JP5192720B2 - Magnetostrictive torque sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetostrictive torque sensor capable of avoiding flow of an excessive current in a detection coil at least, until the passage of a prescribed time from application of power. <P>SOLUTION: This magnetostrictive torque sensor 100 for detecting a torque applied to a shaft 20 is equipped with the shaft 20 coated with magnetostrictive films 30a, 30b, and detection coils 40a, 40b, 40c, 40d for detecting magnetic characteristic change of the magnetostrictive films 30a, 30b. The magnetostrictive torque sensor is also equipped with a current flow shut-off means 70, and the current flow shut-off means 70 will not make excitation voltage applied to the detection coils 40a, 40b, 40c, 40d until the passage of a prescribed time from application of power. In other words, inputting of an excitation signal I and its inverted signal into a bridge circuit is shut off by the current flow shut-off means 70. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention relates to a magnetostrictive torque sensor including a detection coil that detects a change in magnetic characteristics of a magnetostrictive film.

An electric power steering device mounted on a vehicle is configured such that a torque sensor detects torque applied to a steering shaft (shaft) by a driver's steering. The electric power steering apparatus is configured to be provided with a steering assist force according to a torque signal from the torque sensor. As such a torque sensor, a magnetostrictive torque sensor is disclosed (Patent Document 1, Patent Document 2), and this technique has a magnetostrictive film having magnetic anisotropy attached to the surface of a shaft. When a torque is applied to the shaft from the outside, a change in the magnetic permeability of the magnetostrictive film according to the twist is detected as a change in impedance characteristics of a detection coil inserted with the shaft separated. Patent Document 2 discloses a technique in which a change in impedance characteristics of a detection coil is detected by applying a pulsed excitation voltage. In this technique, by applying an excitation voltage with a limited pulse width, the maximum value of the increased current flowing in the detection coil falls within a predetermined range.
Japanese Patent Laid-Open No. 2004-239652 (FIG. 4) Japanese Patent Laying-Open No. 2005-331453 (FIGS. 11 and 12)

When the technique disclosed in Patent Document 2 is used in an electric power steering apparatus, when the ignition switch of the vehicle is turned on, the switching circuit is not started stably, and a DC excitation voltage may be applied to the detection coil. Further, when the switching circuit that generates the rectangular wave excitation voltage is generated by the rectangular wave signal and its inverted signal, the predetermined time until the rectangular wave signal generated by the CPU when the power is turned on is A high level signal and a low level signal are input to the switching circuit, and a DC excitation voltage is applied to the detection coil for this predetermined time.
When a DC excitation voltage is applied, a saturation current determined by the internal resistance of the detection coil flows, the magnetostrictive film and the shaft are magnetized, and the detection signal at the start of torque detection may become unstable. As a result, every time the ignition switch is turned on, a change in steering torque occurs, and there is a problem that the steering feeling deteriorates.
If the application of the DC voltage to the detection coil is continued, disconnection of the detection coil or destruction of the switching element may occur due to excessive current flowing.

  Therefore, an object of the present invention is to provide a magnetostrictive torque sensor capable of avoiding an excessive current flowing through a detection coil at least from when the power is turned on until a predetermined time elapses.

In order to solve the above problem, the magnetostrictive torque sensor according to claim 1 includes a shaft covered with a magnetostrictive film, and a detection coil that detects a change in magnetic characteristics of the magnetostrictive film, and is applied to the shaft. In the magnetostrictive torque sensor for detecting torque, the magnetostrictive torque sensor further includes a plurality of switching elements for applying an excitation voltage to the detection coil, and energization cutoff means, and the energization cutoff means is turned on. From this time to the lapse of a predetermined time, the excitation signal inputted to the plurality of switching elements is cut off so that no excitation voltage is applied to the detection coil.

  According to this, the magnetic characteristics of the magnetostrictive film are changed by the torque applied to the shaft. This change in magnetic characteristics can be detected as a change in inductance of the detection coil. In addition, since the energization cut-off means does not apply the excitation voltage to the detection coil at least from when the power is turned on until a predetermined time elapses, instability when the power is turned on is avoided.

