Abstract:
A multidrop network of multichannel, addressable sensing modules (ASM&#39;s), to be embedded within a composite structure, remotely powered, and interrogated by a personal computer through a non-contacting inductive link. Each ASM contains a microprocessor with non-volatile memory, multiplexer, programmable gain and filter instrumentation amplifier, and sigma delta analog to digital converter (all housed in two thin surface mount packages). An embedded mothernode includes circuitry for power and data reception (into the structure), and data transmission (back out of the structure).  
     The external interrogation system communicates into the network of ASM&#39;s by modulating the AC waveform that delivers power to the embedded electronics. Once addressed, each ASM powers up its programmable (gain &amp; filter) sensing channels (3 full differential or 5 pseudo differential) and data conversion elements. Sensed data are pulse code modulated, including error checking, which serially modulate an RF carrier for wireless transmission out of the composite to the interrogating computer. These advanced, micro-miniature sensing networks may be applied to a wide variety of military, medical, &amp; civil structures.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    High performance materials such as graphite/epoxy composites allow the production of extremely strong, stiff, yet lightweight structures. However, there is a lack of data on their fatigue and failure modes, since these materials have not yet been in service long enough. This lack of statistical databases has limited their widespread acceptance. Furthermore, structures using these new composites tend to be over designed, which, in turn, may reduce or eliminate the benefits of using these materials 1    
           [0002]    Structural health monitoring would allow the realization of the materials full potential, since structures could be designed to their material limits. Embedded sensors could communicate information such as strain, stress, temperature, and pressure, to allow in service tracking of the materials behavior over time. Changes in behavior would indicate the need for service, repair, or replacement of component(s). This would increase our understanding of the characteristics of advanced materials, as well as improve our safety when these new structures are used in critical applications.  
           [0003]    Recent efforts to produce structural monitoring systems have focused on integration of the sensors into the new materials 2 , 3 . Embedded fiber optic sensors, capable of stress/strain measurements over large areas, have been developed, but the problem of connecting these fiber networks to an interrogation system still needs to be addressed. Machining and trimming of composite parts after initial fabrication can damage or destroy direct, contacting type connections to embedded sensors.  
           [0004]    Prior patented art in the medical field includes that of Hogrefe et al. (U.S. Pat. No. 4,561,443; issued Dec. 31, 1985) which describes a coherent inductive communications link for biomedical applications. This two way inductive link provided communications between an external transceiver and an internal transceiver located in a biologically implantable medical device. Digitally formatted command data and programming data was transmitted to the implanted medical device by frequency shift keying in inductive communications link. The internal tranceiver was powered by the inductive field between the internal and external transceivers. Digitally formated data was transmitted to the external transceiver by internal transceiver amplitude modulating field. This patent describes means for communicating with an implanted (or embedded) device, but it does not describe means for conditioning or utilizing sensors or addressing sensor networks.  
           [0005]    The development of improved implantable devices and materials requires knowledge of their in vivo behavior. However, little is known about the actual loads borne by implanted devices in vivo. Direct load measurement would provide extremely valuable information, for the improvement of device designs, and for the rapid rehabilitation of individuals in which devices have been implanted. Multichannel telemetry systems, combined with strain gauges, can provide this information.  
           [0006]    Previous authors have developed and applied implantable telemetry for strain gauged implants 4,5 . However, these systems did not possess the capability of software  
           [0007]    [0007] 3 SPIE Vol. 1798,  Proceedins of the Fiber Optic Smart Structures and Skins V Conference,  1992  
           [0008]    [0008] 4 Bergmann et al.  “Multichannel Strain Gauge Telemetry for Orthopedic Implants.”  J. Biomech., Vol 21, No. 2, Pages 169-176, 1988  
           [0009]    [0009] 5 rohlmann et al.,  “Telemeterized Load Measurement using Instrumented Spinal Internal Fixators in a Patient with Degenerative Instability”,  Spine, Vol 20, No 24, pages 2683-2689, 1995 programmability. By combining advanced, micropower, analog/digital integrated circuits (IC) with new, miniature, low power microprocessors, more versatile implantable telemetry systems can be realized 6 .  
           [0010]    Prior patented art in the field of smart structures includes that of Spillman et al. (U.S. Pat. Nos. 5,440,300, 5,581,248, and  5,703,576).  These patents describe a non-contact power and data interface for smart structures, an embeddable device for contactless interrogation of sensors, and an embeddable DC power supply for smart structure sensors. However, they also do not describe techniques for addressing networks of embedded, multichannel sensing nodes; this limits their utility when multiple sensors are required. These patents also do not make reference to the use of an embedded (remote) microprocessor, and do not use digital encoding methods for conversion of sensor signals and for data communications.  
