Abstract:
A system includes a structure and a component. The component including an energy providing device and a protective cover. The structure has a structure surface. The protective cover has a protective cover attachment surface mounted on the structure surface. The energy providing device is located within the protective cover. The protective cover has a protective cover outer surface. The protective cover outer surface is oriented to face a first direction from which an impact load may strike the protective cover outer surface and shear the protective cover from the structure surface. The protective cover outer surface has an outer surface shape facing the first direction. At least a portion of an impact load striking any region of the protective cover outer surface facing the first direction is transferred by the outer surface shape into a compression load.

Description:
This patent application is a divisional of U.S. patent application Ser. No. 12/211,975, filed Sep. 17, 2008 which is a divisional of U.S. patent application Ser. No. 11/091,244, filed Mar. 28, 2005, now U.S. Pat. No. 7,461,560, which claimed priority of U.S. provisional patent application 60/556,974, filed Mar. 26, 2004. 
     RELATED US PATENT APPLICATIONS AND PAPERS 
     This patent application is related to the following U.S. patent applications: 
     09/731,066 to Townsend, (“the ′066 application”) “Data Collection and Storage Device,” filed Dec. 6, 2000, incorporated herein by reference; 
     10/379,223, to Hamel, et al., (“the ′223 application”) “Energy Harvesting for Wireless Sensor Operation and Data Transmission,” filed Mar. 5, 2003, incorporated herein by reference; 
     10/379,224, to Arms, et al., (“the ′224 application”) “Robotic System for Powering and Interrogating Sensors,” filed Mar. 5, 2003, incorporated herein by reference; and 
     10/769,642, to Arms, et al., (“the ′642 application”) “Shaft Mounted Energy Harvesting for Wireless Sensor Operation and Data Transmission,” filed Jan. 31, 2004 incorporated herein by reference. 
     This patent application is also related to a paper by Arms, S. W. et al., “Power Management for Energy Harvesting Wireless Sensors” (“the power management paper”), Proceedings SPIE Smart Structures and Smart Materials, Paper no. 5763-36, San Diego, Calif., March 2005, incorporated herein by reference. 
    
    
     FIELD 
     This patent application generally relates to a protective cover for a component. More particularly it relates to a cover that protects against a shear force. More particularly it relates to a protective cover for a strain gauge, such as a wireless strain gauge. It also relates to a cover for a strain gauge with an improved moisture barrier. 
     BACKGROUND 
     The quality of data reported by a strain gauge mounted to a metallic substrate depends on the integrity of the adhesive bond between the strain sensor and the substrate. 
     It is generally accepted that the adhesive bond (typically an epoxy) breaks down in the presence of moisture. Swelling of the epoxy due to moisture absorption results in shear stresses at the epoxy/metal interface, and over time, these shear stresses can result in failure of the epoxy bond and de-lamination of the strain gauge. 
     One solution to this problem, often employed on large civil structures, is to package the strain gauge within a sandwich of two hermetically sealed stainless steel ribbons. Laser or electron beam is used to provide the sealing. This strain sensitive ribbon is then spot welded to the structure under test. However, this spot welding process creates localized changes in the steel&#39;s microstructure which may be subject to higher than normal rates of corrosion. For many applications of welded structures, the creation of corrosion focus points is considered unacceptable, as these could result in degradation in the physical appearance, added maintenance costs, or even the initiation of material failure. Therefore protection against moisture is desired. 
     None of the systems for connecting a strain sensor to a structure have been satisfactory in providing a reliable bond that is resistant to moisture degradation without affecting structural properties. In addition, when moisture degradation occurs there has been no way to recognize that data coming from the sensor is not acceptable. Thus, a better system for connecting strain sensors to structures is needed, and this solution is provided by the following. 
