Abstract:
An apparatus and method for detecting acoustic signals from a plurality of acoustic signal sensors. The apparatus comprises a plurality of acoustic signal detection channels. Each channel has an input for receiving acoustic signals from a corresponding acoustic signal sensor and includes circuitry for amplifying the received acoustic signals, removing the D.C. components from the amplified acoustic signals and removing all frequency components from the amplified acoustic signals which are above a predetermined frequency. The apparatus further comprises a circuitry for summing all of the acoustic signals outputted from the acoustic signal detection channels to form a single acoustic signal and for converting the single acoustic signal into a differential signal if at least one acoustic signal sensor senses an acoustic signal and its corresponding acoustic signal detection channel outputs an acoustic signal.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention generally relates to an apparatus and method for detecting acoustic signals from a plurality of acoustic signal sensors. 
     (2) Description of the Prior Art 
     Prior art acoustic signal detection devices typically use complex signal processing circuitry which conditions acoustic signals and performs signal processing functions upon such signals, e.g., Fast Fourier Transforms, to extract desired data. Such a device is disclosed in U.S. Pat. No. 4,017,859. Other prior art devices measure the cross-spectral density of received acoustic signals to determine the acoustic density in a particular direction. Such a device is disclosed in U.S. Pat. No. 4,982,375. Additionally, many prior art devices use complex circuitry to perform phase and magnitude detection and to effect conversion from rectangular to polar coordinates. One such device is described in U.S. Pat. No. 4,953,145. Still, other prior art devices utilize circuitry for the generation of frequency tones. For example, U.S. Pat. No. 3,588,802 utilizes a mechanical vibrator for exciting a frequency tone that is added to the received acoustic signal. Other prior art devices use a pair of hydrophones wherein each hydrophone is dedicated to receiving particular frequency components of acoustic signals. For example, U.S. Pat. No. 4,594,695 discloses a system that utilizes two hydrophones wherein one hydrophone receives a disturbed tracked signal and the other hydrophone receives spurious noises. 
     What is needed is a relatively less complex acoustic signal detection system that provides redundancy whereby the acoustic signal detection system receives and detects acoustic signals from a plurality of acoustic signal sensors (e.g. hydrophones) as long as one of the acoustic signal sensors senses an acoustic signal. Preferably, the redundancy should be realized by the overall design of the acoustic signal detection system so as to substantially increase the probability that acoustic signals sensed by the sensors will still be detected by the acoustic signal detection system even if this system experiences partial component failure. Another desired feature of such an acoustic signal detection system is that it must be relatively simple in construction in order to reduce the costs related to manufacturing, maintenance and repair. 
     Therefore, it is an object of the present invention to provide an apparatus and method for receiving and detecting acoustic signals from a plurality of acoustic signal sensors that fulfills a long-felt need that has not been met by prior art devices and methods. 
     Other objects and advantages of the present invention will be apparent to one of ordinary skill in the art in light of the ensuing description of the present invention. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus and method for detecting acoustic signals from a plurality of acoustic signal sensors. The apparatus comprises a plurality of acoustic signal detection channels. Each acoustic signal detection channel has an input for receiving acoustic signals from a corresponding acoustic signal sensor. Each acoustic signal detection channel further includes circuitry for (i) amplifying the received acoustic signals, (ii) removing the D.C. components from the amplified acoustic signals, and (iii) removing all frequency components from the amplified acoustic signals which are above a predetermined frequency. The apparatus further comprises circuitry for summing all of the acoustic signals outputted from the acoustic signal detection channels to form a single acoustic signal. The apparatus further comprises additional circuitry for converting the single acoustic signal into a differential signal and outputting the differential signal if at least one acoustic signal sensor senses an acoustic signal and the corresponding acoustic signal detection channel outputs an acoustic signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention are believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which like reference numerals refer to like parts and in which: 
     FIG. 1 is a block diagram of the apparatus of the present invention; and 
     FIG. 2 is a diagram, partially in schematic form, showing electrical circuits that are used to realize the apparatus of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, apparatus  10  of the present invention receives acoustic signals from acoustic signal sensors or hydrophones  12  positioned beneath the surface of ocean  14 . The acoustic signals originate from noise source  13 . Apparatus  10  generally comprises acoustic signal detection channels  15  and  16  and circuit  18 . Channel  15  comprises preamplifier  20 , filter circuit  22  and amplifier circuit  24 . Similarly, channel  16  comprises preamplifier  26 , filter circuit  28  and amplifier circuit  30 . Channels  15  and  16  are identical in design and construction. The output of each channel  15  and  16  is coupled into circuit  18 . Circuit  18  is described in detail in the ensuing description. 
