Abstract:
A current sense system includes a current transformer having a primary coil and a secondary coil, wherein the secondary coil has a first and second terminal; a burden resistor connected between the first terminal of the secondary coil and ground; a monitor circuit that measures current in the primary coil by monitoring voltage across the burden resistor; and a built-in test (BIT) circuit connected to the second terminal of the secondary coil. The BIT circuit provides a virtual ground during normal operation, and either a positive voltage or a negative voltage during test operations.

Description:
BACKGROUND 
       [0001]    The present invention relates to built-in test (BIT), and in particular to BIT for a current sensing circuit. 
         [0002]    Current sensing circuits typically employ a current transformer to monitor current in a system. The current in the system passes through the primary coil of the transformer, which generates a proportional current in the secondary coil of the transformer based upon the ratio of turns in each coil. This proportional current flows through a burden resistor in order to create a voltage that is measurable by a monitor circuit. 
         [0003]    Current sensing circuits are tested to ensure proper functionality. In the past, this involved using separate circuitry to inject currents directly to the burden resistor in order to test the monitor circuit. This is not ideal in that it requires external circuitry, and does not test the current transformer itself. 
       SUMMARY 
       [0004]    A current sense system includes a current transformer, a burden resistor, a monitor circuit, and a built-in test (BIT) circuit. The current transformer has a primary coil and a secondary coil. The secondary coil has a first and second terminal. The burden resistor is connected between the first terminal of the secondary coil and ground. The monitor circuit measures current in the primary coil by monitoring voltage across the burden resistor. The BIT circuit is connected to the second terminal of the secondary coil. The BIT circuit provides a virtual ground during normal operation, and either a positive voltage or a negative voltage during test operations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a block diagram illustrating a system for providing built-in-test (BIT) injection for a current sensing circuit according to an embodiment of the present invention. 
           [0006]      FIG. 2  is a flowchart illustrating a method for providing BIT injection for a current sensing circuit according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0007]    The present invention describes a system and method for providing built-in test (BIT) injection for current sensing circuits that employ a current transformer. One terminal of the secondary coil of the current transformer is connected to the output of an operational amplifier (op-amp) circuit. During normal system operation, the output of the op-amp circuit provides a virtual ground, and therefore does not alter the current flowing through the secondary coil. During test, the op-amp circuit can either inject a positive direct-current (DC) current or a negative DC current to the secondary coil of the current transformer, depending on the input to the op-amp. The current through the secondary coil is then measured by a monitor circuit that includes a rectifier and an analog-to-digital converter. This monitor circuit is used during normal system operation to monitor the alternating-current (AC) current through the primary coil of the current transformer. In this way, the current sensing circuit, including the current transformer, can be tested at any time, without external circuitry or physical presence at the system. 
         [0008]      FIG. 1  is a block diagram illustrating system  10  for providing built-in test (BIT) injection for a current sensing circuit according to an embodiment of the present invention. System  10  includes BIT circuit  12 , burden resistor  14 , current transformer  16 , primary coil  18 , secondary coil  20 , signal monitor circuit  22 , primary coil terminals  24   a  and  24   b,  and relay  34 . BIT circuit  12  includes op-amp circuit inputs  26   a  and  26   b,  BIT circuit output line  28 , op-amp circuit  30 , and BIT controller  32 . Signal monitor circuit  22  includes rectifier  36 , analog-to-digital (A-D) converter  38 , and output  40 . Output  40  may be provided to, for example, a microcontroller capable of analyzing a digital signal such as an electronic engine controller (EEC) onboard an engine of an aircraft. BIT controller  32  is any microcontroller capable of outputting voltage to op-amp circuit  30  such as, for example, a field programmable gate array (FPGA). BIT controller  32  may be implemented as a part of an EEC. 
         [0009]    BIT circuit  12  provides three distinct output states to secondary coil  20  on BIT circuit output line  28 . The first state, which is used during normal system operation, provides a virtual ground on output line  28 . This allows the AC current generated in secondary coil  20  during normal system operation to flow through burden resistor  14  as if secondary coil  20  were connected to ground. In the second state, BIT circuit  12  provides a positive voltage on output line  28  in order to inject a positive DC current through secondary coil  20 . In the third state, BIT circuit  12  provides a negative voltage on output line  28  in order to inject a negative DC current through secondary coil  20 . 
