Control unit for electric power steering device

A bending load acting on a steering shaft only with a magnetostrictive torque sensor, without separately providing a sensor for detecting the bending moment. A storage unit stores, as an initial characteristic curve formed from initial detection values, a characteristic curve formed from detection values of each of a first detection coil and a second detection coil when only twisting torque is applied to a steering shaft. A bending load detector provided in an ECU detects a bending load amount acting on the steering shaft based on a difference between each detection value of the first and second detection coils and the initial detection value on the initial characteristic curve corresponding to the detection value.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2010-080928 filed on Mar. 31, 2010 the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control unit for an electric power steering device, and more particularly, to a control unit for an electric power steering device for assisting the steering of a steering wheel based on a detection value of a magnetostrictive torque sensor.

2. Description of Background Art

Electric power steering devices are known for detecting with a torque sensor the twisting torque generated in a steering shaft to assist steering of a steering wheel with an electric motor according to the twisting torque. Such electric power steering device is commonly controlled by a control unit (ECU) of a vehicle.

This type of electric power steering devices include the electric power steering device in which a magnetostrictive torque sensor is used as the torque sensor. See, for example, JP-A No. 2008-83063. The magnetostrictive torque sensor is configured with a magnetostrictive film formed on a surface of the steering shaft and a detection coil provided around the magnetostrictive film. This sensor can be made relatively compact, and therefore has the advantage of contributing to the miniaturization of the vehicle.

Since the magnetostrictive torque sensor is subjected to a bending load, it is necessary to provide plural bearings to reduce bending. However, this causes upsizing of the device and an increase in the production cost.

SUMMARY AND OBJECTS OF THE INVENTION

Accordingly, the present invention has been made under such circumstances, and an object of an embodiment of the present invention is to provide a control unit for an electric power steering device in which a bending load can be detected only by a magnetostrictive torque sensor without separately providing bearings for reducing the bending load acting on a steering shaft. A driver's operation is determined based on the bending load detection value so as to enable assisted control for the electric power steering device according to an operating condition.

As a solution to the above-described problem, the according to an embodiment of the present invention, a control unit for an electric power steering device2is provided with a first magnetostrictive film15and a second magnetostrictive film16with magnetic anisotropic properties opposite in direction to each other, each formed circumferentially on a surface of a steering shaft7, and a first detection coil17and a second detection coil18provided around the first magnetostrictive film and the second magnetostrictive film, respectively, for detecting twisting torque applied to the steering shaft based on detection values of the first detection coil and the second detection coil to adjust an assist amount for the steering shaft with a storage unit31being provided for storing. As initial characteristic curves C1and C2are formed from initial detection values, a characteristic curve formed from detection values of each of the first detection coil and the second detection coil when only twisting torque is applied to the steering shaft. A bending load detector32is provided for detecting a bending load amount acting on the steering shaft based on a difference between each detection value of the first and second detection coils and the initial detection value on the initial characteristic curve corresponding to the detection value.

According to an embodiment of the present invention, when twisting torque is applied in one direction from a neutral position of the steering shaft, the initial characteristic curve has an upwardly convex curve shape that reaches a peak at a predetermined torque, on the other hand, when twisting torque is applied in the other direction from the neutral position of the steering shaft, the initial characteristic curve has a gradually decaying curve shape. Also, the initial characteristic curves of the first detection coil and the second detection coil have opposite characteristics with detection values symmetric with respect to the neutral position of the steering shaft. In addition, the bending load detector detects a twisting torque corresponding to each detection value detected by the first detection coil and the second detection coil from a range having the gradually decaying curve shape on one of the initial characteristic curve of the first detection coil and the initial characteristic curve of the second detection coil, and obtains an initial detection value corresponding to the detected twisting torque from a range having the upwardly convex curve shape on the other initial characteristic curve to detect the bending load amount based on a difference between the initial detection value and the detection value detected by the first detection coil or the second detection coil.

According to an embodiment of the present invention, the control unit for the electric power steering device includes an assist characteristic deciding portion33that determines a vehicle operating condition based on the bending load amount detected by the bending load detector, and adjusts an assist amount for the steering shaft according to this determination state.

According to an embodiment of the present invention, the assist characteristic deciding portion determines the vehicle operating condition based on the bending load amount detected by the bending load detector and a vehicle speed value detected by a vehicle speed sensor for detecting vehicle speed, and adjusts the assist amount for the steering shaft according to this determination state.

