Abstract:
A stress monitoring device of elasto-magneto-electric (EME) effect type, for monitoring stress of a structural component of ferromagnetic materials, includes a magnetic field generating unit, a magneto-electric (ME) sensing unit, a support skeleton, and a signal controlling and conditioning instrument. Under the control of the signal controlling and conditioning instrument, the magnetic field generating unit generates a magnetic field for magnetizing the structural component. The ME sensing unit outputs an electrical signal V ME  characterizing the magnetic field without a need of external power supply. This electrical signal V ME  is analyzed and processed by the signal controlling and conditioning instrument to output a magnetic characteristic value V st  corresponding to the changes of the external forces that are exerted on the component. This stress monitoring device realizes on-line, real-time, and nondestructive monitoring, as well as off-line nondestructive monitoring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of International Patent Application No. PCT/CN2012/085367 with an international filing date of Nov. 27, 2012, designating the United States, now pending, and further claims priority benefits to Chinese Patent Application No. 201110389375.8 filed Nov. 30, 2011. The contents of all of the aforementioned applications, including any intervening amendments thereto, are incorporated herein by reference. Inquiries from the public to applicants or assignees concerning this document or the related applications should be directed to: Matthias Scholl P. C., Attn.: Dr. Matthias Scholl Esq., 14781 Memorial Drive, Suite 1319, Houston, Tex. 77079. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a stress monitoring device of elasto-magneto-electric (EME) effect type, and specifically to a non-destructive stress monitoring device for structural components of ferromagnetic materials. 
     Background of the Invention 
     Several stress monitoring instruments are presently available, such as the pressure sensor, the resistance strain sensor, the vibrating wire strain sensor, the optical fiber grating strain sensor, the piezoelectric acceleration sensor for measuring vibration frequency, and the Elasto-Magnetic cable force sensor based on elasto-magnetic effect. Measuring cable force by applying the pressure sensor is realized by using the precise pressure gauge or hydraulic sensor to measure the hydraulic pressure of the oil cylinder when the cable is stretched by the lifting jack. However, due to the characteristics of the pressure gauge, including the instability in the indicator reading, the necessity to convert the reading to the load value, and large influence by human factor, the pressure sensor is unavailable in the dynamic stress monitoring for the in-service structures. Measuring cable force by applying the resistance strain sensor is based on the principle that the wire resistance varies as the length of the wire changes. Measuring cable force by applying the vibrating wire strain sensor is based on the principle that the vibration frequency of the tensioned metallic string varies as the force resulted from the relative displacement of fixed ends of the string changes. Measuring cable force by applying the optical fiber grating strain sensor is based on the principle that the wavelength of light-wave passing through the optical grating varies with deformation of the optical fiber. In the above mentioned three methods for measuring cable force, the three strain sensors must be sticked on the surface of the structural component, or be welded to the surface of the structural component through the supporting device, or embedded into deforming body. Therefore, when using the above mentioned three methods, the installation is inconvenient and the measurement results are susceptible to influences of external factors. For in-service structures, the resistance strain sensor, vibrating wire strain sensor, or optical fiber grating strain sensor can only measure the changes of the strain/stress (namely, the increment) relative to initial strain/stress after installation or zero setting, but cannot measure the actual absolute value of the strain/stress. Monitoring cable force by measuring vibration frequency uses the quantitative relationship between the cable force and the vibration frequency to convert the vibration frequency tested by the acceleration sensor to the cable force. Because it is simple, cost-saving, and available in online monitoring in-service structures, the method of monitoring cable force by measuring vibration frequency is widely applied in practical engineering. However, there are disadvantages for the method of monitoring cable force by measuring vibration frequency: (1) the relationship between the cable force and the vibration frequency is affected by flexural rigidity, slope, sag, boundary conditions, and the vibration reducing and damping device of the cable, which causes errors in the conversion of the cable force; (2) the converted cable force is static value or averaged value, correspondingly, it is impossible to obtain the variations of the cable force in periodic vibration; and (3) the method is not suitable for monitoring stress of the structural components other than cable. 
     Measuring cable force by applying the Elasto-Magnetic force sensor based on the Elasto-Magnetic effect implements the principle that the magnetization characteristic changes when the ferromagnetic component placed in the magnetic field suffers the stress, and derives the cable force of calibrated ferromagnetic components. Since the method of using Elasto-Magnetic sensor to monitor cable force has advantages in monitoring the actual stress of the in-service structures and realizing non-destructive monitoring, the method overcomes the disadvantages of the other above mentioned methods and thus is a promising method in monitoring stress of the in-service steel structures. At present, there are mainly two types of such Elasto-Magnetic force sensor, namely, the sleeve-type sensor and the bypass-type sensor. On one hand, when used for monitoring stress of the in-service structures, the sleeve-type sensor needs in-situ winding coils, which leads to inconvenient time-consuming operation and heavy workload. Furthermore, because it is hard to control the quality of the coils of the sleeve-type sensor, the accuracy in measurement is low. On the other hand, the utilization of the bypass-type sensor is still in the stage of exploration and has not been promoted to engineering application because of the shortcomings due to the conduction yoke, including large size, heavy weight, and high production cost. No matter the sleeve-type or the bypass-type, the existent Elasto-Magnetic sensors use the secondary coil as the signal detecting element, which results in long measurement cycle (at least 10 seconds for each measurement). Therefore, the existent Elasto-Magnetic sensors cannot realize real-time monitoring and cannot monitor stress variations of the structure in the vibration process (under the action of seismic or/and strong winds). In addition, when using the existent Elasto-Magnetic sensors, it is demanded that the drive coil is large or the magnetic current is high in order to produce strong enough magnetic field, or that the turns of the secondary coil in a winding of a certain length are increased so that the secondary coil could generate sensitive-enough signals to increase the signal-to-noise ratio of the signals for monitoring stress. Furthermore, since the secondary coil is usually wound around the cylindrical support skeletons, the existent Elasto-Magnetic sensors merely detects magnetic field inside the coil, and thus the measured force is the average force of the structural component inside the coil. Hence, the current Elasto-Magnetic sensor could merely measure the uniaxial loads exerted on the components, mainly the cable force, which limits the application of the current Elasto-Magnetic sensor in the components of non-cylindrical cross-section or under complicated loadings. 
