Abstract:
A fiber optic sensor system employs at least one light source that operates to generate light having one or more desired wavelengths. A first optical fiber based sensor transparent to a desired light wavelength operates to sense a magnetic field emitted from a predetermined electrical conductor or a current flowing through the electrical conductor. A temperature sensor that may be another optical fiber based sensor operates to sense an operating temperature associated with the first optical fiber based sensor in response to the light generated by the light source. Signal-processing electronics adjust the sensed current to substantially compensate for temperature induced errors associated with the sensed current in response to the measured operational temperature of the fiber optic sensor.

Description:
BACKGROUND 
       [0001]    This invention relates generally to fiber optic sensing methods and systems, and more particularly, to a fiber optic system and method for compensating temperature induced errors associated with optical current sensor measurements. 
         [0002]    Fiber optic magnetic field or current sensing is strongly temperature dependent. Due to this temperature dependence, such sensing techniques require temperature isolation or temperature measurements and compensation techniques. 
         [0003]    A common principle, applied in state-of-the-art systems is to use metal-wire-bounded thermo elements to measure the temperature. Metal-wire-bounded thermo elements cannot always be employed in electromagnetically harsh environments. Other techniques include self-compensation for temperature during current sensing but these techniques are effective in a limited temperature range or require complicated signal-processing algorithms. 
         [0004]    Fiber optic temperature sensors are better suited for use in electromagnetically harsh environments due to their intrinsic immunity to external electromagnetic fields and have a large measureable temperature range. 
         [0005]    A fiber optic temperature sensing system along with the fiber optic current sensing system would be simpler to implement since both sensing systems are based on the fiber optic sensor platform. 
       BRIEF DESCRIPTION 
       [0006]    Briefly, in accordance with one embodiment, a temperature compensated fiber optic current sensing system comprises: 
         [0007]    a fiber optic transducer configured to sense current flowing through an electrical conductor; 
         [0008]    a fiber optic temperature sensor configured to measure the operational temperature of the fiber optic sensor; and 
         [0009]    signal-processing electronics configured to adjust the sensed current measurement to substantially compensate for temperature induced errors associated with the sensed current in response to the measured operational temperature of the fiber optic current transducer. 
     
    
     
       DRAWINGS 
         [0010]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0011]      FIG. 1  illustrates the temperature dependence of measured current using a fiber optic current sensing system; 
           [0012]      FIG. 2  is a flowchart showing a method of providing a temperature compensated current measurement according to one embodiment of the present invention; 
           [0013]      FIG. 3  is a simplified diagram illustrating a temperature compensated fiber optic current sensing system using a single point fiber optic temperature sensor according to one embodiment of the present invention; 
           [0014]      FIG. 4  is a simplified diagram illustrating a temperature compensated fiber optic current sensing system using a series configuration of fiber optic temperature sensors according to one embodiment of the present invention; 
           [0015]      FIG. 5  is a simplified diagram illustrating a temperature compensated fiber optic current sensing system using one or more continuous distributed fiber optic temperature sensing elements according to one embodiment of the present invention; 
           [0016]      FIG. 6  is a simplified diagram illustrating a temperature compensated fiber optic current sensing system using a parallel configuration of fiber optic temperature sensors according to one embodiment of the present invention; 
           [0017]      FIG. 7  is a simplified diagram illustrating a temperature controller responsive to a temperature compensated fiber optic current sensing system according to one embodiment of the present invention; 
           [0018]      FIG. 8  is a simplified schematic illustrating a temperature compensated fiber optic current sensing system using multiple light sources and multiple photo-detectors combined with a fiber optic current transducer and a separate fiber optic temperature sensor according to one embodiment of the present invention; 
           [0019]      FIG. 9  is a simplified schematic illustrating a temperature compensated fiber optic current sensing system using multiple light sources and multiple photo-detectors in combination with a fiber optic current transducer and a fiber optic temperature sensor that are both integrated with a common optical fiber according to one embodiment of the present invention; 
