STEERING CONTROLLER AND STEERING-SPEED DETECTION METHOD
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a steering controller and a steering-speed detection method, and more particularly, to a steering controller and a steering-speed detection method for assisting a driver in steering.

2. Description of the Related Art
A conventional electric power steering device is provided with a motor for applying a torque to a steering system (steering wheel of a vehicle) so as to assist a driver in its operation. In the conventional electric power steering device, a steering speed of the driver is detected by using a voltage across terminals of a motor (see Japanese Patent Application Laid-open Nos. 2001-171533 and 2001-278083, for example).

Japanese Patent Application Laid-open No. 2 001-171533 describes an electric power steering device which obtains an angular speed of a motor based on a voltage across terminals of the motor.

Japanese Patent Application Laid-open No. 2001-278083 describes an electric power steering device which detects a steering speed based on a current and a voltage of a motor. In Japanese Patent Application Laid-open No. 2001-278083, an offset error in the detected steering speed is removed by a high-pass filter.
Although Japanese Patent Application Laid-open No.

2001-171533 describes the detection of the angular speed of the motor by using the voltage across the terminals of the motor, the case where the offset error is contained in the voltage across the terminals of the motor is not taken into consideration. Therefore, when the offset error is contained in the voltage across the terminals of the motor, there is a problem in that the detected steering speed also contains an offset error.

Although Japanese Patent Application Laid-open No. 2001-278083 describes the removal of the offset error in the steering speed by the high-pass filter, there is a problem in that a low-frequency component of the steering speed is disadvantageously removed by the high-pass filter.

SUMMARY OF THE INVENTION
The present invention has been made to solve the problems described above, and therefore has an object to provide a steering controller and a steering-speed detection method which are capable of obtaining a steering speed without containing an offset error.

According to one embodiment of the present invention, there is provided a steering controller, including: a steering-angle information acquiring section for acquiring information on a steering angle of a steering system of a vehicle; a steering-speed information acquiring section for acquiring first information on a steering speed of the steering system; and a correction section for correcting the first information on the steering speed, which is acquired by the steering-speed information acquiring section, based on the information on the steering angle, which is acquired by the steering-angle information acquiring section, to thereby acquire second information on the steering speed of the steering system.

According to one embodiment of the present invention, the steering controller includes: the steering-angle information acquiring section for acquiring information on a steering angle of a steering system of a vehicle; the steering-speed information acquiring section for acquiring first information on a steering speed of the steering system; and the correction section for correcting the first information on the steering speed, which is acquired by the steering-speed information acquiring section, based on the information on the steering angle, which is acquired by the steering-angle information acquiring section, to thereby acquire second information on the steering speed of the steering system. Therefore, a steering speed without containing an offset error may be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a configuration diagram illustrating a steering controller according to a first embodiment of the present invention and the periphery thereof;
FIG. 2 is a block diagram illustrating a configuration of the steering controller according to the first embodiment of the present invention;
FIG. 3 is a flowchart illustrating an operation of the steering controller according to the first embodiment of the present invention;
FIG. 4 is a flowchart illustrating an operation of a steering-speed computing section of a steering controller according to a second embodiment of the present invention;
FIG. 5 is a block diagram illustrating a part of an internal configuration of the steering-speed computing section of the steering controller according to the second embodiment of the present invention;
FIG. 6 is a block diagram illustrating a part of a variation of the internal configuration of the steering-speed computing section of the steering controller according to the second embodiment of the present invention;
FIG. 7 is a graph showing the effects of the steering controller according to the second embodiment of the present invention;
FIG. 8 is a flowchart illustrating an operation of a steering-speed computing section of a steering controller according to a third embodiment of the present invention; and
FIG. 9 is a block diagram illustrating a part of an internal configuration of the steering-speed computing section of the steering controller according to the third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
FIG. 1 is a configuration diagram illustrating a steering controller according to a first embodiment of the present invention and the periphery thereof. The steering controller is mounted in a vehicle such as an automobile. The vehicle is provided with two front wheels (wheels 3 to be steered) and two rear wheels (drive wheels; not shown) . As illust rated in FIG. 1, the vehicle is provided with a steering wheel 1 which is a steering system to be operated by a driver. A steering shaft 2 is coupled to the steering wheel