     As illustrated in  FIG. 1 , the conventional Elasto-Magnetic stress monitoring device generally comprises an excitation coil, a secondary coil, a support skeleton, a drive circuit, an integrator, a data acquisition and processing module, and a controlling instrument (such as a computer). The support skeleton is installed around the monitored structural component, and the excitation coil and the secondary coil are wound on the support skeleton. When the excitation coil is charged with electricity, a magnetic field is generated to magnetize the structural component to a nearly saturated state. Then, during the demagnetization phase, the magnetic flux passing through the secondary coil is changed and thus the secondary coil output an induced signal. And then, the induced signal is integrated to obtain a detectable electric signal. After data analysis and processing for the detectable electric signal, a characteristic value related to the permeability of the monitored structural component is achieved. Because the permeability of the monitored structural component is related to the stress state, the characteristic value can be converted to the stress using the beforehand calibrated data in the laboratory or on the site where the structural component is installed. 
     Several references in the art recite the Elasto-Magnetic stress monitoring device of  FIG. 1 , including CN 201242481Y, CN 101334325A, CN 101051226A, CN 101013056A, CN 24276011Y. 
     SUMMARY OF THE INVENTION 
     In view of the above described problems, it is one objective of the invention to provide a stress monitoring device having high frequency of sampling to achieve real-time on-line dynamic monitoring and being applicable in monitoring total stress of steel components or other ferromagnetic components in different stress states or of various cross-sectional shapes. 
     To achieve the above objective, the present invention provides a stress monitoring device of elasto-magneto-electric (EME) effect type. 
     According to one aspect of the present invention, there is provided a stress monitoring device of EME effect type for monitoring stress of a structural component of ferromagnetic material, comprising: a magnetic field generating unit, one or more magneto-electric (ME) sensing units, one or more support skeletons, and a signal controlling and conditioning instrument. The magnetic field generating unit is controlled by the signal controlling and conditioning instrument to generate magnetic field in the detected area around the structural component to magnetize the structural component. The ME sensing unit is made of ME laminated composite. Furthermore, the ME sensing unit directly produces an electrical signal V ME  to characterize the magnetic field intensity and magnetic induction intensity without external power supply and signal integration. The support skeleton is used to set up the magnetic field generating unit and fix the ME sensing unit. The signal controlling and conditioning instrument controls the magnetic field generating unit to generate the needed magnetic field, receives the electrical signal V ME  sent from the ME sensing unit, and outputs the final signal V st  after signal conditioning, wherein the final signal V st  is a magnetic characteristic value corresponding to the stress of the structural component. 
     In a class of the embodiment, the magnetic field generating unit is chosen from: (a) excitation coils which generates magnetic field by the excitation electric current provided by a drive circuit; (b) permanent magnets; and (c) the combination of the excitation coils and the permanent magnets. The number of the excitation coils or permanent magnets is one or more connected in series or parallel. 
     In a class of the embodiment, the one or more ME sensing units are arranged on the support skeletons or on the surface of the structural component, at the positions corresponding to one or more detected cross-sections of the structural component. 
     In a class of the embodiment, according to the magnetic field distribution in the detected structural component and the adjacent areas, the ME sensing unit is arranged either on the support skeleton or on the surface of the structural component, at the position where the magnetic field is most sensitive to the stress. 
     In a class of the embodiment, the ME sensing unit is arranged inside or outside of the support skeleton, wherein the outside of the support skeleton includes inner or outer surfaces of the support skeletons, or the surface of the structural component. 
     In a class of the embodiment, the support skeleton is a unitary body or an assembled body of several pieces. 
     In a class of the embodiment, the signal controlling and conditioning instrument comprises a drive circuit and a data acquisition and processing device to control the magnetic field generating unit and collecting and processing data of the electrical signal V ME  sent from ME sensing unit to obtain the magnetic characteristic value V st  which is corresponding to the stress of the structural component. 
     In a class of the embodiment, the source signal of the excitation coils is an AC signal or a pulse signal. 
     In a class of the embodiment, the magnetic field generating unit, the ME sensing unit, the support skeleton unit, the signal controlling and conditioning instrument, or parts thereof, or the entire monitoring device is installed with a protection cover. The protection cover not only shields the external magnetic field to reduce interference with the internal magnetic field and signals, but also protects the entire monitoring device from external damage to prolong the service life of the device. 
