Steering system for vehicle

A vehicle steering system including: (a) a steering operation member; (b) a movement-amount variably transmitting device including: (b-1) a housing fixed to a vehicle body; (b-2) a differential mechanism including (i) a first element connected to the steering operation member, (ii) a second element movable relative to the first element, and (iii) a third element engaged with the first and second elements; and (b-3) a drive source for moving the third element, such that an amount of relative movement of the first and second elements is changeable depending on an amount of movement of the third element; and (c) a wheel turning device connected to the second element to turn a vehicle wheel. The movement-amount variably transmitting device further includes (b-4) a third-element-movement inhibiting device for inhibiting movement of the third element. The steering system further includes (d) a third-element-movement inhibition cancelling device for cancelling inhibition of the movement of the third element.

TECHNICAL FIELD

The present invention relates in general to a steering system that is to be disposed in a vehicle, and more particularly to a steering system having a function enabling change of a ratio of a wheel turning amount of a wheel turning device to an operating amount of a steering operation member.

BACKGROUND ART

In these days, there is a study of, as a steering system that is to be provided in a vehicle, a steering system as disclosed in Patent Documents identified below, which has a so-called VGRS (Variable Gear Ratio Steering) function, i.e., a variable-gear-ratio transmission device enabling change of a ratio of a wheel turning amount of a wheel turning device to an operating amount of a steering operation member (hereinafter abbreviated to as “operation member” where appropriate) such as steering wheel. The system disclosed in Patent Document 1 has a construction in which a housing of the variable-gear-ratio transmission device is rotated together with rotation of a steering shaft. This construction requires drive-source connection cables to be arranged in such a manner that allows the rotation of the housing together with the rotation of the steering shaft. In each of the systems disclosed in Patent Documents 2 and 3, the housing of the transmission device is fixed to the wheel turning device so as not to be rotatable. This construction eliminates necessity of a particular arrangement of the drive-source connection cables, thereby permitting the system as a whole to have a simplified construction.

DISCLOSURE OF THE INVENTION

(A) Outline of the Invention

The variable-gear-ratio transmission device with the VGRS function, as disclosed in the above-identified Patent Documents, includes a drive source and a differential mechanism that includes a movable element movable by the drive source. In general, such a transmission device has a construction in which movement of the movable element of the differential mechanism can be inhibited by a locking mechanism so that an operation applied to the operation member can be transmitted to the wheel turning device at a fixed value as a transmission ratio. This construction is designed in view of a risk that the operation applied to the operation member is not transmitted to the wheel turning device, for example, in event of failure of the drive source. However, each of the systems of Patent Documents 2 and 3 has a problem that a so-called deadlock could be caused by operation of the locking mechanism in a case in which the elements of the differential mechanism are stuck to each other, for example, by entrance of foreign object therebetween, thereby making it difficult to steer the wheel. This is one of problems encountered in the system having the variable-gear-ratio transmission device, described in detail, the system having the variable-gear-ratio transmission device the housing of which is fixed to a body of the vehicle. In the system equipped with such a transmission device, there is still room for improvement for increasing the utility, such as dealing with the above problem. The present invention was made in the light of the background art discussed above, and has an object of the invention to provide a steering system having a high serviceability in the practical use.

For achieving the above object, a vehicle steering system of the present invention is a steering system which has a movement-amount variably transmitting device whose housing is fixedly provided in a body of a vehicle, and which is characterized in that the variable-gear-ratio transmission device has an arrangement in which an amount of a relative movement of a first element connected to a steering operation member and a second element connected to a wheel turning device is changeable depending on an amount of movement of a third element engaged with the first and second elements, and in that the variable-gear-ratio transmission device has a third-element-movement inhibiting device capable of inhibiting the movement of the third element and a third-element-movement inhibition cancelling device configured to cancel inhibition of the movement of the third element, which is made by the third-element-movement inhibiting device.

According to the vehicle steering system of the present invention, even in a case in which elements of a differential mechanism are stuck to each other, for example, the third element can be allowed to be moved by the third-element-movement inhibition cancelling device that is operated to cancel inhibition of the movement of the third element, thereby enabling a vehicle operator to appropriately perform a steering operation.

(B) Modes of the Claimable Invention

There will be described various modes of the invention (hereinafter referred to as “claimable invention” where appropriate) deemed to contain claimable features for which protection is sought. Each of these modes of the invention is numbered like the appended claims and depends from the other mode or modes, where appropriate, for easier understanding of the technical features disclosed in the present specification. It is to be understood that the claimable invention is not limited to the technical features or any combinations thereof which will be described in each of these modes. That is, the scope of the claimable invention should be interpreted in the light of the following descriptions accompanying the various modes and preferred embodiments of the invention. In a limit in accordance with such an interpretation, a mode of the claimable invention can be constituted by not only any one of these modes but also either a mode provided by any one of these modes and additional component or components incorporated therein and a mode provided by any one of these modes without some of components recited therein. It is noted that mode (1) is a mode serves a base of a vehicle steering system as the claimable invention, and the claimable invention can be constituted by mode (1) to which technical features recited in suitably selected one or ones of the other modes are added.

Relationships between the below-described modes and claims are: mode (41) citing mode (1) corresponds to claim1; claim1to which technical features of mode (42) are added corresponds to claim2; claim2to which technical features of mode (5) are added corresponds to claim3; claim3to which technical features of mode (6) are added corresponds to claim4; claim2or4which technical features of modes (7), (9), (10) and (11) are added corresponds to claim5; claim5to which technical features of mode (12) are added corresponds to claim6; claim5or6to which technical features of mode (13) are added corresponds to claim7; claim1to which technical features of mode (43) are added corresponds to claim8; claim8to which technical features of mode (20) are added corresponds to claim9; claim8or9to which technical features of mode (23) are added corresponds to claim10; claim8or10to which technical features of mode (24) are added corresponds to claim11; claim11to which technical features of mode (25) are added corresponds to claim12; claim11or12to which technical features of mode (26) are added corresponds to claim13; any one of claims11through13to which technical features of mode (27) are added corresponds to claim14; any one of claims11through14to which technical features of modes (31) and (32) are added corresponds to claim15; and claim1or15to which technical features of mode (2) are added corresponds to claim16.

(1) A steering system for a vehicle, including:

a steering operation member operable by an operator of the vehicle;

a movement-amount variably transmitting device having: a housing fixedly provided in a body of the vehicle; a differential mechanism constructed to include (a) a first element movably disposed in the housing and connected to the steering operation member so as to be moved by an amount corresponding to an operating amount of the steering operation member, (b) a second element disposed in the housing and movable relative to the first element, and (c) a third element engaged with the first and second elements; and a drive source fixedly provided in the housing and configured to move the third element, such that an amount of a relative movement of the first and second elements is changeable depending on an amount of a movement of the third element; and

a wheel turning device connected to the second element so as to turn a wheel of the vehicle by a wheel turning amount corresponding to an amount of a movement of the second element,

wherein the movement-amount variably transmitting device has a third-element-movement inhibiting device capable of inhibiting the movement of the third element.

As described above, the present mode is a mode reciting components common to various forms of the claimable invention, and has a significance as a base mode for the claimable invention. The steering system of the present mode is, described plainly, a steering system having a VGRS actuator fixedly provided in the vehicle body, wherein the VGRS actuator is equipped with the above-described locking mechanism.

The “wheel turning device” recited in this mode has a construction that is not particularly limited. It is therefore possible to employ any one of various known constructions such as a construction including a steering rod interconnecting wheels and a mechanism configured to axially move the steering rod. As the mechanism for moving the steering rod, for example, a rack pinion mechanism or a ball screw mechanism can be employed.

The “movement-amount variably transmitting device” recited in the present mode is a so-called VGRS actuator that is capable of changing the amount of the relative movement of the first and second elements, i.e., a transmission ratio, based on a parameter such as a running speed of the vehicle. The “housing” constituting the movement-amount variably transmitting device may be fixed to a portion that is not particularly limited, as long as it is fixedly disposed in the vehicle body. As the housing, it is therefore possible to employ any one of various housings such as a housing fixed to a portion of the vehicle body, a housing fixed to a housing of the wheel turning device where the wheel turning device is arranged to move the steering rod in the axial direction in the system, and a housing fixed to a steering column where the system is equipped with the steering column that is constructed to include a tube and a shaft rotatably held in the tube. The “differential mechanism” constituting the movement-amount variably transmitting device has a construction that is not particularly limited, and may be a mechanism, for example, in which the first and second elements are rotatable relative to each other while the third element is engaged with the first and second elements and is rotatable. Specifically, it is possible to employ any one of various known mechanisms such as a mechanism constituted by bevel gears meshing with each other, a planetary gear mechanism, a harmonic gear mechanism (of either two-ring-gears type or cup type) and a cycloid speed reduction mechanism. Further, as the “drive source” that is fixedly provided in the housing, it is possible to employ, for example, an electric motor having a stator fixed to the housing.

The above-described movement-amount variably transmitting device has the “third-element-movement inhibiting device” (e.g., locking mechanism) that is capable of inhibiting the movement of the third element. In the present mode, the third-element-movement inhibiting device can be configured to inhibit the movement of the third element in event of failure of the movement-amount variably transmitting device such as a case of failure of the drive source per se and a case of activation of a protection circuit in response to an excessively large load applied to the drive source. In a case of failure allowing a generally free movement of the third element, an operation applied to the operation member is not transmitted to the wheel turning device. In such a case of the failure, the transmission of the movement from the first element to the second element via the third element can be made with a certain constant transmission ratio, namely, with a predetermined amount of the relative movement of the first and second elements, by inhibiting the movement of the third element by the third-element-movement inhibiting device. Thus, the operation of the operation member can be transmitted to the wheel turning device. That is, the present mode is a mode effective in the system in which the general free movement of the third element is allowed by some factor. It is noted that the third-element-movement inhibiting device has a construction that is not particularly limited and that it is possible to employ any one of various known constructions that make it possible to substantially limit the movement of the third element.

Further, there is also a case of occurrence of failure that disables the third element from being moved by application of a drive force to the third element. Specifically described, for example, in event of failure caused by entrance of foreign object into between the third element and the first or second element, the differential mechanism is placed in a state in which elements of the differential mechanism are stuck to each other, in other words, in a state (hereinafter referred to as “relative-movement disabled state” where appropriate) in which as if the first and second elements were fixed to each other. In such an event, the inhibition of the movement of the third element engaged with the first and second elements would cause a so-called deadlock making it difficult to steer the wheel. Where the housing of the movement-amount variably transmitting device is not fixed to the vehicle body, the wheel can be steered by the operation member even in event of occurrence of the deadlock in the differential mechanism. Therefore, the deadlock is problematic, particularly, in the steering system having the movement-amount variably transmitting device the housing of which is fixed to the vehicle body.

In each of some of the below-described modes, there is a construction that cancels the inhibition of the movement of the third element which is made by the third-element-movement inhibiting device, for the purpose of coping with the above-described relative-movement disabled state. This construction is not particularly limited, and can be any one of various constructions, for example, such as (A) a construction in which the movement of the third element is allowed while the movement of the third element is inhibited by the third-element-movement inhibiting device and (B) a construction in which the third-element-movement inhibiting device is controlled such that the inhibition of the movement of the third element is not performed by the third-element-movement inhibiting device or such that the inhibition of the movement of the third element is released. Owing to such a construction allowing the movement of the third element, even in the relative-movement disabled state, the transmission of the movement from the first element to the second element can be made without the relative movement of the first and second elements. In other words, the wheel can be steered by the operation member with a state of the transmission ratio of 1:1, i.e., with a state in which the movement is transmitted from the first element to the second element at a constant movement ratio of substantially 1:1.

(2) The steering system according to mode (1), wherein the first element and the second element are constructed to include respective circular splines having respective numbers of teeth that are different from each other, and are rotatable, wherein the third element is constructed to include a flexspline which meshes with the circular splines and a wave generator on which the flexspline is fitted, and wherein the drive source is a motor configured to rotate the wave generator, so that the movement-amount variably transmitting device is constructed to include a harmonic gear mechanism.

The present mode is a mode in which the differential mechanism of the movement-amount variably transmitting device is limited to the harmonic gear mechanism (that is called also a harmonic drive mechanism (registered trademark) or a strain wave gearing mechanism). Described in detail, it is a mode in which the differential mechanism is limited to the harmonic gear mechanism having two ring gears. This harmonic gear mechanism is a speed changing mechanism capable of providing a large speed reduction ratio, so that the steering system can be made compact owing to reduction of the drive source in size, according to the preset mode.

(3) The steering system according to mode (1) or (2), wherein the third-element-movement inhibiting device has an engaged portion which is provided on the third element and an engaging portion which is provided in the housing and which is engageable with the engaged portion, such that the movement of the third element is inhibited by engagement of the engaging portion with the engaged portion.

The present mode is a mode in which a limitation is added to construction of the third-element-movement inhibiting device. The third-element-movement inhibiting device recited in the present mode has a construction in which the movement of the third element is inhibited by an interaction of the engaging portion and the engaged portion, namely, a construction including a stopper mechanism.

(4) The steering system according to any one of modes (1)-(3), wherein the movement-amount variably transmitting device has a third-element-movement allowing mechanism configured to allow the movement of the third element when an amount of a movement force forcing the third element to be moved becomes larger than a predetermined threshold during inhibition of the movement of the third element, which inhibition is made by the third-element-movement inhibiting device.

