ANTI-THEFT SYSTEM FOR AUTOMOBILE FIELD OF THE INVENTION The invention relates to an anti-theft system for a car. It refers especially to a closing system through which a blocking of the running of the car is released. DESCRIPTION OF THE PREVIOUS TECHNIQUE A known anti-theft system (US 5,053,774) presents a portable transponder, which receives a coded query signal from a stationary transceiver.
After reception of the coded query signal, a coded response signal is returned to the transceiver. In the coded query signal, energy is transmitted at the same time, through which the coded response signal is activated. The energy is temporarily stored in an accumulator. When there is enough energy in the accumulator, the coded signal is activated. When the transponder and the transceiver are badly coupled together, then in certain circumstances it may take a long time until the accumulator is charged. SUMMARY OF THE INVENTION The problem of the invention is to create an anti-theft system for a car, through which an energy accumulator in the transponder is safely and quickly charged, so that immediately after receiving a coded inquiry signal a coded response signal (designated below as an encoded signal) can be emitted. The problem is solved according to the invention by means of the features of patent claim 1. In this case, by varying the excitation frequency of the coded query signal a secure coupling between transponder and transceiver is achieved. Advantageous configurations of the invention are characterized in the dependent claims. Exemplary embodiments of the invention are explained in detail with the aid of the schematic drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic block diagram of the anti-theft system according to the invention. Figures 2a to 2e show pulse diagrams of a transponder and a transceiver of the anti-theft system. Figure 3 shows a resonance curve of an oscillating circuit. Figure 4 shows a block diagram of a first exemplary embodiment for the generation of an oscillation, and Figure 5 shows a block diagram of a second exemplary embodiment. DETAILED DESCRIPTION OF THE INVENTION The anti-theft system according to the invention has a transceiver 1 arranged in the car (Figure 1), which cooperates with a portable transponder 2 by means of a transformer coupling, when the transponder 2 is in the vicinity of the transceiver 1. Transceiver 1 generates an alternating field, through which energy is transmitted to transponder 2, whereby a capacitor (charge capacitor 3) or accumulator is charged in transponder 2. When sufficient energy is charged in the transponder 2. load capacitor 3, transponder 2 is activated and encoded signals are transmitted back to transceiver 1. For power transmission and data retransmission, transceiver 1 presents an oscillating circuit (hereinafter referred to as an antenna), which is excited with the help of an oscillator 4 for oscillation. For this purpose, the oscillating circuit of the antenna has at least one antenna capacitor 5 and a coil (antenna 6). The antenna 6 can be wound, for example, around the ignition lock.
The transponder 2 also has an oscillating circuit (hereinafter referred to as a transponder oscillating circuit) with a coil 7 and a transponder capacitor 8. When the antenna 6 and the coil 7 are in immediate proximity to each other, then a coupling takes place inductive and in this way a transmission of data or energy. This is the case, for example, when the transponder 2 is arranged in an ignition key. As soon as the ignition key is inserted in the ignition lock and the ignition key is turned, the antenna 6 and the coil 7 are electrically coupled together. The oscillation of the oscillating circuit of the antenna 5, 6 is modulated by means of the transponder 2 in synchronization with an encoded information. To this end, the transponder 2 has a switch 9, which connects in synchronization with the encoded information an additional capacitor 10 in addition to the capacitor of the transponder 8 of the oscillating circuit of the transponder 7, 8. However, the modulation only takes place when the charge capacitor 3 is sufficiently charged with power, to connect the switch 9 in synchronization with the encoded information. The switch 9 is controlled in this case by a transponder control unit (transponder-IC 12), which can be configured as an integrated circuit. As soon as the ignition key is turned in the ignition lock, transceiver 1 generates an alternating field with a large field strength (energy signals) (figure 2a). Within a predetermined time duration (charging phase) the energy signals are generated, here for example 50 ms in length. They have an amplitude of approximately 100 V. According to the quality of the coupling between transponder 2 and transceiver 1, that is, according to the received field strength, these energy signals load the charge capacitor 3 with different speed (figure 2b). When the emission of the output signals is over, the charge capacitor 3 should be charged to a large extent. The transponder 2 recognizes the disconnection of the energy signals, since the transceiver 1 then generates an alternating field with only reduced field strength (in the order of magnitude of some mV). Then the switch 9 is switched within another predetermined time period (charging phase) in synchronization with the encoded information and thus generates the encoded signal (Figure 2c). The encoded signal is a signal with a reduced amplitude, for example of approximately 1 mV, and remains for approximately 20 ms. The amplitude of the coded signal decreases constantly, since the charge capacitor 3 supplies the power for the switch connection and is therefore constantly discharged. The coded signal reacts to the oscillating circuit of the antenna 5, 6, since the antenna 6 and the coil 7 are inductively coupled to each other. Therefore, the oscillation of the oscillating circuit of the antenna 5, 6 is modulated (FIG. 2d). Since the additional capacitor 10 is additionally connected and disconnected to the capacitor of the transponder 8, the oscillating circuit of the antenna 5, 6 is charged to a different extent and therefore the oscillation of the oscillating circuit of the antenna 5, 6 is modulated in its frequency. With an assumed average excitation frequency fE of 129 kHz, in which the oscillating circuit of the antenna 5, 6 is excited, the oscillation frequency of