Current in-service monitoring techniques utilize a dense web of analog sensors connected by individual wires routed to centralized data acquisition and processing units. This traditional approach has a significant weight penalty, can be complex to install and is susceptible to EMI. To resolve these issues, MDC has developed a fully digital SHM solution. The MD7 system is composed of 3 core elements: the IntelliConnector™ miniature node for distributed data acquisition, the VectorLocator™ sensor assembly for guided-wave phased-arrays, and the HubTouch™ data accumulator for remote diagnostic processing. Each element of the MD7 system is networked on a 6-wire serial bus that carries the differential communication, synchronization and power signals, typically using flat-flexible-cable (FFC). Benefits of this distributed infrastructure approach include higher fidelity data through digitizing sensor signals at the point of measurement, reduced computational burden through local signal processing and feature reduction, and overall minimal mass through the elimination of cables, connectors and bulky off-the-shelf hardware.
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Structural Health Monitoring
MD7 Digital SHM System
Present in-service monitoring techniques utilize a dense web of analog sensors connected by individual wires routed to centralized data acquisition and processing units. This traditional approach can be heavy, complex to install and is prone to EMI. To resolve these issues, MDC has developed a fully digital SHM solution. The MD7 system is composed of 3 core elements: the IntelliConnector™ miniature node for distributed data acquisition, the VectorLocator™ sensor assembly for guided-wave phased-arrays, and the HubTouc™ data accumulator for remote diagnostic processing. Each element of the MD7 system is networked on a 6-wire serial bus that carries the differential communication, synchronization and power signals, typically using flat-flexible-cable (FFC). Benefits of this distributed infrastructure approach include higher fidelity data through digitizing sensor signals at the point of measurement, reduced computational burden through local signal processing and feature reduction, and overall minimal mass through the elimination of cables, connectors and bulky off-the-shelf hardware.
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IntelliConnector™
At the core of the MD7 System is the IntelliConnector™, a direct replacement for traditional instrumentation such as oscilloscopes and function generators. Used for distributed data acquisition and signal processing, these “pucks” weigh a mere 12g and are 40 mm in diameter by 6 mm thick. They are potted in urethane to provide resistance to moisture, chemicals, flame and shock loading. Each node provides 6 independent channels of up to 50 MSamples/sec in addition to 8 channels that share up to 3 MSamples/sec, with programmable gain up to 250x. The arbitrary function generator provides a 20 MSample/sec update rate up to 20 Vpp. There is 1 Gbit of internal data buffer and 16 Mbit on-board static storage. More than 100 nodes can be daisy-chained on a serial CAN network using 22-32 VDC. The IntelliConnector™ is capable of synchronously facilitating guided wave (GW), frequency response (FR) and acoustic emission (AE) testing.
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VectorLocator™
Traditional SHM methods require dense sensor meshes to precisely resolve the position of damage events, however this drives system weight and complexity. Furthermore, most prevalent SHM methods rely on wave velocity to triangulate location; however in composite structures velocity is often a function of laminate orientation, and features such as doublers and ply dry-offs can significantly change velocity, adversely affecting accuracy. In response to these dilemmas, MDC has developed the VectorLocator™ method to indicate damage event coordinates using a novel sensor design along with an innovative algorithm that excludes wave velocity information. From a single node location, a ray is generated indicating the direction from which a damage scatter response wave was sensed. A two node system can be used to produce a unique ray intersection specifying the scatter source position. This method can be implemented passively to locate an acoustic event, or actively with guided waves to seek out damage.
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HubTouch™
The system-level element of the MD7 System is the HubTouch™ diagnostic data accumulator. This device serves three main purposes. Its most basic function is to facilitate SHM by driving the CAN bus, conveying testing instructions and synchronizing all of the IntelliConnector™ nodes. Secondly the HubTouch™ collects and stores all of the resulting digitized and pre-processed data transmitted from each of the nodes. Finally it fuses the individual sensor responses into an amalgamated diagnostic picture of the structure being monitored. The HubTouch™ provides a plug-and-play touch-screen LCD interface for programming and executing SHM profiles, an SD-card slot for storing data, and a powerful FPGA platform for deploying embedded diagnostic algorithms. This 100 x 75 x 6 mm hub can be used as a hand-held device with a battery pack, mounted to the structure being monitored, or used as a directly interfaced slot card within a HUMS system.
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PZT-based SHM
The majority of MDC’s extensive SHM experience has made use of piezoelectric material. These elements can be used as sensors by measuring voltage differences across parallel electrodes when cyclically strained, or converselythey can be used as actuators by inducing expansion and contraction with an applied alternating electric field. Materials with piezoelectric properties are particularly attractive for SHM applications due to their high-frequency response and overall wide-bandwidth characteristics. Most research at MDC has indicated piezoceramic elements, specifically PZT (lead zirconatetitanate), to be the most suitable for practical SHM efforts since these wafers have balanced actuator and sensor constants, they are accessible, have well vetted properties and reasonable thermal stability. MDC’s assembly service strives to provide customers with robust PZT packages for repeatable testing using proven techniques to eliminate electrical interference, cross-talk, signal attenuation, and non-uniformities caused by typical fabrication and installation practices such as soldering wires or dilled-hole electrodes.
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CNT-based SHM
MDC has partnered with the Technology Laboratory for Advanced Materials and Structures (TELAMS) in the Department of Aeronautics and Astronautics at the Massachusetts Institute of Technology (MIT) to develop the next generation of advanced SHM technologies through the use of embedded carbon nanotubes (CNTs) to enable multi-physics, multi-functional capabilities within composite laminates. Several studies have shown that CNTs possess exceptional mechanical stiffness (as high as ~1 TPa) and strength, as well as excellent electrical conductivity (~1000x copper) and piezoresistivity (resistivity change with mechanical strain). Thus, they can be used to not only to reinforce composite structures to improve impact and delamination resistance, but also to enable novel SHM and NDE techniques. Vertically or horizontally aligned CNT forests can be transferred to composite pre-preg at room temperature through a “nanostitch” process. Radially aligned CNT can be grown in-situ on dry fiber tows or fabric to create “fuzzy-fiber” reinforced polymers (FFRP).
