For modern access authorization systems and access monitoring systems, use is being increasingly made of electronic security systems or access arrangements in which the authentication of a person who is authorized for access is carried out using a data communication which takes place between a first communication device, which is usually arranged at the object to be accessed such as, for example, a motor vehicle, and a second communication device which is in the possession of the person who is authorized for access, said communication device being, for example, in an electronic key. With respect to the electronic security systems, a differentiation is made between active and passive access arrangements.
In the case of an active access arrangement, an identification code is transmitted from the second communication device to the first communication device, which is arranged, for example, in a vehicle. The transmission is typically brought about by pressing a corresponding pushbutton key on a mobile identification signal generator. In the second communication device, the transmitted identification code is checked, and in the case of success, the security device of the access arrangement is released or locked. Since the identification signal generator has to be intentionally activated by its possessor in order to permit said possessor, for example, to access his motor vehicle, this electronic access system is referred to as an active access arrangement.
In the case of a passive access arrangement such as is shown schematically in FIG. 2, for example, the first communication device KE1 of a motor vehicle FZ transmits interrogation signals NFS with specific field strength at regular intervals. If the second communication device KE2, which is provided in an identification signal generator IG such as a key, is located within the effective range of the first communication device, said second communication device KE2 can receive the interrogation signals of said first communication device and respond thereto with a response signal HFS in order to initiate an authentication process or pre-authentication process. The authentication is carried out by exchanging data telegrams which, inter alia, also transmit the authentication code (CO) from the second communication device to the first communication device. If the authentication is successful, the security device, such as a door lock TS, which is controlled by the access arrangement is released and can then be opened automatically or manually. Since the identification signal generator in the case presented does not have to be intentionally activated by its possessor, this electronic access system is referred to, in contrast to that explained above, as a passive access arrangement. Passive access arrangements are preferably used for what are referred to as keyless vehicle access systems.
The interrogation signal NFS is usually emitted in the inductive frequency range using a low frequency transmitter (LF transmitter), which usually operates in the kHz range, and is received by the LF receiver of the second communication device. The received interrogation signal is decoded and further processed to form a response signal HFS which is transmitted by the second communication device with low transmission power to the RF transceiver device of the first communication device via a radio frequency transmitter (RF transmitter) which is usually operated in the MHz range.
The transmission of the LF interrogation signal is referred to as a wakeup process. The quickly decreasing magnetic field of the interrogation signals which are transmitted by the first communication device limits the effective range of the access arrangement to a functional radius of typically less than ten meters. The functional radius is determined, on the one hand by the transmission power of the LF transmitter and on the other hand by the sensitivity of the LF receiver. The circuit electronics of the second communication device are usually supplied with current by a rechargeable energy store, for example an accumulator. The storage capacity of the energy store is, of course, very low in order to permit small dimensions of the second communication device.
Owing to the small autonomous power supply capacity, the LF receiver of the second communication device is generally designed as a receiver with low energy consumption. The LF receiver has a coil for receiving the magnetic component of the LF radio signals, referred to as the receiving coil. In order to optimize the reception sensitivity of the LF receiver, a suitable capacitor is connected parallel to the receiving coil, as a result of which a parallel resonant circuit is produced, the resonant frequency of which parallel resonant circuit is matched to the LF transmission frequency of the first communication device. The high output signal, achieved during the resonant peak, of the resonant circuit ensures a high level of sensitivity of the receiver to the LF transmission frequency of the first communication device. As a result it is possible to transmit the wakeup signal with a good signal-to-noise ratio.
The further communication between the first and second communication devices generally takes place as a function of the respective distance between the two communication devices. In order to determine the distance between the two devices, after the reception of the wakeup signal the field strength of the LF signal which is emitted by the first communication device is measured at the location of the LF receiver. This is done by measuring the output voltage of the resonant circuit described above. The precision of the determination of the field strength which is achieved with this measuring method is determined substantially by the change in the quality and the resonant frequency of the resonant circuit as a result of component tolerances, especially also as a result of external influences such as, for example, temperature effects. In particular at high quality the influence of a resonance frequency shift due to changes in capacitance or inductance is dominant. The properties of the induction of the oscillatory circuit which is designed to receive the magnetic component of the LF radio signals are highly temperature dependent. If the inductor is embodied as an air coil, the quality of the oscillatory circuit or resonant circuit changes as a result of the strong temperature dependence of the specific resistance of the coil wire. If a ferrite core coil is used, the likewise strong temperature response of the ferrite core is also added to this. Depending on the ambient conditions, the measurement of the same reception field strength can therefore lead to different measurement results.
In order, nevertheless, to obtain a sufficiently reliable measurement, it is customary to damp the oscillatory circuit with, for example, a resistor which is connected parallel to the inductor and capacitor. As a result, the resonance curve of the parallel oscillatory circuit is flattened, causing temperature changes of the capacitor or inductor to have a smaller effect on the resonance of the oscillatory circuit. Furthermore, this stabilizes the quality of the oscillatory circuit to a relatively low value and therefore brings about a more stable output voltage of the resonant circuit. However, a disadvantage with this is that the damping also leads to a reduction in the resonant peak and therefore to a reduction in the sensitivity of the receiving circuit for receiving the wakeup signal.
At present, a compromise is therefore aimed at between the still permissible measuring tolerance and the minimum necessary sensitivity, but said compromise means a departure from the optimum for each of the two operating modes.
If the autonomous power supply is no longer possible because of flat batteries, the passive function of the identification signal generator is also no longer provided. In this case, the release of vehicle devices is replaced by a transponder function. For this purpose, the second communication device is usually introduced into a station which is provided for that purpose and via which, on the one hand, the access control is processed and, on the other hand, an energy store of the second communication device is charged using an inductive LF signal. Subsequent to the charging process a communication can take place between the first communication device and the second communication device for the identification and subsequent release of the vehicle device. For a high level of efficiency of the charging process, the resonant circuit described above must have, as in the case of the reception of the wakeup signal, a high quality. However, the reduction in quality which is associated with the currently practiced compromise solution has an adverse effect on the transponder function.