also been used in quieter facilities approaching the limiting system noise level of 0.003 mV.) This corresponds to a detection capability at 10 m range for a 100 micron particle.

The above is for detection on a bare aluminum plate with uniform impedance. When impedance discontinuities are present (i.e. support frames or ribs) a portion of the signal will be reflected and another portion will be lost due to mode conversion into more dissipative waves. Hence, the detection range will be reduced. This topic requires additional study, but an initial suggestion is that each frame support may reduce signal intensity by 20%.

V e r s i o n : A

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An initial set of measurements has been performed on the small scale test

structure, fabricated to include the features of one proposed lunar habitat (Figure 7-9). These measurements support the above conclusions [59].

Figure 7-9. Small scale test structure designed to evaluate signal propagation

The presence of MLI thermal blankets will have two influences. They will add a small additional contribution to the damping (loss factor) and will slow (and possibly break) the incoming particle. Hypervelocity tests with 1 mm particles developed acoustic signals of approximately 0.8 V p/p on the aluminum layers, and of 0.10 V p/p on the Kapton outer cover layer. This suggests a potentially significantly reduced impact energy is available at the underlying aluminum surface. Additionally, while the hole formed in the front Kapton layer was 1 mm diameter, the hole in the final aluminum foil layer was 3 mm diameter. Either significant mass of the MLI material is breaking off and also impacting the underlying surface, or (less likely) the particle is breaking up. Unfortunately, the signals formed on aluminum plates when covered with thermal blankets has not yet been measured.

The above suggests a rough design formula for the sensor layout. For impact detection we need sensors separated by no more that five frame supports or no more than 10 meters distance (whichever is less). For localization, we need four sensors, preferably separated by no more than three frame supports to reduce complicating reflections. These estimates are tentative and should be refined by additional tests. However, based on the data in hand we can make an estimate. If we consider a cylindrical structure, with a diameter of 8 m and a length of 36 m, placed on its side,

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then this structure would have a top side area of about 400 m2_{. If hull supports are} placed every meter and bulkheads every 4 m, then this structure would require approximately 32 sensors. With 100% sensor system redundancy, the mass of our system would be approximately 2 kg. Its power consumption would be 1 w. The system would be capable of detecting a hypervelocity impact by large (ca. 100 micron) particles and would provide a localized position for the impact. This system could be monitored by a single dedicated microprocessor system, or the data could be fed into the structure's own housekeeping computer for analysis. The detection threshold could be adjustable or it could be fixed depending on the experience gained and the particular application.