     SUMMARY 
     One aspect is a system that includes a structure and a component. The component including an energy providing device and a protective cover. The structure has a structure surface. The protective cover has a protective cover attachment surface mounted on the structure surface. The energy providing device is located within the protective cover. The protective cover has a protective cover outer surface. The protective cover outer surface is oriented to face a first direction from which an impact load may strike the protective cover outer surface and shear the protective cover from the structure surface. The protective cover outer surface has an outer surface shape facing the first direction. At least a portion of an impact load striking any region of the protective cover outer surface facing the first direction is transferred by the outer surface shape into a compression load. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1   a  is a top view of a prior art dielectrometer for cure monitoring of composite materials with a comb like structure; 
         FIG. 1   b  is a top view of a prior art humidity sensor with a comb like sructure; 
         FIG. 2   a  is a top view of one embodiment of a patterned capacitance sensor integrated with a strain gauge; 
         FIG. 2   b  is a top view of another embodiment of a patterned capacitance sensor integrated with a strain gauge in which pads are formed of windows or stripes of metal in the bonding pad area; 
         FIGS. 3   a - 3   c  are cross sectional views of different embodiments of the capacitance sensor integrated with the strain gauge of  FIGS. 2   a ,  2   b  with air, polyimide, and epoxy dielectrics; 
         FIG. 4   a  is a schematic/block diagram of an embodiment including a sensor node and a base station in which the sensor node has both a strain gauge and a moisture sensor; 
         FIG. 4   b  is a schematic/block diagram of an embodiment including a capacitive moisture sensor and a microprocessor with an oscillator; 
         FIG. 5  is a flow chart showing an embodiment of a process to attach and protect a self-testing strain gauge node to a surface of a structure; 
         FIG. 6  is a cross sectional view of an embodiment of a temporary mounting fixture during use for attaching a strain gauge and moisture sensor to a steel structure; 
         FIG. 7  is a three dimensional view of an embodiment of a tape mounted protective cover for protecting a strain gauge and moisture sensor in which the protective cover includes an integrated replaceable sealed battery and openings for wax insertion; 
         FIG. 8  is a three dimensional view of an embodiment of a process for filling a protective cover with molten wax; 
         FIGS. 9   a - 9   c  are cross sectional views similar to those of  FIGS. 3   a - 3   c  with an additional thin film of wax, grease, Waxoyl or anticorrosion formula; and 
         FIGS. 10   a - 10   c  are views of another embodiment of a tape-mounted protective cover for protecting a strain gauge and moisture sensor in which the printed circuit board is mounted to the protective cover, and the protective cover also includes an integrated replaceable sealed battery and openings for wax insertion. 
     
    
    
     DETAILED DESCRIPTION 
     The present inventors recognized that substantial improvement in strain sensor reliability could be achieved by providing an improved moisture barrier and by providing a self testing scheme so that delamination or other problems could be detected and the strain sensor replaced. They recognized that they could provide a for the strain sensor and fill the with wax to substantially improve resistance to moisture penetration. They also recognized that for some dielectrics capacitance of a capacitor adjacent to the strain sensor could provide data about the magnitude of moisture penetration and the potential for degradation of the epoxy bonding the strain sensor to the substrate surface to which it is mounted. They also recognized that the scheme could also be used to monitor the curing of the epoxy or of other polymers. 
     The structure to which the strain sensor may be attached may be a building, a bridge, or a vehicle, such as a car, a truck, a ship, construction equipment, or excavation machinery. The structure can also be the spinning shaft of a motor, pump, generator or other spinning device. 
     A hard-wired system that uses a comb-like structure patterned on polyimide as a dielectrometer for cure monitoring of composite materials are described in a manual, “Eumetric 100A Dielectrometer Cure Monitoring System User&#39;s Guide,” available from Holometrix, formerly Micromet, Newton Centre, Mass. and shown in  FIG. 1   a . The dielectrometer reflects the degree of cross-linking of the polymer chains, which can be related to strength. Higher dielectric constants indicate stronger material properties. 
     The dielectric constant measured in such a device is greatly influenced by the presence of moisture because the dielectric constant of air is one, but the dielectric constant of water is 80. A patterned humidity sensor developed at Dublin City University is described in a paper, “Humidity Sensors,” and includes a comb-like structure, as shown in  FIG. 1   b . This sensor uses polyimide as the moisture sensing dielectric material because of its excellent thermal and electrical stability. It also uses a silicon nitride substrate. 
     Such a capacitance monitoring technique has not previously been used to monitor moisture in the vicinity of a strain gauge&#39;s epoxy bond or attachment to the surface of the structure to which the strain gauge is affixed. 