When the movement of the third element is inhibited in the above-described relative-movement disabled state in which the first and second elements are disabled from being moved relative to each other, there is a case in which a large force is applied from the first or second element to the third element to force the third element to be moved. That is, the term “when an amount of a movement force forcing the third element to be moved becomes larger than a predetermined threshold during inhibition of the movement of the third element”, which is recited in the present mode, is a case of occurrence of a phenomenon that could occur in the relative-movement disabled state, for example. In this sense, it can be considered that the present mode is a mode for coping with the relative-movement disabled state. Further, the present mode is a mode allowing the movement of the third element while leaving the third-element-movement inhibiting device to be activated for inhibiting the movement of the third element. According to the present mode, even in the relative-movement disabled state, the transmission of the movement can be made without the relative movement of the first and second elements, by allowing the movement of the third element, thereby enabling a vehicle operator to appropriately perform a steering operation.

The “movement force” recited in the present mode means a force forcing the third element to be moved, and corresponds to a force which acts on the third element via the first element and which is based on the operating force applied to the operation member, and a force which acts on the third element via the second element and which is based on a force inversely applied from the wheel turning device, for example. The “third-element-movement allowing mechanism” recited in the present mode may be, for example, (A) a mechanism which allows the movement of the third element when the movement force overcomes a force that is generated by the third-element-movement inhibiting device to inhibit the movement of the third element, or (B) a mechanism which is constituted principally by a device such as an electromagnetic clutch such that the movement of the third element is allowed by controlling the device. Where the third-element-movement allowing mechanism is a mechanism operable by a control, the mechanism requires a control device, a drive circuit and the like. Therefore, from a point of view of establishment of simplicity of the movement-amount variably transmitting device, it is preferable that the third-element-movement allowing mechanism is a mechanism which allows the movement of third element by effect of the movement force.

(5) The steering system according to mode (4),

wherein the third-element-movement inhibiting device has an engaged portion which is provided on the third element and an engaging portion which is provided in the housing and which is engageable with the engaged portion, such that the movement of the third element is inhibited by engagement of the engaging portion with the engaged portion,

and wherein the third element is held in frictional engagement with the engaged portion, and the third-element-movement allowing mechanism is constituted by a construction in which the movement of the third element is allowed in a case in which the movement force overcomes a frictional force acting between the third element and the engaged portion.

The present mode is a mode which has a limitation that the third-element-movement inhibiting device has a particular stopper mechanism while the third-element-movement allowing mechanism is a mechanism allowing the movement of the third element by effect of the above-described movement force. Described in detail, the frictional force acting between the third element and the engaged portion is set to have a predetermined amount, so that the movement of the third element relative to the engaged portion is allowed with the engaged portion being held in engagement with the engaging portion, when the amount of the movement force is larger than the predetermined amount of the frictional force. That is, in the present mode, the third-element-movement allowing mechanism is not a mechanism requiring some device to be controlled, so that the movement-amount variably transmitting device can have a relatively simple construction.

(6) The steering system according to mode (5), wherein the third element is rotatable, wherein the engaged portion is provided in a rotatable member that is rotatable relative to the third element, and wherein the third-element-movement allowing mechanism is constructed to include a tolerance ring that is interposed between the rotatable member and the third element.

The present mode is a mode in which the construction of the third-element-movement allowing mechanism is further limited. In the present mode, the third-element-movement allowing mechanism is constructed by simply adding the tolerance ring to the movement-amount variably transmitting device equipped with the third-element-movement inhibiting device that has the stopper mechanism, so that the movement-amount variably transmitting device can be made compact with a simple construction.

(7) The steering system according to any one of modes (1)-(6), including an assisting mechanism configured to assist a wheel turning force for turning the wheel, by an assisting force generated by the assisting mechanism.

(8) The steering system according to mode (7), wherein the assisting mechanism is provided in the wheel turning device.

(9) The steering system according to mode (7) or (8), including a control device configured to control the steering system, wherein the control device is configured to execute an assisting control for controlling the assisting force generated by the assisting mechanism, based on an operating force of the steering operation member.

Each of the above-described modes is a mode relating to a system having a function of assisting a wheel turning force applied to the wheel by, in addition to the operating force applied to the operation member, a force generated by other drive source, namely, a mode relating to a so-called power steering system. It is noted that it is possible to employ, as the “control device”, an electronic control unit which is constituted principally by a computer and which includes a drive circuit for the drive source as needed.

(10) The steering system according to mode (9), wherein the control device is configured to execute an assisting-force increasing control for increasing the assisting force in a state in which the movement of the third element is inhibited by the third-element-movement inhibiting device.

When the movement of the third element is inhibited by the third-element-movement inhibiting device, the transmission ratio of the movement-amount variably transmitting device is fixed so that the operation of the operation member is transmitted to the wheel turning device at a constant transmission ratio. Therefore, there could be a situation in which the operation member has to be operated by an increased amount, and a burden imposed on the operation is possibility increased in such a situation. According to the present mode, when the movement of the third element is inhibited by the third-element-movement inhibiting device, the wheel turning force is assisted by a larger degree thereby reducing the burden imposed on the steering operation performed by the vehicle operator.

(11) The steering system according to mode (10),

wherein the movement-amount variably transmitting device has a third-element-movement allowing mechanism configured to allow the movement of the third element in a case in which an amount of a movement force forcing the third element to be moved becomes larger than a predetermined threshold in a state in which the movement of the third element is inhibited by the third-element-movement inhibiting device,

and wherein the control device is configured to execute the assisting-force increasing control in a relative-movement disabled state in which the first element and the second element are disabled from being moved relative to each other.

As described above, the inhibition of the movement of the third element in the relative-movement disabled state would cause a so-called deadlock in the differential mechanism of the movement-amount variably transmitting device, thereby making it difficult to perform the steering operation. The third-element-movement allowing mechanism is provided with this being taken into account, and enables the movement to be transmitted with the first and second elements being fixed to each other, thereby enabling the wheel to be turned by the operation member. However, since the third-element-movement allowing mechanism works when the movement force acting on the third element becomes larger than a predetermined threshold, the operating force applied to the operation member has to be made large by a certain degree in order to turn the wheel in the relative-movement disabled state. This increases the burden imposed on the vehicle operator who performs the steering operation. According to the present mode, when the wheel is turned by using the third-element-movement allowing mechanism in the relative-movement disabled state, the wheel turning force is effectively assisted whereby the burden imposed on the vehicle operator is effectively reduced.

(12) The steering system according to mode (11), including a relative-movement amount detector configured to detect the amount of the relative movement of the first and second elements,

wherein the control device is configured to execute the assisting-force increasing control, based on the amount of the relative movement detected by the relative-movement amount detector.

It is possible to consider that the first and second elements are disabled from being substantially moved relative to each other, for example, when the amount of the relative movement of the first and second elements is substantially 0 (zero), in other words, when the amount of the movement of the first element and the amount of the movement of the second element are substantially equal to each other. According to the present mode, the relative-movement disabled state can be easily detected by detecting the amount of the relative movement of the first and second elements. Further, according to the present mode, the assisting force can be increased by the assisting mechanism based on the detection. The relative-movement amount detector recited in the present mode is capable of reliably detecting the relative-movement disabled state, so that it is possible to further effectively assist the wheel turning force.

As described above, the amount of the relative movement of the first and second elements is controllable to be changed depending on a parameter such as the running speed of the vehicle. Therefore, during such a control executed on the movement-amount variably transmitting device, a state in which the amount of the relative movement of the first and second elements is substantially 0 (zero) could be established at a certain point of time. However, in the relative-movement disabled state, the amount of the relative movement of the first and second elements is held at substantially 0 (zero) irrespective of a value of the above-described parameter, so that it is possible to recognize the relative-movement disabled state based on this fact. Specifically described, for example, during execution of the control in which the transmission ratio of the movement-amount variably transmitting device can be changed depending on the vehicle running speed, it is possible to recognize the relative-movement disabled state, when the amount of the relative movement of the first and second elements is held at substantially 0 (zero) in spite of change of the vehicle running speed.

As the “relative-movement amount detector” recited in the present mode, for example, it is possible to employ a detector configured to detect the relative movement amount based on the movement amount of the first element and the movement amount of the second element. The movement amount of the first element and the movement amount of the second element may be detected either directly or indirectly. For example, the movement amount of the first element may be detected from an operating amount of the operation member such as an angular position of a steering wheel, while the movement amount of the second element may be detected from a wheel tuning amount such as an amount of displacement of the steering rod.

(13) The steering system according to mode (11) or (12),

wherein the third-element-movement inhibiting device has an engaged portion which is provided on the third element and an engaging portion which is provided in the housing and which is engageable with the engaged portion, such that the movement of the third element is inhibited by engagement of the engaging portion with the engaged portion,

wherein the third-element-movement allowing mechanism is constituted by a construction in which the movement of the third element is allowed when the movement force overcomes a frictional force acting between the third element and the engaged portion that are held in frictional engagement with each other, and

wherein the assisting-force increasing control is a control executed to increase the assisting force in the assisting control, based on the frictional force.

The present mode is a mode of a case in which the third-element-movement allowing mechanism has a construction utilizing effect of the above-described frictional force. Where the third-element-movement allowing mechanism recited in the present mode is employed, the operation force has to be increased by an amount corresponding to the frictional force, so as to make it possible to turn the wheel in the relative-movement disabled state. With this being taken into account, in the present mode, the force assisting the wheel turning force is determined based on the above-described frictional force, so that it is possible to appropriately assist the wheel turning force. It is noted that, in the present mode, the assisting force may be increased by an amount corresponding to the friction force, i.e., by an amount precisely offsetting the frictional force, or also may be increased by an amount corresponding to a part of the frictional force, i.e., by an amount offsetting some percentage of the frictional force.

(14) The steering system according to any one of modes (11)-(13), wherein the control device is configured to control also operation of the drive source of the movement-amount variably transmitting device, and is configured, upon execution of the assisting-force increasing control, to establish a state in which the drive source does not provide a resistance against the movement of the third element.

For example, when the wheel is turned in the relative-movement disabled state by utilizing the above-described third-element-movement allowing mechanism, the drive source connected to the third element could provide a resistance against the turning of the wheel. Described specifically, where the drive source is an electric motor, the motor generates a relatively large braking force based on an electromotive force when input terminals of the motor are short-circuited, and does not generate the braking force based on the electromotive force so as not to provide the resistance against the turning of the wheel when the input terminals of the motor are opened from each other. The present mode is a mode in which the drive source is caused not to provide the resistance against the turning of the wheel. According to the present mode, it is possible to minimize the burden imposed on the steering operation.

(15) The steering system according to any one of modes (9)-(14), including an operating force detector having a deformable member that is elastically deformed depending on the operating force of the steering operation member, and a deformation amount sensor that is configured to detect an amount of deformation of the deformable member, so as to detect the operating force of the steering operation member based on the deformation amount detected by the deformation amount sensor,

wherein the control device is configured to execute the assisting control, based on the operating force of the steering operation member that is detected by the operating force detector.

According to the present mode, the operating force of the operation member can be suitably detected, whereby the wheel turning force can be effectively assisted. In the present mode, specifically, a torsion bar can be employed as the “deformable member”, and a relative-rotation amount sensor configured to detect an amount of relative displacement (relative rotation) of opposite end portions of the torsion bar can be employed as the “deformation amount sensor”. Thus, the “operating force detector” can be configured to detect the operating force of the operation member based on an amount of twisting of the torsion bar that is obtained by the deformation amount sensor.

(16) The steering system according to mode (15), wherein the operating force detector is provided in the movement-amount variably transmitting device.

As in the present mode, where the operating force detector is built in the movement-amount variably transmitting device, the steering system can be made compact.

(17) The steering system according to mode (15) or (16), wherein one and the other of opposite end portions of the deformable member are connected to the steering operation member and the first element, respectively, and wherein the deformation amount sensor is configured to detect an amount of relative displacement of the one and the other of opposite end portions of the deformable member.

(18) The steering system according to mode (15) or (16), wherein one and the other of opposite end portions of the deformable member are connected to the wheel turning device and the second element, respectively, and wherein the deformation amount sensor is configured to detect an amount of relative displacement of the one and the other of opposite end portions of the deformable member.

Each of the above-described two modes recites an additional limitation as to a portion in which the deformable member constituting the operating force detector is provided. Particularly, the latter mode is effective to a case in which the wheel is turned in the relative-movement disabled state by utilizing the third-element-movement allowing mechanism. Since the third-element-movement allowing mechanism works when the movement force acting on the third element becomes a predetermined threshold, as described above, it is not possible to effectively detect an increase amount of the operating force that is required for application of the movement force of the predetermined amount, where the deformable member is disposed between the second element and the wheel turning device. Described in detail, where the deformable member is disposed on a downstream side of a portion that resists the steering operation, it is not possible to expect an increase of the amount of deformation of the deformable member that corresponds to an increase of the operating force. Therefore, in the latter one of the above-described two modes, the execution of the assisting-force increasing control provides a great merit, since the wheel turning force can be suitably assisted by the assisting-force increasing control even if the increase of the operating force is not suitably detected by the operating force detector.