     Referring to FIGS. 1 and 2, acoustic signals received from hydrophones  12  are inputted into inputs  32  and  34  of apparatus  10 . Inputs  32  and  34  are connected to the inputs of preamplifier circuits  20  and  26 , respectively. FIG. 2 shows one embodiment of the internal electrical circuitry of channels  15  and  16  and circuit  18 . Since channels  15  and  16  are identical in design and construction, only channel  15  is described in detail in the ensuing description. Preamplifier circuit  20  amplifies the low level acoustic signals and removes the D.C. (Direct Current) level from these signals. Preamplifier circuit  20  comprises amplifier U 1 . Amplifier U 1  comprises an operational amplifier having an inverting input, a non-inverting input and an output. In one embodiment, amplifier U 1  has the operational characteristics of the commercially available LF147 or LF347 operational amplifiers manufactured by National Semiconductor and Texas Instruments. However, it is to be understood that other operational amplifiers having operational characteristics similar to the aforementioned LF147 or LF347 operational amplifiers can also be used. Amplifier U 1  includes a terminal for connection to a positive power supply voltage source +Vcc. Amplifier U 1  also includes a terminal for connection to a negative power supply voltage source −Vcc. In one embodiment, +Vcc is about +15 VDC and −Vcc is about −15 VDC. 
     Referring to FIG. 2, resistor R 1  is connected between input  32  and the non-inverting input of amplifier U 1 . Resistor R 2  is connected between ground potential and the inverting input of amplifier U 1 . In one embodiment, each resistor R 1  and R 2  has a resistance of about 249Ω. Diodes D 1  and D 2  are connected between ground potential and the non-inverting input of amplifier U 1 . Specifically, the anode of diode D 1  is connected to the non-inverting input of amplifier U 1  and the cathode is connected to ground potential. The cathode of diode D 2  is connected to the non-inverting input of amplifier U 1  and the anode is connected to ground potential. Diodes D 1  and D 2  serve to protect channel  15  from high-level voltage spikes and in particular, electromagnetic pulses (“EMP”). Resistor R 3  functions as a feedback resistor and is connected between the output and inverting input of amplifier U 1 . In one embodiment, resistor R 3  has a resistance of about 1.4 KΩ. 
     In a preferred embodiment, preamplifier circuit  20  is configured to provide a gain between about 500 and 1500. In one embodiment, preamplifier circuit  20  provides a gain of about 1000. The output of amplifier U 1  is coupled to one end of capacitor C 1 . In one embodiment, capacitor C 1  has a capacitance of 0.01 micro-farads. Capacitor C 1  prevents the D.C. component of the amplified acoustic signals from entering filter circuit  22 . 
     Referring to FIGS. 1 and 2, filter circuit  22  functions as a low pass filter. Filter circuit  22  comprises amplifier U 2 . Amplifier U 2  comprises an operational amplifier having an inverting input, a non-inverting input, an output and terminals for connection to +Vcc and −Vcc. In one embodiment, amplifier U 2  has the operational characteristics of the aforementioned LF347 operational amplifier. However, it is to be understood that other operational amplifiers having operational characteristics similar to the LF347 amplifier can also be used. Filter circuit  22  further includes resistors R 4  and R 5  and capacitor C 2 . Amplifier U 2 , resistors R 4  and R 5  and capacitor C 2  are connected to form a low pass filter which has a predetermined cut-off frequency Fc. In a preferred embodiment, the cut-off frequency Fc is between about 100 kHz and 300 kHz. In one embodiment, capacitor C 2  has a capacitance of 100 picofarads and resistor R 5  has a resistance of about 10 KΩ so as to provide a cut-off frequency Fc of about 160 kHz. However, it is to be understood that the capacitance of capacitor C 2  and the resistance of resistor R 5  can be chosen so as to provide a different cut-off frequency Fc. In one embodiment, the resistance of resistor R 4  is about 3.01 KΩ. The gain of the low pass filter realized by amplifier U 2 , resistors R 4  and R 5  and capacitor C 2  is represented by the ratio R 5 /R 4 . Filter circuit  22  further includes resistor R 6  which has one end thereof connected to the output of amplifier U 2  and the other end connected to resistor R 10  which is described in the ensuing description. In one embodiment, resistor R 6  has a resistance of about 49.9Ω. 