           [0011]    However, the use of a microprocessor provides distinct advantages, as it facilitates digital conversion and communications,and improves data fidelity. Furthermore, an embedded microprocessor can be used to send error checking information (checksum byte(s)) from the remote (or embedded) systems to the external interrogation unit. This allows the external unit to detect errors in (wireless) data transmission, and to “flag” those data that may have been altered by interference.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0012]    Development of non contact power/interrogation systems for use with embedded sensing networks would eliminate direct connections and solve the problems associated with them 7 .  
           [0013]    Smart structures should not be limited to specific sensing means, such as fiber optic type, since other sensing technologies can provide very useful information. Examples of these include: foil and piezo-resistive strain gauges, inductive &amp; capacitive sensors, temperature probes, accelerometers, inclinometers, and magnetometers. Recent advances in transducer signal conditioners have produced very small, fully integrated, linear circuits for use with many of these devices 8,9,10 . Intelligent, addressable sensing modules, with built-in signal conditioning and data transmission capabilities, are needed to provide data in a common (digital) format. These modules can then be used to create generalized sensing networks.  
           [0014]    [0014] 7 Townsend, C. P.; Arms, S. W.:  “Method for Remote Powering and Communication with an Embedded Network of Addressable Sensing Modules”,  SPIE&#39;4th Annual Symposium on Smart Structures and Materials, 3-6March, 1997, San Diego, Calif.  
           [0015]    [0015] 8 Analog Devices,  Monolithic Instrumentation Amplifier,  part no. AD620, Amplifier reference manual, Norwood, Mass., 1992  
           [0016]    [0016] 9 Linear Technologies,  Precision, Micropower, Instrumentation Amplifier,  part no. LT1101, Linear Databook, Milipitas, Calif., 1990  
           [0017]    [0017] 10 Analog Devices,  LVDT Signal Conditioner,  part nos. AD598 &amp; Ad698, Special Linear Reference Manual, section 10-23, Norwood, Mass. 1992  
           [0018]    It is the object of this invention to teach a remote powering and communications method, combined with multichannel, microprocessor based sensing modules, especially for use with advanced sensate medical implants. It is further the object of this invention to describe the combination of embedded microproccessors, highly integrated sensor signal conditioners, digital data converters, and the use of networking techniques, especially for smart structure applications, in order to overcome the limitations of previous non contacting power links and analog data transmission systems.  
           [0019]    It is the object of this invention to teach a multidrop network of multichannel, addressable sensing modules (ASM&#39;s), to be embedded within a composite structure, remotely powered, and interrogated by a personal computer through a non-contacting inductive link. Each ASM contains a microprocessor with non-volatile memory, multiplexer, programmable gain and filter instrumentation amplifier, and sigma delta analog to digital converter (all housed in two thin surface mount packages). An embedded mothernode includes circuitry for power and data reception (into the structure), and data transmission (back out of the structure).  
           [0020]    It is the object of this invention to teach an external interrogation system which communicates into the network of ASM&#39;s by modulating the AC waveform that delivers power to the embedded electronics. Once addressed, each ASM powers up its programmable (gain &amp; filter) sensing channels (3 full differential or 5 pseudo differential) and data conversion elements. Sensed data are pulse code modulated, including error checking, which serially modulate an RF carrier for wireless transmission out of the composite to the interrogating computer.  
           [0021]    These networks will allow smart structures to be designed with a broad variety of capabilities. For example, temperature and strain information would be very useful during composite materials fabrication process. Exposure to cyclic strain, stress, pressure, electromagnetic fields, or radiation could be of interest in service. An open network architecture permits the addition or subtraction of various sensing modules, in order to best meet the requirements of a specific application. This capability can provide low cost structural health monitoring, as well as security, intelligence gathering, and control functions to be implemented as needed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1) Concept for remote Interrogation and Activation of Embedded Sensing Networks  
         [0023]    [0023]FIG. 2) Non-Contact Power/Communication with Embedded Sensors Block Diagram  
         [0024]    [0024]FIG. 3) Remote Power Block Diagram  
         [0025]    [0025]FIG. 4) Single Node ASM  
         [0026]    [0026]FIG. 5) Networked ASM  
         [0027]    [0027]FIG. 6) Software Flowchart for Networked (Mother) Node  
         [0028]    [0028]FIG. 7) Reactive Sensor Node  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    FIGS.  1  is a graphic of the overall system concept and FIG. 2 is the system&#39;s functional block diagram. The ASM block diagrams for multidrop networks and single nodes are provided in FIGS. 4 &amp; 5. The remote (embedded) coil ( 1 ) receives energy by tuned inductive coupling; that is, the external powering system automatically adjusts its excitation frequency to match the resonant frequency of the embedded coil / capacitor network. This is important in order to insure efficient power transfer between embedded modules, as variations in the embedded components values (due to manufacturing tolerances, time, and temperature) may shift the embedded systems&#39; resonant frequency.  
