Pulse width modulated cable signal injecting apparatus

[0002] 1. Field of the Invention

[0003] The present invention relates generally to electrical signal injecting apparatus for use with an underwater cable. More specifically, the present invention relates to a portable water tight signal injecting apparatus for inducing a 1024 Hz signal into an underwater cable which allows the user to track the signal with a tracking probe tuned to the same frequency.

[0004] 2. Description of the Prior Art

[0005] Underwater cables are needed for communications, transmission of video and audio data and the like. It has become common practice to bury these cables in sand or under the ocean floor to avoid damage. When a cable breaks or is damaged there will be requirement to locate the cable to effect repairs to the cable or replacement of the cable.

[0006] Accordingly, there is a need to have an apparatus for locating an underwater cable which is buried in sand or under the ocean floor.

[0007] The present invention overcomes some of the difficulties of the prior art including those mentioned above in that it comprises a relatively simple yet highly effective pulse width modulated cable signal injecting apparatus designed to function in an underwater environment. The signal injecting apparatus induces a 1024 hertz tracking signal into a subsea cable allowing an operator to track the signal with a tracking probe tuned to the same frequency.

[0008] A microcontroller generates a continuous 1024 hertz square wave signal which is supplied through a NAND gate to a field effect transistor. When the field effect transistor is switch on a tank circuit comprising a pair of capacitors and the primary of an signal injector coil starts oscillating and will continue to oscillate when the field effect transistor is switch off.

[0009] The power level for the signal to the cable is determined by an operator based on the operator inputs from depressing a pair of push button switches, which are the power up/down push button switches. The microcontroller, responsive to the operator inputs from the push button switches, outputs a digital word to a digital-to-analog converter that represents the required power level. The analog output signal from the digital-to-analog converter is supplied to a control loop circuit which adjust the power level by enabling and disabling the NAND gate. When the NAND gate is enabled, the 1024 HZ signal passes through the NAND gate turning on the field effect transistor and thus causing the primary of a signal injector coil to start oscillating. When the signal injecting coil's current level reaches a point at which the voltage at the positive input of a comparator reaches a threshold voltage at the negative input of the comparator, the output of the comparator will switch from a low to a high which in combination with a Flip-Flop disables the NAND gate turning off the field effect transistor.

[0010] FIGS. 1A-1F, 2, 3, 4, and 5 are a detailed electrical schematic diagram of the pulse width modulated cable signal injecting apparatus which constitutes the preferred embodiment of the present invention.

[0011] Referring to FIGS. 1A-1F and 2, the pulse width modulated cable signal injecting apparatus, designated generally by the reference numeral 20. Injecting apparatus 20, which is designed to function underwater, induces a 1024 hertz signal into a subsea cable. The 1024 Hz signal can then be tracked with a tracking probe tuned to the same frequency. The maximum injecting current provided injecting apparatus 20 is limited to 100 milliamps into a 50 ohm load.

[0012] Injecting apparatus 20 comprises two 12 volt, 24 amp-hr deep discharge, lead-acid batteries 22 and 24 which are used as the power source for apparatus 20. Batteries 22 and 24 are connected to provide 24 volts which is needed to generate the required injection cable current of 100 milliamps for testing an underwater cable for damage or breakage. Batteries 22 and 24 allow signal injecting apparatus to operate at full and half power for 4 hours and 12 hours respectively. Charging of batteries 22 and 24 is accomplished through battery charger jack 26 which is connected to a 24 VDC, 2 amp battery charger (not illustrated).

[0013] Switch SW1 is a power ON/OFF switch which controls a relay 28. Relay 28 includes a coil 30 and a normally open contact 32. Closure of switch SW1 energizes coil 30 closing contact 32 which provides 24 VDC to terminal V_BATswitched. Injecting apparatus 20 also has a varistor 34 which is parallel to normally open contact 32 of relay 28. Varistor 34, which has a threshold voltage of 31 volts, is used to shunt current and protect relay 28 in the event relay 28 opens, i.e. the power is turned off when the injecting apparatus 20 is in a high power mode of operation. Diode D3 is a clamping diode in parallel with the coil 30 of relay 28 which prevents arcing of switch SW1 contacts when switch SW1 is opened.

[0014] The 24 VDC appearing at the terminal V_BATswitched is supplied to a voltage regulator 36 which provides at its output +12 VDC. The output of voltage regulator 36 is connected to the voltage regulator 38. Voltage regulator 38 provides at its output +5 VDC which is supplied to the logic elements of injector apparatus 20 including the V_DDof a microcontroller 40.

