Patent Description:
A current sensor such as a Rogowski coil, air-cored coil, etc. is suitable for detecting large currents because of its flexibility. A large current is sometime carried by a thick metal bus bar. If the metal bus bar is intricately wired, it would be difficult to locate a desired line under test of the bus bar at a position for detecting a current through the line with a current sensor. A flexible Rogowski coil allows a user to make a loop around the line under test.

<FIG> is a schematic block diagram of a current sensor <NUM> having a Rogowski coil <NUM> and an integrator circuit <NUM> usable in a current probe. The Rogowski coil <NUM> has a detecting coil <NUM> and a conducting return wire <NUM> that are made of a wire. A portion of the wire is formed in loops to produce the detecting coil <NUM>. The return wire <NUM> has one end 9b connected to the loop end 12b of the detecting coil <NUM> and is folded back through the center of the detecting coil <NUM> to the loop beginning end 12a of the detecting coil <NUM>. The loop beginning end 12a of the detecting coil <NUM> and the free end 9a of the return wire <NUM> are closely located. In addition, when a user conducts a measurement, the ends 12a and 12b of the detecting coil <NUM> are arranged to be physically close together so that the detecting coil <NUM> constitute a magnetic closed loop around a line under test <NUM>. The loop beginning end 12a of the detecting coil <NUM> is coupled to the integrator circuit <NUM> that typically has a resister <NUM>, a capacitor <NUM> and an operational amplifier <NUM>. The free end 9a of the conducting return wire <NUM> is coupled to ground.

A current Ip flowing in the line under test <NUM> generates magnetic flux that induces a voltage in the Rogowski coil <NUM>. If the frequency of the current Ip becomes higher, the induced voltage also becomes higher. The integrator circuit <NUM> maintains a flat frequency characteristic by lowering the gain of the integrator circuit <NUM> as the frequency of the current Ip increases. <CIT>et al. and <CIT>disclose some applications of a Rogowski coil.

<FIG> is a partial schematic diagram of a device under test showing the use of Rogowski coil sensor <NUM> for measuring a current signal. The device under test in <FIG> is an IGBT (Insulated Gate Bipolar Transistor) <NUM> in inverter circuitry for driving an inductive load, or a three-phase motor <NUM>. The inverter circuitry has a plurality of IGBTs <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> which have respective flywheel diodes <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. Since the three-phase motor <NUM> is an inductive load, it stores energy in the inductance and regenerates a flywheel current that passes through the flywheel diodes <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. A power supply <NUM> provides power to the inverter circuitry. A PWM (Pulse Width Modulation) controller <NUM> is coupled to an IGBT driver <NUM> via a bus to provide a PWM control signal. The IGBT driver <NUM> provides gate drive voltages to the IGBTs <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. The pulse width of the gate drive voltages is modulated according to the PWM control signal.

The positive and negative inputs of a differential probe <NUM> are coupled to the collector and emitter of the IGBT <NUM> respectively to detect a voltage, Vce, between the collector and emitter. The output of the differential probe <NUM> may be connected to a first channel (CH1) of a digital oscilloscope <NUM>. The positive and negative inputs of a differential probe <NUM> are coupled to the gate and emitter of the IGBT <NUM> respectively to detect a voltage, Vge, between the gate and emitter or a gate drive voltage. The output of the differential probe <NUM> may be connected to a third channel (CH3) of the oscilloscope <NUM>. A current probe <NUM> may use a Rogowski coil <NUM> and integrator circuit <NUM> to detect an emitter current Ie of the IGBT <NUM>. The output of the current probe <NUM> is coupled to a second channel (CH2) of the oscilloscope. The digital oscilloscope <NUM> receives the current and voltage signals from the device under test and stores the signals as digital data for display as waveforms.

<FIG> shows voltage and current waveforms of the three channels of the oscilloscope <NUM>. The horizontal and vertical axes are respectively amplitude and time. The time axis of display area A is longer than that of display area B. That is, the voltage and current waveforms of the display area B are zoomed-in version of the voltage and current waveform portions indicated by a box <NUM> in the display area A. Each pulse of the voltage and current waveforms of CH1 and CH2 shows a surge at the rising edge.

