JP2013159201A - Electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To properly restrict steering assist and suppress excessive turn of a steering wheel when a skid of wheels occurs during assist control which is performed when a failure of a torque sensor is detected, whereby the skid can be resolved preferably as a result.SOLUTION: A steering angle deviation computing part 851 computes: a front wheel side steering angle deviation Δθfs which is a deviation between an actual steering angle θs and a front wheel side estimated steering angle θf; and a rear wheel side steering angle deviation Δθrs which is a deviation between the actual steering angle θs and a rear wheel side estimated steering angle θr. A deviation change rate computing part 852 computes a front wheel side deviation change rate Δθfs' which is a change rate of the front wheel side steering angle deviation Δθfs, and a rear wheel side deviation change rate Δθrs' which is a change rate of the rear wheel side steering angle deviation Δθrs. A skid index output part 853 sets a larger value of the front wheel side deviation change rate Δθfs' or the rear wheel side deviation change rate Δθrs' as a skid index SL.

Description

  The present invention relates to an electric power steering apparatus that generates a steering assist torque by driving a motor based on a steering operation of a driver.

  Conventionally, an electric power steering apparatus detects a steering torque applied to a steering wheel by a driver by a torque sensor, and calculates a target assist torque based on the detected steering torque. Then, the steering operation of the driver is assisted by controlling the current flowing through the motor so as to obtain the target assist torque. Such energization control of the motor is called assist control.

  If the torque sensor fails, the target assist torque cannot be calculated and assist control cannot be performed. On the other hand, Patent Documents 1 and 2 propose electric power steering devices that perform steering assist even when a torque sensor fails. The electric power steering apparatus proposed in Patent Document 1 estimates the self-aligning torque transmitted from the road surface to the steering mechanism when a failure of the torque sensor is detected, and based on the estimated self-aligning torque. Calculate the torque command value. Further, the friction coefficient μ between the road surface and the tire is estimated from the operating state of the ABS device (anti-skid control device). Based on the friction coefficient μ, the estimated value of the self-aligning torque increases as the friction coefficient μ increases. It is corrected so that

  In addition, the electric power steering apparatus proposed in Patent Document 2 is estimated using the first estimated steering angle estimated using the wheel speeds of the left and right wheels on the front wheel side and the wheel speeds of the left and right wheels on the rear wheel side. The average estimated rudder angle is obtained by averaging the two estimated rudder angles, and the target assist torque is calculated based on the average estimated rudder angle and the vehicle speed. In addition, based on the steering angle difference representing the difference between the first estimated steering angle and the second estimated steering angle, a slip gain that decreases as the steering angle difference increases is calculated, and this slip gain is used as the target assist torque. By multiplying, the steering assist is reduced when the wheel slips.

JP 2009-6985 A International Publication WO2011 / 048702

  However, the electric power steering apparatus proposed in Patent Document 1 lacks practicality because the correction amount of the torque command value cannot be obtained unless the brake operation is performed. The electric power steering apparatus proposed in Patent Document 2 does not have such a problem, but there is room for improvement in slip detection. In the electric power steering apparatus proposed in Patent Document 2, the first estimated steering angle and the second estimated steering angle are both calculated based on the wheel speeds detected by the wheel speed sensors of the left and right wheels. In general, the detected value of the wheel speed is likely to be affected by a disturbance due to road surface unevenness or the like or a vehicle state. For this reason, even if the wheel is slipping, the values of the first estimated rudder angle and the second estimated rudder angle may match, and in this case, slip may be detected. It is impossible to over-assist.

  The present invention has been made in order to cope with the above-mentioned problem. During the assist control performed when a torque sensor failure is detected, the steering assist is appropriately limited when the wheel slip occurs and steering is performed. The object is to suppress overcutting of the steering wheel and, as a result, to eliminate the wheel slip well.

In order to achieve the above object, the present invention is characterized in that a steering torque sensor (21) for detecting a steering torque input from a steering handle to a steering shaft, and a motor for generating a steering assist torque provided in a steering mechanism ( 20), an abnormality detection means (72) for detecting an abnormality of the steering torque sensor, and when no abnormality of the steering torque sensor is detected, target steering is performed based on the steering torque detected by the steering torque sensor. When an assist control amount (Ta *) is set and an abnormality of the steering torque sensor is detected, a control amount setting means (70) for setting a target steering assist control amount using an alternative parameter different from the steering torque. ) And the drive control of the motor according to the target steering assist control amount set by the control amount setting means The electric power steering apparatus that includes a motor control means (40, 60),
An actual rudder angle detection means (22) for detecting an actual rudder angle, and the left and right wheel speeds of at least one of the front wheel side and the rear wheel side are detected, and an estimated rudder angle is calculated based on the detected wheel speed. Based on the estimated rudder angle calculation means (84), the actual rudder angle detected by the actual rudder angle detection means, and the estimated rudder angle calculated by the estimated rudder angle calculation means, When the slip index setting means (85, 87, 89) for setting the slip index and the steering torque sensor abnormality is detected, the target steering assist control amount is set using an alternative parameter different from the steering torque. Assist limit means (83, 8) for limiting the target steering assist control amount so that the target steering assist control amount decreases as the slip index set by the slip index setting means increases. Lies in having a 88, 90) and.

  In the present invention, when an abnormality of the steering torque sensor is detected, the target steering assist control amount is set using an alternative parameter different from the steering torque. As an alternative parameter, for example, a steering angle can be used. When the rudder angle is used, a target steering assist control amount that increases as the rudder angle increases may be set. As another alternative parameter, for example, the lateral acceleration of the vehicle can be used.

  When steering assist is applied using such alternative parameters, excessive steering assist works if the wheels are slipping. Therefore, in the present invention, the slip index setting means sets a slip index as a wheel slip determination index, and the assist limiting means sets the target steering assist control amount so that the target steering assist control amount decreases as the slip index increases. Limit the amount of control. In order to set the slip index, the present invention includes an actual rudder angle detecting means and an estimated rudder angle calculating means.

  The actual rudder angle detecting means detects an actual rudder angle that is an actual rudder angle detected by the sensor. The actual rudder angle is detected using, for example, a sensor that detects the rotation angle of the steering shaft, a sensor that detects the rotation angle of the motor, a sensor that detects the stroke of the rack bar, a sensor that detects the turning angle of the tire, and the like. be able to. The estimated rudder angle calculation means detects the wheel speed of the left and right wheels on at least one of the front wheel side and the rear wheel side, and calculates the estimated rudder angle based on the detected wheel speed. The estimated steering angle can be expressed, for example, as a function of the difference between the wheel speeds of the left and right wheels with respect to the sum of the wheel speeds of the left and right wheels. The slip index setting means sets a slip index serving as a wheel slip determination index based on the actual steering angle and the estimated steering angle. When wheel slip occurs, the actual rudder angle and the estimated rudder angle take different values. For this reason, the degree of slip can be determined from the magnitude of the deviation between the actual steering angle and the estimated steering angle, and the slip generation timing can be determined from the rate of change of the deviation between the actual steering angle and the estimated steering angle. Any of them can be used as an index for determining wheel slip. In this case, the greater the slip index, the greater the degree of slip or the greater the degree of slip. Therefore, for example, the slip index setting means may set the slip index based on the deviation between the actual steering angle and the estimated steering angle or the rate of change of the deviation.

  In the present invention, since the slip index is set based on the actual rudder angle and the estimated rudder angle, a highly accurate slip index that appropriately represents the slip state (such as the degree of slip and the increasing speed) is set. can do. Accordingly, it is possible to appropriately limit the steering assist, suppress excessive turning of the steering wheel, and satisfactorily eliminate wheel slip. In addition, when performing steering assist using alternative parameters, it is necessary to set the target steering assist control amount to be small in advance in consideration of the occurrence of slip, but in the present invention, the slip detection accuracy is good. Therefore, the target steering assist control amount can be set larger.

  Another feature of the present invention is that the slip index setting means (85, 8541) increases as the change rate (Δθfs ′, Δθrs ′) of the deviation between the actual steering angle and the estimated steering angle increases. The purpose is to set indices (SL, SL1).

  When wheel slip occurs, the deviation between the actual steering angle and the estimated steering angle increases. Therefore, the slip start timing can be satisfactorily detected from the rate of change in deviation between the actual rudder angle and the estimated rudder angle. That is, the start of slip can be detected even when the deviation between the actual steering angle and the estimated steering angle is small. Moreover, it can be expected that the greater the rate of change of this deviation, the greater the degree of slip. Therefore, in the present invention, the slip index setting means sets a slip index that increases as the rate of change in deviation between the steering angle and the estimated steering angle increases. In this case, the rate of change in deviation between the rudder angle and the estimated rudder angle may be set as a slip index, a slip index that changes stepwise in accordance with the rate of change in deviation between the rudder angle and the estimated rudder angle, or A slip index proportional to the rate of change in deviation between the rudder angle and the estimated rudder angle may be set. Thus, according to the present invention, since the detection of slip is not delayed, it is possible to quickly start limiting the steering assist, suppress excessive turning of the steering wheel, and as a result, satisfactorily eliminate wheel slip. Can do.