     The stress monitoring device of the present invention uses ME sensing unit to replace the signal-integration-required secondary coil as the signal detecting element. The ME sensing unit is made of ME laminated composite, and correspondingly the ME sensing unit is able to monitor real-time changes in magnetic field intensity (the response time for the monitoring is in order of milliseconds). The size of the ME sensing unit is small, and correspondingly the weight and size of the device is effectively reduced. The ME sensing unit does not require power supply. The required excitation magnetic intensity is highly reduced due to the high sensitivity of the said ME sensing unit, which effectively reduces the weight and occupation volume of the excitation coil or other type of magnetic field generating unit. By implementing the ME sensing units in different arrangements, monitoring stress of the component in different stress states or of various cross-sectional shapes is realized. The device of the present invention is of high signal sensitivity, high stability and repeatability in testing data, and high measurement precision. Collecting data and processing data in the device of the present invention is automatic so that it is easy to observe stress changes of the measured structural component to realize the automation and continuity of measurement. The application range of the device of the present invention is wide. The installation of the device of the present invention is convenient, and the calibration of the device is simple. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional diagram of the conventional stress monitoring device comprising Elasto-Magnetic cable force sensor; 
         FIG. 2  is a functional diagram of a stress monitoring device of elasto-magneto-electric (EME) effect type in accordance with one embodiment of the invention; 
         FIG. 3A  is an overall diagram for an exemplary structure of the device of  FIG. 2  comprising an exciting coil as the magnetic field generating unit in accordance with one embodiment of the invention; 
         FIG. 3B  is the longitudinal sectional diagram for an exemplary structure of the device of  FIG. 2  comprising an exciting coil as the magnetic field generating unit in accordance with one embodiment of the invention; 
         FIG. 3C  is the transversal sectional diagram for an exemplary structure of the device of  FIG. 2  comprising an exciting coil as the magnetic field generating unit in accordance with one embodiment of the invention; 
         FIG. 4A  is the longitudinal sectional diagram for an exemplary structure of the device of  FIG. 2  comprising a single permanent magnet as the magnetic field generating unit in accordance with one embodiment of the invention; 
         FIG. 4B  is the longitudinal sectional diagram for an exemplary structure of the device of  FIG. 2  comprising two permanent magnets as the magnetic field generating unit in accordance with one embodiment of the invention; 
         FIG. 4C  is the longitudinal sectional diagram for an exemplary structure of the device of  FIG. 2  comprising three permanent magnets as the magnetic field generating unit in accordance with one embodiment of the invention; 
         FIG. 5A  is the longitudinal sectional diagram for an exemplary structure of the device of  FIG. 2  comprising a combination of an exciting coil and a permanent magnet as the magnetic field generating unit in accordance with one embodiment of the invention; 
         FIG. 5B  is the longitudinal sectional diagram for an exemplary structure of the device of  FIG. 2  comprising a combination of two exciting coils and a permanent magnet as the magnetic field generating unit in accordance with one embodiment of the invention; 
         FIG. 5C  is the longitudinal sectional diagram for an exemplary structure of the device of  FIG. 2  comprising a combination of three exciting coils and a permanent magnet as the magnetic field generating unit in accordance with one embodiment of the invention; 
         FIG. 6A  is a schematic perspective diagram illustrating distribution of the magnetic flux lines and a longitudinal arrangement of the ME sensing units for an exemplary structure of the device of  FIG. 3B  which monitors a single cross section of a structural component, in accordance with one embodiment of the invention; 
         FIG. 6B  is a front view of the structure of  FIG. 6A ; 
         FIG. 7A  is a schematic perspective diagram illustrating distribution of the magnetic flux lines and a longitudinal arrangement of the ME sensing units for an exemplary structure of the device of  FIG. 3B  which monitors multiple uniform cross sections of a structural component by displacing several rows of the ME sensing units in one magnetic coil (the arrangement for detection between the magnetic coils and the ME sensing units is a E-MultiME type arrangement), in accordance with one embodiment of the invention; 
         FIG. 7B  is a front view of the structure of  FIG. 7A ; 
         FIG. 8  is schematic diagram illustrating a longitudinal arrangement of the monitored cross sections and the ME sensing units for an exemplary structure of the device of  FIG. 3B  which monitors multiple non-uniform cross sections of a structural component by displacing several rows of the ME sensing units in one magnetic coil (a E-MultiME type detection arrangement), in accordance with one embodiment of the invention; 
         FIG. 9  is a schematic perspective diagram for an exemplary structure of the device of  FIG. 3B  which monitors multiple cross sections of a structural component by using several sets of the magnetic field generating units each combined with one or more ME sensing units (the arrangement for detection between the magnetic field generating units and the ME sensing units is a Multi-EME type arrangement), in accordance with one embodiment of the invention; 
         FIG. 10A  is a longitudinal sectional diagram illustrating distribution of the magnetic flux lines, an arrangement of the ME sensing units, and an arrangement of the skeleton segmentations for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component under a uniaxial load, in accordance with one embodiment of the invention; 
         FIG. 10B  is a transversal sectional diagram for the structure of  FIG. 10A ; 
         FIG. 11A  is a schematic perspective diagram illustrating distribution of the magnetic flux lines, an arrangement of the ME sensing units, and an arrangement of the skeleton segmentations for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component under a one-way bending load M x  around the X axis, in accordance with one embodiment of the invention; 
         FIG. 11B  is a transversal sectional diagram for the structure of  FIG. 11A  which monitors stress of a structural component under a one-way bending load M x  around the X axis; 
         FIG. 11C  is a transversal sectional diagram for the structure of  FIG. 11A  which monitors stress of a structural component under a one-way bending load M y  around the Y axis; 
         FIG. 12A  is a schematic perspective diagram illustrating distribution of the magnetic flux lines, an arrangement of the ME sensing units, and an arrangement of the skeleton segmentations for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component under a biaxial bending load, in accordance with one embodiment of the invention; 