<FIG> shows the voltage and current waveforms of the three channels that corresponding to the display area A of <FIG> wherein the vertical axis is enlarged relative to that of the display area A of <FIG>. The current waveform of the second channel shown in <FIG> has positive half-cycles and negative half-cycles. The current waveform shows current flowing in the emitter of IBGT <NUM> during the positive half cycle as a result of the gate drive voltage turning IBGT <NUM> On and Off using pulse width modulation. During the negative half-cycle, current is drawn through the flywheel diode <NUM> to the motor <NUM>.

Referring again to the current waveform of the second channel of <FIG>, an arrow <NUM> indicates zero ampere level of the current waveform at the left end of the waveform in the display area. <FIG> shows that the zero ampere level of the current waveform fluctuates because the integration process in the integrator circuit <NUM> is not ideal. The fluctuation leads to a measurement error of power. For example, referring to <FIG>, when the gate drive voltage is low, the IGBT <NUM> is off and then the collector-emitter voltage Vce is high and the emitter current Ie should be zero. That is, when the IGBT <NUM> is off, the power loss of the IGBT, or Vce x Ie should be zero. However, the power loss Vce x Ie may show some value due to the fluctuation error.

AC type current probes, such as a current probe using a Rogowski coil cannot detect DC components in a current signal. What is needed is an apparatus and method that can cancel the fluctuation components at a zero ampere level even though the DC component cannot be detected. <CIT>) describes a method and apparatus for accurately detecting a current by reducing errors due to offset values of a current sensor. The method involves detecting a first offset value, immediately after a starting switch of an electric vehicle is turned on, and a second offset value, immediately after the starting switch of the electric vehicle is turned off. The offset values are found by connecting the first and second offset values. <CIT>) describes a current controller and current offset correction method. The current controller is designed to constantly detect an offset value of a current detection system in a state of regular operation of a motor. The offset value is determined based on an A/D converted value detected in falling from the peak of the carrier wave, an A/D converted value detected in rising from the trough of the carrier wave, and modulated wave commands. European patent application <CIT>) describes a motor control method of monitoring the operation of a brushless motor. The current is monitored as it flows into and out of the windings of the motor. A modified output signal is produced by compensating for differences between the actual measured output signal and an ideal output signal. European patent application <CIT>) describes a power-assisted steering system including a current detector. The current detector sequentially detects current flowing to a ground-side switching element in the lower arm whilst the switching element is either on or off. An offset updating unit is included in the system for sequentially updating an offset correction value. A detected current correcting unit is also included, for correcting detected values with the offset correction values.

The present invention is an apparatus for correcting current data samples corresponding to a current signal from a device under test. The apparatus has first acquisition circuitry receiving a current signal from a device under test for generating current data samples corresponding to the current signal and second acquisition circuitry receiving a voltage signal from the device under test corresponding to the current signal by means of a voltage probe for generating voltage data samples corresponding to the voltage signal. The apparatus has a controller that receives the current data samples and voltage data samples and extracts current fluctuation data samples representative of deviations of the current data samples from a zero ampere level, the current fluctuation data samples corresponding to off-periods of the device under test as derived by detecting corresponding off-periods of the device under test in the voltage data samples, generating current fluctuation data samples corresponding to on-periods of the device under test by interpolating the current fluctuation data samples corresponding to the off-periods of the device under test, subtracting the current fluctuation data samples corresponding to the
off-periods and the on-periods of the device under test from the current data samples representing the current signal to generate corrected zero ampere level current data samples, and generating a waveform display using the corrected zero ampere level current data values. The apparatus may also have a user interface for designating a threshold voltage level for the voltage data samples for detecting the off-periods in the device under test.