On the other hand, in the former one of the above-described two modes, the execution of the assisting-force increasing control provides some degree of merit that is not sufficiently great as the merit in the latter mode, since the increase of the operating force can be detected by the operating force detector in the case in which the wheel is turned in the relative-movement disabled state by utilizing the third-element-movement allowing mechanism. However, this can be taken positively in the former mode. That is, the former mode is a mode advantageous from a point of view of simplicity and convenience of the assisting control, since the wheel turning force can be relatively satisfactorily assisted even without execution of the assisting-force increasing control.

(19) The steering system according to any one of modes (1)-(18), including a control device configured to control the steering system, wherein the control device is configured to execute a third-element-movement inhibiting control for inhibiting the movement of the third element by controlling operation of the third-element-movement inhibiting device.

Where the third-element-movement inhibiting device is controlled by the control device as in the present mode, the inhibition of the movement of the third element can be suitably performed, and the inhibition of the movement can be easily released as described later.

(20) The steering system according to mode (19), wherein the control device is configured to execute the third-element-movement inhibiting control in event of failure of the movement-amount variably transmitting device that disenables the drive force from moving the third element.

The term “failure that disenables the drive force from moving the third element” recited in the present mode broadly encompasses various cases such as a case of failure of the drive source per se, a case of disconnection of wire connecting an electrically operated drive source as the drive source and an electric power source and a case of activation of a protection circuit in response to an excessively large load applied to the drive source. As described above, in a case of failure allowing generally free movement of the third element, the operation applied to the operation member is not transmitted to the wheel turning device, but the turning of the wheel can be made with a certain transmission ratio, by inhibiting the movement of the third element by the third-element-movement inhibiting device. It is noted that the above-described failure includes also the above-described relative-movement disabled state, i.e., a state in which the first and second elements are disabled from being moved relative to each other, for example, due to sticking of elements of the differential mechanism.

(21) The steering system according to mode (19) or (20), wherein the control device is configured, in a relative-movement disabled state in which the first element and the second element are disabled from being moved relative to each other, to execute a third-element-movement non-inhibiting-state establishing control for controlling the third-element-movement inhibiting device so as to establish a state in which the movement of the third element is not inhibited.

According to the present mode, the movement of the third element is not inhibited by the third-element-movement inhibiting device even in event of the above-described relative-movement disabled state. Thus, the movement transmission can be made without the relative movement of the first and second elements, whereby the turning of the wheel by operation of the operation member is assured. That is, according to the present mode, it is possible for the vehicle operator to appropriately perform the steering operation. It is noted that the “third-element-movement non-inhibiting-state establishing control” recited in the present mode may be either a control executed for releasing a state in which the movement of the third element is inhibited by the third-element-movement inhibiting device or a control executed for preventing the movement of the third element from being inhibited before the inhibition, as described later. In the latter control, i.e., a control in which the inhibition of the movement of the third element is not made by the third-element-movement inhibiting device, a deadlock is not caused in any moment in event of failure caused by the relative-movement disabled state, thereby making it possible to reduce discomfort given to the vehicle operator during the steering operation.

(22) The steering system according to mode (21), wherein the control device is configured to control also operation of the drive source of the movement-amount variably transmitting device, and is configured, upon execution of the third-element-movement non-inhibiting-state establishing control, to establish a state in which the drive source does not provide a resistance against the movement of the third element.

As described above, when the wheel is turned in the relative-movement disabled state by utilizing the above-described third-element-movement allowing mechanism, the drive source connected to the third element could provide a resistance against the turning of the wheel. According to the present mode with this being taken into account, it is possible to minimize the burden imposed on the steering operation. Since description about the present mode overlaps the description of the previous mode, the description is omitted herein.

(23) The steering system according to mode (21) or (22), including a relative-movement amount detector configured to detect the amount of the relative movement of the first element and the second element,

wherein the control device is configured to execute the third-element-movement non-inhibiting-state establishing control, based on the amount of the relative movement detected by the relative-movement amount detector.

The present mode may be, for example, a mode in which the above-described relative-movement disabled state is detected based on the amount of the relative movement of the first and second elements and then third-element-movement non-inhibiting-state establishing control is executed based on an outcome of the detection. It is therefore possible to reliably establish a state in which the movement of the third element is not inhibited in the relative-movement disabled state. Since description of the present mode overlaps the previous description about the relative-movement amount detector, the description is omitted herein.

(24) The steering system according to any one of modes (21)-(23), wherein the control device is configured, when the movement of the third element is inhibited by the third-element-movement inhibiting device in the relative-movement disabled state in which the first element and the second element are disabled from being moved relative to each other, to execute the third-element-movement non-inhibiting-state establishing control, by executing a third-element-movement inhibition releasing control for releasing the inhibition of the movement of the third element by controlling operation of the third-element-movement inhibiting device.

The present mode is, briefly described, for example, a mode in which the inhibition of the movement of the third element is released based on the relative-movement disabled state during execution of the third-element-movement inhibiting control. In other words, the preset mode is a mode in which it is possible to establish a selected one of a state inhibiting the movement of the third element and a state allowing the movement of the third element in event of failure of the movement-amount variably transmitting device. For example, one of the two states can be selected depending on content of the failure.

(25) The steering system according to mode (24), wherein the control device is configured to execute the third-element-movement inhibition releasing control in a case in which an amount of a movement force forcing the third element to be moved becomes larger than a predetermined threshold in a state in which the movement of the third element is inhibited by the third-element-movement inhibiting device.

The present mode recites an additional limitation regarding recognition of the relative-movement disabled state upon execution of the third-element-movement inhibition releasing control. As described above, the inhibition of the movement of the third element in the relative-movement disabled state would cause a deadlock in the differential mechanism. Then, if the operation member is operated in such a state, for example, there could be occurred a phenomenon that the amount of the movement force acting on the third element becomes larger than a predetermined threshold. The present mode is a mode in which, upon occurrence of such a phenomenon, the control device releases the inhibition of the movement of the third element by the third-element-movement inhibiting device and allows the movement of the third element. In the present mode, as in the above-described mode in which the movement-amount variably transmitting device has the third-element-movement allowing mechanism, the third element can be moved when the amount of the movement force acting on the third element becomes larger than the predetermined threshold.

(26) The steering system according to mode (24) or (25),

wherein the third-element-movement inhibiting device has an engaged portion which is provided on the third element and an engaging portion which is provided in the housing and which is engageable with the engaged portion, such that the movement of the third element is inhibited in an engaged state in which the engaged portion is engaged with the engaging portion, wherein the movement-amount variably transmitting device includes an acting force detector that is configured to detect an acting force acting between the engaging portion and the engaged portion in the engaged state,

and wherein the control device is configured to execute the third-element-movement inhibition releasing control, in a case in which an amount of the acting force detected by the acting force detector becomes larger than a predetermined threshold.

The present mode is a mode which has a limitation that the third-element-movement inhibiting device has a particular stopper mechanism and in which the third-element-movement inhibition releasing control is executed based on an acting force acting between the engaging portion and the engaged portion that are included in the stopper mechanism. It is expected that the stopper mechanism receives a relatively large force as the acting force, for example, when the operation member is operated in event of a deadlock occurring in the differential mechanism. The present mode is a mode in which the relative-movement disabled state is detected based on the acting force and then third-element-movement inhibition releasing control is executed based on an outcome of the detection.

The “acting force” recited in the present mode can be considered as a kind of the above-described movement force, and corresponds to acting and reacting forces acting between the engaging and engaged portions when the engaging and engaged portions are held in engagement with each other. Therefore, the present mode can be considered as one mode of the mode in which the third-element-movement inhibition releasing control is executed when the movement force acting on the third element becomes larger than a predetermined threshold. It is noted that the “acting force detector” has a construction that is not particularly limited, and may be like a load sensor that is provided between the engaging and engaged portions or provided in one of the engaging and engaged portions so as to detect the acting force acting between the engaging and engaged portions. Specifically described, the acting force detector can be constructed to have a distortion gage that is disposed in one of the engaging and engaged portions so as to obtain an amount of distortion of the one of the engaging and engaged portions so that the acting force is detected based on the distortion amount detected by the distortion gage.

(27) The steering system according to any one of modes (24)-(26), wherein the control device is configured to execute the third-element-movement inhibition releasing control, in a case in which an amount of an operating force of the steering operation member becomes larger than a predetermined threshold.

When the operation member is operated in event of a deadlock occurring in the differential mechanism, there is a high possibility that the operating force applied to the operation member becomes large. That is, the present mode is a mode in which, in a case in which the amount of the operating force of the operation member becomes larger than a predetermined threshold, it is regarded that the differential mechanism is placed in the relative-movement disabled state, so that the transmission of the movement from the first element to the second element is enabled by releasing the inhibition of the movement of the third element. It is noted that the steering operation member is connected to the first element so that the operating force of the operation member relates to the movement force that acts on the third element via the first element. Therefore, the operating force of the operation member can be considered as a kind of the movement force, and the present mode can be considered as one mode of the mode in which the third-element-movement inhibition releasing control is executed when the movement force acting on the third element becomes larger than a predetermined threshold.

(28) The steering system according to mode (27), including an operating force detector having a deformable member that is elastically deformed depending on the operating force of the steering operation member, and a deformation amount sensor that is configured to detect an amount of deformation of the deformable member, so as to detect the operating force of the steering operation member based on the deformation amount detected by the deformation amount sensor,

wherein the control device is configured to execute the third-element-movement inhibition releasing control, based on the operating force of the steering operation member that is detected by the operating force detector.

(29) The steering system according to mode (28), wherein the operating force detector is provided in the movement-amount variably transmitting device.

(30) The steering system according to mode (28) or (29), wherein one and the other of opposite end portions of the deformable member are connected to the steering operation member and the first element, respectively, and wherein the deformation amount sensor is configured to detect an amount of a relative displacement of the one and the other of opposite end portions of the deformable member.

Each of the above-described three modes is a mode relating to the operating force detector for detecting the operating force of the operating member. The operating force detector may be substantially the same as the operating force detector that is described in the mode relating to the assisting control. Therefore, description about each of the above-described three modes overlaps the previous description, the description is omitted herein.

(31) The steering system according to any one of modes (24)-(30), including a relative-movement amount detector configured to detect the amount of the relative movement of the first element and the second element,

wherein the control device is configured to execute the third-element-movement inhibition releasing control, based on the amount of the relative movement detected by the relative-movement amount detector.

The present mode is a mode in which the outcome of the detection made by the relative-movement amount detector is utilized upon execution of the third-element-movement inhibition releasing control. Since description about each of the present mode overlaps the description of the previous mode, the description is omitted herein.

(32) The steering system according to mode (31),

wherein the third-element-movement inhibiting device is configured to allow the movement of the third element by an amount within a predetermined threshold range and to inhibit the movement of the third element by an amount exceeding the predetermined threshold range,

and wherein the control device is configured to execute the third-element-movement inhibition releasing control, when the first and second elements are disabled from being moved relative to each other in a state in which the third element is allowed to be moved by the amount within the predetermined threshold range.

The present mode is a mode for enabling the detection of the above-described amount of the relative movement in a case in which the movement of the third element is inhibited by the third-element-movement inhibiting device. In the present mode, the third-element-movement inhibiting device is configured not to completely inhibit the movement of the third element, but to allow the third element to be moved by an amount within a predetermined threshold range while inhibiting the third element from being moved by the amount exceeding the predetermined threshold range. It is noted that the “within the predetermined threshold range” recited in the present mode may be, for example, a small range allowing a backlash or play. According to the present mode, it can be determined whether the differential mechanism is placed in the relative-movement disabled state or not, based on the amount of the relative movement, even in a state in which the third-element-movement inhibiting device works.

(33) The steering system according to mode (32), wherein the third-element-movement inhibiting device has an engaged portion which is provided on the third element and an engaging portion which is provided in the housing and which is engageable with the engaged portion, such that the movement of the third element is inhibited by engagement of the engaging portion with the engaged portion, and such that the movement of the third element by an amount corresponding to a play provided between the engaging portion and the engaged portion is allowed as the movement of third element by the amount within the predetermined threshold range.

The present mode is a mode which recites a limitation that the third-element-movement inhibiting device has a particular stopper mechanism and a limitation that the above-described predetermined threshold range corresponds to an amount of a backlash or play in a state of the engagement of the engaging portion and the engaged portion. According to the present mode, the movement of the third element by a relatively small amount makes it possible to determine whether the first and second elements are moved relative to each other or not.

(41) The steering system according to any one of modes (1)-(3), further including a third-element-movement inhibition cancelling device configured to cancel inhibition of the movement of the third element, which is made by the third-element-movement inhibiting device.

The present mode is a mode that generalizes some of the above-described modes. The term “third-element-movement inhibition cancelling device” can be considered as a generic term of means for coping with the relative-movement disabled state, and the present mode can be considered as a mode generic to the above-described modes each having the purpose of coping with the relative-movement disabled state. That is, the third-element-movement inhibition cancelling device is constituted principally by a mechanism allowing the movement of the third element, as described above. The third-element-movement inhibition cancelling device may be configured to allow the movement of the third element in a state in which the movement of the third element is being inhibited by the third-element-movement inhibiting device, or to control the third-element-movement inhibiting device so as not to cause the third-element-movement inhibiting device to inhibit the movement of the third element.