         FIG. 12B  is a transversal sectional diagram for the structure of  FIG. 12A  which monitors stress of a structural component under biaxial bending load M xy  around the X axis and Y axis; 
         FIG. 13A  is a schematic perspective diagram for an exemplary structure of the device of  FIGS. 7A and 7B  which monitors a structural component under a torque M z  by displacing several rows of the ME sensing units in one magnetic coil and displacing one or more ME sensing units in each row of the ME sensing units (a E-MultiME type detection arrangement), in accordance with one embodiment of the invention; 
         FIG. 13B  is schematic diagram of the stress distribution of the structural component under a torque M z  of  FIG. 13A ; 
         FIG. 13C  is a schematic perspective diagram for an exemplary structure of the device of  FIG. 9  which monitors a structural component under a torque M z  by displacing one set of the magnetic field generating units combined with the ME sensing units around one of the multiple cross sections (a Multi-EME type detection arrangement), in accordance with one embodiment of the invention; 
         FIG. 14  is a transversal sectional diagram illustrating an arrangement of the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component having a circular cross section, in accordance with one embodiment of the invention; 
         FIG. 15A  is a transversal sectional diagram illustrating a first arrangement of the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component having a rectangular cross section, in accordance with one embodiment of the invention; 
         FIG. 15B  is a transversal sectional diagram illustrating a second arrangement of the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component having a rectangular cross section, in accordance with one embodiment of the invention; 
         FIG. 15C  is a transversal sectional diagram illustrating a third arrangement of the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component having a rectangular cross section, in accordance with one embodiment of the invention; 
         FIG. 16A  is a transversal sectional diagram illustrating a first arrangement of the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component having a T-type cross section, in accordance with one embodiment of the invention; 
         FIG. 16B  is a transversal sectional diagram illustrating a second arrangement of the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component having a T-type cross section, in accordance with one embodiment of the invention; 
         FIG. 16C  is a transversal sectional diagram illustrating a third arrangement of the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component having a T-type cross section, in accordance with one embodiment of the invention; 
         FIG. 17  is a transversal sectional diagram illustrating an arrangement of the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component having an irregular cross section, in accordance with one embodiment of the invention; 
         FIG. 18A  is a transversal sectional diagram illustrating a first relative arrangement between the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C , in accordance with one embodiment of the invention; 
         FIG. 18B  is a transversal sectional diagram illustrating a second relative arrangement between the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  in accordance with one embodiment of the invention; 
         FIG. 18C  is a transversal sectional diagram illustrating a third relative arrangement between the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C , in accordance with one embodiment of the invention; 
         FIG. 18D  is a transversal sectional diagram illustrating a fourth relative arrangement between the ME sensing units and the support skeleton for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C , in accordance with one embodiment of the invention; 
         FIG. 19A  is a schematic perspective diagram illustrating a first arrangement of the ME sensing units in a structure having E-MultiME type arrangement between the ME sensing units and the magnetic field generating units for an exemplary structure of the device of  FIG. 13A  which monitors stress of a structural component having a rectangular cross section, in accordance with one embodiment of the invention; 
         FIG. 19B  is schematic diagram of the stress distribution in the structural component under a torque M z  of  FIG. 19A ; 
         FIG. 19C  is a schematic perspective diagram illustrating a second arrangement of the ME sensing units in a structure having E-MultiME type arrangement between the ME sensing units and the magnetic field generating units for an exemplary structure of the device of  FIG. 13A  which monitors stress of a structural component having a rectangular cross section, in accordance with one embodiment of the invention; 
         FIG. 19D  is schematic diagram of the stress distribution in the structural component under a torque M z  of  FIG. 19C ; 
         FIG. 20A  is a schematic perspective diagram illustrating a first arrangement of the ME sensing units in a structure having Multi-EME type arrangement between the ME sensing units and the magnetic field generating units for an exemplary structure of the device of  FIG. 13C  which monitors stress of a structural component having a rectangular cross section, in accordance with one embodiment of the invention; 
         FIG. 20B  is a schematic perspective diagram illustrating a second arrangement of the ME sensing units in a structure having Multi-EME type arrangement between the ME sensing units and the magnetic field generating units for an exemplary structure of the device of  FIG. 13C  which monitors stress of a structural component having a rectangular cross section, in accordance with one embodiment of the invention; 
         FIG. 21A  is schematic diagram of structures for applying two ME sensing units to detect stress of a structural component under a uniaxial loading, in accordance with one embodiment of the invention; 
         FIG. 21B  is the test result of  FIG. 21A  illustrating the relationship between the external force F and the magnetic characteristic value V st  for the structural component; 
         FIG. 21C  is schematic diagram of structures for applying two ME sensing units to detect stress of a structural component under an axial force N and a bending moment M, in accordance with one embodiment of the invention; 
         FIG. 21D  is the test result of  FIG. 21C  illustrating the relationship between the external forces (M and N) and the magnetic characteristic value V st  for the structural component. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 2  illustrates an exemplary embodiment of a stress monitoring device of EME effect type of the present invention. The stress monitoring device of the present invention comprises a magnetic field generating unit, one or more ME sensing units, one or more support skeletons, and a signal controlling and conditioning instrument. In this invention, the ME sensing units are used to replace the secondary coil of the conventional Magneto-Elastic cable force sensor shown in  FIG. 1  to generate sensing signals. The ME sensing units possess the following advantages: 1) being capable of producing a strong output signal in proportional to the external force (i.e., good linearity in the mechanical-magneto-electric coupling); 2) being sensitive to the variances of the external force; and 3) being installed easily and stably. 