The above apparatus provides a platform for implementing a method for correcting current data samples representative of a current signal from a device under test. The method includes a step of receiving, by a first acquisition circuitry, a current signal using a current sensor coupled to a device under test and generating current data samples representative of the current signal from the device under test. A voltage signal corresponding to the current signal from the device under test is received by a second acquisition circuitry using a voltage probe and voltage data samples are generated representative of the voltage signal. Current fluctuation data samples representative of deviations of the current data samples from a zero ampere level are extracted with the current fluctuation data samples corresponding to off-periods of the device under test as derived by detecting corresponding off-periods of the device under test in the voltage data samples. The current fluctuation data samples of the off-periods of the device under test are interpolated to generate current fluctuation data samples corresponding to on-periods of the device under test. The current fluctuation data samples corresponding to the off-periods and the on-periods of the device under test are subtracted from the current data samples representing the current signal to generate corrected zero ampere level current data samples with the corrected zero ampere level current data samples being displayed.

The current sample correcting method has additional steps of designating an error cancel range around the zero ampere level of the corrected zero ampere level current data samples, and modifying the corrected zero ampere level current data samples by changing the corrected zero ampere level current data samples within the error cancel range to zero.

The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with appended claims and attached drawings.

<FIG> is a block diagram of an oscilloscope <NUM> for correcting current measurements from a device under test (DUT) <NUM> according to the present invention. The oscilloscope <NUM> has multiple input channels CH1, CH2, CH3 and CH4 with each input channel having acquisition circuitry <NUM>CH1 <NUM>CH2, <NUM>CH3, and <NUM>CH4. The output of each acquisition circuit <NUM>CH1 <NUM>CH2, <NUM>CH3, and <NUM>CH4 is coupled to a controller <NUM>. The controller <NUM> is coupled to I/O circuitry <NUM>, memory <NUM>, a display device <NUM>, such as a liquid crystal display, cathode ray tube or the like, and a mass storage device <NUM>, such as a hard disk drive, thumb drive, compact disk, or the like. The mass storage device may store program instructions for operating the oscilloscope <NUM> as well as program instructions for performing steps for correcting current measurements from the device under test (DUT) <NUM> according to the present invention. The DUT <NUM>, such as the inverter circuitry of <FIG>, is coupled to the input channels <NUM>CH1 <NUM>CH2, <NUM>CH3 via signal acquisition probes <NUM><NUM>, <NUM><NUM>, and <NUM><NUM> that respectively correspond to differential probe <NUM>, current probe <NUM> and differential probe <NUM> in <FIG>.

Each of the channel acquisition circuitry <NUM>CH1 <NUM>CH2, <NUM>CH3, and <NUM>CH4 includes, illustratively, input conditioning circuitry, such as an input amplifier, analog-to-digital conversion circuitry, trigger circuitry supporting acquisition memory, and the like. The supporting acquisition memory may be a portion of the memory <NUM>. Acquisition circuitry <NUM>CH1 <NUM>CH2, <NUM>CH3, and <NUM>CH4 operate to digitize one or more of the signals from the DUT <NUM> to produce one or more respective digital data sample streams suitable for use by the controller <NUM>. Acquisition circuitry <NUM>CH1 <NUM>CH2, <NUM>CH3, and <NUM>CH4 in response to commands received from controller <NUM>, change trigger conditions, decimator functions, and other acquisition related parameters. Each of the acquisition circuitry <NUM>CH1 <NUM>CH2, <NUM>CH3, and <NUM>CH4 couples its respective digital data sample streams to the controller <NUM>.

The controller <NUM> includes processor circuitry, support circuitry and memory to process the received digital data sample streams to generate waveform data for presentation on the display device <NUM>. The processor circuitry cooperates with the support circuitry, such as power supplies, clock circuits, cache memory, and the like, as well as circuits that assist in executing software routines stored in memory <NUM>. As such, it is contemplated that some of the process steps discussed herein as software processes may be implemented within hardware, for example, as circuitry that cooperates with processor circuitry to perform various steps. Controller <NUM> also interfaces with input/output (I/O) circuitry <NUM>. For example, I/O circuitry <NUM> may comprise a keypad, pointing device, touch screen, or other means adapted to provide user input and output to the controller <NUM>. Controller <NUM>, in response to such user input, adapts the operations of the acquisition circuitry <NUM>CH1 <NUM>CH2, <NUM>CH3, and <NUM>CH4 to perform various data acquisitions, triggering, processing, and display communications, among other functions.