           [0020]      FIG. 10  is a simplified schematic illustrating a temperature compensated fiber optic current sensing system using a common light source and a common photo-detector combined with a fiber optic current transducer and a separate fiber optic temperature sensor according to one embodiment of the present invention; 
           [0021]      FIG. 11  is a simplified schematic illustrating a temperature compensated fiber optic current sensing system using a common light source and common photo-detector in combination with a fiber optic current transducer and a fiber optic temperature sensor that are both integrated with a common optical fiber and driving a common detector unit according to another embodiment of the present invention; 
           [0022]      FIG. 12  is a simplified schematic illustrating a temperature compensated fiber optic current sensing system using a common light source in combination with a fiber optic current transducer and a fiber optic temperature sensor that may or may not be integrated with a common optical fiber and driving corresponding detectors according to one embodiment of the present invention; 
           [0023]      FIG. 13  is a simplified schematic illustrating a temperature compensated fiber optic current sensing system using multiple light sources in combination with a fiber optic current transducer and a fiber optic temperature sensor that may or may not be integrated with a common optical fiber and a common detector according to one embodiment of the present invention; 
           [0024]      FIG. 14  depicts a physical temperature compensated fiber optic current sensing system using one or multiple fiber Bragg grating sensors to implement fiber optic temperature sensors and fiber optic current transducer to measure current based on Faraday effect, corresponding to system architecture represented by  FIGS. 4 and 8 ; and 
           [0025]      FIG. 15  depicts a physical temperature compensated fiber optic current sensing system using Gallium-Arsenide material (GaAs) optical reflectivity based fiber temperature sensing technology and discrete Faraday Garnet crystal based current sensing technology to implement a system architecture represented by  FIGS. 3 and 8 . 
       
    
    
       [0026]    While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
       DETAILED DESCRIPTION 
       [0027]    Embodiments of the invention described herein with reference to  FIGS. 1-15  are directed to a temperature compensated fiber optic sensor system for magnetic field or current sensing. Particular embodied magnetic field or current sensors described herein are based on the Faraday effect in optical materials such as an optical fiber core or a Faraday garnet. More specifically, these embodiments are based on polarimetric sensing principles where the angle of polarized light rotates with respect to the strength of a magnetic field generated by current flow. 
         [0028]    The embodied fiber optic temperature sensors described herein employ intrinsic and/or extrinsic fiber optic sensing methods that may include, without limitation, fiber Bragg grating measurements, Raman scattering, Brillouin scattering, Fabry-Perot interferometric measurements, Mach-Zehnder interferometric measurements, Michelson interferometric measurements, Sagnac interferometric measurements, microbending measurements, macrobending measurements, polarimetric measurements, pyrometric measurements, reflectivity measurements, and emissivity measurements. The location of the temperature sensor points can be separate from or co-located with an optical magnetic/current sensor such as a magnetic field sensitive optical fiber or Faraday garnet. 
         [0029]    Combining both fiber optic magnetic field/current sensors and fiber optic temperature sensors on one optical fiber according to one embodiment, provides a cost effective system that can be manufactured with enhanced performance. Since the Faraday effect is strongly temperature dependent, the measured temperature can be used to compensate for any temperature-induced error in the current/magnetic field measurements. 
         [0030]      FIG. 1  illustrates the variability of the current measurement with changing temperature. The figure shows the non-linear characteristics of the temperature dependence. A fiber optic current transducer system that operates in an extended temperature zone has to be compensated for this temperature-induced error. 
         [0031]      FIG. 2  identifies the functional blocks in order to implement a temperature compensated fiber optic current transducer. Temperature measurement  202 , along with the current measurement  204  is fed into a signal processor  206 . The signal processor  206  uses these two inputs to produce a more accurate current measurement  208  that does not include errors induced by temperature. 
         [0032]      FIG. 3  is a simplified diagram that illustrates a temperature compensated fiber optic current sensing system  10  using a single point fiber optic temperature sensor  12  according to one embodiment of the present invention. Fiber optic current sensing system  10  can be seen to include a light source  14  that can be a laser light or a broadband light source according to particular embodiments. Fiber optic current sensing system  10  also includes a fiber optic current transducer  16  that may operate using the Faraday effect. 