       Remotely Powering the Embedded ASM Networks  
       [0030]    [0030]FIG. 3 provides a schematic diagram of the circuit used to deliver energy from the external (exciter) coil ( 3 ) to the embedded (receive) coil. A power amplifier ( 2 ) capable of delivering sufficient power to external exciter coils is employed to generate a magnetic field (which penetrates the structure under test); this field is received by an embedded coil, which in turn is used to power the embedded electronics within the composite structure.  
         [0031]    For our tests with 10 mm thick fiberglass/graphite/epoxy composite, we found that a minimum of 5 Watts was required from the external power amplifier in order to achieve good performance. This power may vary greatly depending on a variety of factors, including the type of material under test, the spacing between exciter and receiver coils, and the relative alignment of the exciter and receiver coils. The exciter coil used in our tests was 3.5 turns 1 inch diameter of #16 A.W.G. magnet wire. It was driven by a 10 turn secondary winding on the output transformer. The exciter coil was wound on one half of a Ferroxcube pot core of 4C4 material. This ferrite material has a high saturation flux density up through 30 MHz; this material raises the impedance of the exciter coil, and reduces the drive current relative to an air core exciter coil.  
         [0032]    The receiver coil was a printed circuit coil design with 10 turns, 1.062 inch diameter, embedded in polyimide “flex-strip” material. This coil was used along with a bridge of IN914 diodes ( 4 ) and a 2.5 μF ( 5 ) filter cap to form a DC power supply to power the internal electronics. The 10 mm conductive graphite composite material was used as the test sample that the powering system was designed around, as this composite provide the greatest challenge for efficient power transfer. Through this material we were able to deliver 3.2 volts DC to a 100 Ohm load (or 96 mW of power). This allows us maximum of 32 mA of excitation current for the design of the internal system. This exceeded our target specification of 25 mA at 3.0V DC (75 mW) through air. We have demonstrated that we can deliver more power when the material is non-conductive. When the receive coil was spaced 10 mm from the exciter coil and the transmission medium was air, we were able to couple in 180 mW of power to the internal system.  
         [0033]    The rectifier (diode bridge, capacitor, and regulator) convert the AC energy into 3 volts DC, which is used to power up the phase locked loop (PLL), AD7714, PIC16C, and RF SAW oscillator. The PLL (and integrated VCO) is phase locked to the incoming signal, and the PLL error voltage shifts in amplitude in accordance with the frequency shift modulation. The error voltage represents the conversion of incoming FSK signals into a serial data stream for direct communication to the embedded micro controller.  
       Communications Into the Composite Structure  
       [0034]    To minimize component number and system complexity, it is advantageous to use the oscillator of the power supply module in order to communicate command signals from the external interrogation system to the embedded sensor network. In the preferred embodiment, command signals from the interrogating personal computer (PC) ( 7 ) modulate the low frequency power oscillator using frequency shift keying (FSK), to transmit command information through the inductive power/communication link. This modulation is achieved by applying the command data to a voltage controlled oscillator. The output of this voltage controlled oscillator (VCO) ( 8 ) is used to control the input of a power amplifier. The center frequency of the VCO is chosen to match the optimum frequency as determined by testing of the proposed composite structure. The power amplifier then drives the external coil at the required levels to achieve adequate power to the embedded coil.  
         [0035]    The embedded coil is connected to a phase-locked loop (PLL) FSK decoder ( 6 ) (FIG. 2). The FSK decoder recovers the command signals and communicates them over the command data line. Command signals will be a data request from the personal computer to a specifically addressed ASM ( 9 ) in the embedded network. The command signals will be received by each ASM in the embedded network, but only the specifically addressed ASM will respond by sending its data. This method will insure that the network of ASM&#39;s will not interfere with one another, since only one ASM will transmit its data at a time.  
