ELECTRIC CIRCUIT ASSEMBLY APPARATUS

Abstract: A sensor buoy, such as a sonobuoy, is described as one example of apparatus comprising an electrical circuit assembly having a housing surrounding the assembly, the housing com­ prising a thermoplastic shell blow-moulded around the circuit assembly. The described sensor buoy has a terminal weight and a sensor for deployment beneath the surface of the water after the buoy impacts the water with a cable connecting the sensor to the circuit assembly. The circuit assembly is arranged to process the output of the sensor. An antenna is arranged to transmit a signal corresponding to the output.

Description: ELECTRIC CIRCUIT ASSEMBLY APPARATUS Description This invention, in its broadest context, relates to apparatus incorporating an electric circuit assembly. Such apparatus may take many forms and be used in many industries. For reasons such as the provision of strength or protection or in the provision of the apparatus as a construction made up of smaller sub-assemblies, there will usually be a housing in which the circuit assembly is located. As will become clear from the detailed description which follows we have found that in the particular example of a sensor buoy, one example of such electric circuit assembly apparatus, there are particular advantages in the fabrication of the housing by a particular process, namely blow-moulding, which, so far as we are aware has never previously been proposed for electric circuit assembly apparatus. Though these advantages are of particular importance for sensor buoys typically launched from aircraft and having a sensor carried within the buoy until water impact which is then deployed beneath the water surface, signals from the sensor being transmitted to the surface, and typically back to the aircraft, by underwater cable and floating transmitter, the arrangement being particularly useful for the detection of submarines, in which case the sensor will be an acoustic device, these same advantages have a much more general applicability. Accordingly, in one aspect of the present invention, we provide apparatus comprising an electrical circuit assembly and a housing surrounding said assembly, characterised in that said housing comprises a thermoplastic shell blow-moulded around said circuit assembly. In the preferred arrangements described in detail hereinbelow, the apparatus comprises a sensor buoy, such as a sonobuoy, having a terminal weight and sensor for deployment beneath the surface of the water after said buoy impacts the water, a cable connecting said sensor to said circuit assembly; and said circuit assembly is arranged to process the output of said sensor, and an antenna is arranged to transmit a signal corresponding to said output. In a second and alternative aspect of this invention, there is provided a method of fabricating a sensor buoy such as a sonobuoy, comprising the steps of: installing a circuit assembly on a ballast cup or other base; extruding a tube of molten thermoplastic material fully around said circuit assembly and base; and moulding said tube between sections of a mould and injecting high pressure gas into the interior of said material so as to blow-mould a thermoplastic shell around said circuit assembly. In preferred embodiments, the blow-moulded housing forms a seamless, watertight flotation compartment for the circuit assembly, the terminal weight and sensor (e.g., hydrophone) are contained within a ballast cup at the base of the housing and are deployed upon water impact; the blow-moulded housing is moulded tightly around the exterior of the ballast cup to provide a water tight seal, even during water impact; the circuit assembly consists of a vertically-oriented circuit board supported on the ballast cup; an air space separates the board from the housing (which povides insulation from the heat of the blow-moulding process); an antenna is supported on a tab extending through the blow-moulded housing at its top; water cooling is supplied to the moulds used in the blow-moulding process to reduce the temperature elevation of the electronic components; annular grooves are provided on the exterior of the ballast cup for improving the water tightness of the seal to the blow-moulded housing; and aerodynamic fins are moulded into the top of the housing for aerodynamic orientation. An antenna attached to the buoy housing is suitably adapted to be deployed during the free-fall of the buoy, prior to its water impact. In preferred embodiments, the antenna is wrapped around the housing; the housing includes a shallow groove for receiving the wrapped-around antenna; the antenna is a blade of greater width than thickness; and the antenna is adapted to provide aerodynamic drag and stability during the buoy's fall. A water-impact-actuated release means is suitably provided comprising a plate extending across the bottom opening of the ballast cup and adapted to release the terminal weight and sensor upon water impact but not upon high inertial loading. In preferred embodiments, a weakened region is provided near one tab and the terminal weight is spaced from the plate in the vicinity of that tab so that inertial loading on the weight is resisted by unweakened regions of the plate and so that space is provided between the plate and the weight in the vicinity of the weakened region to enable the water impact force to push the plate inward and thereby break the plate at the weakened region. The buoy may have an extended spheroid housing, i.e., one