According to an embodiment of the present invention, the assist characteristic deciding portion determines the vehicle operating condition based on the bending load amount detected by the bending load detector, the vehicle speed value of the vehicle speed sensor when this bending load amount is detected, and the twisting torque values from the first detection coil and the second detection coil when this bending load amount is detected, and adjusts the assist amount for the steering shaft according to this determination state.

According to an embodiment of the present invention, the assist characteristic deciding portion decides the assist amount for the steering shaft based on the vehicle speed value of the vehicle speed sensor and the twisting torque values from the first detection coil and the second detection coil. When a bending load amount is detected by the bending load detector, the assist characteristic deciding portion determines the vehicle operating condition based on the bending load amount detected by the bending load detector, and the vehicle speed value of the vehicle speed sensor when this bending load amount is detected, and adjusts the decided assist amount for the steering shaft according to this determination state.

According to an embodiment of the present invention, the assist characteristic deciding portion performs damper feel control for the steering shaft based on the bending load amount detected by the bending load detector, the vehicle speed value of the vehicle speed sensor when this bending load amount is detected, and the twisting torque values from the first detection coil and the second detection coil when this bending load amount is detected.

According to an embodiment of the present invention, the assist characteristic deciding portion determines the vehicle operating condition based on the bending load amount detected by the bending load detector, a vehicle speed value detected by a vehicle speed sensor for detecting vehicle speed, the twisting torque values from the first detection coil and the second detection coil, and a tilt angle value from a tilt angle sensor for detecting a vehicle tilt angle to adjust the assist amount for the steering shaft according to this determination state and perform damper feel control.

According to an embodiment of the present invention, the bending load amount acting on the steering shaft can be quantitatively detected only by the magnetostrictive torque sensor, thereby eliminating the need to separately provide a bending load detecting sensor and allowing miniaturization of the vehicle and a reduction in the production costs. More specifically, the characteristic curves have the properties that, even when a bending load is applied, there is little characteristic change in the ranges having the gradually decaying curve shape on the initial characteristic curves stored in the storage unit, and therefore can be utilized as absolute values for use in uniquely obtaining twisting torque from the detection values of the first detection coil and the second detection coil. Thus, each value in the ranges having the gradually decaying curve shape can be used as the basis for detecting a twisting torque. Also, each initial detection value of the first detection coil and the second detection coil under no-bending-load condition is obtained from the initial characteristic curve as a storage value corresponding to this twisting torque, and a comparison can be made in terms of the difference between the initial detection value and the actual measurement value, thereby allowing the determination whether or not the steering shaft is subjected to bending and the measurement of the bending load amount, without the need to separately provide a bending load detecting sensor. Thus, miniaturization of the vehicle and a reduction in production costs can be realized.

According to an embodiment of the present invention, the assist characteristics for the electric power steering device according to operating conditions can be obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.FIG. 1shows the configuration of an electric power steering system1including a control unit according to this embodiment. In this embodiment, the electric power steering system1is mounted on a so-called all-terrain vehicle (ATV), and is composed of an electric power steering device2and an ECU3serving as a control unit for controlling the electric power steering device2.

As shown inFIG. 1, the electric power steering device2is provided with a steering system4, a magnetostrictive torque sensor5, and an electric motor6. The steering system4includes a steering shaft7rotatably supported by a body frame that is not shown, a steering wheel8provided on an upper end of the steering shaft7, a pitman arm9provided on a lower end of the steering shaft7, and a tie rod10provided at both ends of the pitman arm9. Front wheels11are coupled to the respective tie rods10so that the steering force from the steering wheel8is transmitted to the front wheels11through the steering shaft7and the tie rods10.

A worm wheel12is provided on a lower portion of the steering shaft7, and the electric motor6for imparting assist force to the steering shaft7is disposed adjacent to the worm wheel12. The magnetostrictive torque sensor5is provided for detecting the twisting torque acting on the steering shaft7, and is disposed above the electric motor6and the worm wheel12. The magnetostrictive torque sensor5is accommodated in a sensor housing13, and the electric motor6is fixed to a reduction gear case14for covering the worm wheel12.

