Abstract:
A system for measuring a mixture ratio associated with a two-part fluid, at least one of the two parts including conductive particles. The system includes a pair of electrodes, a circuit, and an output. The electrodes are disposed on opposite sides of the two-part fluid and sense the dielectric strength of the fluid. The circuit communicates with the electrodes to sense the dielectric strength and outputs a signal representing the dielectric strength. Preferably, the circuit includes a capacitive bridge, an input for a set-point, and an output for an error signal. Also, the system can include a housing for the electrodes. A timer may also be provided to measure the time elapsed from the beginning of the mixing of the two-part fluid. Preferably, the two-part fluid is an adhesive with aluminum particles that is made from a resin and a hardener. Methods of producing two-part fluids are also provided.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to mixing multi-part fluids and, more particularly, to mixing two-part fluids in which one of the two parts includes conductive particles.  
       BACKGROUND OF THE INVENTION  
       [0002]     Two-part adhesives provide high strength joints that require little, if any, machining to assemble. Typically, the two-part adhesive is made at or just before the time the adhesive must be applied to the mating surfaces of the joint. Appropriate quantities of the two parts are brought into contact and mixed thoroughly. Once mixed, the two-part fluid begins to harden (or cure or set) and must be applied to the joint before the cure becomes too advanced. Accordingly, the two-part adhesive is usually applied to one of the surfaces immediately and the two surfaces are clamped together for the required time to cure the adhesive. After the adhesive cures, the surfaces are unclamped and the assembled joint is used in higher level assemblies.  
         [0003]     In some applications the adhesive joint must conduct either electricity, heat, or both electricity and heat while carrying a load. Because of the nature of the compounds formed in these two-part fluids it is unlikely that an adhesive can be found with both the desired load carrying properties and the desired thermal or electric conductivity for any given application. To provide the desired conductivity, conductive particles are frequently introduced into one of the two pre-mix parts of the adhesive. The concentration of particles is pre-determined so that when the two parts are mixed, the particles are of a sufficient concentration to provide the desired conductivity. Frequently, though, the mixture ratio of the two-part fluid varies due to disturbances in the mixing system and other sources of error. When the mixture ratio varies from the optimum, the concentration of the conductive particles also changes. As a result, the conductivity of the joint is affected. Likewise, the load carrying capabilities of the joint can change also.  
         [0004]     In many applications, an automated mixer is used to mix batches of the two-part adhesive. Typically, the flow rate of each of the two parts of the mixture is determined at the beginning of the production run and again at the end of the run. While these spot checks detect some variations from the optimum mixture ratio, the spot checks do not continuously monitor the system. If a variation occurs between the initial and final checks, non-optimum adhesive can be created and applied to the joint(s) being made with the adhesive.  
         [0005]     Because the mixture ratio influences the properties of the joint, monitoring the mixture ratio of two-part fluids in real time and on a continuous basis would improve quality and reduce post-cure inspection processes.  
       SUMMARY OF THE INVENTION  
       [0006]     The invention provides apparatus and methods for monitoring the mixing of multi-part fluids. In a first preferred embodiment, the present invention provides a system for mixing a two-part epoxy adhesive. The system includes an automatic mixer that has two reservoirs, one for the resin and one for the hardener. Pumps force the two materials through metered nozzles and then into a static mixing tube. At the end of the static mixing tube a sensor monitors the mixture ratio of the mixed two-part fluid to ensure that the adhesive is at the proper mixture ratio. A hose can be attached to the end of the static mixing tube to pump the adhesive to the bond locations of the joint. More particularly, the invention provides an in-line mix monitor for use with Hysol® EA 9394 two-part epoxy available from the Henkel Loctite Corporation of Dusseldorf, Germany. This embodiment takes advantage of the fine particles of aluminum that the EA 9394 epoxy hardener contains to sense the mixture ratio of the mixed EA 9394 epoxy. These principles apply to any multi-part fluid (e.g. a potting material) in which one part contains conductive material.  
         [0007]     In a second preferred embodiment, the present invention provides a system for measuring a mixture ratio associated with a two-part fluid, in which at least one of the two-parts includes conductive particles. The system includes a pair of electrodes, a circuit, and an output. The electrodes are disposed on opposite sides of the two-part fluid and sense the dielectric strength of the fluid. The circuit communicates with the electrodes to sense the dielectric strength and outputs a signal representing the dielectric strength. Preferably, the circuit includes a capacitive bridge, an input for a set-point, and an output for an error signal. Also, the system can include a housing for the electrodes. A timer may be provided to measure the time elapsed from the beginning of the mixing of the two-part fluid. Preferably, the two-part fluid is an adhesive made from a resin and a hardener, one of which contains conductive particles.  