         [0036]    To test the FSK concept, a personal computer was used to control the frequency of the power supply. The frequency of the power supply was modulated in proportion to the output of the computers RS232 port. The frequency deviation of the system was adjustable via a potentiometer. The frequency of the supply was set to 500 kHz with a deviation in frequency of +/−7% for a change in input bit level.  
         [0037]    The demodulation system consisted of a micropower phase locked loop (PLL) and six passive components. The system is designed to lock onto to the center frequency of 500 kHz and track this signal. An error signal proportional to the difference between the center frequency and the output is produced at the output of the phase locked loop circuit. This error output is fed to an voltage comparator that produces an output wave form that is compatible with the microprocessor. The data format transmitted through the link is in eight bit asynchronous serial data format (1 start bit, 8 data bits 1 stop bit).  
         [0038]    FSK data transmission possesses and advantage over ASK in that the modulation of the carrier does not significantly effect the power transfer. ASK will also be more sensitive to unintended changes in the magnitude of the power signal that may occur over the full temperature range, in the presence of electrical noise, or coil misalignment. The FSK method is much more immune to error in all of the above environments, resulting in a more robust data transfer link. The FSK system also offers higher speed transfer capabilities at no cost in power transfer, so if increased sensor data bandwidth is required in the future the FSK system will be capable of this.  
         [0039]    The FSK receiver implementation is extremely small in size and component count. The supply current of the internal PLL based FSK system is only 510 microamps at 500 kHz input frequency. Based on the test results the FSK system was chosen as the communication link for communicating with the internal network. The system was tested for bit error rate at a baud rate of 2400 baud (bits per second ) with the primary coil transmitting to the secondary coil through a 10 mm sample of conductive graphite composite. The bit error rate was determined to be 3.7×10−8 error/bit (0.04 ppm).  
         [0040]    This allows the user to interrogate the micro controller (for information regarding previous tests or stored sensor data), or to reprogram sampling parameters, filter parameters, and analog gain on a channel-by-channel basis. In this way, optimization of data sampling from each channel may be performed to maximize resolution or bandwidth, as required by the application.  
         [0041]    Frequency shift keying (FSK) has been utilized for this application because it offers a robust method of data communication, and there are plenty of off-the-shelf components that support FSK generation and detection. Furthermore, the carrier remains active during modulation, which allows continuous power transfer during interrogation. This may be critical for embedded sensing systems that have several active integrated circuits.  
       ASM (Both Single Node &amp; Networked) Architecture  
       [0042]    ASM&#39;s are capable of being remotely addressed by the personal computer through the FSK communications scheme described earlier. Once activated, the addressed ASM will power the sensing circuitry, read the A/D converter, and send pulse code modulated (PCM) digital data out. It is important to note that single node (non-networked) ASM&#39;s may also be employed. The single node ASM&#39;s are similiar in topology to the networked versions, however, they do not require an RS485 line driver. A block diagram of a wireless, single node ASM is provided in FIG. 4 ( 16 ).  
         [0043]    The main circuit was designed to provide high resolution measurement from any sensor that can be incorporated into a DC Wheatstone full bridge or half bridge configuration. At the heart of the ASM is a tiny,  20  pin, small shrink outline package (SSOP), micropower, 8 bit microcontroller ( 9 ). This microcontroller provides a high performance RISC based system in a microminiature low power package. The microcontroller serves to read command data from the network interface, program and acquire data from the analog to digital converter ( 10 ), provide multiplexed power to the sensor bridges, and transmit data onto the network bus. The microcontroller interfaces to the network bus using a micropower half duplex CMOS RS485 line driver ( 12 ). A block diagram of a wireless, networked node ASM is provided in FIG. 4 ( 17 ).  
         [0044]    A flowchart of the software architecture used to interrogate ASM nodes is provided in FIG. 8. To begin, the microcontroller reads data from the command data line, if this data addresses this particular ASM, it will switch power to the strain gauge bridge, signal conditioner, and A/D converter. Switching power is accomplished by utilization of one of the microcontroller&#39;s digital output lines. These lines are capable of driving up to 25 milliamps of DC current, and may be programmed in order to switch the 3 volt supply to the strain gauge bridge, signal conditioner, and A/D converter.  