  Another feature of the present invention is that the slip index setting means (87) sets a first slip index (SL1) that increases as the rate of change in deviation between the actual steering angle and the estimated steering angle increases. A first slip index setting unit (8541), a second slip index setting unit (8542) for setting a second slip index (SL2) that increases with an increase in the deviation between the actual steering angle and the estimated steering angle. The assist limiting means (88) is configured so that the target steering assist control amount decreases as the first slip index increases, and the target steering assist increases as the second slip index increases. The target steering assist control amount is limited so that the control amount becomes small.

  In the present invention, the target steering assist control amount is limited based on both the deviation between the actual steering angle and the estimated steering angle and the rate of change of the deviation. In this case, the first slip index setting unit sets the first slip index that increases as the rate of change in deviation between the actual steering angle and the estimated steering angle increases, and the second slip index setting unit estimates the actual steering angle. A second slip index that increases with an increase in deviation from the steering angle is set. Then, the assist limiting means makes the target steering so that the target steering assist control amount decreases as the first slip index increases, and the target steering assist control amount decreases as the second slip index increases. Limit the amount of assist control.

  Immediately after the occurrence of slip, the deviation between the actual rudder angle and the estimated rudder angle increases, so the rate of change of this deviation is large. For this reason, the start timing of slip can be quickly detected by the first slip index. Thereafter, the rate of change of the deviation becomes small, but if the slip is occurring, the deviation between the actual rudder angle and the estimated rudder angle maintains a value corresponding to the degree of slip. For this reason, it is possible to detect that the slip continues by using the second slip index. Therefore, the target steering assist control amount can be limited quickly and for an appropriate period. As a result, excessive turning of the steering wheel can be suppressed, and wheel slip can be satisfactorily eliminated.

  Another feature of the present invention is that the assist limiting means (88) uses a slip index having a larger degree of limiting the target steering assist control amount among the first slip index and the second slip index. Thus, the target steering assist control amount is limited.

  The first slip index becomes a large value immediately after the occurrence of the slip, but thereafter becomes a small value even if the slip occurs. On the other hand, the second slip index is a small value immediately after the occurrence of the slip, but thereafter takes a value corresponding to the degree of slip during the period in which the slip occurs. Therefore, according to the present invention, immediately after the occurrence of slip, the target steering assist control amount is decreased by increasing the first slip index, and thereafter, the target steering assist control amount is decreased by increasing the second slip index. Therefore, the target steering assist control amount can be limited quickly and for an appropriate period. As a result, excessive turning of the steering wheel can be suppressed, and wheel slip can be satisfactorily eliminated.

  Another feature of the present invention resides in that the slip index setting means (87, 89) sets a slip index that increases as the deviation between the actual steering angle and the estimated steering angle increases.

  By detecting the deviation between the actual rudder angle and the estimated rudder angle, the degree of wheel slip can be appropriately detected. Therefore, a slip index that appropriately reflects the degree of slip can be obtained by setting a slip index that increases as the deviation between the actual rudder angle and the estimated rudder angle increases. Thereby, it is possible to suppress overcutting of the steering wheel and to satisfactorily eliminate wheel slip. In this case, the deviation between the rudder angle and the estimated rudder angle may be set as a slip index, or a slip index that changes stepwise according to the deviation between the rudder angle and the estimated rudder angle, or the rudder angle and the estimated rudder angle. A slip index proportional to the deviation from the corner may be set.

  Another aspect of the present invention is a change gradient adjustment that reduces a ratio of a change amount of the target steering assist control amount restriction to a change amount of the slip indicator, as compared to a case where the slip indicator increases when compared with a decrease amount. Means (S23, S24, S25, S26) are provided. Further, when the first slip index or the second slip index decreases, the change in the limit of the target steering assist control amount with respect to the change amount of the first slip index or the second slip index is larger than when the first slip index or the second slip index increases. There is a change gradient adjusting means (S23, S24, S25, S26) for reducing the ratio of the amounts.

  The assist limiting means tightens the target steering assist control amount more strictly (increases the control amount) in accordance with the increase in the slip index, and loosens the target steering assist control amount limitation in accordance with the decrease in the slip index (control amount). , The assist limit responsiveness is determined by the ratio of the change amount of the target steering assist control amount restriction to the slip index change amount. When wheel slip occurs, it is necessary to quickly limit the steering assist, so it is desirable to increase the ratio of the change amount of the target steering assist control amount limit to the change amount of the slip index. If the ratio is used (with the same responsiveness) and the limit on the steering assist is relaxed as the slip decreases, the steering assist increases rapidly. In this case, the driver may feel uncomfortable due to torque fluctuation.

  Therefore, in the present invention, the change gradient adjusting means reduces the ratio of the change amount of the restriction of the target steering assist control amount to the change amount of the slip index, as compared with the case where the slip index increases when the slip index decreases. As a result, when wheel slip occurs, the steering assist can be quickly limited, and when the slip becomes small, the steering assist limit can be slowly relaxed. As a result, according to the present invention, it is possible to achieve both the elimination of the slip and the reduction of the steering discomfort.

  Another feature of the present invention is that the assist limiting means acquires vehicle speed information representing a vehicle speed, and relaxes the limit of the target steering assist control amount when the vehicle speed is low compared to when the vehicle speed is high (91). is there.

  Generally, the rudder angle and rudder angular speed are larger as the vehicle speed is lower and smaller as the vehicle speed is higher. For this reason, the steering angle deviation and the rate of change of the steering angle deviation tend to be excessively large as the vehicle speed decreases. Therefore, in the present invention, when the vehicle speed is low, the restriction on the target steering assist control amount is relaxed compared to when the vehicle speed is high. As a result, excessive assist limitation during low-speed traveling can be avoided.

  In the above description, in order to help the understanding of the invention, the reference numerals used in the embodiments are attached in parentheses to the configurations of the invention corresponding to the embodiments. It is not limited to the embodiment defined by the reference numerals.

1 is a schematic configuration diagram of an electric power steering apparatus according to an embodiment of the present invention. It is a functional block diagram of assist ECU. It is a graph showing a normal time assist map. It is a functional block diagram of an abnormal time assist torque calculation part. It is a graph showing a basic assistance map at the time of abnormality. It is explanatory drawing explaining the tread and wheelbase of a vehicle. It is a functional block diagram of the slip parameter | index setting part which concerns on 1st Embodiment. It is a graph showing the gain map which concerns on 1st Embodiment. It is a functional block diagram of the slip parameter | index setting part which concerns on 2nd Embodiment, and a gain setting part. It is a graph showing the 2nd gain map concerning a 2nd embodiment. It is a graph showing transition of the actual steering angle which concerns on 1st, 2nd embodiment, an estimated steering angle, the change rate of deviation, and the amount of assistance. It is a functional block diagram of the slip parameter | index setting part which concerns on 3rd Embodiment. It is a graph showing the upper limit map concerning the 1st modification. It is an input-output block diagram of the vehicle speed variable coefficient multiplication part which concerns on a 2nd modification. It is a graph showing the vehicle speed gain map which concerns on a 2nd modification. It is a flowchart showing the gain return process routine which concerns on a 4th modification. It is a flowchart showing the assist limitation gain setting routine which concerns on a 5th modification. It is a graph showing the assist restriction | limiting gain characteristic which concerns on a 5th modification.

  Hereinafter, an electric power steering apparatus according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic configuration of an electric power steering apparatus 1 for a vehicle according to the embodiment.

  The electric power steering apparatus 1 is configured according to a steering mechanism 10 that steers steered wheels by a steering operation of a steering handle 11, a motor 20 that is assembled to the steering mechanism 10 and generates steering assist torque, and an operating state of the steering handle 11. The electronic control unit 100 that controls the operation of the motor 20 is provided as a main part. Hereinafter, the electronic control unit 100 is referred to as an assist ECU 100.

  The steering mechanism 10 is a mechanism for turning the left and right front wheels Wfl and Wfr by rotating the steering handle 11, and includes a steering shaft 12 connected so that the steering handle 11 is integrally rotated at the upper end. A pinion gear 13 is connected to the lower end of the steering shaft 12 so as to rotate integrally. The pinion gear 13 meshes with a gear portion 14 a formed on the rack bar 14 and constitutes a rack and pinion mechanism together with the rack bar 14.

  The rack bar 14 has a gear portion 14 a housed in the rack housing 16, and both left and right ends thereof are exposed from the rack housing 16 and connected to the tie rod 17. A stopper 18 constituting a stroke end is formed at a portion where the rack bar 14 is connected to the tie rod 17, and the lateral movement stroke of the rack bar 14 is mechanically caused by contact between the stopper 18 and the end of the rack housing 16. Is regulated. The other ends of the left and right tie rods 17 are connected to knuckles 19 provided on the left and right front wheels Wfl and Wfr. With such a configuration, the left and right front wheels Wfl and Wfr are steered to the left and right according to the axial displacement of the rack bar 14 as the steering shaft 12 rotates about the axis.