[0015] Microcontroller 40 monitors the battery voltage of batteries 22 and 24 a voltage divider circuit consisting of resistors R16 and R17 which are respectively 100 k-ohm and 10 k-ohm resistors. If the battery voltage provided to the PE0 input of microcontroller 40 is below approximately 20 volts, the PC 0 output of microcontroller 40 is set high or the logic one state. This logic one signal is supplied to an inverter 64 which inverts the signal resulting in a logic zero at the output of inverter 64. There is a also a current limiting resistor R8 connected to the output of inverter 64. A low at the output of inverter 64 turns on a light emitting diode 44 which indicates a low battery voltage.

[0016] Referring to FIG. 4, power control switches SW2 and SW3, which along with D Flip-Flops 46 and 48 comprise a debounce circuit 50 for power up and power down operator control by the operator. Power control switches SW2 and SW3 are momentary contact switches. The Q1 output of Flip-Flop 46 transitions from a high state to a low state to a high state every time a user closes switch SW2, thus providing a pulse at the Q1 output of Flip-Flop 46 every time switch SW2. Similarly, the Q2 output of Flip-Flop 48 transitions from a high state to a low state to a high state every time a user closes switch SW3, thus providing a pulse at the Q2 output of Flip-Flop 48 every time switch SW3.

[0017] The high to low pulse provided at the Q1 output of Flip-Flop 46 is supplied to an inverter 52 which inverts the pulse and then supplies the inverted pulse to a NAND gate 56. The high to low pulse provided at the Q2 output of Flip-Flop 48 is supplied to an inverter 54 which inverts the pulse and then supplies the inverted pulse to NAND gate 56. The resulting pulse occurring at the output of NAND gate 56 is a high to low pulse which occurs if either switch SW2 or SW3 is pressed.

[0018] Referring to FIGS. 1A and 4, the three pulse signals identified as PC1, STRA and PC2 are supplied to the PC1, PC2 and the STRA inputs of microcontroller 40. Microcontroller 40 reads and then processes the PC1, PC2 and STRA pulse signals to determine which switch SW2 or SW3 is being pressed and then adjust the power level of the 1024 Hz signal provided injecting apparatus 20 accordingly. Switch SW2 is the power up switch and SW3 is the power down switch, thus, PC1 is the power up pulse signal and PC2 is the power down pulse signal.

[0019] Referring to FIG. 3, injecting apparatus 20 also has a seven segment light emitting diode (LED) display 57 which has its inputs connected through inverters 58, 60, 62, 64, 66, 68 and 70 to the PB0-PB6 outputs of microcontroller 40. Inverters 58, 60, 62, 64, 66, 68 and 70 buffer and invert the signals from microcontroller 40 to LED display 57, while resistors R7 and R8 limit the current to the individual segments of LED display 57.

[0020] Referring to FIGS. 1A and 5, microcontroller 40 generates a continuous 1024 hertz square wave signal at its PA6 output which is supplied to the first input of a NAND gate 72. This signal passes through and is inverted by NAND gate 72 when NAND gate 72 receives a logic one enable signal at its second input of NAND gate 72. Inverter 74 inverts the 1024 hertz square wave signal and then supplies the signal to the gate of a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) 76. Inverter 74 is a MOSFET driver.

[0021] A signal injector coil 77 is attached to a water tight electronics canister (not illustrated) through an external connector. The water tight canister contains the batteries and electronics for injecting apparatus 20

[0022] The primary of signal injector coil 77, capacitor C8 and capacitor C11 comprise a tank circuit 80. Capacitor C8 is a 10 uF capacitor, while capacitor C11 is a 0.22 uF capacitor. When MOSFET 76 is switched on tank circuit 80 starts oscillating and will continue to oscillate when MOSFET 76 is switch off. Tank circuit 80 oscillations increase the power coupled to the secondary cable (not shown).

[0023] A pair of zener diodes D1 and D2 are used to prevent the drain voltage of MOSFET 76 from exceeding approximately 100 volts which could damage MOSFET 76. Small energy transients which are faster than the diodes D1 and D2 will generally exceed 100 volts, but do not provide sufficient energy to damage MOSFET 76. A jumper wire applied around diode D2 may be used to bypass diode D2 leaving only diode D1 operational. This, in turn, allows for different diode combinations for different breakdown voltages and current requirements for MOSFET 76. The tank circuit 80 oscillations do not normally exceed 80 volts peak. Accordingly, a 91 volt zener diode will protect MOSFET 76 from high voltage spikes, but not breakdown from tank circuit 76 oscillations during normal operation of injecting apparatus 20.

[0024] MOSFET 76 is a current sensing device which provides an accurate fraction of the drain current through its current sense lead C and its Kelvin sense lead K. A current of approximately 1/3000 of I_Dis supplied from the current sense lead c of MOSFET 76 to the negative input of an operational amplifier 82. Operational amplifier 82 converts this current to a negative voltage signal and also acts as a low pass filter removing transients caused when MOSFET 76 turns on. The negative voltage signal from operational amplifier 82 is supplied to an operational amplifier 84. This signal is inverted to a positive going waveform/signal and filtered by amplifier 84 which is operating as a low pass filter.

