1. Field of the Invention
The present invention relates to the improvement of reliability, or the reduction of failure rate, of an automotive vehicle comprising a redundant controller including a plurality of redundant elements.
2. Discussion of Related Art
An automotive vehicle is comprised of a great number of components such as parts, subassemblies, etc. Therefore, the reliability of the automotive vehicle as a whole cannot be increased to a sufficiently high degree, i.e., the failure rate of the vehicle cannot be decreased to a sufficiently low degree, unless the reliability of each of the components can be increased to a significantly high degree, i.e., unless the failure rate of the each component can be decreased to a significantly low degree. In fact, however, there are some limits to the improvements of reliability of components, and there are some cases where, though the improvements of reliability of components are technically possible, the production costs of the improved components are significantly increased and accordingly those components are not practically usable. Hence, it has been practiced to make an important component of an automotive vehicle redundant, i.e., double, triple, or more, so that even if one of the redundant components may fail, the automotive vehicle as a whole can continue to function. Meanwhile, there has been a tendency that various important functions of an automotive vehicle are electronically controlled. For example, an electronically controlled braking system is used in a commercially available automotive vehicle, and use of an electronically controlled steering system is on the table. However, generally, electronic parts are more sensitive than mechanical parts, and accordingly respective rates of malfunctions and/or failures of the electronic parts are higher than those of the mechanical ones. Therefore, there is a greater need to make electronic parts redundant.
However, it is not easy to achieve a target, or a higher, degree of reliability of an automotive vehicle as a whole, and various countermeasures have been proposed. For example, Patent Document 1 (Japanese Patent Publication No. 9-151780) discloses two electronic control units which control respective objects in an automotive vehicle and each one of which includes a primary CPU (central processing unit) and a secondary CPU that monitor each other with respect to their operations. If the respective operations of the two CPUs are both normal, the each one control unit sends, to the other control unit, an electric signal indicative of the fact. Thus, the two electronic control units monitor each other with respect to their operations. Patent Document 2 (Japanese Patent Publication No. 6-15333) discloses a plurality of electric motors which are connected, parallel to each other, to a battery. When the voltage of the battery is not normal, the supply of electricity from the battery is cut, first, from one of the electric motors that least influences the running function of a vehicle, and is cut last from a power steering motor as the last one of the electric motors. Patent Document 3 (Japanese Patent Publication No. 5-125971) discloses an electrically operated rear-wheel steering apparatus. When a steering angle of rear wheels of a vehicle is abnormally fixed because of a failure of the steering apparatus, then a degree of opening of a throttle is restrained to keep safety. More specifically described, in the case where the abnormally fixed steering angle of the rear wheels is large enough to influence substantially the running function of the vehicle, the degree of opening of the throttle is largely restrained; and in the case where the abnormally fixed steering angle is not so large as to influence the running function, the degree of opening of the throttle is not restrained so largely.