  A motor 20 is assembled to the steering shaft 12 via a reduction gear 25. The motor 20 rotationally drives the steering shaft 12 about its axis through the reduction gear 25 by the rotation, and gives an assist force to the turning operation of the steering handle 11.

  A steering torque sensor 21 and a steering angle sensor 22 are assembled to the steering shaft 12 at an intermediate position between the steering handle 11 and the reduction gear 25. The steering torque sensor 21 detects, for example, a twist angle of a torsion bar (not shown) interposed in an intermediate portion of the steering shaft 12 by a resolver or the like, and inputs from the steering handle 11 to the steering shaft 12 based on the twist angle. The detected steering torque Tr is detected. As for the steering torque Tr, the operation direction of the steering wheel 11 is identified by positive and negative values. For example, the steering torque Tr when the steering handle 11 is steered in the right direction is indicated by a positive value, and the steering torque Tr when the steering handle 11 is steered in the left direction is indicated by a negative value. In this embodiment, the torsion angle of the torsion bar is detected by a resolver, but it can also be detected by another rotational angle sensor such as an MR sensor.

  The steering angle sensor 22 is provided at an intermediate position between the steering torque sensor 21 and the reduction gear 25 on the steering shaft 12, and detects the steering angle θs. As will be described later, for the steering angle, the detected steering angle detected by the steering angle sensor 22 and the estimated steering angle estimated by calculation based on the wheel speed are used. Therefore, the steering angle detected by the steering angle sensor 22 is used. θs is called an actual steering angle θs. For the steering angle, the steering direction is identified by a positive / negative value. For example, the steering angle in the right direction is indicated by a positive value, and the steering angle in the left direction is indicated by a negative value. In the present embodiment, the steering angle sensor 22 detects the rotation angle of the steering shaft 12, but detects the rotation angle of the motor 20 and the turning angles of the left and right front wheels Wfl and Wfr, which are physical quantities corresponding to the actual steering angle. You may comprise as follows. Moreover, you may comprise so that the displacement stroke of the axial direction of the rack bar 14 may be detected.

  Next, the assist ECU 100 will be described. As shown in FIG. 2, the assist ECU 100 calculates a target control amount of the motor 20 and outputs a switch drive signal corresponding to the calculated target control amount, and is output from the electronic control circuit 50. And a motor drive circuit 40 that energizes the motor 20 in accordance with the switch drive signal.

  Various motors 20 can be employed. For example, when a DC brushless motor is used, a three-phase inverter may be used as the motor drive circuit 40. When a brushed motor is used, an H bridge circuit may be used as the motor drive circuit 40. . In the present embodiment, description will be made assuming that a DC brushless motor is used.

  The electronic control circuit 50 includes a microcomputer including a CPU, ROM, RAM, and the like, various input / output interfaces, a switch drive circuit that supplies a switch drive signal to the motor drive circuit 40, and the like.

  Focusing on the function, the electronic control circuit 50 calculates the motor assist amount so that a current corresponding to the target assist torque Ta * flows to the motor 20 and the target assist torque calculator 70 for calculating the target assist torque Ta *. And a motor control unit 60 that outputs a switch drive signal corresponding to the motor energization amount to the motor drive circuit 40. The processing in each functional unit is repeatedly executed at a predetermined short period by the microcomputer.

  The target assist torque calculation unit 70 includes a normal assist torque calculation unit 71, an abnormal assist torque calculation unit 80, an abnormality detection unit 72, and a control switching unit 73. The normal assist torque calculation unit 71 receives the abnormality determination flag Ffail output from the abnormality detection unit 72. When the abnormality determination flag Ffail is “0”, the target assist torque Ta1 is calculated and the abnormality determination flag Ffail is calculated. Is “1”, the calculation block stops the calculation process. The abnormality assist torque calculation unit 80 receives the abnormality determination flag Ffail output from the abnormality detection unit 72, calculates the target assist torque Ta2 when the abnormality determination flag Ffail is “1”, and calculates the abnormality determination flag Ffail. Is “0”, the calculation block stops the calculation process.

  The abnormality detection unit 72 determines whether or not the steering torque sensor 21 is abnormal. When it is determined that there is no abnormality, the abnormality determination flag Ffail is set to “0”, and when it is determined that there is an abnormality. Sets the abnormality determination flag Ffail to “1”, and outputs the set abnormality determination flag Ffail to the normal assist torque calculation unit 71, the abnormal assist torque calculation unit 80, and the control switching unit 73.

  The control switching unit 73 inputs the target assist torque Ta1 calculated by the normal assist torque calculating unit 71 and the target assist torque Ta2 calculated by the abnormal assist torque calculating unit 80, and the abnormality determination flag Ffail is “0”. ", The target assist torque Ta1 is selected, and when the abnormality determination flag Ffail is" 1 ", the target assist torque Ta2 is selected. Then, the selected target assist torque Ta1 (or Ta2) is set to the target assist torque Ta *, and the target assist torque Ta * is output to the motor control unit 60.

  Details of each functional unit of the target assist torque calculation unit 70 will be described later.

  The motor control unit 60 includes a current feedback control unit 61 and a PWM signal generation unit 62. The current feedback control unit 61 receives the target assist torque Ta * output from the control switching unit 73, and generates the target assist torque Ta * by dividing the target assist torque Ta * by the torque constant of the motor 20. The target current I * required for the calculation is calculated. Then, a motor current Im (referred to as an actual current Im) detected by a current sensor 41 provided in the motor drive circuit 40 is read, and a deviation between the target current I * and the actual current Im is calculated, and this deviation is used. The target voltage V * is calculated so that the actual current Im follows the target current I * by proportional-integral control. Then, a PWM control signal (switch drive signal) corresponding to the target voltage V * is output to the switching element of the motor drive circuit (inverter) 40. As a result, the motor 20 is driven and an assist torque that follows the target assist torque Ta * is applied to the steering mechanism 10.

  In this embodiment, since a DC brushless motor is used, the current feedback control unit 61 inputs the motor rotation angle θm detected by the motor rotation angle sensor 23 provided in the motor 20 and rotates the motor. The angle θm is converted into an electrical angle, and the phase of the target current is controlled based on the electrical angle.

  Next, each functional unit of the target assist torque calculation unit 70 will be described in detail. The normal assist torque calculation unit 71 inputs the vehicle speed V detected by the vehicle speed sensor 24 and the steering torque Tr detected by the steering torque sensor 21, and refers to the normal assist map shown in FIG. The assist torque Ta1 is calculated. The normal assist map is stored in the normal assist torque calculator 71 and is association data in which the relationship between the steering torque Tr and the target assist torque Ta1 is set for each of a plurality of representative vehicle speeds V. The target assist torque Ta1 is set such that it increases as the magnitude (absolute value) of the torque Tr increases and decreases as the vehicle speed V increases. The target assist torque Ta1 is calculated so as to work in the direction of the steering torque Tr.

  In calculating the target assist torque Ta1, various compensation torques may be added to the target assist torque Ta1. For example, a friction compensation torque that compensates for the frictional force in the steering mechanism 10 may be added. Moreover, you may make it add the friction viscosity compensation torque which compensates a viscosity component in addition to a friction component.

  When the abnormality determination flag Ffail output from the abnormality detection unit 72 is “0”, the normal-time assist torque calculation unit 71 repeatedly executes such calculation processing at a predetermined short cycle, and the target assist torque that is the calculation result Ta1 is output to the control switching unit 73.

  The abnormality detection unit 72 determines whether the steering torque sensor 21 is abnormal. The steering torque sensor 21 can calculate the steering torque by detecting the torsion angle of the torsion bar provided in the middle of the steering shaft 12, and the rotation angle at one end and the rotation angle at the other end of the torsion bar are calculated. The torsion angle is detected from the angle difference. The steering torque sensor 21 includes a rotation angle sensor such as a resolver or an MR sensor in order to detect the rotation angle. In addition to the calculated value of the torsion angle corresponding to the steering torque Tr, the detection signal of the rotation angle sensor is also sent to the assist ECU 100. Output. Note that only the detection signal of the rotation angle sensor may be output to the assist ECU 100 and the assist ECU 100 may calculate the steering torque.

  The rotation angle sensor provided in the steering torque sensor 21 outputs a voltage signal corresponding to the rotation angle. Therefore, when the voltage value of the output signal is out of the proper range, it is considered that a disconnection or a short circuit has occurred in the rotation angle sensor. Also, for example, when using a rotation angle sensor whose output voltage periodically changes in a sine wave shape, such as a resolver, even when the output voltage is fixed to a constant value, disconnection or short-circuiting may occur. It is thought that it occurred.

