Patent Application
20090039864
The present invention relates to sensors, and more specifically, to the wireless detection and monitoring of corrosion.
Corrosion is a serious concern for such surfaces as aerospace structures. Despite the tendency toward composite vehicles, there are still many metal-to-metal and metal-to-composite interfaces in legacy and next-generation air transportation vehicles. Large air transport customers have expressed a need for a simple, light weight, inexpensive corrosion sensors that can detect corrosion at these interfaces. Their objective is to detect corrosion with a low cost inspection protocol that can detect corrosion damage before the damage becomes extensive. Often they find damage when the cost of mitigation is expensive. This expense could have been averted if the damage had been detected sooner.
Unfortunately the corrosion can take place in hard to reach places making traditional manual nondestructive inspections time consuming and difficult. An alternative is to design a sensor that can be installed during, or after vehicle fabrication, that could be used to monitor the interface condition at the location of fastener, between interface regions of structural layers and other corrosion initiation sites. Battery-less, wireless operation would make any potential sensor very attractive for use in difficult to access locations.
In one embodiment, and by way of example only, a wireless corrosion detector includes a surface acoustic wave (SAW) sensor tag consisting primarily of a SAW filter, an antenna connected to the input port and at least one electro-resistive (ER) sensor coupled to the output port.
In another embodiment, again by way of example only, a wireless corrosion system includes a processor. A transceiver device is coupled to the processor. A corrosion detector includes at least one electro-resistive (ER) sensor coupled to an output port of a surface acoustic wave (SAW) sensor tag. The SAW sensor tag is interrogated by a radio frequency (RF) signal generated by the transceiver. The SAW sensor tag rebroadcasts a response RF signal incorporating amplitude variation and delay. The response RF signal is received by the transceiver and interpreted by the processor as corrosive activity.
In an additional embodiment, again by way of example only, a method of detecting corrosion includes fixing a corrosion detector to a surface subject to corrosion. The corrosion detector includes at least one electro-resistive (ER) sensor coupled to an output port of a surface acoustic wave (SAW) sensor tag. The corrosion detector is interrogated with a radio frequency signal (RF). A response RF signal incorporating amplitude variation and delay is received. The amplitude variation and delay incorporated in the response RF signal is interpreted as a measure of the corrosion activity that has taken place and been witnessed by the sensor in the vicinity of the sensor.
The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.
The claimed subject matter exploits two emerging technologies, which when combined provide an inexpensive, battery-less and wireless sensor for detecting the presence of corrosion in difficult to reach places. Electro-resistive (ER) sensors may be used for corrosion monitoring in powered and wired configurations. Such ER sensors can include smart washers for helicopter gearbox fasteners and atmosphere-exposed ER sensors that witness the environmental loading experienced by a vehicle. A surface acoustic wave (SAW)-based sensor tag consists of a SAW filter with an antenna at input port and an impedance varying sensor at its output port. Variations in the filter's acoustic impedance lead to variations in reflectance for a received interrogating RF wave.
In an implementation where ER sensors are combined with such a SAW sensor tag, the reflectance of the acoustic wave by the SAW filter and the re-emitted RF signal is in proportion to the impedance of the respective ER sensor tied to SAW filter output port. By combining such ER sensors with SAW tags, the impedance of the corrosion monitoring sensor may be measured without battery power or wired connections. Appropriate packaging may allow this sensor solution to monitor corrosion at layered interfaces, fasteners or as environmental witnesses.
Such an implementation may be made by combining one or several electro-resistive sensors at the output port of a SAW based sensor tag. A variety of form factors can be envisioned for the electro-resistive (ER) sensor. To monitor corrosion at the interface between multiple metal layers or a metal-composite interface the electro-resistive sensor could be fabricated from, or connected to, the SAW ‘transceiver’ tag using a multilayer flex circuit design. Such a thin ‘smart washer’ could be installed during assembly and buried between layers of a structure subject to corrosion, with the SAW sensor tag emerging at the faying edge of a lap splice or similar joining structure.
A single SAW sensor tag could be wired to a set of ER sensors in various combinations of parallel and series topologies. Each SAW sensor tag connected to multiple ER sensors would be capable of monitoring a region of the structure for corrosion. The degree of corrosion detection location specificity could be adjusted by varying the number of ER sensors served by a SAW tag or by using a more sophisticated SAW tag design that is capable of differentiating between output port sensors based on added communication complexities.
After installation, the presence of corrosion is sensed through changes in resistance of the ER sensor in contact with an uncorroded, or relatively uncorroded, portion of the structure being monitored. The ER sensor's electrical resistance changes due to the presence of corrosion products or due to the breakdown in a sacrificial protective coating due to corrosion activity.
The SAW filter is a bandpass, frequency selective device that converts electrical energy into acoustic energy. When the SAW filter is interrogated by an appropriate RF signal the RF energy is converted into an elastic wave that travels down the SAW filter experiencing a transit time delay. When the converted ultrasonic signal reaches the end of the SAW delay line it is reflected by the output port load and it travels back up the delay line to be re-transmitted by the antenna connected to the input port of the SAW tag. Variations in the ER sensor impedance can be seen as amplitude variations and delay in the rebroadcast RF signals. Reference RF reflection using dummy loads within the same SAW-ER sensor package could be used to calibrate the amplitude response removing common shared sources for signal amplitude variation.
Turning to
Again, as previously described, as the elastic wave reaches the end of the output IDT 26, it is reflected by the output port load (including the sensor 34), and travels back along the SAW filter 12 to the antenna 14 where it is re-transmitted. Variations in the ER sensor 34 impedance can be seen as amplitude variations and delay in the rebroadcast RF signals.
Turning to
Variations in the amplitude and delay can be interpreted by the controller/processor as evidence that corrosion has taken place in the vicinity of the sensor. Again, for example, reference RF reflections using dummy loads within the same SAW-ER sensor package but experiencing different time delays that the ER sensor responses, could be used to calibrate the amplitude response removing common shared sources for signal amplitude variation.
A set of ER corrosion sensors 34 can be coupled in series (not shown), and then coupled in parallel with a SAW sensor 12. In this scenario, ER sensors 34 can be selectively placed in a desired region to be monitored. In addition, various ER sensors 34 can be placed in various layers of material, where a flex circuit element (not shown) connects the ER sensors 34 to each other, or to the SAW sensor 12. As one skilled in the art will appreciate, a number of configurations of ER sensors 34, in several form factors, can be implemented to suit a particular application.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention.