As shown inFIG. 2, the magnetostrictive torque sensor5includes a first magnetostrictive film15and a second magnetostrictive film16formed on the steering shaft7and vertically aligned, and a first pair of detection coils17and a second pair of detection coils18provided around the first magnetostrictive film15and the second magnetostrictive film16, respectively. The first magnetostrictive film15and the second magnetostrictive film16are each annularly formed over the whole circumference on an outer peripheral surface of the steering shaft7, and made of a magnetostrictive film with a magnetostrictive property changed according to torque, such as an Ni—Fe-based alloy film formed by vapor plating. The first magnetostrictive film15and the second magnetostrictive film16have magnetic anisotropic properties opposite in direction to each other. The magnetostrictive direction of the second magnetostrictive film16is different from that of the first magnetostrictive film15.

More specifically, for example, the magnetostrictive direction of the first magnetostrictive film15has an angle of 45 degrees with respect to the axial direction of the steering shaft7, while the magnetostrictive direction of the second magnetostrictive film16has an angle of −45 degrees with respect to the axial direction of the steering shaft7so that different impedances are outputted from the first detection coil17and the second detection coil18.

The first detection coil17and the second detection coil18are each composed of upper and lower coil bobbins19, four in total, allowing the steering shaft7to pass through; coils20wound on the respective coil bobbins19; and two pairs of upper and lower magnetic yokes21for accommodating the coil bobbins19and the coils20, and are accommodated in the sensor housing13formed into a cylindrical shape in such a manner so as to surround the steering shaft7and elongated in the axial direction of the steering shaft7. The sensor housing13holds the yokes21with the outer periphery of each yoke21abutting on an inner peripheral surface of the sensor housing13to position the coils20around the first magnetostrictive film15and the second magnetostrictive film16. Two pairs of upper and lower couplers22are provided on the coils20so that detection signals from the coils20are extracted through these couplers22.

The reduction gear case14has a box shape for covering the worm wheel12provided on the steering shaft7while covering the steering shaft7, and the electric motor6is fixed to an outer wall surface thereof. The electric motor6is fixed to the reduction gear case14, with a shaft portion23of the electric motor6extending to the inside of the reduction gear case14, and a worm gear24provided on a leading end of the shaft portion23is brought into a meshing engagement with the worm wheel12. Thus, the assist force from the drive motor6is imparted to the steering shaft7. Also, the sensor housing13includes bearings25and26for rotatably supporting the steering shaft7at upper and lower portions thereof, respectively, and the reduction gear case14includes a bearing27for rotatably supporting the steering shaft7at a lower portion thereof.

As shown inFIG. 1, the ECU3is electrically connected to the magnetostrictive torque sensor5and the electric motor6, and also is electrically connected to a vehicle speed sensor28for detecting vehicle speed and a tilt angle sensor29for detecting a vehicle tilt angle. The ECU3detects a bending load amount based on a twisting torque detection signal from the magnetostrictive torque sensor5, and can output a predetermined control signal to the electric motor6using an assist characteristic deciding portion33based on the twisting torque, the bending load amount, the vehicle speed value from the vehicle speed sensor28, and the tilt angle value from the tilt angle sensor29.

The ECU3includes a storage unit31, a bending load detector32, and the assist characteristic deciding portion33. The storage unit31is typically composed of storage means such as a ROM, and stores, as an initial characteristic curve formed from initial detection values, a characteristic curve formed from detection values of each of the first detection coil17and the second detection coil18when only twisting torque is applied to the steering shaft7. It should be noted that the detection values of the first detection coil17and the second detection coil18when only twisting torque is applied to the steering shaft7, are referred to as “initial detection values”, as described above, and a curve plotted in coordinates from these initial detection values is referred to as an initial characteristic curve.

FIG. 3conceptually shows the initial characteristic curves stored in the storage unit31. InFIG. 3, the horizontal axis represents the torque (T) added to the steering shaft7and the vertical axis represents the impedance (Z) of the first detection coil17and the second detection coil18outputted by the added torque. InFIG. 3, it is to be noted that C1denotes the initial characteristic curve of the first detection coil17, and C2denotes the initial characteristic curve of the second detection coil18. As is clear fromFIG. 3, when twisting torque is applied in one direction from a neutral position of the steering shaft, the initial characteristic curves C1and C2have a convex curve shape (ranges R1and R2) that reaches a peak at a predetermined torque, on the other hand, when twisting torque is applied in the other direction from the neutral position of the steering shaft7, the initial characteristic curves C1and C2have a gradually decaying curve shape (ranges R3and R4). Furthermore, since the first magnetostrictive film15and the second magnetostrictive film16have mutually different magnetostrictive directions (more correctly, in a symmetrical manner), the initial characteristic curves C1and C2have opposite characteristics with detection values symmetrical with respect to the neutral position of the steering shaft7.