     The present strain sensing system has the ability to monitor and report on the integrity of its own encapsulation by monitoring the moisture content of the epoxy or the moisture content adjacent to the epoxy. Self-testing of the integrity of the encapsulation is accomplished by measuring the capacitance of a capacitance sensor that is sensitive to the presence of moisture in the vicinity of the strain gauge/epoxy glue line attachment to the metal or other material of the structure to which it is affixed. 
     Patterned capacitance sensor  20  is integrated with and provided around the periphery of strain gauge  22 , as shown in top view in  FIGS. 2   a ,  2   b . Patterned capacitance sensor  20  includes interdigitated comb metal plates  20   a ,  20   b  on polyimide substrate  24 , such as a Kapton substrate, as shown in cross section in  FIG. 3   a . As shown, patterned capacitance sensor  20  is located on three sides of small 5000 ohm strain gauge  22 , such as the Micro-Measurements model N3K-06-S022H-50C/DP. Capacitance sensor  20  is preferably un-encapsulated and its polyimide substrate  24  is preferably in direct contact with the same epoxy adhesive  26  used to affix strain gauge  22  to surface  28  of structure  30 . 
     Capacitance sensor  20  and strain sensor  22  are both fabricated by lithographically providing metal lines  20   a ,  20   b ,  22 ′ on polyimide substrate  24 , as shown in  FIG. 3   a . Sensor assembly  32 , including capacitance sensor  20  and strain sensor  22 , are preferably epoxy bonded with epoxy  26  to surface  28  of structure  30 , such as a machine, bridge, vehicle or any other structure. In one embodiment, shown in  FIG. 3   a , polyimide cap  34  is provided to protect metal lines  20   a ,  20   b ,  22 ′ from mechanical damage. Capacitance of capacitance sensor  20  changes as moisture content of air dielectric  36  between plates  20   a,    20   b  of capacitance sensor  20  changes. 
     In another embodiment, polyimide dielectric  36 ′, or another polymer that has a dielectric constant sensitive to the presence of moisture, is provided between plates  20   a,    20   b  of capacitance sensor  20 , as shown in  FIG. 3   b . Alternatively, polyimide cap  34  is omitted and mounting epoxy is itself provided on the surface of capacitance sensor  20  and between metal plates  20   a ,  20   b  to provide epoxy dielectric  36 ″ between plates  20   a,    20   b  of capacitance sensor  20 , as shown in  FIG. 3   c . Should moisture reach mounting epoxy  26 , it will also be present in air dielectric  36 , polyimide dielectric  36 ′, or epoxy dielectric  36 ″, and change the capacitance of capacitance sensor  20 . 
     In another approach, capacitance of the strain sensor itself is used as the moisture sensor. While electrical contact to the surface  28  of structure  30  would provide a two plate capacitance with polyimide substrate  24  and mounting epoxy  26  serving as the dielectric in that case, no electrical contact to the structure surface is actually needed. With a high frequency signal applied across strain gauge  22 , as described herein above for separate capacitance sensor  20 , changes in dielectric properties in its neighborhood could be detected, including changes from moisture penetration adjacent strain gauge  22 . 
     The change in capacitance of capacitance sensor  20  is detected by capacitance signal conditioning circuit  50 , A/D converter  52 , and microprocessor  54  and transmitted externally by transmitter  56  through antenna  58 , as shown in  FIG. 4   a . These components are all located on circuit board  59  that is also bonded to surface  28  of structure  30 . Base station  62  receives transmission from antenna  58  and from other sensor nodes that may be nearby. Signal conditioning circuit  50  includes, sine or square wave oscillator  70  that provides a high frequency signal to capacitive divider  72  that includes capacitance sensor  20  and reference capacitor  74 . Reference capacitor  74  has an inorganic dielectric and is insensitive to changes in humidity. Output of capacitance divider  72  will track changes in capacitance in humidity sensitive capacitance sensor  20 , and this signal is amplified in AC amplifier  76 , rectified in full wave synchronous rectifier  78 , and filtered in low pass filter  80  to provide a DC output proportional to the difference in capacitance between capacitors  20  and  74 . If this number stays constant then capacitance sensor  20  has not changed and humidity has not entered. Thus, the present invention provides self-testing of the integrity of the epoxy bond between sensor assembly  32  and structure surface  28  and wireless transmission of the integrity data. 
