Closed-loop magnetoresistive current sensor system having active offset nulling

[0003] The invention pertains to current sensors and particularly to magnetoresistive (MR) current sensors. More particularly, it pertains to MR sensors having low offsets.

[0004] The present current sensor is a closed-loop, non-contact, fast-responding, wide-bandwidth, field nulling current sensor. This sensor reveals parameters equivalent to other closed-loop approaches such as the closed loop Hall effect current sensor in the related art. It is the use of the MR sensor having a set/reset and offset nulling loop leads to ultra low offsets and ultra low offset drifts over all conditions in current measurement. It also has very low offset drift over wide ranges of operating temperatures. Because of the ultra low offset, the sensor provides very accurate measurements of current. It has an auto-zero offset reduction circuit, which has a separate feedback path to remove offset and offset drift from the circuit. The magnetic signal and offset signals are driven to their respective nulls by independent feedback loops. The offset feedback loop is not used in Hall effect type current sensors. The transient noise and spikes caused by the set/reset circuit are greatly reduced with this new feedback scheme. The sensor has a frequency response from DC to over 150 kHz and has a response time of less than one microsecond.

[0005] FIG. 1 is an illustration of an MR closed loop current sensing system.

[0006] FIG. 2 is a functional diagram of the MR current sensing system.

[0007] FIG. 3 is a schematic of the MR current sensing system.

[0008] An overview of a closed loop MR current sensor system 10 is shown in FIG. 1. Input primary current 12 can be in a conductor having one to N turns, where N can be any practical number. The magnetic flux 86 from current 12 of the primary current carrying conductor is concentrated by a high permeability core 26 and dropped across a gap 17 to produce a magnetic field 86. An inductive feedback or rebalance coil 25 produces a magnetic field 99 to oppose magnetic field 86. MR die 11 senses the difference between magnetic fields 86 and 99. The output of MR die 11 is amplified, among other things, by an electronics loop 91, which includes an operational amplifier electronics 30, demodulator 31, buffer amplifier electronics 32, set/reset circuit 90, modulator 36 and integrator 37. Electronics loop 91, along with coil 25, drives magnetic field 86 in gap 17 to a null value with a magnetic field 99. A path 97 provides set/reset signals to MR die 11. Circuit 90 sets and resets or rotates the magnetic vector of MR die 11 180 degrees at a 1 kHz rate. This technique is used to produce an ultra low offset current sensor 10. Gap 17 is small relative to the core 26 cross-section and length to provide better accuracy and improved shielding from stray external magnetic fields.

[0009] A conceptual layout of MR sensor system 10 is shown in FIG. 2. System 10 has an anisotropic MR Wheatstone bridge 11 next to a conductor(s) carrying a current 12 that generates a magnetic field 86. This magnetic field is typically concentrated across MR sensor 11 through use of a magnetic core 26. In FIG. 3, MR sensor 11 is comprised of four Permalloy (NiFe) resistors 13-16 that are connected end to end, and the magnetic field 86 (B) of current 12 and 99 of rebalance coil 25 changes the resistance of resistors 13, 14, 15 and 16. In the set/reset process, a high current (unrelated to the current being measured or the current being fed back) pulse is sent through the set/reset strap by a MOSFET driver 78. This “sets” the magnetization of the Permalloy in one direction such that a positive magnetic field (generated by the conductor or feedback coil or any other field source) will cause the resistance of two resistors to increase and the other two resistors to decrease. If a pulse of equal amplitude but opposite direction is then sent through the set/reset strap, this is a “reset” condition and each resistor will change resistance opposite to the set condition. The set/reset process and the magnetization of the Permalloy are independent of the conductor with current that is being measured. Resistors 13 and 15 increase in resistance for a first direction of current 12 flow and resistors 14 and 16 decrease in the set condition. For the first direction of current 12 flow, resistors 13 and 15 decrease and resistors 14 and 16 increase in resistance in the reset condition. The resistance changes are proportional to the magnitude of the current 12 flow. Current 12 may have its magnitude measured in amperes and the magnetic field measured in Gauss. K_B17 is a transfer function inherent to sensor 11. Function 17 represents the effect of the magnetic field 86 of current 12 (and the field 99 generated inductive feedback coil 25) on the resistors of sensor 11. Modulator or multiplier 18 of FIG. 2 is where the set/reset signal comes to sensor 11 from an oscillator 20. The signal is a one kHz square wave 21. The signal may be another kind of signal and/or have a different frequency. Summer 19 conceptually represents the superposition of a V_off1(MR offset voltage) to an output signal of sensor 11. K_A23 represents the transfer function for a gap in core 26, which may be given in ampere-turns a device having N₁24 turns in the first winding, which is the current 12 carrying wire. N₂25 represents the number of turns of the second winding (inductive feedback coil), which couples the negative magnetic feedback (99) to sensor 11. The feedback coil has about 1000 turns. The magnetic core superposes the magnetic fields (99 and 86) from the feedback coil 25 and the current 12, therefore it is represented by summer 26.