having a cylindrical mid section and semispherical end portions. In preferred embodiments, the antenna is connected to the top semispherical portion; the bottom semispherical portion has a flat surface generally at its centre for causing the buoy to dig into the water upon water impact rather than to skip along the surface. The teachings of the present invention have numerous advantages, both in relation to buoys and to other forms of apparatus. They can provide a simpler and less costly construction, and one that, in the case of a buoy, can safely be allowed to fall freely (without a parachute or similar drag device) to the water surface. Forming the housing by blow-moulding creates a seamless structure with no high stress points; and it avoids the problem that two-piece construction housings have of fracturing, for example, upon impact. In the case of buoys, the need for reinforcement bands, which complicate the manufacturing process and add to the weight of a sonobuoy, is eliminated; and suprisingly the blow-moulding process, even through an extremely hot process, does not harm the temperature sensitive electronic components. Around the ballast cup of a buoy, blow-moulding provides an excellent seal, one that withstands water impact. Because the buoy antenna is released at launching, it provides additional drag and stability during free flight to ensure that the buoy impacts the water at the desired velocity and in a nose down orientation so that the shear plate will break; launch release also eliminates the need for an antenna release mechanism at impact, which complicates the buoy's manufacture and increases its weight. The extended spheroid shape of the buoy housing also has advantages; it provides a high centre of buoyancy and low centre of gravity to reduce the sonobuoy's rolling motion and provide a more stable platform for transmission of the alarm signal, it provides high drag and stability during free fall, it has good impact strength, and the preferred flattened bottom prevents the buoy from skipping across the water at initial impact. The invention is hereinafter more particularly described by way of example only with reference to an embodiment of sensor buoy, but it is to be understood that the invention is not restricted to this particular form of apparatus alone. A detailed description of the fabrication, structure and deployment of a preferred embodiment of sensor buoy is hereinafter described by reference to and as shown in the accompanying drawings, in which: Figure 1 is a group of four diagrammatic views of the deployment of said embodiment into the water; Figure 2 is an exploded perspective view of said embodiment; Figure 3 is a sectional view of the ballast cup and hydrophone/terminal weight assembly of the apparatus of Figures 1 and 2; Figure 4 is a sectional view taken a#long the line 4-4 of Figure 3 showing the sonobuoy after the shear plate has broken; Figure 5 is a block diagram of the signal processor of said embodiment; and Figures 6a to 6g are perspective views of steps in the manufacture of said embodiment. Referring to Figures 1 and 2, there is shown a sonobuoy consisting of a printed circuit board 1, ballast cup 2, omni-directional hydrophone 3, terminal weight 5, shear plate 7, battery pack 10, transmitting antenna 12, and polyethylene buoy housing 14. Mounted to the printed circuit board is a conventional transmitter, microprocessor chip 31 (TMS 1000, Texas Instruments), integrated circuit 30, and associated discrete components. Three flat pack batteries (6 Volt Polaroid P-80 Polapulse) are contained in case 22 that snaps on to the printed circuit board. Antenna tab 13 is riveted to the top of the board. The board is riveted at the bottom to the ballast cup. At the same time, rubber plug 4 attached to the bottom of the board is inserted through a hole in the top surface of the ballast cup. This rubber plug acts as a seal when the buoy is in the water. Also, the plug houses two salt water electrodes 9 that are connected to the circuit board and that protrude from the plug's bottom, The hydrophone consists of a flexural transducer, which is a small thin piezoelectric ceramic disc 6 mounted on a brass plate. The ceramic disc surface is silvered and an electrode is soldered to this surface. Another electrode is soldered to the brass ring. One end of each of two conductors in cable 11 is connected to each electrode. The transducer is placed on a plastic ring which is cemented into the zinc terminal weight and then potted over. The plastic ring and potting serve to protect and isolate the connections. Around the outside of the terminal weight is wound at least 300 feet of three-strand cable. Two strands are conductors for carrying the output signal of the hydrophone. The third strand is Kevlar (Dupont), a high strength-to-weight ratio polymer. Only a very thin (200 Denier) strand of Kevlar is needed, thus reducing the cross section of the cable and thus its drag in the water. The cable at the outside end of the spool is fed through rubber grommet 8 that fits into a hole in the ballast cup and the two wires contained in the cable are connected to the circuit board. The terminal weight/hydrophone assembly is placed inside the ballast cup, and shear plate 7, Qith its two retaining tabs, is inserted into grooves on the inside of the ballast cup. As shown in¯ Fig. 3, the left retaining tab of the shear plate supports the terminal weight and will prevent the terminal weight from prematurely falling out of the ballast cup