JP2012107939A - Magnetic sensor using magnetoresistive element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor chip which can detect the continuous change of a magnetic field as a highly accurate digital value and can provide the value at low cost.SOLUTION: A magnetic sensor having an MR element comprises; an MR oscillator in which an oscillation period changes corresponding to the intensity of an external magnetic field; a fixed oscillator oscillating with a fixed oscillation period; and an integrator for counting a square wave output by the MR oscillator based on a reset signal output by the fixed oscillator and outputting the wave as a digital value. The MR oscillator, the fixed oscillator and the integrator of the magnetic sensor are formed into one semiconductor chip.

Description

  The present invention relates to a magnetic sensor, and more particularly to a magnetic sensor that includes an oscillator using an MR (Magneto-resistive) element and outputs a detected magnetic field strength as a digital value.

The in-vehicle sensor is composed of a sensor element that detects a physical quantity and components that are centered on an application specific integrated circuit (ASIC) that targets the sensor element. Various methods are adopted as sensor elements for detecting the rotation of the wheels of an automobile. There are those using a resolver, those using a magnetoresistive element, and those using an optical encoder. A sensor ASIC that controls sensor elements and detects and converts an output includes an amplifier (such as an instrumentation amplifier), a band limiting filter, a buffer, an oscillator, an A / D converter, and a comparator.

The ASIC is sometimes disposed on the engine side, and is required to operate in an environment exposed to a wide temperature range and large electromagnetic noise. There is a great need for a sensor ASIC that operates in a wide temperature range, is resistant to noise, and has high detection sensitivity. Such an ASIC tends to have a large circuit scale and a high chip unit price.

In Patent Document 1, an inverter connected in series, a resistor R connected to the input terminal of the first inverter and connected to the input of the first inverter, an inverter output having a signal opposite in phase to the output described above, and the first An oscillator composed of a capacitor connected to the input of an inverter is disclosed. When this oscillator is used as an IC (Integrated Circuit) oscillator, it is difficult to adjust the oscillation frequency. In order to reduce the current consumption of the oscillator, it is preferable to reduce the W (channel width) / L (channel length) ratio of the transistors constituting the first-stage inverter.

The resistor R can be a sensor using a CdS (cadmium sulfide) cell or a magnetoresistive element. In such a configuration, since the change in the magnetic field is output with a frequency, a subsequent circuit that converts the frequency into a digital signal is required. Since the oscillation frequency is significantly affected by the offset and sensitivity of the magnetoresistive element, various corrections are required for use as a magnetic sensor.

The CR oscillation circuit disclosed in Patent Document 2 has a correction function. Here, the oscillation frequency is made variable by turning on / off an arbitrary switch using a resistor array in which a resistor and a switch are connected in parallel.

Patent Document 3 shows a circuit example of a magnetic sensor for a rotation sensor of an automobile. The circuit is used in a sensor signal processing device, and the amplitude of the amplified sensor signal is within an allowable range. Since the sensor signal is weak, it is amplified before performing signal processing such as binarization.

The sensor signal processing apparatus compares the output value of the amplifier with a predetermined maximum value, and outputs a first signal when the output value of the amplifier is larger than the predetermined maximum value, and the output of the amplifier A second comparator that outputs a second signal when the value of the amplifier is less than a predetermined minimum value by comparing the value with a predetermined minimum value; and a capacitor connected to a reference voltage terminal of the amplifier; A discharge switching element that operates by the first signal from the first comparator to discharge the capacitor and lowers the reference voltage in the amplifier, and a capacitor that operates by the second signal from the second comparator. A charging switching element for charging and raising a reference voltage in the amplifier is provided.

  The rotation angle sensor 3 has a pair of MR elements 4A and 4B arranged to face the gear 2. As the gear 2 rotates, the voltage at the midpoint 5 of the MR elements 4A, 4B changes, and a signal is output from the rotation angle sensor 3. A signal from the rotation angle sensor 3 is input to the amplifier 8. The output terminal of the amplifier 8 is connected to the comparator 11 and the comparator 12, respectively. The output signal of the amplifier 8 is compared with 3.8V and 0.2V by the comparator 11 and the comparator 12.