         [0033]    Fiber optic temperature sensor  12  may be independent from optic fiber current transducer  16  according to one embodiment. According to one aspect, temperature sensor  12  may comprise, for example, Gallium-Arsenide material (GaAs), which is optically transparent at light wavelengths above about 850 nm due to its material band edge. The position of this band edge is temperature dependent and shifts approximately 0.4 nm per degree Kelvin. This information is transmitted to corresponding temperature sensor opto-electronics  24  along an optical fiber  26 . The temperature information is then transmitted to signal-processing electronics  28  that may be, for example, a digital signal processor (DSP). The signal-processing electronics  28  processes the measured current signals generated via the current transducer  16  along with the measured temperature signals generated via the temperature sensor  12 , to generate a temperature compensated current signal measurement. Fiber optic temperature sensor  12  may comprise a desired portion of the optical fiber  26  according to another embodiment, wherein the desired portion includes, for example, one or more fiber sensors. 
         [0034]      FIG. 4  is a simplified diagram illustrating a temperature compensated fiber optic current sensing system  30  using a series configuration of temperature sensors  32  according to one embodiment of the present invention. Fiber optic current sensing system includes a light source  14  that is a laser light source according to one embodiment or a broadband light source according to another embodiment, and further includes a fiber optic current transducer  16  that may operate using the Faraday effect. 
         [0035]    According to one embodiment, temperature sensors  32  comprise multiple fiber sensors, intrinsic or extrinsic, at discrete points in or along the optical fiber  26 . The properties of light passing through the fiber sensors are temperature dependent in well-known fashion; and so operating principles of fiber temperature sensors are not discussed further herein to preserve brevity and enhance clarity in better understanding the principles described herein. Light signals generated via temperature sensors  32  are transmitted to corresponding temperature sensor opto-electronics  24  along optical fiber  26 . The temperature information is then transmitted to signal-processing electronics  28  that may include, for example, and without limitation, a digital signal processor (DSP). The signal-processing electronics  28  processes the current signals generated via the fiber optic current transducer  16  along with the temperature signals generated via the plurality of fiber optic temperature sensors  32  to generate a temperature compensated current measurement signal. 
         [0036]      FIG. 5  is a simplified diagram illustrating a temperature compensated fiber optic current sensing system  190  using one or more continuous distributed temperature sensors  192  according to one embodiment of the present invention. Fiber optic current sensing system  190  functions in substantially the same fashion as temperature compensated fiber optic current sensing systems  10  and  30  described above, with the exception of using a continuous distributed temperature sensing configuration for measure and transmit temperature signals to corresponding temperature sensor opto-electronics  24 . 
         [0037]      FIG. 6  is a simplified diagram illustrating a temperature compensated fiber optic current sensing system  40  using a parallel configuration of fiber optic temperature sensors  42  according to one embodiment of the present invention. Fiber optic current sensing system  40  functions in substantially the same fashion as temperature compensated fiber optic current sensing systems  10  and  30  described above, with the exception of using a parallel configuration of fiber optic temperature sensors  42  and a plurality of corresponding optic fibers  44  that provide a communication path for transmitting temperature signals to corresponding temperature sensor opto-electronics  24 . 
         [0038]      FIG. 7  is a simplified diagram illustrating a temperature compensated fiber optic current sensing system  50  has a temperature controller  56 , that is responsive to temperature measured by temperature sensor  12  and temperature sensing electronics  24 , according to one embodiment of the present invention. 
         [0039]    According to one embodiment, the temperature sensor  12  measures the temperature and transmits the information via fiber optic cable  26  to temperature sensor opto-electronics  24  which yields a temperature measurement that can be used by a temperature controller  56  via a data communication link  55  to control a heating and or a cooling element  52 . According to another embodiment the temperature measurement from temperature sensing opto-electronics  24  can simultaneously be used via data communication link  55  by the signal-processing electronics  28  that may include, for example, and without limitation, a digital signal processor (DSP) to yield a temperature compensated current measurement. This may be the case if the heating/cooling element is not fast enough or has limited heating/cooling capabilities. 