     In the present invention, under the control of signal controlling and conditioning instrument, the magnetic field generating unit creates a magnetic field in the area where the ME sensing units are applied. The ferromagnetic structural component is thus magnetized. Under the action of the external forces, the magnetic properties of the component are changed, which causes changes in the magnetic intensity and induction intensity in the position of the component and the neighboring area. The magnetic field generating unit is of various kinds in need thereof. The ME sensing unit used to monitor the changes of the magnetic intensity and induction intensity directly generates an electrical signal V ME  characterizing the magnetic intensity or induction intensity without the process of signal integration. The electrical signal V ME  is analyzed and processed by the signal controlling and conditioning instrument to output a final signal, namely, the magnetic characteristic value V st  which is corresponding to the stress state of the component. Therefore, nondestructive monitoring stress and external force of a structural component of a ferromagnetic material is realized by using the monitoring device of the present invention. 
       FIGS. 3A, 3B, and 3C  are respectively the overall schematic diagram, longitudinal sectional diagram, and cross-sectional diagram for a first exemplary structure of a stress monitoring device of EME effect type of  FIG. 2 . The magnetic field generating unit is an exciting coil  34 . A support skeleton  33  is installed outside the ferromagnetic component  31 , the exciting coil  34  is wound around the support skeleton, and two ME sensing units  32   a  and  32   b  are placed inside of the support skeleton  33 . 
     The number of the ME sensing unit may be one or more; the ME sensing unit may be installed interior or exterior of the support skeleton; the support skeleton may be an integral body or an assembled body of more than pieces for the purposes of convenient production process and installation process; the exciting coil may be one coil or more coils connected in series or parallel; and the signal source of the exciting coil may be an AC signal or pulse signal. 
       FIGS. 4A, 4B, and 4C  are longitudinal sectional diagrams for a second exemplary structure of the stress monitoring device of  FIG. 2 . The magnetic field generating unit is a permanent magnet  43  which forms a magnetic field loop together with the monitored component  41  via a yoke  42 . The number of the permanent magnet  43  may be one ( 43 , see  FIG. 4A ), two ( 43   a ,  43   b , see  FIG. 4B ), three ( 43   a ,  43   b ,  43   c , see  FIG. 4C ), or more. 
       FIGS. 5A, 5B, and 5C  are longitudinal sectional diagrams for a third exemplary structure of the stress monitoring device of  FIG. 2 . The magnetic field generating unit is the combination of an exciting coil  44  and a permanent magnet  43 . The exciting coil  44  may include one coil ( 44 , see  FIG. 5A ), two coils ( 44   a ,  44   b , see  FIG. 5B ), three coils ( 44   a ,  44   b ,  44   c , see  FIG. 5C ), or more coils connected in series or parallel; and the signal source of the exciting coil  44  may be an ac signal or pulse signal. Although the number of the permanent magnet  43  shown in  FIGS. 5A, 5B, and 5C  is one, it may be one, two, or more as shown in  FIGS. 4A, 4B, and 4C . 
       FIGS. 6A and 6B  are respectively a schematic perspective diagram and a front view for a first exemplary structure of the stress monitoring device of  FIG. 3B . The structure of  FIGS. 6A and 6B  is used for monitoring a single cross section of a structural component. The ME sensing units are arranged according to the patterns of the forces applied to the component and the distribution of the magnetic flux lines around the component. The magnetic flux lines  54  and the axial arrangement of the ME sensing unit  51  are shown for the case that the component  52  is exerted a uniaxial loading. Preferably, the ME sensing unit  51  is arranged in the position where magnetic intensity is the most sensitive to stress change. For example, for an uniaxial loading component  51 , according to the distribution of the magnetic flux lines  54  shown in these two figures, the ME sensing unit  51  is arranged in the position corresponding to the middle position of exciting coil  53  where the magnetic flux lines are the densest and the changes of the magnetic intensity are the largest when the stress changes. Nevertheless, the ME sensing unit  51  may be arranged in other places. 