When the output signal of the amplifier 8 deviates from 3.8 V or 0.2 V, the MOS transistor 18 or 21 is turned on and the capacitor 25 is charged or discharged. The reference voltage of the amplifier 8 is changed via the operational amplifier 27 in accordance with the change in the potential of the capacitor 25 due to the charging / discharging. This method focuses on the passage of the gear 2 and binarizes the magnetic field change, and is not suitable for continuous change detection.

The magnetic sensor disclosed in Patent Document 4 outputs the output signal of the magnetic field sensor as a digital signal. A magnetic sensor is composed of an oscillation circuit including a magnetic sensor element and a counter circuit connected to the oscillation circuit, and a magnetic field change signal detected by the magnetic sensor element is converted into a digital signal, thereby outputting the output signal of the magnetic field sensor as a digital signal. is doing. The magnetic sensor first outputs a signal for a change in the magnetic field as an analog signal. In order to digitize an analog signal, an A / D converter is necessary. This sensor has a shape in which a substrate is provided with a CMOS (Complementary Metal Oxide Semiconductor) inverter IC and a magnetoresistive element, and the circuit scale becomes large, so it is not suitable for miniaturization. In addition, since the time constant is set by the magnetoresistive element and the capacitance, it is vulnerable to variations in the magnetoresistive element, and a problem remains in terms of offset and sensitivity.

JP-A-2-274118 JP 2006-191262 A Japanese Patent Laid-Open No. 7-167876 Japanese Patent Application No. 2001-367134

The sensor ASIC is connected to an A / D converter and a comparator via an amplifier (such as an instrumentation amplifier), a band limiting filter, and a buffer. Such an ASIC tends to have a large circuit scale and a high chip unit price. The demand for reducing the cost of in-vehicle sensors is severe. Therefore, it is desired to develop a semiconductor chip that can detect a continuous magnetic field change as a highly accurate digital value and provide it at low cost.

A magnetic sensor according to the present invention includes an MR element, an MR oscillator whose oscillation period changes in accordance with the intensity of an external magnetic field, a fixed oscillator that oscillates at a constant oscillation period, and a rectangular wave output from the MR oscillator. An integrator that counts based on a reset signal output from the fixed oscillator and outputs a digital value is provided, and the MR oscillator, the fixed oscillator, and the integrator are formed in one semiconductor chip.

The magnetic sensor according to the present invention outputs the change in the magnetic field as a digital value using the MR element and the detection circuit.

It is a figure which shows the whole structure of the magnetic sensor in connection with this invention. It is a figure for demonstrating the usage of the magnetic sensor in connection with this invention. It is a figure for demonstrating the method of using the magnetic sensor in connection with this invention as a rotation sensor. It is a figure for demonstrating another method of using the magnetic sensor in connection with this invention as a rotation sensor. It is a figure for demonstrating the waveform which a magnetic sensor outputs when a gearwheel rotates forward. It is a figure for demonstrating the waveform which a magnetic sensor outputs when a gearwheel reversely rotates. It is a figure for demonstrating the waveform which a magnetic sensor outputs, when a gearwheel changes from normal rotation to reverse rotation. It is a figure for demonstrating the structure of the MR oscillation circuit concerning this invention. It is a figure for demonstrating the output characteristic of MR oscillation circuit. It is a figure for demonstrating one form of the integrator of the magnetic sensor in connection with this invention. It is a figure for demonstrating another form of the integrator of the magnetic sensor in connection with this invention. It is a figure for demonstrating another form of the MR oscillation circuit in connection with this invention. It is a figure for demonstrating one form of the differential amplifier of MR oscillation circuit. It is a figure for demonstrating another form of the differential amplifier of MR oscillation circuit. It is a figure for demonstrating another form of the MR oscillation circuit in connection with this invention. It is a figure for demonstrating the oscillation frequency of MR oscillation circuit. It is a figure for demonstrating the voltage characteristic of MR oscillation circuit. It is a whole block diagram which shows another form of the magnetic sensor in connection with this invention. It is a figure for demonstrating the method of using a magnetic sensor as a rotation sensor. It is a figure for demonstrating the waveform which a magnetic sensor outputs when a gearwheel rotates forward. It is a figure for demonstrating the waveform which a magnetic sensor outputs when a gearwheel reversely rotates. It is a figure for demonstrating the output which binarized the waveform which a magnetic sensor outputs, when a gearwheel rotates forward. It is a figure for demonstrating the output which binarized the waveform which a magnetic sensor outputs, when a gearwheel reversely rotates.