the oscillating circuit of the antenna 5, 6 is modified, for example, from 123 kHz to 134 kHz due to the frequency modulation through the encoded signal. The modulated oscillation of the oscillating circuit of the antenna 5, 6 is detected by a demodulator 13 and evaluated in a control and evaluation unit 14. To this end, the periodic durations or the frequencies of the modulated oscillation are measured. When the frequency of the modulated oscillation is below a threshold value of for example 129 kHz, then a high level of the modulated signal is recognized and when the frequency is above 129 kHz, then a low level is recognized ( figure 2e). In this way, the coding information of the transponder 2 is demodulated from the modulated oscillation. The coded information is compared in the control or evaluation unit 14 with a predetermined theoretical coded information. In case of coincidence of the two, a release signal is issued to a security team in the car. Such a safety device can be a door lock or a gear lock. When the encoded signal is authorized and correct, the door lock is unlocked or the travel lock is deactivated, so that a motor start is possible. The oscillating circuit of the antenna 5, 6 is forced by means of the oscillator 4 with an excitation variable at an oscillation with an excitation frequency fE. The voltage or output current of the oscillator 4 can be used as the excitation variable. The oscillator 4 oscillates with the oscillator frequency f0. A frequency divider 15 can be additionally arranged between the oscillator 4 and the oscillating circuit of the antenna 5, 6, which reduces the frequency of the oscillator f0 to the desired excitation frequency fE. A stationary forced oscillation of the oscillating circuit of the antenna 5, 6, which then oscillates with the excitation frequency fE, is obtained via the excitation variable. Each oscillating circuit has its own frequency or also called resonance frequency fR, which is determined by means of the components of the oscillating circuit, that is, essentially by the antenna 6 and the antenna capacitor 5. The intensity generated (field strength / amplitude) of the oscillation is maximum when the oscillating circuit is excited with the excitation frequency fE equal to the resonance frequency fR (the working point P¿ of the oscillating circuit is then at the resonance point P0; see in this respect figure 3). In this case, the majority of the energy is transmitted to the transponder 2, so that the charge capacitor 3 can be charged quickly. The power balance is illustrated with the help of a resonance curve (figure 3), in which the frequency f is plotted on the abscissa (x-axis) and the intensity of the oscillation I due to the magnitude of excitation, that is, the amplitude of the excitation voltage or of the excitation current, is represented in the ordinate (y-axis). If the excitation frequency fE (operating point P) deviates from the resonance frequency fR, then the intensity I of the oscillation is reduced and less energy is transmitted to the transponder 2. The working point P0 is obtained when the frequency of excitation fE is equal to the resonance frequency fR. Depending on the difference between the two frequencies, the working point is at smaller intensities I (see working point P? Or P2). If the work point is below a power limit 16 calculated in advance, less field energy is obtained in the transponder 2. Therefore, by virtue of the too small energy transmitted, the charge capacitor 3 does not it can be loaded quickly or not enough. The transponder 2 does not then modulate the oscillating circuit of the antenna 5, 6 and the modulation is interrupted in the meantime. In the design of such an anti-theft system it is certainly ensured that the excitation frequency fE largely coincides with the resonance frequency fR. However, by virtue of the tolerances of the components in both the transceiver 1 and also in the transponder 2, the resonance frequency fR and the excitation frequency fE can be deviated from one another, so that no optimal transmission of energy to the transponder 2. Even with small deviations of the two frequencies from each other, a high reduction of the intensity (field strength) can take place with high quality oscillating circuits (narrow quality curve represented by dashes in figure 3) of the alternate field transmitted. An optimum power balance would be given if the components of the oscillator circuits and the oscillator 4 were selected in such a way that they have only small deviations from the theoretical values and therefore the same relations always reign. However, for this, a very high cost must be borne. Also external influences, such as temperature fluctuations, can have an influence on the components, so that the relationships change quickly. This can happen even if the maximum energy is not transmitted. In order for the charging capacitor 3 to always be fully and safely charged, it is provided according to the invention that the excitation frequency fE is varied at least in the charging phase. As soon as the transponder 2 is turned with the ignition key in the ignition lock, the energy of the oscillator 4 is switched on. The oscillator 4 starts oscillating at a predetermined frequency f ". The oscillation frequency fE, which may be equal to the frequency of the oscillator f0, is modified within the charging phase in a predetermined frequency range. This is sufficient for the working point of the maximum power transmission (see FIG. 3) to be reached at least once approximately, in order to sufficiently charge the charging capacitor 3. The excitation frequency fE can be modified in stages in this case predetermined within the predetermined frequency range. For example, the excitation frequency fE can be modified from 129 kHz ± 3% (where the predetermined frequency range is 129 kHz ± 3%) in steps of 500 Hz. The excitation frequency fE can also be modified continuously within the frequency range predetermined. Additionally it is possible to modify the resonance frequency fR of the oscillating circuit of the antenna 5, 6 at any excitation frequency fE in predetermined steps. To this end, different impedances to the oscillating circuit of the antenna 5, 6 can be additionally connected or disconnected at any excitation frequency fE, whereby the resonance frequency fR is modified with respect to the excitation frequency fE.
