Hydraulic power steering system

A first valve opening degree correction value computation unit computes a correction value (first valve opening degree correction value) for the absolute value of a valve opening degree command value on the basis of a rotation speed deviation computed by a rotation speed deviation computation unit in a pump driving motor control unit. A second valve opening degree computation unit computes a correction value (second valve opening degree correction value) for the valve opening degree command value on the basis of the sign of the valve opening degree command value and the first valve opening degree correction value. A correction value addition unit adds the second valve opening degree correction value to the valve opening degree command value set by the valve opening degree command value setting unit.

INCORPORATION BY REFERENCE/RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2012-089380 filed on Apr. 10, 2012 the disclosure of which, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a hydraulic power steering system.

2. Discussion of Background

There is a conventional hydraulic power steering system that generates steering assist force by supplying hydraulic fluid from a hydraulic pump via a hydraulic control valve to a power cylinder coupled to a steering mechanism of a vehicle. In a common hydraulic power steering system, a hydraulic control valve is mechanically coupled to a steering member, such as a steering wheel, via a steering shaft, and the opening degree of the hydraulic control valve is adjusted on the basis of an operation of the steering member.

Japanese Patent Application Publication No. 2006-306239 (JP 2006-306239 A) describes a hydraulic power steering system that controls the opening degree of a hydraulic control valve with the use of an electric motor (valve driving motor) without mechanically coupling the hydraulic control valve to a steering member.

In the hydraulic power steering system in which the opening degree of the hydraulic control valve is controlled with the use of the valve driving motor, feedback control is executed on an electric motor (pump driving motor) for driving a hydraulic pump such that the rotation speed of the pump driving motor becomes a commanded pump rotation speed. However, the rotation speed of the pump driving motor may fluctuate due to fluctuations in load, or the like. For example, when a rotor of the hydraulic control valve is returned from an end position, at which hydraulic fluid supplied to one of cylinder chambers of a power cylinder becomes the maximum, toward a neutral position, the rotation speed of the pump driving motor fluctuates.

In this way, when the rotation speed of the pump driving motor fluctuates, steering assist force varies although the opening degree of the hydraulic control valve remains unchanged. Therefore, a steering feel may deteriorate.

SUMMARY OF THE INVENTION

The invention provides a hydraulic power steering system with which fluctuations in steering assist force due to fluctuations in the rotation speed of a pump driving motor are suppressed and a steering feel is improved.

According to a feature of an example of the invention, an opening degree command value set by opening degree command value setting means is corrected on the basis of a deviation between a rotation speed command value set by rotation speed command value setting means and a rotation speed of a pump driving motor, which is detected by rotation speed detection means, and a valve driving motor is controlled on the basis of the opening degree command value obtained through the correction. Thus, when the rotation speed of the pump driving motor fluctuates, it is possible to suppress fluctuations in steering assist force. Therefore, it is possible to improve a steering feel.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1is a schematic view that shows the schematic configuration of a hydraulic power steering system1according to an embodiment of the invention. The hydraulic power steering system1is used to apply steering assist force to a steering mechanism2of a vehicle. The steering mechanism2includes a steering wheel3, a steering shaft4, a pinion shaft5, and a rack shaft7. The steering wheel3serves as a steering member, and is operated by a driver in order to steer the vehicle. The steering shaft4is coupled to the steering wheel3. The pinion shaft5is coupled to the distal end portion of the steering shaft4, and has a pinion gear6. The rack shaft7has a rack7athat is in mesh with the pinion gear6, and serves as a steered shaft extending in the lateral direction of the vehicle.

Tie rods8are coupled to respective ends of the rack shaft7. The tie rods8are coupled to knuckle arms11that support right and left steered wheels9,10, respectively. Each of the knuckle arms11is provided so as to be pivotable about a corresponding one of kingpins12. When the steering wheel3is operated to rotate the steering shaft4, the rotation of the steering shaft4is converted by the pinion gear6and the rack7ainto linear motion in the axial direction of the rack shaft7. The linear motion is converted into rotational motion of each knuckle arm11about the corresponding kingpin12. Thus, the right and left steered wheels9,10are steered.