The bending load detector32is designed to detect the bending load amount with reference to the above-described initial characteristic curves C1and C2. The bending load detector32detects a twisting torque corresponding to each detection value detected by the first detection coil17and the second detection coil18from the range R3or R4having the gradually decaying curve shape on one of the initial characteristic curve C1of the first detection coil17and the initial characteristic curve C2of the second detection coil18, and obtains an initial detection value corresponding to the detected twisting torque from the range R3or R4as described above from the range R1or R2having the upwardly convex curve shape on the other initial characteristic curve C1or C2to detect the bending load amount based on a difference between the initial detection value and the detection value (actual measurement value) detected by the first detection coil17or the second detection coil18.

This bending load detector32is designed to detect the generation of a bending load and a bending load amount, by utilizing the properties that, in the case where a bending load is applied to the steering shaft7, there is little difference between the initial detection value Z1, Z2and the actual measurement values P1, P2in the ranges R3and R4having the gradually decaying curve shape on the initial characteristic curves C1and C2, while there is a greater difference ΔS there between in the ranges R1and R2having the upwardly convex curve shape. Referring toFIGS. 3, C3and C4show actual characteristic curves (actual characteristic curves C3, C4under bending load) formed from respective detection values of the first detection coil17and the second detection coil18with twisting torque applied in a state in which a bending load is applied to the steering shaft7. As can be seen when compared with the initial characteristic curves C1and C2, the actual characteristic curves C3and C4have the above-described properties, that is, there is little difference between the initial detection values Z1, Z2and the actual measurement value values P1, P2in the ranges R3and R4(the range enclosed by the chain double-dashed line) having the gradually decaying curve shape on the initial characteristic curves C1and C2, respectively, while there is a difference therebetween in the ranges R1and R2having the upwardly convex curve shape.More specifically, as shown inFIG. 3, for example, when an actual detection (measurement) value indicated by P1is detected in the first detection coil17and a an actual detection (measurement) value indicated by P2is detected in the second detection coil18. The bending load detector32detects a twisting torque T1corresponding to the actual detection (measurement) value P2detected by the second detection coil18with reference to the range R4having the gradually decaying curve shape on the initial characteristic curve C2of the second detection coil18, and then obtains an initial detection value Z1with reference to the range R1having the upwardly convex curve shape on the initial characteristic curve C1corresponding to the twisting torque T1to perform a comparison between the initial detection value Z1and the actual measurement value P1detected by the first detection coil17(on initial characteristic curve C1). Thereafter, if in the above comparison there is a difference (ΔS), the bending load detector32determines that a bending load is generated, and detects a bending load amount according to the difference ΔS. It should be noted that the bending load detector32calculates the bending load amount according to whether the difference ΔS is large or small, and modifications of this calculation include a calculation method using a predetermined arithmetic expression, and a calculation method in which the bending load amount according to the difference ΔS is previously obtained, and this information is stored in the storage unit31or the like to perform calculations by comparison and reference.

In addition, based on the bending load amount detected by the bending load detector32, the vehicle speed value detected by the vehicle speed sensor28, the twisting torque value from the first detection coil17and the second detection coil18, and the tilt angle value from the tilt angle sensor29, the assist characteristic deciding portion33decides an assist amount for the steering shaft7and a damper feel and determines an operating condition to control the damper feel while adjusting the assist amount for the steering shaft7according to this determination state.

It should be noted that the assist amount serves to facilitate a rotational operation of the steering shaft7by an occupant and refers to the level of the output value of the electric motor6. It also should be noted that the damper feel means preventing the steering shaft7from being rotated by an external force (such as a road surface reaction force), and sending, to the electric motor6, electricity42that is in opposite phase to a vibration41of the twisting torque applied to the steering shaft7so as to cancel out this vibration, as shown inFIG. 4. Thus, the electric motor6is allowed to finely-operate in the direction opposite to the vibration of the twisting torque, thereby causing hard rotation of the steering shaft7. The strength of the damper feel is adjusted depending on the amount of power supply. The decision of the damper feel refers to the level of power supply, and the control of a damper feel means adjusting the power supply.

