Hydraulic power steering apparatus

In a hydraulic power steering apparatus, hydraulic pressure for generating steering assist power is varied by varying opening of a variable throttle portion by movement of a spool. The spool is disposed in a provisional origin by driving a stepping motor in one direction according to a number of steps for setting provisional origin from a position at a time of start of control, and the number of steps at a time of completing of the driving is set to a provisional origin for calculation. In a return to origin permission state where the spool is disposed in a provisional origin and the number of steps from the provisional origin for calculation is zero, a number of steps at a time when the driving of the stepping motor according to a number of steps obtained by subtracting the number of steps for setting provisional origin from a number of steps for setting origin is completed is set to an origin for calculation, and the movement of the spool is stopped at the origin with a stopper. After setting the origin for calculation, a number of steps from the origin for calculation instead of the provisional origin for calculation is set according to the vehicle driving condition for positioning the spool.

BACKGROUND OF THE INVENTION

The present invention relates to a hydraulic power steering apparatus in which the steering assist power can be varied by controlling a stepping motor according to a vehicle driving condition.

In order to vary the steering assist power according to the vehicle driving condition, for example, vehicle speed in a hydraulic power steering apparatus, a stepping motor that moves a spool of a variable throttle valve provided in a hydraulic circuit is driven according to the number of steps that is set according to the vehicle driving condition (see Patent Document 1).

In order to set accurately the degree of opening of a variable throttle portion in such variable throttle valve, it is necessary to perform accurate positioning of the spool by controlling the stepping motor. For this purpose, the origin corresponding to the actual reference position of the spool is required to match the origin for calculation that serves as a reference for calculating the number of steps.

Accordingly, a stopper that can stop the movement of the spool in one direction is provided in the origin, the number of steps for setting origin that is required for moving the spool to the origin is predetermined, and the stepping motor is driven according to the number of steps for setting origin at the time when the control is started. As a result, the spool is positioned in the origin at the time when the driving of the stepping motor according to the number of steps for setting origin is completed therefore, the value of the number of steps at this time is set as the origin for calculation.Patent Document 1: Japanese Patent Application Laid-open No. 2000-229578

SUMMARY OF THE INVENTION

When the stepping motor is controlled according to, for example, vehicle speed, the variable throttle portion is closed by positioning the spool in the origin when the vehicle speed is zero, and as the vehicle speed increases, the degree of opening of the variable throttle portion is increased by moving the spool away from the origin. As a result the steering assist power is decreased when the vehicle speed increased. Such control of the stepping motor is usually started and ended as the ignition switch of the vehicle is turned on and off. A typical driver turns the ignition switch off when the vehicle speed is zero or close to zero, whereby the spool is disposed in the origin or close thereto when the control of the stepping motor ends.

However, there are also drivers who turn the ignition switch off during the vehicle travels, for example, in downhill, so that the vehicle travels to the stop position by inertia. In such case because the vehicle speed at the time when the control of the stepping motor has ended is high, the spool is disposed at a distance from the origin. For this reason, the number of steps for setting origin is set on the assumption that the spool is disposed in a position farthest from the origin in order that the spool can be moved reliably to the origin when the control of the stepping motor is started again.

In this case, because the number of steps for setting origin is large, a long time is required to complete the driving according to the number of steps for setting origin at a time when the control of the stepping motor is started; therefore, a long time elapses before the origin for calculation is set. Because the steering assist power cannot be controlled according to the vehicle driving condition before the origin is set, the interval in which the steering feeling is degraded is increased at the initial stage of the driving of the vehicle.

It is an object of the present invention to provide a hydraulic power steering apparatus that can resolve the above-described problems.

The hydraulic power steering apparatus in accordance with the present invention comprises a variable throttle valve that has a variable throttle portion in which the degree of opening is varied by movement of a spool and varies hydraulic pressure for generating steering assist power by this variation of the degree of opening; a stepping motor for moving the spool in one direction and the other direction; means for driving the stepping motor according to a number of steps a stopper that can stop the movement of the spool in the one direction at an origin; means for storing a predetermined number of steps that is required to move the spool to the origin as a number of steps for setting origin; means for storing a number of steps that is less than the number of steps for setting origin and is predetermined for moving the spool in the one direction from a position at a time when a control of the stepping motor starts as a number of steps for setting provisional origin; means for setting a number of steps at a time when the driving of the stepping motor according to the number of steps for setting provisional origin is completed to a provisional origin for calculation, on the assumption that the spool is positioned in a provisional origin when the driving of the stepping motor according to the number of steps for setting provisional origin is completed; means for setting a number of steps from the provisional origin for calculation that corresponds to an amount of movement of the spool from the provisional origin in the other direction according to a vehicle driving condition; means for determining whether it is in a return to origin permission state in which conditions for return to the origin are satisfied, these conditions including a condition that the spool is disposed in the provisional origin and a condition that the number of steps from the provisional origin for calculation set according to the vehicle driving condition is zero; and means for setting a number of steps at a time when the driving of the stepping motor according to a number of steps obtained by subtracting the number of steps for setting provisional origin from the number of steps for setting origin is completed to an origin for calculation, in the return to origin permission state; wherein instead of the number of steps from the provisional origin for calculation, the number of steps from the origin for calculation that corresponds to the amount of movement of the spool from the origin in the other direction is set according to the vehicle driving condition, after the origin for calculation is set; and the spool is positioned by driving the stepping motor according to the number of steps that is set according to the vehicle driving condition.