  The abnormality detection unit 72 detects an abnormality of the steering torque sensor 21 as described above based on the output voltage of the rotation angle sensor (determines whether there is an abnormality). Then, according to the abnormality determination result of the steering torque sensor 21, the abnormality determination flag Ffail is set to “1” (abnormal) or “0” (abnormal).

  Next, the abnormality assist torque calculation unit 80 will be described. The normal assist torque calculation unit 71 described above calculates the target assist torque Ta1 based on the steering torque Tr, but cannot calculate the target assist torque Ta1 when the steering torque sensor 21 fails. Therefore, the abnormal assist torque calculator 80 calculates the target assist torque Ta2 instead of the normal assist torque calculator 71 when an abnormality of the steering torque sensor 21 is detected.

  A plurality of embodiments of the abnormal assist torque calculation unit 80 will be described.

<First Embodiment>
As shown in FIG. 4, the abnormal assist torque calculation unit 80 includes a basic assist torque calculation unit 81, an assist limit calculation unit 82, and an assist limit unit 83. The basic assist torque calculation unit 81 inputs the vehicle speed V detected by the vehicle speed sensor 24 and the actual steering angle θs detected by the steering angle sensor 22, and refers to the abnormal basic assistance map shown in FIG. The basic assist torque Tbase is calculated. The abnormal basic assist map is stored in the basic assist torque calculation unit 81, and is association data in which the relationship between the actual steering angle θs and the basic assist torque Tbase is set for each of a plurality of representative vehicle speeds V. It has a characteristic of setting a basic assist torque Tbase that increases as the magnitude (absolute value) of the actual steering angle θs increases and increases as the vehicle speed V increases. The basic assist torque Tbase is set to the same sign as the actual steering angle θs. Accordingly, if the actual steering angle θs is rightward, the basic assist torque Tbase that works in the right steering direction is set, and if the actual steering angle θs is leftward, the basic assist torque Tbase that works in the left steering direction is set. . The basic assist torque calculation unit 81 outputs the calculated basic assist torque Tbase to the assist restriction unit 83.

  The assist limit calculation unit 82 includes an estimated rudder angle calculation unit 84, a slip index setting unit 85, and a gain setting unit 86. The estimated rudder angle calculation unit 84 uses the left front wheel speed Vfl detected by the left front wheel speed sensor 26, the right front wheel speed Vfr detected by the right front wheel speed sensor 27, and the left rear wheel speed sensor 28. The detected left rear wheel speed Vrl and the right rear wheel speed Vrr detected by the right rear wheel speed sensor 29 are inputted, and two estimated steering angles are calculated. One of the two estimated rudder angles is an estimated rudder angle θf calculated from the left front wheel speed Vfl and the right front wheel speed Vfr, and the other is the left rear wheel speed Vrl and the right rear wheel speed. The estimated steering angle θr calculated from the wheel speed Vrr. Hereinafter, the estimated steering angle θf is referred to as a front wheel side estimated steering angle θf, and the estimated steering angle θr is referred to as a rear wheel side estimated steering angle θr.

ここで、Ｇは予め設定されている舵角換算用のギヤ比（オーバーオールギヤ比）を表し、ａは左右後輪Ｗrl，Ｗrrのトレッド、ｂは車両のホイールベースを表す（図６参照）。 The front wheel side estimated steering angle θf is calculated by the following equation (1), and the rear wheel side estimated steering angle θr is calculated by the following equation (2). Here, the front wheel side estimated steering angle θf and the rear wheel side estimated steering angle θr are the steering angles converted around the steering shaft.

Here, G represents a preset gear ratio (overall gear ratio) for rudder angle conversion, a represents the tread of the left and right rear wheels Wrl, Wrr, and b represents the vehicle wheel base (see FIG. 6).

  Since the wheel speeds Vfl, Vfr, Vrl, and Vrr detected by the wheel speed sensors 26 to 29 include noise, the estimated rudder angle calculation unit 84 calculates the wheel speeds Vfl, Vfr, Vrl, Vrr, or the estimated rudder angle. It is preferable to perform noise removal filter processing on θf and θr.

  The estimated rudder angle calculation unit 84 outputs the calculated front wheel side estimated rudder angle θf and rear wheel side estimated rudder angle θr to the slip index setting unit 85. As shown in FIG. 7, the slip index setting unit 85 includes a rudder angle deviation calculation unit 851, a deviation change rate calculation unit 852, and a slip index output unit 853. The steering angle deviation calculation unit 851 includes a front wheel side deviation calculation unit 8511 and a rear wheel side deviation calculation unit 8512.

  The front wheel side deviation calculation unit 8511 inputs the actual steering angle θs detected by the steering angle sensor 22 and the estimated front wheel side estimated steering angle θf calculated by the estimated steering angle calculation unit 84, and the absolute value of the actual steering angle θs. A front wheel side steering angle deviation Δθfs which is a deviation between | θs | and the absolute value | θf | of the front wheel side estimated steering angle θf is calculated (Δθfs = (|| θs | − | θf ||)). The rear wheel side deviation calculating unit 8512 inputs the actual steering angle θs detected by the steering angle sensor 22 and the rear wheel side estimated steering angle θr calculated by the estimated steering angle calculation unit 84, and the actual steering angle. A rear wheel side steering angle deviation Δθrs, which is a deviation between the absolute value | θs | of θs and the absolute value | θr | of the estimated rear wheel side steering angle θr, is calculated (Δθrs = (|| θs | − | θr ||). ).

  The steering angle deviation calculation unit 851 outputs the calculated front wheel side steering angle deviation Δθfs and rear wheel side steering angle deviation Δθrs to the deviation change rate calculation unit 852. The deviation change rate calculation unit 852 includes a front wheel side deviation change rate calculation unit 8521 and a rear wheel side deviation change rate calculation unit 8522. The front wheel side deviation change rate calculation unit 8521 calculates a front wheel side deviation change rate Δθfs ′, which is a change rate with respect to time, of the front wheel side steering angle deviation Δθfs by differentially calculating the front wheel side steering angle deviation Δθfs. The rear wheel side deviation change rate calculation unit 8522 calculates a rear wheel side deviation change rate Δθrs ′, which is a change rate with respect to time, of the rear wheel side steering angle deviation Δθrs by differentiating the rear wheel side steering angle deviation Δθrs. .

  Deviation change rate calculation unit 852 outputs the calculated front wheel side deviation change rate Δθfs ′ and rear wheel side deviation change rate Δθrs ′ to slip index output unit 853. The slip index output unit 853 sets and outputs a slip index SL according to a predetermined rule based on the front wheel side deviation change rate Δθfs ′ and the rear wheel side deviation change rate Δθrs ′. For example, the slip index output unit 853 sets and outputs the larger one of the front wheel side deviation change rate Δθfs ′ and the rear wheel side deviation change rate Δθrs ′ as the slip index SL. That is, when the front wheel side deviation change rate Δθfs ′ is compared with the rear wheel side deviation change rate Δθrs ′, and the front wheel side deviation change rate Δθfs ′ is larger than the rear wheel side deviation change rate Δθrs ′, the front wheel side deviation change rate is changed. When the rate Δθfs ′ is set to the slip index SL (SL ← θfs ′), and the rear wheel side deviation change rate Δθrs ′ is larger than the front wheel side deviation change rate Δθfs ′, the rear wheel side deviation change rate Δθrs. 'Is set as the slip index SL (SL ← Δθrs').

  Alternatively, the slip index output unit 853 calculates an average value ((Δθfs ′ + Δθrs ′) / 2) of the front wheel side deviation change rate Δθfs ′ and the rear wheel side deviation change rate Δθrs ′, and uses the average value as the slip index SL. You may make it set to. Alternatively, a value (Δθfs ′ · Δθrs ′) obtained by multiplying the front wheel side deviation change rate Δθfs ′ and the rear wheel side deviation change rate Δθrs ′ may be calculated and set as the slip index SL.

  If slip occurs in any of the front, rear, left and right wheels, the turning centers of the front, rear, left and right wheels are not the same, and as a result, the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle deviation Δθrs increase. Accordingly, it can be determined that the greater the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle deviation Δθrs, the greater the degree of slip. When the wheel is slipping, if the steering assist is performed with the same force as when the wheel is not slipping, the wheel is over-assisted, the steering wheel is easily turned too much, and the slip is promoted. Therefore, it is necessary to quickly grasp the timing of shifting to the slip state and limit the steering assist.

  Therefore, in the present embodiment, the front wheel side deviation change rate Δθfs ′, which is the change rate of the front wheel side steering angle deviation Δθfs, and the rear wheel side deviation change rate Δθrs ′, which is the change rate of the rear wheel side steering angle deviation Δθrs. Based on this, the timing at which the grip state (the tire is not slipping on the road surface) to the slip state is quickly detected. For this purpose, the slip index setting unit 85 sets a slip index SL as a slip determination index using the front wheel side deviation change rate Δθfs ′ and the rear wheel side deviation change rate Δθrs ′.