     An alternative embodiment to determine change in capacitance of capacitance sensor  20  is shown in  FIG. 4   b . An AC signal generated by a program running on microprocessor  54  derived from the microprocessor clock is provided across outputs  82 ,  84  of microprocessor  54 . Input  86  receives a signal resulting from RC delay across resistor  88  and capacitance sensor  20 . This delay will change as moisture level increases between plates  20   a ,  20   b  of capacitive capacitance sensor  20 . Microprocessor  54  detects the presence of moisture based on the delay between output signal  82  and input signal  86 . 
     Uni-axial, bi-axial and triaxial strain gauges, such as those available from Vishay Micromeasurements, Raleigh, N.C. can be used, such as part numbers CEA-06-125UW-350, CEA-06-125UT-350, and CEA-06-125UA-350. Principal strain magnitudes and strain directions can be computed, as described in a textbook by James W. Dally &amp; William F. Riley, “Experimental Stress Analysis”, Third Edition, Chapter 9, Strain-Analysis Methods, pp 311-315 publisher: McGraw-Hill, Inc., NY, NY (c) 1991, 1978, 1965 by Dally and Reilly. These gauges include resistors, and the resistance changes both from changes in strain and from changes in moisture. The gauges do not include ability to detect moisture and do not include ability to distinguish a change in resistance due to a change in moisture from a change in resistance due to a change in strain. The deleterious effects of moisture and some ways to waterproof are described in the Dally &amp; Riley book on pages 196-197. The present patent application provides a way to detect both strain and moisture and to protect against moisture. 
     Microprocessor  54  can receive data from capacitance sensor  20  related to any change in dielectric constant of its dielectric  36 ,  36 ′,  36 ″ and can report this change to base station  62 , as shown in  FIG. 4   a . Information concerning a degraded capacitance sensor  20  that indicates the presence of moisture in dielectric  36 ,  36 ′,  36 ″ between plates  20   a ,  20   b  transmitted to base station  62 , which will sound an alarm, store the data in memory, and mark that particular sensor assembly  32  for replacement. Sensor assemblies  32  that exhibit capacitance within a tolerance will remain in service transmitting data from surface  28  of structure  30  to which they are mounted. Thus, the present invention provides for self-testing and maintenance of sensors to ensure that they are reliably providing accurate data and that the bonding to structure surfaces has not degraded from moisture penetration. 
       FIG. 4   a  also shows an energy source, such as a battery or an energy harvesting device. These supply Vsupply to processor  54 . Processor  54  can control power Vcc to capacitance signal conditioning  50  and A/D converter  52 . Processor  54  can also control power Vcc&#39; to strain gauge  22 , strain gauge signal conditioning DC AMP, and the strain gauge A/D converter. Power Vtx can also be provided to transmitter  56  under control of processor  54 . Also processor  54  can write data to non-volatile memory  57 . A more detailed circuit diagram for a single strain gauge bridge is provided in  FIG. 16  of the ′642 application. Multiple strain gauge bridges can be provided, as shown in  FIG. 2  of the ′066 application, which includes a multiplexer. 
     Strain gauges have long been bonded to metal surfaces and the process for bonding a strain gauge to a metal surface is well known in the art. A combination of heat and pressure have been used to cure a thin glue line of two-part epoxy between the strain sensing element and the metallic substrate. Over 24 hours is needed at room temperature. About two hours is needed at an elevated temperature of about 150 C. Two-part epoxy with such extended cure time has been used for best results. However, this extended time process has not been easy to deploy in the field, especially if many strain sensor nodes need be attached to a structure. Compromises are typically made to facilitate quick curing, such as the use of cyanoacrylates (super-glues) or one-part epoxies. However, these room temperature, fast-curing adhesives do not provide as strong a bond as extended cure time two-part epoxy, greatly limiting the use of such glue-bonded strain gauges for long term structural health monitoring applications. 