A steering angle sensor31is arranged around the steering shaft4. The steering angle sensor31is used to detect a steering angle θh that is the rotation angle of the steering shaft4. In the present embodiment, the steering angle sensor31is used to detect a rotation amount (rotation angle) of the steering shaft4in each of the forward and reverse directions from a neutral position of the steering shaft4. The steering angle sensor31outputs an amount of rotation to the left from the neutral position as a positive value, and outputs an amount of rotation to the right from the neutral position as a negative value. The pinion shaft5is provided with a torque sensor32that is used to detect a steering torque Th.

The hydraulic power steering system1includes a hydraulic control valve14, a power cylinder16and a hydraulic pump23. The hydraulic control valve14is, for example, a rotary valve, and includes a rotor housing (not shown) and a rotor (not shown) that is used to change a direction in which hydraulic fluid flows. For example, a hydraulic control valve described in JP 2006-306239 A may be used as the hydraulic control valve14. The opening degree of the hydraulic control valve14is controlled by rotating the rotor of the hydraulic control valve14with the use of an electric motor15(hereinafter, referred to as “valve driving motor15”). The valve driving motor15is formed of, for example, a three-phase brushless motor. A rotation angle sensor33is arranged near the valve driving motor15. The rotation angle sensor33is formed of, for example, a resolver, and is used to detect a rotation angle θB of the rotor of the valve driving motor15.

The hydraulic control valve14is connected to the power cylinder16that applies steering assist force to the steering mechanism2. The power cylinder16is coupled to the steering mechanism2. Specifically, the power cylinder16has a piston17and a pair of cylinder chambers18,19. The piston17is formed integrally with the rack shaft7. The cylinder chambers18,19are defined by the piston17. The cylinder chambers18,19are connected to the hydraulic control valve14via a fluid passage20and a fluid passage21, respectively.

The hydraulic control valve14is arranged at an intermediate part of a fluid circulation passage24that passes through a reservoir22and the hydraulic pump23that is used to generate steering assist force. The hydraulic pump23is formed of, for example, a gear pump. The hydraulic pump23is driven by an electric motor25(hereinafter, referred to as “pump driving motor25”) to draw the hydraulic fluid stored in the reservoir22and supply the hydraulic fluid to the hydraulic control valve14. Excess hydraulic fluid is returned from the hydraulic control valve14to the reservoir22via the fluid circulation passage24.

The pump driving motor25is used to drive the hydraulic pump23. Specifically, the output shaft of the pump driving motor25is coupled to the input shaft of the hydraulic pump23. When the output shaft of the pump driving motor25rotates, the input shaft of the hydraulic pump23rotates, and the hydraulic pump23is driven. The pump driving motor25is formed of a three-phase brushless motor. A rotation angle sensor34is arranged near the pump driving motor25. The rotation angle sensor34is formed of, for example, a resolver and is used to detect a rotation angle θP of the rotor of the pump driving motor25.

When the rotor of the hydraulic control valve14is rotated by the valve driving motor15in one direction from a reference rotation angular position (neutral position), the hydraulic control valve14supplies the hydraulic fluid to one of the cylinder chambers18,19of the power cylinder16via a corresponding one of the fluid passages20,21, and returns the hydraulic fluid in the other one of the cylinder chambers18,19to the reservoir22. In addition, when the rotor of the hydraulic control valve14is rotated by the valve driving motor15in the other direction from the neutral position, the hydraulic control valve14supplies the hydraulic pressure to the other one of the cylinder chambers18,19via the other one of the fluid passages20,21, and returns the hydraulic fluid in the one of the cylinder chambers18,19to the reservoir22.

When the rotor of the hydraulic control valve14is at the neutral position, the hydraulic control valve14keeps the pressures in the cylinder chambers18,19of the power cylinder16equal to each other, and hydraulic fluid circulates through the fluid circulation passage24. When the rotor of the hydraulic control valve14is rotated by the valve driving motor15, the hydraulic fluid is supplied to one of the cylinder chambers18,19of the power cylinder16, and the piston17moves in the vehicle-width direction (the lateral direction of the vehicle). Thus, steering assist force acts on the rack shaft7.

The valve driving motor15and the pump driving motor25are controlled by an electronic control unit (ECU)40. A steering angle θh detected by the steering angle sensor31, a steering torque Th detected by the torque sensor32, a signal output from the rotation angle sensor33, a signal output from the rotation angle sensor34, a vehicle speed V detected by a vehicle speed sensor35, a signal output from a current sensor36(seeFIG. 2), and the like, are input into the ECU40. The current sensor36is used to detect a current that flows through the valve driving motor15.