  At the time of transition from the grip state to the slip state, the steering angle deviation increases, that is, the rate of change of the steering angle deviation becomes a positive value. Therefore, in the present embodiment, the front wheel side deviation change rate Δθfs ′ and the rear wheel side deviation change rate Δθrs ′ are when the deviation changes in the increasing direction (when the change rate becomes a positive value). When the slip index SL is set as described above and the deviation is changing in the decreasing direction (when the rate of change is negative), the slip index SL is set to zero. Accordingly, the slip index SL represents an index that increases as the speed at which the slip increases.

  When the slip index output unit 853 sets the slip index SL, the slip index output unit 853 outputs the slip index SL to the gain setting unit 86. The gain setting unit 86 receives the slip index SL and calculates the assist limit gain K with reference to the gain map shown in FIG. The gain map is stored in the gain setting unit 86. In a range where the slip index SL is equal to or less than the reference value SLref, the assist limit gain K is set to 1 (K = 1.0), and the slip index SL is the reference. In a range exceeding the value SLref, there is a characteristic of setting an assist limit gain K that decreases as the slip index SL increases. The gain setting unit 86 outputs the calculated assist limit gain K to the assist limit unit 83.

  The assist limiter 83 receives the basic assist torque Tbase output from the basic assist torque calculator and the assist limit gain K output from the assist limit calculator 82, and multiplies the basic assist torque Tbase by the assist limit gain K. Thus, the target assist torque Ta2 is obtained (Ta2 = K × Tbase). The assist limiting unit 83 outputs the target assist torque Ta2 that is the calculation result to the control switching unit 73.

  According to this electric power steering apparatus 1, even when the steering torque sensor 21 is out of order, the abnormal assist torque calculating unit 80 calculates the target assist torque Ta * instead of the normal assist torque calculating unit 71. The steering assist can be continued. In this case, the abnormal assist torque calculation unit 80 sets the basic assist torque Tbase based on the vehicle speed V and the actual steering angle θs, and further changes in deviation between the actual steering angle θs and the estimated steering angles θf and θr. A slip index SL is set based on the rate. Then, by multiplying the basic assist torque Tbase by the slip index SL, a target assist torque Ta * is calculated such that the steering assist decreases as the slip index SL increases. As a result, the start of slip can be quickly detected to limit the steering assist, and the steering handle 11 can be prevented from being overcut by over-assist.

  Further, when such assist limitation is performed, the slip index SL is set based on the rate of change in deviation between the actual steering angle θs and the estimated steering angles θf and θr, and therefore the timing for appropriately shifting to the slip state. Can be captured. That is, when the slip is determined using the change rate of the deviation (|| θf | − | θr ||) between the estimated rudder angles, the error is included in each of the two estimated rudder angles θf and θr. However, in this embodiment, since the deviation between the actual steering angle θs and the estimated steering angles θf and θr is used instead of the estimated steering angles, the deviation change rate is used. Therefore, it is possible to appropriately grasp the timing for shifting to the slip state.

  Further, since the slip change rate SL is calculated using the two deviation change rates (Δθfs ′, Δθrs ′), the start of the slip can be detected appropriately. In this case, if the rate of change in which the degree of limiting the steering assist is increased is set in the slip index SL, the start of slip can be detected more quickly. Further, when the slip index SL is set using the average value of the change rates, it is possible to detect the start of slip more accurately.

  Further, in the assist control when the steering torque sensor 21 fails, when the occurrence of slip is assumed, the target assist torque Ta * cannot be increased so much, but in this embodiment, the start of slip can be detected quickly. Therefore, the target assist torque Ta * can be set larger, and the steering feeling becomes good.

Second Embodiment
Next, a second embodiment of the abnormality assist torque calculation unit 80 will be described. The abnormality assist torque calculation unit 80 according to the second embodiment includes a slip index setting unit 87 and a gain setting unit 88 instead of the slip index setting unit 85 and the gain setting unit 86 of the first embodiment. The configuration is the same as that of the first embodiment (see FIG. 4). As shown in FIG. 9, the slip index setting unit 87 includes a steering angle deviation calculation unit 851, a deviation change rate calculation unit 852, and a slip index output unit 854. The steering angle deviation calculation unit 851 and the deviation change rate calculation unit 852 are the same as those in the first embodiment. Only the configuration different from the first embodiment will be described below.

  The slip index output unit 854 includes a first slip index output unit 8541 and a second slip index output unit 8542. The first slip index output unit 8541 performs the same processing as the slip index output unit 853 of the first embodiment. That is, the front wheel side deviation change rate Δθfs ′ and the rear wheel side deviation change rate Δθrs ′ are input from the deviation change rate calculation unit 852, and the larger deviation change rate is set as the slip index SL. Alternatively, an average value of the front wheel side deviation change rate Δθfs ′ and the rear wheel side deviation change rate Δθrs ′ is set as the slip index SL. Alternatively, a value obtained by multiplying the front wheel side deviation change rate Δθfs ′ and the rear wheel side deviation change rate Δθrs ′ is set in the slip index SL.

  The slip index SL set by the first slip index output unit 8541 is referred to as a first slip index SL1. The first slip index output unit 8541 outputs the first slip index SL1 to the gain setting unit 88.

  The second slip index output unit 8542 sets a slip index corresponding to the deviation between the actual steering angle and the estimated steering angle. As described above, when slip occurs in any of the front, rear, left and right wheels, the turning centers of the front, rear, left and right wheels are not the same, and as a result, the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle deviation Δθrs are growing. Accordingly, it can be determined that the greater the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle deviation Δθrs, the greater the degree of slip. Therefore, the second slip index output unit 8542 sets a second slip index SL representing the degree of slip based on the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle deviation Δθrs.

  For example, the second slip index output unit 8542 sets the larger one of the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle deviation Δθrs as the second slip index SL2. Alternatively, an average value ((Δθfs + Δθrs) / 2) of the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle deviation Δθrs is calculated, and the average value is set as the second slip index SL2. Alternatively, a value (Δθfs · Δθrs) obtained by multiplying the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle deviation Δθrs may be calculated and set as the second slip index SL2. Second slip index output unit 8542 outputs the set second slip index SL2 to gain setting unit 88.

  The gain setting unit 88 inputs the first slip index SL1 and the second slip index SL2. As illustrated in FIG. 9, the gain setting unit 88 includes a first gain setting unit 881, a second gain setting unit 882, and a gain integration unit 883. Similar to the gain setting unit 86 of the first embodiment, the first gain setting unit 881 stores a gain map (referred to as a first gain map) shown in FIG. 8, and refers to this first gain map. Then, a first assist limit gain K1 corresponding to the first slip index SL1 is calculated. The first gain setting unit 881 outputs the first assist limit gain K1 that is the calculation result to the gain integration unit 883.

  The second gain setting unit 882 stores the second gain map shown in FIG. 10, and calculates the second assist limit gain K2 corresponding to the second slip index SL2 with reference to the second gain map. In the second gain map, the second assist limit gain K2 is set to 1 (K2 = 1.0) in the range where the second slip index SL is equal to or less than the reference value SL2ref, and the second slip index SL2 is the reference value. In a range exceeding SL2ref, there is a characteristic of setting a second assist limit gain K2 that decreases as the second slip index SL2 increases. The second gain setting unit 882 outputs the second assist limit gain K2 that is the calculation result to the gain integration unit 883.

  The gain integrating unit 883 inputs the first assist limit gain K1 and the second assist limit gain K2, and sets the final assist limit gain K to be output to the assist limiter 83. For example, the gain integration unit 883 sets the smaller assist assist gain K of the first assist limit gain K1 and the second assist limit gain K2. That is, the limit gain that increases the degree of assist limit is selected and set to the assist limit gain K.

  The gain integrating unit 883 may set the average value of the first assist limit gain K1 and the second assist limit gain K2 as the assist limit gain K. Alternatively, the gain integration unit 883 may set the product of the first assist limit gain K1 and the second assist limit gain K2 as the assist limit gain K.

  The gain integration unit 883 outputs the assist limit gain K that is the calculation result to the assist limit unit 83.

  In the second embodiment, the first slip index SL1 is set from the rate of change in deviation between the actual rudder angle and the estimated rudder angle, and the second slip index SL2 is set from the deviation between the actual rudder angle and the estimated rudder angle. . Since the first slip index SL1 and the second slip index SL2 have different units, they cannot be simply compared. Therefore, the first assist limit gain K1 and the second assist limit gain K2 corresponding to the slip indexes SL1 and SL2 are calculated. Thereby, the first assist limit gain K1 and the second assist limit gain K2 can be regarded as slip indexes. In this case, the smaller the control gains K1 and K2, the larger the slip index may be considered. Then, a final assist limit gain K is set based on both assist limit gains K1 and K2.