     An improved system for in-field connection of a strain gauge to a metal or non-metal structural surface, using optimum epoxy formulations, and with subsequent waterproof encapsulation of the strain gauge and its signal conditioning, data logging, and wireless communication electronics, is needed. The finished package must be low profile, durable, low cost, and suitable for long term deployment. With the self-testing feature described herein above providing wirelessly transmission of information about the ingress of moisture, such a package has potential for much wider use individually or in a network of many such nodes than currently available packages. The application of wireless sensors with data logging elements, signal conditioning electronics and bidirectional electronics has been described in the ′066 patent application. 
     In addition to providing the self testing for moisture and the wireless transmission of this self-test data feature, the present inventors also provided an improved process to attach and protect their fully integrated, self-testing strain gauge sensor node to a surface of a structure, as shown in the flow chart in  FIG. 5 . In the first step, surface  28  of structure  30  to which sensor assembly  32  is to be mounted is properly cleaned, as shown in step  200 . The surface can be a steel surface or it can be a plastic, composite or any other material. 
     In one embodiment strain gauge  22  and moisture sensing capacitance sensor  20  will have already been pre-wired to circuit board  59 , or they can be integral with circuit board  59 . Circuit board  59  contains supporting electronics and is fully tested for proper operation at the factory. Circuit board  59  can be fabricated of fiberglass materials, such as FR4 or of ceramic materials, such as low temperature co-fired ceramics. Circuit board  59  can also be fabricated of thin flexible insulative materials, such as polyimide. In this embodiment strain gauge  22 , moisture sensing capacitance sensor  20 , and circuit board  59  can be affixed to the structure using a UV-cured epoxy adhesive, as shown in step  201 . UV light is provided to adhesive located under strain gauge  22 , moisture sensing capacitance sensor  20 , and edges of circuit board  59  accessible to UV light. 
     In another embodiment, circuit board  59  may be mounted to protective cover  89 , as shown in  FIGS. 10   a - 10   b . In this embodiment lead wires from strain gauge  22  and moisture sensing capacitance sensor  20 , affixed with a UV-cured epoxy adhesive, are plugged into a receptacle extending from circuit board  59 . Circuit board  59  mounted in protective cover  89  can be protected with wax, silicone grease, or another protective material in the factory with only wires and/or a receptacle extending for mating with lead wires from strain gauge  22  and moisture sensing capacitance sensor  20 . In this embodiment, protective cover  89  encloses strain gauge  22 , moisture sensing capacitance sensor  20 , circuit board  59 , and lead wires there between. 
     The strain and moisture sensing elements are glued directly to the structure&#39;s steel substrate, as shown in steps  202  to, using a process more fully described herein below. For attachment to a steel portion of structure  32 , magnetic mounts  90  are used to temporarily attach specially designed mounting fixture  92 , as shown in step  202  and in  FIG. 6 . Fixture  92  includes frame  94  held in position by magnetic mounts  90 . 
     Sensor assembly  32  is applied to surface  28  of structure  30  with epoxy as shown in step  203 . Threaded plunger  96  provides compression on sensor assembly  32  including strain gauge  22  and capacitance moisture sensor  20 . Threaded plunger  96  is tightened as shown in step  204 , to provide compression force on sensor assembly  32 . 
     Thermoelectric heating element  98  provides heat to more rapidly cure epoxy (not shown) beneath sensor assembly  32  while it is being compressed. Heating element  98  is turned on to cure epoxy as shown in step  205 . Temperature and pressure are monitored with temperature sensor  104  and pressure sensor  102 , as shown in step  206 , and information may be fed back to heating element  98  and threaded plunger  96  or to the operator allowing control over the amount of pressure and heat applied to the assembly. Optionally, capacitance sensor  20  can be used to monitor the state of cure during this step, as shown in step  207  and waiting step  208 , and to provide feedback about changes in the dielectric constant of the epoxy during the curing process, as described herein above for the embodiment of  FIG. 3   c . To accomplish this a temporary power source is provided to circuit board  59  during curing. Rubber pad  100  insures a stable pressure and an even pressure distribution during curing. 
