Piezoelectric Sensors in the Cold Realms of Space
High powered plasma-based weapons’ fire impacts a future human space vessel, causing stereotypical sparks to fly off every computer terminal on the bridge. ‘Hull integrity is failing, Captain!’ exclaims some poor low-level officer on the star ship. How does he know? What does he even mean? The answer is not in the far-flung fantasy of a science fiction movie. The technology that would allow that poor ensign to make his dire proclamation is here today in the form of structural health monitoring. More specifically, that doomed craft was likely using piezoelectric sensors to look for cracks within the structure. Why?
After asking why, the second question to ask those future designers of space ships would be, ‘What technology are you using to monitor hull integrity?’ They would answer, ‘Piezoelectric sensors, of course.’ That broaches another question: what are piezoelectric sensors? To answer that, we need to examine piezoelectric materials. A piezoelectric is a material that is electromechanically coupled. That means that if the material is deformed, it will produce a voltage, and if a voltage is applied, the material will deform. To give an example, a quartz crystal will give off a measureable voltage when a force is applied that bends it. The converse is also true; if a voltage is applied to the quartz crystal, it will change shape. Quartz is not the only piezoelectric material. Many crystals display the piezoelectric effect. To integrate the piezoelectric effect into sensors, undeformed piezoelectric sensors are mounted onto a structure. Vibrations are sent through the structure, and the sensors produce voltages based on their deformations due to the vibrations. The undamaged structure’s response becomes the baseline. If a crack forms or a bolt loosens, the outputs from the sensors will change because the vibrations through the structure change. Upon comparison with the baseline, damage to the structure becomes obvious because of a shift in the output voltages.
Informed readers (especially of the engineering ilk) may skeptically proclaim, ‘Why put all the effort into a system to drive vibrations throughout an entire structure when we can use foil strain gauges? They would be much simpler and less expensive.’ (For those readers who are not full time engineers, a foil strain gauge is a flat wire that can be mounted to the outside of a structure. As the structure deforms, the strain gauge stretches, causing the resistance of the gauge and the voltage drop across it to increase. That voltage difference is used to measure the deformation at each strain gauge’s location.) The answer is that foil strain gauges cannot detect cracks or other flaws that would cause catastrophic failure. They require a way to relate the loading, stress, and strain at each strain gauge location. That becomes tedious, even for a computer. Piezoelectric sensors do not suffer from the same setback. A comparison with the baseline is enough to predict damage and failure.
Most piezoelectric sensor testing has taken place on earth outside of the space environment (the space environment is the low pressure, low temperature, high radiation environment common in space). Common applications of piezoelectric sensors are in commercial aircraft and bridges. That is important because some research shows that piezoelectric sensors are permanently changed by the space environment. That would be a real problem for future astronauts trying to determine if the thin shell protecting them is going to rupture. This permanent change is likely caused by the extreme temperature range and radiation in space. Insulation may prevent such permanent changes. Something like Mylar film, which would help insulate the sensors from some electromagnetic radiation, can be placed on the sensors to protect them from the hostile surroundings of space. Another option that could work for manned missions is to put the sensors on the inside of the ship. The sensors would experience earth-like conditions (except for the lack of gravity).
- Sandia National Laboratories, Sensors may monitor aircraft for defects, 2007: Accessed 28 October 2013.