The assist characteristic deciding portion33performs the decisions of an assist amount and a damper feel, the determination of an operating condition, adjustment of the assist amount, control of the damper feel based on plural pieces of prestored characteristic information. These pieces of characteristic information are prestored in the storage unit31.

FIGS. 5 to 8show various characteristic information (MAP) to which the assist characteristic deciding portion33refers.FIG. 5conceptually shows assist amount characteristic information M1for use in the decision of the assist amount stored in the storage unit31;FIG. 6conceptually shows damper feel characteristic information M2for use in the control of the damper feel;FIG. 7conceptually shows assist-up characteristic information M3for use in the adjustment (increase) of the assist amount; andFIG. 8conceptually shows assist-down characteristic information M4for use in the adjustment (decrease) of the assist amount.

Referring toFIG. 5, the assist amount characteristic information M1is information for use in setting an assist current according to the twisting torque and the vehicle speed. The assist amount characteristic information M1includes low-speed assist characteristic information51that specifies an assist amount according to the twisting torque during low-speed operating; medium-speed assist characteristic information52that specifies an assist amount according to the twisting torque during medium-speed operating; and high-speed assist characteristic information53that specifies an assist amount according to the twisting torque during high-speed operating.

In this example, the assist amounts of the high-speed assist characteristic information53, the medium-speed assist characteristic information52, and the low-speed assist characteristic information51are set to increase in this order. Also, in each of the low-, medium-, and high-speed characteristic information, the assist amount is set to gradually increase according to the twisting torque in a relatively small range of the twisting torque, while the assist amount is set constant in a relatively large range of the twisting torque. Further, threshold value ranges set as the relatively small ranges of the twisting torque, vary among the low, medium, and high speeds. In this embodiment, the assist characteristic deciding portion33has the respective threshold value ranges for determining the low, medium, and high speeds based on the vehicle speed value (for example, the low speed is equal to or higher than 0 km/h and below 10 km/h, and the medium speed is equal to or higher than 10 km/h and below 20 km/h). The assist characteristic deciding portion33determines these three types and then refers to the assist amount characteristic information M1to decide the assist amount.

Referring toFIG. 6, the damper feel characteristic information M2is information for use in setting the strength of the damper feel according to the vehicle speed, the twisting torque, and the bending load amount (bending force). In this damper feel characteristic information M2, the setting is made such that the higher the vehicle speed value, the larger the degree of increase in the damper feel, while the lower the vehicle speed value, the smaller the degree of increase in the damper feel. In other words, there are set a determination threshold value61(for example, 30 km/h or more) as a high vehicle speed value and a determination threshold value62(for example, equal to or higher than 0 km/h and below 5 km/h) as a low vehicle speed value, and the setting is made such that the higher the vehicle speed value, the larger the amount of increase in the damper feel with respect to the bending load amount. Also, the setting is made such that the larger the bending load amount, the more the damper feel (the larger the amount of increase in the damper feel). In this embodiment, the assist characteristic deciding portion33sets the strength of the damper feel based on the vehicle speed value, the twisting torque value, and the bending load amount (bending force), with reference to the damper feel characteristic information M2. It should be noted that as for speed values between the determination threshold values61and62, the assist characteristic deciding portion33linearly calculates the values between the determination threshold values61and62with the vehicle speed value and the bending load amount as binomial parameters to set the strength of the damper feel.

Referring toFIG. 7, the assist-up characteristic information M3is information for use in setting the assist-up amount according to the vehicle speed and the bending load amount (bending force). In this assist-up characteristic information M3, the setting is made such that the lower the vehicle speed value, the larger the degree of increase in the assist-up amount, while the higher the vehicle speed value, the smaller the degree of increase in the assist-up amount. In other words, there are set a determination threshold value71(for example, equal to or higher than 0 km/h and below 5 km/h) as a low vehicle speed value and a determination threshold value72(for example, 30 km/h or more) as a high vehicle speed value, and the setting is made such that the lower the vehicle speed value, the more the increase in the assist-up amount with respect to the bending load amount. Also, the setting is made such that the larger the bending load amount, the larger the assist-up amount.