         [0008]     In a third preferred embodiment, the present invention provides a two-part fluid producing system. The system includes a source for the first part of the two-part fluid, a source for a second part of the fluid, a mixer, a sensor, and an output. In the present embodiment, the first part of the fluid contains conductive particles which, preferably, are aluminum. The mixer communicates with both sources to mix the two parts. The sensor communicates with the mixer, receives the mixed two-part fluid, and senses the dielectric strength of the fluid. The sensor also outputs a signal that is representative of the sensed dielectric strength. Preferably, the system includes an input for a set-point and a fluid control device that adjusts the amount of one of the parts of the two-part fluid based on the set-point and the dielectric strength of the fluid. A timer for measuring the elapsed time from the beginning of the mixing operation may also be included in the system.  
         [0009]     In yet another preferred embodiment, the present invention provides a method of producing a two-part fluid. The method generally includes mixing the parts of the fluid, sensing the dielectric strength of the mixed fluid, and determining a mixture ratio from the sensed dielectric strength. Preferably, the mixing is adjusted based on the determined mixture ratio and a pre-selected set-point. The time since the mixing began may also be measured.  
         [0010]     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate exemplary embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings:  
         [0012]      FIG. 1  illustrates a structure assembled with a two-part adhesive that was mixed in accordance with the principles of the present invention;  
         [0013]      FIG. 2  illustrates a cross section of the structure of  FIG. 1  taken along the line  2 - 2 ;  
         [0014]      FIG. 3  illustrates a system constructed in accordance with a preferred embodiment of the present invention;  
         [0015]      FIG. 4  illustrates a circuit constructed in accordance with another preferred embodiment of the present invention;  
         [0016]      FIG. 5  illustrates a method in accordance with yet another preferred embodiment of the present invention; and  
         [0017]      FIG. 6  illustrates the results of a test using the circuit of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     Referring to the accompanying drawings in which like reference numbers indicate like elements,  FIG. 1  illustrates a joint constructed in accordance with the principles of the present invention.  
         [0019]     During the assembly of an aircraft it is often desirable to use room temperature paste bonding (with, for example, an epoxy adhesive) of the primary aerospace structures rather than fasteners for assembly. The advantages of using paste bonding include a reduction in the overall weight of the assembled aircraft and a reduction in the expense of assembling the aircraft. If the resin-hardener mixture ratio (for the epoxy adhesive) is outside of a pre-selected band, the bond will perform in a non-optimal manner. Therefore, the present invention provides apparatus for, and methods of, continuously monitoring the mixture ratio of multi-part fluids and, more particularly, two-part adhesives.  
         [0020]     A joint  10  constructed in accordance with the principles of the present invention is illustrated by  FIG. 1 . The joint  10  includes two structures which, in  FIG. 1 , include a rib  12  and a panel  14 . The structures  12  and  14  may be of any material suitable for adhesive bonding with the surfaces to be bonded prepared by, for example, cleaning, degreasing, or surface roughening. Between the rib  12  and the panel  14  a layer of adhesive  16  bonds the two structures together so that the adhesive  16  carries a load between the two structures  12  and  14 . Due to operational considerations it may also be desired that the joint  10  allow heat or electricity to be conducted across the adhesive  16 . Thus, the designer of the joint  10  typically calls for a two-part adhesive with conductive particles  18  (see  FIG. 2 ) to be employed as the adhesive  16 .  
         [0021]      FIG. 3  schematically illustrates a system for mixing a two-part fluid that is constructed in accordance with the principles of the present invention. For epoxies, the system  100  includes a resin reservoir  102 , a hardener reservoir  104 , a static mixer  106 , a hose with applicator  108 , a dielectric strength sensor  110 , a controller  112 , and a pair of fluid control devices (e.g. valves or metered pumps, orifices, or nozzles)  114  and  116  associated with the two reservoirs  102  and  104 . The resin reservoir  102  and the hardener reservoir feed resin and hardener, respectively, into the mixer  106  via the fluid control devices  114  and  116 . The fluid control devices are configured to provide the resin and hardener at flow rates that ensure that the mixture ratio in the mixer  106  is at a preselected set-point. As the resin and hardener flow into the mixer  106  the mixer  106  begins mixing the two components (i.e. parts) of the two-part fluid. The mixing proceeds as the two parts flow through the length of the mixer  106  which is configured to ensure that complete mixing of the two parts occurs before the fluid exits the mixer  106 .  
