Method, a device and a system for preventing false alarms in a theft-preventing system
The invention relates to a method, a device and a system for preventing false alarms in a theft-preventing system comprising a magnetic field for detecting at least one metal object in a first detection zone and means for determining a time-difference; said method comprising the steps of detecting a first change in a parameter indicative of the magnetic field in said first detection zone; detecting a second change in the parameter indicative of the magnetic field in said first detection zone; determining a time-difference between the detection of said first change in said parameter and the detection of said second change in said parameter; and performing an action based on said time-difference. In this way, false alarms due to a metal detector falsely detecting for example movement of metal-doors in a first detection zone may be reduced/eliminated. Additionally, the false alarms due to an opening and/or closing of a door with metal parts acting as an active tag may be reduced/eliminated.
1. A method of preventing false alarms in a theft-preventing system comprising a magnetic field created by an active metal detector and determining a first detection zone; said method comprising the steps of
detecting a first change in a parameter indicative of the magnetic field in said first detection zone due to a metal object entering the magnetic field created by the active metal detector;
detecting a second change in the parameter indicative of the magnetic field in said first detection zone due to the metal object leaving the magnetic field created by the active metal detector;
determining a time-difference between the detection of said first change in said parameter and the detection of said second change in said parameter;
performing an action based on said time-difference.
2. A method according to claim 1 , wherein the step of detecting a first change in a parameter indicative of the magnetic field in said first detection zone comprises detecting a decrease in the parameter.
3. A method according to claim 1 , wherein the step of detecting a second change in the parameter indicative of the magnetic field in said first detection zone comprises detecting an increase in the parameter.
4. A method according to claim 1 , wherein the parameter is chosen from the group consisting of
amplitude of the magnetic field; and
phase of the magnetic field.
5. A method according to claim 1 , wherein the step of performing an action based on said time-difference comprises
performing a first action if said time-difference is above a first value; and
performing a second action if said time-difference is below or equal to said first value.
6. A method according to claim 5 , wherein the second action is chosen from the group consisting of
sounding an alarm; transmitting a silent alarm to e.g. a pager;
setting off a visual alarm; and
transmitting a signal to a video-surveillance system indicating to the video-surveillance system to start monitoring.
7. A method according to claim 5 , wherein the first action comprises refraining from sounding an alarm.
8. A method according to claim 5 , wherein the first value is chosen from the group consisting of
substantially a second; and
in the interval 0.5 seconds to 3 seconds.
9. A method according to claim 1 , wherein the theft-preventing system further comprises a tag-based theft-preventing system for detecting a tagged object in a second detection zone determined by an electromagnetic field.
10. A method according to claim 9 , wherein the tag-based system is chosen from the group consisting of
resonance circuit system;
RFID system; and
Diode system.
11. A method according to claim 9 , wherein an overlap between said first detection zone and said second detection zone is chosen from the group of
coinciding;
disjoint; and
partially overlapping.
12. A device for preventing false alarms in a theft-preventing system comprising a magnetic field created by an active metal detector and determining a first detection zone; said device comprising
means for detecting a first change in a parameter indicative of the magnetic field in said first detection zone due to the metal object leaving the magnetic field created by the active metal detector;
means for detecting a second change in the parameter indicative of the magnetic field in said first detection zone due to the metal object leaving the magnetic field created by the active metal detector;
means for determining a time-difference between the detection of said first change in said parameter and the detection of said second change in said parameter;
means for performing an action based on said time-difference.
13. A device according to claim 12 , wherein the means for detecting a first change in a parameter indicative of the magnetic field in said first detection zone are adapted to detect a decrease in the parameter.
14. A device according to claim 12 , wherein the means for detecting a second change in the parameter indicative of the magnetic field in said first detection zone are adapted to detect an increase in the parameter.
15. A device according to claim 12 , wherein the parameter indicative of the magnetic field is chosen from the group consisting of
an amplitude of the magnetic field; and
a phase of the magnetic field.
16. A device according to claim 12 , wherein the means for performing an action based on said time-difference are adapted to
perform a first action if said time-difference is above a first value; and
perform a second action if said time-difference is below or equal to said first value.
17. A device according to claim 16 , wherein the second action is chosen from the group consisting of
sounding an alarm;
transmitting a silent alarm to e.g. a pager;
setting off a visual alarm; and
transmitting a signal to a video-surveillance system indicating to the video-surveillance system to start monitoring.
18. A device according to claim 16 , wherein the first action comprises refraining from sounding an alarm.
19. A device according to claim 16 , wherein the first value is chosen from the group consisting of
substantially a second; and
in the interval 0.5 seconds to 3 seconds.
20. A device according to claim 12 , wherein the theft-preventing system further comprises a tag-based system for detecting a tagged object in a second detection zone.
21. A device according to claim 20 , wherein the tag-based system is chosen from the group consisting of
resonance circuit system;
RFID system; and
Diode system.
22. A device according to claim 20 , wherein the overlap between said first detection zone and said second detection zone is chosen from the group of
coinciding;
disjoint; and
partially overlapping.
23. A system for preventing false alarms in a theft-preventing system, comprising:
a tag-based theft-preventing device for detecting a tagged object and indicating the detection of a tagged object,
a clock,
an active metal detector, wherein the metal detector creates a magnetic field having a magnetic field parameter indicative of an opening and/or closing of a door, and
means for determining that the tag-based theft-preventing device indicates the detection of a tagged object substantially at the same time as the metal detector indicates a change in the magnetic field parameter indicative of an opening and/or closing of a door and for restraining the system from sounding an alarm IF it is determined that the tag-based theft-preventing device indicates the detection of a tagged object substantially at the same time as the metal detector indicates a change in the magnetic field parameter indicative of an opening and/or closing of a door.