Recently, there is increasing demand on electronic devices for higher safety and reliability in communications. For example, in vehicle control systems, there have been studying a system related to a safety of driver such as a brake system and a steering system. In that system, a desired controlling force is calculated by a control device using information as an input which has been digitized by a measuring device from a controlling force of a driver, which is then transmitted by way of signal transmission to a final controlled object, such as a brake mechanism, a motor arranged near a front or rear wheel axle or a hydraulic machine. According to this system, a mechanical system directly connecting the driver to the controlled object is omitted, so that it is possible to save space and to enhance arrangement flexibility of a driver seat and/or driving mechanism. (the system is called as “Steer-by-Wire” etc.)
Similarly, other than communications between electronic devices, there is also demand for higher safety and reliability in communications between control units arranged in a common electronic device. In particular, arising demand therefor is made in communications between control units in an electronic device, such as a control device directed to vehicle running whose failure affects to threaten directly the safety of the driver. In the following, description will be made citing an electric power steering device, as a specific example.
An electric power steering device, one of the vehicle control systems, is a device to assist driver's steering, which achieves assistance by applying an assist force using a motor on a joining section (column shaft, rack and pinion) between a steering wheel and tires. The assist force of the motor is typically calculated by a control unit on the basis of the steering force of the driver detected by a sensor, and is output by controlling a motor driving unit. When signal transmission is used in electric power steering control, a failure in signal transmission leads to an abnormality in the assist force of the motor. In some cases, the abnormality in the assist force of the motor makes influence on drivers steering to finally threaten the safety of the driver.
In a conventional electric power steering device, as shown in FIG. 16, reliable communications are made between two different control units (for example, Patent Document 1). Information necessary for the control, such as vehicle information and torque information measured by a sensor 61, is input in a control unit 62, and a current controlled by the control unit 62 in response to the input information is input in the motor 63, so that the electric power steering device is actuated according to a torque output by the motor 53 in response to the current. In the control process of the control unit 62, firstly, a first control unit 64 calculates a motor-current command value using input information from the sensor 61, and transmits the command value to a second control unit 67 by way of a high-speed communication path 65 and a low-speed communication path 66. The second control unit 67 controls a motor driving unit 69 to output a current according to the received motor-current command value. The motor driving unit 69 generates a current under control of the second control unit 67, and outputs it to the motor 63.
In the control process of the control unit 62, the motor-current command value transmitted from the first control unit, 64 to the second control unit 67 finally determines the torque to be output by the motor 63, and thus a higher safety and reliability is required thereto. This is the reason why the communication path is made redundant as the high-speed communication path 65 and the low-speed communication path 66. The way to utilize redundant communication data in the redundant communication paths is that the data communicated through the high-speed communication path 65 is normally utilized, and when an abnormality occurs in the high-speed communication path 65, data switching is made so that the data communicated through the low-speed communication path 66 is utilized. In this way, in the plurality of communication paths, when an abnormality occurs in the high-speed communication path that is utilized for communication in normal time, the path is switched to the low-speed communication path, to thereby continue the control and ensure the reliability.