  The energization cut-off means also includes a relay that cuts off the power supply of the bridge circuit that generates the excitation voltage. According to this, since the relay cuts off the power supply of the bridge circuit, no excitation voltage is generated.

According to a second aspect of the present invention, there is provided a magnetostrictive torque sensor that includes a shaft coated with a magnetostrictive film and a detection coil that detects a change in magnetic characteristics of the magnetostrictive film, and detects torque applied to the shaft. The magnetostrictive torque sensor is further provided with a switching element that is driven by an output signal of the CPU and applies an excitation voltage to the detection coil, and an energization interruption means. The energization interruption means is predetermined when the power is turned on. Until the passage of time, the excitation signal input to the switching element is cut off so that no excitation voltage is applied to the detection coil .

  The invention according to claim 3 is the magnetostrictive torque sensor according to claim 1 or 2, wherein the torque sensor is mounted on a vehicle, and the power is turned on by turning on an ignition switch of the vehicle. It is characterized by being. According to this, an excitation voltage is not applied to the detection coil from when the ignition switch is turned on until a predetermined time has elapsed.

  According to a fourth aspect of the present invention, in the magnetostrictive torque sensor according to any one of the first to third aspects of the present invention, the magnetostrictive torque sensor further includes control means for supplying the excitation signal, and the control means includes the excitation signal. When transmission becomes possible, an energization permission signal is supplied to the energization cut-off means. According to this, when the excitation signal can be transmitted, the energization permission signal is supplied to the energization cutoff means, and the cutoff of the excitation signal by the energization cutoff means is released.

  In addition, when the excitation voltage is a rectangular wave excitation signal and a rectangular wave voltage generated from this inverted signal, the predetermined time from when the power is turned on until the excitation signal is generated is at least the excitation signal or this Since one of the inverted signals becomes a DC voltage, the DC voltage is applied to the detection coil. As a result, a saturation current flows through the detection coil, and an error may occur in torque detection when the excitation signal is generated. However, the DC voltage is not applied to the detection coil by not applying the excitation voltage to the detection coil during a predetermined time from when the power is turned on to when the excitation signal is generated. The pulse width of the excitation signal needs to be shortened so that the increased current flowing through the detection coil is not saturated.

  According to the present invention, it is possible to provide a magnetostrictive torque sensor capable of avoiding an excessive current flowing through the detection coil at least from when the power is turned on until a predetermined time elapses.

(First embodiment)
A magnetostrictive torque sensor according to an embodiment of the present invention detects torque applied to a steering shaft (shaft) of an electric power steering apparatus as will be described later.
A magnetostrictive torque sensor 100 shown in the configuration diagram of FIG. 1 is detected in series with magnetostrictive films 30a and 30b that are attached to the entire circumference of two adjacent axial directions of a shaft (steering shaft) 20. The coils 40a and 40b, the detection coils 40c and 40d connected in series in the opposite direction, the bridge circuit 10 for applying a rectangular wave-like excitation voltage to the detection coils 40a, 40b, 40c and 40d, and the detection coils 40a and 40b. The differential amplifier 50 that outputs the differential voltage between the connection point VS1 and the connection point VS2 of the detection coils 40c and 40d, the energization cutoff means 70 that cuts off the application of the excitation voltage by the bridge circuit 10, and these elements and means are controlled. A CPU 60 that calculates the torque intensity applied to the shaft 20 and its direction is provided.

  The bridge circuit 10 includes four switching elements 10a, 10b, 10c, and 10d. The switching elements 10a and 10c are p-channel MOSFETs, and the switching elements 10b and 10d are n-channel MOSFETs. The drains of the switching element 10a and the switching element 10c are connected to the detection coil power source, and the sources of the switching element 10b and the switching element 10d are grounded. The source of the switching element 10a, the drain of the switching element 10b, and the terminal S1 are connected, and the source of the switching element 10c, the drain of the switching element 10d, and the terminal S2 are connected. The gate of the switching element 10a, the switching element 10b, and the gate terminal G1 are connected, and the gate of the switching element 10c, the gate of the switching element 10d, and the gate terminal G2 are connected.