Embodiment 1 FIG.
The configuration of the magnetic sensor according to the present invention will be described with reference to FIG. The magnetic sensor 1 includes an MR oscillator 2, a fixed oscillator 3, and an integrator 4, and these elements are integrally formed in a CMOS-IC (semiconductor chip) 10. The MR oscillator 2 using the MR element 5 changes the oscillation frequency according to the change in magnetism around the MR element 5. The fixed oscillator 3 outputs an RST signal for resetting the integrator 4. The integrator 4 counts the output signal of the MR oscillator 2 as it is, and resets the count by the RST signal of the fixed oscillator 3.

The MR element 5 constituting the MR oscillator 2 is formed in the upper layer of the silicon chip. A film-like MR element 5 is formed on the upper layer of the CMOS-IC 10, and the MR element 5 is connected to the inside of the CMOS silicon chip using a through hole. When the position of the magnet 11 arranged around the CMOS-IC 10 on which the magnetic sensor is mounted changes, the change in the magnetic field is transmitted to the MR element 5 and the resistance value of the MR element 5 changes.

An oscillator is a general term for circuits and devices that generate repetitive signals. The fixed oscillator 3 has a constant oscillation frequency and uses a known solid oscillator oscillation circuit or the like. In the solid oscillator oscillation circuit, the oscillation frequency is determined by connecting a component (solid oscillator) that causes natural vibration by applying a voltage such as a crystal oscillator or a ceramic oscillator in the circuit. It is known that a circuit using a crystal resonator has very high oscillation frequency accuracy.

The usage of the magnetic sensor 1 will be described with reference to FIGS. In FIG. 2, as a method of changing the resistance value of the MR element 5, a method of detecting a change in the distance between the magnet 11 and the MR element 5 (upper stage) and a slide of the magnet 11 in a direction parallel to the MR element 5 are shown. The method (bottom) is shown. The MR element 5 constituting the MR oscillator 2 is formed in the upper layer (front side) of the CMOS-IC 10. The magnet 11 may be arranged on the front side of the CMOS-IC 10 (left side) or may be arranged on the back side (right side). In any case, as the magnet 11 moves, the transmission period of the MR oscillator 2 changes.

The magnetic sensor 1 can also constitute a rotation sensor. FIG. 3 shows a first configuration when the magnetic sensor according to the present application is used as a rotation sensor. In addition to the CMOS-IC 10, the rotation sensor 23 requires a gear 21 made of iron or the like attached to a rotation shaft such as a wheel, and a magnet 11 attached to the back side of the CMOS-IC 10. FIG. 4 shows a second embodiment of the rotation sensor according to the present application. The rotation sensor 24 requires a gear 22 made of iron or the like attached to a rotation shaft such as a wheel. The gear 22 has a magnet with a tooth tip.

When the rotations of the gears 21 and 22 are positive rotations, the gears 21 and 22 approach the MR element 5 alternately, so that the output of the magnetic sensor 1 has a sinusoidal waveform as shown in FIG. When the gears 21 and 22 rotate in the reverse direction, the phase of the output of the magnetic sensor 1 is reversed as shown in FIG. When reverse rotation occurs during normal rotation, the waveform is such that the sine wave is turned back halfway as shown in FIG. Thus, the magnetic sensor 1 can detect the rotation of the gears 21 and 22.