It should be noted that the assist-up characteristic information M3is referred to by the assist characteristic deciding portion33when the twisting torque is relatively large. When the twisting torque is relatively large, the vehicle speed is low, and the bending load is large, the vehicle is likely to be in an operating condition such as the condition of “downhill road” operation, “stationary swing,” or “bank road (reverse steering)” operation. On the other hand, when the vehicle speed is high and the bending load is large, the vehicle is likely to be in an operating condition such as the condition of “rough road operating,” “full braking,” or “jump”. Therefore, in the assist-up characteristic information M3, the setting is made such that when the vehicle speed is low and the bending load amount is large, the assist amount is increased to a large degree, while, when the vehicle speed is high and the bending load amount is small, the assist amount is increased to a small degree.

In addition, referring toFIG. 8, the assist-down characteristic information M4is information for use in setting the assist-down amount according to the speed value and the bending load amount (bending force). In this assist-down characteristic information M4, the setting is made such that the lower the vehicle speed value, the larger the degree of increase in the assist-down amount, while the higher the vehicle speed value, the smaller the degree of increase in the assist-down amount. In other words, a determination threshold value81is set (for example, equal to or higher than 0 km/h and below 5 km/h) as a low vehicle speed value and a determination threshold value82is set (for example, 30 km/h or more) as a high vehicle speed value, and the settings are made such that the lower the vehicle speed value, the more the increase in the assist-down amount with respect to the bending load amount. Also, the setting is made such that the larger the bending load amount, the larger the assist-down amount.

It should be noted that the assist-down characteristic information M4is referred to by the assist characteristic deciding portion33when the twisting torque is relatively small. When the twisting torque is relatively small, the vehicle speed is low, and the bending load is large, the vehicle is likely to be in an operating condition such as the condition of an “uphill road” operation or a “bank road (normal steering)” operation. Therefore, in the assist-down characteristic information M4, the setting is made such that when the vehicle speed is low and the bending load amount is large, the assist amount is decreased to a large degree, while, when the vehicle speed is high and the bending load amount is small, the assist amount is decreased to a small degree. In this embodiment, the assist characteristic deciding portion33determines whether the twisting torque is high or low, and then refers to the assist-up characteristic information M3or the assist-down characteristic information M4to set the assist-up amount or the assist-down amount based on the vehicle speed value and the bending load amount (bending force). It should be noted that as for speed values between the determination threshold value71(81) and the determination threshold value72(82), the assist characteristic deciding portion33linearly calculates the values between the determination threshold values71(81) and72(82) with the vehicle speed value and the bending load amount as binomial parameters to set the assist-up amount and the assist-down amount. It should be noted that, referring toFIGS. 4 and 5, the assist-up correction means increasing the output of the assist current in the direction of arrow UP, while the assist-down correction means decreasing the output of the assist current in the direction of arrow DOWN.

Next, one example of the assist control for the electric power steering device2by the ECU3configured as above will be described with reference to the flowchart shown inFIG. 9.

In step S1, the ECU3reads a vehicle speed value from the vehicle speed sensor28, and also reads a twisting torque value from the magnetostrictive torque sensor5. In step S2, an assist amount and a damper feel are decided by the assist characteristic deciding portion33. It should be noted that the assist characteristic deciding portion33decides the assist amount by comparing the vehicle speed value and twisting torque value read in step S1with the assist amount characteristic information M1shown inFIG. 5, and as for the damper feel, sets a predetermined electric power value.

In step S3, the ECU3reads a bending load using the bending load detector32to determine in step S4whether or not bending is detected. If bending is detected, the process goes to step S5. If not detected, the process ends and is again repeated from step S1. Thereafter, in step S5, the ECU3reads the vehicle speed value and the twisting torque when the bending load is detected, and in step S6, performs a damper feel correction using the assist characteristic deciding portion33. It should be noted that the assist characteristic deciding portion33performs the damper feel correction by comparing the vehicle value and twisting torque read in step S5with the damper feel characteristic information M2shown inFIG. 6.

In step S7, the ECU3determines, using the assist characteristic deciding portion33, whether the twisting torque read in step S5is higher or lower than a predetermined twisting torque value. If the twisting torque is higher, the process goes to step S8, on the other hand, if the twisting torque is below the predetermined twisting torque, the process goes to step S9. It should be noted that the determination threshold value of the predetermined twisting torque value changes depending on vehicle speed and such information is also stored in the storage unit31.

Thereafter the ECU3performs, using the assist characteristic deciding portion33, a correction for the increase in assist amount in step S8, and a correction for the decrease in assist amount in step S9. It should be noted that the assist characteristic deciding portion33refers, in step S8, to the assist-up characteristic information M3shown inFIG. 7, and refers, in step S9, to the assist-down characteristic information M4shown inFIG. 8to decide an amount of assist adjustment.