  According to this configuration, the bridge circuit 10 is an inverter in which the terminal S1 becomes the power supply potential by setting the terminal G1 to the ground potential, and the terminal S1 becomes the ground potential by setting the gate terminal G1 to the power supply potential of the detection coil power supply. The circuit is configured. Similarly, in the bridge circuit 10, the terminal S2 becomes the power supply potential by setting the gate terminal G2 to the ground potential, and the terminal S2 becomes the ground potential by setting the gate terminal G2 to the power supply potential.

  The magnetostrictive films 30a and 30b are, for example, films having magnetic anisotropy such as Fe—Ni or Fe—Cr, and have anisotropy in opposite directions at two adjacent axial directions of the shaft 20, respectively. It is attached. Therefore, when torque is applied to the shaft 20 in one direction, a magnetic permeability difference appears in the magnetostrictive films 30a and 30b, and when torque is applied in the reverse direction, a change in permeability appears in the reverse direction.

  Here, a method for applying the magnetostrictive films 30a and 30b and a method for imparting anisotropy will be described. First, the shaft 20 is heat-treated so that the Rockwell hardness is HRC 40 to 65, and then the magnetostrictive films 30a and 30b are coated on the outer peripheral surfaces of the shaft 20 at two locations in the axial direction by plating or vapor deposition. To wear. Next, by twisting the shaft 20, a counterclockwise torque T (for example, about 10 kgf · m (98 N · m)) is applied to the magnetostrictive film 30a, and in this state, the magnetostrictive film 30a is made to have a high frequency using a coil. When the counterclockwise torque T is removed after the magnetostrictive film 30a is heated and cooled at about 300 ° C. for several seconds and then cooled, the anisotropy is imparted to the magnetostrictive film 30a. Next, by twisting the shaft 20 in the opposite direction, a clockwise torque T (for example, about 10 kgf · m (98 N · m)) is applied to the magnetostrictive film 30b. Is vibrated at a high frequency in the same manner as the magnetostrictive film 30a, thereby heating the magnetostrictive film 30b at about 300 ° C. As a result, the magnetostrictive film 30b is given anisotropy in the opposite direction to the magnetostrictive film 30a.

  The series circuit of the detection coils 40a and 40b or the series circuit of the detection coils 40c and 40d detects the magnetic permeability difference between the magnetostrictive films 30a and 30b as an inductance difference. When an excitation voltage is applied to both ends of the series circuit, the magnetic permeability difference between the magnetostrictive films 30a and 30b is detected as potential fluctuations at the connection points VS1 and VS2. That is, the difference between the intermediate potential before torque generation and the potentials of the connection points VS1 and VS2 corresponds to the torque. At this time, since the detection coils 40c and 40d are connected in the reverse direction, the potential at the connection point VS1 and the potential at the connection point VS2 vary in the reverse direction. Thereby, twice the sensitivity can be obtained as compared with the case where either one of the potentials is detected. In addition, the detection coil is connected in series to obtain a differential output of two potential signals, thereby canceling fluctuation due to temperature change.

