The present invention relates to a receiver for a tire condition monitoring apparatus applied to a vehicle, and more specifically, the present invention pertains to a technology for receiving tire condition detection signals wirelessly transmitted from transmitters mounted on vehicle tires by a receiver provided on a vehicle body in a suitable manner.
Wireless type tire condition monitoring apparatuses have been proposed for permitting a driver to check the conditions of vehicle tires from the interior of the passenger compartment (for example, refer to Japanese Patent No. 3596490). One such tire condition monitoring apparatus includes transmitters and a receiver. Each transmitter is attached to one of vehicle tires, and the receiver is provided in the body of the vehicle. Each transmitter detects the condition of the corresponding tire, that is, the internal air pressure and the internal temperature, and transmits a tire condition detection signal on a radio wave from a transmission antenna. The receiver receives the tire condition detection signal from each transmitter through a reception antenna, and displays the information of the tire condition on a display provided in the passenger compartment as necessary.
In the above-mentioned tire condition monitoring apparatus, the radiation direction of the radio wave from each transmitter changes as the associated tire is rotated, and various vehicle body components that have influence on the radio waves exist between the transmitters and the reception antenna. Thus, electromagnetic interference such as multipath fading easily occurs, and the radio wave from each transmitter is not reliably received by the receiver. FIG. 4 is a graph showing an example of the relationship between the rotational angle of the tire and the radio wave reception level at the receiver. As shown in FIG. 4, points where the reception level decreases extremely, that is, null points occur while the tire rotates once, for example, due to multipath fading. At the null points, the reception level becomes less than a permissible level, and thus the radio wave from the transmitter cannot be substantially received.
To solve the above-mentioned problem, a tire condition monitoring apparatus that employs a space diversity system has been proposed in, for example, Japanese Patent No. 4041958. The receiver for tire condition monitoring apparatus disclosed in the above publication includes units each having a reception antenna. The units can be connected in series. Each unit includes a demodulated signal generating section, a reception level signal generating section, a comparator circuit, and a selection circuit. The demodulated signal generating section demodulates a reception signal supplied from the reception antenna of the associated unit, and outputs a demodulated signal to the selection circuit. The reception level signal generating section generates a reception level signal that represents the level of the reception signal, and outputs the reception level signal to the comparator circuit and the selection circuit. The comparator circuit compares the reception level signal of the associated unit with the reception level signal input from another unit. Then, the comparator circuit generates a selection indicator signal corresponding to the comparison result and outputs the signal to the selection circuit. The selection circuit performs switching operation so as to select the reception signal having greater level from the reception signal of the associated unit and the reception signal of the other unit based on the selection indicator signal. More specifically, the selection circuit receives the demodulated signal and the reception level signal from another unit in addition to the demodulated signal and the reception level signal of the associated unit. Then, when the reception level signal of the associated unit is greater than the reception level signal of the other unit, the selection circuit selects and outputs the demodulated signal and the reception level signal of the associated unit. When the reception level signal of the associated unit is smaller than the reception level signal of the other unit, the selection circuit selects and outputs the demodulated signal and the reception level signal of the other unit.
Therefore, when at least two or more of the above-mentioned units are connected in series, the reception signal having the greatest level is output from the final stage unit. That is, in the receiver of the apparatus disclosed in the above-mentioned Japanese Patent No. 4041958, among signals simultaneously received by spatially separated reception antennas, the signal having the greatest level can be selected and used.
The receiver of the apparatus disclosed in the above-mentioned Japanese Patent No. 4041958 requires several units each including the demodulated signal generating section, the reception level signal generating section, the comparator circuit, and the selection circuit to achieve space diversity. Therefore, the configuration of the receiver is complicated and costs are increased.
To simplify the configuration and reduce the costs, the configuration of the apparatus disclosed in the above-mentioned Japanese Patent No. 4041958 may be changed to be provided with a plurality of reception level signal generating sections respectively generating reception level signals representing the level of the reception signals sent from reception antennas, a single comparator circuit, which compares the reception level signals and generates a selection indicator signal corresponding to the comparison result, a single selection circuit, which performs switching operation to select the reception signal having the greatest level based on the selection indicator signal, and a single demodulated signal generating section, which demodulates the reception signal selected by the selection circuit. With this configuration, the comparator circuit, the selection circuit, and the demodulated signal generating section may be provided only one each.
In the above-mentioned configuration, it takes a certain time to determine the level of the reception signals sent from the reception antennas and perform switching operation to select the reception signal having the greatest level. However, the transmitters of the tire condition monitoring apparatus are generally configured to intermittently transmit a relatively short data frame (for example, a length of several microseconds to several tens of microseconds) at predetermined time intervals (for example, one minute intervals). Thus, even if the level of the reception signals are determined and switching operation is performed to select the reception signal having the greatest level while receiving such a short data frame, the data frame cannot actually be completely demodulated due to the time required for the switching operation, and results in the occurrence of a bit error.