[0025] The positive going voltage, which represents the drain current to MOSFET 76, is supplied to the positive input of voltage comparator 86. A power level control signal is supplied to the negative input of voltage comparator 86.

[0026] Microcontroller 40 generates the power level control to signal based on the operator inputs from depressing switches SW2 and SW3, which are the power up/down push button switches. Microcontroller 40 outputs a digital word to a digital-to-analog converter 90 that represents the power level. The analog output signal provided by converter 90 is buffered by operational amplifier 92 and then supplied to the negative input of voltage comparator 86. A J-K Flip-Flop 88 completes the control loop with the E clock signal for Flip-Flop 88 being provided by microcontroller 40.

[0027] When the 1024 Hz square wave signal from microcontroller 40 is low, it resets Flip-Flop 88 which causes the Q1 output of Flip-Flop 88 to transition high enabling NAND gate 72. Enabling NAND gate 72 allows the 1024 Hz square wave signal to pass through NAND gate 72, inverter 74 and turn on MOSFET. The coil current starts to rise when the 1024 Hz square wave is at the positive part of its cycle, the time constant depending on the secondary load. A lower secondary impedance will result in more secondary current with the resultant being a greater cancellation of primary flux and consequently a faster rise time. The current will rise to the point at which the voltage at the positive input of comparator 86 reaches the threshold voltage at negative input of comparator 86. When this occurs, the output of comparator 86 will switch from a low to a high.

[0028] On the next falling edge of the E clock signal from microcontroller 40, the {overscore (Q)} output of Flip-Flop 88 will transition low. A low output at the {overscore (Q)} output of Flip-Flop 88 will disable NAND gate 72 and turn off MOSFET 76 which turns off the coil current.

[0029] The microcontroller digital word supplied to the digital-to-analog converter 90 sets the maximum primary current for signal injector coil 77 to about 12 amperes at the maximum power level set by the operator through switches SW2 and SW3. With a 50 ohm secondary load, the primary current reaches 12 amperes in approximately 20 msecs which is approximately one half of the period of the 1024 HZ signal. Using a nominal 50 ohm load, the primary current reaches a maximum level at the end of the positive pulse of the 1024 Hz signal and comparator 86 does not trip and disable NAND gate 72. Using a lower impedance secondary load, the primary current will reach the maximum current before 20 msec and the current will be shut off before damage to MOSFET 76 occurs.

[0030] Injecting apparatus 20 includes a switched capacitor voltage converter 37 which inverts the 5 VDC supplied to its V+ input to a corresponding negative voltage of −5 VDC for the operational amplifiers 82, 84 and 92 shown in FIG. 5. Injector apparatus 20 also includes a computer interface 39 (FIG. 1B) which allows an operator to interface microcontroller 40 to an external computer. A supply voltage supervisor 41 (FIG. 1E) is connected to the /RESET input of microcontroller 40. Supply voltage supervisor 41 monitors the supply voltage for under voltage conditions. During power up, when the supply voltage attains a value approaching 1 volt, the /RESET output of supply voltage supervisor becomes active low to prevent undefined operation.

[0031] The signal injector coil 77 consists of a split-core 3transformer with four turns of #20 transformer wire distributed evenly across half of the core. Power is transmitted to the injector coil via an electrical cable which connects to the amplifier canister (J2). The percentage of available power applied to the coil is controlled by the “Power Up”/“Power Down” push button switches SW2 and SW3 located on a amplifier front panel.

[0032] Each half of the injector core is molded into a mating cylindrical PVC housing using a polyether polyurethane as a bonding matrix. The secondary transformer winding is spliced to a 12-foot cable, and the transformer and splice are encapsulated in the urethane matrix. The cable exits the housing through a strain relief and terminates to a two-pin male wet mateable underwater connector.

[0033] The two halves of the signal injector coil 77 are hinged together with stainless steel fasteners. After placement of the coil around the cable, the two core faces are drawn tightly together and locked into place using four stainless steel latches.

[0034] The transformer core material is a silicon steel alloy. The exposed faces are very susceptible to rust and corrosion. After use, corrosion is minimized by flushing the faces with fresh water and by keeping the core surfaces covered with a thin coat of silicon compound.

[0035] Appendix A is a program listing of the computer software program for microcontroller 40.

[0036] From the foregoing, it may readily be seen that the present invention comprises a new, unique and exceedingly cable signal injecting apparatus for use in locating an undersea cable which constitutes a considerable improvement over the known prior art. Many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.