The foregoing embodiment of the present invention includes the bending load detector32provided in the ECU3, for storing, in the storage unit31provided in the ECU3, as the initial characteristic curves C1and C2formed from the initial detection values, the characteristic curves formed from respective detection values of the first detection coil17and the second detection coil18when only twisting torque is applied to the steering shaft7, and detecting the bending load amount acting on the steering shaft7based on the differences between the detection values detected by the first detection coil17and the second detection coil18, and the initial detection values on the initial characteristic curves C1and C2corresponding to the respective detection values. Thus, the bending load amount can be quantitatively detected only by the magnetostrictive torque sensor5, thereby eliminating the need to separately provide a bending load detecting sensor and allowing miniaturization of the vehicle and a reduction in production costs.

In addition, this embodiment includes the assist characteristic deciding portion33that detects an operating condition of the vehicle based on the bending load amount detected by the bending load detector32, or the like, and adjusts the assist amount for the steering shaft7according to this determination state. Thus, the assist characteristics for the electric power steering device2according to operating conditions can be properly obtained.

Next, a modification of the process of the ECU3will be described with reference concurrently to the flowcharts shown inFIGS. 10 and 11. In this process, firstly in step S21, the ECU3reads a vehicle speed value from the vehicle speed sensor28, and also reads a twisting torque value from the magnetostrictive torque sensor5. In step S22, an assist amount and a damper feel are decided by the assist characteristic deciding portion33. It should be noted that the assist characteristic deciding portion33decides the assist amount by comparing the vehicle speed value and twisting torque value read in step S21with the assist amount characteristic information M1shown inFIG. 5, and as for the damper feel, sets a predetermined electric power value.

In step S23, the ECU3reads a bending load using the bending load detector32to determine whether or not bending is detected. If bending is not detected, the process returns to step S21. If bending is detected, the process goes to step S24. In step S24, it is detected whether or not the vehicle is moving. If the vehicle is moving, the process goes to step S25. If the vehicle is not moving, the process goes to step S26. In step S26, the ECU3performs, using the assist characteristic deciding portion33, a correction for the increase in assist amount. The reason for this increase in assist amount is because of the presumption that the vehicle is in a stationary swing condition.

In step S25, the ECU3determines whether or not the vehicle speed is high, on the basis of whether or not the vehicle speed is higher than a predetermined vehicle speed value. If the vehicle speed is high, the process goes to step S29shown inFIG. 11. If not, the process goes to step S27. In step S27, whether or not the vehicle speed is medium is determined on the basis of whether or not the vehicle speed is higher than a predetermined vehicle speed value. If the vehicle speed is medium, the process goes to step S28. If not, it is determined that the vehicle speed is low, and the process goes to step S34shown inFIG. 11.

After the determination that the vehicle speed is high, the ECU3determines in step S29whether or not the bending load is “large,” on the basis of whether or not the bending load is higher than a predetermined value. If the bending load is “large,” the process goes to step S30. If not, the process goes to step S31. In step S31, whether or not the bending load is “moderate” is determined on the basis of whether or not the bending load is higher than a predetermined value. If the bending load is “moderate,” the process goes to step S32. If not, it is determined that the bending load is small, the process goes to step S33.

After the determination that the bending load is “large”, the ECU3performs in step S30, using the assist characteristic deciding portion33, a correction for increasing the damper feel. After the determination that the bending load is “moderate,” the ECU3performs in step S32a correction for slightly increasing the damper feel. After the determination that the bending load is small, a normal characteristic is maintained in step S33. It should be noted that, in this modification, when the bending load is “large,” the vehicle is presumed to be in a condition such as jump or landing, and the damper feel is increased. When the bending load is “moderate,” the vehicle is presumed to be in a condition such as full braking, and the damper feel is slightly increased. After the above damper feel correction, the process is again repeated from step S21.

On the other hand, in step S28, after it is determined in step S27that the vehicle speed is medium, both of the assist amount and the damper feel are linearly controlled. This is the same manner as the process shown inFIG. 9based on the damper feel characteristic information M2, the assist-up characteristic information M3, and the assist-down characteristic information M4shown inFIGS. 6 to 8in which the twisting torque is also referred to.

