Abstract:
The fastener tension monitoring system provides for automatic notification when a structural tensile fastener, such as a bolt, is loosened. The system incorporates piezoelectric material in the core of the fastener shank or deposited around the shank. The piezoelectric material communicates electrically with an electrical storage device, a microprocessor, and a transmitter installed on the fastener head. Vibration or other changes in the tensile force developed by the fastener result in electrical impulses being generated by the piezoelectric material, generating electrical energy that is stored in the storage device and used to operate the microprocessor. In the event that an out of tolerance condition is sensed by the microprocessor, the transmitter is activated to send a signal to that effect. A receiver and computer are provided to monitor any such signals. The piezoelectric material may include pyroelectric material as well for the generation of electrical energy due to temperature changes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to automated monitoring devices, and particularly to a fastener tension monitoring system, particularly for monitoring threaded bolts, that generates electrical energy from forces imparted to the fastener and uses the energy to transmit data regarding those forces. 
     2. Description of the Related Art 
     Large structures (such as buildings, bridges, factories and manufacturing plants, ships, aircraft, etc.) are conventionally assembled using tensile fasteners, such as bolts, among other assembly means. A great deal of knowledge has been accrued over the years regarding the sizes and types of fasteners required for any given structure, and the gripping force that must be provided by the fasteners to provide a safe and sound structure. 
     While the structures and fasteners may be assembled properly during the construction process, various conditions act upon structural joints over time to cause the fasteners to loosen. Vibration due to traffic (on bridges) and the operation of associated machinery can act to loosen bolts and rivets in the structure. Corrosion can reduce the strength of a fastener and/or the adjacent structural material in a structure. The fastener itself may stretch slightly over time, resulting in a loosened joint. 
     As a result periodic inspections are required of many, if not most, large structures to detect any substandard joints or connections, and to repair those joints or connections as required. Such inspections are, of course, costly in terms of man-hours, and often require the operation of the structure to be shut down during the course of the inspection. This is particularly true of ships and aircraft, but may also hold true for various factories, manufacturing plants, pipelines, and similar structures, depending upon the depth of the inspection required. In most cases such inspections will not turn up any problems, with problems that are detected normally being a small minority of the total structural assembly. Nevertheless, the time required to perform the inspection, and the time lost from being unable to use the structure, remain the same whether problems are found or not. 
     Thus, a fastener tension monitoring system solving the aforementioned problems is desired. 
     SUMMARY OF THE INVENTION 
     The fastener tension monitoring system includes a bolt having a piezoelectric element incorporated therewith. The piezoelectric element may include pyroelectric properties as well, i.e., the generation of electrical energy as the material changes temperature state. The piezoelectric or pyroelectric material may be incorporated in a hollowed portion of the shank of the bolt, or the shank may be coated with the material. Electrical leads are extended from the piezoelectric/pyroelectric element through the head of the bolt, where they are connected to an electrical energy storage device, such as a small electrical storage cell (battery) or a capacitor atop the head of the bolt. Changes in the gripping force developed by the bolt due to vibration or other forces induced in the structure result in the generation of a small amount of electrical energy that is stored in the electrical energy storage device (battery or capacitor). If a pyroelectric element is used, temperature changes induced in the bolt due to environmental temperature changes also cause the element to generate a small amount of electrical energy for storage in the electrical energy storage device. 
     Processing circuitry and a transmitter are also installed atop the head of the bolt. The electrical energy storage device communicates electrically with the processing circuitry and transmitter. As the tensile force developed in the bolt changes, the electrical properties of the piezoelectric material also change. One of the electrical properties affected is the impedance of the material. The processing circuitry uses the electrical energy developed by the tensile force changes, or previously stored in the electrical energy storage device due to piezoelectric or pyroelectric generation, to process the changes as evidenced by changes to the electrical impedance or other changes to the electrical properties, and to transmit a signal of these changes. The system includes a receiver for receiving the transmitted signal from the bolt, and a computer for monitoring the received signal. 
     These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an environmental side elevation view in section of a threaded bolt installation in which the bolt incorporates a first embodiment of a fastener tension monitoring system according to the present invention. 
         FIG. 2  is a side elevation view in section of a threaded bolt installation in which the bolt incorporates a second embodiment of a fastener tension monitoring system according to the present invention. 
         FIG. 3A  is a block diagram of the components incorporated into a fastener in the fastener tension monitoring system according to the present invention. 
