Anti-theft system for a motor vehicle

US5804888A · kind A · utility

Assignee

• Siemens Aktiengesellschaft · DE

Inventors

• Robert Murr · Barbing, DE
• Bernhard Foerstl · Kelheim, DE
• Thomas Roehrl · Barbing, DE
• Herbert Zimmer · Regensburg, DE

Key dates

• Filing date: Aug 28, 1995
• Grant date: Sep 8, 1998
• Expiry date: Aug 28, 2015

Classification

• Technology area (CPC G): Physics
• CPC primary: G07C2009/00777
• WIPO field: Control
• WIPO sector: Instruments

Abstract

An anti-theft system for a motor vehicle includes a portable transponder carrying an item of code information. A stationary antenna has a resonant circuit with components and a resonant frequency being determined by the components. An oscillator oscillates at an oscillator frequency and has an output variable being used as an exciter variable with an exciter frequency for compelling an oscillation of the resonant circuit. The oscillation is amplitude-modulated as a function of the code information of the transponder. An evaluation unit receives the oscillation of the resonant circuit. The evaluation unit detects the modulated oscillation and demodulates the code information out of the oscillation. A comparator compares the code information with command code information and supplies an enable signal if a match occurs. A security unit receives the enable signal. The exciter frequency or the resonant frequency of the resonant circuit is varied if initially no code information is recognized by the evaluation unit.