         [0040]    According to one embodiment, a temperature controller  56  is electrically or optically coupled to a heating/cooling element  52  strategically placed in close proximity to the fiber optic current transducer  16  such that the heating/cooling element  52  can effectively heat and cool the fiber optic current transducer  16 . Heating/cooling element  52  may also work in combination with an insulator element  54  to cool down or heat up the fiber optic current transducer  16 . If the temperature controller  56  is electrically powered, the level of current passing through heating/cooling element  52  is therefore controlled in a manner that causes the fiber optic current transducer  16  to operate within a temperature stabilized operating environment. 
         [0041]      FIG. 8  is a simplified block diagram illustrating a temperature compensated fiber optic current sensing system  60  using multiple light sources  62 ,  64  transmitting light to a fiber optic current transducer  66  and a fiber optic temperature sensor  68  to generate current and temperature signals received by corresponding detectors  70 ,  72 , according to one embodiment of the present invention. 
         [0042]      FIG. 9  is a simplified block diagram illustrating a temperature compensated fiber optic current sensing system  74  using multiple light sources  62 ,  64  transmitting light to a fiber optic current and temperature sensor  76  to generate current and temperature signals received by multiple detectors  70 ,  72  according to one embodiment of the present invention. The current and temperature sensing elements  76  are integrated with an optical fiber common to both sensors. 
         [0043]      FIG. 10  is a simplified block diagram illustrating a temperature compensated fiber optic current sensing system  78  using a common light source  80  transmitting light to a fiber optic current transducer  82  and a fiber optic temperature sensor  84  to generate current and temperature signals received via a common detector  86  according to one embodiment of the present invention. 
         [0044]      FIG. 11  is a simplified block diagram illustrating a temperature compensated fiber optic current sensing system  88  using a common light source  80  transmitting light to a fiber optic current and temperature sensor  76  to generate current and temperature signals received by a common detector  86  according to one embodiment of the present invention. The current and temperature sensing elements  76  are integrated with an optical fiber common to both sensors. 
         [0045]      FIG. 12  is a simplified block diagram illustrating a temperature compensated fiber optic current sensing system  90  using a common light source  80  transmitting light to a fiber optic current and temperature sensor  76  to generate current and temperature signals received by and a plurality of detectors  70 ,  72  according to one embodiment of the present invention. The current and temperature sensing elements  76  are integrated with an optical fiber that may or may not be common to both sensors. Detector  70  operates to measure the current represented by the current signal, while detector  72  operates to measure the temperature represented by the temperature signal. 
         [0046]      FIG. 13  is a simplified block diagram illustrating a temperature compensated fiber optic current sensing system  92  using multiple light sources  62 ,  64  transmitting light to a fiber optic current and temperature sensor  76  to generate current and temperature signals received by a common detector  86  according to one embodiment of the present invention. The current and temperature sensing elements  76  may or may not be integrated with an optical fiber common to both sensors. The fiber optic current transducer is responsive to light transmitted from light source  62 , while the fiber optic temperature sensor is responsive to light transmitted from light source  64 . Detector  86  operates to measure the current represented by the current signal and also to measure the temperature represented by the temperature signal. The embodiments described above with reference to  FIGS. 1-13  are not so limited however; and it shall be understood that many other embodiments can be formulated using the inventive concepts and principles described herein. 
         [0047]      FIG. 14  depicts a physical temperature compensated fiber optic current sensing system  100  according to one embodiment, using one or multiple fiber Bragg grating sensors  102  to implement fiber optic temperature sensors and fiber optic current transducer  110  to measure current based on Faraday effect, corresponding to system architecture represented by  FIGS. 4 and 8 . Fiber optic current transducer signals are transmitted along optical fiber  110  while fiber optic temperature signals are transmitted along optical fiber  108 . Temperature sensor detector  106  receives temperature signals via optical fiber  108  while current sensor detector  104  receives current signals via a separate corresponding optical fiber  110 . Detector unit  106  includes a light source for the fiber optic temperature sensor(s) while detector unit  104  includes a light source for the fiber optic current transducer(s). The signal-processing unit  112  receives temperature information from detector  106  via a data communication link  114  and the current information from detector  104  via a data communication link  116  to generate a temperature compensated current measurement. 