         FIG. 3B  is block diagram of the components incorporated in the receiver in the fastener tension monitoring system according to the present invention. 
     
    
    
     Similar reference characters denote corresponding features consistently throughout the attached drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The fastener tension monitoring system provides a means for measuring variations in the strain placed upon an installed tensile fastener, and particularly a bolt, and for generating the required electrical energy to provide a short range RF transmission signal to a receiver for monitoring and processing the signal.  FIG. 1  of the drawings provides an elevation view in section of a threaded bolt  110  having a head  112  and an elongate shank  114  extending from the head. At least the distal portion  116  of the shank  114  may be threaded, and the proximal grip length portion  118  may be smooth and devoid of threads, although the entire length of the shank  114  may be threaded. A nut  120  is threaded onto the threaded distal portion  116  of the shank  114  to fasten two or more structural components, e.g., structural components S 1  and S 2 , to one another. Other conventional components, e.g., washers, etc., are not shown for clarity in the drawing. 
     A receptacle  122  is formed through the head  112  and into at least the proximal portion  118  of the shank  114  adjacent the head  112 . The receptacle  122  is filled with a piezoelectric material  124  that adheres to the inner walls of the receptacle  122 . The piezoelectric material may be any of a number of such well-known materials, e.g., lead zirconate titanate (PZT), etc. The piezoelectric material has at least one electrical property that changes when the physical force applied to the material changes. The material  124  may be a pure piezoelectric compound or mixture, or may alternatively include at least some fraction of pyroelectric material to generate electrical energy as the temperature of the bolt  110  changes. An example of such pyroelectric material is lithium tantalate (LiTaO 3 ), which actually exhibits both pyroelectric and piezoelectric properties to make it a desirable material for use in the fastener tension monitoring system. By adhering the PZT (and/or LiTaO 3 ) material to the inner walls of the receptacle  122  of the bolt  110 , slight changes in the elongation of the bolt due to loosening of the bolt and/or movement of the structural components S 1  and S 2  are imparted to the PZT or LiTaO 3 , thus changing its electrical property(ies). One such electrical property that is affected by changes in the physical force imparted to the material is the electrical impedance of the material. 
     A cap  126  is attached to the head  112  of the bolt  110 . The cap contains conventional microcircuitry  128  comprising an impedance reader, e.g., model AD5933 or other suitable unit, a microcontroller, and a very low power transmitter therein. A small electrical energy storage device  130 , e.g., an electrical storage cell or battery, capacitor, etc., is also installed within the cap  126 . The very small electrical currents generated by the piezoelectric material  124  due to slight variations in tension, and thus strain, applied to the bolt  110  are sufficient to charge the electrical energy storage device  130  over time. The electrical energy generated by the piezoelectric material  124  provides sufficient energy to operate the microcontroller to measure changes in impedance (or other electrical property) of the piezoelectric material  124  as forces on the bolt  110  change, and also to operate the short range transmitter. A corresponding receiver, discussed further below, receives any transmissions from the transmitter for processing and monitoring. Each cap  126  and its microcircuitry  128  may be provided with a code to identify the specific location of the associated bolt  110 , thus enabling the receiver and processor to identify the specific bolt  110  that has loosened in its installation. While a cap  126  is shown and described for the embodiment of  FIG. 1 , it should be understood that sufficiently miniaturized microcircuitry may allow the entire component package of the microcircuitry  128  and the electrical energy storage device  130  to be disposed completely within the head  112  of the bolt  110 , depending upon the size of the bolt. 
       FIG. 2  provides an elevation view in section of another embodiment of the fastener tension monitoring system, wherein the piezoelectric and/or pyroelectric material is disposed circumferentially about the shank of the bolt. The bolt  210  of  FIG. 2  is configured externally much like the bolt  110  of  FIG. 1 , having a head  212  and an elongate shank  214  extending therefrom. The distal portion  216  of the shank  214  is threaded, and the proximal grip length portion  218  of the shank  214  may be smooth and devoid of threads. A nut  120 , identical to the nut  120  of the assembly illustrated in  FIG. 1 , is secured to the threaded distal end portion  216  of the shank  214  to grip the two structural components S 3  and S 4  between the head  212  and the nut  120 . 
