Abstract:
Apparatus and methods are provided for a system for measurement of a current in a conductor such that the conductor current may be momentarily directed to a current measurement element in order to maintain proper current without significantly increasing an amount of power dissipation attributable to the current measurement element or adding resistance to assist in current measurement. The apparatus and methods described herein are useful in superconducting circuits where it is necessary to monitor current carried by the superconducting elements while minimizing the effects of power dissipation attributable to the current measurement element.

Description:
GOVERNMENT INTEREST STATEMENT 
     This invention was made with Government support under Contract DE-FC36-93CH10580 awarded by the United States Department of Energy. The Government may have certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to measurement of current. 
     BACKGROUND 
     One of the chief features of superconducting wires is that they have extremely low (in the case of high-temperature superconductors) or zero (in the case of low temperature superconductors) resistance and hence dissipate little, if any, power. In many applications where they are used, it is necessary to monitor the current carried by the superconductor. The additional resistance and power dissipation associated with current-sensing systems, such as resistive current sensors, tends to defeat some of the advantage of using superconducting wire. Apparatus and techniques are needed to measure current in a conductor such as a superconducting wire in a straight forward manner without introducing additional resistance and power dissipation. 
     SUMMARY 
     The above mentioned problems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification. In various embodiments, apparatus and methods provide a system for measurement of a current in a conductor such that the conductor current may be momentarily directed to a current-measurement element. These and other aspects, embodiments, advantages, and features will become apparent from the following description and the referenced drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of features of an embodiment of a current measurement apparatus coupled to a conductor. 
         FIG. 2  illustrates a block diagram of a configuration of an embodiment of a current measurement apparatus coupled to a conductor. 
         FIG. 3  illustrates a block diagram of a configuration of an embodiment of a current measurement apparatus coupled to a conductor. 
         FIG. 4  illustrates a block diagram of a configuration of an embodiment of a current measurement apparatus coupled to a conductor including a switch to completely eliminate current flow to a current-measurement element. 
         FIG. 5  illustrates a block diagram of a system having an embodiment of a current measurement apparatus coupled to a conductor. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of the invention, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the present invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments disclosed herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. The following detailed description is, therefore, not to be taken in a limiting sense. 
     In an embodiment, current in a conductor may be momentarily directed through a current-measurement element to determine the amount of current in the conductor. Such a current measurement method may be used in a system where continuous current sensing is not required. In addition, such a current measurement method may be used in a system where power dissipation in a current-sensing element is of concern. For example, in situations where it is necessary that the current-sensing take place in a cryogenic environment, it may be desirable to eliminate power dissipation in a current-sensor. In an embodiment, current in a superconducting element may be momentarily directed through a current-measurement element to determine the amount of current in the superconducting element. The superconducting element may be, but is not limited to, a superconducting coil or a superconducting wire. The measured current may be compared against a threshold current level for operating the conductor. In an embodiment, if the measured current is less than the threshold current, a superconducting element may be charged to increase its current to a level above the threshold. In an embodiment, a superconducting coil may be in a persistent current mode in which current is continually flowing in the superconducting coil without constant coupling to a power source. 
       FIG. 1  illustrates a block diagram of features of an embodiment of a current measurement apparatus  100  coupled to a conductor  105 . Current measurement apparatus  100  may include a current-measurement element  110  and a switch  120 . Switch  120  may be a low-resistance switch that can be used to shunt current around current-measurement element  110  when current-measurement element  110  is not being used to measure the current flowing in conductor  105 . Switch  120  provides a means to reduce the power dissipation of current-measurement element  110  that may be applicable to situations where continuous current measurement is not required. Conductor  105  may be an electrical conductor through which current flows such as a current-carrying wire. Conductor  105  may be a superconducting element such as a superconducting wire and/or a superconducting coil. Conductor  105  may be a current-carrying element in a cryogenic environment. 
     Current-measurement element  110  may be realized in various configurations. Current-measurement element  110  may be a resistor. Measurement of the voltage across the resistor, having a known resistance, determines the current through the resistor. Current-measurement element  110  may be a semiconductor device configured to function as a resistor. Current-measurement element  110  is not limited to a resistor or a semiconductor device configured to function as a resistor but may be realized as other components or elements having known characteristics to provide a measure of the current through the component. 
     Switch  120  may momentarily direct current from conductor  105  to current-measurement element  110 . The switch may be realized as a semiconductor switch, a relay, other electronic and/or electro-mechanical switch, or a device to selectively direct current to current-measurement element  110 . Semiconductor devices, such as MOSFETS, may be adapted as low-resistance shunt switches, especially when they are cooled to cryogenic temperatures, such as for embodiments in which current sensing takes place at cryogenic temperatures. In addition, semiconductor devices may be controlled in a straight forward manner. A semiconductor switch may include using a transistor arrangement in which one or more transistors may be turned on or off to momentarily provide a current path to current-measurement element  110 . A semiconductor switch may include a MOSFET or other FET type device. The semiconductor switch may include bipolar transistors arranged to momentarily provide a current path to current-measurement element  110 . A variety of devices and device arrangements may configured in various embodiments as a switch to momentarily provide a current path to current-measurement element  110 . 