     Swivel  106  allows aluminum plate  108  along with heating element  98  freedom of movement to accommodate a tilted surface. Aluminum plate  108  provides for uniform distribution of heat from heating element  98 . 
     After curing is complete the mounting fixture is removed, as shown in step  209 . Next protective cover  89  is installed on sensor assembly  32  and its supporting electronics on printed circuit board as shown in step  210 . Finally remaining space in protective cover  89  is filled with wax, as shown in step  211 . 
     An alternative method for quickly attaching a strain gauge to the substrate is to use an ultraviolet (UV) light curable epoxy. These epoxies are advantageous in that they are cured to provide a strong bond in a matter of seconds with exposure to UV light. They have advantage in that, before exposure to the UV light, the strain gauge can be re-positioned as needed, and then a few seconds exposure fixes the gauge in place. A potential problem is that UV light cannot penetrate the polyimide materials commonly used in strain gauge construction. However, the present inventors found that fiberglass resin backed strain gauges used for high performance transducers become clear when UV epoxy is placed on their backing, transmit UV, and allow UV curable epoxy to be used. 
     In preliminary experiments the present inventors bonded several fiberglass resin backed strain gauges from Micro-Measurements, Inc., Atlanta, Ga., with a UV curable epoxy from Epoxy Technology, Inc. Destructive testing of the glue line indicated that a strong bond had been achieved beneath the strain sensing elements. However, testing showed delamination and that the epoxy had not been cured beneath the large copper bonding tab areas. Clearly the UV light did not reach these areas. The present inventors designed a custom strain gauge with windows or stripes of metal in the copper bonding pad area to let sufficient UV light through to cure the epoxy in these areas, as shown in  FIG. 2   b . In addition to soldering, attachment of lead wires to these pads can be accomplished with electrically conductive UV curable epoxy, available from Allied Chemical Co., division of Honeywell, Plymouth, Minn. 
     In the next step in the packaging process protective cover  89  is provided and mounted on surface  28  of structure  30  to enclose sensor assembly  32  and circuit board  59  with its antenna  58 , as shown in step  204  and in  FIG. 7 . Preferably, cover  89  is fabricated of a clear polycarbonate material. Protective cover  89  can include high strength aggressive contact adhesive tape  112 , available from 3M Corp., Minneapolis, Minn., on its bottom edges for securing to surface  28  of structure  30 . The technician doing the mounting will remove the protective polyethylene film (not shown) covering adhesive tape  112  and visually align battery compartment plug  114  with its mate battery connection header  115  on circuit board  59 . Battery compartment plug  114  is wired to battery compartment  116  into which battery  118  can be inserted and sealed with O-ring seal  120  on threaded battery cover  122  that encloses battery  118  in threaded hole  124 . Cover  89  will then be pressed onto surface  28  of structure  30  to provide a high strength bond there between. 
     To maintain a long life for battery  118 , the power management paper describes techniques to reduce power consumption, extending the life of battery  118 . These energy saving strategies are also useful when energy harvesting systems are deployed, such as those describes in the ′223 patent application to Hamel and the ′642 patent application to Arms. The energy harvesting methods could be used to eliminate battery  118  and energy can be stored on a capacitor, as described in these patent applications. The present inventors found that low leakage electrochemical batteries exhibited characteristics that were favorable for use with energy harvesting. Battery  118  can be a rechargeable battery and energy harvesting can be used to recharge battery. Alternatively, electromagnetic energy can be provided to recharge battery  118  as described in the ′224 application. Alternatively, a charger can be plugged into the sensor node to charge battery  118 . 
     The ′642 application also provides a scheme for performing automatic and wireless shunt calibration and for adjusting offsets and gains wirelessly. 
     Next, wax moisture barrier  130  is provided to protect components on circuit board  59  and sensor assembly  32  including strain sensor  22  and capacitance sensor  20  as shown in  FIGS. 7 ,  8 . It is well known that microcrystalline wax is the most effective organic barrier material currently available for protecting strain gauge circuits from moisture. Wax is reported to be superior to butyl rubber and silicone rubber because both of these materials absorb moisture from the environment while wax rejects moisture. But there are several disadvantages of using wax barriers, including weak mechanical properties, a tendency to become brittle at extremely low temperatures, and a low melting point of 170 deg F. or 80 degrees C. 