A circuit example of the MR oscillator 2 is shown in FIG. The single-phase CR oscillator 2a includes an MR resistor Rv, a fixed resistor Rs, a capacitor Cs, a first inverter, a second inverter, and a third inverter. The MR resistance Rv converts a linear magnetic field change into a resistance value change. Here, the first inverter indicates a portion where the PMOS transistor Mp1 and the NMOS transistor Mn1 are arranged in series. Similarly, the second inverter includes a PMOS transistor Mp2 and an NMOS transistor Mn2, and the third inverter includes a PMOS transistor Mp3 and an NMOS transistor Mn3. The fourth inverter (Mp4 and Mn4) is a buffer. If a parasitic capacitance or the like is added to the node Vf, the oscillation frequency is reduced. Therefore, the node Vf is not seen from the outside by the fourth inverter so that the time constant of the internal oscillator is not changed. Node Vo is an output terminal of the CR oscillator 2a.

The operation of the single-phase CR oscillator 2a will be described with reference to FIG. The voltage change of the node Vg, the node Vm, and the node Vf when the CR oscillator 2a is operating is shown. When the node Vm changes to the “H” state, the amplitude of the node Vg once changes to the “H” state and then decreases according to the current from the resistor (Rs + Rv). Eventually, the node Vg becomes lower than the threshold voltage of the first inverter, and at that moment, the node Vm becomes “L”. From the voltage change of the first inverter at the node Vg, the oscillation period T of the CR oscillator 2a is expressed as T = 2.2 × Cs × (Rv + Rs). Since the resistance Rv can be changed by the magnetic field applied to the MR element 5, the oscillation frequency T changes according to the change of the magnetic field applied to the CR oscillator 2a. The interval tw1 is equal to the interval tw2.

If only a variable resistor is used to determine the oscillation frequency, it cannot be ensured that the resistance value of all oscillators can oscillate stably. Here, the resistance that determines the oscillation frequency is the MR resistance Rv and the fixed resistance Rs so that the variable width can be limited. When used as a magnetic sensor, the frequency of the MR oscillator 2a using the MR element is increased, the frequency of the normal fixed oscillator 3 for resetting the integrator 4 is set low, and the bus width of the integrator 4 is increased. The resolution of the magnetic sensor can be increased.

A circuit example of the integrator 4 is shown in FIG. The digital filter 6a corresponds to a low-pass filter, and includes a cumulative adder, a switch, a frequency divider (consisting of a counter or the like), and a difference unit. The cumulative adder is composed of a D-FF (delay flip-flop) and an adder. The differentiator also includes a D-FF and a subtracter. A signal from the MR oscillator 2 using the MR element is connected to the node DIN, and a signal from the fixed oscillator 3 is connected to the node CK. This connection may be reversed. The digital filter 6a continues to count the signal from the node DIN, but the signal is transmitted to the differentiator only at a certain interval provided by the switch. The difference value subtracts the current value and the previous value at regular intervals to obtain the digital value of the current sensor output.

It is also possible to improve the performance by increasing the order of the digital filter. For example, when a third-order filter is used, three cumulative adders and three differentiators are provided as shown in FIG. The digital filter 6b is composed of a three-stage cumulative adder, a switch, a frequency divider (consisting of a counter or the like), and a differentiator having the same number of stages as the cumulative adder. The cumulative adder includes a D-FF and an adder. The differentiator also includes a D-FF and a subtracter. A signal of the MR oscillator 2 using the MR element is connected to the node DIN, and a fixed oscillator 3 is connected to the node CK. This connection may be reversed.

The digital filter 6b continues to count the signal from the node DIN, but is transmitted to the differentiator only at a certain interval by the switch. The difference value is subtracted from the current value and the previous value to obtain the digital value of the current sensor output. Although the counter as an integrator corresponds to a first-order low-pass filter, the digital filter 6b can realize a third-order filter, so that a high-performance magnetic sensor can be realized. Further, a simple configuration can be realized by using an up counter with reset. It is also possible to obtain a digital value by counting the clock of the oscillator and resetting it at the timing of the fixed oscillator 3.

Embodiment 2. FIG.
The configuration of the magnetic sensor according to the second embodiment will be described with reference to FIG. The magnetic sensor according to the second embodiment uses a differential CR oscillator as the MR oscillator 2. A differential CR oscillator 2b using MR elements is shown in FIG. The differential CR oscillator 2b includes an MR resistor (Rvp and Rvn), a fixed resistor (Rsp and Rsn), a capacitor (Csp and Csn), a first differential amplifier A1, a second differential amplifier A2, and a third difference. It comprises a dynamic amplifier A3. The differential amplifiers A1 to A3 are configured by NMOS transistors (Mnc1 and Mn1-2) and PMOS transistors (Mpc1 to Mpc2) as shown in FIGS.