1. The two wheels 3 to be steered are coupled to the steering shaft
2. When the driver steers the steering wheel 1, the right and left wheels 3 to be steered are steered in accordance with the rotation of the steering shaft 2. A steering-angle sensor 4 for detecting a steering angle is provided to the steering wheel 1. A torque sensor 5 is provided to the steering shaft 2 . The torque sensor 5 detects a steering torque exerted on the steering shaft 2. A motor 6 is coupled to the steering shaft 2 through an intermediation of a speed reduction mechanism 7 . A steering assist torque generated by the motor 6 is applied to the steering shaft 2. A speed of the vehicle is detected by a vehicle-speed sensor 8. A current flowing through the motor 6 (hereinafter referred to as "motor current") is detected by a current sensor 9. A voltage across terminals of the motor 6 (hereinafter referred to as "motor voltage") is detected by a voltage sensor 10.

In this case, the steering-angle sensor 4 composes a steering-angle information acquiring section for acquiring information on the steering angle of the steering system of the vehicle. However, the steering-angle information acquiring section is not limited to the above-mentioned case. Any steering-angle information acquiring section for acquiring the information on the steering angle of the steering system of the vehicle can be used as long as the information on the steering angle of the steering system of the vehicle is obtained. Therefore, the steering-angle information acquiring section may be composed, for example, of a yaw-rate sensor. The yaw-rate sensor is a sensor for detecting a yaw rate (speed of change of an angle of rotation in a direction in which the vehicle turns).

Detection values by the steering-angle sensor 4, the torque sensor 5, the vehicle-speed sensor 8, the current sensor 9, and the voltage sensor 10 are input to a control unit 11. The control unit 11 computes a target steering assist torque to be generated by the motor 6 based on the above-mentioned detection values, and controls a value of a current to be supplied to the motor 6 based on the computed target steering assist torque.

FIG. 2 is a diagram illustrating a configuration of the steering controller according to the first embodiment of the present invention. As illustrated in FIG. 2, the steering controller includes the plurality of sensors including the steering-angle sensor 4, the torque sensor 5, the vehicle-speed sensor 8, the current sensor 9, and the voltage sensor 10, and the control unit 11. As illustrated in FIG. 2, the control unit 11 includes a computation device 40 and a current driver 19. The computation device 4 0 includes a microcomputer which includes a memory (not shown) having a ROM and a RAM. The current driver 19 supplies a current to the motor 6 to drive the motor 6.

In the first embodiment, the following steering controller is described as an example. Specifically, the steering controller performs control for assisting the driver in steering (hereinafter referred to as "steering assist control") and control for the purpose of returning the steering wheel 1 to a neutral position even in a region where the steering wheel 1 does not return to the neutral position because of a small road-surface reaction force torque (hereinafter referred to as "steering-wheel return control").

As illustrated in FIG. 2, the computation device 40 includes a steering-speed computing section 12, a steering-holding determining section 13, a target steering-speed setting section 14, a first steering-assist-torque computing section 15, a steering-state determining section 16, a second steering-assist-torque computing section 17, and an adder section 18.

The steering-speed computing section 12 receives detection values from the steering-angle sensor 4, the current sensor 9, and the voltage sensor 10, and computes the steering speed based on the input detection values. The steering-speed computing section 12 acquires the motor current from the current sensor 9 and the motor voltage from the voltage sensor 10. The steering-speed computing section 12 obtains an induced voltage of the motor 6 based on the motor current and the motor voltage. A method of computing the induced voltage is described in a second embodiment of the present invention. Then, the steering-speed computing section 12 estimates the steering speed based on the induced voltage. However, the estimated steering speed contains an offset error due to the presence of an oxide film formed on a contact portion and degradation with elapse of time. Therefore, the steering-speed computing section 12 obtains a correction value for correcting the estimated steering speed based on the steering angle acquired from the steering-angle sensor 4. The steering-speed computing section 12 corrects the estimated steering speed by using the correction value, and outputs the corrected steering speed.