[0010] The output of sensor 11 goes to a summer 27 which adds an offset V_off228 of the operational amplifier electronics 30 and the voltage offset feedback to the output signal of sensor 11 to cancel the offsets. K_C29 represents the transfer function, such as gain, of the operational amplifier electronics 30. The output of electronics 30 goes to demodulator 31 to be demodulated. Items 46-49 are actually a part of demodulator 31. An input signal 21 from oscillator 20 sets the demodulator 31 frequency. The demodulator sensor signal goes on to K_D32, which is the transfer function of the final operational or current buffer amplifier electronics 32 for sensor system 10. It has a large gain at DC and rolls off at above 1 kHz. The output goes to a transfer function 33 of the inverse of the sum of the load resistance R_L34 of system 10 and the inductive impedance (R_ind+jωL_ind) of winding or coil 25. Z_ind=(R_ind+jωL_ind), where Z_indis the impedance of the inductive feedback coil which is the sum of the inductive coil's resistance (R_ind) and reactance (jωL_ind) where L_indis the inductive coil's inductance. The output of the current sensor is given in current and is typically run through a load (RL) to measure a voltage out. An offset feedback signal from output 35 goes to a modulator 36. This offset feedback signal could also come from the output of 33 (tied after 32 on the schematic). Signal 21 from oscillator 20 actuates modulator 36. The modulated offset feedback signal goes to an integrator 37 that provides the integrated voltage offset feedback signal (DC) to summer 27 of operational amplifier electronics 30 with the modulated signal level at about 1 kHz filtered.

[0011] FIG. 3 is a schematic of sensor system 10. Sensor 11 has the Wheatstone bridge of resistors 13-16, connected end-to-end as described above. The bridge outputs go to the inverting and non-inverting inputs, respectively, via conductors 93 and 94 and 4.02K ohm resistors 39 and 40, of differential amplifier 38 of operational amplifier electronics 30. A 40.2 K ohm resistor 41 connects the non-inverting input to ground, and a 40.2 K feedback resistor 42 connects the output of amplifier 38 to its inverting input. The output of amplifier 38 goes to the inverting input of amplifier 43 via a 4.02 K ohm resistor 44. The output of amplifier 43 is connected to its inverting input via a 40.2 K ohm resistor 45. The non-inverting input of amplifier 43 is connected to ground. A resistor to ground is not necessary here (for this op-amp only) since any offset because of mis-matched input resistance is removed by the offset feedback. This removes one resistor. The output goes to the inverting input of amplifier 47 of demodulator 31, via a 4.02 K ohm resistor 46. The output is connected to the inverting input via a 4.02 K ohm resistor 48. The non-inverting input is connected to ground via a 2 K ohm resistor 49. Operational amplifiers 38, 43 and 47 are BA4560 amplifiers, although many other models could be used.

[0012] An input to a first terminal of a normally open switch 50 of demodulator 31 is connected to the output of amplifier 43. An input to a first terminal of a normally closed switch 51 is connected to the output of inverting amplifier 47. The output terminals of switches 50 and 51 are connected to the inverting input of amplifier 52 of output buffer electronics 32, via a 2 K ohm resistor 53. The actuators of switches 50 and 51 of demodulator 31 are actuated with signal 21 from oscillator 20. Switches 50 and 51 of demodulator 31 are in an ADG413 quad SPST analog switch device.

[0013] The non-inverting input of amplifier 52 is connected to ground via a 2 K ohm resistor 54. The output of amplifier 52 is connected to its inverting input via a 56 picofarad capacitor 57. This connection arrangement may be varied depending on the stability of the amplifier 52. The output of amplifier 52 goes to the bases of NPN buffer output transistor 58 and PNP buffer output transistor 59. The collectors of transistors 58 and 59 are connected to +15 volts and −15 volts, respectively. The emitters of transistors 58 and 59 are connected together, and are connected to the output of amplifier 52 via a 316 ohm resistor 60. The common emitter connection of transistors 58 and 59 is the output of buffer electronics 32. This output is connected to the inverting input of amplifier 52 via a 0.033 microfarad capacitor 55 and a 5.11 K ohm resistor 56 connected in series. A diode 61 has a cathode and an anode connected to the collector and emitter, respectively, of transistor 58. A diode 62 has a cathode and an anode connected to the emitter and collector, respectively, of transistor 59. Amplifier 52 is a BA4560 device; transistor 58 is a BCX55 device; and transistor 59 is a BCX52 device.