         [0045]    This is advantageous, since the individual bridge currents can represent a significant current draw, especially when powered up simultaneously. By multiplexing power to the sensors synchronously with the analog to digital conversion, we deliver power to only one bridge at a time. For five Wheatstone bridge circuits comprised of 1000 ohm strain gauges, we reduces the bridge current demand from 15 milliamps to 3 milliamps. Excitation may also be selectively applied to only the gauge (or ASM node) that the PC (through the mother node) requests data from. This selective gage excitation greatly reduces the total supply of the system.  
         [0046]    A single chip (AD7714, Analog Devices, Norwood, Mass.) ( 10 ) is used to interface the analog outputs of the sensing devices to the microcontroller. The highly integrated AD7714 represents a single chip solution to strain gauge signal conditioning in an off-the-shelf, thin-shrink surface mount package. It contains a multiplexer, precision programmable-gain instrumentation amplifier, programmable digital filter, 24 bit analog to digital (AID) converter, and direct interface to the embedded micro controller. The AD7714 is capable of conditioning three full differential bridge inputs, or 5 pseudo-differential inputs. The large dynamic range afforded by the 24 bit A/D converter allows the use of bridge circuits with appreciable offsets, without requiring trimming networks or potentiometers. The architecture of the AD7714 also features the capability to remove offsets due to bridge unbalances, without sacrificing dynamic range of the A/D converter. This is accomplished through a one time calibration procedure, which we have implemented. This approach represents a significant reduction in size, complexity and cost.  
         [0047]    After allowing sufficient time for the analog circuitry to settle, the microcontroller will then perform an A/D conversion of the signal conditioners output. The A/D converter will be referenced to the 3 volt supply line, which is also used to power the strain gauge Wheatstone bridge. This technique provides ratiometric output from the A/D converter, which greatly reduces bridge measurement errors due to drift in the DC supply voltage.  
         [0048]    After reading the A/D converter, power is shut down to the bridge, conditioner, and AID converter. The microcontroller transmits the reading, along with a synch byte and checksum byte for error checking. These data bytes are sent through a tri-state line driver, in low impedance mode, on to the ASM data line. ASM&#39;s that have not been addressed are isolated from the ASM data line, because their line drivers have been placed in high impedance mode.  
         [0049]    In the event an error is detected, the personal computer will re-address the ASM in order to try to obtain error free data; however, in the event of more than three unsuccessful attempts, the interrogation system will move on to request data from other ASM&#39;s on the network. Once the ASM&#39;s microcontroller has completed data transmission, the microcontroller then puts its line driver into high impedance mode. This allows other ASM&#39;s to communicate over the ASM data line without loading their line drivers, or damaging other ASM&#39;s. This hardware and software architecture facilitates the addition of ASM&#39;s into the network, which may be accomplished by simply tying each additional ASM to the network&#39;s four lines.  
       Data communication from the remote network  
       [0050]    Digital data is sent to the PIC16C, which then modulates by (ASK) the high frequency RF surface acoustic wave (SAW) oscillator for transmission out of the composite material. The SAW oscillator typically operates in the (narrow) band of 303.825 MHz, and is precisely matched to its companion receiver in its oscillation frequency. This insures “lock” of the received signal, since drift of the SAW oscillator transmitter/receiver pair over time and temperature is negligible. This also makes initial manufacture of the devices much simpler as no initial adjustments (tuning) of the transmitter or receiver are required. Data is directly modulated onto the carrier from the microcontroller using simple amplitude modulation of the RF signal.  
         [0051]    The external receiver ( 13 ) utilizes a matched SAW resonator and a RS- 232  level shifter to convert the digital data from the receiver (0 to 3 volts) into levels that are compatible with the personal computers (+/−9V) serial communications port.  
         [0052]    Recent advances in wireless communications devices allows these systems to achieve high data transfer rates (9600 to 19,200 baud). We are have found data reception to be extremely reliable over short distances (15 ft). The use of PCM permits transmission of a checksum byte at the end of each data set (or full channel sweep); at the receiver, this checksum is compared to the actual sum of the received bits in order to detect RF transmission errors.  
         [0053]    The system was tested by transmitting known digital data along with a checksum byte, the received data was collected by the computer, checked for an error by computing the checksum and saving this data to the disk drive of an interrogating personal computer. To be representative of a typical composite materials application, the system was tested by transmitting data through a 10 mm carbon composite sample to the receiver. The duration of the test was 48 hours running continuously. The bit error rate was determined to be only 10×10−6. Errors due to RF transmission are detected by the error checking routines so errors do not propagate into data collection.  