According to the present invention, in the initial stage of the control of the stepping motor, when the driving of the stepping motor according to the number of steps for setting provisional origin is completed, a number of steps at this time is set to a provisional origin for calculation, on the assumption that the spool is positioned in the provisional origin at this time. The number of steps from the provisional origin for calculation corresponding to the amount of movement of the spool from the provisional origin in the other direction is set according to the vehicle driving condition. The spool is positioned by driving the stepping motor according to the number of steps set according to the vehicle driving condition, and thus the steering assist power that corresponds to the vehicle driving condition can be obtained.

Because the number of steps for setting provisional origin is less than the number of steps for setting origin, the provisional origin for calculation can be set faster than the origin for calculation. As a result, the steering assist power corresponding to the vehicle driving condition can be obtained faster than in the conventional apparatuses, and it is possible to shorten the interval in which the control of the stepping motor according to the driving condition is restricted at the initial stage of the driving of the vehicle.

Although a difference can occur between the provisional origin for calculation and the origin for calculation because the provisional origin does not always matches the origin, the steering feeling can still be improved in comparison with the case in which the steering assist power does not at all correspond to the vehicle driving condition. Furthermore, the provisional origin for calculation matches the origin for calculation when the provisional origin matches the origin; therefore, sufficient steering feeling can be obtained within a short interval.

Further, because the spool is positioned in the provisional origin in the return to origin permission state, the spool is disposed in the origin when the driving of the stepping motor according to the number of steps obtained by subtracting the number of steps for setting provisional origin from the number of steps for setting origin is completed, and the value of the number of steps at this time can be set to the origin for calculation. As a result, the provisional origin can be reliably matched with the origin. Moreover, because the origin for calculation is set when the vehicle driving condition under which the movement of the spool from the provisional origin in the other direction is not required is satisfied, such setting can be performed without unfavorable effect on the steering feeling.

It is preferred that when the return to origin permission state is dissolved during the driving of the stepping motor according to the number of steps obtained by subtracting the number of steps for setting provisional origin from the number of steps for setting origin, a number of steps at this time is set to a new provisional origin for calculation, on the assumption that the spool is positioned in a new provisional origin at this time. As a result, even when the provisional origin does not match the origin, the difference between the provisional origin for calculation and the origin for calculation can be gradually decreased before the origin for calculation is set, and the unfavorable effect on the steering feeling can be reduced.

It is preferred that the vehicle driving condition includes the vehicle speed, and the amount of movement of the spool from the provisional origin or the origin in the other direction increases with the increase in the vehicle speed. As a result, the vehicle speed is usually zero or close to zero when the control of the stepping motor is completed; therefore, the spool is disposed in the origin or close thereto. As a result, in the usual case, the provisional origin can be matched with the origin by stopping the movement of the spool with the stopper when the driving of the stepping motor according to the number of steps for setting provisional origin is completed in the initial stage of the next control, and thus sufficient steering feeling can be obtained within a short period.

In this case, it is preferred that the hydraulic pressure acting on a hydraulic actuator for generating steering assist power is decreased with the increase in the amount of movement of the spool from the provisional origin or the origin in the other direction, and the control of the stepping motor is started when the vehicle speed becomes equal to or less than a preset value. As a result, when the ignition switch is turned on during the vehicle travels by inertia, for example, in downhill, the spool does not move in the direction for increasing the steering assist power to set the provisional origin for calculation, and thus the increase in steering assist power at the time when the vehicle speed increases can be prevented and the steering stability can be maintained.

With the hydraulic power steering apparatus in accordance with the present invention, the interval in which the control of the steering assist power corresponding to the driving condition is restricted at the initial stage of the driving of the vehicle can be shortened, and the steering feeling can be improved.

DETAILED DESCRIPTION OF THE INVENTION

In a rack and pinion type hydraulic power steering apparatus1shown inFIG. 1, a torsion bar6that is connected via a pin4to an input shaft2joined to a steering wheel that is not shown in the figure is joined via serrations5to an output shaft3. The input shaft2is supported via a bearing8by a valve housing7and supported via a bushing12by the output shaft3. The output shaft3is supported via bearings10,11by a rack housing9. Vehicle wheels for steering that are not shown in the figure are joined to a rack16engaged with a pinion15formed in the output shaft3. As a result, by the transmission of the rotation of the input shaft2caused by steering operation via the torsion bar6and output shaft3, the pinion15is rotated and the rack16moves in the lateral direction of the vehicle, thereby steering angle of the vehicle varies.