In this case, the steering-speed computing section 12 composes a steering-speed information acquiring section and a correction section. The steering-speed information acquiring section acquires first information on the steering speed of the steering system. The correction section corrects the first information, which is acquired by the steering-speed information acquiring section, based on the information on the steering angle of the steering system, which is acquired by the steering-angle information acquiring section (steering-angle sensor or yaw-rate sensor), to thereby acquire second information on the steering speed of the steering system. In this case, the induced voltage of the motor 6 is described as an example of the first information on the steering speed of the steering system. However, the first information on the steering speed of the steering system is not limited thereto, and may be the steering speed obtained from a device for detecting the steering speed, such as a resolver. A specific operation of the steering-speed computing section 12 is described later in the second and third embodiments of the present invention.

The steering-holding determining section 13 determines whether or not the steering system of the vehicle is in a state of steering holding. Specifically, the steering-holding determining section 13 determines whether or not the steering wheel 1 is kept at an approximately constant steering angle. The steering-holding determining section 13 uses the steering speed computed by the steering-speed computing section 12 to determine that the steering system is in the "state of steering holding" when the steering speed is smaller than a threshold value, and outputs a steering angle at the steering speed which is smaller than the threshold value. On the other hand, when the steering speed is equal to or larger than the threshold value, the steering-holding determining section 13 determines that the steering system is not in the "state of steering holding", and outputs the steering angle at the time of the previous determination for the steering holding without updating the steering angle to be output.

The target steering-speed setting section 14 receives the steering angle from the steering-angle sensor 4, the vehicle speed from the vehicle-speed sensor 8, and the steering angle from the steering-holding determining section 13. The target steering-speed setting section 14 computes a target steering speed based on the received values. As a computationmethod, for example, the following methodmay be used. Specifically, a lookup table which stores target steering speeds for the above-mentioned values is stored in advance. The values of the steering angle from the steering-angle sensor 4, the vehicle speed from the vehicle-speed sensor 8, and the steering angle from the steering-holding determining section 13 are input to the lookup table to obtain a corresponding target steering speed. Alternatively, the following method may also be used. Specifically, an arithmetic expression (or a calculation formula) for obtaining the target steering speed is stored. The values of the steering angle from the steering-angle sensor 4, the vehicle speed from the vehicle-speed sensor 8, and the steering angle from the steering-holding determining section 13 are entered into the arithmetic expression to obtain the target steering speed.

The first steering-assist-torque computing section 15 receives the steering speed computed by the steering-speed computing section 12 and the target steering speed computed by the target steering-speed setting section 14. The first steering-assist-torque computing section 15 obtains a deviation between the steering speed and the target steering speed, and computes a first steering assist torque based on the obtained deviation. The first steering assist torque is a torque for returning the steering wheel 1 to the neutral position. As a method of computing the first steering assist torque, the following method may be used. Specifically, a lookup table which stores a value of the first steering assist torque for the deviation between the steering speed and the target steering speed is stored in advance. A value of the deviation is input to the lookup table to obtain a corresponding value of the first steering assist torque. Alternatively, the following method may also be used. Specifically, an arithmetic expression (or a calculation formula) for obtaining the first steering assist torque based on the deviation between the steering speed and the target steering speed is stored in advance. The value of the deviation is entered into the arithmetic expression to obtain the first steering assist torque.

The steering-state determining section 16 receives the first steering assist torque computed by the first steering-assist-torque computing section 15. The steering-state determining section 16 determines whether or not the first steering assist torque computed by the first steering-assist-torque computing section 15 is needed. When determining that the first steering assist torque is needed, the steering-state determining section 16 outputs the first steering assist torque. When determining that the first steering assist torque is not needed, the steering-state determining section 16 outputs zero. In this case, whether or not the steering wheel is

in a state in which the steering wheel is released by the driver (hereinafter referred to simply as "released state") is determined based on the steering torque from the torque sensor 5. When the steering wheel is in the released state, the first steering assist torque is to be applied. When the steering wheel is not in the released state, the first steering assist torque is not to be applied. The first embodiment describes the following case as an example of the determination of the released state . Specifically, it is determined that the steering wheel is in a steering state when a detection value (absolute value) by the torque sensor 5 is equal to or larger than a threshold value, whereas it is determined that the steering wheel is in the released state when the detection value is smaller than the threshold value.
The second steering-assist-torque computing section 17 receives the steering torque from the torque sensor 5. The second steering-assist-torque computing section 17 computes a second steering assist torque for assisting the driver in steering based on the steering torque. As a method of computing the second steering assist torque, the following method may be used. Specifically, a lookup table which stores a value of the second steering assist torque for the steering torque is stored in advance. A value of the steering torque is input to the lookup table to obtain a corresponding value of the second steering assist torque from the lookup table. Alternatively, the following method may also be used. Specifically, an arithmetic expression (or a calculation formula)