due to inertial forces during handling before deployment, The right retaining tab is scored at 40 (Figs. 2 and 4) and is spaced below the terminal weight as a result of the tapered shape of the bottom of the weight. The spacing between the plate and the weight in the vicinity of the weakened (scored) region allows the plate to be pushed inward at water impact; thus tearing or rupturing the plate. On the other hand, the space prevents inertial loads on the weight from being applied to the weakened region, thus assuring that the plate will not accidentally rupture during handling. To form the buoy housing, the printed circuit board and ballast cup assembly are placed (Fig. 6a) on a support stand (not shown). While the two halves of housing mold 19 are spaced apart, extruder 20 extrudes a tube of high density polyethylene around the electronicsand ballast/hydrophone assembly (Fig. 6b). The thickness of the tube varies from 0.100 inches at the bottom to 0.075 inches at the top. Next, the two halves of the mold are pressed together (Fig. 6c), compressively molding the polyethylene around the base of the ballast cup and around tab 13 at the top of the printed circuit board. A needle 21 is now inserted into the pinched off tube and high pressure air is blown into the tube, forcing the tube to assume the shape of the inside of the mold. This blow molding operation is a hot process. To keep the maximum temperature to which the components on the circuit boards are exposed below their maximum temperature of 1850, the molding is done quickly and the molds themselves are water cooled. The result òf the blow molding operation is a one piece seamless housing with no structural weak points. The housing is trimmed (Figs. 6d-6f), leaving an extended spheroid (cylindrical middle section with semispherical upper and lower ends) and two stabilizing fins 15. Two scuttle plugs 16 (Fig. 2), made of Polyox (DuPont), a water soluble plastic, are pressed into holes formed after molding in the midsection of the housing. Steel antenna 12 is then riveted (Fig. 6f) to tab 44 which is made from the same antenna material and which extends from the circuit board 1. (The full antenna is not installed prior to molding to reduce the length of extruded polyethylene required.) A shallow groove 42 (Fig. 1) is formed during molding completely around the outside of the housing to form a receptacle for the steel antenna when it is wrapped around the housing and inserted in launching cylinder 17 (Fig. 6g). The components of the signal processor, which controls the detection operation of the sonobuoy, are shown in Fig. 5. The hydrophone signal is preamplified, and then filtered and amplified to leave only the signal and noise in the frequency band of interest. Any DC bias is subtracted out through the use of a microprocessor-controlled autobalance circuit and the resulting AC signal is rectified. When the amplitude of the AC signal at the analog-to-digital converter input is greater than a threshold voltage stored in the microprocessor, the latter will modulate the transmitter via the alarm output, and the transmitter will send out this signal via'the antenna. This will continue for as long as the target is present, or 15 minutes, the life of the batteries. If the transmitter is never used, the batteries will operate the rest of the circuitry for about four hours. The buoy is 4.5 inches in diameter, 5.6 inches in height, and 1.4 lb in weight. Its center of gravity is 1.7 inches from its base, and its center of buoyancy is 1.95 inches from its base. The antenna is 18 inches long and 0.5 inches wide. Operation The sonobuoys are deployed as shown in Fig. 1. One buoy at a time is launched from a cylinder (each of which contains six buoys) carried by an aircraft. Antenna 12 springs back to its relaxed position upon the sonobuoy's exit from the cylinder, and the antenna acts as a tail to provide, along with the fins, drag and stability for the sonobuoy as it falls freely to the water. The fins are particularly helpful to initially orienting the buoy immediately after launch. Upon impact, the scored right retaining tab 40 of the shear plate 7 (Fig. 3) will break. This releases the hydrophone/terminal weight assembly from the ballast cup, allowing the assembly to fall down into the water until the spool of cable completely unwinds. The release of the assembly will also expose the salt water electrodes to the salt water, completing a circuit that turns on the power supply. The signal processor initializes the system by setting the initial gain of the integrator and the initial threshold level. The sonobuoy is now ready for its normal mode of operation. In its normal mode of operation, the sonobuoy listens for submarines by measuring the ambient noise detected by the hydrophone across a frequency range. When the detected noise rises above a stored threshold level, the transmitter sends out an alarm signal as long as the target is present or until the battery power runs out. If no submarine is detected, the transmitter is silent. The scuttle plugs will erode after about eight hours, and the sonobuoy will sink. Persons skilled in the art of electric circuit assembly apparatus will have no difficulty envisaging other embodiments of such apparatus than the specific sensor buoy hereinabove described in detail with reference to the accompanying drawings. The blow-moulding process herein disclosed can readily be employed to enclose other types of circuit assemblies than those for use in a sensor buoy.