Memory <NUM> may include volatile memory, such as SRAM, DRAM, among other volatile memories. Memory <NUM> may also include non-volatile memory devices, such as a disk drive or a tape medium, among others, or programmable memory, such as an EPROM, among others.

Although controller <NUM> of <FIG> is depicted as a general purpose computer that is programmed to perform various control functions in accordance with the present invention, the invention may be implemented in hardware such as, for example, an application specific integrated circuit (ASIC). As such, it is intended that the processor circuitry, as described herein, be broadly interpreted as being equivalently performed by hardware, software, or by a combination thereof.

Referring to <FIG>, there is shown a block diagram of the oscilloscope <NUM> from <FIG> having processing blocks in a processor circuitry block <NUM> of the controller <NUM> illustrating steps for correcting current measurements from a device under test (DUT) <NUM> according to the present invention. The oscilloscope <NUM> in <FIG> shows CH1 acquisition circuitry <NUM>CH1 receiving a voltage signal via differential voltage probe <NUM><NUM> corresponding to probe <NUM> in <FIG>. CH2 acquisition circuitry <NUM>CH2 receives a current signal from the current probe <NUM><NUM> corresponding to probe <NUM> in <FIG>. CH3 acquisition circuitry <NUM>CH3 receives a current signal from the differential voltage probe <NUM><NUM> corresponding to probe <NUM> in <FIG>. Each voltage and current signal is amplified, digitized and stored as voltage data samples or current data samples in a circulating buffer memory of each respective acquisition circuitry <NUM>CH1, <NUM>CH2, <NUM>CH3.

Referring to <FIG>, there is shown a flow chart representative of the steps for correcting current measurements from a device under test according to the present invention. The respective voltage data samples and current data samples generated in each of the respective acquisition circuitry <NUM>CH1, <NUM>CH2, <NUM>CH3 are captured in each of the respective circulating buffer memories according to a trigger signal when a trigger condition is satisfied as represented by step <NUM>. The captured voltage data samples and current data samples are coupled to the controller <NUM> and stored in memory <NUM>. A threshold level voltage Vref may be designated by a user through a user interface <NUM> displayed on display <NUM> or retrieving a previously stored threshold level voltage Vref as shown in step <NUM>. The threshold level Vref may be designated for the voltage Vce between the collector and emitter or the gate drive voltage Vge between the gate and emitter of IGBT <NUM> to determine On or Off periods of the IGBT <NUM>.

The current data samples and the voltage data samples are provided to an extraction block <NUM> in <FIG> for extracting current fluctuation data samples from the acquired current data samples. The current fluctuation data samples correspond to Off-periods and On-periods of the IGBT <NUM> and represent deviations of the current data samples from a zero ampere level. The current fluctuation data samples for Off-periods of the current data samples are derived by detecting corresponding Off-periods of the Vce or Vge voltage data samples as shown in step <NUM>. Preferably, a central portion of each Off-period of the voltage Vce or Vge is detected and the corresponding portion of the Off-period current data sample is extracted as current fluctuation data sample. As shown in <FIG>, the current signal of the Off-periods corresponds to zero ampere level having current fluctuations.

The current fluctuation data samples for the OFF-periods are provided to an interpolation block <NUM> in <FIG> for extracting zero ampere level current fluctuation data samples for On-period of the IGBT <NUM>. As shown in <FIG>, the current data samples of the On-periods are high so that a DC component of the current signal cannot be extracted. A sin(x)/x function may be used on the Off-period current fluctuation data samples to generate the On-period current fluctuation data samples as shown in step <NUM>. Alternatively, the Off-period current fluctuation data samples are interpolated for the On-periods by simply connecting between them with linear lines. <FIG> the current waveform <NUM> at the emitter of IGBT <NUM> having zero ampere level current fluctuations and a current waveform <NUM> representative of the current fluctuation data samples.