for obtaining the second steering assist torque based on the steering torque is stored in advance. The value of the steering torque is entered into the arithmetic expression to obtain the second steering assist torque.
The adder section 18 receives the output value from the steering-state determining section 16 (specifically, the first steering assist torque from the first steering-assist-torque computing section 15 or zero) and the second steering assist torque from the second steering-assist-torque computing section 17. The adder section 18 adds the output value from the steering-state determining section 16 and the second steering assist torque to compute a final steering assist torque.
The current driver 19 supplies a current based on the steering assist torque obtained in the adder section 18 to the motor 6 to drive the motor 6. As a result, the motor 6 generates a necessary steering assist torque.
Next, an operation of the steering controller according to the first embodiment is described. FIG. 3 is a flowchart illustrating an operation of the steering controller according to the first embodiment. The operation illustrated in FIG. 3 is repeatedly executed for each control cycle.
First, in Step SI, the control unit 11 acquires the steering angle from the steering-angle sensor 4, the motor voltage from the voltage sensor 10, the motor current from the current sensor 9, the steering torque applied by the driver from the torque sensor

5, and the vehicle speed from the vehicle-speed sensor 8.
In Step S2, the steering-speed computing section 12 first obtains the induced voltage based on the motor current and the motor voltage. Next, the steering-speed computing section 12 computes the steering speed by using the steering angle and the induced voltage to output the computed steering speed.
In Step S3, the steering-holding determining section 13 determines whether or not the steering system of the vehicle is in the state of steering holding. When determining that the steering system is in the state of steering holding, the steering-holding determining section 13 outputs the steering angle at the time. On the other hand, when determining that the steering system is not in the state of steering holding, the steering angle at the time of the previous determination for the steering holding is output without being updated.
In Step S4, the target steering-speed setting section 14 computes the target steering speed based on the steering angle and the vehicle speed acquired in Step SI and the steering angle acquired in Step S3 to set the target steering speed.
In Step S5, the first steering-assist-torque computing section 15 computes the first steering assist torque based on the deviation between the target steering speed and the steering speed.
In Step S6, the steering-state determining section 16 determines whether or not the first steering assist torque computed in the first steering-assist-torque computing section 15 is needed.

When determining that the first steering assist torque is needed, the steering-state determining section 16 outputs the first steering assist torque. When determining that the first steering assist torque is not needed, the steering-state determining section 16 outputs zero. In this case, as the determination whether or not the first steering assist torque is needed, whether or not the steering wheel is released by the driver is determined based on the steering torque from the torque sensor 5. When the steering wheel is in the released state, the steering-state determining section 16 outputs the first steering assist torque. When the steering wheel is not in the released state, the steering-state determining section 16 outputs the first steering assist torque as zero.
In Step S7, the second steering-assist-torque computing section 17 computes the second steering assist torque for assisting the driver in steering based on the steering torque acquired in Step SI.
In Step S8, the adder section 18 adds the first steering assist torque output from the steering-state determining section 16 and the second steering assist torque output from the second steering-assist-torque computing section 17 to output the result of addition as the final steering assist torque.
In Step S9, the current driver 19 supplies the current to the motor 6 based on the final steering assist torque obtained in Step S8 to drive the motor 6.
In the steering-wheel return control performed by the steering

controller described above, whether or not the steering system is in the state of steering holding is determined based on the steering speed and the threshold value. Therefore, when the offset error is contained in the steering speed or the steering speed has a" low resolution, there is a fear of erroneous determination for the steering holding. Moreover, the first steering assist torque for returning the steering wheel 1 to the neutral position is generated so that the steering speed follows the target steering speed. Therefore, when the steering assist torque contains the offset error, an appropriate steering assist torque is not obtained. As a result, there is a fear in that a steering-wheel return feeling is degraded. Therefore, in the first embodiment, processing of computing the steering speed described later in the second and third embodiments is used in the steering-speed computing section 12. As a result, the steering speed with a quick response, a high resolution, and a reduced offset error can be obtained. Consequently, a preferred steering-wheel return feeling in accordance with the target steering speed can be obtained.
As described above, in the first embodiment, the steering controller includes the steering-angle information acquiring section for acquiring the information on the steering angle of the steering system of the vehicle, the steering-speed information acquiring section for acquiring the first information on the steering speed of the steering system, and the correction section for correcting the first information on the steering speed, which is