[0014] The output of first buffer electronics 32 is connected to a first terminal of winding 25. Winding 25 provides the magnetic feedback to MR sensor 11 via a magnetic core 26 and gap 23. Core 26 and gap 23 are represented in FIG. 2 but not shown in FIG. 3. A second terminal of winding 25 is connected to the anode of diode 63 and the cathode of diode 64. The cathode of diode 63 is connected to the collector of transistor 58. The anode of diode 64 is connected to the collector of transistor 59. Diodes 61, 62, 63 and 64 function as voltage clamps for circuit protection.

[0015] The second terminal of winding 25 is the current output 65 of sensor system 10. Output 65 is connected to ground via a 4,700 picofarad capacitor. Output 65 is connected to one end of a 100 ohm RL resistor 34. The other end of resistor 34 is connected to ground. This is the resistor used here, but the customer usually chooses this value. It can be anything from zero to one megohm, although the device does not operate over its full range of current with larger resistors. Voltage output 35 across resistor 34 is the indication of the amount of current 12 flowing through the wire proximate to MR sensor 11. Also, output 65 is connected as an offset feedback signal to the non-inverting input of a buffer amplifier 66. Amplifier 66 is an LT1013 device. Although the LT1013's low offset and offset drift properties are not required, it is used as this buffer versus a cheaper amplifier because it is a spare on the chip anyway.

[0016] The output of amplifier 66 is connected to its inverting input and to input terminals of a normally open switch 67 and a normally closed switch 68. Switches 67 and 68 are of modulator 36. Actuators of switches 67 and 68 are actuated by signal 21 from oscillator 20. The output terminals of switches 67 and 68 are connected to the inverting and non-inverting inputs of an amplifier 69 of integrator 37 via 24.9 K ohm resistors 70 and 71, respectively. Amplifier 69 is an LT1013 device. A 0.47 microfarad capacitor connects the output of amplifier 69 to its inverting input. A 0.47 capacitor 73 connects the non-inverting input of amplifier 69 to ground. The output of amplifier 69 is connected to a summer 27 point at the non-inverting input of amplifier 43, via 46.4 K ohm resistors 74 and 75. The inter-connection of resistors 74 and 75 is connected to ground through a 0.47 microfarad capacitor 76. The signal from amplifier 69 to the summer 27 point is a voltage offset signal that is used for nulling out offset signals (V_off1) 22 in MR sensor 11 and offset signals (V_off2) of amplifier electronics 30.

[0017] Demodulator 31 and modulator 36 have normally open switches 50 and 67, and normally closed switches 51 and 68. The switches, as noted above, are paired off in demodulator 31 and modulator 36. In each pair, one switch is open and one switch is closed at any one given time period. All of the four switches 50, 51, 67 and 68 are actuated by clock pulse 21. Since the sensed signal was modulated in MR sensor 11 at one kHz, demodulator 31 demodulates the signal back down to DC (and modulates DC signals up to one kHz). Switches 67 and 68 take one kHz square wave 21 and alternately switch the incoming signal to the non-inverting and the inverting inputs of operational amplifier 69. This in effect modulates any DC signal and demodulates a one kHz signal. Amplifier 69 then acts as an integrator to pass the DC signal as feedback to operational amplifier electronics 30. Resistors 74 and 75 and capacitor 76 also act to filter this voltage.

[0018] The set/reset mechanism 90 for MR sensor 11 is initiated by a signal 21 from oscillator 20. Oscillator 20 consists of an LM555 chip 77 with a terminal 1 to ground. A terminal 2 is connected to threshold terminal 6 that is connected to ground via a 0.01 microfarad capacitor 80 and to VCC (+5 volts DC) via a 49.9 K ohm resistor 82. An output terminal 3 provides about one kHz square wave output 21 to the input of a high speed MOSFET driver 78. Also, chip 77 has a reset terminal 4 connected to VCC(+5 Volts DC), and a control terminal 5 connected to ground via a 0.01 microfarad capacitor 79. Discharge terminal 7 is connected to terminal 6 via a 21.5 k ohm resistor 81. VCC terminal 8 is connected to +5 volts DC.

[0019] The output of set/reset driver 78 is connected to set/reset coil or strap 83 of MR sensor 11, via a 0.01 microfarad capacitor 84 and an eight ohm resistor 85 connected in series.

[0020] Associated with the power supply, there are a 5.1 volt Zener diode 87 and 2 K resistor 88 tied between ground and +15 volts. This creates a 5 volt supply for switched 67, 68, 50 and 51 (through Vp) and device 77. The approach creates a 5 volt supply, and improves performance over the tying those pins to +15 volts. This configuration decreases the “glitches” on the output each time the set or reset occurs, for the price of two low-cost components.

[0021] Although the invention has been described with respect to a specific preferred embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present application. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.