  The differential amplifier 50 is composed of an operational amplifier, and amplifies the potential difference between the connection point VS1 and the connection point VS2. The CPU 60 functions as a control unit and includes an A / D converter 60a, and converts the analog output voltage of the differential amplifier 50 into a digital signal for processing. Further, the CPU 60 calculates the strength and direction of the torque applied to the shaft 20 and outputs a torque signal T. Further, the CPU 60 generates an excitation signal I which is a rectangular wave signal, and this excitation signal I is input to the gate terminal G2 of the bridge circuit 10 via an energization cutoff means 70 which will be described later, and the inverter 55 and the energization cutoff means 70 are connected. Through the gate terminal G1. Further, the excitation signal I is inhibited from generating a rectangular wave voltage for a predetermined time (signal activation time) from when the CPU power is turned on, and a low level is output. For this reason, the inverted signal of the excitation signal I is at the high level for this predetermined time. The CPU 60 generates an energization permission signal for notifying that the excitation signal I can be transmitted after a predetermined time has elapsed after the CPU power supply is turned on. Note that the transmission line for transmitting the energization permission signal is pulled down by the resistor R. The differential amplifier 50 is driven by an OP Amp power source, and the CPU 60 is driven by a CPU power source. Both power sources are 5V.

  The energization cut-off means 70 which is a characteristic configuration of the present embodiment cuts off the transmission of the excitation signal I and its inverted signal to the gate terminals G1 and G2 of the bridge circuit 10 for a predetermined time, and permits this cut-off using the energization permission signal. To do. Specifically, by setting the gate terminal G1 and the gate terminal G2 to the ground potential or the power supply potential, the terminals S1 and S2 have the same potential, and no current flows through the detection coils 40a, 40b, 40c, and 40d. By this energization cut-off means 70, application of the excitation voltage to the detection coils 40a, 40b, 40c, 40d is cut off for a predetermined time after the CPU power supply is turned on. That is, both the excitation signal I and its inverted signal are blocked from being input to the bridge circuit 10.

Next, the operation of the magnetostrictive torque sensor 100 will be described. Each part of the magnetostrictive torque sensor 100 provided in the electric power steering device is activated when an ignition switch of the vehicle is turned on.
2A shows the time change of the output voltage of the ignition switch (IG-SW), FIG. 2B shows the time change of the detection coil power supply, and FIG. 2C shows the time change of the gate IC power supply. FIG. 2 (d) is a diagram showing the time change of the CPU power supply. The power supply voltage [V] of each part rises by turning on the ignition switch.
FIG. 2E shows the waveform of the excitation signal I, and a rectangular wave voltage having a pulse width t1 and a duty of 1/2 is generated after a predetermined signal activation time has elapsed since the ignition switch was turned on. This rectangular wave voltage is applied to the gate terminal G2 of the bridge circuit 10 through the energization cut-off means 70, and the voltage having the inverted waveform is applied to the gate terminal G1. The solid line in FIG. 2 (f) is the waveform of the energization permission signal, and transitions from the low level state to the high level state when the excitation signal I can be transmitted by the CPU 60. By this energization permission signal, the energization cut-off means 70 permits the excitation signal I and its inverted signal to be input to the gate terminals G1, G2.

  FIG. 2G shows the waveform of the detection coil voltage. When a high level signal of the excitation signal I is applied to the gate terminal G2 of the bridge circuit 10, and when a low level signal that is the inverted signal is applied to the gate terminal G1, the terminal S2 becomes the ground potential and the terminal S1 becomes the power supply potential. It becomes. On the other hand, when a low level signal of the excitation signal I is applied to the gate terminal G2, and when a high level signal that is the inverted signal is applied to the gate terminal G1, the terminal S1 becomes the ground potential and the terminal S2 becomes the power supply potential. . As a result, a rectangular wave AC voltage that alternately reverses positive and negative voltages is applied to the series circuit of the detection coils 40a and 40b and the series circuit of the detection coils 40c and 40d.

  FIG. 2 (h) shows a waveform of the detection coil current. When a positive voltage is applied to the detection coils 40a, 40b, 40c, and 40d, a current that monotonously increases with time using the inverse of the inductance as a proportional coefficient flows. . Further, when a negative voltage is applied to the detection coils 40a, 40b, 40c, and 40d, a current that monotonously decreases flows with time. At this time, the increase current or decrease current is limited by the pulse width t1, and no saturation current flows. Since the magnetic permeability that defines the inductance has nonlinear characteristics, the current does not change linearly.

