Communications: electrical – Selective – Interrogation response
Reexamination Certificate
2000-01-07
2004-05-04
Horabik, Michael (Department: 2635)
Communications: electrical
Selective
Interrogation response
C340S010200, C340S010100, C340S010400, C340S572100
Reexamination Certificate
active
06731198
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The invention relates to a security system of a base station and at least one transponder. More particularly, the invention relates to a security system in which a data dialogue takes place between the base station and the transponder wherein the base station sends data to the transponder on a carrier frequency in the LF (low frequency) range and the transponder sends back data to the base station on a carrier frequency in the RF (radio frequency) range.
2. The Prior Art
Transponders are mobile electronic switching circuits, which serve primarily for the identification of persons, animals, or objects, and allow for data exchange with a base station without contact. Transponders are also used to track moving objects for logistical purposes or for securing items of value.
To monitor and control larger areas, an LF range is used for data transfer from the base station to the transponder. In the LF range, the impairment of the field through objects, especially of a metallic type, is very low and the penetration depth in materials is very high. However, a relatively high transmitting power is required for this.
The feedback transmission from the transponder to the base station on the other hand is by means of a carrier frequency in the RF range, since relatively large distances can be bridged here with very small antennae and a very low transmitting power. Because of the low battery capacity of the transponder a low transmitting power is of great significance in the interest of a long service life.
One problem consists in that transmissions in the RF range can be easily impaired through shadows or manipulation. In this way it would be possible to operate a jammer in the RF range used by the transponder or the transponder could be screened by wrapping in RF-impermeable metal foil.
In the older DE 19745963 A1, an anti-theft system is described wherein a base station and a transponder are operating in the LF range and at the same frequency. In order for the base station not to hinder the reception of the weak transponder signals, an antenna for the transponder signals with its considerably higher transmitting power must be arranged outside the effective range of the base station. Alternatively, the base station and the transponder must only send alternately. Moreover the receiver must additionally be blocked for the transponder signals while the base station is transmitting. Otherwise the adaptation times, in which the receiver of the transponder signals would able to adapt to the weak transponder signals after the fading of the strong signals of the base station, would be too long.
The spatial separation of the antenna for the transponder signals from the effective range of the base station would have to be so large because of the considerably higher transmitting power of the base station compared with the transmitting power of the transponder, that its realization in practice is excluded. The alternate allocation of transmitting times has the disadvantage that limiting the transmitting time further restricts the effective maximum data rate because of the lower maximum data rate with carrier frequencies in the NF range. In practice, this arrangement severely impairs or even renders impossible the simultaneous monitoring of several transponders.
SUMMARY OF THE INVENTION
The invention is based on creating a security system which still enables a bi-directional exchange of data between transponder and base station with adequate data rate even with interferences of a carrier frequency in the RF range.
In accordance with the invention, this task is solved by providing a security system having a base station and at least one transponder between which a data dialogue takes place wherein the base station sends data to the transponder on a carrier frequency in the LF range and the transponder sends back data to the base station on a carrier frequency in the RF range. Further details and advantageous embodiments of the invention are discussed below.
According to the invention, the transponder incorporates an additional transmitter for the transmission of data to the base station. The transmitter may have an electromagnetic carrier frequency in the LF range, an audible carrier frequency in the ultrasonic range or an optical carrier frequency in the infrared range. By switching over to this additional transmitter it is possible to avoid interference in the RF range. Although more energy is generally required to operate this transmitter, such is consciously accepted for data transmission in emergency cases.
Particularly advantageous is an electromagnetic carrier frequency in the LF range, for this has a very large penetration depth as already mentioned above. In this way, a data dialogue between the transponder and the base station may be maintained even if data transmission in the RF range would no longer be possible because of screening. Such screening could arise, for example, through the wrapping of the transponder in aluminum foil. On the other hand, the electromagnetic field is only affected to a minor extent because of the low penetration depth but not attenuated so severely that data transmission would no longer be possible.
Since the transmitting frequencies of transponder and base station in the LF range are different, they could be distinguished through means of selection. Transponders and base station can therefore be transmitting simultaneously and independent of each other. This enables the full utilization of the maximum data range available with the carrier frequency used and increases the probability that the weak transponder signal can be safely received and reliably evaluated even with field strength fluctuations or a rapid crossing of the local receiving range. The carrier frequency of the transmitter of the base station is preferably in the LF range up to 150 kHz. This low operating frequency distinguishes the system. A large penetration depth in materials is present in the system, and the homogeneity of the electrical fields is only affected to a minor degree by objects in the vicinity of the transmitter antennae. Installation even in the immediate vicinity of metal components is therefore possible. Contrary to high-frequency fields, no reflections occur which could lead to undesirable and changeable nulls or increases of the field strength. Moreover, the fields are hardly affected even by objects that have been dynamically introduced or translocated to the detection area. In addition to this, screening for manipulation purposes is not effective or only to a limited extent.
The carrier frequency of the first transmitter of the transponder in the RF range is preferably above 10 MHz. On the one hand this high frequency has the advantage that data communication in duplex mode is possible without reciprocal impairment of the transmission channels. On the other hand the high frequency has the advantage that the transponder requires only a very small antenna and a low transmitting power. In addition, atmospheric interferences and interferences from switching processes of other electrical devices are absent in this frequency range. Secure data transmission is thereby ensured in most cases.
However, the high frequency has the disadvantage that the electromagnetic fields can be very easily shadowed or screened. In order to compensate for this disadvantage in the case of interference, the carrier frequency of the second transmitter of the transponder in the NF range is also dimensioned up to 150 kHz. Consequently the same transmission characteristics as with the carrier frequency of the base station apply to data transmission to the transponder.
An exchange of data for testing the transmission paths is preferably initialized between the base station and the transponder at intervals and the result evaluated. This measure serves to provide information on the quality of the transmission paths. Consequently, it also provides information on a possible interruption or interference in one of the directions or in both directions. In this way i
Scheffler Hartmut
Stobbe Anatoli
Antaloli Stobbe
Brown Vernal
Collard & Roe P.C.
Horabik Michael
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