         [0022]     Variations may occur in the flow rate of one, or both, of the resin and the hardener. Therefore, the system  100  includes the dielectric strength sensor  110  at the discharge of the mixer  106 . The sensor  110  senses the dielectric strength of the mixed two-part fluid that flows between a pair of electrodes held in a spaced-apart relationship by a known distance. Because the distance between the electrodes and the configuration of the electrodes are known, the only variable that influences the capacitance of the sensor  110  is the dielectric strength of the two-part fluid between the electrodes. Further, because the dielectric strength of the fluid does not depend noticeably on the mixture ratio of the two-part fluid (absent the conductive particles), the only source of variation of the capacitance of the sensor  110  is the concentration of the conductive particles in the mixed fluid. Since the concentration of the conductive particles depends on the mixture ratio it is possible to determine the mixture ratio based on the sensed dielectric strength. Thus, the sensor  110  monitors the mixture ratio of the two-part fluid and sends a signal representative of the mixture ratio to the controller  112 . In turn, the controller compares the dielectric strength (or actual mixture ratio) to a user selected set-point and generates an error signal to drive the control members  114  and  116 . As a result, the mixture ratio of the two-part fluid is under real-time, continuous, closed loop control. If the mixture ratio deviates from the set-point, the controller  112  can generate an audible or visual message to the operator and can even log the event.  
         [0023]     While the exemplary adhesive  116  is a two-part adhesive with conductive particles  118 , the mixing of any multi-part fluid with conductive particles in at least one of the parts can be monitored in accordance with the principles of the present invention. For instance, the mixing of two-part polyurethanes, RTV rubbers, silicones, and acrylics, can be monitored in accordance with these principles. This list is not exhaustive and is not intended to limit the scope of the present invention. Nor is the scope of the invention limited to multi-part adhesives. The multi-part fluids of the present invention can be used to form coatings, elastomers, molded products, and many other products without departing from the scope of the present invention. The conductive particles may be made of any electrically conductive material. Aluminum is preferred in one embodiment although the particles can be made from any metal. Many of these two part fluids are commercially available and come with the conductive particles mixed into one of the parts. In the alternative, the user can mix the conductive particles into one part without departing from the scope of the present invention. The choice of materials (i.e. the parts of the fluid and the material of the conductive particles) is based on the end use to which the mixed fluid will be applied. Likewise, the concentration of conductive particles in the one part is determined generally by the requirements associated with the end use of the two part fluid (e.g. how much electrical resistance is to be allowed across a given adhesive joint). The system  100  is then adjusted to detect variations in the concentration of the particles (of the selected material) in the mixed fluid at the desired mixture ratio.  
         [0024]     With reference now to  FIG. 4 , a circuit  112  is schematically illustrated that includes, or connects to, the sensor  110  of  FIG. 3  and related electronics. Generally, the circuit  112  of the current preferred embodiment includes the sensor  110 , a capacitive bridge  118 , a volt meter  120 , a waveform generator  122 , and a signal amplifier  124 . The sensor  110  further includes a pair of electrodes  126  and  128  and a volume between the electrodes for a dielectric. When installed in the system  100  of  FIG. 3 , the sensor is configured so that the mixed two-part fluid from the mixer  110  fills the dielectric volume  130 . Also, the pair of electrodes  126  and  128  is connected to the capacitive bridge  118  as the variable capacitor to be sensed by the volt meter  120 . The waveform generator  122  and signal amplifier  124  are connected in series and are further connected to the capacitive bridge  118  and sensor  110  in a manner to impose a varying voltage across the sensor  110 . Because of the varying dielectric strength of the fluid in the sensor  110 , a varying voltage difference indicative of the capacitance of the sensor  110  will develop across the bridge  118  and be sensed by the volt meter  120 . Thus, the circuit  112  allows the dielectric strength of the as-mixed, two-part fluid to be determined. As a result, the mixture ratio of the two-part fluid can be determined from the sensed voltage.  
         [0025]     With continuing reference to  FIG. 4 , an alternative embodiment of the present invention also includes a microprocessor  126 , a timer  128 , and a signal conditioner  130 . The signal conditioner  130  is connected to allow the microprocessor  126  to sense the voltage measured by the volt meter  120 . The timer  128  allows the microprocessor to determine the time elapsed from the time that mixing of the two-part fluid began. As will be seen, the elapsed time can also bear on the determination of the mixture ratio. In the alternative, the timer  128  is obviated by knowing how long it takes the fluid to flow from the initial mixing point in the mixer  106  to the sensor  110 . The time required for the fluid to travel between that point and the sensor  110  may be determined empirically or determined from the measured flow rates of the two parts (or of the two-part fluid).  