  In addition, when the timing of the excitation signal I and the timing of the energization permission signal are shifted as shown by the broken line in FIG. 2F, the negative pulse voltage having the time width t2 is generated as shown by the broken line in FIG. Applied, a negative decreasing current flows for this time t2. Then, when a positive pulse voltage with a pulse width t1 is applied, the current increases with time from the negative current value, and when the negative pulse voltage is applied, the increase in current stops, and the current again Decrease. That is, by varying the time width t2, the average current is reduced, and further, by making the time width t2 substantially equal to the pulse width t1, the detection coils 40a, An alternating current can be passed through 40b, 40c, and 40d. In other words, an alternating current can be caused to flow by appropriately setting the value of the detection coil current at the rising or falling timing of the excitation signal I.

  FIG. 3A is a diagram showing a state where the detection coils 40 a, 40 b, 40 c, and 40 d are wound around the shaft 20. Detection coils 40a and 40c are loosely inserted in two axial directions on the surface of the magnetostrictive film 30b attached to the shaft 20, and detection coils 40b and 40d are loosely inserted in two axial directions on the surface of the magnetostrictive film 30a. . The winding direction of the winding is the same direction, and the magnetic fields H1, H2, H3, and H4 are not canceled. Further, a rectangular wave voltage is applied between the detection coils 40a and 40d and the detection coils 40b and 40c, a connection point VS1 between the detection coil 40a and the detection coil 40b is drawn, and the detection coil 40c and the detection coil 40d are connected. Connection point VS2 is drawn out.

  FIG. 3B shows a magnetic field H [A / m] and magnetic flux density (magnetizing force) B = 4πI [when a rectangular wave AC voltage is applied to the detection coils 40a, 40b, 40c, and 40d and an alternating current flows. It is the figure which showed the relationship with T]. By appropriately selecting the pulse width t1 of the excitation signal, the magnetic flux can be prevented from being saturated. In addition, by flowing a positive / negative symmetrical alternating current, the absolute value of the magnetic flux density becomes about half that of only a positive or negative pulse current. In addition, the relationship between the current and the magnetic flux density (magnetizing force) can maintain linearity.

  As described above, according to the present embodiment, the energization cut-off means 70 avoids any one or both of the excitation signal I and its inverted signal being input to the gate terminals G1 and G2 of the bridge circuit 10. The Thereby, it is avoided that a DC voltage is applied to the detection coils 40a, 40b, 40c, and 40d by a DC signal generated at the signal activation time, and a saturation current depending on the internal resistance is detected by the detection coils 40a, 40b, 40c, It does not flow to 40d. For this reason, the magnetostrictive films 30a and 30b do not saturate during the signal activation time. Further, since the average current can be changed by changing the time difference t2 which is the difference between the end time of the signal activation time and the cutoff end time of the energization cutoff means 70, the current flowing through the detection coils 40a, 40b, 40c and 40d Can be exchanged.

(Second Embodiment)
In the first embodiment, the excitation signal I and its inverted signal are cut off using the energization cut-off means 70 in order to avoid the DC voltage generated during the signal start-up time being input to the gate terminal G1, A time corresponding to the time can be generated using a differentiating circuit, and a signal can be blocked using a gate IC.

FIG. 4 is a configuration diagram of the magnetostrictive torque sensor 110. In FIG. 4, since the bridge circuit 10, the shaft 20, the magnetostrictive films 30a and 30b, the detection coils 40a, 40b, 40c, and 40d, the differential amplifier 50, and the CPU 60 are the same as those in the first embodiment, the description thereof is omitted. Only the point will be described.
The excitation signal I generated by the CPU 60 is directly input to the gate terminal G2 as a right FET drive signal. A differentiation circuit 80 including a capacitor C and a resistor R generates a differential waveform of the rising voltage of the gate IC power supply. The NOR operation of the differential waveform and the excitation signal I is performed using the gate IC85. The NOR signal as the calculation result is input to the gate terminal G1 as the left FET drive signal.

