Abstract:
Remote sensors and methods of remote sensing are disclosed. A remote sensor includes a first circuit and a second circuit. The first circuit includes a first coil, a magnetic field generator for driving a current through the first coil to generate a magnetic field, and circuitry for determining loading of the magnetic field. The second circuit includes a second coil located proximate the first coil and a voltage-to-current converter for converting a voltage at an input of the second circuit to current and applying the current to the second coil. The current in the second coil registers as a loading of the magnetic field generated by the first coil. The loss, in response to the loading of the magnetic field, is measurable by the first circuit.

Description:
[0001]    This application claims priority to U.S. provisional patent application 61/904,269 of Steven R. Widener, filed on Nov. 14, 2013 for NON-CONTACT REMOTE VOLTAGE/CURRENT MEASUREMENT TECHNIQUE, which is hereby incorporated by reference for all that is disclosed therein. 
     
    
     BACKGROUND 
       [0002]    Some voltage measuring circuits measure voltages on remote circuits wherein there is a substantial common mode voltage difference between the two circuits. Isolation protects the measuring circuit from being damaged by the voltages generated in the remote circuit that are being measured. In some circuits, isolated voltage or current measurements are required for safety and/or operational concerns. One example of this type of measurement is a voltage measurement used in power supply feedback for the purpose of voltage regulation. Some of these power supplies have an optically coupled device to relay information about a voltage of concern across an isolation barrier to a measurement circuit. However, these optically coupled devices are expensive and unreliable. 
       SUMMARY 
       [0003]    This disclosure has several aspects. One aspect of the disclosure is a remote sensor that includes a first circuit and a second circuit. The first circuit includes a first coil, a magnetic field generator for driving a current through the first coil to generate a magnetic field, and circuitry for determining loading of the magnetic field. The second circuit includes a second coil located proximate the first coil and a voltage-to-current converter for converting a voltage at an input of the second circuit to current and applying the current to the second coil. The current in the second coil registers as a loading of the magnetic field generated by the first coil. The loss, in response to the loading of the magnetic field, is measurable by the first circuit. 
         [0004]    In some aspects of the remote sensor, the first circuit includes an inductance to digital converter. In other aspects, the magnetic field in the first coil induces a voltage in the second coil and the voltage induced in the second coil may drive at least one electronic component on the second circuit. The second circuit may include a rectifier and/or a filter coupled to the second coil, for rectifying and filtering the voltage induced in the second coil. 
         [0005]    In some aspects of the remote sensor, the voltage-to-current converter includes a differential amplifier, a transistor, and a resistance. The differential amplifier has a first input and a second input, wherein the first input is coupled to the input of the second circuit. The base of the transistor is coupled to the output of the amplifier. The resistance is coupled between the emitter of the transistor and a common node. Feedback is coupled between the emitter of the transistor and the second input of the amplifier. 
         [0006]    A trans-impedance amplifier may be coupled between the input of the second circuit and the voltage-to-current converter, wherein the trans-impedance amplifier is for converting a current flowing at the input to the second circuit to a voltage. In some versions of the remote sensors, a resistance is coupled across the input of the second circuit, wherein the remote sensor is for measuring current flowing through the resistance. A voltage offset may be coupled to the input, for offsetting the voltage at the input of the second circuit. 
         [0007]    Other aspects of the remote sensor include a third coil that is coupleable to the first circuit, wherein the third coil is operable to generate a magnetic field. A fourth coil is located proximate the first coil, wherein the magnetic field generated by the third coil induces a voltage in the fourth coil. A resistance having a predetermined value is coupled to the fourth coil. In some aspects, the fourth coil and the resistance are for inducing a predetermined loss in the magnetic field generated by the third coil. The third coil may be coupleable to the magnetic field generator. 
         [0008]    Another aspect of the disclosure includes a method for measuring a remote voltage. An example of the method includes generating a magnetic field in a first coil, inducing a voltage in a second coil in response to the magnetic field, converting the remote voltage to a loading current, applying the loading current to the second coil, measuring the loading in the magnetic field in response to the loading current, and correlating the loading in the magnetic field with the magnitude of the remote voltage. 
         [0009]    Other aspects include rectifying the voltage induced in the second coil and driving at least one electronic component on the second circuit with the rectified voltage. Another variation includes rectifying the voltage induced in the second coil, filtering the rectified voltage, and driving at least one electronic component on the second circuit with the filtered voltage. Other variations include driving a current through a resistance to generate the remote voltage. 