A hydraulic cylinder20is provided as a hydraulic actuator for generating steering assist power. The hydraulic cylinder20comprises a cylinder tube formed by a rack housing9and a piston21integrated with the rack16. A rotary type hydraulic control valve30is provided to supply hydraulic fluid according to steering resistance to oil chambers22,23that are partitioned by the piston21. In the control valve30, a cylindrical first valve member31that is inserted into the valve housing7so as to be able to rotate relatively is joined via a pin29to the output shaft3so as to rotate together, and a second valve member32that is inserted into the first valve member31coaxially so as to be able to rotate relatively is formed integrally with the outer peripheral portion of the input shaft2so as to rotate together. An inlet port34connected to a pump70, a first port37connected to one oil chamber22of the hydraulic cylinder20, a second port38connected to the other oil chamber23, a first outlet port36connected to a tank71, and a second outlet port61connected to the tank71via a variable throttle valve60are provided in the valve housing7, and are connected to each other via flow paths provided between the inner periphery of the valve member31and the outer periphery of the valve member32.

As shown inFIG. 3andFIG. 4, on the inner periphery of the first valve member31, a plurality of right steering grooves50aare connected via flow paths53and the first port37to the oil chamber22, a plurality of left steering grooves50bare connected via flow paths54and the second port38to the oil chamber23, and a plurality of communication grooves50care formed. On the outer periphery of the second valve member32, a plurality of hydraulic fluid supply grooves51aare connected to the pump70via hydraulic fluid supply paths55and the inlet port34, a plurality of first hydraulic fluid discharge grooves51bare connected to the tank71via a flow path52b(seeFIG. 1) and the first outlet port36through a space between the input shaft2and the torsion bar6from the flow paths52a, and a second hydraulic fluid discharge grooves51care connected to the variable throttle valve60via flow paths59and the second outlet port61. The grooves are disposed with spacing to each other in the circumferential direction, the first hydraulic fluid discharge grooves51bare disposed between the right steering grooves50aand the left steering grooves50b, the second hydraulic fluid discharge grooves51care disposed between the communication grooves50c, and the hydraulic fluid supply grooves51aare disposed between the right steering grooves50aand the communication grooves50cand between the left steering grooves50band the communication grooves50c. Throttle portions A, A′, B, B′, C, C′, D, D′ are formed between the edges along the axial direction of the grooves50a,50b,50cformed in the first valve member31and the edges along the axial direction of the grooves51a,51b,51cformed in the second valve member32.

As shown inFIG. 5, the edges along the axial direction of grooves51a,51b,51cformed in the second valve member32have chamfered portions. As shown inFIG. 4andFIG. 5, a width W of the chamfered portions (surrounded by □ inFIG. 3) of the edges along the axial direction of the hydraulic fluid supply grooves51ain the throttle portions A′, C′ between the hydraulic fluid supply grooves51aand the communication grooves50c, a width W′ of the chamfered portions (surrounded by Δ inFIG. 3) of the edges along the axial direction of the second hydraulic fluid discharge grooves51cin the throttle portions B′, D′ between the communication grooves50cand the second hydraulic fluid discharge grooves51c, and a width W″ of the chamfered portions (surrounded by ∘ inFIG. 3) of the edges along the axial direction of the other grooves formed in the second valve member32satisfy the relationship W>W′>W″. In comparing the relative rotation angles (i.e. closure angles) of the two valve members31,32required to close completely the throttle portions A, A′, B, B′, C, C′, D, D′ from a state in which there is no steering resistance (that is a state shown inFIG. 4andFIG. 5), a closure angle θr of the throttle portions A′, C′ is larger than a closure angle θs of the throttle portions B′, D′, and the closure angles θr, θs are larger than the closure angle θt of the other throttle portions A, B, C, D. As a result, the throttle portions located between the first valve member31and the second valve member32can be divided into a first group consisting of a plurality of throttle portions A, B, C, D and a second group consisting of a plurality of throttle portions A′, B′, C′, D′ that have a closure angle larger than that of the throttle portions A, B, C, D that belong to the first group, and the steering resistance required to close the throttle portions A′, B′, C′, D′ that belong to the second group is larger than the steering resistance required to close the throttle portions A, B, C, D that belong to the first group. Further, the throttle portions that belong to the second group are divided into two types, i.e., throttle portions B′, D′ and throttle portions A′, C′ that have a closure angle larger than that of the throttle portions B′, D′.

