1. Field of the Invention
The present invention relates to a vehicle wheel information acquiring device that includes a vehicle wheel sensor fixed to a vehicle wheel and a vehicle-body-side device fixed to a vehicle body and that causes the vehicle wheel sensor to transmit a vehicle wheel state quantity such as a tire pressure to the vehicle-body-side device using a radio signal and causes the vehicle-body-side device to acquire the vehicle wheel state quantity.
2. Description of Related Art
In the related art, a device is known which detects a vehicle wheel state quantity such as a tire pressure and notifies a driver of the vehicle wheel state quantity. For example, in a tire pressure detector, proposed in Japanese Patent Application Publication No. 2006-312342 (JP 2006-312342 A), each vehicle wheel is provided with a sensor that detects a tire pressure, tire pressure information is transmitted from the sensors using a radio signal, and a vehicle-body-side device receives the radio signal and acquires the tire pressure information. When it is determined on the basis of received vehicle wheel information that the tire pressure is lowered, the vehicle-body-side device displays the intent on an annunciator to notify a driver of the intent.
However, since the sensor revolves around an axle with the rotation of the corresponding vehicle wheel, the received intensity of the radio signal transmitted from the sensor in the vehicle-body-side device varies depending on the rotation position of the vehicle wheel. Accordingly, a vehicle wheel rotation position at which the vehicle-body-side device cannot receive the radio signal is present. FIG. 8 illustrates an example of a relationship between rotation positions (0° to 360°) of a vehicle wheel and received signal intensity indices. As can be seen from the drawing, the received signal intensity depends on the rotation position of the vehicle wheel (the revolution position of the sensor) and the received signal intensity may be less than a reception limit depending on the rotation position of the vehicle wheel. Accordingly, when a vehicle stops at a vehicle wheel rotation position at which the radio signal cannot be received, vehicle wheel information such as a tire pressure cannot be acquired before start of driving at the time of the next start of the running.