FIG. 2is a block diagram that shows the electrical configuration of the ECU40. The ECU40includes a microcomputer41, a drive circuit (inverter circuit)42and a drive circuit (inverter circuit)43. The drive circuit42is controlled by the microcomputer41, and supplies electric power to the valve driving motor15. The drive circuit43is controlled by the microcomputer41, and supplies electric power to the pump driving motor25. The current sensor36is provided on a power supply line that connects the drive circuit42to the valve driving motor15.

The microcomputer41includes a CPU and memories (a ROM, a RAM, and the like), and executes predetermined programs to function as a plurality of functional processing units. The functional processing units include a valve driving motor control unit50that controls the valve driving motor15and a pump driving motor control unit70that controls the pump driving motor25.

The pump driving motor control unit70includes a steering angular velocity computation unit71, a pump rotation speed command value setting unit82, a rotation angle computation unit73, a rotation speed computation unit74, a rotation speed deviation computation unit75, a PI control unit76and a PWM control unit77. The steering angular velocity computation unit71subjects a value output from the steering angle sensor31to temporal differentiation to compute a steering angular velocity. The pump rotation speed command value setting unit72sets a pump rotation speed command value (motor rotation speed command value) VP* that is a command value of the rotation speed (number of revolutions) of the hydraulic pump23(a command value of the rotation speed of the pump driving motor25) on the basis of the steering angular velocity computed by the steering angular velocity computation unit71.

Specifically, the pump rotation speed command value setting unit72sets a pump rotation speed command value VP* on the basis of a map that stores the correlation between the steering angular velocity and the pump rotation speed command value VP*.FIG. 3is a graph that shows an example of a manner of setting the pump rotation speed command value VP* with respect to the steering angular velocity. The pump rotation speed command value VP* is set such that the pump rotation speed command value VP* takes a predetermined lower limit when the steering angular velocity is zero and the pump rotation speed command value VP* monotonously increases with an increase in the steering angular velocity.

The rotation angle computation unit73computes a rotation angle θP of the pump driving motor25on the basis of a signal output from the rotation angle sensor34. The rotation speed computation unit74computes a rotation speed (number of revolutions) VP of the pump driving motor25on the basis of the rotation angle θP of the pump driving motor25, which is computed by the rotation angle computation unit73. The rotation speed deviation computation unit75computes a deviation ΔVP (=VP*−VP) between the pump rotation speed command value VP* set by the pump rotation speed command value setting unit72and the rotation speed VP of the pump driving motor25, which is computed by the rotation speed computation unit74.

The PI control unit76carries out PI computation on the rotation speed deviation ΔVP computed by the rotation speed deviation computation unit75. That is, the rotation speed deviation computation unit75and the PI control unit76constitute rotation speed feedback control means for bringing the rotation speed VP of the pump driving motor25to the pump rotation speed command value VP*. The PI control unit76carries out PI computation on the rotation speed deviation ΔVP to thereby compute a control voltage value that is a value of control voltage that should be applied to the pump driving motor25.

The PWM control unit77generates a drive signal on the basis of the control voltage value computed by the PI control unit76and the rotation angle OP of the pump driving motor25, which is computed by the rotation angle computation unit73, and supplies the drive signal to the drive circuit43. Thus, a voltage corresponding to the control voltage value computed by the PI control unit76is applied from the drive circuit43to the pump driving motor25.

The valve driving motor control unit50includes an assist torque command value setting unit51, a valve opening degree command value setting unit52, a valve opening degree command value correction unit60, a rotation angle computation unit53, a rotation angular deviation computation unit54, a proportional-integral (PI) control unit55, a motor current computation unit56, a current deviation computation unit57, a PI control unit58and a pulse width modulation (PWM) control unit59.

The assist torque command value setting unit51sets an assist torque command value TA* on the basis of a detected steering torque Th detected by the torque sensor32and a vehicle speed V detected by the vehicle speed sensor35. The assist torque command value TA* is a command value of assist torque that should be generated by the power cylinder16. Specifically, the assist torque command value setting unit51sets an assist torque command value TA* on the basis of a map that stores the correlation for each vehicle speed, between the detected steering torque and the assist torque command value.FIG. 4is a graph that shows an example of a manner of setting the assist torque command value with respect to the detected steering torque.

