| 摘要 |
The radio frequency (RF) security system includes a central control unit and a plurality of portable transmitters (up to 128 transmitters) which are in radio frequency communication with the central control unit. This communication is one-way from the portable transmitters to the central control unit. The central control unit and the portable transmitters both include microprocessors and associated memory. Each portable transmitter is assigned a unique unit binary code. In order to detect destruction of the transmitter unit, a powerline is imbedded in an elongated band which is placed on the wrist of a child or attached to an inanimate object. When the band is severed, the powerline is severed and the microprocessor in the portable transmitter is shut down. During normal operation (without the band being severed), the portable transmitter has an RF transmitting circuit which is fed the unique unit code and which frequency modulates (FM) the RF carrier signal with the unit code. The resulting FM signal is transmitted to the central control unit. When power is severed to the microprocessor, the RF transmitter in the portable transmitter continuous emitting an RF carrier signal. The central control unit, in addition to the microprocessor and memory, includes a keypad input device, an antenna system, an RF directional detection circuit, a threshold detection circuit, an identification circuit, distance measuring circuit, and several displays. One display shows the orientation or bearing as well as the distance between the central control unit and each portable transmitter unit. This is accomplished by the directional detection circuit generating phase differential signals which are analyzed by the microprocessor in order to determine the relative position and a distance measuring circuit which determines distance by the relative strength of the received RF signal. The threshold detection circuit determines when the received RF signal fails below a certain threshold. At that time, the threshold detection circuit issues an alarm which stops the scan cycle of the microprocessor. Upon issuance of an alarm, the unique unit code is displayed to the operator so that the operator can easily determine which transmitter has been severed or which transmitter has left the security region (approximately 1,000 feet).
|