  FIG. 5 is a signal waveform of each part, and FIGS. 5A to 5E are the same as those in the first embodiment, and thus description thereof is omitted. FIG. 5 (f) shows an output waveform of the differentiation circuit 80. This waveform instantly generates a voltage substantially equal to the power supply voltage simultaneously with the rise of the gate IC power supply (FIG. 6 (c)). It decreases with the time constant with the unit R. FIG. 5G shows the waveform of the right-side FET drive signal, which is the waveform of the excitation signal I (FIG. 5E) itself. FIG. 6H shows the waveform of the left FET drive signal, and an inverted signal of the excitation signal I is output. However, it is not output when the output voltage of the differentiation circuit 80 is equal to or higher than a certain voltage. The time constant of the differentiating circuit 80 is appropriately selected according to the signal activation time. FIG. 6 (i) shows a detection coil voltage waveform, and FIG. 6 (j) shows a detection coil current waveform. These are basically the same waveforms as those in the first embodiment. However, the rise time of the left FET drive signal can change depending on the relationship between the output of the differentiation circuit and the threshold voltage of the gate IC 85.

(Third embodiment)
In each of the above embodiments, the rectangular wave signals input to the gate terminals G1 and G2 of the bridge circuit 10 are controlled, but the detection coils 40a, 40b, 40c, and 40d are controlled even when the power supply voltage applied to the bridge circuit 10 is controlled. It is possible to avoid the application of a DC voltage to.
FIG. 6 is a configuration diagram of the magnetostrictive torque sensor 120. In FIG. 6, since the bridge circuit 10, the shaft 20, the magnetostrictive films 30a and 30b, the detection coils 40a, 40b, 40c, and 40d, the differential amplifier 50, the inverter 55, and the CPU 60 are the same as those in the first embodiment, description thereof is omitted. Only the differences will be described.

  The relay 95 which is an energization interruption means includes a relay coil 95a and a contact point 95b, and supplies the detection coil power to the switching elements 10a and 10c of the bridge circuit 10 by energizing the relay coil 95a. The timer means 90 energizes the relay coil 95a after a set time after the detection coil power supply is turned on. This set time is the signal activation time described above.

The operation of this embodiment will be described with reference to the signal waveform of FIG.
Since the waveforms in FIGS. 7A to 7E are the same as those in the first embodiment, description thereof will be omitted. FIG. 7F shows a relay drive signal generated by the timer means 90. This signal is delayed by the signal activation time of the excitation signal I (FIG. 7 (e)) after the ignition switch (IG-SW) (FIG. 7 (a)) is turned ON. The right FET drive signal in FIG. 7G has the same waveform as the excitation signal I, and the left FET drive signal in FIG. FIG. 7 (i) shows a detection coil voltage waveform, and a rectangular wave AC voltage similar to that in the first embodiment is applied after the signal activation time has elapsed. FIG. 7J shows the detection coil current, which is the same current as in the first embodiment. That is, according to the present embodiment, a DC voltage is not applied to the detection coils 40a, 40b, 40c, and 40d due to the DC voltage of the left FET drive signal generated during the signal activation time.

(Example of use)
Next, an electric power steering apparatus in which the magnetostrictive torque sensors 100, 110, 120 of the embodiment are used will be described with reference to FIG.
In the electric power steering apparatus 200, when the steering handle 210 is rotated, the shaft 20 which is a steering shaft directly connected to the steering handle 210 rotates the pinion 260 constituting the rack and pinion mechanism 270, thereby moving the rack shaft 250. To change the direction of the steered wheel 220. At this time, the control device 230 drives and controls the electric motor 240 according to the torque signal T detected by the magnetostrictive torque sensors 100, 110, and 120 using the detection coil 40. The electric motor 240 operates to reduce the steering torque of the steering handle 210 by rotating the pinion 260 via the power transmission mechanism 280. If the driver's steering torque is TH, the torque transmitted to the pinion 260 is TP, and the constant related to the magnitude of the auxiliary torque by the motor 240 is KA, the relationship TH = TP / (1 + KA) is established. To do. Further, the electric power steering apparatus 200 includes a steer_by_wire in which the steering handle 210 and the steered wheel 220 are mechanically separated.