acquired by the steering-speed information acquiring section, based on the information on the steering angle, which is acquired by the steering-angle information acquiring section, to thereby acquire the second information on the steering speed of the steering system. Therefore, the first information on the steering speed, which contains the offset error, is corrected based on the information on the steering angle. Therefore, the second information on the steering speed, which does not contain the offset error, can be obtained.
In particular, in the embodiment illustrated in FIG. 2, the steering controller includes the steering-angle sensor 4 for detecting the steering angle of the steering wheel 1 which is the steering system of the vehicle, the current sensor 9 for detecting the current flowing through the motor 6 (motor current) which applies the torque to the steering system, the voltage sensor 10 for detecting the voltage across the terminals of the motor 6 (motor voltage), the steering-speed computing section 12 for obtaining the steering speed of the steering system based on the steering angle detected by the steering-angle sensor 4, the current detected by the current sensor 9, and the voltage detected by the voltage sensor 10, the first steering-assist-torque computing section 15 for computing the steering assist torque based on the steering speed of the steering system, which is obtained by the steering-speed computing section 12, to thereby output the computed steering assist torque as the first steering assist torque, and the current driver 19 for driving

the motor 6 based on the first steering assist torque. The steering-speed computing section 12 estimates the steering speed based on the current and the voltage as the first steering speed (first information on the steering speed), computes the correction value (information on the steering angle) for correcting the first steering speed based on the steering angle, corrects the first steering speed by using the correction value, and outputs the corrected first steering speed as the steering speed of the steering system (second information on the steering speed). With the configuration described above, in the first embodiment, the first steering speed containing the offset error (first information on the steering speed) is corrected by the correction value obtained in accordance with the steering angle (information on the steering angle). Therefore, the steering speed without containing the of f set error (second information on the steering speed) can be obtained.
In the first embodiment, the induced voltage of the motor 6 is obtained based on the current detected by the current sensor 9 and the voltage detected by the voltage sensor 10 . Then, the first steering speed (first information on the steering speed) is obtained based on the induced voltage. As a result, the steering speed with a high resolution can be obtained.
Moreover, in the first embodiment, the steering controller further includes the torque sensor 5 provided to the steering system, for detecting the steering torque of the steering system, and the second steering-assist-torque computing section 17 for computing

the second steering assist torque based on the steering torque of the steering system, which is detected by the torque sensor 5. The current driver 19 drives the motor 6 based on the value obtained by adding the first steering assist torque and the second steering assist torque in the adder section 18 . In the manner described above, the steering assist torque is computed by using both the first steering assist torque for returning the steering wheel 1 to the neutral position and the second steering assist torque for assisting the driver in steering. Thus, both the steering-wheel return control and the steering assist control can be performed.
Second Embodiment
In the second embodiment, computation of the steering speed, which is executed in the steering-speed computing section 12 included in the control unit 11, is described referring to a flowchart of FIG. 4. Specifically, the flowchart of FIG. 4 illustrates in detail the processing in Step S2 of the flowchart of FIG. 3.
Overall configuration and operation of the steering controller according to the second embodiment are the same as those of the first embodiment described above, and therefore the description thereof is herein omitted. Components common to the first embodiment described above are denoted by the same reference symbols . Therefore, differences from the first embodiment are mainly described below.
As illustrated in FIG. 4, first, inStepSll, the steering-speed computing section 12 uses a motor voltage EM from the voltage sensor 10 and a motor current IM from the current sensor 9 to obtain an