(42) The steering system according to mode (41), wherein the movement-amount variably transmitting device has a third-element-movement allowing mechanism configured to allow the movement of the third element in a case in which an amount of a movement force forcing the third element to be moved becomes larger than a predetermined threshold in a state in which the movement of the third element is inhibited by the third-element-movement inhibiting device,

and wherein the third-element-movement inhibition cancelling device is constituted by the third-element-movement allowing mechanism.

Since the present mode has the above-described third-element-movement allowing mechanism, the present mode provides substantially the same effects as described regarding the modes each having the third-element-movement allowing mechanism. Further, in the present mode, it is also possible to employ technical features included in modes relating to the modes each having the third-element-movement allowing mechanism.

(43) The steering system according to mode (41) or (42), including a control device configured to control the steering system, wherein the control device is configured to control operation of the third-element-movement inhibiting device so as to execute a third-element-movement inhibiting control for inhibiting the movement of the third element,

wherein the third-element-movement inhibition cancelling device is constituted by the control device that is configured, in a relative-movement disabled state in which the first element and the second element are disabled from being moved relative to each other, to execute a third-element-movement non-inhibiting-state establishing control for controlling operation of the third-element-movement inhibiting device so as to establish a state in which the movement of the third element is not inhibited.

The present mode is a mode for controlling the third-element-movement inhibiting device to establish a state in which the movement of the third element is not inhibited in the relative-movement disabled state. According to the present mode, the present mode provides substantially the same effects as described regarding the modes each reciting the third-element-movement non-inhibiting-state establishing control. Further, in the present mode, it is also possible to employ technical features included in modes relating to the modes each reciting the third-element-movement non-inhibiting-state establishing control.

BEST MODE FOR CARRYING OUT THE INVENTION

There will be described embodiments of the present invention, by reference to the accompanying drawings. It is to be understood that the claimable invention is not limited to the following embodiments, and may be otherwise embodied with various changes and modifications, such as those described in the foregoing “MODES OF THE INVENTION”, which may occur to those skilled in the art.

First Embodiment

(A) Construction of Steering System

FIG. 1is a schematic view showing an overall construction of a steering system of a first embodiment of the invention. The present steering system is a so-called electrical power steering system, and substantially sectioned into an operating device10, a wheel turning device12, a VGRS actuator14as a movement-amount variably transmitting device and an electronic control unit16(hereinafter abbreviated to as “ECU16” where appropriate) as a control device. That is, the steering system is constructed to include these components.

The operating device10is constructed to include a steering wheel20as a steering operation member and a steering column22(hereinafter abbreviated to as “column22” where appropriate). The column22is constructed to include a steering shaft24having an end portion to which the steering wheel20is attached, and a steering tube26(hereinafter abbreviated to as “tube26” where appropriate) as a shaft housing in which the steering shaft2420is rotatably held. The tube26is fixed to a reinforcement of an instrument panel whereby the column22is fixedly disposed in a body of a vehicle. Further, in the column22, an operation angle sensor28is provided in the portion of the shaft24to which the steering wheel20is attached. The operation angle sensor28is provided to detect an operating angle of the steering wheel20as an operating amount of the operation member.

The wheel turning device12is constituted principally by a housing30that is fixed to the vehicle body (chassis, described in detail) and a steering rod32that is provided in the housing30so as to be movably in an axial direction (i.e., in a lateral direction of the vehicle). The wheel turning device12has a pinion shaft34as an input shaft to which a steering force is applied from the operating device10. The steering rod32has a rack38formed therein and meshing with a pinion36that is formed in the pinion shaft34, so that the pinion shaft34and the steering rod32is connected to each other through a rack pinion mechanism (seeFIG. 2). Owing to such a construction, the wheel turning device12is configured to move the steering rod32in the above-described axial direction by rotation of the pinion shaft34. The steering rod32has opposite end portions that are connected to end portions of respective right and left tie rods42through respective ball joints40. The other end portions of the respective right and left tie rods42are connected through respective ball joints44to respective knuckle arm portions50that are included in respective steering knuckles48holding respective right and left steerable wheels46. Further, the wheel turning device12is equipped with an assisting mechanism52that is configured to assist a wheel turning force required for turning the wheels46, so that the axial movement of the steering rod32is assisted by the assisting mechanism52. Although not being shown in the drawings, a thread groove (external thread) is formed in the steering rod32, and a nut and an electric motor are provided in the housing30of the wheel turning device12. The nut carries bearing balls, and is held in thread engagement with the thread groove of the steering rod32. The electric motor is configured to rotate the nut. That is, the assisting mechanism52has a ball screw mechanism constituted by the thread groove and the nut, such that the axial movement of the steering rod32is assisted by a drive force of the electric motor. It is noted that the wheel turning device12is provided with a wheel-turning amount sensor54that is configured to detect an amount of movement of the steering rod32.

The VGRS actuator14is a device having a function of transmitting rotation of the steering shaft24(which is caused in response to rotation of the steering wheel20) to the wheel turning device12. As shown inFIG. 1, the VGRS actuator14is fixed to the wheel turning device12through a housing80of the actuator14that is fastened to the housing30of the wheel turning device12. An input shaft82of the VGRS actuator14includes an end portion which extends out from the housing80and which is connected an end portion of an intermediate shaft86via a universal joint84. The other end portion of the intermediate shaft86is connected, via a universal joint88, an end portion of the steering shaft24that is opposite to the above-described end portion to which the steering wheel20is attached.

FIG. 2shows a cross section of the VGRS actuator14. The VGRS actuator14is constructed to include the housing80, the input shaft82provided rotatably relative to the housing80, the output shaft90provided rotatably relative to the housing80, and a movement-amount variably transmitting mechanism92that is capable of changing a rotation ratio as a ratio between rotations of the respective input and output shafts82,90while transmitting the rotation of the input shaft82to the output shaft90. The housing80is constituted by three sub-housings (an upper housing94, a lower housing96and a locking-mechanism-portion housing98) that are assembled. The lower housing96has a flange portion100in which faster-receiving holes102provided in respective four positions that are equi-angularly positioned on a circumference. The flange portion100is attached to a frame portion104provided in the housing30of the wheel turning device12such that a flange surface of the flange portion100is in contact with a frame surface of the frame portion104. The frame portion104has four internally threaded holes106positioned in respective positions corresponding to the respective four faster-receiving holes102, so that the flange portion100and the frame portion104are fastened to each other through bolts108as fasteners while each of the faster-receiving holes102and a corresponding one of the internally threaded holes106are aligned with each other. With the housing80of the VGRS actuator14being thus fixed to the housing30of the wheel turning device12, the VGRS actuator14is fixed to the wheel turning device12, whereby the housing80is unrotatable relative to the vehicle body.

The input shaft82is constituted by an upper shaft110, a lower shaft112and a torsion bar114that are integral with one another. The upper shaft110includes an extending portion that extends out from an upper portion of the housing80, and a serration is formed in an outer periphery of the extending portion. The upper shaft110is connected, at the extending portion formed with the serration, to the universal joint84, so that the rotation is inputted from the operating device10to the upper shaft110. The upper shaft110has a stepped bore having a lower portion that provides a large diameter portion in which the lower shaft112is introduced. Bearings116are interposed between an inner circumferential surface of the large diameter portion of the upper shaft110and an outer circumferential surface of the lower shaft112, whereby the upper shaft110and the lower shaft112are rotatable relative to each other. The lower shaft112includes a flange portion118provided by its lower portion, and has a blind hole which opens at its upper end portion and which extends in the axial direction. The torsion bar114is held in serration engagement at one of its opposite end portions with a bottom portion of the blind hole of the lower shaft112, and is held in serration engagement also at the other end portion with an upper end portion of the upper shaft110. Owing to such a construction, the input shaft82allows twisting of the torsion bar114, so that the input shaft82per se is twisted by an amount corresponding to an amount of the twisting of the torsion bar114.

The output shaft90includes a shaft portion120provided by its lower portion, and an annular portion122located on an upper side of the shaft portion120. The annular portion122is formed integrally with the shaft portion120, and has a diameter larger than that of the shaft portion120. The flange portion118of the lower shaft112constituting the input shaft82is positioned along a flange portion of the output shaft90interconnecting the annular portion122and the shaft portion120, so that the flange portion118is surrounded by the annular portion122. The shaft portion120is hollow, and a lower end portion of the lower shaft112constituting the input shaft82is introduced in an upper portion of a hole of the shaft portion120. Between an outer circumferential surface of the lower end portion of the lower shaft112and an inner circumferential surface of the hole of the shaft portion120, there is interposed a bushing124such that the lower shaft112and the shaft portion120is rotatable relative to each other. The upper shaft110constituting the input shaft82is rotatably held at its outer periphery by the upper housing94via a bearing126, while the shaft portion120of the output shaft90is rotatably held at its outer periphery by the lower housing96via a bearing128. Owing to the construction as described above, the input shaft82and the output shaft90are disposed coaxially with each other, and are rotatable relative to each other and relative to the housing80.

The pinion shaft34as an input shaft of the wheel turning device12is held, at its portion that is lower than the pinion36, by the housing30via a bushing130, and is held, at its portion that is higher than the pinion36, by the housing30via a bearing132, so that the pinion shaft34is provided rotatably relative to the housing30. An outer serration134is formed in an upper end portion of the pinion shaft34, while an inner serration136is formed in a lower end portion of the shaft portion120of the output shaft90. Thus, the output shaft90and the pinion shaft34are held in serration engagement with each other, in a state in which the VGRS actuator14is attached to the wheel turning device12. Owing to such a construction, the rotation of the output shaft90is transmitted to the pinion shaft34.

As described above, the steering rod32is held by the housing30, movably in the axial direction. The steering rod32is positioned relative to the housing30such that the rack38formed in the steering rod32meshes with the pinion36of the pinion shaft34. In a portion located on a back side of a portion of the steering rod32in which the rack38is formed, there is provided a mechanism for backing up the steering rod32. Described in detail, a support member140is disposed in a hole provided in the housing30, for supporting the steering rod32from its back side, and a cap142is held in thread engagement with an end portion of the hole of the housing30. A compression coil spring144is provided between the support member140and the cap142. Owing to such a construction, the steering rod32is backed up, and a suitable meshing of the rack38and the pinion36with each other is assured.

The movement-amount variably transmitting mechanism92employs a harmonic gear mechanism. In the VGRS actuator14, a motor150(electric motor) is provided as a drive power source of the harmonic gear mechanism. A motor shaft152as an output shaft of the motor150is hollow, and is disposed such that the input shaft82, described in detail, the lower shaft112is introduced in the motor shaft152per se. Between an inner circumferential surface of the motor shaft152and an outer circumferential surface of the lower shaft112, there are interposed bearings154,156, so that the motor shaft152is held rotatably relative to the lower shaft112so as to be rotatable relative to the housing80. On an outer peripheral portion of the motor shaft152, a plurality of permanent magnets158are fixedly disposed and arranged in a circumferential direction. The permanent magnets158constitute a rotor of the motor150. Meanwhile, a plurality of polar bodies160(each provided by a core and a coil wound on the core) are fixedly disposed on an inner surface of the housing80, so as to be opposed to the permanent magnets158. Each of the polar bodies160serves as a stator pole so that the plurality of polar bodies160constitute a stator. Owing to such a construction, the motor150serves as a so-called brushless motor. It is noted that a rotational angular position of the rotator, i.e., a rotational position (that can be called also a rotational angle or a rotational phase) of the motor shaft152, is detected by a resolver164that is disposed between an attached ring162attached to an upper end portion of the motor shaft152and an inner surface of the housing80. The motor50is driven by a drive circuit (not shown in the drawings) under a control by which ones of the polar bodies160, selected according to the rotational angular position of the rotor, are energized. Further, a rotational velocity of the motor150, more precisely, a rotational velocity of the motor shaft152, is controlled also by utilizing a signal indicative of detection made by the resolver164.

The movement-amount variably transmitting mechanism92is constructed to include a stator gear180as a first circular spline (first ring gear), a driven gear182as a second circular spline (second ring gear), a flexible gear184as a flex spline meshing with the stator and driven gears180,182, and a wave generator186supporting the flexible gear184and configured to generate a wave. Hereinafter, there will be described a construction and a function of the movement-amount variably transmitting mechanism92, by reference toFIG. 3that is a schematic view showing the movement-amount variably transmitting mechanism92as seen in the axial direction.

The stator gear180is a ring gear having internal teeth, and is fixed to the input shaft82, described in detail, to an outer peripheral portion of the flange portion118of the lower shaft112, so that the stator gear180is unrotatable relative to the input shaft82. The driven gear182is a ring gear having internal teeth, and is fixed to an upper end portion of an inner peripheral portion of the annular portion122of the output shaft90, so that the driven gear182is unrotatable relative to the output shaft90. The stator gear180and the driven gear182are disposed coaxially with each other. The stator gear180as an input side gear is positioned on a lower side of the driven gear182as an output side gear. That is, the stator gear180as the input side gear is closer than the driven gear182as the output side gear, to a lower portion of the VGRS actuator14that provides an output side portion of the VGRS actuator14. Meanwhile, the driven gear182as the output side gear is positioned on an upper side of the stator gear180as the input side gear. That is, the driven gear182as the output side gear is closer than the stator gear180as the input side gear, to an upper portion of the VGRS actuator14that provides an input side portion of the VGRS actuator14. Thus, the stator gear180and the driven gear182are arranged in the axial direction, as if their respective positions were switched to each other. The number of the teeth of the stator gear180and the number of the teeth of the driven gear182are different from each other. The number of the teeth of the stator gear180is 102, while the number of the teeth of the driven gear182is 100. The flexible gear184is a ring gear having external teeth, and is relatively thin so as to have a flexibility. The number of the teeth of the flexible gear184is 100 as that of the driven gear182. It is noted that the numbers of the teeth of the gears may be switched, for example, so that the stator gear180has 100 teeth while the driven gear182has 102 teeth.