The detected steering torque Th is expressed, for example, such that torque for steering to the left takes a positive value and torque for steering to the right takes a negative value. In addition, the assist torque command value TA* takes a positive value when assist torque for steering to the left is generated by the power cylinder16, and takes a negative value when assist torque for steering to the right is generated by the power cylinder16.

The assist torque command value TA* with respect to a positive value of the detected steering torque Th takes a positive value, and the assist torque command value TA* with respect to a negative value of the detected steering torque Th takes a negative value. When the detected steering torque Th is a small value that falls within the range of −T1to T1, the assist torque command value TA* is set to zero. When the detected steering torque Th falls outside the range of −T1to T1, the assist torque command value TA* is set such that the absolute value of the assist torque command value TA* increases as the absolute value of the detected steering torque Th increases. In addition, the assist torque command value TA* is set such that the absolute value of the assist torque command value TA* decreases as the vehicle speed V detected by the vehicle speed sensor35increases. The assist torque command value TA* set by the assist torque command value setting unit51is transmitted to the valve opening degree command value setting unit52.

The valve opening degree command value setting unit52sets a valve opening degree command value (motor rotation angle command value) θB* on the basis of the assist torque command value TA* set by the assist torque command value setting unit51. The valve opening degree command value θB* is a command value of the opening degree of the hydraulic control valve14(command value of the rotation angle of the valve driving motor15). In the present embodiment, the rotation angle of the valve driving motor15at the time when the rotor of the hydraulic control valve14is at the neutral position is zero degrees. Then, when the rotation angle of the valve driving motor15is larger than zero degrees, the opening degree of the hydraulic control valve14is controlled such that assist torque for steering to the left is generated by the power cylinder16. On the other hand, when the rotation angle of the valve driving motor15is smaller than zero degrees, the opening degree of the hydraulic control valve14is controlled such that assist torque for steering to the right is generated by the power cylinder16. Note that, as the absolute value of the rotation angle of the valve driving motor15increases, the absolute value of assist torque generated by the power cylinder16increases.

The valve opening degree command value setting unit52sets the valve opening degree command value θB* on the basis of a map that stores the correlation between the assist torque command value TA* and the valve opening degree command value θB*.FIG. 5is a graph that shows an example of a manner of setting the valve opening degree command value θB* with respect to the assist torque command value TA*. The valve opening degree command value θB* with respect to a positive value of the assist torque command value TA* takes a positive value, and the valve opening degree command value θB* with respect to a negative value of the assist torque command value TA* takes a negative value. The valve opening degree command value θB* is set such that the absolute value of the valve opening degree command value θB* increases as the absolute value of the assist torque command value TA* increases. The valve opening degree command value θB* set by the valve opening degree command value setting unit52is transmitted to the valve opening degree command value correction unit60.

The valve opening degree command value correction unit60corrects the valve opening degree command value θB*, which is set by the valve opening degree command value setting unit52, on the basis of the rotation speed deviation ΔVP (=VP*−VP) computed by the rotation speed deviation computation unit75in the pump driving motor control unit70. The valve opening degree command value θB*′ obtained through correction by the valve opening degree command value correction unit60is transmitted to the rotation angular deviation computation unit54. The details of the valve opening degree command value correction unit60will be described later.

The rotation angle computation unit53computes the rotation angle θB of the valve driving motor15on the basis of a signal output from the rotation angle sensor33. The rotation angle θB computed by the rotation angle computation unit53is transmitted to the rotation angular deviation computation unit54. The rotation angular deviation computation unit54computes a deviation ΔθB (=θB*′−θB) between the valve opening degree command value θB*′, which is obtained through correction by the valve opening degree command value correction unit60, and the rotation angle θB of the valve driving motor15, which is computed by the rotation angle computation unit53.

The PI control unit55carries out PI computation on the rotation angular deviation ΔθB computed by the rotation angular deviation computation unit54. That is, the rotation angular deviation computation unit54and the PI control unit55constitute rotation angle feedback control means for bringing the rotation angle θB of the valve driving motor15to the corrected valve opening degree command value θB*′. The PI control unit55computes a current command value for the valve driving motor15by carrying out PI computation on the rotation angular deviation ΔθB.

The motor current computation unit56detects a motor current that flows through the valve driving motor15on the basis of a signal output from the current sensor36. The current deviation computation unit57computes a deviation between the current command value obtained by the PI control unit55and the motor current computed by the motor current computation unit56. The PI control unit58carries out PI computation on the current deviation computed by the current deviation computation unit57. That is, the current deviation computation unit57and the PI control unit58constitute current feedback control means for bringing the motor current that flows through the valve driving motor15to the current command value. The PI control unit58computes a control voltage value that is a value of control voltage that should be applied to the valve driving motor15by carrying out PI computation on the current deviation.

The PWM control unit59generates a drive signal on the basis of the control voltage value computed by the PI control unit58and the rotation angle θB of the valve driving motor15, which is computed by the rotation angle computation unit53, and supplies the drive signal to the drive circuit42. Thus, a voltage corresponding to the control voltage value computed by the PI control unit58is applied from the drive circuit42to the valve driving motor15.

The valve opening degree command value correction unit60will be described in detail. The valve opening degree command value correction unit60includes a first valve opening degree correction value computation unit61, a second valve opening degree correction value computation unit62and a correction value addition unit63. The first valve opening degree correction value computation unit61computes a correction value (hereinafter, referred to as “first valve opening degree correction value Δθ1*”) for the absolute value |θB*| of the valve opening degree command value θB* on the basis of the rotation speed deviation ΔVP (=VP*−VP) computed by the rotation speed deviation computation unit75in the pump driving motor control unit70. The first valve opening degree correction value computation unit61computes the first valve opening degree correction value Δθ1* on the basis of for example, a map that stores the correlation between the rotation speed deviation ΔVP and the first valve opening degree correction value Δθ1*.FIG. 6is a graph that shows an example of a manner of setting the first valve opening degree correction value Δθ1* with respect to the rotation speed deviation ΔVP.

When the rotation speed deviation ΔVP is a value close to zero within the range from −A (A>0) to A, the first valve opening degree correction value Δθ1* is fixed to zero. In a range in which the rotation speed deviation ΔVP is larger than the predetermined value A, the first valve opening degree correction value Δθ1* is set so as to monotonously increase (linearly in the example ofFIG. 6) from zero with an increase in the rotation speed deviation ΔVP. That is, when the pump rotation speed command value VP* is larger than the rotation speed VP of the pump driving motor25and the deviation ΔVP (=VP*−VP) is larger than the predetermined value A, the first valve opening degree correction value Δθ1* takes a positive value, and the first valve opening degree correction value Δθ1* increases as the rotation speed deviation ΔVP increases. In the range in which the rotation speed deviation ΔVP is larger than the predetermined value A, the first valve opening degree correction value Δθ1* may be set so as to nonlinearly increase with an increase in the rotation speed deviation ΔVP.

On the other hand, in a range in which the rotation speed deviation ΔVP is smaller than the predetermined value −A, the first valve opening degree correction value Δθ1* is set so as to monotonously (linearly in the example shown inFIG. 6) decrease from zero with a decrease in the rotation speed deviation ΔVP. That is, when the pump rotation speed command value VP* is lower than the rotation speed VP of the pump driving motor25and the deviation ΔVP (=VP*−VP) is smaller than the predetermined value −A, the first valve opening degree correction value Δθ1* takes a negative value, and the first valve opening degree correction value Δθ1* decreases as the rotation speed deviation ΔVP decreases. In the region in which the rotation speed deviation ΔVP is smaller than the predetermined value −A, the first valve opening degree correction value Δθ1* may be set so as to nonlinearly decrease with a decrease in the rotation speed deviation ΔVP. The first valve opening degree correction value Δθ1* computed by the first valve opening degree correction value computation unit61is transmitted to a second valve opening degree correction value computation unit62.

The second valve opening degree correction value computation unit62computes a correction value (hereinafter, referred to as “second valve opening degree correction value Δθ2*”) for the valve opening degree command value θB* on the basis of the sign of the valve opening degree command value θB* set by the valve opening degree command value setting unit52and the first valve opening degree correction value Δθ1* computed by the first valve opening degree correction value computation unit61. Specifically, when the valve opening degree command value θB* set by the valve opening degree command value setting unit52is larger than or equal to zero (zero or a positive value), the second valve opening degree correction value computation unit62computes the first valve opening degree correction value Δθ1*, which is computed by the first valve opening degree correction value computation unit61, as it is, as the second valve opening degree correction value Δθ2*. Then, the second valve opening degree correction value computation unit62transmits the computed second valve opening degree correction value Δθ2* to the correction value addition unit63.

On the other hand, when the valve opening degree command value θB* set by the valve opening degree command value setting unit52is a negative value, the second valve opening degree correction value computation unit62computes a value (−Δθ1*), which is obtained by inverting the sign of the first valve opening degree correction value Δθ1* computed by the first valve opening degree correction value computation unit61, as the second valve opening degree correction value Δθ2*. Then, the second valve opening degree correction value computation unit62transmits the computed second valve opening degree correction value Δθ2* to the correction value addition unit63.

The correction value addition unit63adds the second valve opening degree correction value Δθ2*, which is transmitted from the second valve opening degree correction value computation unit62, to the valve opening degree command value θB* set by the valve opening degree command value setting unit52. The steering assist force (assist torque) that is generated by the power cylinder16varies on the basis of the rotation speed (number of revolutions) of the pump driving motor25and the valve opening area (valve opening degree) of the hydraulic control valve14. Specifically, the steering assist force increases as the rotation speed of the pump driving motor25increases and as the valve opening area of the hydraulic control valve14increases. When the rotation speed VP of the pump driving motor25fluctuates due to fluctuations in load, or the like, and the rotation speed VP of the pump driving motor25becomes lower than the pump rotation speed command value VP*, the steering assist force tends to fluctuate so as to be decreased.

In such a case, the valve opening degree command value correction unit60corrects the valve opening degree command value θB* such that the absolute value of the valve opening degree command value θB* set by the valve opening degree command value setting unit52increases. Specifically, when the valve opening degree command value θB* is a positive value (a command value for generating assist torque for steering to the left), the second valve opening degree correction value Δθ2* that is added to the valve opening degree command value θB* becomes a positive value. When the valve opening degree command value θB* is a negative value (a command value for generating assist torque for steering to the right), the second valve opening degree correction value Δθ2* that is added to the valve opening degree command value θB* becomes a negative value.

At this time, the valve opening degree command value θB* is corrected such that the absolute value of the valve opening degree command value θB* increases as the deviation ΔVP (=VP*−VP) between the pump rotation speed command value VP* and the rotation speed VP of the pump driving motor25increases. That is, the valve opening degree command value θB* is corrected such that the steering assist force increases. Thus, it is possible to suppress fluctuations in steering assist force. Therefore, it is possible to improve a steering feel.

On the other hand, when the rotation speed VP of the pump driving motor25fluctuates due to fluctuations in load, or the like, and the rotation speed VP of the pump driving motor25becomes higher than the pump rotation speed command value VP*, the steering assist force tends to fluctuate so as to be increased. In such a case, the valve opening degree command value correction unit60corrects the valve opening degree command value θB* such that the absolute value of the valve opening degree command value θB* set by the valve opening degree command value setting unit52decreases. Specifically, when the valve opening degree command value θB* is a positive value (a command value for generating assist torque for steering to the left), the second valve opening degree correction value Δθ2* that is added to the valve opening degree command value θB* becomes a negative value. When the valve opening degree command value θB* is a negative value (a command value for generating assist torque for steering to the right), the second valve opening degree correction value Δθ2* that is added to the valve opening degree command value θB* becomes a positive value.

At this time, the valve opening degree command value θB* is corrected such that the absolute value of the valve opening degree command value θB* decreases as the deviation ΔVP (=(VP*−VP)<0) between the pump rotation speed command value VP* and the rotation speed VP of the pump driving motor25decreases. That is, the valve opening degree command value θB* is corrected such that the steering assist force reduces. Thus, it is possible to suppress fluctuations in steering assist force. Therefore, it is possible to improve a steering feel.

The embodiment of the invention is described above. However, the invention may be implemented in various other embodiments. For example, in the above-described embodiment, the first valve opening degree correction value computation unit61computes the first valve opening degree correction value Δθ1* on the basis of the map that stores the correlation between the rotation speed deviation ΔVP and the first valve opening degree correction value Δθ1*. Alternatively, the first valve opening degree correction value computation unit61may compute the first valve opening degree correction value Δθ1* from the rotation speed deviation ΔVP on the basis of a predetermined arithmetic expression.

Other than the above, various modifications may be made within the scope of the invention.