       FIGS. 7A and 7B  respectively a schematic perspective diagram and a front view for a second exemplary structure of the stress monitoring device of  FIG. 3B . The structure of  FIGS. 7A and 7B  is used for monitoring multiple cross sections of a structural component having uniform cross sections. For a structural component exerted axial forces similar to that of the component  52  in this embodiment, the ME sensing units are arranged in several cross sections as two ME sensing units  51   a  and  51   b  in this embodiment are arranged in two cross sections, to better monitor the stress state along the axis of the component in the direction where the forces are applied. When the component is applied with a axially constant force, the average value detected from the different ME sensing units in the different cross sections represents the stress of the component, which effectively improving the precision and reliability in measurement. 
       FIG. 8  is a schematic diagram illustrating a longitudinal arrangement of the monitored cross sections and the ME sensing units for a third exemplary structure of the stress monitoring device of  FIG. 3B . The structure of  FIG. 8  is used for monitoring multiple cross sections of a structural component having non-uniform cross sections. In this embodiment, three couples of ME sensing units ( 65   a ,  65   b ), ( 65   a ,  65   b ), and ( 65   a ,  65   b ) are respectively arranged on the cross sections  62 ,  63 , and  64  to monitor stress, and the arrangement for detection between excitation coil and the ME sensing units is a E-MultiME type arrangement (i.e., several rows of the ME sensing units are arranged in one excitation coil). For a structural component comprising non-uniform cross sections or being exerted forces changed along the cross sections, detection of the forcing state of the component is achieved by monitoring multiple cross sections of the component, as is the case in this embodiment. Detecting forcing state of the component by monitoring multiple cross sections of the component of this embodiment has an advantage in the case when the component is exerted non-axial forces including bending force and torque. 
       FIG. 9  is a schematic perspective diagram illustrating a longitudinal arrangement of the ME sensing units and the magnetic field generating units for a fourth exemplary structure of the device of  FIG. 3B . The structure of  FIG. 9  is used for monitoring multiple cross sections of a structural component by displacing several sets of the magnetic field generating units each combined with one or more ME sensing units on the multiple cross sections (a Multi-EME detection type). As shown in the figure, an individual set of the magnetic field generating unit combined with several ME sensing units ( 69 ,  70 ) is arranged on each of the different cross sections  61 . Although  FIG. 9  merely illustrates an example involving a structural component having uniform cross sections, the structural layout of the figure is applicable in the case where the detected structural component comprises non-uniform cross sections. For a structural component comprising non-uniform cross sections or being exerted forces changed along the cross sections, detection of the forcing state of the component is achieved by monitoring multiple cross sections of the component, as is the case in this embodiment. Detecting forcing state of the component by monitoring multiple cross sections of the component of this embodiment has an advantage in the case when the component is exerted non-axial forces including bending force and torque. 
     Due to its small size and light weight, the ME sensing unit occupies small space for location and accordingly is able to precisely detect local magnetic strength and induction strength to achieve local stress of the component. Therefore, the combination of the stresses in different locations of the component may provide a precise forcing state of the component of various forms when multiple ME sensing units are used to detect the different locations of the components. For example, for detecting a structural component under axial forces, one ME sensing unit may be used to output a value representing the stress, or multiple ME sensing units may be used to output a average value representing the stress to reduce measurement error due to the non-uniformity of the component. Before the elastic instability of the structural component occurs, i.e., when only axial forces are exerted on the component, the conventional elasto-magnetic cable force sensor is able to measure the uniaxial forcing state like the sensor comprising the ME sensing units of the present invention. However, once the elastic instability of the component occurs, i.e., when bending forces or toques are exerted on the component, the conventional elasto-magnetic cable force sensor cannot detect the forcing state changes due to the bending forces or torques, while the present sensor having multiple ME sensing units is able to do so. 
     The following exemplary embodiments illustrate that multiple ME sensing units are used to monitor stress of the structural components under different forcing states. 
       FIGS. 10A and 10B  are respectively a longitudinal sectional diagram and a cross-sectional diagram for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component under a uniaxial load. In this embodiment, the component  71  is subjected to an axial force F, a ME sensing unit  72  and two support skeleton segmentations  73  are used for detection. The distribution of the magnetic flux lines  75  and the arrangement of the ME sensing unit  72 , as well as the partitioning form of support skeleton  73  are shown in the figure. Even though the shown support skeleton  73  is segmented into two pieces  73   a , and  73   b , the support skeleton  73  may be formed by one or more pieces according to the needs of actual manufacture and installation. The number of the ME sensing unit  72  may be one or more as needed. 
       FIGS. 11A, 11B, and 11C  are respectively a schematic perspective diagram, and two cross-sectional diagrams for an exemplary structure of the device of  FIGS. 3A, 3B , and  3 C which monitors stress of a structural component under a bending load. The distribution of the magnetic flux lines, the arrangement of the ME sensing units  83   a ,  83   b , and  83   c , and the form of support skeleton segmentations  84   a ,  84   b , and  84   c  are shown in the figure.  FIG. 11A  is a schematic perspective diagram for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component under a one-way bending load M x  around the X axis.  FIG. 11B  is a cross-sectional diagram for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component under a one-way bending load M x  around the X axis.  FIG. 11C  is a cross-sectional diagram for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component under a one-way bending load M y  around the Y axis. At least two or more ME sensing units under are needed for the case of one-way bending to obtain an idealized monitoring result. 
       FIGS. 12A and 12B  are respectively a schematic perspective diagram and a cross-sectional diagram for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C  which monitors stress of a structural component under a bending load. The distribution of the magnetic flux lines, the arrangement of the ME sensing units  102   a ,  102   b ,  102   c , and  102   d  and the form of support skeleton segmentations  103   a ,  103   b ,  103   c , and  103   d  are shown in the figure. The structures of the figures are for the purpose of monitoring stress of a structural component under a biaxial bending moment M XY  around both the X axis and Y axis. At least three or more ME sensing units under are needed for the case of one-way bending to obtain an idealized monitoring result. 
       FIG. 13A  is a schematic perspective diagram for exemplary structure of the device of  FIGS. 7A, 7B  (the arrangement for detection between excitation coil and the ME sensing units is a E-MultiME type arrangement). As shown in the figure, multiple rows of the ME sensing units are arranged in one magnetic coil  113 , and several ME sensing units ( 112   a ,  112   c , and  112   e ) and ( 112   b ,  112   d , and  112   f ) are arranged in each row, for the purpose of monitoring the stress distribution of the component  111  in torsion, wherein the load is a torque M Z . 
       FIG. 13B  is schematic diagram of the stress distribution of the structural component  111  under a torque M z  of  FIG. 13A . 
       FIG. 13C  is a schematic perspective diagram for exemplary structure of the device of  FIG. 9  (the arrangement for detection between excitation coil and the ME sensing units is a Multi-EME type arrangement). As shown in the figure, an individual set of magnetic field generating unit ( 116  or  117 ) combined with several ME sensing units ( 114   a ,  114   b , and  114   c ) or ( 115   a ,  115   b , and  115   c ) is arranged for each of the multiple sections, to monitor the stress distribution of the component  111  in torsion, wherein the load is a torque M Z . 
     Similarly, for the components subjected to other forms of forcing state, the arrangement of the ME sensing units and the segmentations of the support skeletons are determined by the forcing characteristic. 
     The conventional Magneto-Elastic cable force sensor can only monitor the component under uniaxial loading, mainly cable force. For the conventional Magneto-Elastic cable force sensor, since the secondary coil can only measure the change of the magnetic field in the whole windings area and the secondary coil is usually wound around the cylindrical support skeleton, the measured force is an average force inside the entire secondary coil. Therefore, the conventional Magneto-Elastic cable force sensor could not detect local stress or force on the component. Correspondingly, the conventional Magneto-Elastic cable force sensor is not applicable to detect stress of the non-cylindrical components, such as many types of cross-sections: circular cross-section, rectangular cross-section, L-type cross-section, and T-type cross-section. On contrast the stress monitoring device of EME effect type in the present invention is able to monitor the stress distribution and forcing state of the components having different shapes of cross sections by placing the ME sensing units in multiple locations around the cross sections. The ME sensing units are symmetrically placed for the components with symmetrical cross sections, and dispersedly and locally placed for the components of asymmetrical cross sections. In the present invention, the specific displacements of the elements in the stress monitoring device of EME effect type are determined according to the shapes of the cross section of the monitored components. In the invention, the locations and number of the ME sensing units are determined according to the shapes of the cross section of the specific component and the stress distribution of the component. Generally, the ME sensing units are placed at the locations of stress transition or the characteristic points of the stress distribution (e.g. the maximum stress point). 
     The following exemplary embodiments illustrate that multiple ME sensing units are used to monitor stress of the structural components having different shapes of cross sections. 
       FIG. 14  shows a transversal sectional diagram for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C . The figure illustrates an arrangement of the ME sensing units and the support skeleton for detecting a circular cross-sectional structural component  121 . An ME sensing unit  122  and a unitary support skeleton  123  are taken in this embodiment, nevertheless, multiple ME sensing units and multiple integrated support skeletons may be adopted. 
       FIGS. 15A, 15B, and 15C  show a transversal sectional diagram for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C . The figure illustrates an arrangement of the ME sensing units  132 ,  142 , and  152  and the support skeletons  133 ,  143 , and  153  for detecting rectangular cross-sectional structural components  131 ,  141 , and  151 . 
       FIGS. 16A, 16B, and 16C  show a transversal sectional diagram for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C . The figure illustrates an arrangement of the ME sensing units  162 ,  172 , and  182  and the support skeletons  163 ,  173 , and  183  for detecting rectangular cross-sectional structural components  161 ,  171 , and  181 . 
       FIG. 17  shows a transversal sectional diagram for an exemplary structure of the device of  FIGS. 3A, 3B, and 3C . The figure illustrates an arrangement of the ME sensing units  192  and the form of support skeleton segmentations  193  for detecting irregular cross-sectional structural component  191 . 
     The following exemplary embodiments illustrate the relative installation positions of the multiple ME sensing units and the support skeletons. 
       FIG. 18A  shows the first exemplary arrangement for the relative installation positions of the ME sensing unit and support skeleton of the device of  FIGS. 3A, 3B, and 3C . The figure shows that the ME sensing unit  202  is placed inside of the support skeletons  203   a ,  203   b.    