The wave generator186functions as a generally elliptic-shaped cam, and is constructed to include a generally elliptic-shaped support plate188and a bearing190that is fitted on an outer peripheral end of the support plate188. The support plate188has an axial hole in its center, and is connected to the motor shaft152that is fitted in the axial hole, such that the support plate188is unrotatable relative to the motor shaft152. The bearing190is mounted on the support plate188, with an outer periphery of the support plate188being fitted in an inner race192of the bearing190. An outer race194of the bearing190is relatively thin so as to have a flexibility. The flexible gear184is mounted on an outer periphery of the bearing190such that the flexible gear184is unrotatable relative to the outer race194of the bearing190. The flexible gear184is deformed by the wave generator186so as to have an elliptic shape. Therefore, the flexible gear184meshes, at two portions thereof that lie on or in neighborhood of a long axis of the elliptic shape, with the stator gear180and the driven gear182, while being completely separated, at portions thereof that line on or around a short axis of the elliptic shape, from the stator gear180and the driven gear182.

Upon rotation of the stator gear180in a state in which the motor shaft152is inhibited from being rotated, the flexible gear184is circulated, together with the outer race194of the bearing190, along an ellipse, while being elastically deformed with the meshing portions thereof being changed as the circulation thereof. As a result of the circulation of the flexible gear184, the driven gear182, which also meshes with the flexible gear184, is rotated in the same direction as the rotation of the stator gear180. Described in detail, in this instance, the driven gear182is rotated relative to the stator gear180at a rotation ratio of 102/100 that corresponds to a gear ratio between these gears180,182. Hereinafter, this rotation ratio will be referred to as “reference rotation ratio” where appropriate.

There will be described a case in which the wave generator186is rotated by rotating the motor shaft152, on the assumption that the stator gear180is fixed, in the interest of simplifying the description. When the wave generator186is rotated, the flexible gear184is rotated while being elastically deformed with positions of the meshing being changed as the rotation thereof. Since the stator gear180and the driven gear182are different from each other with respect to the number of teeth, there is caused, between the stator gear180and the driven gear182, a rotational angle difference (rotational phase difference) whose amount corresponds the difference with respect to the number of teeth. Described specifically, the rotational angle difference whose amount corresponds to two teeth is caused per one rotation of the wave generator186. Described in detail, when the wave generator186is rotated by one rotation in clockwise direction as seen inFIG. 3, the driven gear182is rotated relative to the stator gear180by an amount corresponding to two teeth in counterclockwise direction as seen inFIG. 3.

Actually, since the stator gear180is rotated together with rotation of the input shaft82, the rotation ratio of the driven gear182to the stator gear180, i.e., a transmission ratio of the movement-amount variably transmitting mechanism92, is determined according to the amount of the relative rotation of the stator gear180and the wave generator186. Described specifically, when the stator gear180and the wave generator186are rotated in the same direction, the driven gear182is rotated at a rotational velocity that is reduced to be lower than a rotational velocity corresponding to the above-described reference rotation ratio. When the stator gear180and the wave generator186are rotated in respective directions opposite to each other, the driven gear182is rotated at a rotational velocity that is increased to be higher than the rotational velocity corresponding to the above-described reference rotation ratio. Since a degree of the increase or reduction of the rotational velocity is dependent on both of the rotational velocity of the stator gear180and the rotational velocity of the wave generator186, the transmission ratio can be changed in an arbitrary manner, by changing the rotational velocity of the motor150depending on the rotational velocity of the stator gear180. The movement-amount variably transmitting mechanism92is thus configured to transmit the rotation of the input shaft82to the output shaft90and to change a ratio of the rotational amount of the output shaft90to the rotational amount of the input shaft82, in other words, the transmission ratio as a ratio of the rotational velocity of the output shaft90to the rotational velocity of the input shaft82.

Owing to the above-described constructions, in the movement-amount variably transmitting mechanism92, the lower shaft112of the input shaft82rotatably disposed in the housing80and the stator gear180connected to the lower shaft112function as a first element, while the output shaft90rotatably disposed in the housing80and the driven gear182connected to the output shaft90function as a second element. Further, the flexible gear184meshing with the stator gear180and the driven gear182, the wave generator186fitted in the flexible gear184, the motor shaft152connected to the wave generator186and other components function as a third element. These three elements cooperate to constitute a differential mechanism. As described above, the rotation ratio during the state of inhibition of the rotation of the motor shaft152is equal to the above-described reference rotation ratio. During this state, i.e., a state of inhibition of the movement of the third element, the amount of the relative movement of the first and second elements corresponds to the reference rotation ratio, irrespective of the movement amount of the first element.

Further, the VGRS actuator14as the movement-amount variably transmitting device is equipped with a motor-shaft rotation locking mechanism200(hereinafter simply referred to as “locking mechanism200” where appropriate) as a third-element-movement inhibiting device capable of inhibiting the rotation of the motor shaft152that constitutes the third element. The locking mechanism200will be described by reference toFIG. 4(illustrating only elements disposed within the housing98) that is a cross sectional view (taken along line A-A inFIG. 2). The locking mechanism200is constructed to include an electromagnetically-operated solenoid202as a drive source, a lock lever206pivotable about a fixed pin204that is fixedly disposed on an inner surface of the locking-mechanism-portion housing98, and a lock holder208as a rotatable member that is fitted on an outer periphery of the motor shaft152via a tolerance ring207. The tolerance ring207is fitted on the motor shaft152, and is constructed to include an annular member having a corrugated portion that functions as a spring. The tolerance ring207supports the annular lock holder208fitted on an outer periphery of the tolerance ring207, owing to an elastic force of the corrugated portion. The tolerance ring207normally inhibits rotation of the lock holder208relative to the motor shaft152, owing to a frictional force generated by the elastic force.

The lock lever206is biased by a spring212that is disposed with the fixed pin204being introduced therein, so as to be pivoted in a direction causing a distal end portion210(as an end portion of the lock lever206) to be moved toward the lock holder208. The other end portion of the lock lever206is connected to the solenoid202. The solenoid202is configured, upon energization thereof, to pivot the lock lever206in a direction causing the distal end portion210to be moved away from the lock holder208. The distal end portion210of the lock lever206functions as an engaging portion, while each of recessed portions214formed in an outer periphery of the lock holder208functions as an engaged portion, so that the distal end portion210is engageable with the recessed portions214.

Upon energization of the solenoid202, the lock lever206is pivoted against a biasing force generated by the spring212, whereby the engagement of the distal end portion210of the lock lever206and the recessed portion214of the lock holder208is released thereby allowing the rotation of the motor shaft152(seeFIG. 4(a)). Upon deenergization of the solenoid202, the lock lever206is engaged at the distal end portion210with the recessed portion214by the biasing force of the spring212, thereby inhibiting the rotation of the motor shaft152(seeFIG. 4(b)). While the rotation of the motor shaft152is being inhibited, the transmission of the rotation is made according to the above-described reference rotation ratio.

There will be described a case in which a movement force acts on the motor shaft152to force the motor shaft152to be rotated during inhibition of the rotation of the motor shaft152by the above-described locking mechanism200. When the movement force forcing the motor shaft152to be rotated is larger than the frictional force acting between the tolerance ring207and the lock holder208, the motor shaft152is allowed to be rotated even in the state in which the locking mechanism200is activated to inhibit the rotation of the motor shaft152. Owing to such a construction, the VGRS actuator14has a third-element-movement allowing mechanism configured to allow the movement of the third element by effect of the movement force even during the inhibition of the movement of the third element by the third-element-movement inhibiting device, when an amount of the movement force acting on the third element becomes larger than a threshold. That is, in the present VGRS actuator14, the third-element-movement allowing mechanism is constituted to include the tolerance ring207. Further, the present steering system includes a third-element-movement inhibition cancelling device configured to cancel the inhibition of the movement of the third element made by the third-element-movement inhibiting device, since the VGRS actuator14has the third-element-movement allowing mechanism.

In the distal end portion210of the lock lever206, there is provided a distortion gage216that is configured to detect an amount of deformation of the lock lever206that could be caused by an interaction of the lock lever206and the lock holder208. In the present steering system, it is therefore possible to estimate, based on the deformation amount of the lock lever206, an acting force acting between the lock lever206and the lock holder208when the lock lever206and the lock holder208are engaged with each other. Thus, the VGRS actuator14is arranged to have an acting force detector configured to detect the acting force, which acts between the lock lever206and the lock holder208according to the movement force forcing the motor shaft152to be rotated.

Further, the VGRS actuator14has two resolvers220,222in addition to the above-described resolver164. The resolver220is provided between the upper shaft110(in which an upper end portion of the torsion bar114is fitted) and an inner surface of the housing80, so as to function as a detector for detecting a rotational angular position of the upper shaft110. The resolver222is provided between an attached ring224that is fixedly provided in an outer peripheral portion of the lower shaft112(in which a lower end portion of the torsion bar114is fitted) and the inner surface of the housing80, so as to function as a detector for detecting a rotational angular position of the lower shaft112. It is possible to detect, from signals indicative of detections made by these two resolvers220,222, an amount of a relative rotational displacement of the upper shaft110and the lower shaft112as an amount of a relative displacement of the upper and lower end portions of the torsion bar114. In the steering system, the operating force (operating torque, described in detail) applied to the steering wheel20is estimated based on the relative rotational displacement amount, i.e., an amount of twisting deformation of the torsion bar114. Thus, the present steering system is arranged to have an operating force detector configured to detect the operating force of the steering wheel20based on the above-described relative displacement amount of the upper and lower end portions of the torsion bar114. The operating force detector is constructed to include the torsion bar114serving as a deformable member and a relative-displacement amount sensor that is constituted by the resolvers220,222serving as an end-portion displacement amount sensor and another end-portion displacement amount sensor. In other words, the operating force detector is provided in the VGRS actuator14, and is constructed to include the torsion bar114serving as the deformable member that is disposed between the steering wheel20and the stator gear180constituting the first element, and a deformation amount sensor configured to detect the twisting deformation amount of the torsion bar114. It is possible to estimate, based on the detection signals supplied from the resolvers220,222, also a direction in which the steering wheel20is operated.

The above-described resolver222is capable of detecting a rotational angle of the stator gear180connected to the lower shaft112, based on the detected rotational angular position of the lower shaft112. The wheel-turning amount sensor54provided in the wheel turning device12is capable of detecting a rotational angle of the pinion shaft34, i.e., a rotational angle of the driven gear182, based on the detected movement amount of the steering rod32. Therefore, from detection signals supplied from the resolver222and the wheel-turning amount sensor54, it is possible to detect a relative rotational angle of the stator gear180and the driven gear182as the amount of the relative movement of the first and second elements. Thus, the present steering system is arranged to have a relative-movement amount detector configured to detect the amount of the relative movement of the first and second elements.

(B) Controls of Steering System

The present steering system constructed as described above is controlled by the ECU16, which is constituted principally by a computer. To the ECU16, there are connected various sensors such as the above-described operation angle sensor28, wheel-turning amount sensor54, resolvers164,220,222, distortion gage216and wheel velocity sensors230provided in the wheels (only one of the wheel velocity sensors230provided in one of the wheels46is shown inFIG. 1). Further, the ECU16has drive circuits (drivers) for driving the motor of the assisting mechanism52of the wheel turning device12, the motor150of the VGRS actuator14and the solenoid202of the locking mechanism200, and is constructed such that the computer is configured to control the motor of the assisting mechanism52, motor150and solenoid202through the drive circuits.

i) Basic Controls

The ECU16executes two controls as basic controls. One of the two controls is a control (hereinafter referred to as “transmission ratio control” where appropriate) that relates to the above-described movement-amount variably transmitting mechanism92included in the VGRS actuator14. As described above, the VGRS actuator14has the movement-amount variably transmitting mechanism92configured to change the transmission ratio γ, i.e., a ratio of the rotational amount of the pinion shaft34of the wheel turning device12to the rotational amount of the steering shaft24, and executes a transmission ratio control for controlling the transmission ratio γ. Specifically described, a running speed v of the vehicle is estimated based on wheel velocities detected by the wheel velocity sensors230provided in the respective wheels, and the movement-amount variably transmitting mechanism92(rotational direction and rotational velocity of the motor150, described in detail) is controlled so as to establish the transmission ratio γ corresponding to the estimated running speed v.