  Immediately after the occurrence of slip, the deviation between the actual rudder angle and the estimated rudder angle increases, so the rate of change of this deviation is large. For this reason, the first slip index SL1 is greater than the second slip index SL2, and the first assist limit gain K1 that is the greater degree of assist limit is selected. Thereafter, the rate of change of the deviation becomes small, but if the slip is occurring, the deviation between the actual rudder angle and the estimated rudder angle maintains a value corresponding to the degree of slip, so the second slip The second assist limit gain K2 is selected so that the index SL2 is larger than the first slip index SL1 and the degree of assist limitation is larger. As described above, in the second embodiment, the target steering assist control amount decreases as the first slip index SL1 increases, and the target steering assist control amount decreases as the second slip index SL2 increases. The target steering assist control amount is limited so as to decrease.

  Therefore, according to the second embodiment, not only the start of the slip is quickly detected using the first slip index SL1 and the steering assist is limited, but also the period during which the slip continues using the second slip index SL2. The steering assist can be limited during the operation. FIG. 11 is a graph showing a difference in transition of the assist amount (target assist torque Ta *) between the first embodiment and the second embodiment. In both the first embodiment and the second embodiment, the assist limitation is started when the change rate of the steering angle deviation exceeds the reference value at time t1, and the assist amount is zero, that is, the steering assist is stopped at time t2. . In the first embodiment, the steering assist is resumed from time t3 when the rate of change in the increasing direction of the steering angle deviation is reduced to a predetermined value. On the other hand, in the second embodiment, since the steering angle deviation is large at time t3, the steering assist is still stopped and the steering assist is resumed from time t4 when the steering angle deviation is small.

  In the first embodiment, there is a possibility that the steering assist may be started in the middle of the elimination of the slip. However, in the second embodiment, since the stop of the steering assist can be maintained more appropriately, more Slip can be effectively eliminated. Moreover, since the setting of the assist stop period can be arbitrarily set by setting the second gain map or the processing of the gain integration unit 883, the slip cancellation function and the steering assist function can work well.

<Third Embodiment>
Next, a third embodiment of the abnormality assist torque calculation unit 80 will be described. The abnormality assist torque calculation unit 80 according to the third embodiment is provided with a slip index setting unit 89 and a gain setting unit 90 in place of the slip index setting unit 85 and the gain setting unit 86 of the first embodiment. Is the same as that of the first embodiment (see FIG. 4). As shown in FIG. 12, the slip index setting unit 89 includes a steering angle deviation calculation unit 851 and a slip index output unit 855. The rudder angle deviation calculator 851 is the same as that in the first embodiment. The slip index output unit 855 performs the same processing as the second slip index output unit 8542 of the second embodiment. The gain setting unit 90 performs the same processing as the second gain setting unit 882 of the second embodiment.

  In the slip index setting unit 89 of the third embodiment, a slip index SL corresponding to the deviation between the actual steering angle and the estimated steering angle is set and output. For example, the slip index output unit 855 inputs the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle deviation Δθrs calculated by the steering angle deviation calculation unit 851, and the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle. The larger one of the deviations Δθrs is set as the slip index SL. Alternatively, an average value ((Δθfs + Δθrs) / 2) of the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle deviation Δθrs may be calculated, and the average value may be set as the slip index SL. Alternatively, a value (Δθfs · Δθrs) obtained by multiplying the front wheel side steering angle deviation Δθfs and the rear wheel side steering angle deviation Δθrs may be calculated and set as the slip index SL. The slip index output unit 855 outputs the set slip index SL to the gain setting unit 90.

  The gain setting unit 90 stores the gain map shown in FIG. 8, refers to this gain map, calculates the assist limit gain K corresponding to the slip index SL, and assists the assist limit gain K that is the calculation result. Output to the limiter 83.

  In the third embodiment, a slip index SL corresponding to the deviation between the actual rudder angle and the estimated rudder angle is set without using the rate of change in deviation between the actual rudder angle and the estimated rudder angle. An assist limit gain K that decreases as the value increases is set. Therefore, the steering assist can be limited over a period during which slip occurs. Moreover, since the slip index SL corresponding to the deviation between the actual steering angle θs and the estimated steering angles θf, θr is set without using the deviation between the estimated steering angles θf, θr, the estimated steering angles θf, θr are set. The influence of the included error is small, and the slip state can be detected appropriately. Thereby, over-assist can be prevented and excessive turning of the steering wheel 11 can be suppressed. As a result, the slip can be effectively eliminated.

  Next, modified examples applicable to the above three embodiments will be described.

<First Modification>
In each of the above embodiments, as a method of limiting the steering assist when the slip is detected, the basic assist torque Tbase is multiplied by the assist limiting gain K. However, the limitation of the steering assist can be implemented by reducing the upper limit value of the target assist torque Ta *. For example, the processing of the gain setting units 86, 88, 90 is changed as follows. Since this process is not a process for setting the assist limit gain but a process for setting the upper limit value of the target assist torque Ta *, hereinafter, the gain setting units 86, 88, 90 will be referred to as the upper limit value setting units 86, 88. , 90.

  The upper limit value setting units 86 and 90 corresponding to the modified examples of the first and third embodiments store the upper limit value map shown in FIG. 13 and refer to the upper limit value map to determine the upper limit value corresponding to the slip index SL. Calculate Tlim. This upper limit map sets the upper limit value Talim of the target assist torque Ta * to the basic upper limit value Talim0 (upper limit value when no slip is detected) in a range where the slip index SL is equal to or less than the reference value SLref. In the range where the slip index SL exceeds the reference value SLref, there is a characteristic of setting an upper limit value Talim that decreases as the slip index SL increases. The upper limit setting units 86 and 90 output the upper limit value Talim, which is the calculation result, to the assist limiting unit 83.

  The upper limit value setting unit 88 corresponding to the modification of the second embodiment stores a first upper limit value map and a second upper limit value map (not shown), and refers to the first upper limit value map, and the first slip index. The first upper limit value Talim1 is set from SL1, and the second upper limit value Talim2 is set from the second slip index SL2 with reference to the second upper limit value map. The first upper limit map and the second upper limit map have the same characteristics as the upper limit map shown in FIG. 13, and the horizontal axis is the first slip index SL1 in the first upper limit map. What is necessary is just to set it as 2nd slip parameter | index SL2 in an upper limit map. The upper limit setting unit 88 sets an upper limit value Talim based on the first upper limit value Talim1 and the second upper limit value Talim2. For example, the smaller one of the first upper limit value Talim1 and the second upper limit value Talim2 is set as the upper limit value Talim. Or you may make it set the average value of 1st upper limit Talim1 and 2nd upper limit Talim2 to upper limit Talim. The upper limit value setting unit 88 outputs the upper limit value Talim as a calculation result to the assist limiting unit 83.

  The assist limiting unit 83 compares the basic assist torque Tbase with the upper limit value Talim, and if the basic assist torque Tbase does not exceed the upper limit value Talim, sets the basic assist torque Tbase to the target assist torque Ta2 (Ta2 ← Tbase When the basic assist torque Tbase exceeds the upper limit value Talim, the upper limit value Talim is set to the target assist torque Ta2 (Ta2 ← Talim). Therefore, it is possible to appropriately limit the steering assist according to the slip index SL.

<Second Modification>
In the second modification, a function for making the assist limit gain K variable according to the vehicle speed is added. As shown in FIG. 14, the abnormal-time assist torque calculation unit 80 includes a vehicle speed variable coefficient multiplication unit 91. The vehicle speed variable coefficient multiplication unit 91 inputs the assist limit gain K calculated by the gain setting unit 86 (88, 90) and the vehicle speed V detected by the vehicle speed sensor 24. The vehicle speed variable coefficient multiplication unit 91 stores the vehicle speed gain map shown in FIG. 15 and calculates the vehicle speed gain Kv corresponding to the vehicle speed V with reference to the vehicle speed gain map. In the vehicle speed gain map, the vehicle speed gain Kv is set to a constant reference value Kv0 in a range where the vehicle speed V is less than or equal to the reference value Vref, and the vehicle speed decreases as the vehicle speed V increases in a range where the vehicle speed V exceeds the reference value Vref. It has a characteristic for setting the gain Kv.

  The vehicle speed variable coefficient multiplication unit 91 sets a value obtained by multiplying the assist limit gain K calculated by the gain setting unit 86 (88, 90) by the vehicle speed gain Kv as a final assist limit gain K (K ← Kv · K) and output to the assist limiting unit 83.

  Generally, the rudder angle and rudder angular speed are larger as the vehicle speed is lower and smaller as the vehicle speed is higher. Further, as can be seen from the above formulas (1) and (2), the lower the vehicle speed, the greater the estimated steering angle sensitivity with respect to the wheel speed fluctuation. For this reason, regarding the steering angle deviation and the rate of change of the steering angle deviation, an excessively large value is easily calculated at a lower vehicle speed. Therefore, in the second modification, the limitation of the target steering assist control amount during low-speed traveling is relaxed by correcting the assist limitation using the vehicle speed gain Kv. As a result, it is possible to avoid excessive assist limitation during low-speed traveling.