     It will be noted that the bolt  210  of  FIG. 2  differs from the bolt  110  of  FIG. 1  in that it has no central receptacle formed through the head  212  and upper or proximal portion of the shank  214 . Rather, the piezoelectric and/or pyroelectric material  224  is applied externally as a circumferential sleeve around the unthreaded portion of the shank  214 . The piezoelectric and/or pyroelectric material  224  is adhered to the exterior surface of the bolt shank  214 , so that it will be subject to the same elongation and contraction stresses that occur in the bolt shank  214  if the tension in the bolt  210  changes. The piezoelectric and/or pyroelectric material  224  of the embodiment of  FIG. 2  may be chemically identical to the piezoelectric and/or pyroelectric material  124  of the embodiment illustrated in  FIG. 1 , but differs in physical configuration due to its placement about the circumference of the unthreaded portion  218  of the bolt shank  214 . 
     A cap  226  is attached to the head  212  of the bolt  210 . The cap contains conventional microcircuitry  128  comprising an impedance reader, a microcontroller, and a very low power transmitter therein. The microcircuitry of the cap  226  is essentially identical to that contained in the cap  126  of the bolt  110  of  FIG. 1 , and accordingly need not be described further. The only difference between the cap  126  and the cap  226  is the provision for spaced apart electrical leads extending from the circumferentially applied piezoelectric and/or pyroelectric material  224 , rather than having the leads immediately adjacent one another, as shown in  FIGS. 1 and 2 . Accordingly, lead passages  222   a  and  222   b  are provided through the bolt head  212  to allow the microcircuitry  128  and the electrical energy storage device  130 , identical to those components described further above in  FIG. 1 , to communicate electrically with the piezoelectric and/or pyroelectric material  224 . 
       FIGS. 3A and 3B  respectively provide block diagrams of the electronic and microelectronic components of the fastener tension monitoring system.  FIG. 3A  represents the components installed with the tensile fastener, while  FIG. 3B  represents the components installed remotely from the fastener. 
     The piezoelectric material (PZT)  124  of  FIG. 3A  is exemplary. It is understood that the embodiments of the piezoelectric and/or pyroelectric material  124  and  224  used in the present systems are chemically identical to one another. They differ only in their physical configurations, and may be used interchangeably as the PZT material  124  shown in  FIG. 3A . The polarity of the generated electrical energy produced by the piezoelectric and/or pyroelectric material  124  will vary, depending upon the direction of movement of the fastener (extension or contraction of its length) and the direction of the temperature change (warmer or colder), in the case of pyroelectric material. Accordingly, the piezoelectric and/or pyroelectric material  124  communicates electrically with a rectifier  310  disposed in the microelectronic circuitry  128  disposed in the caps  126  and  226  of the various embodiments of the fastener tension monitoring system. Rectified (dc) electrical current then passes to an electrical energy storage and management device, i.e., the electrical storage cell or battery or capacitor  130  disposed in the caps  126 ,  226  of the various embodiments. 
     The piezoelectric and/or pyroelectric material  124  also communicates electrically with circuitry to measure changes in the electrical properties of the material  124  as changing physical forces are transferred from the fastener to the material. The present inventors have found that the measurement of the electrical impedance of the material  124  is perhaps the optimum means of determining the variations in the physical forces occurring. Accordingly, impedance measuring circuitry  312  is provided, and the piezoelectric and/or pyroelectric material  124  communicate electrically with the impedance measuring componentry  312 . 
     The impedance measuring circuitry  312 , in turn, communicates electronically with a microprocessor and electronic memory chip  314  for the storage of data and the processing of that data. The electrical energy required to operate the microprocessor and memory chip  314  is received from the electrical energy storage device  130 . The microprocessor and memory chip  314  provides the data to a very low power radiofrequency (RF) transmitter  316 , which, in turn, transmits the data via a wireless link to a receiver. The transmitter  316  may be a transceiver capable of receiving RF signals as well. In this manner, a signal transmitted back to the transceiver  316  may be used to clear and reset the microprocessor and memory  314 , or to perform some other desired function. 
     The RF signal transmitted by the transmitter or transceiver  316  is received by a nearby receiver  318 , shown in the block diagram of  FIG. 3B . The receiver  318  may include RF transmission means, i.e., may be a transceiver, in order to transmit signals to the transceiver  316  that is physically located with the fastener. The receiver or transceiver  318  communicates electronically with a conventional computer  320  for storing and processing the data received, and displaying such data. The computer may be programmed using known, conventional means to transmit or display a signal in the event that tolerances of the fastener exceed certain predetermined parameters, and/or to retain a record of the data received from the fastener by means of its transmitter or transceiver  316 . 
     It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.