     Referring to FIG. 2, filter-circuit  22  further comprises amplifier U 3  and resistors R 7  and R 8 . Amplifier U 3  can be realized by the commercially available LM741 operational amplifier, manufactured by several manufacturers including National Semiconductor and Texas Instruments. Amplifier U 3  can also be realized by the commercially available LF147 and LF347 operational amplifiers which were previously discussed herein. Amplifier U 3  and resistors R 7  and R 8  are connected together to provide an inverting buffer amplifier. Similar to amplifiers U 1  and U 2 , amplifier U 3  includes terminals for connection to +Vcc and −Vcc. Resistor R 7  functions as a feedback resistor and is connected between the output and inverting input of amplifier U 3 . Resistor R 8  is connected between the inverting input of amplifier U 3  and the output of amplifier U 2 . In a preferred embodiment, resistors R 7  and R 8  have resistances that provide unity gain. In one embodiment, resistors R 7  and R 8  each have a resistance of 3.01 KΩ. Filter circuit  22  further includes resistor R 9  that is connected between the output of amplifier U 3  and one end of resistor R 11 . In one embodiment, resistor R 9  has a resistance of about 49.9Ω. 
     Referring to FIGS. 1 and 2, the output signals of amplifiers U 2  and U 3  are inputted into amplifier circuit  24 . Amplifier circuit  24  comprises amplifier U 4 , resistors R 10 , R 11 , R 12 , R 13  and variable resistor or potentiometer R 14 . Amplifier U 4  includes an inverting input, a non-inverting input, an output and +Vcc and −Vcc terminals. In one embodiment, amplifier U 4  can be realized by the commercially available LM741, LF147 or LF347 operational amplifiers previously discussed herein. Amplifier U 4 , resistors R 10 , R 11 , R 12 , R 13  and variable resistor R 14  are connected to provide an inverting summing circuit with unity gain. Resistor R 10  is connected between one end of resistor R 6  and the inverting input of amplifier U 4 . Similarly, resistor R 11  is connected between one end of resistor R 9  and the non-inverting input of amplifier U 4 . Resistor R 12  is a feedback resistor and is connected between the output and inverting input of amplifier U 4 . In one embodiment, resistors R 10 , R 11  and R 12  each have a resistance of about 10 KΩ. Resistor R 13  is connected between the non-inverting input of amplifier U 4  and the negative power supply −Vcc. Resistor R 13  functions as biasing resistor. In one embodiment, resistor R 13  has a resistance of about 9 KΩ. Variable resistor R 14  provides the ability to adjust offset voltages. In one embodiment, variable resistor has a resistance range between about 0Ω and 200Ω. 
     Referring to FIG. 2, amplifier circuit  24  further comprises diodes D 3  and D 4 . Diode D 3  is connected between the +Vcc power supply voltage and the +Vcc terminal of amplifier U 4  such that diode D 3  is forward biased. In this configuration, diode D 3  blocks current from flowing into the +Vcc power supply voltage source. Similarly, diode D 4  is connected between the −Vcc power supply voltage and the −Vcc terminal of amplifier U 4  such that diode D 4  is forward biased. In this configuration, diode D 4  blocks current from flowing into the −Vcc power supply voltage source. Amplifier circuit  24  further includes output resistor R 15 . Resistor R 15  is connected between the output of amplifier U 4  and the input to amplifier circuit  18 . In one embodiment, resistor R 15  has a resistance of about 10 KΩ. 