         [0026]     Turning now to  FIG. 5 , a method in accordance with a preferred embodiment of the present invention is illustrated. The method  300  generally includes mixing a two-part fluid, sensing the mixture ratio of the fluid, and adjusting the mixing process to maintain the mixture ratio at a set-point. More particularly,  FIG. 5  shows the two-part fluid being mixed in operation  302  and the dielectric strength of the mixed fluid being sensed in operation  304 . In operation, it has been found that the sensed dielectric strength decreases exponentially as the time from the beginning of the mixing increases. Therefore, it has been found helpful to begin a timer to measure the elapsed time since the mixing began as indicated at reference  306 . From the sensed dielectric strength and the elapsed time, the mixture ratio can be determined. See operation  308 . A comparison can be made between the mixture ratio of the mixed fluid and the desired set-point as in operation  210 . If necessary, or desired, the method  300  may repeat the steps  302 ,  304 ,  306 ,  308 ,  310 , and  312  until the mixing of the two-part fluid is discontinued. See operation  314 .  
         [0027]     A test was performed to confirm that it is possible to sense the difference between the EA 9394 adhesive that is mixed properly and adhesive which is not mixed properly. A mixture ratio of hardener to resin of 17/100 (0.17) was used as the desired mixture ratio. Two other mixture ratios of the EA 9394 hardener and ratio were tested, namely 0.05 and 0.34. A sample of adhesive for each of the three mixture ratios was mixed thoroughly and injected into separate but otherwise identical test cells. The dielectric strength of each sample was measured over time using the capacitive bridge  118  of  FIG. 4  operating with a 10 kHz sinusoidal signal as supplied by the waveform generator  122  and signal amplifier  124 . The peak-to-peak voltage across the bridge  118  was measured with the volt meter  120 .  FIG. 6  shows the results of the test.  
         [0028]     The x-axis  492  of the graph  400  of  FIG. 6  indicates the elapsed time from the beginning of the mixing of the samples. The y-axis shows the peak-to-peak voltage across the bridge  118 . The curves  405 ,  417 , and  434  show the results for the 0.05, 0.17, and 0.34 mixture ratio samples, respectively. As is apparent, there is a characteristic decay in the voltage as a function of time from the initiation of the mixing. Despite the decay, a large voltage difference exists between the curves  405 ,  417 , and  434  for at least the first hour and longer. Thus, both the voltage and the elapsed time can be measured to determine whether the measured voltage for a given sample is tracking on the  417  curve (or other curve corresponding to a desired mixture ratio). Note should also be made that the curves  405 ,  417 , and  434  indicate that within the first few minutes the circuit  112  was able to detect a 2 mV difference in the measured voltage per a 0.01 change in the mixture ratio. This result is sufficiently detectable that the difference in mixture ratios may be sensed with commercially available instrumentation.  
         [0029]     Thus, an in-line adhesive mixing monitor such as the sensor  110  of  FIGS. 3 and 4  can be employed to monitor the mixture ratio of a two-part fluid. Further, the monitoring can be performed continuously and in real time. In a preferred embodiment, the sensor  110  can be constructed from a pair of electrodes and a housing which holds the electrodes a pre-selected distance apart with the two-part fluid flowing between the electrodes. If the fluid system is pressurized, the housing can also be configured to contain the pressure while holding the electrodes in the spaced-apart relationship. Appropriate fluid fittings can also be included with the housing. A capacitive bridge may then be connected across the electrodes and, with shielding to prevent extraneous signals from interfering, used to sense the dielectric strength of the fluid. Additionally, a microprocessor, such as the microprocessor  126  of  FIG. 4 , may be employed to sense the dielectric strength (or the peak-to-peak voltage across the sensor  110 ) of the two-part fluid, determine the elapsed time (from timer  128 ), and determine the mixture ratio.  
         [0030]     In view of the foregoing, it will be seen that the several advantages of the invention are achieved. More particularly, apparatus and methods have been provided to determine continuously, and in real time, the mixture ratio of a two-part fluid. Further, the joints created with the two-part fluid have been improved because the mixture ratio of the fluid can now be held at a pre-selected set-point to ensure that the optimal load-bearing and conductive properties of the two-part adhesives are optimal. Because the adhesive remains optimal throughout its application to the joint, the amount of adhesive used can be reduced to lighten the joint without sacrificing joint strength. Accordingly, the payload carrying capacity of mobile vehicles (e.g. aircraft or spacecraft) constructed in accordance with the principles of the present invention can be increased.  
         [0031]     The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.  
         [0032]     As various modifications could be made in the constructions and methods herein described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.