induced voltage EE of the motor 6, which corresponds to the steering speed.
The induced voltage EE of the motor 6 is obtained by Expression (1) by using the motor voltage EM and the motor current IM.
EE=EM-IM-RM-Eb (1)
In Expression (1) , RM is an armature resistance of the motor 6, and Eb is a voltage drop of a brush of the motor 6.
Next, in Step S12, a motor rotation speed VM is computed based on the induced voltage EE by Expression (2).
VM=EE/Kp (2)
In Expression (2), Kp is an induced-voltage coefficient.
Next, in Step S13, a steering speed Gomega is estimated based on the motor rotation speed VM. The motor rotation speed and the steering speed have a relationship based on a structural mechanism such as a reduction gear, the steering wheel, a rack of a tire shaft, and a pinion. In other words, the motor rotation speed and the steering speed are proportional to each other. Therefore, based on the motor rotation speed, the estimated steering speed 9omega is obtained by Expression (3) . The motor rotation speed is obtained based on the induced voltage. Therefore, the estimated steering speed Gomega is hereinafter referred to as "estimated steering speed based on the induced voltage" (first steering speed).
eomega=GgearxVM (3)
In Expression (3) , Ggear is a factor of proportionality determined by the structural mechanism described above.

However, the voltage drop Eb of the brush of the motor 6 changes under the effects of the oxide film formed on the contact portion and degradation with elapse of time. Therefore, the induced voltage EE contains the amount of such changes as an offset error. As a result, the steering speed 9omegaestimatedbased on the induced voltage EE also contains the offset error.
Therefore, in Step S14, the steering-speed computing section 12 uses the information obtained based on the steering angle to correct the steering speed eomega- FIG. 5 illustrates a configuration of the correction performed in Step S14. FIG. 5 illustrates a high-pass filter 21, a low-pass filter 22, and an adder 23. The steering speed 9omega is input to the high-pass filter 21. A steering speed s9h computed based on the steering angle is input to the low-pass filter 22. The adder 23 adds an output from the high-pass filter 21 and an output from the low-pass filter 22.
The configuration illustrated in FIG. 5 uses a speed obtained based on a steering angle 9h in a low-frequency region which is affected by the offset error contained in the induced voltage EE, and uses the estimated steering speed 9omega based on the induced voltage EE in a high-frequency region, so as to compute a final steering speed 9hybrid • A transfer function Gl of the high-pass filter 21 is expressed by: Gl=Ts/(Ts+1), whereas a transfer function G2 of the low-pass filter 22 is expressed by: G2=l/(Ts+l). Then, the configuration illustrated in FIG. 5 can be expressed by Expression (4).

In Expression (4) , T is a time constant of a cut-off frequency of the high-pass filter 21 and the low-pass filter 22, and s is a Laplace operator.
In the configuration illustrated in FIG. 5, in order to remove the offset error contained in the estimated steering speed 6oniega (first steering speed) based on the induced voltage EE, filtering with the high-pass filter 21 is performed. The cut-off frequency of the high-pass filter 21 is set to a value that allows the offset error to be removed. Therefore, by the filtering with the high-pass filter 21, the offset error in the steering speed 0omega can be removed. However, when the steering speed is at a low frequency equal to or lower than the cut-off frequency of the high-pass filter 21, the steering speed is removed together with the offset error. Therefore, the steering speed at the low frequency cannot be obtained by the processing with the high-pass filter 21.
Therefore, in the configuration illustrated in FIG. 5, the steering speed s9h (second steering speed) obtained based on the steering angle 0h is computed to obtain the steering speed at the low frequency. Then, the filtering with the low-pass filter 22 is performed on the steering speed s9h obtained based only on the steering angle 0h- The cut-off frequency of the low-pass filter 22 is set to the same value as the cut-off frequency of the high-pass filter 21. By the filtering with the low-pass filter 22, the steering speed

at the low frequency which is equal to or lower than the cut-off frequency can be obtained.
In the above-mentioned manner, the cut-off frequencies of the high-pass filter 21 and the low-pass filter 22 are set to the same value. Then, the results of output from the high-pass filter 21 and the low-pass filter 22 are added by the adder 23 . In this manner, when the frequency of the steering angle is larger than the cut-off frequency, the steering speed obtained by using the induced voltage EE is used. On the other hand, when the frequency of the steering angle is equal to or lower than the cut-off frequency, the steering speed obtained by using the steering angle 9h is used. As a result, an accurate steering speed 9hyt>rid without containing the offset error can be obtained.
Expression (4) described above can be converted equivalently into Expression (5).
Expression (5) expresses filtering with a high-pass filter 2 6 performed on a sum of the estimated steering speed 9omega based on the induced voltage EE and a value obtained by dividing the steering angle 9hby a time constant T (= 9omega+©h/T) to obtain the final steering speed 9hybrid- Expression (5) is expressed in the form of a block diagram as illustrated in FIG. 6. FIG. 6 illustrates a divider 24,