Claims: CLAIMS 1. Apparatus comprising an electrical circuit assembly and a housing surrounding said assembly, characterised in that said housing comprises a thermoplastic shell blow moulded around said circuit assembly. 2. Apparatus according to Claim 1, further characterised in that it comprises a sensor buoy, such as a sonobuoy, having a terminal weight and sensor for deployment beneath the surface of the water after said buoy impacts the water, a cable connecting said sensor to said circuit assembly; and in that said circuit assembly is arranged to process the output of said sensor and an antenna is arranged to transmit a signal corresponding to said output. 3. A buoy according to Claim 2, further characterised in that said blow moulded thermoplastic shell forms a water-tight flotation compartment for supporting said circuit assembly and antenna on the surface of the water. 4. A buoy according to Claim 2 or Claim 3, further characterised in that said housing contains said terminal weight and sensor prior to°their deployment. 5. A buoy according to Claim 4, further characterised in that said buoy is adapted to be ejected from an airplane and to fall to the water without a parachute or other drag device and in that there is provided a waterimpact-actuated release means for deploying said terminal weight and sensor. 6. A buoy according to Claim 4 or Claim 5, further characterised in that said housing comprises a ballast cup at its base, said blow-moulded shell being moulded around the exterior of said ballast cup and said terminal weight and sensor being stowed within said ballast cup prior to their deployment. 7. A buoy according to both Claim 5 and Claim 6, further characterised in that said release means comprises a plate extending across the bottom opening of said ballast cup and adapted to rupture upon water impact. 8. A buoy- according to Claim 6 or Claim 7, further characterised in that said circuit assembly includes at least one circuit board supported on said ballast cup and completely surrounded by said blow-moulded shell. 9, A buoy according to Claim 8, further characterised in that said circuit board is generally vertically oriented when said buoy is floating upright and is separated from said shell by an air space. 10. A buoy according to Claim 8 or Claim 9, further characterised in that a tab extends from said circuit board through said blow-moulded shell, said shell being moulded around said tab, and said antenna being connected to the portion of said tab extending outside of said shell. 11. Apparatus according to Claim 1, further characterised in that it comprises Åa sensor buoy, such as a sonobuoy; in that said housing is adapted to be ejected from an aircraft and to fall freely without the assistance of a parachute to the water surface; and in that said circuit assembly includes a transmitter, and is connected to an antenna attached to said housing, said antenna extending from said housing and being adapted to be deployed during the fall of said buoy. 12. A buoy according to Claim 11, further characterised in that said antenna and housing are adapted to permit said antenna to be stowed prior to ejection by wrapping the antenna around the exterior of said housing. 13. A buoy according to Claim 12, further characterised in that said antenna is a blade with a with much greater than its thickness. 14. A buoy according to Claim 13, further characterised in that said housing includes a groove around its exterior for receiving said blade antenna when said antenna is wrapped therearound prior to ejection. 15. A buoy according to any of Claims 2 to 14 further characterised in that said antenna is adapted tc provide aerodynamic drag and stability during the fall of said buoy to the water surface. 16. A buoy according to any of Claims 2 to 15 further characterised in that said circuit assembly inside said blow-moulded housing comprises all the electronic circuitry of said buoy. 17. A buoy according to any of Claims 2 to 16, wherein said buoy is a sonobuoy, and is further characterised in that said sensor is a hydrophone and said circuit assembly includes means for detecting the presence of a submarine from the output of said hydrophone and means for causing said transmitter to transmit an alarm via-said antenna to a remote receiver. 18. Apparatus according to any preceding claim, further characterised in that said blow moulded shell is moulded in one piece without seams. 19. A method of fabricating a sensor buoy such as a sonobuoy, comprising the steps of: installing a circuit assembly on a ballast cup or other base; extruding a tube of molten thermoplastic material fully around said circuit assembly and base; and moulding said tube between sections of a mould and injecting high pressure gas into the interior of said material so as to blow mould a thermoplastic shell around said circuit assembly and base. 20. A method according to Claim 19, further characterised in that said mould sections are cooled and said extruding and moulding steps are performed rapidly enough not to damage electronic components of said circuit assembly. 21. A sensor buoy such as a sonobuoy, said buoy comprising a terminal weight and sensor for deployment beneath the surface of the water after said buoy impacts the water, a.floatable housing containing said weight and sensor prior to their deployment upon impact and for remaining on the surface after deployment, a cable for connecting said sensor to said housing, and water-impactactuated release means comprising a plate extending across the bottom opening of said ballast cup and adapted to release said weight and sensor upon water impact but not upon high inertial loading. 22. A buoy according to Claim 21, further characterised in that said plate comprises first and second tabs extending into receptacles in said ballast cup; in that a weakened region is provided in the vicinity of said first tab; and in that said terminal weight is spaced from said plate at the vicinity of said first tab and weakened region and abuts said plate at the vicinity of said second tab so that inertial loads on said weight are resisted by the unweakened area of said plate in the vicinity of said second tab and so that water impact on said plate can press said plate inwardly at the vicinity of said first tab to thereby tear or rupture said weakened region and thereby deploy said weight and sensor. 23. A buoy according to Claim 22, further characterised in that buoy has a flat surface generally at the centre of its bottom surface for causing the buoy to dig into the water upon impact rather than to skip along the surface.

Inventors: GAGNON, DAVID P., HUDSON, ALAN T., JOHNS, DAVID W., LANGENHEIN, JAMES W., JR.
Applicants: SIPPICAN OCEAN SYSTEMS, INC.
Application year: 1984
Main classification: B63B22
IPC codes: B29C49, B63B22, G01D21