The differential CR oscillator 2b using MR resistors (Rvp and Rvn) changes the oscillation frequency in accordance with the change in magnetism on the MR element. This signal is counted by the integrator 4 as it is, and the integrator 4 is reset by the fixed oscillator 3. When Rvp = Rv + ΔRv and Rvn = Rv−ΔRv, the oscillation period T of the differential CR oscillator 2b is expressed as T = 2.2 × Cs × (Rv + 2ΔRv + Rs). Since ΔRv can be changed by the magnetic field applied to the MR element, the oscillation frequency changes according to the magnetic field applied to the oscillator.

Here again, the resistors that determine the oscillation frequency are the MR resistor Rv and the fixed resistor Rs, and the variable width is limited. Compared with the circuit of the first embodiment, the circuit configuration of the oscillator that detects the magnetic field is a differential configuration, which makes it more resistant to power supply noise. Further, since the resistance value of the MR element changes twice, the signal detection sensitivity is also increased.

Embodiment 3 FIG.
The magnetic sensor according to the third embodiment is different from the magnetic sensor according to the first embodiment in the configuration of the MR oscillator. FIG. 15 shows the configuration of relaxation oscillator 2c using the MR element according to the third embodiment. The relaxation oscillator 2c includes an MR element (Rvp and Rvn), a fixed resistor (Rsp and Rsn), a capacitor (Cs1 and Cs2), an N-type MOS transistor (Mnc1 to 3 and Mn1 to 3), and a P-type MOS transistor (Mp1). Consists of

The oscillation frequency f of the relaxation oscillator 2c is expressed by the equation shown in FIG. Here, gm is the transconductance (A / V) of Mn1 and Mn2, and Cl is the sum of the gate capacities of Mp1 and Mn3. The relaxation oscillator 2c using the MR element changes the oscillation frequency in accordance with the change in magnetism on the MR element. This signal is counted by the integrator 4 as it is, and the integrator 4 is reset by the fixed oscillator 3. The inverters Mp1, Mn3, and Mnc1 are buffers that prevent the time constant of the internal oscillator from being changed.

FIG. 17 shows waveforms of the node Von, the node Vop, the node Vcp, and the node Vcn when the relaxation oscillator 2c is operated. When the node Vop is “H” and the node Von is “L”, Mn1 is in an OFF state and Mn2 is in an ON state. Then, the node Vcn is in an equilibrium state with the current supplied from Mn2 and the current drawn from Mnc3. On the other hand, the voltage at the node Vcp decreases because current is drawn from Mnc2. When the node Vcp decreases, Mn1 is turned on next, and an equilibrium state is reached between the current supplied from Mn1 and the current drawn from Mnc2. The voltage of the node Vop decreases and Mn2 is turned off. By repeating this, relaxation oscillator 2c outputs a rectangular wave from node Vop and node Von.

In the third embodiment, as compared with the circuit of the first embodiment, the configuration of the oscillator is a fully differential configuration, so that it is more resistant to power supply noise. Further, since the resistance change of the MR element is doubled, the signal detection sensitivity is also increased. Furthermore, this relaxed oscillator can oscillate at a higher frequency than the CR oscillator using the MR element in the first and second embodiments. Therefore, a larger sampling ratio can be realized than a magnetic sensor using a CR oscillator, and the output of the magnetic sensor can be made highly accurate.

Embodiment 4 FIG.
FIG. 18 shows a fourth embodiment of the magnetic sensor according to the present application. The CMOS-IC 10 includes two adjacent magnetic sensors 1 (30a) and 2 (30b), a phase detection circuit 25 connected to them, and a fixed oscillator. The fixed oscillator is shared by the magnetic sensor 1 (30a) and the magnetic sensor 2 (30b). The output of the magnetic sensor 1 (30a) and the output of the magnetic sensor 2 (30b) are input to the phase detection circuit 25. The phase detection circuit 25 binarizes the outputs of the magnetic sensor 1 (30a) and the magnetic sensor 2 (30b).