an adder 25, and the high-pass filter 26. The divider 2 4 divides the steering angle 6h by the time constant T. The adder 25 adds the estimated steering speed 9omega based on the induced voltage EE and the output of the divider 24. The high-pass filter 26 performs the filtering on the output of the adder 25. In Expression (5), T is a time constant of the cut-off frequency of the high-pass filter 22, and s is a Laplace operator. A transfer function Gl of the high-pass filter 26 is expressed by: Gl=Ts/(Ts+1).
In Expression (4) , it is necessary to perform the filtering with the high-pass filter 21 on the estimated steering speed 6omega obtained based on the induced voltage EE and the filtering with the low-pass filter 22 on the steering speed computed based on the steering angle 9h- Therefore, in the configuration expressed by Expression (4) illustrated in FIG. 5, the computation of the filtering is required to be performed twice.
On the other hand, in the configuration expressed by Expression (5) illustrated in FIG. 6, the computation can be performed by one filtering. Therefore, the configuration expressed by Expression (5) has the effect of alleviating a computation load.
The effects of the second embodiment are now described.
The voltage drop Eb of the brush of the motor 6 changes under the effects of the oxide film formed on the contact portion and degradation with elapse of time. Therefore, the induced voltage EE contains the change amount thereof as the offset error. As a result, conventionally, there is a problem in that the estimated

steering speed eomega obtained based on the induced voltage EE also contains the offset error.
On the other hand, the steering angle detected by the steering-angle sensor 4 sometimes has a low resolution. When the resolution of the steering-angle sensor 4 is 9step and a speed computation cycle is Tstepf a resolution of the steering speed which can be computed by using the steering-angle sensor 4 is obtained as 9step/Tstep- For example, when the resolution 9step of the steering-angle sensor 4 is 1 deg and the speed computation cycle Tstep is 10 ms, the resolution of the steering speed becomes 100 deg/s . Therefore, as the resolution of the steering speed, the obtained resolution is low. In order to enhance the resolution of the steering speed, it is conceivable to set the speed computation cycle Tstep larger or to execute low-pass filtering on the computed steering speed. In both cases, however, there arises a problem in that a response of the computation of the steering speed is disadvantageously delayed.
The steering angle detected by the steering-angle sensor 4 is information obtained through a CAN network. Therefore, an update cycle sometimes becomes longer. In such a case, the speed computation cycle Tstep becomes equal to or longer than the update cycle. As a result, the response of the computation of the steering speed is delayed. Specifically, there arises a problem in that a frequency band of the speed computation cannot be set high. Therefore, there arises the following trade-off problem when the steering speed is

to be computed by using the steering-angle sensor 4. Specifically, when the resolution of the steering speed is to be enhanced, the response of the computation of the steering speed is delayed. On the other hand, when the response of the computation of the steering speed is to be increased, the resolution of the steering speed becomes lower.
In the second embodiment, the steering-speed computing section 12 can obtain the steering speed in the high-frequency band from which the offset error is removed based on the estimated steering speed Gomega obtained based on the induced voltage EE, which contains the offset error, owing to the effects of the high-pass filter 21. On the other hand, based on the steering angle 9h, the steering speed with the high resolution without containing the offset error can be obtained owing to the effects of the low-pass filter 22 . By using both of the steering speeds, the steering speed with a quick response, a high resolution, and a reduced offset error can be obtained.
FIG. 7 is a graph showing the result when the cut-off frequencies of the high-pass filter 21 and the low-pass filter 22 illustrated in FIG. 5 are set to 0.3 Hz. In FIG. 7, a solid line 30 indicates the steering speed according to the second embodiment, and a dotted line 31 indicates the steering speed based on the induced voltage containing the offset error. From FIG. 7, it is understood that the effects of the offset error in the induced voltage EE are eliminated when the steering speed is in the vicinity of zero and a good result of the computation of the steering speed is obtained

also for the low-frequency region.
As described above, also in the second embodiment, after the first information on the steering speed is corrected based on the information on the steering angle, the second information on the steering speed is acquired. Therefore, the same effects as those of the first embodiment are obtained.

