CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent Application No. 2018-082555, which was filed on Apr. 23, 2018, the disclosure of which is herein incorporated by reference in its entirety.

BACKGROUND

Technical Field

The following disclosure relates to a steering system in which steering of a wheel in accordance with an operation of an operation member by a driver is achieved by a steering device.

Description of Related Art

A steering system described in Japanese Patent Application Publication No. 2010-36806, for instance, is proposed in the field of vehicle steering systems. A controller in the steering system is configured to execute two processes, i.e., a process of controlling a steering torque (which is one sort of a main process) and a failure diagnosis process of diagnosing a failure of the controller (which is one sort of an auxiliary process). In the steering system, the controller executes the failure diagnosis process on a higher priority basis than the process of controlling the steering torque.

SUMMARY

A steering system in which steering of a wheel in accordance with an operation of the operation member is performed by a steering device under control of the controller, namely, what is called steer-by-wire steering system, has been actively developed in recent years. The controller in such a steering system suffers from a relatively heavy load in a process in which the controller controls the steering device to perform the steering of the wheel in accordance with the operation of the operation member. (The process will be hereinafter referred to as “steering process” where appropriate). Thus, in the case where the controller executes both of the main process including the steering process and the auxiliary process relating to an operation of the steering system, the problem of the load on the controller is serious. The problem is not solved by the conventional technique of executing the auxiliary process on a higher priority basis described in the above Publication. An attempt to solve the problem by use of a high-performance controller inevitably pushes up a cost of the steering system, so that the steering system equipped with such a controller does not have high utility. Accordingly, the present disclosure is directed to a steer-by-wire steering system having high utility.

In one aspect of the disclosure, a steering system includes a controller configured to parallelly execute a main process including a process in which the controller controls a steering device to perform steering of the wheel in accordance with an operation of an operation member and an auxiliary process relating to an operation of the steering system and configured to decrease an execution ratio of the main process and increase an execution ratio of the auxiliary process when a condition that a running speed of a vehicle is not lower than a threshold speed is satisfied.

When the running speed of the vehicle becomes higher, a force by which the wheel is steered need not be so large. Accordingly, even if the execution ratio of the main process is decreased when the running speed of the vehicle is high, any particularly serious problem does not arise. By decreasing the execution ratio of the main process, the execution ratio of the auxiliary process can be increased by an amount corresponding to the decrease in the execution ratio of the main process, thus enabling the auxiliary process to be thorough and sufficient one. According to the steering system of the present disclosure, by changing the execution ratios of the main process and the auxiliary process in the controller, a high-performance steering system can be constructed while preventing a processing load of the controller from becoming excessive.

There will be exemplified and explained various forms of the present disclosure. Each of the forms is numbered and depends from the other form or forms, where appropriate. This is for easier understanding of the forms of the present disclosure, and it is to be understood that combinations of constituent elements that constitute the present disclosure are not limited to those described in the following forms. That is, it is to be understood that the present disclosure shall be construed in the light of the following description of various forms and embodiments. It is to be further understood that, as long as the present disclosure is construed in this way, any form in which one or more constituent elements is/are added to or deleted from any one of the following forms may be considered as one form of the present disclosure.

(1) A steering system of a vehicle, comprising an operation member to be operated by a driver, a steering device configured to steer a wheel, and a controller configured to control the steering system,

wherein the controller is configured to parallelly execute a main process including a process in which the controller controls the steering device to perform steering of the wheel in accordance with an operation of the operation member and an auxiliary process relating to an operation of the steering system and configured to decrease an execution ratio of the main process and increase an execution ratio of the auxiliary process when a condition that a running speed of the vehicle is not lower than a threshold speed is satisfied.

The “controller” in this form is an ordinary device whose main constituent element is a computer including a processor, a RAM, a ROM, and so on. Here, an amount of all processes executed by the controller per unit time is defined as an overall processing amount. In this case, the “execution ratio of the main process” and the “execution ratio of the auxiliary process” in the controller are respectively regarded as a ratio of a processing amount of the main process to the overall processing amount and a ratio of a processing amount of the auxiliary process to the overall processing amount. The execution ratio may be referred to as a processing load, a processing burden or the like of the controller in each of the main process and the auxiliary process. As later explained, the processing load of the controller is prevented from becoming excessive by changing the execution ratio of the main process and the execution ratio of the auxiliary process depending on circumstances. In the case where the main process should be executed on a higher priority basis than the auxiliary process, the execution ratio of the main process is increased. On the other hand, in the case where a large amount of processes are not required for the main process, the execution ratio of the main process may be decreased. In this case, by increasing the execution ratio of the auxiliary process by an amount corresponding to the decrease in the execution ratio of the main process, it is possible to execute a sufficient and thorough auxiliary process.