  In the second modification, the assist restriction is corrected by multiplying the assist restriction gain K by the vehicle speed gain Kv. However, by combining the first modification, the upper limit value Tlim is multiplied by the vehicle speed gain Kv. The assist limit may be corrected (Tlim ← Kv · Tlim).

<Third Modification>
In each of the above-described embodiments, the two estimated steering angles θf and θr are calculated, and the steering angle deviations Δθfs and Δθrs and the deviation change rates Δθfs ′ and Δθrs are calculated based on the two estimated steering angles θf and θr, respectively. 'Is calculated, but only one of them may be calculated. For example, the estimated steering angle calculation unit 84 calculates only the front wheel side estimated steering angle θf, the steering angle deviation calculation unit 851 calculates only the front wheel side steering angle deviation Δθfs, and the deviation change rate calculation unit 852 calculates the front wheel side deviation change rate. Only Δθfs ′ may be calculated. Alternatively, the estimated steering angle calculation unit 84 calculates only the rear wheel side estimated steering angle θr, the steering angle deviation calculation unit 851 calculates only the rear wheel side steering angle deviation Δθrs, and the deviation change rate calculation unit 852 calculates the rear wheel side. Only the deviation change rate Δθrs ′ may be calculated. In this case, the slip index output unit 853 in the first embodiment may output the slip index SL corresponding to one of the deviation change rates. In addition, the first slip index output unit 8541 in the second embodiment outputs the first slip index SL1 corresponding to any one of the deviation change rates, and the second slip index output unit 8542 outputs any one of the steering angle deviations. The corresponding second slip index SL2 may be output. In addition, the slip index output unit 855 in the third embodiment may output the slip index SL corresponding to one of the steering angle deviations.

<Fourth Modification>
In each of the embodiments described above, the steering assist is limited using the assist limit gain K that is set according to the slip index SL. Therefore, for example, from the state where the assist limit gain K is zero (K = 0), that is, the state where the steering assist is stopped, the slip index SL suddenly decreases and the assist limit gain K increases rapidly. There is. At this time, if the steering operation is being performed, the driver feels uncomfortable due to a change in the steering assist. Therefore, in the fourth modified example, when the assist limit gain K once decreases to zero, even if the assist limit gain K set in the gain map is increased thereafter, the assist limit gain K is at least set only for a set time. Is maintained at zero, and a process for returning the assist limit torque to the normal value (value based on the gain map) at the timing when the basic assist torque Tbase returns to zero is added.

  FIG. 16 is a flowchart illustrating a gain return processing routine performed by the gain setting unit 86. In addition, although the modified example applied to 1st Embodiment is demonstrated here, it can apply similarly in 2nd, 3rd embodiment. As described above, the gain setting unit 86 calculates the assist limit gain K corresponding to the slip index SL with reference to the gain map, and repeatedly executes this gain return processing routine at a predetermined short period.

  In step S11, the gain setting unit 86 determines that the assist limit gain K (n-1) calculated immediately before (one calculation cycle before) is zero, and this time, the assist limit gain calculated with reference to the gain map. It is determined whether K (n) is not zero. That is, it is determined whether or not the slip index SL has decreased from the state in which the steering assist is stopped (K = 0) to the return state (0 <K <1). The gain setting unit 86 once ends this routine when the condition of step S11 is not satisfied. In this case, the gain setting unit 86 outputs the assist limit gain K (n) to the assist limit unit 83.

  When the gain setting unit 86 repeats such processing and determines “Yes” in step S11, the gain setting unit 86 determines in step S12 whether the set time has been continued. That is, it is determined whether or not “Yes” is continuously determined for a set time after “Yes” is first determined in step S11. If the set time has not been continued (S12: No), in step S13, the assist limit gain K (n) is set to zero, and this routine is terminated once. That is, the assist limit gain K (n) calculated from the gain map does not become zero, but the assist limit gain K (n) is forcibly set to zero. Accordingly, the assist limit gain K (n) set to zero is output to the assist limiter 83. Therefore, the steering assist is not resumed yet.

  The gain setting unit 86 repeats such processing. If “Yes” is determined in step S12, it is determined in step S14 whether or not the basic assist torque Tbase is zero (Tbase = 0). If the basic assist torque Tbase is not zero, the assist limit gain K (n) is forcibly set to zero in step S13. Therefore, the steering assist is not resumed yet.

  On the other hand, if the basic assist torque Tbase is zero, in step S15, the value calculated from the gain map is set as the assist limit gain K (n). That is, at this point, the assist limit gain K (n) calculated with reference to the gain map is employed. Therefore, the assist restriction unit 83 performs assist restriction according to the assist restriction gain K (n).

  As described above, according to the fourth modification, after the slip index SL increases and the steering assist is stopped (K = 0), the first condition that the steering assist stop state is maintained at least for the set time, and While the second condition that the basic assist torque Tbase is zero is not satisfied, the assist limit gain K is forcibly maintained at zero and the steering assist is stopped. When the above two conditions are satisfied, the assist limit gain K setting method is returned to the normal one and set to a value corresponding to the slip index SL. Therefore, even when the slip state is released, the steering assist does not change suddenly, and the driver does not feel uncomfortable.

  In the fourth modification, two conditions are set, but only the first condition or only the second condition may be set. Further, “zero” in the assist limit gain K and the basic assist torque Tbase does not mean only complete zero, but includes substantially zero, that is, a value near zero that can be regarded as zero.

<Fifth Modification>
When the slip index is increasing, lowering the steering assist earlier can increase the slip cancellation effect, and when the slip index is decreasing, the torque fluctuation is better when the steering assist is returned (increased) more slowly. It is possible to reduce the uncomfortable feeling caused by steering. Therefore, in the fifth modification, the variation range of the assist limit gain K with respect to the variation range of the slip index is set to be different between when the slip index is increasing and when the slip index is decreasing.

  FIG. 17 is a flowchart showing an assist limit gain setting routine performed by the gain setting unit 86. In addition, although the modified example applied to 1st Embodiment is demonstrated here, it can apply similarly in 2nd, 3rd embodiment. The gain setting unit 86 repeatedly executes an assist limit gain setting routine at a predetermined short cycle.

  In step S21, the gain setting unit 86 determines whether or not the assist limit gain K (n-1) calculated immediately before (one calculation cycle before) is not 1. When the assist limit gain K (n-1) is 1 (K (n-1) = 1.0), the slip index setting unit 85 calculates this time with reference to the gain map in step S22. The assist limit gain K (n) is set based on the slip index SL (n), and this routine is temporarily terminated. When such a process is repeated and the slip index SL increases and the determination in step S21 is “No”, that is, the assist limit gain K (n−1) calculated immediately before (one calculation cycle before) is 1. When there is no longer any gain, the gain setting unit 86 calculates the slip index SL (n−1) calculated immediately before (one calculation cycle before) from the slip index SL (n) calculated this time by the slip index setting unit 85 in step S23. It is determined whether or not the subtracted value (SL (n) −SL (n−1)) is a positive value. That is, it is determined whether or not the slip index SL is increasing.

  If the gain setting unit 86 determines “Yes” in step S23, that is, if the slip index SL is increasing, the gain setting unit 86 sets the change gradient α to the first change gradient α1 in step S24. In Step S23, when it is determined that “No”, that is, the slip index SL is decreasing (including the case where the slip index SL has not changed), the change gradient α is set to the second change gradient α2 in Step S25. To do.

  As shown in FIG. 18, the first change gradient α1 represents the degree of decrease when the assist limit gain K is decreased, that is, the ratio of the decrease width of the assist limit gain K to the increase width of the slip index SL. The gradient α2 represents the degree of increase when the assist limit gain K is increased, that is, the ratio of the increase width of the assist limit gain K to the decrease width of the slip index SL. In this case, the first change gradient α1 is set to a larger value than the second change gradient α2.

Subsequently, in step S26, the gain setting unit 86 calculates the change amount (SL (n) −SL (n) per one calculation cycle of the slip index SL from the assist limit gain K (n−1) calculated immediately before. -1)) is subtracted by a value obtained by multiplying the change gradient α, thereby calculating the assist limit gain K (n) in the current calculation cycle. This can be expressed by the following formula.
K (n) = K (n-1) -α (SL (n) -SL (n-1))

  Since it may be considered that the assist limit gain K (n) calculated in step S26 is not in the range of 0 to 1.0, the gain setting unit 86 executes the processes of steps S27 to S30. In step S27, the gain setting unit 86 determines whether or not the assist limit gain K (n), which is the above calculation result, is a negative value. The limiting gain K (n) is set to zero (K (n) = 0). If it is not a negative value, it is determined in step S29 whether the assist limit gain K (n) is a value greater than 1. If the value is greater than 1, the assist limit gain is determined in step S30. K (n) is set to 1 (K (n) = 1.0). When the assist limit gain K (n) is in the range of 0 to 1.0 (S27: No), the assist limit gain K (n) is set to the value calculated in step S26.

