1. Field of the Invention
The invention relates to a device for monitoring and wirelessly signaling a pressure or a change in pressure in pneumatic tires on vehicles.
2. Description of Related Art
A device for monitoring and wirelessly signaling a pressure or a change of pressure in pneumatic tires on vehicles, where a pressure sensor, a transmitter, a control circuit and a battery are combined in a unit referred to as “wheel electronics”, which is mounted on the rim well of the respective vehicle wheel, has been disclosed by ATZ Automobiltechnische Zeitschrift, Vol. 102, No. 11/2000, pp. 950 to 956. The wheel electronics are mounted in a housing and are then potted so that the no access to, and no change of, the battery is possible. Consequently, the service life of the wheel electronics mounted on the respective wheel is limited by the battery life. It is, therefore, important that current consumption be kept as low as possible so as to achieve the longest
possible service life for the battery and, in parallel to that, for the wheel electronics.
Instead of operating the wheel electronics continuously, one therefore effects pressure measurements at predetermined time intervals only, and the pressure values obtained typically are not transmitted every time a measurement is made, but rather from time to time only, for example once per minute. Between the different measuring operations, the wheel electronics are caused to assume a power-saving condition. It is the transmitting operation that momentarily consumes the greatest amount of energy. It loads the battery with a current pulse that may lead to an internal voltage drop in the battery. In an extreme case, the voltage may drop so far that regular operation of the electronic circuits supplied by the battery is no longer guaranteed. This risk exists not only, but especially, with aged batteries: Even if the battery has not reached the end of its service life, but still contains some residual charge, ageing combined with the associated passivation may result in increased internal resistance of the battery. And cold temperatures also increase the internal resistance of batteries; every driver of a car knows that the vehicle battery looses capacity in winter so that eventually it may no longer be in a position to function properly, especially if it is already aged, when the temperature drops below a given limit.
Further, U.S. Pat. No. 6,218,937 B1 discloses a device having the features of the preamble of Claim 1, which comprises, in addition to the battery, a back-up capacitor intended to ensure that the transmitter will be adequately supplied with power. However, such a back-up capacitor reduces the service life of the battery.
DE 101 30 035 A1 describes a device having the features of the preamble of Claim 1, except for the back-up capacitor. The described device comprises a control switch for controlling the excitation of components of the transmitter module. The control switch comprises an input, which is connected to the battery via a filter and/or a voltage regulator being neither described nor shown in detail.