  According to this use example, the signal activation time when the ignition switch is turned on does not drive the bridge circuit 10, so that the detection signal at the start of torque detection does not become unstable, and the magnetostrictive film and the shaft are It is not magnetized unnecessarily. As a result, the steering torque does not change and the steering feeling does not deteriorate.

(Comparative example)
Next, the configuration and operation in the case where the energization cutoff means is not provided will be described as a comparative example. In FIG. 9, the bridge circuit 10, the shaft 20, the magnetostrictive films 30a and 30b, the detection coils 40a, 40b, 40c, and 40d, the differential amplifier 50, the inverter 55, and the CPU 60 are the same as those in the above-described embodiments, and thus description thereof is omitted. To do. That is, in this comparative example, the excitation signal I and this inverted signal are directly input to the gate terminals G1 and G2 of the bridge circuit 10, and the detection coil power supply is directly applied to the drains of the switching elements 10a and 10c.

Next, the operation of this comparative example will be described with reference to FIG.
Since the waveforms in FIGS. 10A to 10D are the same as those in each of the embodiments, the description thereof will be omitted. The excitation signal I in FIG. 10E is a rectangular wave signal waveform with a duty of 1/2 generated by the CPU 60 after the signal activation time has elapsed. This excitation signal and the inverted signal are input to the gate terminals G1 and G2 of the bridge circuit 10, and the waveform of the detection coil voltage shown in FIG. 10 (f) is generated. In this waveform, except for the signal activation time, a rectangular wave AC voltage is used in the same manner as in each of the above embodiments, but during the signal activation time, a DC voltage is applied to the detection coils 40a, 40b, 40c, and 40d. For this reason, the detection coil current waveform shown in FIG. 10G gradually increases as the detection coil power supply rises, and a saturation current defined by the internal resistance of the detection coil flows. Then, when the polarity of the detection coil voltage is reversed to negative, the current starts to decrease. At this time, the current decrease width is defined by the pulse width t1 of the excitation signal. When the polarity of the detection coil voltage is reversed again, the current of the detection coil increases. For this reason, the average current flowing through the detection coils 40a, 40b, 40c, and 40d is offset by ΔI, and the magnetostrictive films 30a and 30b are magnetized. FIG. 10 (h) shows the waveform of the right FET drive signal, and FIG. 10 (i) shows the waveform of the left FET drive signal.

  Next, it will be described with reference to FIG. 11 that the magnetostrictive films 30a and 30b of this comparative example are magnetized. FIG. 11A shows a state in which the DC voltage V is applied and the DC current I flows during the signal activation time, and is different from FIG. 3A in that it is magnetized in one direction. FIG. 11B shows a state in which a saturated direct current flows and the magnetization force 4πI is saturated. FIG. 11C is a diagram showing the relationship between the residual magnetic flux density [T] and the torque sensor midpoint voltage, and the midpoint voltage (the voltages at the connection points VS1 and VS2) varies as the residual magnetic flux density increases. . Note that a rectangular wave AC voltage is applied to the detection coils 40a, 40b, 40c, and 40d by the bridge circuit 10, but the potential at the connection points S1 and S2 is either the power supply potential or the ground potential. The voltages of VS1 and VS2 are based on the midpoint potential (2.5V) of the power supply voltage (5V).