  The gain setting unit 86 once ends the routine when the assist limit gain K (n) is set in this way. Then, similar processing is repeated at a predetermined short cycle.

  According to the fifth modification, when the slip index is decreasing, the degree (change speed) for changing the assist limit is made smaller than when the slip index is increasing. For example, when applied to the first embodiment, when the rate of change in the increasing direction of the rudder angle deviation is decreasing, the degree to which the assist restriction is changed is made smaller than when increasing. Further, when applied to the third embodiment, when the steering angle deviation is reduced, the degree to which the assist restriction is changed is made smaller than when the steering angle deviation is increasing. Further, when applied to the second embodiment, both characteristics are provided. In other words, when the rate of change in the increasing direction of the rudder angle deviation is decreasing, and when the rudder angle deviation is decreasing, the degree to which the assist limit is changed compared to when the rudder angle deviation is increasing. Make it smaller. Therefore, according to the fifth modified example, it is possible to achieve both the elimination of the slip and the reduction of the steering discomfort.

  Although the electric power steering apparatus 1 according to the plurality of embodiments and modifications has been described above, the present invention is not limited to the above-described embodiments and modifications, and various modifications can be made without departing from the object of the present invention. Is possible.

  For example, a plurality of modifications can be combined. For example, in the fourth modification and the fifth modification, the steering assist is limited using the assist limit gain K. However, the steering assist is limited using the upper limit value Tlim in combination with the first modification. You can also do it. In this case, upper limit values Tlim (n) and Tlim (n-1) may be used instead of the assist limit gains K (n) and K (n-1). Further, “K = 1” may be replaced with “Talim = Talim0”, and “K = 0” may be replaced with “Talim = 0”.

  In the present embodiment, the basic assist torque Tbase is calculated using the actual steering angle θs as an alternative parameter of the steering torque. However, the estimated steering angle can be used instead of the actual steering angle θs. Further, lateral acceleration acting on the vehicle can be used as an alternative parameter of the steering torque without using the steering angle. In this case, a target steering assist amount (for example, basic assist torque Tbase) that increases as the lateral acceleration increases may be set. The lateral acceleration may be detected by a sensor or may be obtained by calculation.

  In the present embodiment, the assist limit gain K is linearly decreased as the slip index SL increases in a range where the slip index SL exceeds the reference value SLref. A configuration in which the limiting gain K is decreased stepwise may be used. For example, the assist limit gain K may be changed in two steps as the slip index SL increases.

  In this embodiment, the rudder angle deviation or the rate of change of the rudder angle deviation is set in the slip index SL, but instead, the rudder angle deviation or the rate of change of the rudder angle deviation is increased. Accordingly, a slip index that increases stepwise may be set.

  Further, in this embodiment, the basic assist torque Tbase has its characteristics switched according to the vehicle speed, but it may not be made to respond to the vehicle speed. Further, the basic assist torque Tbase may be added with a compensation torque that changes according to the steering angular speed, for example, a friction compensation torque that compensates for the frictional force in the steering mechanism 10 or a viscosity compensation torque that compensates for the viscosity. .

  Further, in the present embodiment, the steering angle sensor 22 and the wheel speed sensors 26 to 29 are provided to detect the actual steering angle and the wheel speed. However, these sensors are not provided, and are provided in the vehicle. Information representing the actual steering angle and wheel speed detected by the vehicle ECU (for example, the vehicle attitude control ECU) may be acquired via a communication line.

  In the present embodiment, the column assist type electric power steering device that applies the torque generated by the motor 20 to the steering shaft 12 has been described. However, the rack assist type that applies the torque generated by the motor to the rack bar 14 is described. An electric power steering device may be used.

  DESCRIPTION OF SYMBOLS 1 ... Electric power steering apparatus, 10 ... Steering mechanism, 11 ... Steering handle, 12 ... Steering shaft, 20 ... Motor, 21 ... Steering torque sensor, 22 ... Steering angle sensor, 24 ... Vehicle speed sensor, 26-29 ... Wheel speed sensor , 40 ... motor drive circuit, 50 ... electronic control circuit, 60 ... motor control unit, 70 ... target assist torque calculation unit, 71 ... normal assist torque calculation unit, 72 ... abnormality detection unit, 73 ... control switching unit, 80 ... Abnormal assist torque calculation unit, 81 ... basic assist torque calculation unit, 82 ... assist limit calculation unit, 83 ... assist limit unit, 84 ... estimated rudder angle calculation unit, 85, 87, 89 ... slip index setting unit, 86, 88 , 90 ... Gain setting section (upper limit setting section), 91 ... Vehicle speed variable coefficient multiplication section, 100 ... Electronic control unit (assist ECU), 851 Steering angle deviation calculation unit, 852... Deviation change rate calculation unit, 853, 854, 855... Slip index output unit, 881... First gain setting unit, 882. Second gain setting unit, 883. Side deviation calculation unit, 8512 ... Rear wheel side deviation calculation unit, 8521 ... Front wheel side deviation change rate calculation unit, 8522 ... Rear wheel side deviation change rate calculation unit, 8541 ... First slip index output unit, 8542 ... Second slip index Output unit, Ffail: abnormality determination flag, K, K1, K2 ... assist limit gain, Kv: vehicle speed gain, SL, SL1, SL2 ... slip index, Ta * ... target assist torque, Ta1 ... target assist torque, Ta2 ... target assist Torque, Tr ... Steering torque, V ... Vehicle speed, Vfl ... Left front wheel speed, Vfr ... Right front wheel speed, Vrl ... Left rear wheel speed, Vrr ... Right rear wheel speed, α ... Change gradient , Δθfs: Front wheel side steering angle deviation, Δθrs: Rear wheel side steering angle deviation, Δθfs ': Front wheel side deviation change rate, Δθrs': Rear wheel side deviation change rate, θf ... Estimated front wheel side steering angle, θr ... Rear wheel side Estimated rudder angle, θs ... Actual rudder angle.

Claims (8)

A steering torque sensor for detecting a steering torque input from the steering handle to the steering shaft;
A motor provided in the steering mechanism for generating steering assist torque;
An abnormality detecting means for detecting an abnormality of the steering torque sensor;
When an abnormality of the steering torque sensor is not detected, a target steering assist control amount is set based on the steering torque detected by the steering torque sensor, and when an abnormality of the steering torque sensor is detected, A control amount setting means for setting a target steering assist control amount using an alternative parameter different from the steering torque;
An electric power steering apparatus comprising: motor control means for driving and controlling the motor according to the target steering assist control amount set by the control amount setting means;
An actual rudder angle detecting means for detecting an actual rudder angle;
An estimated rudder angle calculating means for detecting the wheel speeds of the left and right wheels on at least one of the front wheel side and the rear wheel side, and calculating an estimated rudder angle based on the detected wheel speed;
Slip index setting means for setting a slip index as a wheel slip determination index based on the actual steering angle detected by the actual steering angle detection means and the estimated steering angle calculated by the estimated steering angle calculation means When,
When an abnormality of the steering torque sensor is detected and the target steering assist control amount is set using an alternative parameter different from the steering torque, the target is increased as the slip index set by the slip index setting means increases. An electric power steering apparatus comprising: an assist limiting unit that limits the target steering assist control amount so that the steering assist control amount becomes small.   2. The electric power steering apparatus according to claim 1, wherein the slip index setting unit sets a slip index that increases as the rate of change in deviation between the actual steering angle and the estimated steering angle increases. The slip index setting means includes
A first slip index setting unit that sets a first slip index that increases as the rate of change in deviation between the actual rudder angle and the estimated rudder angle increases;
A second slip index setting unit that sets a second slip index that increases with an increase in deviation between the actual steering angle and the estimated steering angle;
The assist limiting means is
The target steering assist control is performed such that the target steering assist control amount decreases as the first slip index increases, and the target steering assist control amount decreases as the second slip index increases. 3. The electric power steering apparatus according to claim 2, wherein the amount is limited. The assist limiting means is
The target steering assist control amount is limited by using a slip index having a larger degree of limiting the target steering assist control amount among the first slip index and the second slip index. 3. The electric power steering device according to 3.   5. The slip index setting unit sets a slip index that increases with an increase in a deviation between the actual steering angle and the estimated steering angle. 6. Electric power steering device.   When the slip index decreases, it comprises a change slope adjusting means for reducing a ratio of a change amount of the target steering assist control amount restriction to a change amount of the slip index as compared with a case where the slip index increases. The electric power steering device according to any one of claims 1, 2, and 5.   When the first slip index or the second slip index decreases, the change amount of the limit of the target steering assist control amount with respect to the change amount of the first slip index or the second slip index is larger than when the first slip index or the second slip index increases. 5. The electric power steering apparatus according to claim 3, further comprising change gradient adjusting means for reducing the ratio.   8. The assist limiting unit acquires vehicle speed information indicating a vehicle speed, and relaxes the limit of the target steering assist control amount when the vehicle speed is low compared to when the vehicle speed is high. The electric power steering device according to any one of claims.

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