Anti-theft system for a motor vehicle

• US 5,838,074 A
• Filed: 12/11/1996
• Issued: 11/17/1998
• Est. Priority Date: 12/11/1995
• Status: Expired due to Term

• Current Assignee: Continental Automotive GmbH (Continental AG)
• Original Assignee: Siemens AG
• Inventors: Loeffler, Maximilian, Zimmer, Herbert
• Primary Examiner(s): Gaffin, Jeffrey A.
• Assistant Examiner(s): Ganjian, Peter
• Application Number:
  US08/763,906
• Time in Patent Office: 706 Days
• Field of Search: 307/9.1-10.8, 180/287, 180/167, 340/425.5, 340/426, 340/435, 340/825.31, 340/825.34, 340/825.36, 340/825.44, 340/825.54, 340/825.69, 340/825.72, 364/423.098, 364/423.99, 364/424.045, 235/128, 235/380, 235/382, 235/382.5, 380/3, 342/42, 342/52, 342/51, 342/53
• US Class Current: 340/5.61
• CPC Class Codes: B60R 25/24 using electronic identifiers containing a code not memorised by the user
• CPC Class Codes: G01S 13/753 using frequency selective elements, e.g. resonator
• CPC Class Codes: G01S 13/758 using a signal generator powered by the interrogation signal

Abstract

An anti-theft system for a motor vehicle includes a transceiver disposed in a motor vehicle and a portable transponder. If a magnetic alternating field of the transceiver is turned on, energy signals are transmitted inductively to the transponder, where they charge an energy storing device. As soon as the alternating field is turned off, a transponder oscillating circuit oscillates at its resonant frequency. This oscillation is transmitted inductively back to the transceiver. In the process, the resonant frequency is measured. Then, the exciter frequency of the alternating field is adapted to the resonant frequency of the transponder, so that the inductive energy transmission is as effective as possible in subsequent charging operations.

Claims

1. An anti-theft system for a motor vehicle, comprising:

a transceiver disposed in a motor vehicle; and

a portable transponder;

said transceiver having an oscillator, an oscillating circuit excited to oscillate at a predetermined first frequency by said oscillator, causing energy signals to be transmitted at a first frequency to said transponder when said transponder is disposed in the immediate vicinity of said transceiver;

said transponder having a transponder oscillating circuit and an energy storing device, said energy storing device receiving the energy signals from said transceiver and is charged by the energy signals, said charged energy storage device causing said transponder oscillating circuit to oscillate at its resonant frequency and transmit data signals at a resonant frequency back to said transceiver;

said transceiver having a frequency counter receiving the data signals and measuring the resonant frequency of said transponder; and

said transceiver having a control unit connected to said frequency counter and to said oscillator for controlling said oscillator to excite said oscillating circuit with a second frequency approximately matching the measured resonant frequency of said transponder.

2. The anti-theft system according to claim 1, wherein said transponder has a code generating unit storing a piece of code information in memory and generating a code word from the code information, the code word is transmitted back to said transceiver in the oscillation of said transponder oscillating circuit and amplitude-modulated as a function of the code information.

3. The anti-theft system according to claim 1, wherein said control unit demodulates a code word from an amplitude-modulated oscillation, compares the code word with a desired piece of code information and generates an enable signal if the code word and the desired piece of code information agree.

4. The anti-theft system according to claim 1, wherein said energy storing device is a capacitor.

5. The anti-theft system according to claim 1, wherein said energy storing device is a rechargeable battery.

6. The anti-theft system according to claim 1, wherein a value of the measured resonant frequency is stored in memory in said transceiver and is re-stored only if it deviates from a previously measured value.

7. The anti-theft system according to claim 1, wherein said transponder has a code generating unit storing a computation formula in memory and generating a code word from the computation formula, the code word is transmitted back to said transceiver in the oscillation of said transponder oscillating circuit and amplitude-modulated as a function of the computation formula.