The two valve members31,32are elastically rotated by torsion of the torsion bar6due to the steering resistance, so that the degree of opening of the throttle portions A, B, C, D, A′, B′, C′, D′ vary, and the hydraulic pressure acting on the hydraulic cylinder20varies according to the variation in this degree of opening, with the result that the steering assist power corresponding to the steering resistance is generated.FIG. 4shows a state in which no steering operation is performed, wherein the throttle portions A, B, C, D, A′, B′, C′, D′ located between the two valve members31,32are completely open, the inlet port34and the outlet ports36,61communicate with each other via a flow path27between the valve members, the oil flowing from the pump70to the control valve30returns to the tank71, and no steering assist power is generated. In a case where the two valve members31,32rotate due to the steering resistance generated by steering operation to the right, as shown inFIG. 3, the degree of opening of the throttle portions A, A′ increases, the degree of opening of the throttle portions B, B′ decreases, the degree of opening of the throttle portions C, C′ decreases, and the degree of opening of the throttle portions D, D′ increases. As a result, the hydraulic10fluid, the pressure of which corresponds to the steering resistance, is supplied to the oil chamber22for generating right steering assist power in the hydraulic cylinder20due to the flow of the hydraulic fluid shown by arrows in the figure, while the hydraulic fluid flows back from the oil chamber23for generating left steering assist power to the tank71, with the result that the steering assist power for turning the vehicle to the right is applied from the hydraulic cylinder20to the rack16. In a case where the vehicle is steered to the left, the first valve member31and the second valve member32rotate in the direction opposite to the direction in the case where it is steered to the right, the degree of opening of the throttle portions A, A′ decreases, the degree of opening of the throttle portions B, B′ increases, the degree of opening of the throttle portions C, C′ increases, and the degree of opening of the throttle portions D, D′ decreases; therefore, the steering assist power for turning the vehicle to the left is applied from the hydraulic cylinder20to the rack16.

As shown inFIG. 1andFIG. 6, the variable throttle valve60communicating with the second outlet port61has a second valve housing7′ joined to the valve housing7, a spool (movable member)62inserted into an insertion hole66formed in the second valve housing7′ so as to be able to move linearly along the axial direction (up-down direction inFIG. 1andFIG. 6), a threaded member64screwed into the spool62, and a stepping motor80that rotates the threaded member64. Since the central axis of the threaded member64is parallel to the central axis of the spool62so that the two central axes are mutually eccentric, whereby the spool62is prevented from rotating concurrently with the threaded member64when the threaded member64rotates. The two central axes can also coincide with each other, if the concurrent rotation can be prevented, for example, by providing the second valve housing7′ with a protrusion that is fitted in an axial groove formed in the outer periphery of the spool62so as to be able to move relatively in the axial direction. A circumferential groove62ais formed in the outer periphery of the spool62, a circumferential groove66ais formed in the inner periphery of the insertion hole66, and the space between the two circumferential grooves62a,66aserves as a variable throttle portion67. The degree of opening of the variable throttle portion67is varied by the linear movement of the spool62in the axial direction.

A control device100constituted of a computer mounted in the vehicle is provided as means for driving the stepping motor80according to the number of steps. The control device100is connected to a vehicle speed sensor101that detects vehicle speed as a vehicle driving condition, and sends a pulse signal corresponding to the number of steps that corresponds to the detected vehicle speed to the motor80. As a result the stepping motor80is driven so as to be rotated according to the number of steps corresponding to the vehicle speed, and the stepping motor80rotates the threaded member64in one rotation direction when the vehicle speed is low to displace the spool62in one direction (downward direction in the figure), and rotates the threaded member64in the other rotation direction when the vehicle speed increases to displaces the spool62in the other direction (upward direction in the figure). As a result, because the spool62is displaced upward direction in the figure when the vehicle speed is high, the degree of opening of the variable throttle portion67increases as shown inFIG. 6, while because the spool62is displaced downward direction when the vehicle speed is low, the degree of opening thereof decreases as shown inFIG. 7.

A communication path58through which the circumferential groove66ain the inner periphery of the insertion hole66communicates with the second outlet port61is formed in the second valve housing7′. A diametrical hole62cthrough which the circumferential groove62ain the outer periphery of the spool62communicates with a through hole62dof the spool62is formed in the spool62. The through hole62dof the spool62communicates with a space below the spool62in the insertion hole66. A communication flow path76through which the space below the spool62communicates with the first outlet port36is formed in the valve housing7and the second valve housing7′ so as to be located by the spool62in the diametrical direction. A drain flow path62his formed parallel to the through hole62din the spool62so that the space above the spool62communicates with the space below the spool. As a result, the hydraulic fluid supplied from the pump70reaches the variable throttle portion67via the flow path27between the valve members, the second outlet port61, and the communication path58, and then reaches the tank71from the variable throttle portion67via the communication path76and the first outlet port36. As a result, a hydraulic circuit shown inFIG. 2is formed. To be more precise, the throttle portions A, B, C, D that belong to the first group in the hydraulic control valve30and the throttle portions A′, B′, C′, D′ that belong to the second group are parallel to each other. The variable throttle portion67of the variable throttle valve60is disposed in series with the throttle portions A′, B′, C′, D′ that belong to the second group between them and the tank71. The variable throttle valve60is connected in series with the throttle portions A′, B′, C′, D′ that belong to the second group so that it is possible to vary a ratio of the flow rate of hydraulic fluid that is controlled by the throttle portions A, B, C, D belonging to the first group to the flow rate of hydraulic fluid that is controlled by the throttle portions A′, B′, C′, D′ belonging to the second group. As a result, the hydraulic pressure for generating the steering assist power can be varied by varying the degree of opening of the variable throttle portion67.

As shown inFIG. 6andFIG. 7, one end of the insertion hole66is closed by a stopper68that is screwed into the second valve housing7′, and the other end of the insertion hole is closed by a cover94′. As shown inFIG. 7, the linear movement of the spool62in the one direction to decrease the degree of opening of the variable throttle portion67is stopped by the stopper68. The position at which the movement of the spool62is stopped by the stopper68is treated as an origin. The amount of screwing of the stopper68into the second valve housing7′ is able to be varied, so that the position of the origin can be adjusted so as to be varied. A compression coil spring90is disposed between the spool62and the stopper68to prevent the spool62from shaking.

The control device100stores a predetermined number of steps that is required to move the spool62by the stepping motor80to the origin as a number of steps for setting origin. It is preferred that the number of steps for setting origin is set on the assumption that the spool62is disposed in a position farthest from the origin, so that the spool62can be reliably moved to the origin when the control of the stepping motor80is started. In the present embodiment, the number of steps at the time when the spool62is disposed farthest from the origin is set to 217 steps, and the number of steps for setting origin is set to 220 steps. As a result, even after the movement of the spool62is stopped by the stopper68, the stepping motor80rotates in a step-out state; therefore the spool62can be reliably positioned in the origin.

The control device100stores a number of steps that is less than the number of steps for setting origin and is predetermined for moving the spool62in the one direction from a position at a time when the control of the stepping motor80starts as a number of steps for setting provisional origin. The number of steps for setting provisional origin in the present embodiment is set to 70 steps, but this number is not particularly limited, provided that it is less than the number of steps for setting origin and makes it possible to shorten the interval in which the control of the steering assist power according to the driving condition is restricted at the initial stage of the control.

The control device100sets a number of steps at a time when the driving of the stepping motor80according to the number of steps for setting provisional origin is completed to a provisional origin for calculation, on the assumption that the spool62is positioned in a provisional origin at this time.FIG. 8shows relationship of circuit voltage V of the control device100, ON/OFF state of an alarm lamp (not shown in the figure), vehicle speed v, and a position of the spool62from the origin to time t that is elapsed after the ignition switch is turned on. InFIG. 8, the vehicle speed v is treated as zero. In a state in which the steering assist power is not controlled according to the vehicle driving condition after the control with the control device100is started, the alarm lamp is lighted to warn the driver. When a circuit voltage V reaches a value Va necessary for the control with the elapse of time t0since the ignition switch of the vehicle is on, the alarm lamp is turned on and the control of the stepping motor80with the control device100is started. By driving the stepping motor80according to the number of steps, the movement of the spool62in the one direction starts from a position PAat the time when the control of the stepping motor80is started. When the stepping motor80is driven so as to be rotated by the number of steps for setting provisional origin (70 steps) with the elapse of time t1, the position PBof the spool62at this time is treated as the provisional origin, and the value of the number of steps at this time is set to, for example, zero as a provisional origin for calculation and stored in the control device100. As a result, the control device100becomes a state in which a number of steps from the provisional origin for calculation that corresponds to an amount of movement of the spool62from the provisional origin in the other direction is set according to vehicle driving condition, and the steering assist power is controlled according to the driving condition therefore, the alarm lamp is turned off. The spool62is positioned by driving the stepping motor80according to the number of steps from the provisional origin for calculation set according to the vehicle driving condition. Here, the amount of movement of the spool62from the provisional origin in the other direction increases with the increase in the vehicle speed v, and the hydraulic pressure acting on the hydraulic cylinder20decreases with the increase in the amount of movement of the spool62from the provisional origin in the other direction provided that the steering resistance is even. The vehicle driving condition in the present embodiment is vehicle speed, but other driving condition such as a steering angle can be also included in addition to the vehicle speed, or other driving condition such as a steering angle can be adopted instead of the vehicle speed as the vehicle driving condition. The control device100judges whether it is in a return to origin permission state in which conditions for return to the origin are satisfied, these conditions include a condition that the spool62is disposed in the provisional origin and a condition that the number of steps from the provisional origin for calculation set according to the vehicle driving condition is zero. In the present embodiment, the conditions for return to the origin are satisfied when the spool62is disposed in the provisional origin and the vehicle speed v is zero, but alternatively, the conditions for return to the origin can be also satisfied when the spool62is disposed in the provisional origin and the vehicle speed v is equal to or less than a value larger than zero. Further, the conditions for return to the origin can be also satisfied by satisfying an additional condition. For example, a condition that the control device100is not in a failsafe control mode is adopted as the additional condition, and the conditions for return to the origin are not satisfied in the failsafe control mode in which the control device100disposes the spool62in a position farthest from the origin in the other direction by the failsafe control when no signal from the vehicle speed sensor101is inputted into the control device100.