         [0010]    Other aspects include driving a current through a third coil and inducing a magnetic field in a fourth coil in response to the current in the third coil, wherein the fourth coil causes a predetermined loading in the magnetic field. A variation of this aspect includes correlating the loading in the magnetic field with the distance between the first coil and the second coil, wherein the magnitude of the remote voltage is a function of the distance. 
         [0011]    Another aspect of the disclosure includes a first circuit and a second circuit. The first circuit includes a first coil and a proximity sensor for driving a current through the first coil to generate a magnetic field and to determine loading of the magnetic field. The second circuit includes a second coil located proximate the first coil. A a voltage-to-current converter converts a voltage at an input of the second circuit to current and drives the current through the second coil. The current in the second coil registers as a loading of the magnetic field generated by the first coil, wherein the loss is measurable by the proximity sensor. A rectifier coupled to the second coil rectifies a voltage induced in the second coil by the magnetic field, wherein the rectified voltage is for driving at least one electronic device on the second circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a block diagram of a remote sensing system for measuring a remote voltage. 
           [0013]      FIG. 2  is a graph depicting the relationship between the sensed voltage of  FIG. 1  and the output of the proximity sensor of  FIG. 1 . 
           [0014]      FIG. 3  is a detailed schematic diagram illustrating an implementation of the remote sensing system of  FIG. 1 . 
           [0015]      FIG. 4  is a detailed schematic diagram illustrating an implementation of the remote sensing system that includes a trans-impedance amplifier. 
           [0016]      FIG. 5  is a detailed schematic diagram illustrating an implementation of the remote sensing system that includes a voltage offset coupled to the input. 
           [0017]      FIG. 6  is a flow chart illustrating the operation of a remote sensing system. 
           [0018]      FIG. 7  is a diagram illustrating implementation of a remote sensing system. 
           [0019]      FIG. 8  is a schematic illustration of a calibration circuit for a remote sensing system. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Remote current and/or voltage sensing systems and methods of measuring remote voltages and currents are disclosed herein. Reference is made to  FIG. 1 , which is a block diagram of a remote voltage sensing system  100 . The system  100  includes a first circuit  102  and a second circuit  104 . In the example of  FIG. 1 , the first circuit  102  is a circuit that is conventionally used to determine the distance between a target object (not shown) and a first coil L 1 . The distance measurement or proximity measurement is conventionally achieved by inducing a magnetic field in the target object and calculating the distance as a function of the losses associated with the target object. The first circuit  102  includes a proximity sensor  105  that calculates distance as a function of losses resulting from the magnetic field induced in the target object. 
         [0021]    The system  100  induces a magnetic field in a second coil L 2  rather than a target object. The second coil L 2  is located a distance  106  from the first coil L 1 . In some examples of the system  100 , the distance  106  is approximately thirty-nine mils (0.99 mm) and the first coil L 1  and the second coil L 2  are fixed relative to each other so that the distance  106  is a fixed distance. The second circuit  104  further includes an input  110 , which in the example of  FIG. 1  is coupled to a sensed voltage V SEN . The input  110  is also coupled to a voltage-to-current converter  112  that sinks a current I1, wherein the current I1 is proportional to the voltage at the input  110 . The current I1 loads a tank circuit  104  that includes the second coil L 2  and a capacitor C 1 . The current I1 is sometimes referred to as the loading current. A magnetic field is generated by the first circuit  102  by driving current through a tank circuit  114  that includes the first coil L 1  and a capacitor C 2 . The magnetic coupling of the second coil L 2  to the first coil L 1  results in a loss detected by the proximity sensor  105  due to the magnetic field generated by the first coil L 1 . The loss is registered as the virtual resistance R S  by the proximity sensor  105  and is used to calculate the distance between the first coil L 1  and the second coil L 2 . The distance measurement calculated by the proximity sensor  105  is a function of the sensed voltage V SEN  because the distance  106  between the coils L 1  and L 2  is fixed. More specifically, the voltage V SEN  is converted to a current that simulates a distance measurement calculated by the first circuit  102 . The sensed voltage V SEN  is then readily calculated as a function of the distance calculated by the proximity sensor  105 . More specifically, the proximity sensor  105  measures magnetic losses that are correlated to the sensed voltage V SEN . Accordingly, the system  100  measures the sensed voltage V SEN  without any physical connection between the first circuit  102  and the second circuit  104 . 