(Modification)
The present invention is not limited to the above-described embodiment, and various modifications such as the following are possible, for example.
(1) In each of the above embodiments, an excitation voltage of a rectangular wave AC voltage is applied to the detection coils 40a, 40b, 40c, and 40d, but a predetermined time elapses after the power is turned on even when a positive or negative pulse voltage is applied. Until the excitation voltage is not applied, the DC voltage is not applied to the detection coils 40a, 40b, 40c, and 40d during the signal activation time until the activation of the switching circuit after power-on is stabilized.
(2) In each of the above embodiments, the detection coils 40c and 40d are connected in the opposite direction of the detection coils 40a and 40b to obtain a differential output between the connection point VS1 and the connection point VS2. When applied, the potential fluctuates in the same direction, and the differential output becomes near the zero point. In other words, when an abnormality occurs, the zero point is lost, and this can be detected and used as an abnormality detection means.

It is a block diagram of the magnetostrictive torque sensor which is one Embodiment of this invention. It is a figure which shows the time change of each part of the magnetostrictive torque sensor which is one Embodiment of this invention. It is the figure which showed a mode that the detection coil was wound around the shaft, and the rectangular wave alternating voltage was applied, and the figure which showed the relationship between a magnetic field and magnetizing force. It is a block diagram of the magnetostrictive torque sensor which is other embodiment of this invention. It is a figure which shows the time change of each part of the magnetostrictive torque sensor which is other embodiment of this invention. It is a block diagram of the magnetostrictive torque sensor which is further another embodiment of this invention. It is a figure which shows the time change of each part of the magnetostrictive torque sensor which is further another embodiment of this invention. It is a block diagram of an electric power steering device. It is a block diagram of the magnetostrictive torque sensor which is a comparative example of this invention. It is a figure which shows the time change of each part of the magnetostrictive torque sensor which is a comparative example of this invention. FIG. 3 is a diagram showing a state where a detection coil is wound around a shaft and a DC voltage is applied, a diagram showing a relationship between a magnetic field and a magnetizing force, and a diagram showing a relationship between a residual magnetic flux density and a torque sensor midpoint voltage. .

Explanation of symbols

10 Bridge circuit 10a, 10b, 10c, 10d Switching element 20 Shaft (steering shaft)
30a, 30b Magnetostrictive film 40, 40a, 40b, 40c, 40d Detection coil 50 Differential amplifier 55 Inverter 60 CPU (control means)
60a A / D converter 70 Current cut off means 80 Differentiating circuit 85 Gate IC
90 Timer means 95 Relay (energization interruption means)
95a Relay coil 95b Contact 100, 110, 120 Magnetostrictive torque sensor 200 Electric power steering device 210 Steering handle 220 Steering wheel 230 Controller 240 Electric motor 250 Rack shaft 260 Pinion 270 Rack pinion mechanism 280 Power transmission mechanism

Claims (4)

In a magnetostrictive torque sensor that includes a shaft coated with a magnetostrictive film, and a detection coil that detects a change in magnetic characteristics of the magnetostrictive film, and detects torque applied to the shaft.
The magnetostrictive torque sensor further includes a plurality of switching elements that apply an excitation voltage to the detection coil, and an energization cutoff unit.
The energization cut-off means cuts off an excitation signal input to the plurality of switching elements from a time when power is turned on until a predetermined time elapses, thereby preventing an excitation voltage from being applied to the detection coil. Magnetostrictive torque sensor. In a magnetostrictive torque sensor that includes a shaft coated with a magnetostrictive film, and a detection coil that detects a change in magnetic characteristics of the magnetostrictive film, and detects torque applied to the shaft.
The magnetostrictive torque sensor further includes a switching element that is driven by an output signal of a CPU and applies an excitation voltage to the detection coil, and an energization cutoff unit.
The energization cut-off means cuts off an excitation signal input to the switching element from a time when power is turned on until a predetermined time elapses, thereby preventing an excitation voltage from being applied to the detection coil. Torque sensor. The torque sensor is mounted on a vehicle,
The magnetostrictive torque sensor according to claim 1 or 2, wherein the power is turned on by turning on an ignition switch of the vehicle. A control means for supplying the excitation signal;
4. The magnetostriction according to claim 1, wherein the control unit supplies an energization permission signal to the energization cutoff unit when the excitation signal can be transmitted. 5. Type torque sensor.

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