Abstract:
A sensor includes a piezoelectric hydrophone and at least one accelerometer. In calibration mode, the hydrophone is connected to a source of a known electrical signal and outputs a mechanical/acoustic signal that the accelerometer detects. Comparison of the known electrical signal to the output of the accelerometer allows calibration of the accelerometer. In operation mode, both the hydrophone and the accelerometer are connected to a data acquisition unit.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 60/382,583, filed May 24, 2002, whose disclosure is hereby incorporated by reference in its entirety into the present disclosure. 
     
    
     
       FIELD OF INVENTION  
         [0002]    The present invention is directed to a field or institute calibrated sensor system for detecting motion and pressure changes in, e.g., underwater environments to be used in activities such as geophysical exploration, depth detection and anti-submarine warfare.  
         BACKGROUND OF THE INVENTION  
         [0003]    Acoustic sensors are used in underwater environments for a variety of purposes, such as geophysical exploration, depth detection and anti-submarine warfare. Because of the nature of the environment, these sensors often provide the only information on the physical conditions of the surroundings. Therefore, it is vital that the sensors be accurate and calibrated properly.  
           [0004]    It is known in the art to use piezoelectric elements in acoustic sensors, particularly those in underwater environments. U.S. Pat. No. 4,536,862 to Sullivan discloses a sensor having piezo-electric elements attached to conductive plates. When external forces act on the sensor, the conductive plates and corresponding piezo-electric elements flex in response thereto, with the piezo-electric elements generating an electrical signal in relation to the applied force. However, Sullivan is silent on the manner in which the sensor is calibrated.  
           [0005]    U.S. Pat. No. 5,995,451 to Evans et al. discloses an underwater sensor using piezo-electric elements and accelerometers. The reference discloses a first calibration method for the piezo-electric elements and the accelerometers that use a function generator, a speaker and a sound pressure level meter. A second calibration method places the sensor in a fluid stream with a known flow rate so that a calibration table can be produced. However, neither of those calibration methods can be implemented in a self-contained sensor or in the field.  
           [0006]    The Sullivan reference fails to disclose a calibration method and the components that are needed to perform a calibration procedure. The Evans et al. reference discloses a calibration method, which in the first instance requires a speaker and a sound pressure level meter, and in the second instance requires the user to produce a calibration chart. The Evans et al. reference therefore requires several components for the calibration procedure, specifically the speaker and the sound pressure level meter, and it also requires that the calibration take place prior to deployment of the sensors.  
         SUMMARY OF THE INVENTION  
         [0007]    Therefore, it is an object of the invention to have an underwater sensor system that is self-contained, field or institute calibrating, and made with a minimal number of components. It is a further object of the invention to have a sensor system that is able to calibrate itself after deployment, or that is capable of being calibrated in the field or in the institute and then monitoring and verifying its calibration  
           [0008]    To achieve the above and other objects, the sensor system of the present invention includes a sensor element attached to a signal generator and a data acquisition unit. The sensor element contains a piezo-electric hydrophone and at least one accelerometer which measure the pressure and motion changes, respectively, in underwater environments and sends the information to the data acquisition unit. In a preferred embodiment, the sensor includes three mutually orthogonal accelerometers for three-dimensional motion detection.  
           [0009]    The sensor system is also able to calibrate itself by sending a known electrical signal from the signal generator to the hydrophone, which vibrates at a known frequency and level in response to the electrical signal, producing mechanical motion that emulates an acoustic signal that is picked up by the nearby accelerometers. The accelerometers transmit the information to the data acquisition unit which correlates the known input electrical signal with the accelerometer output signal to ensure that the accelerometers are properly calibrated.  