         [0022]    Having summarily described the system  100 , it will now be described in greater detail. The first circuit  102  includes the proximity sensor  105 , which in some examples includes or is based around an inductance-to-digital converter, such as the LDC 1000 manufactured by the Texas Instruments Corporation of Dallas, Tex., USA. The inductance-to-digital converter drives a current through the first coil L 1 , which generates a magnetic field. When a target object that includes a conductive material, such as a metal target, is brought into the proximity of the first coil L 1 , the magnetic field induces circulating currents (eddy currents) within the target object. The eddy currents are a function of the distance, size, and composition of the target. The eddy currents generate their own magnetic field, which opposes the magnetic field generated by the first coil L 1 . The magnetic field mechanism is similar to a transformer, where the first coil L 1  is the primary coil and the eddy currents are the secondary core. The inductive coupling between the primary coil and the secondary coil depends on the distance and shapes of the cores. Hence, the resistance and inductance of the secondary coil (eddy current) is detected as a distant dependent resistive and inductive component in the coil L 1 . As described above, the system  100  replaces the target object with the second coil L 2 . 
         [0023]    The proximity sensor  105  generates current that causes an alternating magnetic field to be generated by the tank circuit  114 , which includes the first inductor L 1  and the capacitor C 2 . The parallel coupling of the capacitor C 2  and the first inductor L 1  minimizes the power consumption of the first circuit  102 . The parallel coupling of the first inductor L 1  and the capacitor C 2  also forms a resonator so that the power consumption is reduced to the eddy currents and inductor losses that are modeled by the resistor R S . The resistor R S  is sometimes referred to as a virtual resistor. Some inductance-to-digital converters simultaneously measure the impedance and resonant frequency of the resonator formed by the tank circuit  104 . The inductance-to-digital converters accomplish this task by regulating the oscillation amplitude in a closed loop configuration to a constant level while monitoring the energy dissipated by the resonator. By monitoring the amount of power injected into the resonator, the inductance-to-digital converter determines the value of Rs. The converters can also measure the oscillation frequency of the LC circuit, which is the frequency that determines the inductance of the LC circuit. 
         [0024]    Rather than calculating parameters of a target, the first circuit  102  calculates parameters of the effects generated by the loading of the second coil L 2  on the magnetic field generated by the first coil L 1 . The coupling of the magnetic field by the second coil L 2  simulates eddy current losses that are proportional to the sensed voltage V SEN  at the input  110  to the voltage-to-current converter  112 . The loading of the magnetic field resulting from the second coil L 2  is measured by the first circuit  102 , wherein the distance determined by the first circuit  102  is proportional to loading by the second coil L 2 . The loading of the second coil L 2  is a function of the sensed voltage V SEN , so the distance measurement calculated by the first circuit  102  is a function of the sensed voltage V SEN .  FIG. 2  is an example graph showing the relation between the sensed voltage V SEN  and the distance  106  calculated by the first circuit  102 . 
         [0025]    Reference is made to  FIG. 3 , which is a detailed schematic diagram illustrating an implementation of the remote sensing system  100  of  FIG. 1 .  FIG. 3  illustrates a system  300  that includes a first circuit  302  and a second circuit  304 . The first circuit  302  is similar to the first circuit  102  of  FIG. 1 . The second circuit  304  includes a tank circuit  306  that includes the second coil L 2  and the capacitor C 1 . The tank circuit  306  is tuned to the frequency emitted by the first coil L 1 . The tank circuit  306  inductively couples the magnetic field so a loss is detected by the first circuit  302 , wherein the loss is proportional to the sensed voltage V SEN . The tank circuit  306  also converts the magnetic field induced on it by the first circuit  302  to an alternating voltage across the coil L 2  and the capacitor C 1 . 
         [0026]    The tank circuit  306  is coupled to a rectifier  310  that includes four diodes D 1 -D 4 . In some aspects of the rectifier  310 , the diodes D 1 -D 4  are BAT54 type diodes. The rectifier  310  performs full wave rectification on the voltage across the coil L 2  and the capacitor C 2 . Accordingly, the voltage at a node N 1  is a full wave rectified sine wave. The node N 1  is coupled to a filter  312  that includes a diode D 5  and a capacitor C 3 . In some aspects of the filter  312 , the diode D 5  is a BAT54 type diode and the capacitor C 3  has a value of approximately 0.1 uf. The filter  312  filters out AC components from the rectified sine wave so that the voltage at a node N 2  is substantially a DC voltage that is capable of providing power to components in the second circuit  304 . 