Described more in detail, as shown inFIG. 5, while the running speed v of the vehicle is within a range between a controllable minimum speed vMINand a controllable maximum speed vMAX, the transmission ratio γ is reduced with increase of the running speed v and is increased with reduction of the running speed v. Further, since the rotational velocity dφ of the motor shaft152corresponding to the transmission ratio γ is determined according to the rotational velocity of the stator gear180, a target value of the rotational velocity dφ of the motor shaft152is determined by a calculation made based on the rotational velocity dδ of the steering wheel20that is estimated based on the operating angle of the steering wheel20detected by the operation angle sensor28in the present transmission ratio control. Then, a command indicative of the determined target value of the rotational velocity dφ is issued to the drive circuit. The motor150is controlled, by the drive circuit, based on the rotational angular position of the motor shaft152detected by the resolver164, such that the motor shaft152is rotated at the target value of the rotational velocity dφ that is indicated by the command. Owing to such a control, the transmission ratio γ corresponding to the vehicle running speed v is established. According to the present transmission ratio control, when the vehicle runs fast, the wheel turning amount of the steerable wheel46is reduced relative to the operating angle (operating amount) of the steering wheel20so as to improve a running stability of the vehicle. When the vehicle runs slowly, the wheel turning amount of the steerable wheel46is increased relative to the operating angle of the steering wheel20so as to facilitate the steering operation.

Another one of the controls executed by the ECU16is a control (hereinafter referred to as “assisting control” where appropriate) relating to the above-described assisting mechanism52of the wheel turning device12. The ECU16estimates the operating torque T as the operating force applied to the steering wheel20, based on the relative rotational displacement amount Δτ (twisting deformation amount of the torsion bar114) of the upper and lower end portions of the torsion bar114detected by the resolvers220,222, and controls the assisting mechanism52, described in detail, controls an electric power W that is to be supplied to the electric motor included in the assisting mechanism52, such that the assisting force FAcorresponding to the estimated operating torque is generated. The supplied electric power W or assisting force FAis set depending on the operating torque T or relative rotational displacement amount Δτ, as shown inFIG. 6. Thus, the supplied electric power W is determined depending on a current value of the relative rotational displacement amount Δτ, and a command indicative of a determined value of the supplied electric power W is issued to the drive circuit for driving the electric motor, so that the drive circuit is operated to supply the determined value of the supplied electric power W to the electric power. Owing to such a control, the turning of the wheel is assisted according to the operating torque T. Described specifically, when the relative rotational displacement amount Δτ becomes large with increase of twisting of the torsion bar114, it is estimated that the operating torque T is large so that a relatively large amount of the electric power W is supplied to the electric motor whereby the assisting force FAgenerated by the assisting mechanism52is made large. When the relative rotational displacement amount Δτ becomes small with reduction of twisting of the torsion bar114, it is estimated that the operating torque T is small so that a relatively small amount of the electric power W is supplied to the electric motor whereby the assisting force FAgenerated by the assisting mechanism52is made small.

ii) First Failure Control

In event of a failure disenabling the motor150from rotating the motor shaft152constituting the third element, the ECU16executes a first failure control in place of the above-described transmission ratio control. This first failure control is a third-element-movement inhibiting control, i.e., a control executed for inhibiting the rotation of the third element by operating the locking mechanism200as the above-described third-element-movement inhibiting device, described in detail, by locking the motor shaft152with deenergization of the solenoid202. Specifically, the failure disenabling the motor150from rotating the motor shaft152corresponds to, for example, a case in which the drive force cannot be applied to the motor shaft152due to disconnection or the like, and a case in which the motor150becomes unable of generating the drive force as a result of activation of a protection circuit in response to an excessively large load applied to the motor150. In such cases, the motor shaft152is allowed to be relatively freely rotated, whereby the wave generator186is allowed to be freely rotated in the movement-amount variably transmitting mechanism92. Therefore, in such cases, the transmission of the rotation between the stator gear180and the driven gear182could not be appropriately performed. For coping with such a situation, the first failure control is executed in the present steering system.

Described specifically, the ECU16monitors a state of supply of the electric power to the motor150in the drive circuit driving the motor150, and detects disconnection of the motor150and overload to the motor150, based on an outcome of the monitoring, so that supply of the electric power to the solenoid202is stopped upon detection of the disconnection or the overload, whereby the rotation of the motor shaft152is inhibited. By such an execution of the first failure control, the rotation ratio of the driven gear182to the stator gear180is fixed to the reference rotation ratio, whereby the turning of the wheel is performed at a fixed value of the transmission ratio γ that corresponds to the reference rotation ratio.

iii) Second Failure Control

As a factor causing the overloading of the motor150, there is a state in which as if the elements of the movement-amount variably transmitting mechanism92in the form of the stator gear180, driven gear182, flexible gear184(engaged with the gears180,182) and wave generator186are stuck to one another, in other words, a relative-rotation disabled state (as a kind of “relative-movement disabled state”) in which the stator gear180and the driven gear182are not rotatable relative to each other. The failure accompanied by the relative-rotation disabled state could be caused, for example, by entrance of foreign object into the movement-amount variably transmitting mechanism92. In the relative-rotation disabled state, even if the motor shaft152is intended to be rotated at a rotational velocity corresponding to the transmission ratio, the motor shaft152cannot be rotated at the rotational velocity, so that the motor150is overloaded if such a state is continued.

In event of occurrence of the failure accompanied by the relative-rotation disabled state, if the above-described first failure control is executed to inhibit the rotation of the motor shaft152, the movement-amount variably transmitting mechanism92suffers a so-called deadlock impeding the steering operation. For coping with such a problem, the ECU16executes a second failure control. In this second failure control, there are executed a third-element-movement inhibition releasing control, i.e., a control for releasing the inhibition of the rotation of the motor shaft152that is made by the locking mechanism200, and a control for avoiding the motor150from providing a resistance against the rotation of the motor shaft152. Specifically described, the electric power is supplied to the solenoid202, and the motor150is placed, by the drive circuit, in a free state (hereinafter referred to as “motor free state” where appropriate), i.e., in a state in which the input terminals of the motor150are electrically disconnected from the electric power source while being opened from each other, so that the electric power is not supplied to the motor150while a force based on an electromotive force is not generated by the motor150.

The ECU16executes the second failure control upon satisfaction of any one of three conditions. A first one (hereinafter referred to as “first condition” where appropriate) of the conditions is that an amount of the movement force acting on the third element becomes larger than a predetermined threshold. Specifically described, the first condition is that, when the lock lever206is engaged with the lock holder208, an acting force AF as a force acting therebetween as a result of action and reaction thereof is larger than a predetermined threshold AF0. As described above, in the present steering system, since the VGRS actuator14has the acting force detector in the form of the distortion gage216, the acting force AF is detected based on the detection signal supplied from the distortion gage216. It is determined whether the first condition is satisfied or not based on the detected acting force AF, and then the second failure control in place of the first failure control is executed when it is determined that the first condition is satisfied.

A second one (hereinafter referred to as “second condition” where appropriate) of the conditions is that an amount of the operating force applied to the steering operation member becomes larger than a predetermined threshold. When the steering wheel20is intended to be rotated by the vehicle operator in event of a deadlock occurring in the movement-amount variably transmitting mechanism92, there is a high possibility that the operating force applied to the steering wheel20becomes large. The present steering system has the above-described operating force detector which is constructed to include the two resolvers220,222provided between the steering wheel20and the stator gear180, namely, which is based on the relative rotational displacement amount Δτ of the torsion bar114, as described above, so that it is determined whether the second condition is satisfied or not, based on an outcome of the detection made by the operating force detector. Described specifically, when an amount of the operating torque T as the detected operating force is larger than a predetermined threshold T0, the occurrence of the deadlock is recognized so that it is determined whether the second condition is satisfied or not based on the recognition. The second failure control is executed when it is determined that the second condition is satisfied.

A third one (hereinafter referred to as “third condition” where appropriate) of the conditions is that the first element and the second element are not substantially moved relative to each other. Specifically described, the third condition is that a rotational amount ΔθSof the stator gear180constituting the first element and a rotational amount ΔθDof the driven gear182constituting the second element are substantially equal to each other. Each of the rotational amounts ΔθS, ΔθDis a rotational amount within a certain short length of time and accordingly can be considered as a rotational velocity. For example, when the VGRS actuator14simply suffers disconnection of the motor150, for example, the rotation ratio of the driven gear182to the stator gear180corresponds to the reference rotation ratio. However, during the relative-rotation disabled state, the rotation ratio is constantly about 1/1. The third condition is a condition established with this fact being taken into account. It is noted that the locking mechanism200of the present steering system is provided with a play240between the lock lever206and the lock holder206, described in detail, between the distal end portion210of the lock lever206and each recessed portion214of the lock holder208, so as to allow the rotation of the third element by an amount within a predetermined threshold range that corresponds to the play240while inhibiting the rotation of the third element. According to such a construction, in the present VGRS actuator14, even in the relative-rotation disabled state, the rotation ratio of the driven gear182to the stator gear180is detectable owing to the rotation by the amount corresponding to the play240.

Described specifically, firstly, the rotational amount ΔθSof the stator gear180is recognized based on the detected value detected by the resolver222that is capable of detecting the rotational angle of the stator gear180, and the rotational amount ΔθDof the driven gear182is recognized based on the detected value detected by the wheel turning amount sensor54that is capable of detecting the rotational angle of the driven gear182. Then, on condition that the rotational amount ΔθSof the stator gear180is not 0 (zero), it is determined whether a difference between the rotational amount Δθsof the stator gear180and the rotational amount ΔθDof the driven gear182is smaller than a predetermined threshold Δθ0(that is set to a value extremely close to 0) or not. When the difference is smaller than the predetermined threshold Δθ0, it is determined that the third condition is satisfied whereby the second failure control is executed.

As described above, when the first failure control is executed, the ECU16is configured to determine whether the failure is caused by the relative-rotation disabled state or not, by determining (A) whether the acting force acting between the engaging portion and the engaged portion of the locking mechanism exceeds the predetermined threshold, (B) whether the operating force applied to the steering operation member exceeds the predetermined threshold, and (C) whether the first and second elements are substantially movable relative to each other. That is, when at least one of the above-described third conditions is satisfied, it is determined that the transmitting mechanism92is placed in the relative-rotation disabled state, and the ECU16executes the second failure control. It is noted that either of the determinations regarding the above-described first and second conditions can be regarded as a determination as to whether the amount of the movement force acting on the third element exceeds a predetermined threshold or not.

Owing to the second failure control executed as described above, in the VGRS actuator14, there is established a state in which as if the input shaft82and output shaft90were integral with each other so that the rotations of the input shaft82and output shaft90are allowed to be rotated thereby making it possible to appropriately perform the steering operation. Further, in this state, it is possible to avoid the motor150from being overloaded. In the second failure control, the third-element-movement inhibiting device is controlled to release the inhibition of the movement of the third element, which is made by the third-element-movement inhibiting device. Therefore, the present steering system is given the third-element-movement inhibition cancelling device owing to an arrangement in which the ECU16as the control device is configured to execute the second failure control, described in detail, the third-element-movement inhibition releasing control. Since there are three conditions as conditions for initiation of the second failure control, as described above, it is possible to consider that the present steering system has three kinds of third-element-movement inhibition cancelling devices. Further, the third-element-movement inhibition releasing control is a control for establishing a state in which the movement of the third element is not inhibited. In this sense, the third-element-movement inhibition releasing control is a kind of third-element-movement non-inhibiting-state establishing control.

iv) Modified Manner of Execution of Second Failure Control

The above-described second failure control is a control executed for establishing a state in which the movement of the third element is not inhibited by the third-element-movement inhibiting device, by releasing the inhibition of the movement of the third element after the movement of the third element has been once inhibited by the third-element-movement inhibiting device. However, in place of such an execution of the control, it is also possible to directly execute the second failure control in the relative-rotation disabled state. For example, the above-described determination regarding the third condition, i.e., the determination as to whether the first and second elements are substantially movable relative to each other, may be made also before activation of the locking mechanism200. This is because, as described above, the rotation ratio of the driven gear182to the stator gear180is constantly about 1/1 during the relative-rotation disabled state. Therefore, it is also possible to always make the determination regarding the third condition in a normal state and to execute the second failure control upon satisfaction of the third condition. That is, also with the thus modified manner of execution of the control, it is possible to carry out the third-element-movement non-inhibiting-state establishing control.

Upon initiation of the second failure control from the normal state, the motor shaft152of the third element may be inhibited from being driven, by inhibiting the execution of the transmission ratio control so as to place the motor150into the above-described free state. However, since the rotation of the motor shaft152is not inhibited, it is not necessary execute a control relating to activation of the locking mechanism200. It is noted that the transmission ratio control is executed during the normal state so that there is a case in which the rotation ratio of the driven gear182to the stator gear180is set to 1/1 due to the execution of the transmission ratio control when the vehicle running speed v has a certain value. In view of this, for assuring an accurate recognition of the relative-rotation disabled state, it is preferable to make the determination regarding the above-described third condition by also seeing if the vehicle running speed v is being changed or not.

v) Relationship between Third-Element-Movement Allowing Mechanism and Second Failure Control

The present steering system is given, in addition to the third-element-movement inhibition cancelling device provided in the execution of the second failure control, another third-element-movement inhibition cancelling device provided by the arrangement in which the VGRS actuator14has the above-described third-element-movement allowing mechanism. As described above, the third-element-movement allowing mechanism has a function of allowing the rotation of the motor shaft152relative to the lock holder208when the movement force acting on the motor shaft152is larger than the frictional force generated by the tolerance ring207. The movement force is a force which is generated based on the operating force applied to the steering wheel20and an input inversely applied from the wheel turning device12, and which acts on the motor shaft152, for example, via the stator gear180and the driven gear182. That is, the third-element-movement allowing mechanism is configured to allow the rotation of the motor shaft152owing to an effect of the movement force even when the locking mechanism200is activated to inhibit the rotation of the motor shaft152.