The Off-period and On-period current fluctuation data samples are provided to a subtraction block <NUM> along with the current data samples representative of the current signal at the emitter of IGBT <NUM>. The subtraction block <NUM> subtracts the Off-period and On-period zero ampere level current fluctuation data samples from the original current data samples to generate corrected current data samples as shown in step <NUM> and representatively shown by the corrected current waveform <NUM> in <FIG>. The corrected zero ampere level current data samples may be stored in memory <NUM> for further processing by the controller <NUM> or displayed as a waveform on display <NUM> as shown by the decision step <NUM>.

The corrected zero ampere level current data samples sometime have white noise and/or offset errors. The white noise is mainly due to noise of the current sensor <NUM> with the integrator circuit <NUM> and internal noise of the digital oscilloscope <NUM>. The offset error is mainly due to the interpolated On-period zero ampere current fluctuation data samples may not be ideal. To cancel the noise and/or offset errors, a user may designate a noise cancel range Ncr around the zero ampere level through the user interface <NUM> as represented in step <NUM>. The noise cancel range Ncr may be +/- <NUM>% of the current amplitude around the zero ampere level, for example. However, it depends on characteristics of a device under test. The noise cancel range Ncr may be designated using an absolute value Nacr, that is, Ncr is from - Nacr to + Nacr. In this case, noise/offset canceled current data Y is: <MAT> "y" represents the corrected zero ampere level current data samples. "IF (y > Nacr, <NUM>, <NUM>)" means if y > Nacr is true, it is <NUM> but if not it is <NUM>. "IF (y < - Nacr, <NUM>, <NUM>)" is similar. This process modifies the corrected zero ampere level current data samples within the noise cancel range Ncr zero to cancel the white noise and/or offset errors around the zero level as shown in step <NUM>.

Then, a calculate power block <NUM> calculates power by multiplying Ie data and Vce data according to program stored in the mass storage device <NUM> as shown in step <NUM>. For example, conduction power loss of the IGBT <NUM> may be evaluated by multiplying Ie data and Vce data. The conduction power loss arises due to that the collector-emitter voltage Vce is not zero even during the On-periods. Note that the conduction power loss can be evaluated more accurately since the corrected zero ampere level current data samples are corrected. Also, the corrected zero ampere level current data samples during the Off-period are revised to zero. The calculated power values and modified corrected zero ampere level current data samples may be displayed on a screen of the display <NUM> as shown in step <NUM>.

Claim 1:
A method for correcting, and displaying on a display screen of a test and measurement instrument, current data samples representative of a current signal from a device under test (<NUM>) comprising steps of:
receiving, by a first acquisition circuitry (<NUM>CH2), the current signal by using a current sensor (<NUM>) coupled to the device under test (<NUM>) and generating the current data samples representative of the current signal from the device under test (<NUM>) (operation <NUM>), said current sensor including a non-ideal integrator;
receiving, by a second acquisition circuitry (<NUM>CH1), a voltage signal corresponding to the current signal from the device under test (<NUM>) by using a voltage probe (<NUM><NUM>) and generating voltage data samples representative of the voltage signal (operation <NUM>);
extracting current fluctuation data samples representative of deviations of the current data samples from a zero ampere level, said deviations caused by the integrator of the current sensor, the current fluctuation data samples corresponding to off-periods of the device under test (<NUM>) as derived by detecting corresponding off-periods of the device under test (<NUM>) in the voltage data samples (operation <NUM>);
interpolating the current fluctuation data samples of the off-periods of the device under test (<NUM>) for generating current fluctuation data samples corresponding to on-periods of the device under test (<NUM>) (operation <NUM>);
subtracting the current fluctuation data samples corresponding to the off-periods and the on-periods of the device under test (<NUM>) from the current data samples representing the current signal to generate corrected zero ampere level current data samples (operation <NUM>); and
generating a waveform display using the corrected zero ampere level current data samples (operation <NUM>), and displaying the waveform display on the display screen of the test and measurement instrument.