In the return to origin permission state, the control device100sets a number of steps at a time when the driving of the stepping motor80according to a number of steps obtained by subtracting the number of steps for setting provisional origin from the number of steps for setting origin is completed to an origin for calculation. As shown inFIG. 8, in a case where it is in the return to origin permission state when the provisional origin for calculation is set with an elapse of time t1, if the driving of the stepping motor80is continued and the stepping motor80is driven so as to be rotated by the number of steps (150 steps) obtained by subtracting the number of steps for setting provisional origin from the number of steps for setting origin in the return to origin permission state, the spool62is positioned in the origin Pcwith an elapse of time t2, and the value of the number of steps at this time is set to, for example, zero as an origin for calculation and stored in the control device100. After this origin for calculation is set, instead of the number of steps from the provisional origin for calculation, the number of steps from the origin for calculation corresponding to the amount of movement of the spool62from the origin in the other direction is set by the control device100according to the vehicle driving condition. As a result, the spool62is positioned by driving the stepping motor80according to the number of steps from the origin set according to the vehicle driving condition. Here, the amount of movement of the spool62from the origin in the other direction increases with the increase in the vehicle speed v, and the hydraulic pressure acting on the hydraulic cylinder20decreases with the increase in the amount of movement of the spool62from the origin in the other direction provided that the steering resistance is even.

In the control device100, when the return to origin permission state is dissolved during the driving of the stepping motor80according to the number of steps obtained by subtracting the number of steps for setting provisional origin from the number of steps for setting origin, a number of steps at this time is set to a new provisional origin for calculation on the assumption that the spool62is positioned in a new provisional origin at this time.

For example, as shown inFIG. 9, it is assumed that after the provisional origin for calculation is set with an elapse of time t1, the return to origin permission state is dissolved because the vehicle speed v exceeds zero at the time when time t3elapses, and the vehicle speed v reaches va, and then it becomes to the return to origin state when the vehicle speed v becomes to zero at the time when time t4elapses. In this case, a position PB′ of the spool62at the time when time t3elapses is treated as a new provisional origin, and a number of steps at this time is set to, for example, zero as a new provisional origin for calculation and stored anew in the control device100. As a result, after the elapse of time t3and before the elapse of time t4, the control device100sets the number of steps from the new provisional origin for calculation corresponding to the amount of movement of the spool62in the other direction from the provisional origin according to the vehicle speed v, and the spool62is positioned by the driving of the stepping motor80according to the number of steps from the new provisional origin for calculation. Because it becomes to the return to origin permission state with the elapse of time t4, the state in which the stepping motor80is driven so as to be rotated by the number of steps (150 steps) obtained by subtracting the number of steps for setting provisional origin from the number of steps for setting origin in the return to origin permission state is realized again, the spool62is positioned in the origin Pcwith an elapse of time t5, and a number of steps at this time is set to an origin for calculation that is set to, for example, zero and stored in the control device100.

The control of the stepping motor80is started when the vehicle speed v is equal to or less than a preset value vb, in the present embodiment, the control is started when the vehicle speed v is equal to or less than 5 km/h. For example, as shown inFIG. 10, in a case where the vehicle travels on a hill or the like at a speed vChigher than 5 km/h since the ignition switch is turned on, the control of the stepping motor80by the control device100is not started even if the circuit voltage V reaches the value Vanecessary for control with the elapse of time t0after turning on the ignition switch of the vehicle. The stepping motor80is driven according to the number of steps when the vehicle speed v becomes 5 km/h with an elapse of time t0′, and when the stepping motor80is driven so as to be rotated by the number of steps for setting provisional origin (70 steps) with an elapse of time t1′, the position PBof the spool62at this time is treated as a provisional origin, and a provisional origin for calculation is set. In a case where it is in the return to origin permission state after an elapse of the time t1′, if the driving of the stepping motor80is continued and the stepping motor is driven so as to be rotated by the number of steps (150 steps) obtained by subtracting the number of steps for setting provisional origin from the number of steps for setting origin in the return to origin permission state, the spool62is positioned in the origin Pcwith an elapse of time t2′, and the origin for calculation is set.