In view of those described above, the third-element-movement allowing mechanism and the second failure control are common to each other with respect to function of allowing the rotation of the motor shaft152, so that it can be considered that the third-element-movement inhibition cancelling device is disposed in a redundant manner in the present steering system. It is noted that the amount of the movement force for overcoming the frictional force generated by the tolerance ring207is set to be larger than the above-described predetermined threshold in the determination regarding the first condition and the above-described predetermined threshold in the determination regarding the second condition so that the second failure control is initiated, in general, prior to the allowance of the rotation of the motor shaft152by the third-element-movement allowing mechanism. Therefore, in the present steering system, the third-element-movement allowing mechanism is the third-element-movement inhibition cancelling device serving as a backup device that is provided for a fail-safe purpose. However, it may be modified such that the rotation of the motor shaft152is allowed by the third-element-movement allowing mechanism, prior to or concurrently with the execution of the second failure control. Further, the present steering system may be modified also such that the third-element-movement allowing mechanism is not provided or such that the second failure control is not executed.

vi) Flows of Controls

The above-described assisting control and actuator control are executed in accordance with an assisting control program and an actuator control program that are shown in flow charts ofFIGS. 7 and 8, respectively. Each of these control programs is repeatedly executed at a short time interval (e.g., several milliseconds to several tens of milliseconds). Hereinafter, the assisting control and the actuator control (including the transmission ratio control, first failure control and second failure control) will be described by reference to the respective flow charts.

As shown inFIG. 7, in the assisting control for control the assisting force that is to be generated by the assisting mechanism52of the wheel turning device12, step S1(hereinafter abbreviated to as “S1” as well as the other steps, where appropriate) is first implemented to obtain the relative rotational displacement amount Δτ of the upper and lower end portions of the torsion bar114that is detected by the resolvers220,222. Then, S2is implemented to determine the electric power W that is to be supplied to the electric motor of the assisting mechanism52, based on the relative rotational displacement amount Δτ. Next, in S3, a command indicative of the supplied electric power W is issued to the drive circuit. Owing to such a control, in the present steering system, the wheel turning is always assisted according to the operating force applied to the steering wheel20.

As shown inFIG. 8, a failure flag F is used in the actuator control. The failure flag F is set to 0 (zero) as an initial flag value. When the motor150is disconnected or overloaded, the flag value becomes 1 (one). When the transmitting mechanism92is placed in the above-described relative-rotation disabled state, the flag value becomes 2 (two).

S13is implemented to determine whether the motor150is disconnected or not. S14is implemented to determine whether the motor150is overloaded. When the motor150is neither disconnected nor overloaded, S15through S19are implemented to execute the transmission ratio control. That is, the vehicle running speed v is estimated, and the transmission ratio γ is recognized based on the estimated vehicle running speed v with reference to the map shown inFIG. 5. Next, the rotational velocity dδ of the steering wheel20is estimated, and the rotational velocity Δφ at which the motor shaft152is to be rotated is determined based on the estimated rotational velocity dδ and the recognized transmission ratio γ. Then, a command indicative of the determined rotational velocity Δφ is issued to the drive circuit of the motor150.

When the motor150is disconnected or overloaded, S20is implemented to activate the locking mechanism200so as to lock the motor shaft152. That is, the execution of the first failure control is initiated whereby the rotation of the motor shaft152is inhibited. Upon initiation of the first failure control, the failure flag F is set to 1 (one) in S21.

After the failure flag F has been set to 1, S22through S25are implemented to make determinations. The determination made in S22corresponds to the above-described determination regarding the first condition. In S22, it is determined whether the acting force AF acting between the lock lever206and the lock holder208is larger than the predetermined threshold A or not. The determination made in S23corresponds to the determination regarding the second condition. In S23, it is determined whether the operating torque T exceeds the predetermined threshold T0or not. The determinations made in S24and S25correspond to the determination regarding the third condition. When the stator gear180is being rotated, it is determined whether the difference between the rotational amount ΔθSof the stator gear180and the rotational amount ΔθDof the driven gear182is smaller than the predetermined threshold Δθ0or not. When any one of the first through third conditions is satisfied, S26is implemented to operate the locking mechanism200to release the locking of the motor shaft152and to establish the above-described motor free state by the drive circuit. That is, the execution of the second failure control is initiated. Upon initiation of the second failure control, the failure flag F is set to 2 (two) in S27.

Once after the first failure control has been initiated, the above-described determinations regarding the first through third conditions are repeated as a result of an affirmative determination in S12while the locking of the motor shaft152is being maintained. Once after the second failure control has been initiated, the locking of the motor shaft152is held released while the motor free state is maintained as a result of an affirmative determination in S11.

vii) Functional Construction of Control Device

The ECU16, which is configured to carry out procedures according to the above-described assisting control program and actuator control program, can be considered to have a functional construction as shown inFIG. 9. Described in detail, the ECU16has an assisting control portion B1configured to carry out procedures according to the assisting control program and an actuator control portion B2as a movement-amount variably transmitting device control portion configured to carry out procedures according to the actuator control program.

Described in detail, the actuator control portion B2has a transmission-ratio control portion B21as a functional portion configured to carry out procedures according to S15through S19of the actuator control program, a first-failure control portion B22as a functional portion configured to carry out a procedure according to S20depending on the determination made in S13or S14, and a second-failure control portion B23as a functional portion configured to carry out procedures according to S26depending on the determination made in any one of S22through S25. The first-failure control portion B22is a functional portion configured to cause the locking mechanism200to inhibit the rotation of the motor shaft152as the third element under a particular situation, and can be considered as a third-element-movement inhibiting control portion. The second-failure control portion B23is a functional portion configured, in a state in which the rotation of the motor shaft152is inhibited, to execute a control for allowing the rotation of the motor shaft152, by controlling the activation of the locking mechanism200in such a manner that releases the inhibition of the rotation of the motor shaft152, and can be considered as a third-element-movement inhibition releasing control portion. Further, the second-failure control portion B23can be considered as a kind of third-element-movement non-inhibiting-state establishing control portion as a functional portion configured to establish a state in which the movement of the third element is not inhibited.

Described more in detail, the second-failure control portion B23can be considered to include three functional portions. One of them is an acting-force-basis releasing control portion B231configured to release the inhibition of the rotation of the motor shaft152based on the determination made in S22, namely, to execute a control for releasing the inhibition of the rotation of the motor shaft152when the amount of the acting force acting in the locking mechanism200exceeds a predetermined threshold. Another one of them is an operating-force-basis releasing control portion B232configured to release the inhibition of the rotation of the motor shaft152based on the determination made in S23, namely, to execute a control for releasing the inhibition of the rotation of the motor shaft152when the amount of the operating force applied to the steering wheel20exceeds a predetermined threshold. Then, still another one of them is a relative-movement-basis releasing control portion B233configured to release the inhibition of the rotation of the motor shaft152based on the determination made in S25, namely, to execute a control for releasing the inhibition of the rotation of the motor shaft152when the stator gear180as the first element and the driven gear182as the second element become disabled from being substantially moved relative to each other.

Second Embodiment

(A) Construction of Steering System

FIG. 10is a cross sectional view of the movement-amount variably transmitting device in the form of a VGRS actuator that is provided in a steering system of a second embodiment of the present invention. This VGRS actuator250is fixedly provided in the wheel turning device12, like the above-described VGRS actuator14provided in the steering system of the first embodiment. Since an overall construction of the present steering system is substantially the same as that of the steering system shown inFIG. 1, description of that is omitted. Further, since the VGRS actuator250is constructed to include elements similar or substantially the same as those of the VGRS actuator14, the same reference signs will be used to identify the similar or substantially the same elements, and description of these elements will be simplified.

Like the VGRS actuator14in the first embodiment, the VGRS actuator250is constructed to include a housing252, an input shaft254, an output shaft256and the movement-amount variably transmitting mechanism92. The housing252is constructed to include three sub-housings (an upper housing260, a lower housing262and a locking-mechanism-portion housing264). The upper housing260and the lower housing262are fastened to each other through bolts266as fasteners, and are easily separable from each other. The lower housing262is integral with the housing30of the wheel turning device12, so as not to be easily separable from the housing30. From a different point of view, the lower housing262can be considered as a part of the housing30of the wheel turning device12. That is, the VGRS actuator250can be considered as a form in which the housing252is integral with the housing30of the wheel turning device12. Owing to such a construction, the VGRS actuator250is fixed to the wheel turning device12, so that the housing252is provided in the vehicle body, unrotatably relative to the vehicle body.

The input shaft254is configured to have a stepped shaft portion272and a flange portion274. The shaft portion272has a blind hole270formed to extend from its lower end portion in an axially upward direction. The flange portion274is formed integrally with the shaft portion272, and is contiguous to the lower end portion of the shaft portion272. The input shaft254includes an extending portion that extends out from an upper portion of the housing252, and a serration is formed in an outer periphery of the extending portion. The input shaft254is connected, at the extending portion formed with the serration, to the universal joint84, so that the rotation is inputted from the operating device10to the input shaft254.

The output shaft256is constituted by a main shaft280, a pinion shaft282and a torsion bar284that are integral with one another. The main shaft280serves as a principal shaft and is located in an upper portion of the output shaft256. The pinion shaft282serves also as an input shaft of the wheel turning device12and is located in a lower portion of the output shaft256. The torsion bar284interconnects the main shaft280and the pinion shaft282. The main shaft280is hollow, and is formed integrally with a flange portion286and an annular portion288. The flange portion286extends from an axially intermediate portion of the main shaft280in a direction perpendicular to the axial direction. The annular portion288extends from an outer periphery of the flange portion286in the axial direction. The pinion shaft282has, in its upper portion, a blind hole axially extending from it upper end portion. The pinion36is formed in an axially intermediate portion of the pinion shaft282. A lower end portion of the main shaft280is introduced in the blind hole of the pinion shaft282via a bushing290, such that the main shaft280and the pinion shaft282are rotatable relative to each other. The torsion bar284, which is disposed inside the main shaft280, is fixed at its upper end portion to an upper end portion of the main shaft280through a pin292, so that the torsion bar284and the main shaft280are unrotatable relative to each other. Further, the torsion bar284is held in serration engagement with a bottom portion of the blind hole of the pinion shaft282, so that the torsion bar284and the pinion shaft282are unrotatable relative to each other. Owing to such a construction, the output shaft256allows twisting of the torsion bar284, so that the output shaft256per se is twisted by an amount corresponding to an amount of the twisting of the torsion bar284.

The upper portion of the main shaft280constituting the output shaft256is introduced in the blind hole270of the input shaft254, and an bearings300are interposed between an inner circumferential surface of the blind hole270and an outer circumferential surface of an upper portion of the main shaft280, so that the input shaft254and the main shaft280are rotatable relative to each other. The input shaft254is rotatably held in an inner surface of the upper housing260via a bearing302. Further, the pinion shaft282constituting the output shaft256has an intermediate portion that is held by the lower housing262and the housing30of the wheel turning device12via a bearing304, and a lower end portion that is held by the housing30via a bushing306, so that the pinion shaft282is rotatably held by the lower housing262and the housing30. Owing to the construction as described above, the input shaft254and the output shaft256are disposed coaxially with each other, and are rotatable relative to each other and relative to the housing252. The wheel turning device12including the pinion shaft282has a construction that is substantially the same as that of the first embodiment, and description of the construction is omitted.

Like in the first embodiment, the movement-amount variably transmitting mechanism92employs a harmonic gear mechanism. In the VGRS actuator250, the motor150is provided as a drive power source of the harmonic gear mechanism. The motor shaft152as an output shaft of the motor150is hollow, and is disposed such that the input shaft254and the output shaft256are introduced in the motor shaft152per se. Described in detail, the motor shaft152is rotatably held by the input shaft254via a bearing320and a bushing322, so as to be rotatable relative to the housing252. Like in the first embodiment, the permanent magnets158constituting the rotor are disposed on the outer peripheral portion of the motor shaft152, while the polar bodies160constituting the stator are disposed on the inner surface of the housing252, so that the motor150serves as a so-called brushless motor. Further, like in the first embodiment, the rotational position of the motor shaft152is detected by the resolver164, so that an outcome of the detection made by the resolver164is utilized, for example, in a control for selecting ones of the polar bodies160that are to be energized and a control for controlling the rotational velocity of the motor150.