         [0010]    In one embodiment, op-amp circuit  30  is an operational amplifier implemented as a differential amplifier with input  26   a  as the non-inverting input and input  26   b  as the inverting input. Op-amp circuit inputs  26   a  and  26   b  are connected to BIT controller  32 . BIT controller  32  provides four possible binary input combinations to op-amp circuit  30 . These inputs may be generated, for example, using outputs of metal-oxide-semiconductor field-effect transistors (MOSFETs) that produce output voltages of, for example, 3.3 volts. If the voltage on input  26   a  is a logic high value and the voltage on input  26   b  is a logic low value, the output provided on BIT circuit output line  28  will be a known positive voltage based upon the characteristics of the differential amplifier. If the voltage on input  26   a  is a logic low value and the voltage on input  26   b  is a logic high value, the output provided on BIT circuit output line  28  will be a known negative voltage based upon the characteristics of the differential amplifier. If the voltages on both inputs  26   a  and  26   b  are logic low values, there will be no output of BIT circuit  12  and the output provided on BIT circuit output line  28  will act as a virtual ground for system  10 . The final state, both inputs  26   a  and  26   b  being logic high values, will also produce a virtual ground, but is not necessary because it is redundant with the state of both inputs  26   a  and  26   b  being logic low values. 
         [0011]    During normal system operation, BIT controller  32  provides logic low voltage on both inputs  26   a  and  26   b.  AC current flows through primary coil  18 , and in turn an AC current is generated in secondary coil  20  that is proportional to the current in primary coil  18 . BIT circuit  12  acts as a virtual ground so AC current flows through secondary coil  20  and burden resistor  14 . This creates an AC voltage across burden resistor  14 . Rectifier  36  receives the AC voltage across burden resistor  14  and provides a unidirectional voltage to A-D converter  38 . A-D converter  38  then provides a digital output representative of the root mean square (RMS) voltage from rectifier  36 . This digital output is representative of the AC current present in primary coil  18 . Output  40  is used, for example, by a controller that implements digital logic to verify that the digital output is representative of the expected current through primary coil  18 . 
         [0012]    During a first BIT injection test, BIT controller  32  provides a logic high voltage on input  26   a  and a logic low voltage on input  26   b  such that a positive voltage is generated at the output of op-amp circuit  30 , creating a DC current through secondary coil  20 . Although not required, the current flowing through primary coil  18  may be terminated prior to initiating this BIT injection test using relay  34 . This allows for easier testing in that there is no AC current present in the system during the test. The positive voltage generated at the output of op-amp circuit  30  creates a DC current that flows through secondary coil  20  and burden resistor  14 . This creates a positive DC voltage across burden resistor  14 . This positive voltage passes through the positive voltage path of rectifier  34  and is provided to A-D converter  38 . A-D converter  38  provides a digital output to controller  38  representative of the voltage provided by rectifier  36 . Output  40  is then verified to determine that the BIT injection test was performed successfully using, for example, digital logic. If the test is unsuccessful, a failed test is indicated. A failed test indication is implemented as any common indication method, such as setting a bit or illuminating an light emitting diode (LED). 
         [0013]    During a second BIT injection test, BIT controller  32  provides a logic low voltage on input  26   a  and a logic high voltage on input  26   b  such that a negative voltage is generated at the output of op-amp circuit  30 . Although not required, the current flowing through primary coil  18  may be terminated prior to initiating this BIT injection using, for example, a relay. The negative voltage generated at the output of op-amp circuit  30  creates a DC current that flows through secondary coil  20  and burden resistor  14 . This creates a negative voltage across burden resistor  14 . This negative voltage passes through the negative voltage path of rectifier  36 , is converted into a positive voltage and provided to A-D converter  38 . A-D converter  38  provides a digital output on output  40  representative of the voltage provided by rectifier  36 . Output  40  is then verified to determine that the BIT test was performed successfully using, for example, digital logic. By injecting both a positive and a negative DC voltage, rectifier  36  can be fully tested. If the test is unsuccessful, a failed test is indicated. 
         [0014]      FIG. 2  is a flowchart illustrating a method  60  for providing BIT injection for a current sensing circuit according to an embodiment of the present invention. At step  62 , BIT controller provides a logic low voltage to both inputs of op-amp circuit  30  to provide a virtual ground during normal system operation. AC current is flowing through primary coil  18  which generates an AC current in secondary coil  20  that flows through burden resistor  14 . Monitor circuit  22  provides a digital output on output  40  for verification. At step  64 , BIT controller  32  provides a logic high voltage to the non-inverting input of op-amp circuit  30  and a logic low voltage to the inverting input of op-amp circuit  30  in order to provide positive current injection to secondary coil  20 . At step  66 , monitor circuit  22  provides a digital output on output  40  to be verified. If the injection test was successful, method  60  proceeds to step  68 . If the injection test was unsuccessful, method  60  proceeds to step  70 . At step  68 , BIT controller  32  provides a logic low voltage to the non-inverting input of op-amp circuit  30  and a logic high voltage to the inverting input of op-amp circuit  30  in order to provide negative current injection to secondary coil  20 . At step  72 , monitor circuit  22  provides a digital output on output  40  to be verified. If the injection test was successful, method  60  proceeds to step  74 . If the injection test was unsuccessful, method  60  proceeds to step  70 . At step  74 , the injection tests were successful and normal system operation resumes. At step  70 , an unsuccessful test is indicated. 
         [0015]    In this way, the present invention describes a system and method for providing built-in test (BIT) for circuits that employ current sensing through the use of a current transformer. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.