The magnetic sensor according to the fourth embodiment can be utilized as a rotation sensor as shown in FIG. The rotation sensor 31 includes a gear 21 made of iron or the like, a CMOS-IC 10, and a magnet 11. The CMOS-IC 10 is provided with two adjacent MR elements 5a and 5b. The MR element 5a is included in the MR oscillation of the magnetic sensor 1 (30a). The MR element 5b is included in the MR oscillation of the magnetic sensor 2 (30b). A distinction between forward rotation and reverse rotation of the gear can be determined from the output of the phase detection circuit 25.

When the gear 21 rotates forward, the outputs indicated by the magnetic sensor 1 and the magnetic sensor 2 are displayed in FIG. The solid line is the waveform of the magnetic sensor 1, and the broken line is the waveform of the magnetic sensor 2. FIG. 21 shows an output when the gear 21 rotates in the reverse direction. As in FIG. 20, the solid line is the waveform of the magnetic sensor 1, and the broken line is the waveform of the magnetic sensor 2. Since the magnetic sensor 1 and the magnetic sensor 2 are adjacent to each other, a phase shift occurs between the two waveforms. The rotation direction is detected by detecting the phase advance / delay by the phase detection circuit 25.

FIG. 22 shows characteristics obtained when the waveform in the case of normal rotation shown in FIG. 20 is binarized. FIG. 23 shows characteristics obtained when the waveform in the case of reverse rotation shown in FIG. 21 is binarized. The rotation direction can be determined by looking at the polarity ("H" / "L") of the magnetic sensor 1 with reference to the rising edge of the waveform of the magnetic sensor 2, for example. In the case of FIG. 20 in which the gear 21 is rotating forward, the output of the magnetic sensor 1 is “L” at the rising edge of the magnetic sensor 2. In the case of FIG. 21 in which the gear is rotating in reverse, the output of the magnetic sensor 1 at the rising edge of the magnetic sensor 2 is “H”. Thus, the rotation and direction of rotation of the gear can be detected.

In the fourth embodiment, the gear 22 having teeth made of magnets as shown in FIG. 4 can also be used. As the MR element, for example, a GMR (Giant Magneto Resistive) element or a TMR (Tunneling Magneto Resistive) element can be used to achieve higher sensitivity and higher accuracy.

  DESCRIPTION OF SYMBOLS 1 Magnetic sensor, 2 MR oscillator, 3 Fixed oscillator, 4 Integrator, 5 MR element, 6 Digital filter, 10 MOS-IC, 11 Magnet

Claims (5)

An MR oscillator having an MR element, the oscillation period of which varies according to the intensity of the external magnetic field, a fixed oscillator that oscillates at a constant oscillation period, and a reset that outputs the rectangular wave output from the MR oscillator to the fixed oscillator A magnetic sensor comprising an integrator that counts based on a signal and outputs a digital value, wherein the MR oscillator, the fixed oscillator, and the integrator are formed on a single semiconductor chip. A first MR oscillator having a first MR element, the oscillation period of which varies according to the strength of the external magnetic field, a fixed oscillator that oscillates at a constant oscillation period, and a rectangle output by the first MR oscillator A first integrator that counts a wave based on a reset signal output from the fixed oscillator and outputs it as a digital value; and a second MR element, the oscillation period of which varies according to the strength of the external magnetic field. 2 MR oscillators, a second integrator that counts a rectangular wave output from the second MR oscillator based on a reset signal output from the fixed oscillator, and outputs a digital value; and the first integrator; A phase detection circuit for detecting a phase difference between the first MR oscillator and the first MR oscillator from an output of the second integrator;
The first MR oscillator, the second MR oscillator, the fixed oscillator, the first integrator, the second integrator, and the phase detection circuit are formed on one semiconductor chip. Magnetic sensor to do. 3. The magnetic sensor according to claim 1, wherein the MR oscillator is a single-phase CR oscillator. The magnetic sensor according to claim 1, wherein the MR oscillator is a differential CR oscillator. 3. The magnetic sensor according to claim 1, wherein the MR oscillator is a relaxation type oscillator.

Images