The flowcharts inFIG. 11andFIG. 12show the control procedure of the stepping motor80with the control device100. First, an origin setting flag is turned off (Step S1), the vehicle speed v is read (Step S2), it is judged whether the vehicle speed v is equal to or less than a preset value vb(Step S3), where the vehicle speed v is greater than vb, the processing flow returns to Step S2of the control sequence, and where the vehicle speed v is equal to or less than vb, the stepping motor80is driven (Step S4), and the spool62is moved in the one direction. Then, it is judged whether the number of steps N for rotation of the stepping motor80is reached to the number of steps for setting provisional origin (this number is 70 in the present embodiment) (Step S5), if not, the processing flow returns to Step S4of the control procedure and the driving is continued. If the number of steps N for rotation is reached to the number of steps for setting provisional origin in Step S5of the control procedure, the number of steps N for rotation at this time is set to zero as the provisional origin for calculation (Step S6), a value α obtained by adding the number of steps N for rotation at the present time to the number of steps for setting provisional origin is found (Step S7), the vehicle speed v is read (Step S8), and it is judged whether it is in the return to origin permission state (Step S9). If the number of steps N for rotation at the present time is zero and the vehicle speed v is zero, it is judged that it is in the return to origin permission state, and if not, it is judged that it is not in the return to origin permission state. When it is judged that it is in the return to origin permission state, the stepping motor80is driven (Step S10), the spool62is moved in the one direction, and it is judged whether the number of steps N for rotation of the stepping motor80is reached to a value obtained by subtracting the value α from the number of steps for setting origin (this number is 220 in the present embodiment) (Step S11), if it is not reached, the processing flow returns to Step S7of the control procedure. If the number of steps N for rotation is reached to the value obtained by subtracting the value α from the number of steps for setting origin in Step S11of the control procedure, the number of steps N for rotation at this time is set to zero as the origin for calculation (Step S12), the driving of the stepping motor80is stopped (Step S13), the origin setting flag is turned on (Step S14), and it is judged whether the control is to be ended, for example, according to a judgment whether the ignition switch is on or off (Step S15). In a case where it is judged that it is not in the return to origin permission state in Step S9of the control procedure, the number of steps N0′ from the provisional origin for calculation that corresponds to the amount of movement of the spool62in the other direction from the provisional origin is set according to the vehicle driving condition (Step S16). In the present embodiment, because the number of steps at the time when the spool62is disposed in a position farthest from the origin is set to 217 steps, the upper limit of N0′ is set so that N0′+α is equal to or less than 217 steps. Then, the stepping motor80is driven (Step S17), and it is judged whether the number of steps N for rotation of the stepping motor80is reached to N0′ (Step S18), if it is reached, the processing flow returns to Step S15of the control procedure and it is judged whether the control is to be ended, if it is not reached, the processing flow returns to Step S8of the control procedure. If the control is not to be ended in Step S15of the control procedure, it is judged whether the return to origin flag is on (Step S19), if the return to origin flag is not on, the process flow returns to Step S8of the control procedure, and if the return to origin flag is on, the vehicle speed v is read (Step S20), and the number of steps N0from the origin for calculation that corresponds to the amount of movement of the spool62from the origin in the other direction is set according to the vehicle driving condition (Step S21). Further, in the present embodiment, because the number of steps at the time when the spool62is disposed in the position farthest from the origin is set to 217 steps, the upper limit of N0is set such that N0becomes equal to or less than 217 steps. Then, the stepping motor80is driven (Step S22), and it is judged whether the number of steps N for rotation of the stepping motor80is reached to N0(Step S23), if it is reached, the driving of the stepping motor80is stopped (Step S24), and the processing flow returns to Step S15of the control procedure and it is judged whether the control is to be ended, if it is not reached, the processing flow returns to Step S20of the control procedure.

According to the above-described embodiment, in the initial stage of the control of the stepping motor80, when the driving of the stepping motor80according to the number of steps for setting provisional origin is completed, a number of steps at this time is set to a provisional origin for calculation, on the assumption that the spool62is positioned in the provisional origin at this time. The number of steps N0′ from the provisional origin for calculation corresponding to the amount of movement of the spool62from the provisional origin in the other direction is set according to the vehicle driving condition, and the spool62is positioned by driving the stepping motor80according to this number of steps N0′, whereby the steering assist power corresponding to the vehicle driving condition can be obtained. Because the number of steps for setting provisional origin is less than the number of steps for setting origin, the provisional origin for calculation can be set faster than the origin for calculation. As a result, the steering assist power corresponding to the vehicle driving condition can be obtained faster than in the conventional apparatuses, and it is possible to shorten the interval in which the control of the stepping motor80according to the driving condition is restricted at the initial stage of the driving of the vehicle. Although a difference can occur between the provisional origin for calculation and the origin for calculation because the provisional origin does not always matches the origin, the steering feeling can still be improved in comparison with a case in which the steering assist power does not at all correspond to the vehicle driving condition. Furthermore, the provisional origin for calculation matches the origin for calculation when the provisional origin matches the origin; therefore, sufficient steering feeling can be obtained within a short interval.

Further, because the spool62is positioned in the provisional origin in the return to origin permission state, the spool62is disposed in the origin when the driving of the stepping motor80according to the number of steps obtained by subtracting the number of steps for setting provisional origin from the number of steps for setting origin is completed, and the value of the number of steps at this time can be set to the origin for calculation. As a result, the provisional origin can be reliably matched with the origin. Moreover, because the origin for calculation is set when the vehicle driving condition under which the movement of the spool62from the provisional origin in the other direction is not required is satisfied, such setting can be performed without unfavorable effect on the steering feeling.