         [0027]    The sensed voltage V SEN  is coupled to a voltage-to-current converter  314 . The voltage-to-current converter  314  includes a differential amplifier  316 , a transistor Q 1 , and a resistor R 1 . In the example of  FIG. 3 , the differential amplifier  316  is an operational amplifier and the sensed voltage V SEN  is coupled to the non-inverting input of the operational amplifier. In some aspects of the voltage-to-current converter  314 , the differential amplifier is an OPA348 type device and is powered by the voltage at the node N 2 . The amplifier  316  may be selected for rail-to-rail capabilities and low quiescent current to avoid inducing non-linearities in the voltage-to-current transfer function. It is noted that the current powering the amplifier  316  may be much less than the current that loads the voltage at the node N 1 . The output of the amplifier  316  is coupled to the base of the transistor Q 1 . The emitter of the transistor Q 1  is coupled to the inverting input of the amplifier  316 . The collector of the transistor Q 1  is coupled to the node N 1 . A resistor R 1  is coupled between the emitter of the transistor Q 1  and a common node N 3 , which may be a ground. In some aspects of the voltage-to-current converter, the transistor Q 1  is an MMBT3904 type device and the resistor R 1  has a value of approximately 4.99 kΩ. In some implementations, the transistor Q 1  may be a MOSFET in order to minimize the voltage-to-current transfer function errors that are due to the limited Hfe of the transistor Q 1 . 
         [0028]    As described above, the tank circuit  306  generates the AC voltage in response to a magnetic field induced on the second inductor L 2 . The AC voltage is rectified by the rectifier  310 . The AC components are removed from the rectified signal to form a substantially DC voltage at node N 2  that powers the amplifier  316 . Accordingly, the second circuit  304  does not require a separate power supply. 
         [0029]    As described above, the proximity determination made by the first circuit  302  is a function of the sensed voltage V SEN . The sensed voltage V SEN  is presented to the non-inverting input of the amplifier  316 , which provides a high impedance for the sensed voltage V SEN . The amplifier  316  adjusts its output so that the voltage across the resistor R 1  is substantially the same as the sensed voltage V SEN . The voltage across the resistor R 1  causes a constant current of magnitude V SEN /R 1  to flow through the emitter of the transistor Q 1  and load the tank circuit  306  and the rectifier  310  with a current I2, which is sometimes referred to as a loading current. The field induced by the current I2 appears to the first circuit  302  to represent a loss, noted as the virtual resistor R S , that must be replenished. This loss is reported in the proximity sensor  105  as a proximity reading and is a function of the sensed voltage V SEN  for a given physical geometry of coils L 1  and L 2 . 
         [0030]    In some examples, the loss calculated by the first circuit  302  is calibrated to the sensed voltage V SEN  so that the sensed voltage V SEN  is readily calculated. More specifically, the proximity determined by the first circuit  302  is a function of the sensed voltage V SEN . Accordingly, the sensed voltage V SEN  is readily determined based on the function. In other examples, look-up tables or the like store information regarding the correlation between proximity and the sensed voltage V SEN . A processor or similar device (not shown in  FIG. 3 ) may solve the function or retrieve data from the look-up table to determine the sensed voltage V SEN . The above-described function or the look-up table may include correction factors to account for non-linearities in the transfer function between distance and sensed voltage V SEN , temperature dependency, and corrections determined from calibration information. In addition, other factors including humidity and aging may be used to improve the measurement accuracy. 
         [0031]    Some embodiments of the second circuit  304  include a sensing resistor (not shown in  FIG. 3 ) coupled across the input  320  to measure current. The current flow through the sensing resistor generates the sensed voltage V SEN  which is determined by the first circuit  302 . Reference is made to  FIG. 4 , which is a system  400  that includes a trans-impedance amplifier  402  coupled to the second circuit  404 . The trans-impedance amplifier  402  measures very small sensing currents I SEN . The trans-impedance amplifier  402  includes an amplifier  408  with the inverting input coupled to the sensing current I SEN . A resistor R SEN  provides feedback to the inverting input. A voltage reference V REF  is coupled to the non-inverting input of the amplifier  408 . The voltage reference V REF  provides offset to the amplifier  408  to enable adequate operating headroom for positive and/or negative currents. 
         [0032]      FIG. 5  is an embodiment of a system  500  that senses positive or negative voltages or currents. The second circuit  504  includes an offset reference voltage V REF  coupled to the amplifier  316 . In the example of  FIG. 5 , the sensed voltages V SEN  that are equal to zero are processed by the second circuit  504  as a fixed offset voltage level. As the sensed voltage V SEN  transitions positive or negative, the calculated sensed voltage V SEN  will be more or less than the offset level. The proximity sensor  105  or other calculating device coupled thereto may mathematically remove the offset level and provide bipolar measurements for the sensed voltage V SEN . 