     The present inventors found that problems associated with the weak mechanical strength of wax  130  could be avoided by providing wax  130  inside polycarbonate protective cover  89 ,  110  to control and protect wax  130  from mechanical damage as shown in  FIG. 8 . They provided injection gun  132  filled with liquid polycrystalline wax  130  to fill protective cover  89 ,  110  and encapsulate sensor assembly  32  and electronics on circuit board  59  inside cover  89 ,  110  after cover  89 ,  110  has been mounted to surface  28  of structure  30 . Inlet filling tube  134  is connected to threaded wax inlet hole  136  of protective cover  89 ,  110  using polytetraflourethylene (Teflon) tubing. Molten wax  130  is injected into cover  89 ,  110  through inlet hole  136  until cover  89 ,  110  is visually full of wax  130  and wax  130  begins to be extruded out of outlet hole  138  and into outlet tube  140  in cover  89 ,  110  as shown in  FIG. 8 . 
     The present inventors found that a variety of protective materials can be used, including wax, grease, a foam protective agent, and anticorrosion formulas, such as ACF-50. Thin film 150 of wax, grease, Waxoyl or anticorrosion formula is shown in  FIGS. 9   a - 9 c. Waxoyl is available from Waxoyl AG, Basel, Switzerland. ACF-50 is available from Lear Chemical Research Corp, Mississauga, Ontario, Canada. 
     The foam protective agent can be a urethane expanding foam, which can be obtained from a manufacturer, such as Fomo Products, Inc., Norton, Ohio. This urethane foam is available in many forms although a 2 component aerosol would be easiest to use in this application due to the 2 minute cure time and its ability to be sprayed through a long tube into the enclosure opening. This material is water proof, expanding, bonds to many surfaces, and is slightly flexible. The expansion will ensure that all of the components including the strain gauge and electronics are thoroughly coated. 
     In addition, vent  170 , such as a vent provided by W. L. Gore and Associates, Inc., Newark, Del., may be provided to provide pressure equalization without allowing moisture to pass, as shown in  FIGS. 10   a ,  10   b ,  10   c . 
     Protective cover  160  has a rounded convex outer surface with curvature extending from the structure surface to which it is attached, as shown in  FIG. 10   a . This shape facilitates transfer of many impact loads that might shear the adhesive bond into compression loads. Thus a falling object is less likely to cause breakage of adhesive tape  112  holding protective cover  160  to the structure surface. Protective cover  160  includes printed circuit board  59  mounted to mounting bosses  164  on bottom surface  166  with screws  168 , as shown in  FIG. 10   b . Protective cover bottom surface  166  includes a planar portion that is for mounting on the structure surface. Other than at a penetration extending through the protective cover outer surface, such as battery compartment  116 , all sidewalls of the protective cover outer surface have the rounded convex outer surface with curvature extending from the structure surface to which it is attached. 
     Alternatively, as shown in  FIGS. 7 and 8 , other than at a penetration extending through the sidewalls of protective cover  89 ,  110 , such as for battery compartment  116 , threaded wax inlet hole  136 , or outlet hole  138 , all parts of the sidewalls of protective cover  89 ,  110  are tilted, and the tilting sidewalls extend toward each other from surface  28  of structure  30 . 
     Protective cover  160  also includes adhesive tape  112  for adhesively attaching protective cover  160  to a structure surface, as shown in  FIG. 10   c . Inlet hole  136  and outlet hole  138  are provided as described herein above, as shown in  FIG. 10   a . Inlet hole  136  and outlet hole  138  may be threaded to accommodate a plug and o-ring for sealing purposes after filling is complete. Vent  170 , such as a vent provided by W. L. Gore and Associates, Inc. Newark, Del., can be provided as well. 
     Battery compartment  116  is also provided with its cover  122  and o-ring seal  120 . Positive return  172  extends from the positive terminal of battery  118  to printed circuit board  59 . Spring  174  for the negative terminal of battery  118  is also provided. 
     While the disclosed methods and systems have been shown and described in connection with illustrated embodiments, various changes may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. 
     The examples given are intended only to be illustrative rather than exclusive.