         [0048]    Fiber optic current sensing system  100  is based on the Faraday effect, which is a magnetically induced birefringence and leads to the rotation of the plane of polarization of a traveling light wave. The Faraday effect can be observed in diamagnetic and paramagnetic material like optical fibers using either a polarimetric method to measure the rotation of a linear polarization or an interferometric method to measure the non-reciprocal phase shift. 
         [0049]      FIG. 15  depicts a physical temperature compensated fiber optic current sensing system  140  according to one embodiment, using Gallium-Arsenide material (GaAs) optical reflectivity based fiber temperature sensing technology to implement a system architecture represented by  FIGS. 3 and 8 . Fiber optic current sensing system  140  includes a GaAs chip  142  that operates to reflect signals in response to light generated by a light source  144 . Fiber optic current transducer signals are transmitted along optical fiber  158  while fiber optic temperature signals are transmitted along optical fiber  160 . 
         [0050]    GaAs chip  142  comprises a direct band-edge material, which is optically transparent at light wavelengths above about 850 nm due to its internal material band edge. However, the position of this band edge is temperature dependent and shift about 0.4 nm per degree Kelvin. Other materials that may be used as direct band edge temperature sensors include without limitation, type III-V and type II-VI materials. Type III-V materials may include, for example, Gallium Arsenide, Indium Phosphide, Gallium Phosphide, Gallium Nitride, Aluminum Nitride, Indium Gallium Phosphide, Gallium Arsenide Phosphide, Indium Phosphide Arsenide, Aluminum Gallium Arsenide, Gallium Indium Arsenide Phosphide and Indium Arsenide. Type II-VI materials may include, for example, Zinc Telluride, Cadmium Sulphide, Cadmium Telluride, Cadmium Selenide, Zinc Selenide, Zinc Sulphide Selenide, Zinc Cadmium Sulphide, Zinc Oxide, Indium Selenide and Zinc Sulphide. 
         [0051]    The current transducer head  148  comprises small crystal faraday garnet material exhibiting magneto optic sensitivity (high Verdet constant) that is at least an order of magnitude higher than those of typical paramagnetic and diamagnetic optical fiber based materials. Sensor head  148  measures the current based on the Faraday effect, which is a magnetically induced birefringence and leads to the rotation of the plane of polarization of a traveling light wave transmitted through the faraday garnet. A signal-processing unit  150  receives temperature information from detector  144  via data communication link  152  and the current information from detector  154  via data communication link  156  to generate a temperature compensated current measurement. 
         [0052]    Current and temperature information can be simultaneously determined by incorporating an optical fiber temperature sensing element directly into the fiber optic current sensing system, by placing the optical fiber temperature sensing element in the proximity of the Faraday crystal garnet, or along side of the optical fiber. The resultant integrated system will share many similar optical components, thus reducing the cost and size of a fiber optic sensor system. 
         [0053]    In summary explanation, a temperature compensated fiber optic current sensing system combines magnetic field or current sensing and temperature sensing to compensate temperature sensitive current measurements. According to one embodiment, the magnetic field or current transducer is based on the Faraday effect in optical materials such as diamagnetic and/or paramagnetic optical fiber cores or ferromagnetic garnets. According to one aspect, the sensor system employs polarimetric sensing principles where the angle of polarized light rotates with respect to the strength of a magnetic field or current flow. The sensor system further employs temperature sensing based on one or more intrinsic and extrinsic fiber optic sensing methods. The optical fiber temperature sensing methods and/or elements can include, without limitation, measurements based on measurement techniques selected from fiber Bragg grating measurements, Raman scattering, Brillouin scattering, Fabry-Perot interferometric measurements, Mach-Zehnder interferometric measurements, Michelson interferometric measurements, Sagnac interferometric measurements, microbending measurements, macrobending measurements, polarimetric measurements, pyrometric measurements, reflectivity measurements, and emissivity measurements. 
         [0054]    Combining both sensors on one fiber provides a cost effective system. Since the Faraday effect is strongly temperature dependent, the measured temperature can be used to calibrate in real-time the current/magnetic field measurements. The location of the temperature sensor points can be at separate optical components or can be combined along with the optical magnetic field and current transducer such as magnetic field sensitive optical fiber or Faraday garnet(s). 
         [0055]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.