     In an embodiment, switch  120  momentarily directs conductor current through current-measurement element  110  for a period of time such that this period of time is short with respect to the period of time for which current does not flow in current-measurement element  110 . By momentarily directing conductor current to current-measurement element  110 , switch provides a short duty cycle for current measurement. In an embodiment, switch  120  may be opened for a time limited to the time necessary to measure the voltage across current-measurement element  110 . 
       FIG. 2  illustrates a block diagram of a configuration of an embodiment of a current measurement apparatus  200  coupled to a conductor  205 . Current measurement apparatus  200  and conductor  205  may be used in a manner as discussed with respect to the arrangement of  FIG. 1 . Current measurement apparatus  200  may include a switch  220  connected in parallel with current-measurement element  210  across which a voltage may be measured to determine the current in conductor  205 . Current-measurement element  210  may be a resistor having a known resistance or other component configured as a resistor having a known or calculable resistance. To reduce the power drain caused by current-measurement element  210 , conductor  205  may be operated with switch  220  in a normally closed position such as to effectively provide a short circuit across current-measurement element  210  in which essentially no current flows through current-measurement element  210 . This effective short circuit may be realized by a switch having a significantly small resistance compared with the resistance of current-measurement element  210 . When switch  220  is momentarily opened, current flows from conductor  205  through current-measurement element  210 , which provides a voltage that can be measured to determine the current flow. The current may be determined as the ratio of the measured voltage to the resistance of current-measurement element  210 . 
     In an embodiment, conductor  205  may be a superconducting coil, where current measurement apparatus  200  provides a current-sense system that is in series with superconducting coil  205 . This series configuration may couple to a power source or other means in which power is provided to superconducting coil  205 . This coupling may provide constant current to superconducting coil  205 . Switch  220  having a small resistance relative to current-measurement element  210  dissipates less power than having current-measurement element  210  in series with superconducting coil  105 . Thus, once the current of superconducting coil  205  is measured by directing the current through current-measurement element  210 , switch  220  may be closed to provide an effective short across current-measurement element  210 . In an embodiment, switch  220  may be opened for a time limited to the time necessary to measure the voltage across current-measurement element  210 . 
       FIG. 3  illustrates a block diagram of a configuration of an embodiment of a current measurement apparatus  300  coupled to a conductor  305 . Current measurement apparatus  300  and conductor  305  may be used in a manner as discussed with respect to the arrangement of  FIG. 1 . In an embodiment, conductor  305  may be a superconducting element  305  such as a superconducting wire or superconducting coil. Current measurement apparatus  300  provides a current-sense system that is in series with superconducting element  305 . In an embodiment, a superconducting coil  305  may operate in a persistent current mode. A persistent current mode is an arrangement in which a superconducting coil is brought to an operating current, is provided with a short circuit across the superconducting coil, is disconnected from the power source that provided the current to the superconducting coil, and operates with current continually flowing through it. With no resistance, a superconducting loop, which includes the superconducting coil in a closed loop, may operate with current flowing through it indefinitely. However, there is some resistance associated with superconducting coils and the leads providing the closed loop configuration, especially those superconducting coils operating at elevated temperatures. Thus, the current in a superconducting coil operating in a persistent current mode may decay over time. Such decay may be relatively slow. 
     In an embodiment, conductor  305  is a superconducting coil  305 , where current measurement apparatus  300  includes a switch  320  connected in parallel with current-measurement element  310  across which a voltage may be measured to determine the current in superconducting coil  305 . Current-measurement element  310  may be a resistor or other component configured as a resistor. To reduce the power drain caused by current-measurement element  310 , superconducting coil  305  may be operated with switch  320  in a normally closed position such as to effectively provide a short circuit across current-measurement element  310  in which essentially no current flows through current-measurement element  310 . This effective short circuit may be realized by a switch having a significantly small resistance compared with current-measurement element  310 . Power dissipation in switch  320  having a small resistance will cause the current to decay in superconducting coil  305 . Switch  320  may be selected based on acceptable current decay rates for superconducting coil  305 . When switch  320  is momentarily opened, current flows from superconducting coil  305  through current-measurement element  310 , which provides a voltage across current-measurement element  310 . This voltage may be measured to determine the current flow. Once the current is measured, switch  320  may be closed to once again provide an effective short across current-measurement element  310 . In an embodiment, switch  320  may be opened for a time limited to the time necessary to measure the voltage across current-measurement element  310 . 
       FIG. 4  illustrates a block diagram of a configuration of an embodiment of a current measurement apparatus coupled to a conductor including a switch to completely eliminate current flow to a current-measurement element. Current measurement apparatus  400  and conductor  405  may be used in a manner as discussed with respect to the arrangement of  FIG. 1 . In an embodiment, conductor  405  may be, but is not limited to, a semiconductor coil. 