       FIG. 18B  shows the second exemplary arrangement for the relative installation positions of the ME sensing unit and support skeleton of the device of  FIGS. 3A, 3B, and 3C . The figure shows that the ME sensing unit  212  is placed outside of the support skeletons  213   a ,  213   b , and specially, on the outer surface of the support skeletons  213   a ,  213   b.    
       FIG. 18C  shows the third exemplary arrangement for the relative installation positions of the ME sensing unit and support skeleton of the device of  FIGS. 3A, 3B, and 3C . The figure shows that the ME sensing unit  215  is placed outside of the support skeletons  214   a ,  214   b , and specially, on the inner surface of the support skeletons  214   a ,  214   b.    
       FIG. 18D  shows the fourth exemplary arrangement for the relative installation positions of the ME sensing unit and support skeleton of the device of  FIGS. 3A, 3B, and 3C . The figure shows that the ME sensing unit  218  is placed outside of the support skeletons  217   a ,  217   b , and specially, on the surface of the monitored component  31 . 
       FIG. 19A  shows an exemplary arrangement of the ME sensing units in a structure having E-MultiME type arrangement between the ME sensing units and the magnetic field generating units of the device of  FIG. 13A . As shown in  FIG. 19A , the component  221  has a rectangular cross section, and the ME sensing units  222   a ,  222   b ,  222   c ,  222   d ,  222   e ,  222   f  are placed outside of the support skeleton  223 .  FIG. 19B  shows the stress distribution of the monitored rectangular cross-sectional component  221  of  FIG. 19A  which is subjected to a torque M Z . 
       FIG. 19C  shows another exemplary arrangement of the ME sensing units in a structure having E-MultiME type arrangement between the ME sensing units and the magnetic field generating units of the device of  FIG. 13A . As shown in  FIG. 19A , the component  221  has a rectangular cross section, and the ME sensing units  224   a ,  224   b ,  224   c ,  224   d ,  224   e ,  224   f  are placed outside of the support skeleton  223 .  FIG. 19D  shows the stress distribution of the monitored rectangular cross-sectional component  221  of  FIG. 19C  which is subjected to a torque M Z . 
       FIG. 20A  shows a first arrangement of the ME sensing units in a structure having Multi-EME type arrangement between the ME sensing units and the magnetic field generating units for an exemplary structure of the device of  FIG. 13C . As shown in FIG.  20 A, the component  231  has a rectangular cross section, and the ME sensing units  233   a ,  233   b ,  233   c ,  231   a ,  231   b ,  231   c  are placed outside of the support skeleton  234 ,  235 . 
       FIG. 20B  shows a second arrangement of the ME sensing units in a structure having Multi-EME type arrangement between the ME sensing units and the magnetic field generating units for an exemplary structure of the device of  FIG. 13C . As shown in  FIG. 20B , the component  231  has a rectangular cross section, and the ME sensing units  237   a ,  237   b ,  237   c ,  236   a ,  236   b ,  236   c  are placed inside of the support skeleton  234 ,  235 . 
       FIG. 21A  shows structures by using the stress monitoring device of EME effect type of the invention to detect stress of a structural component under a uniaxial force F. Two ME sensing units S 1  and S 2  are applied to detect stress of the component.  FIG. 21B  is the test result of  FIG. 21A  illustrating the relationships between the external force F and the magnetic characteristic value V st  corresponding to the two ME sensing units S 1  and S 2  for the structural component. Because the forcing state is uniform for the monitored cross sections, the results obtained from the electrical signals that are output from the two ME sensing units S 1  and S 2  and processed by the signal controlling and conditioning instrument are the same, the relationships between the external force F and magnetic characteristic value V st  with respect to the two ME sensing units S 1  and S 2  are monotonous. The relation curve between the external force F and magnetic characteristic value V st  when the component is subjected to a uniaxial loading may be linear, piecewise linear, or nonlinear. In practical application, the detected stress is determined by the calibrated curve or calibrated table and the magnetic characteristic values V st  corresponding to the two ME sensing units S 1  and S 2 . 
       FIG. 21C  shows structures by using the stress monitoring device of EME effect type of the invention to detect stress of a structural component under a bending force M and a axial force N. Two ME sensing units S 1  and S 2  are applied to detect stress of the component.  FIG. 21D  is the test result of  FIG. 21C  illustrating the relationships between the external force M or N and the magnetic characteristic value V st  for the structural component. Because the detected cross sections are stressed unevenly, the results obtained from the electrical signals that are output from the two ME sensing units S 1 , and S 2  and processed by the signal controlling and conditioning instrument are different. The forcing state of the component, including the bending moment M and axial force N are determined by the calibrated data and the corresponding magnetic characteristic values V st  of the two ME sensing units S 1  and S 2 . 
     In addition, the magnetic field generating unit, the ME sensing unit, the signal controlling and conditioning instrument, and the support skeletons, each or all or the whole combination thereof in the present invention may be covered with or without a protection cover. The protection cover (the element  35  as shown in  FIG. 3C ) is not only able to screen out the external magnetic field to reduce interference between the external magnetic field and the internal magnetic field and signals of the device, but also able to protect the covered element from external damage and correspondingly prolong the service life of the device. 
     While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.