       Compatibility with Various Sensing Devices  
       [0054]    The system has been tested successfully with Wheatstone bridge sensors, including accelerometers, foil strain gauges, and strain gauge based force transducers. In addition, we have successfully tested ASM nodes with an inductive displacement transducer. The inductive displacement transducer requires an AC excitation waveform as oppose to the DC which is required by strain gage bridges (FIG. 7). The AC waveform is rectified using a simple peak detector and the output of the peak detector is fed to the analog to digital converter ( 15 ).  
         [0055]    In the preferred ASM embodiment, the bridge signal conditioner and analog to digital converter are located adjacent to the sensing elements (strain gauges). Use of flexible, polyimide circuitry allows the sensing elements and ASM electronics to be fully integrated, since strain gauges and temperature sensors may be etched on to the polyimide during manufacture. Pads on the flexible circuitry would also allow the use of semiconductor strain gauges (or other DC bridge) devices. The close proximity of the sensing elements, signal conditioner, and A/D converter is advantageous, since this construction provides very low noise data for transmission onto the network.  
         [0056]    Polyimide backed etched metal foil strain gauges are widely used for stress analysis and structural model verification. This basic construction is also used to create commercially available temperature sensors and crack propagation detectors. Their extremely thin profile, versatility, and low cost make them ideal for use in critical composite strucures.  
         [0057]    ASM&#39;s can measure the output of up to five Wheatstone bridge circuits. Each module is compatible with a wide variety of (DC) bridge type sensors, including: strain gauge rosettes, piezoresistive strain gauges, foil temperature sensors (thermocouples and resistance temperature detectors), magnetic field sensors, accelerometers, and pressure transducers.  
         [0058]    This capability is generally useful, since it may be extended to provide the measurement of many physical quantities, such as: strain direction/magnitude, crack propagation, inclination/orientation, magnetic field intensity/direction, and temperature gradients. We have also demonstrated that sensors employing inductive or capacitive technologies may also be used with our ASM&#39;s (with modifications to the front-end active and passive components) to perform AC excitation/demodulation.  
         [0059]    Accelerometers are widely used for structural testing and monitoring; they may be used to measure both acceleration and inclination (with respect to Earth&#39;s gravity vector). ASM&#39;s with on board frequency and amplitude detection capability have also been designed, FIG. 8 is a representative schematic block diagram of an enhanced (accelerometer type) ASM node. We note that this node may also be used in conjunction with sensors that are suited to measurement by frequency counting, such as: vibrating wire strain gauges, and resonant (capacitive and inductive tank oscillator) sensors.  
         [0060]    Vibration monitoring nodes could perform the function of a miniature (and very elegant) frequency analyzer. By utilization of many of these nodes, placed strategically on or within a structure, engineers will gain valuable insight into the structure&#39;s vibratory response. This information is important for mathematical modeling of the structure&#39;s inherent stiffness.  
         [0061]    Differential capacitive transducers possess several advantages over foil type resistance and piezoresistive sensors; including: capability of micro power operation, and ability to withstand high temperature environments 11 . Inductive transducers possess advantages of excellent robustness, moisture resistance, non-contact operation, capability of measurement of large deformations, crack propagation, and gap displacements.  
         [0062]    The use of strain gauge rosettes is widespread in the aeronautical, naval, and aerospace industries as a means of mathematical model verification and structural testing. Stress analysis applications typically require the measurement of principal strain magnitude and direction 12 . This is usually accomplished by application of a strain gauge rosette 13  to the structure under test. The ASM module described in this invention can accept up to three true differential or 5 pseudo differential inputs (as it has an on-board multiplexer), therefore it can be interfaced with an off-the-shelf strain rosette.  
       Embedding in Composite Structures  
       [0063]    By placing these components on a flexible polyimide substrate, ASM&#39;s may be directly bonded to the surface of a composite structure&#39;s main load bearing component. The material&#39;s final protective overcoat may be used to embed the ASM network, internal power/communications module, and inductive link within the composite structure.  
         [0064]    The small size of our embedded nodes facilitates their application within thick composite structures or in very tight, space critical applications. By changing the relative locations of IC&#39;s on the thin (polyimide or laminate) substrate, ASM&#39;s may be adapted to the physical limitations posed by an application, and can be as thin as .080″, which can be embedded in many composite materials.  
         [0065]    Structures with a broad variety of capabilities may be developed by embedding sensing networks of strain, crack propagation, acceleration, pressure, magnetic fields, etc. Applications include health monitoring of thick composite structures, bridges, dams, and buildings.