In this form, it is assumed that the load imposed on the controller by execution of the main process when the vehicle is running at a relatively high speed is light, and a thorough and sufficient auxiliary process is executed in high-speed running of the vehicle. This form enables construction of a high-performance steering system while preventing the processing load of the controller from becoming too large. When the running speed of the vehicle is relatively high, the steering of the wheel by the steering device can be performed with a comparatively small force, so that the load that acts on the steering device is comparatively light. This form may be regarded as a form in which the load of the controller by execution of the main process is lightened and the auxiliary process is thoroughly and sufficiently executed when the vehicle running speed is relatively high.

In general, there is an upper limit in the overall processing amount of the controller as a whole. From the viewpoint of making the most of the overall processing amount, the controller is desirably execute, all the times, the processes such that the overall processing amount is close to the upper limit. In the case where the controller executes the processes to such an extent that the overall processing amount is close to the upper limit, an increase in the execution ratio of the main process results in a decrease in the execution ratio of the auxiliary process, and an increase in the execution ratio of the auxiliary process results in a decrease in the execution ratio of the main process. In the case where the execution ratio of the main process and the execution ratio of the auxiliary process are changed, a time period for which each process is executed may be changed by changing details of each process or the number of executions of each process per unit time may be changed without changing details of each process.

In the case where the controller includes a plurality of control units each including a computer, each of the plurality of control units may change the execution ratio of the main process and the execution ratio of the auxiliary process. Alternatively, only at least one control unit, as a part of the plurality of control units, may change the execution ratio of the main process and the execution ratio of the auxiliary process. In either configuration, the execution ratio of the main process and the execution ratio of the auxiliary process in the controller as a whole are changed. Further, the execution ratio of the main process or the execution ratio of the auxiliary process may be 0 irrespective of whether the execution ratios are those in the controller as a whole or the execution ratios are those in each control unit. That is, the execution ratio of the main process and the execution ratio of the auxiliary process may be changed such that one of the main process and the auxiliary process is not to be executed in the controller as a whole or in each control unit.

The “process in which the controller controls the steering device to perform steering of the wheel in accordance with the operation of the operation member” included in the main process is a basic process in the steer-by-wire steering system. (This process will be hereinafter referred to as “steering process” where appropriate.) For instance, the process is a control process for controlling the steering device such that a steering angle of the wheel becomes equal to an angle that corresponds to an operating angle of a steering wheel as the operation member. The steer-by-wire steering system is often equipped with a reaction force actuator configured to give an operation reaction force to the operation member for making a steering feeling felt by a driver appropriate one. The main process may include, in addition to the steering process, a process for controlling an operation reaction force generated by the reaction force actuator and other processes in association with the steering process. (The process for controlling the operation reaction force will be hereinafter referred to as “reaction force process” where appropriate.)

The auxiliary process relating to the operation of the steering system is not directly relevant to the operation of the steering system, and the sort of the auxiliary process is not limited. For instance, the auxiliary process includes a process of confirming soundness of the steering system. Specifically, the auxiliary process includes a breakdown-tendency recognition process of recognizing that the steering system has a tendency to break down, a system appropriateness determining process of determining whether or not the steering system is in a state in which the steering of the wheel can be appropriately performed, and so on, as later explained.

(2) The steering system according to the form (1), wherein the auxiliary process includes a breakdown-tendency recognition process of recognizing that the system has a tendency to break down.

This form includes features as to the sort of the auxiliary process. The “breakdown-tendency recognition process” may include an urgent process for detecting that an electric failure or the like is occurring in the steering system right now. In the case where the steering system is configured to change the execution ratio of the auxiliary process, the breakdown-tendency recognition process may include, as a principal process, a process of recognizing that the steering system will probably break down after a lapse of a certain time if the steering system keeps operating. For instance, as the breakdown-tendency recognition process, there may be employed a process of recognizing a condition of the steering system that is not so serious and urgent, such as a condition of fatigue of constituent components of the steering device, the reaction force actuator and the like. In a steer-by-wire steering system in which the wheel is steered by a force of a drive source of the steering device without depending on the operation force applied to the operation member by the driver, it is particularly meaningful to recognize the tendency of the breakdown of the steering device.