After the determination that the vehicle speed is not medium, the ECU3determines in step S34(FIG. 11) whether or not the bending load is “large,” on the basis of whether or not the bending load is higher than a predetermined value. If the bending load is “large,” the process goes to step S35. If not, the process goes to step S36. In step S36, whether or not the bending load is “moderate” is determined on the basis of whether or not the bending load is higher than a predetermined value. If the bending load is “moderate,” the process goes to step S37. If not, it is determined that the bending load is small and the process goes to step S38.

After the determination that the bending load is “large,” the ECU3performs in step S35, using the assist characteristic deciding portion33, a correction for increasing the assist amount to a small degree. After the determination that the bending load is “moderate,” a normal characteristic is maintained in step S37. After the determination that the bending load is “small,” the ECU3performs in step S38a correction for increasing the assist amount to a slightly large degree. It should be noted that, in this modification, when the bending load is “large,” the vehicle is presumed to be operating on a uphill road, a downhill road or a bank road, and a correction for increasing the assist amount to a small degree is performed. Here, after this assist amount correction, the process is again repeated from step S21. Through the foregoing process, the assist characteristics for the electric power steering device2according to operating conditions can be also properly obtained.

Next, another modification of the process of the ECU3will be described. In this process, motor rotational acceleration caused by a reaction force applied to the electric motor6is detected. When the absolute value of the detection value is larger than a predetermined value (or equal to or more than a predetermined value), reduction control of the current value for the electric motor6is performed, thereby creating a damper feel. On the other hand, when the absolute value of the detected motor rotational acceleration is equal to or less than a predetermined value (or smaller than a predetermined value), the current value for the electric motor6is increased according to the twisting torque to increase the assist current. Furthermore, in this process, as for the damper feel, the current decrease amount is adjusted, and as for the increase in assist amount, the current increase amount is adjusted, according to the steering angle of the steering wheel.

To be more specific, referring toFIG. 12, looking at the left half area on the drawing sheet, the motor rotational acceleration is detected as indicated by line A (a solid heavy line) inFIG. 12, and assume that this detection value is larger than a predetermined value. In this case, in this process, the normal assist current for the electric motor6as indicated by line B (a chain double-dashed line) inFIG. 12is decreased as indicated by a down-pointing arrow (increase in damper feel) on the drawing sheet, and therefore the current (damper-feel-increase motor current) indicated by line C (a chain line) inFIG. 12is outputted to the electric motor6, so that the damper feel is provided. On the other hand, looking at the right half area on the drawing sheet, when the motor rotational acceleration is “0,” the normal assist current B is increased as indicated by an up-pointing arrow (increase in assist amount) on the drawing sheet according to the detection torque (twisting torque) indicated by D (a dotted line) inFIG. 12, and therefore the current (assist-increase motor current) indicated by line E (a solid line, a thin line) inFIG. 12is outputted to the electric motor6, so that the damper feel control is not performed. In addition, inFIG. 12, line F denotes the steering angle of the steering wheel, in which the upper side on the drawing sheet denotes the displacement angle at the time of steering to the right, while the lower side on the drawing sheet denotes the displacement angle at the time of steering to the left. As can be seen when comparing the line F with the lines B, C, and E, as the steering wheel is closer to a neutral position, the current decrease amount for the case of the damper feel control and the current increase amount for the case of the increase in assist amount are made larger. As the steering wheel is closer to steering limit positions, the current decrease amount for the case of the damper feel control and the current increase amount for the case of the increase in assist amount are made smaller.

As the assist current and damper feeling deciding processing of the ECU3, this processing may be employed. After that, alternatively, the decided assist current may be adjusted according to the bending load. It should be noted that inFIG. 12, the vertical axis represents the motor current and the steering angle of the steering wheel and serves as an index of the lines B, C, D, and F, and the lines A and D are shown for the convenience of the description taken in connection with the lines B, C, D, and F.

While it has been described in the foregoing embodiment that in the processes of the ECU3described usingFIGS. 9,10, and11, the assist characteristic is decided out of consideration of the tilt angle value of the tilt angle sensor29. However, since the determination of the operating condition including the tilt angle value allows still further fragmentation of the operating condition, thereby enabling high-precision control of the assist amount and the damper amount according to the operating condition. In concrete terms, if the operating conditions, such as whether on a uphill road or a downhill road, are determined based on the tilt angle value, further high-precision assist characteristic can be obtained.