     In an embodiment, current measurement apparatus  400  provides a current-sense system that is in series with a superconducting coil  405 . In conjunction with this configuration, superconducting coil  405  may operate in a persistent current mode. Current measurement apparatus  400  may include a switch  420  connected in parallel with a series combination of current-measurement element  410  and a switch  430 . A voltage may be measured across current-measurement element  410  to determine the current in superconducting coil  405 . Current-measurement element  410  may be a resistor or other component configured as a resistor. To reduce the current decay caused by current-measurement element  410 , superconducting coil  405  may operate with switch  420  in a normally closed position such as to effectively provide a short circuit across the series combination of current-measurement element  410  and switch  430 . In an embodiment, switch  430  may be open when switch  420  is closed so that no current flows through current-measurement element  410 . Switch  420  having a small resistance will cause the current to decay in superconducting coil  405 . Switch  420  may be selected based on acceptable current decay rates for superconducting coil  405 . When switch  420  is momentarily opened and switch  430  is momentarily closed, current flows from superconducting coil  405  through current-measurement element  410 , which provides a voltage. This voltage may be measured to determine the current flow. Once the current is measured, switch  420  may be closed and switch  430  opened, to once again eliminate current flow to current-measurement element  410 . In an embodiment, switch  430  may be closed to effectively generate the configuration of  FIG. 3 . With switch  430  closed, the effective short circuit provided by a closed switch  420  may be realized by switch  420  having a significantly small resistance compared with current-measurement element  410 . In an embodiment, switch  420  is opened and switch  430  is closed for a time limited to the time necessary to measure the voltage across current-measurement element  410 . 
       FIG. 5  illustrates a block diagram of a system  500  having an embodiment of a current sense apparatus  502  coupled to a conductor  505 . Conductor  505  may be any current-carrying element of system  500 . In various embodiments, the current sense apparatus  502  may be configured in manner as shown in  FIGS. 1-4 . In addition to current sense apparatus  502 , system  500  may include a power supply  525  and a control unit  515 . Control unit  515  may manage operation of switches  507 ,  527 , and  520 . Switches  507 ,  527 , and  520  may each be realized as a semiconductor switch, a relay, other electronic and/or electro-mechanical switch, or a device to selectively direct current to current-measurement element  510 . A semiconductor switch may include using a transistor arrangement in which one or more transistors may be turned on or off to momentarily provide a current path to current-measurement element  510 . A semiconductor switch may include a MOSFET or other FET type device. 
     In an embodiment, system  500  may include a superconducting coil  505  in which a switch  507  may be operated to place superconducting coil in a persistent current mode. With switch  527  closed and switch  507  open, power supply  525  may charge superconducting coil  505  to a desired operating current level. In embodiment, switch  520  may be open while charging superconducting coil  505 , so that current sense apparatus  502  will be active. With current sense apparatus  502  active, control unit  515  may monitor the coil current. When the desired current level is achieved, control unit  515  may close switch  507  to short superconducting coil  505  for persistent current operation. Control unit  515  may open switch  527  to disconnect power supply  525 . Alternatively, control unit  515  may shut off power supply  525 , in which case, depending on the type of power supply, switch  527  may be eliminated from the configuration for system  500 . Current will flow through the shorted loop with a time constant determined by the coil inductance and the net series resistance in the loop. Control unit  515  may also close switch  520  so that current sense apparatus  502  does not dissipate energy. Control unit  515  may periodically open switch  520  so that current sense apparatus  502  may provide a measure of the coil current. 
     At a point at which control unit  515  determines that the coil current has dropped by a pre-specified amount or has dropped below a threshold level, control unit  515  may open switch  507 , close switch  527  (or turn on power supply  525 ) to couple power from power supply  525  to superconducting coil  505 , and open switch  520  to monitor the coil current. These switches remain in these states until the coil current is increased to the desired level. In an embodiment, the threshold is set at 95% of a selected operating current for superconducting coil  505 . The threshold may be set at other levels depending on the application. After the desired level for the coil current is achieved, superconducting coil  505  may be placed back in the persistent current mode as described above. Control unit  505  may continue the cycle of periodically monitoring the coil current and recharging the current in superconducting coil  505 . 
     In an embodiment, system  500  may be a system that uses current sensing apparatus  502  to periodically measure the current in conductor  505  in a manner similar to that discussed herein. In an embodiment, system  500  includes a superconducting coil  505  configured as a direct current (DC) magnet. In an embodiment, system  500  includes a rotating machine. In an embodiment, system  500  includes a superconducting motor or generator. In an embodiment, system  500  is configured to use a current sensing scheme in which a very low resistance switch may momentarily direct current from conductor  505  to current-measurement element  510  to determine the current of conductor  505 . Current-measurement element  510  may effectively be out of the circuit in which current flows through conductor  505  during times in which a current measurement is not being made. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of embodiments of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive, and that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Combinations of the above embodiments, and other embodiments, will be apparent to those of skill in the art upon studying the above description.