         [0033]      FIG. 6  is a flow chart  600  describing the operation of the system  300  of  FIG. 3 . In step  602 , a magnetic field is generated in the first coil  1 . In step  604 , a voltage is induced in the second coil L 2  in response to the magnetic field. The remote voltage, which is V SEN , is converted to a loading current in step  606 . In step  608 , the loading current is applied to the second coil L 2 . The loss in the magnetic field in response to the loading current is measured at step  610 . The loss of the magnetic field is correlated to the remote voltage in step  612 . 
         [0034]      FIG. 7  is a block diagram of a remote sensing system  700  implemented to measure a remote sensed voltage V SEN  generated by a remote voltage source  701 . As with the previous examples, the system  700  includes a first circuit  702  and a second circuit  704 . The first circuit  702  has a portion  706  that includes a first coil (not shown in  FIG. 7 ) as depicted by the first coil L 1  of  FIG. 1 . A portion  708  of the second circuit  704  includes a second coil (not shown in  FIG. 7 ) as depicted by the second coil L 2  of  FIG. 1 . A gap  710  having a fixed width is located between the portion  706  and the portion  708  so that the portions  706  and  708  are located a fixed distance from one another. In some aspects of the system  700 , the gap  710  is air. In other aspects of the system  700 , the gap  710  is filled with a solid structure that secures the portions  706  and  708  and enables a magnetic field to permeate through the gap  710 . 
         [0035]    The second circuit  704  includes a voltage-to-current converter  712  that is coupled to the portion  708  by conductors  714 . In some examples, the portion  708  is on a substrate that includes the voltage-to-current converter  712 . The voltage-to-current converter  712  is coupled to the remote voltage source  701  by conductors  716 . In some examples of the system  700 , the remote voltage source  701  is located on the same substrate as the voltage-to-current converter  712 . The first circuit  702  includes a proximity sensor  720 , which, in the example of  FIG. 7 , is located on the same substrate as the portion  706  that includes the first coil. In some examples, the proximity sensor  720  is located on a separate substrate than the portion  706 . 
         [0036]    The system  700  includes a processor  724  that is coupled to the first circuit  702 . The processor  724  receives data generated by the proximity sensor  720  and calculates the sensed voltage V SEN  based on the above-described magnetic losses. More specifically, the processor  724  receives proximity data generated by the proximity sensor  720  and correlates the proximity data to the sensed voltage V SEN  as described with reference to  FIG. 2 . In some examples, the processor  724  and the proximity sensor  720  are a single device. 
         [0037]    The proximity sensor  720  generates a magnetic field in the coil in the portion  706 . The magnetic field permeates through the gap  710  where it induces a voltage across the coil in the portion  708 . The voltage-to-current converter  712  converts the sensed voltage V SEN  to a loading current that flows through the coil in the portion  708 . The proximity sensor  720  detects the loading current in the second coil as a loss and correlates the loss to the distance of the gap  710 . The processor  724  correlates the distance of the gap to the sensed voltage V SEN  and outputs data indicating the sensed voltage V SEN . 
         [0038]      FIG. 8  is a schematic diagram of a remote sensing system  800  with a calibration circuit  802  coupled thereto. The system  800  includes switches  806  and  808  that couple the proximity sensor  810  to either the first circuit  814  or the calibration circuit  802 . The switches  806  close and the switches  808  open to couple the proximity sensor  810  to the first circuit  814 . Alternatively, the switches  806  open and the switches  808  close to couple the proximity sensor to the calibration circuit  802 . 
         [0039]    The calibration circuit  802  includes a target circuit  820  and an induction circuit  822 . The target circuit  820  has a resistance R 4  that is connected in parallel with a capacitor C 5  and an inductor L 3 . In some examples, the resistance has a value of approximately 10 KΩ and the capacitor C 5  and inductor L 3  have the same values as the second inductor L 2  and capacitor C 1  in the second circuit  304 . The induction circuit  822  includes a coil L 4  and a capacitor C 6 . The proximity sensor  810  measures the above-described magnetic loss by way of a virtual resistance R S2  to determine the loss in the target circuit  820  caused by the resistance R 4 . The calibration circuit  802  may be nearly co-located with the coils L 1  and L 2  of the first circuit  302  and the second circuit  304 , so that variability in coupling and the impact of nearby metal objects is minimized through differential measurement techniques. The calculation of the resistance R S2  through the proximity sensor  810  determines scale and offset factors for the voltage or current measurement because the resistance R 4  is known and presents a known magnetic loss as seen by the first circuit  802 . 
         [0040]    While an illustrative and presently preferred embodiment of the disclosure have been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.