     Referring to FIGS. 1 and 2, the output of amplifier circuit  24  is fed into circuit  18 . Circuit  18  comprises two stages. The first stage is a summing circuit which is comprised of amplifier U 5  and resistors R 17  and R 18 . In one embodiment, amplifier U 5  is configured to have the operating characteristics of the commercially available LF147 operational amplifier previously described herein. However, it is to be understood that amplifier U 5  can be configured to have the operational characteristics of the other commercially available operational amplifiers previously described herein. Resistor R 15  of circuit  24  is connected between the output of amplifier U 4  and the inverting input of amplifier U 5 . Similarly, the output of circuit  30  is connected to the inverting input of amplifier U 5 . Resistor R 17  is a feedback resistor connected between the output and inverting input of amplifier U 5 . In one embodiment, resistor R 17  has a resistance of about 10 KΩ. Resistor R 18  provides offset compensation. In one embodiment, resistor R 18  has a resistance value of about 2.5 KΩ. The output of amplifier U 5  is connected to output terminal  36  and is also fed into the second stage of circuit  18 . The second stage functions as a differential amplifier and comprises amplifier U 6  and resistors R 19 , R 20  and R 21 . In one embodiment, amplifier U 6  is configured as an operational amplifier that has operational characteristics similar to the commercially available LF147 operational amplifier. However, amplifier U 6  can also be configured as any of the commercially available operational amplifiers previously described herein. Resistor R 19  is connected between the output of amplifier U 5  and the inverting input of amplifier U 6 . Resistor R 21  is a feedback resistor and is connected between the inverting input and the output of amplifier U 6 . In one embodiment, resistors R 19  and R 21  each have a resistance of about 10Ω. Resistor R 20  provides offset compensation. In one embodiment, resistor R 20  has a resistance of about 2.5 KΩ. The output of amplifier U 6  is connected to output terminal  38 . 
     The magnitude of the signal measured between output terminals  36  and  38  represents the difference in magnitudes between the acoustic signals outputted from channels  15  and  16 . Apparatus  10  outputs a signal between terminals  36  and  38  if at least one of hydrophones  12 , and the detection channel to which it is connected, are functioning properly. Specifically, apparatus  10  outputs a signal between terminals  36  and  38  if at least one of hydrophones  12  senses an acoustic signal and the corresponding acoustic signal detection channel outputs an acoustic signal. Thus, the internal circuitry of apparatus  10  provides built-in redundancy thereby ensuring that acoustic signals are detected even if one of the hydrophones and/or one of the detection channels have failed. 
     Although apparatus  10  has been described as having two detection channels that are connected to the corresponding hydrophones, it is to be understood that apparatus  10  can be configured to have a plurality of detection channels wherein each detection channel is connected to a corresponding hydrophone. 
     Output terminals  36  and  38  can be connected to peripheral electronic analysis equipment such a computer, oscilloscope, video monitor, cathode-ray-tube, liquid-crystal-display, etc. Analog-to-digital conversion circuitry, and driver or buffer circuitry, well known in the art, may be needed to couple the signal outputted at terminals  36  and  38  to the aforementioned analysis equipment. 
     Although the foregoing description is in terms of the resistors and capacitors in apparatus  10  having the stated resistances and capacitances, respectively, it is to be understood that the resistors and capacitors can have different resistance and capacitance values, respectively. It is also to be understood that decoupling capacitors are connected between the +Vcc or −Vcc terminals of all amplifiers and ground potential in a manner well known in the art. Additionally, amplifiers U 1 , U 2 , U 3 , U 4 , U 5  and U 6  can also be realized by discrete components such as NPN or PNP transistors, or n-channel or p-channel field effect transistors. 
     In one embodiment, +Vcc is about +15 VDC and −Vcc is about −15 VDC. However, it is to be understood that the circuits described herein can be configured to operate with positive and negative power supply voltage sources having other magnitudes, e.g. +12 VDC, −12 VDC, etc. 
     Thus, the system of the present invention achieves the objects set forth above. Specifically, the system of the present invention: 
     a) utilizes a plurality of acoustic signal detection channels that provide redundancy to ensure that acoustic signals will be detected in the event of failure of any of the hydrophones or acoustic signal detection channels; 
     b) provides accurate and consistent measurements; 
     c) can be implemented with a variety of hardware components; and 
     d) can be implemented at a relatively low cost. 
     While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.