Further, in the second embodiment (embodiment illustrated in FIG. 5), the second information on the steering speed is acquired based on the frequency component extracted from the steering-speed information computed based on the information on the steering angle and the frequency component extracted from the first information on the steering speed. Therefore, the steering speed with the quick response, the high resolution, and the reduced offset error can be obtained. Specifically, in the embodiment illustrated in FIG. 5, the steering-speed computing section 12 computes the steering speed of the steering system as the second steering speed based on the steering angle 9h detected by the steering-angle sensor 4, adds the correction value to the value obtained by performing the high-pass filtering on the estimated steering speed 90mega (first steering speed) based on the induced voltage EE by using the value obtained by performing the low-pass filtering on the second steering speed as the correction value to correct the estimated steering speed 9omega (first information on the steering speed) based on the induced voltage EE, and outputs the corrected estimated steering speed as the steering speed of the steering system (second information on the steering speed). In other words, the above-mentioned operation is equivalent to the operation of obtaining the steering speed at the high frequency obtained by removing the offset error from the estimated steering speed 90mega based on the induced voltage EE, which contains the offset error, and obtaining the steering speed at the low frequency with the high resolution without the offset error based on the steering angle 9h- By using both the steering speeds in this manner, the steering speed with the quick response, the high resolution, and the reduced offset error can be obtained.

In the second embodiment (embodiment illustrated in FIG. 6) , the low-frequency component of the first information on the steering speed is corrected based on the information on the steering angle. Therefore, the steering speed with the quick response, the high resolution, and the reduced offset error can be obtained. Specifically, in the embodiment illustrated in FIG. 6, the steering-speed computing section 12 uses the value (information on the steering angle) obtained by dividing the steering angle 9h detected by the steering-angle sensor 4 by the time constant T as the correction value to output the value obtained by performing the high-pass filtering on the value obtained by adding the correction value to the estimated steering speed 9omega (first information on the steering speed) based on the induced voltage EE as the steering speed of the steering system (second information on the steering speed) . In other words, the above-mentioned operation is equivalent to the operation of obtaining the steering speed at the high frequency-obtained by removing the offset error from the estimated steering speed 60mega based on the induced voltage EE, which contains the offset error, and obtaining the steering speed at the low frequency with the high resolution without the offset error based on the steering angle 9h. By using both the steering speeds in the above-mentioned manner, the steering speed with the quick response, the high resolution, and the reduced offset error can be obtained.

Third Embodiment

In the third embodiment, a computation method different from that in the second embodiment for the computation of the steering speed, which is executed in the steering-speed computing section 12 included in the control unit 11, is described referring to a flowchart of FIG. 8. Specifically, the flowchart of FIG. 8 illustrates in detail the processing in Step S2 of the flowchart of FIG. 3.

In the third embodiment, components common to the first and second embodiments are denoted by the same reference symbols. Therefore, differences from the first and second embodiments are mainly described below. The third embodiment has a configuration in which the steering speed obtained based on the induced voltage when a change in the steering angle is small is stored in the memory as the correction value and the estimated steering speed 90mega based on the induced voltage EE is corrected by using the correction value described above.

FIG. 8 is a flowchart illustrating an operation of the steering-speed computing section 12 of the control unit 11 according to the third embodiment. The flowchart of FIG. 8 differs from the flowchart of FIG. 4 referred to in the second embodiment in that Steps S21 and S22 are added and Step S23 is provided in FIG. 8 in place of Step S14 of FIG. 4.

In the third embodiment, first, in Step S21, when determining that a change in the steering angle which is obtained from the steering-angle sensor 4 is small, the control unit 11 obtains the estimated steering speed 9omega based on the induced voltage EE and stores the obtained estimated steering speed Gomega as a correction value 9ref in a memory 28 (see FIG. 9) . The correction value 9ref is now described.

When determining that the change amount in the steering angle 9h which is obtained from the steering-angle sensor 4 is smaller than a threshold value, specifically, determining that the steering angle 9h falls within a region in the vicinity of zero (-e<9h