The movement-amount variably transmitting mechanism92is constituted to include the stator gear180, driven gear182, flexible gear184and wave generator186, like in the first embodiment. The stator gear180is fixed to an outer periphery of the flange portion274of the input shaft254, while the driven gear182is fixed to the annular portion288of the main shaft280constituting the output shaft256. Owing to such a construction, in the movement-amount variably transmitting mechanism92, the input shaft254rotatably provided in the housing252and the stator gear180connected to the input shaft254function as the first element, while the main shaft280of the output shaft256rotatably provided in the housing252and the driven gear182connected to the main shaft280serve as the second element. The other portions of the movement-amount variably transmitting mechanism92are substantially the same as those in the first embodiment with respect to constructions, movements and functions, so that descriptions thereof are omitted herein. Further, the VGRS actuator250has a motor-shaft rotation locking mechanism200as the third-element-movement inhibiting device and the tolerance ring207. That is, the VGRS actuator250has the third-element-movement allowing mechanism configured to allow the rotation of the motor shaft152even in a case in which the rotation of the motor shaft152is inhibited by the locking mechanism200, and also the third-element-movement inhibition cancelling device constructed to include the third-element-movement allowing mechanism. Since the constructions of the locking mechanism200and the tolerance ring207are substantially the same as those in the first embodiment, descriptions thereof are omitted.

The VGRS actuator250has three resolvers330,332,334in addition to the above-described resolver164. The resolver330is disposed between the input shaft254and an inner surface of the housing252, and is capable of detecting a rotational angular position of the input shaft254. The resolver332is disposed between a lower portion of the main shaft280constituting the output shaft256and the inner surface of the housing252, and is capable of detecting a rotational angular position of the main shaft280. The resolver334is disposed between the pinion shaft282constituting the output shaft256and the inner surface of the housing252, and is capable of detecting a rotational angular position of the pinion shaft282. It is therefore possible to detect an amount of a relative rotational displacement of the main shaft280and the pinion shaft282, from detection signals supplied from the resolvers332,334, so that the resolvers332,334function as the operating force detector that is configured to detect the operating force applied to the steering wheel20based on the relative rotational displacement amount. That is, the present VGRS actuator250has the operating force detector built therein, and the operating force detector is constructed to include the deformable member in the form of the torsion bar284that is disposed between the wheel turning device12and the driven gear182constituting the second element, and the deformation amount sensor configured to detect an amount of twisting deformation of the torsion bar284.

Further, from the detection signals supplied from the resolvers330,332or from the detection signals supplied from the resolvers330,334, it is possible to detect a difference or ratio between the rotational angle of the input shaft254and the rotational angle of the pinion shaft282. Like in the first embodiment, an outcome of the detection made by the resolvers is utilized in the determination of the amount of the assisting force and the rotational direction of the wave generator186and also in the control of the rotation ratio between the input shaft254and the output shaft256. Further, from the detection signals supplied from the resolvers330,332, it is possible to detect a relative rotational angle of the stator gear180and the driven gear182as the amount of the relative movement of the first and second elements. Thus, the present steering system is arranged to have the relative-movement amount detector configured to detect the amount of the relative movement of the first and second elements.

(B) Controls of Steering System

The controls executed during either the normal state or failure state in the steering system of the present embodiment is substantially the same as those of the above-described first embodiment, except for some parts only which will be described.

In the VGRS actuator14in the first embodiment, the torsion bar114is provided in the input shaft82. On the other hand, in the VGRS actuator250in the present embodiment, the torsion bar284is provided in the output shaft256. That is, the operating force detector is provided for the output shaft256. Therefore, in a state in which the rotation of the motor shaft152is inhibited by the locking mechanism200in the relative-rotation disabled state, i.e., in a state in which the deadlock occurs in the movement-amount variably transmitting mechanism92, the torsion bar114is not twisted even when a large amount of the operating force is applied to the steering wheel20. Therefore, the operating force cannot be detected by the above-described operating force detector. In view of this, in the controls executed in the steering system of the present embodiment, the determination regarding the second condition (S23in the flow chart ofFIG. 8) is not performed, and the second failure control based on the determination regarding the second condition is not executed. Thus, in the steering system of the present embodiment, there is not exists the operating-force-basis releasing control portion B232(seeFIG. 9) as the functional force included in the ECU15of the system of the above-described embodiment.

Further, in the controls executed in the steering system of the present embodiment, when the determination regarding the third condition is performed, the rotational amount ΔθSof the stator gear180is recognized based on the detected value detected by the resolver330, and the rotational amount ΔθDof the driven gear182is recognized based on the detected value detected by the resolver332.

Third Embodiment

A steering system of the third embodiment has the same construction as the steering system of the second embodiment. However, the steering system of the third embodiment is different from the steering system of the second embodiment with respect to controls, specifically, controls related to VGRS actuator250and the assisting mechanism52of the wheel turning device12.

Regarding the controls related to the VGRS actuator250, the first failure control, i.e., the control for coping with the disconnection and overloading of the motor150is executed as in the first and second embodiments, but the second failure control, i.e., the control for coping with the relative-rotation disabled state, in which the stator gear180and the driven gear182are disabled from being rotated relative to each other due to sticking of the elements, is executed in a different manner. In the present embodiment, the second failure control is executed not only after execution of the first failure control but also without execution of the first failure control when the relative-rotation disabled state is recognized.

Described in detail, when the second failure control is executed after execution of the first failure control, the motor150is simply placed in the above-described motor free state, without releasing the locking of the motor shaft152, which has been made in the first failure control. When the second failure control is executed without execution of the first failure control, the motor150is placed in the motor free state and the motor shaft152is locked by the locking mechanism200, for example, for purpose of preventing overloading of the motor150.

Since the second failure control is executed as described above, the rotation of the motor shaft152is inhibited by the locking mechanism200during the execution of the second failure control. However, owing the provision of the above-described tolerance ring207, when the amount of the movement force acting on the motor shaft152exceeds the amount of the frictional force of the tolerance ring207, the rotation of the motor shaft152is allowed. Owing to the function of this tolerance ring207, i.e., owing to the function of the third-element-movement allowing mechanism, even in the case in which the motor shaft152is locked by the locking mechanism200during the relative-rotation disabled state, the wheel can be turned in a state in which the stator gear180and the driven gear182are integral with each other, i.e., in a state of the transmission ratio of 1:1, without a deadlock occurring in the movement-amount variably transmitting mechanism92.

However, during the relative-rotation disabled state, the steering operation has to be performed with application of a sufficiently large amount of the operating force that enables the movement force acting on the motor shaft152to overcome the frictional force generated by the tolerance ring207. Therefore, a large burden is imposed on the vehicle operator. With this taken into account, in the present embodiment, during the execution of the second failure control, the assisting-force increasing control is executed for increasing the assisting force by the assisting mechanism52.

During the normal state, the assisting mechanism52is caused to generate the assisting force FAas shown inFIG. 11(a). On the other hand, during the execution of the assisting-force increasing control, an assisting force ΔFAis additionally generated as shown inFIG. 11(b). That is, there is added the assisting force ΔFAcorresponding to the frictional force so that the movement force that precisely offsets the frictional force acts on the motor shaft162. Therefore, as shown inFIG. 11(c), during the execution of the assisting-force increasing control, the assisting force FAis generated by the assisting mechanism52by an amount that is larger than in the normal state. The added assisting force ΔFAis transmitted to the motor shaft152via the output shaft256and the driven gear182, and serves as a force acting against the frictional force generated by the tolerance ring207. Consequently, the wheel turning can be made, theorically, by the same amount of the operating force as that applied to the steering wheel20in the normal state. Thus, in the present embodiment, the burden imposed on the vehicle operator is alleviated during the relative-movement disabled state, by increasing the assisting force generated by the assisting mechanism52. Since the assisting force FAis controlled by controlling the supplied electric power W supplied to the electric motor of the assisting mechanism52, as described above, the supplied electric power is increased by an electric power amount ΔW that corresponds to the added assisting force ΔFAfor increasing the assisting force in the actual execution of the assisting-force increasing control.

In the steering system of the present embodiment, the actuator control is executed according to the actuator control program shown inFIG. 12. Like in the control executed in the above-described embodiments, in procedures according to this program, S33and S34are implemented to determine whether the motor is disconnected or overloaded. When the motor is either disconnected or overloaded, S44and S45are implemented whereby the first failure control is initiated so that the motor shaft152is locked by the locking mechanism200and the failure flag F is set to 1 (one). When the motor is neither disconnected nor overloaded, S35through S37are implemented to perform determinations. These determinations are substantially the same as the determination regarding the third condition in the above-described embodiments, and it is determined, when the stator gear180is being rotated, whether the difference between the rotational amount Δθsof the stator gear180and the rotational amount ΔθDof the driven gear182is smaller than the predetermined threshold Δθ0or not, namely, whether the transmitting mechanism92is placed in the relative-rotation disabled state or not. However, S35is implemented to determine whether the vehicle running speed v is being changed or not, so that S36and S37are implemented to perform the determination regarding the relative-rotation disabled state, upon satisfaction of a condition that the vehicle running speed v is being changed. When the first failure control is not executed, S39through S43are implemented to execute the transmission ratio control. Therefore, the above-described condition is provided for assuring an accurate recognition of the relative-rotation disabled state.

When the transmitting mechanism92is placed in the relative-rotation disabled state, S46through S48are implemented to initiate the second failure control, so that the motor shaft152is locked by the locking mechanism200, the motor150is placed in the above-described motor free state, and the failure flag is set to 2 (two). When the transmitting mechanism92is not placed in the relative-rotation disabled state, S39through S43are implemented to carry out procedures for the transmission ratio control in the same manner as in the above-described embodiments.

Once after the first failure control has been initiated, the determinations made in S36and S37, i.e., the determination regarding the relative-rotation disabled state is handled by the determination performed in S32. In S32, since the determination is performed in a state in which the locking of the motor shaft152is maintained, it is performed by utilizing the play240between the distal end portion210of the lock lever206and each recessed portion214of the lock holder208, as described above. When the transmitting mechanism92is placed in the relative-rotation disabled state, the second failure control is initiated by implementations of S46and steps following S46. When the transmitting mechanism92is not placed in the relative-rotation disabled state, the execution of the first failure control is continued. Once after the second failure control has been initiated, it is continued depending on the determination of S31.

Further, in the present steering system according to the present embodiment, the assisting control is executed according to the assisting control program shown inFIG. 13. In this control, S53is implemented to make the determination utilizing the failure flag F, i.e., the determination as to whether the transmitting mechanism92is placed in the relative-rotation disabled state or not. When the transmitting mechanism92is not placed in the relative-rotation disabled state, the wheel turning force is assisted in the same manner as in the first and second embodiments. When the transmitting mechanism92is placed in the relative-rotation disabled state, S55is implemented to determine whether the steering operation is being carried out or not. Only when the steering operation is being carried out, S56is implemented to increase the electric power W supplied to the electric motor of the assisting mechanism52so as to increase the assisting force, as described above. It is noted that the determination as to whether the steering operation is being carried out or not is made based on a slight change of the rotational angle of the steering wheel20that is detected by the operation angle sensor28, and that the direction of the steering operation is also specified base on the change when it is recognized that the steering operation is being carried out.

The ECU16, which is configured to perform procedures according to the above-described actuator control program and assisting control program, can be considered to have functional constructions as shown inFIG. 14. Described in detail, the ECU16has an assisting control portion C1configured to perform procedures according to the assisting control program. This assisting control portion C1has a functional portion configured to procedures of S55, S56depending on a determination of S53, namely, an assisting-force increasing portion C11configured to increase the assisting force by operation of the assisting mechanism52in the relative-rotation disabled state.

Further, the ECU16has an actuator control portion C2as the movement-amount variably transmitting device control portion configured to perform procedures according to the actuator control program. Like in the system of the above-described embodiments, the actuator control portion C2has a transmission ratio control portion C21and a first failure control portion C22as the third-element-movement inhibiting control portion. The actuator control portion C2further has a second failure control portion C23as a functional portion configured to perform procedures of S46through S48depending on the detection of the relative-rotation disabled state.

Although the steering system of the present embodiment is a system having the constructions as described above, it can be modified as follows. For example, while the present system employs the VGRS actuator250as in the second embodiment, it can employ also the VGRS actuator14as in the first embodiment. In the VGRS actuator14, the torsion bar114is disposed between the stator gear180and the steering wheel20. It might be considered that the operating force has to be made large due to the frictional force of the tolerance ring207during the execution of the second failure control. However, in the case of the employment of the VGRS actuator14, the amount of twisting deformation of the torsion bar114is increased as a result of the increase of the operating force, because of the position of provision of the torsion bar114. Consequently, where the VGRS actuator14is employed, it is possible to expect that the assisting force can be increased by a certain degree even without relying on the assisting-force increasing control. Therefore, where the VGRS actuator14is employed, the merit provided by the assisting-force increasing control is smaller than where the VGRS actuator250is employed as in the second embodiment.

In the above-described system, the assisting force ΔFAis added such that the movement force precisely offsetting the frictional force generated by the tolerance ring207acts on the motor shaft152in the execution of the assisting-force increasing control. However, in place of adding such an amount of the assisting force ΔFA, a different amount of the assisting force ΔFAmay be added such that the movement force offsetting some percentage of the frictional force acts on the motor shaft152.

In the above-described system, as the determination made prior to the initiation of the execution of the second failure control during the execution of the first failure control, only the determination substantially the same as the determination regarding the third condition in the above-described embodiments is performed, namely, only the determination based on the difference between the rotational amount ΔθS of the stator gear180and the rotational amount ΔθD of the driven gear182is performed. However, in addition to or in place of such a determination, the determination regarding the first condition in the above-described embodiment, i.e., the determination based on the acting force acting on the locking mechanism200may be performed so that the second failure control is executed depending on an outcome of this determination.