(3) The steering system according to the form (1) or (2), wherein the auxiliary process includes a system appropriateness determining process of determining whether or not the steering system is in a state in which the steering of the wheel is appropriately performed.

There may be cases in which the steering of the wheel in accordance with the operation of the operation member is not performed due to some reason though the system is not broken down. For instance, a case is expected in which the operation amount of the operation member and the steering amount of the wheel differ from each other beyond a certain degree. Further, in an arrangement in which a plurality of sensors having the same function are provided as a redundant system, a case is expected in which detection values of the sensors differ from one another beyond a certain degree. In this form, a process of confirming the presence or absence of such cases is executed as the auxiliary process. In this respect, such cases may be regarded as cases that occur when the steering system is already suffering from the breakdown. Thus, the system appropriateness determining process may be regarded as one sort of a system breakdown determining process.

(4) The steering system according to any one of the forms (1) through (3),

wherein the steering device is configured to be operated by a drive current supplied thereto, and

wherein the controller is configured to decrease the execution ratio of the main process and increase the execution ratio of the auxiliary process when both of the condition that the running speed of the vehicle is not lower than the threshold speed and a condition that the drive current is not larger than a threshold current are satisfied.

When the drive current is large, the load that acts on the steering device is large. On the other hand, when the drive current is small, the load that acts on the steering device is small. This form takes it into account. Accordingly, this form allows the auxiliary process to be thorough and sufficient with higher reliability when the load of the steering device is small.

(5) The steering system according to any one of the forms (1) through (4),

wherein the controller is configured to be operable in a first mode and operable in a second mode in which the execution ratio of the main process is set to be lower than that in the first mode and the execution ratio of the auxiliary process is set to be higher than that in the first mode, and

wherein the controller is configured to switch an operation mode thereof from the first mode to the second mode so as to decrease the execution ratio of the main process and increase the execution process of the auxiliary process.

Although the execution ratio of the main process and the execution ratio of the auxiliary process may be continuously changed, the execution ratios may be set to respective specific values. As recited in this form, two mutually different set execution ratios may be determined for each of the main process and the auxiliary process. Further, as the operation mode of the controller, the first mode may be set such that the execution ratio of the main process is equal to a higher one of the two set execution ratios while the execution ratio of the auxiliary process is equal to a lower one of the two set execution ratios, and the second mode may be set such that the execution ratio of the main process is equal to a lower one of the two set execution ratios while the execution ratio of the auxiliary process is equal to a higher one of the two set execution ratios. By switching the operation mode of the controller between the thus set first and second modes, the execution ratios of the main process and the auxiliary process can be simply changed depending on circumstances.

(6) The steering system according to any one of the forms (1) through (5),

wherein the steering device includes a plurality of drive sources, and the controller includes a plurality of control units each of which is configured to control at least one of the plurality of control units, and

wherein the steering device is configured to steer the wheel by a drive force of at least one of the plurality of drive sources controlled by any of the plurality of control units.

(7) The steering system according to the form (6), wherein at least one of the plurality of control units is at least one both process execution unit configured to selectively execute one of the main process and the auxiliary process, and at least one of the at least one both process execution unit is configured to execute the auxiliary process in place of the main process, so that the controller decreases the execution ratio of the main process and increases the execution ratio of the auxiliary process.

According to the above two forms, the wheel is steered by the drive force of at least one of the plurality of drive sources controlled by any of the control units that are executing the main process. It is thus possible to change the execution ratio of the main process by changing the number of control units that execute the main process. According to the latter form, the at least one both process execution unit is configured to execute a selected one of the main process and the auxiliary process. It is thus possible to easily change the execution ratio of the main process and the execution ratio of the auxiliary process because a selected one of the main process and the auxiliary process is executed by the at least one both process execution unit. The number of both process execution units is not limited. For instance, only one of the control units may be the both process execution unit. Alternatively, two or more control units may be the both process execution units. In the case where two or more control units are the both process execution units, all of the two or more both process execution units may be configured to execute a selected one of the main process and the auxiliary process or only a part of the two or more both process execution units may be configured to execute a selected one of the main process and the auxiliary process.

(8) The steering system according to the form (7),

wherein the controller includes, as the at least one both process execution unit, at least two both process execution units, and

wherein the at least two both process execution units alternately execute the auxiliary process in place of the main process.

This form enables the at least two both process execution units to evenly execute the auxiliary process. Further, in the event of a failure of any one of the both process execution units, it is possible to permit other both process execution unit to execute the auxiliary process.