A circuit arrangement for modifying the excitation frequency fE in predetermined steps is shown in FIG. 4. In this case, an excitation circuit 17 is driven synchronized by a clock frequency CLK. A control unit of the frequency sequence 18 predetermines the frequency steps, with which the programmable driving circuit 18 must be driven. As soon as the ignition key is turned in the ignition lock, that is, as soon as the power supply is turned on (see the ON / OFF signal, which is conducted through an AND-gate 20, in the figures 4 and 5), the oscillating circuit of the antenna 5, 6 is excited through an amplifier 19 with the predetermined excitation frequency fE. The excitation frequency fE can also be modified continuously with the aid of a voltage controlled oscillator 4 (VCO) (FIG. 5). For this purpose, an appropriate control signal (sawtooth voltage from a sawtooth generator 21) is placed at the input of the oscillator 4. The oscillating circuit of the antenna 5, 6 is designed in the embodiment -conditioned by its components - in such a way that its resonance frequency fR is approximately 129 kHz. The frequency of the oscillator f0 can be, for example, approximately 4 MHz. To use the oscillator 4 also for the excitation of the oscillating circuit, a frequency divider-1/32 is provided between the oscillator 4 and the oscillating circuit of the antenna 5, 6. This results in an excitation frequency fE of approximately 129 kHz. By modifying the frequency of the oscillator f0 around a predetermined value, the excitation frequency fE can be modified so that at least once a working point is reached, in which approximately the maximum energy is transmitted. Accordingly, in this way it is achieved that sufficient energy is transmitted to the transponder 2 and the charge capacitor 3 is loaded safely and quickly. Instead of a fixed frequency divider 15, a digital frequency divider can also be used, which divides the frequency of the oscillator f0 to the excitation frequency fE. A digital frequency divider of this type has an adjustable scale-free ratio. The control or evaluation unit 14 can be made by a microprocessor or by a functionally equivalent circuit arrangement. Therefore, the function of the demodulator 13 can also be assumed by the microprocessor. The theoretical encoded information with which the encoded information supplied by the transponder 2 is compared is stored in a memory (ROM, EEPROM) not shown. The encoded information can be stored on the transponder 2 equally in memories of this type. It may also be provided that the excitation frequency fE is constantly modified within the charging phase, but in this case the frequency range is not predetermined, but the excitation frequency fE is modified until the predetermined time duration has expired. of the loading phase. Therefore, no fixed frequency range is traversed, but only the frequency is modified for a period of time. In the embodiments, the charging phase lasts approximately 50 ms. Within this time period, the excitation frequency fE is modified according to the invention and within this time period the charging capacitor 3 is charged inductively by transceiver 1. The excitation frequency fE can also travel several times over the frequency range predetermined, so that a working point with the maximum oscillation intensity I is reached several times. The predetermined frequency range and the duration of time, during which the energy signal is sent to transponder 2, depend on the magnitude of the charge capacitor 3 and the necessary energy in the transponder 2. The switch 9 can also be realized by means of an integrated circuit, in which the additional capacitor 10 is also contained. The switch 9 and the additional capacitor 10 can also be contained in the transponder IC 12. Instead of the additional capacitor 10, an inductivity can also be connected additionally. However, for the invention it is only essential that the oscillating circuit of the transponder 7, 8 be modified as a function of the encoded information and in this way the oscillation of the oscillating circuit of the antenna 5, 6 is modulated. In this case, it can be use a modulation of pulse frequency, amplitude and width.
NOVELTY OF THE INVENTION Having described the present invention is considered as a novelty, and therefore, the content of the following claims is claimed as property: