1. Technical Field
The present disclosure relates to a travel control device for a vehicle including a steering transmission ratio variable device and a power steering device.
2. Description of the Related Art
As a travel control device for vehicles such as a motor vehicle, there has hitherto been well known a travel control device including a steering transmission ratio variable device and an electric power steering device in a steering transmission system between a steering wheel and steered wheels. The steering transmission ratio variable device is arranged between an upper steering shaft and a lower steering shaft, and is configured to rotate the lower steering shaft with respect to the upper steering shaft, thereby changing a steering transmission ratio. This type of travel control device is allowed to control a steering angle of the steered wheels without requiring a steering operation by the driver, thereby being capable of optimally controlling the steering transmission ratio, namely, a ratio of a change amount in a steering angle of the steered wheels with respect to an operation amount of the steering wheel depending on a vehicle speed or the like.
Further, there is also known a travel control device including a driving support device configured to calculate a target steering angle of the steered wheels for causing the vehicle to travel along a predetermined travel path, and to control the electric power steering device and the steering transmission ratio variable device so that an actual steering angle of the steered wheels conforms to the target steering angle. The travel control device including the driving support device may cause the vehicle to travel along the predetermined travel path, and may attain desired steering characteristics without requiring the steering operation by the driver.
For example, in Japanese Patent Application Laid-open No. 2011-31770, there is described a travel control device configured to carry out driving support control of controlling the electric power steering device, to thereby control the steering angle of the steered wheels so that the vehicle travels along a predetermined travel path, and to suppress a behavior change of the vehicle generated during the driving support control. With the travel control device described in Japanese Patent Application Laid-open No. 2011-31770, the vehicle can be caused to travel along the predetermined travel path while suppressing the behavior change of the vehicle.
However, there are the following problems in a related-art travel control device in which the electric power steering device is installed in the steering transmission system between the steered wheels and the steering transmission ratio variable device like the travel control device described in Japanese Patent Application Laid-open No. 2011-31770. Specifically, when the electric power steering device is controlled through the driving support control and the steering angle of the steered wheels is changed, a steering member like a lower steering shaft between the electric power steering device and the steering transmission ratio variable device is rotated. A friction force acts between members rotating relatively to each other in the steering transmission ratio variable device. Thus, when those members cannot rotate relatively to each other due to the friction force, an upper steering shaft on a steering wheel side and the lower steering shaft on a steered wheel side with respect to the steering transmission ratio variable device do not rotate relatively to each other, but integrally rotate.
A steering angle sensor configured to detect a steering angle required to control the steering transmission ratio and the like is installed on the upper steering shaft, and thus the steering angle detected by the steering angle sensor changes as the upper steering shaft rotates. The steering transmission ratio variable device is controlled so as to attain a predetermined steering transmission ratio based on a changing steering angle, and thus the steering transmission ratio variable device tries to rotate the lower steering shaft with respect to the upper steering shaft even when the driver is not steering. However, the electric power steering device is generating a steering torque so as to change the steering angle of the steered wheels, and thus the steering transmission ratio variable device cannot rotate the lower steering shaft, while the upper steering shaft is rotated by a reaction force.
In particular, while the driving support control is being carried out, the driver does not need to actively carry out the steering, a force of the driver for gripping the steering wheel is low, and a so-called hands-off state may be brought about. Therefore, a repetition of the above-mentioned phenomenon causes a repetition of a reciprocal rotation of the upper steering shaft. As a result, a minute vibration of the steering wheel and a vibration change in a steering reaction force are liable to be generated, and the driver may feel a sense of discomfort and a sense of annoyance.
In the travel control device described in Japanese Patent Application Laid-open No. 2011-31770, as shown in FIG. 8 of Japanese Patent Application Laid-open No. 2011-31770, when the driving support control is being carried out, the steering transmission ratio is changed toward a small side (so-called slow side) compared to a case in which the driving support control is not being carried out over an entire vehicle speed range. Thus, influence of the steering operation by the driver exerted on the change in the steering angle of the steered wheels can be reduced, thereby increasing a travel stability of the vehicle, but the problem of the minute vibration of the steering wheel and the vibration change in the steering reaction force cannot be solved.