Further, when the return to origin permission state is dissolved during the driving of the stepping motor80according to the number of steps obtained by subtracting the number of steps for setting provisional origin from the number of steps for setting origin, a number of steps at this time is set to a new provisional origin for calculation, on the assumption that the spool62is positioned in a new provisional origin at this time as a result, even when the provisional origin does not match the origin, the difference between the provisional origin for calculation and the origin for calculation can be gradually decreased before the origin for calculation is set, and the unfavorable effect on the steering feeling can be reduced.

Because the vehicle driving condition includes the vehicle speed and the amount of movement of the spool62from the provisional origin or the origin in the other direction increases with increase in the vehicle speed, the vehicle speed is usually zero or close to zero when the control of the stepping motor80is completed; therefore, the spool62is disposed in the origin or close thereto. As a result, in the usual case, the provisional origin can be matched with the origin by stopping the movement of the spool62with the stopper when the driving of the stepping motor80according to the number of steps for setting provisional origin is completed in the initial stage of the next control, and thus sufficient steering feeling can be obtained within a short period.

The hydraulic pressure acting on the hydraulic cylinder20for generating steering assist power decreases with the increase in the amount of movement of the spool62from the provisional origin or the origin in the other direction, and the control of stepping motor80starts when the vehicle speed becomes equal to or less than a preset value therefore, when the ignition switch is turned on during the vehicle travels by inertia, for example, in downhill, the spool62does not move in the direction for increasing the steering assist power to set the provisional origin for calculation, and thus the increase in the steering assist power at the time when the vehicle speed increases can be prevented and the steering stability can be maintained.

Further, the maximum value of flow passage area corresponding to the degree of opening of the variable throttle portion67is equal to or more than the maximum value of flow passage area of the throttle portions A′, B′, C′, D′ that belong to the second group (it is a maximum value of total flow passage area of the throttle portions B′, C′ during steering to the right, and it is a maximum value of total flow passage area of the throttle portions A′, D′ during steering to the left), or is increased to a value so that it does not function as a throttle. In the present embodiment, the minimum value of the flow passage area of the variable throttle portion67is zero. InFIG. 13, a solid line X shows a variation characteristic of the flow passage area of the throttle portions A, B, C, D that belong to the first group to the relative rotation angle of the two valve members31,32(it is a variation characteristic of the total flow passage area of the throttle portions B, C during steering to the right, and it is a variation characteristic of the total flow passage area of the throttle portions A, D during steering to the left). An alternate long and short dash line U shows a variation characteristic of the flow passage area of the throttle portions A′, C′ to the relative rotation angle of the two valve members. An alternate long and short dash line V shows a variation characteristic of the flow passage area of the throttle portions B′, D′ to the relative rotation angle of the two valve members. A solid line Y shows a characteristic obtained by combining the variation characteristic of the flow passage area of the throttle portions A′, C′ and the variation characteristic of the flow passage area of the throttle portions B′, D′. A broken line R shows the flow passage area of the variable throttle portion67at the time when the vehicle travels at a medium speed. When the vehicle travels at a low speed, the variable throttle portion67is completely closed; therefore, the hydraulic pressure acting on the hydraulic cylinder20is controlled according to the variation characteristic line X of the flow passage area of the throttle portions A, B, C, D of the first group. When the vehicle travels at a high speed, the flow passage area of the variable throttle portion67reaches a maximum, and the hydraulic pressure acting on the hydraulic cylinder20is controlled according to the combined characteristic of the variation characteristic line Y of the flow passage area of the throttle portions A′, B′, C′, D′ of the second group and the variation characteristic line X of the flow passage area of the throttle portions A, B, C, D of the first group. When the vehicle travels at a medium speed, the flow passage area of the variable throttle portion67is larger than the minimum value and smaller than the maximum value of the flow passage area of the throttle portions A′, B′, C′, D′ that belong to the second group, and so the steering assist power is applied according to the characteristic obtained by combining the characteristic line R of the flow passage area of the variable throttle portion67with the variation characteristic line X of the flow passage area of the throttle portions A, B, C, D that belong to the first group until the flow passage area of the throttle portions A, B, C, D that belong to the first group becomes zero (until the relative rotation angle becomes θainFIG. 13), the steering assist power becomes a constant value determined by the flow passage area of the variable throttle portion67after the throttle portions A, B, C, D that belong to the first group are completely closed and before the flow passage area of the throttle portions A′, B′, C′, D′ that belong to the second group becomes less than the flow passage area of the variable throttle portion67(between θaand θbof the relative rotation angle inFIG. 13), and then the steering assist power is applied according to the variation characteristic line Y of the flow passage area of the throttle portions A′, B′, C′, D′ that belong to the second group after the flow passage area of the throttle portions A′, B′, C′, D′ that belong to the second group becomes less than the flow passage area of the variable throttle portion67.