           [0010]    The sensor system is self-contained and is able to calibrate itself without any external components or preliminary steps. In this way, the sensor system can calibrate itself after deployment into the field so that the accuracy of its readings is ensured.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    A preferred embodiment of the present invention will be set forth in detail with reference to the drawings, in which:  
         [0012]    [0012]FIG. 1 shows a schematic of the sensor system of the present invention; and  
         [0013]    [0013]FIG. 2 shows a flow chart of the calibration and operation modes of the sensor system of FIG. 1.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    Referring now in detail to the drawings, FIG. 1 shows a sensor system  10  comprising a sensor element  12 , a signal generator  14  and a data acquisition unit  16 . The sensor system  10  has two modes, a first calibration mode that calibrates the sensor element  12 , and a second operating mode that detects pressure and motion changes in underwater environments to be used for activities such as geophysical exploration, depth detection and anti-submarine warfare.  
         [0015]    The sensor element  12  includes a high impedance crystal hydrophone  18  and three accelerometers  20  within the same housing and in close proximity to each other. The housing is any housing suitable for use with a hydrophone. The sensor  12  is mounted within the housing so as to permit the sensor to communicate with the surrounding water. Each accelerometer  20  measures the forces in a different axis, specifically, the x, y and z-axes. The hydrophone  18  contains a piezo-electric element that is able to detect the pressure changes in the surrounding environment.  
         [0016]    The hydrophone  18  is attached to a first circuit  22  with a pair of switches S 1  and S 2  that move between the calibration mode and the operation mode. When in the calibration mode, the switches S 1  and S 2  are attached to nodes  15  to form a closed loop with the signal generator  14 . In the operating mode, the switches S 1  and S 2  are attached to nodes  17  which are connected to the data acquisition unit  16  and by-pass the signal generator  14 . A signal conditioning element  24 , a pre-amplifier in the preferred embodiment, may be added between the hydrophone  18  and the data acquisition unit  16  to modify a signal from the hydrophone  18  before it is collected in the data acquisition unit  16 . The accelerometers  20  are electrically connected to the data acquisition unit  16  by a second circuit  26  that runs parallel to the first circuit  22 .  
         [0017]    The calibration and operation of the sensor system  10  will now be described with reference to both FIG. 1 and FIG. 2. It is determined in step  202  whether the system  10  should be in the calibration mode or the operation mode. In the calibration mode, the switches S 1  and S 2  are attached in step  204  to nodes  15  to form a closed loop between the hydrophone  18  and the signal generator  14 . The signal generator  14  sends a known electrical signal in step  206  to the hydrophone  18 , causing the piezo-electric element to vibrate, producing a mechanical signal emulating an acoustic signal at a known level and frequency. The mechanical signal is detected in step  208  by the nearby accelerometers  20 , which transmit an electrical output signal to the data acquisition unit  16 . A computer, either in the data acquisition unit  16  or provided for separately, then compares the known input signal to the accelerometer output signal in step  210  to ensure in step  212  that the accelerometers  20  are operationally properly calibrated. The calibration can be implemented in any suitable way; for example, the comparison carried out in step  210  can be used to produce a calibration table.  
         [0018]    In the operating mode, the switches S 1  and S 2  are attached in step  214  to nodes  17  and connect the hydrophone  18  to the data acquisition unit  16  and by-pass the signal generator  14 . Both the hydrophone  18  and the accelerometers  20  are connected in parallel to the data acquisition unit  16  and transmit information to the data acquisition unit  16 . The piezo-electric element in the hydrophone  18  produces an electrical signal based on pressure changes in the environment in step  216  and sends the signal to the data acquisition unit  16 . The accelerometers  20  are in a neutrally buoyant state; they produce an electrical signal based on any changes in speed or direction of the surrounding water in step  218  and transmit the signal to the data acquisition unit  16 .  
         [0019]    Although certain presently preferred embodiments of the present invention have been specifically described herein, it will be apparent to those skilled in the art to which the invention pertains that variations and modifications of the various embodiments shown and described herein may be made without departing from the spirit and scope of the invention. For example, statements of intended use are illustrative rather than limiting. Also, while three accelerometers are disclosed, more or fewer could be used instead; for example, if only one-dimensional detection is required, only one accelerometer need be provided. Therefore, the present invention should be construed as limited only by the appended claims.