1. Field of the Invention
The invention relates to a power output apparatus, a vehicle including the power output apparatus, and a control unit and method for the power output apparatus.
2. Description of the Related Art
A power output apparatus is described, for example, in Japanese Patent Application Publication No. JP-2003-143707. In the described power output apparatus, an engine, an electric power generator, and an output shaft are connected to a planetary gear unit, and a drive motor is connected to the output shaft. In the power output apparatus, the rotational speed of the drive motor may be calculated by multiple methods. For example, the rotational speed of the drive motor may be calculated based on the rotational speed of the output shaft, the position of the rotor of the drive motor, and the engine speed and the rotational speed of the electric power generator. Thus, the rotational speed of the drive motor is calculated more accurately. As a result, the drive motor is controlled more reliably.
Some power output apparatuses including a transmission, arranged between a drive motor and an output shaft, that performs the shifting operation while outputting power, in addition to an engine, a planetary gear unit, an electric power generator, and the drive motor, make a synchronous determination, in which it is determined whether the engine and the drive motor operate in synchronization with each other, in order to smoothly change the gear ratio in the transmission, based on the rotational speed of the output shaft detected by an output shaft rotational speed sensor or the rotational speed of the output shaft estimated based on the engine speed and the rotational speed of the electric power generator, and the rotational speed of the drive motor. If the gear ratio is changed in the transmission when the rotational speed of the output shaft cannot be estimated, the following inconvenience may occur. For example, if a malfunction in the output shaft rotational speed sensor occurs during the shifting operation, the power output apparatus is unable to make an appropriate synchronous determination. This is because, the appropriate rotational speed of the output shaft can neither be detected by the output shaft rotational speed sensor nor estimated based on the engine speed and the rotational speed of the electric power generator. As a result, the components of the transmission may wear out or a shock may be imparted to the vehicle.