Near field presence detection device

1. A near field presence detection device provided for monitoring an area around a mouth of a reading member, in particular a card reading member, said device comprises a presence detector having a transmitter and a receiver, said transmitter and receiver being provided to operate with a carrier frequency above 18 KHz in said area, said detector being provided to detect a change of field impedance due to a foreign object placed within said area and to generate a detection signal thereupon, said detector being further provided to generate a preliminary alarm signal if a signal level of said detection signal generated by said detector exceeds a predetermined threshold range, said detector being connected to a verification unit provided for determining upon receipt of said preliminary alarm signal if said preliminary alarm signal subsists within a predetermined period (t 2 −t 1 ) and for producing an effective alarm signal if said preliminary alarm signal subsists within said predetermined period (t 2 −t 1 ), characterized in that said detector is connected to an integrator, said integrator being provided for receiving said detection signal and for integrating said detection signal over a time period Δt in order to obtain an integrated value ({overscore (V)}) for said detection signal, said verification unit comprising an input for receiving said integrated value and being provided for determining said threshold range on the basis of said integrated value.

2. A detection device as claimed in claim 1 , characterized in that said receiver comprises a sampling unit provided for sampling receipt signal, and wherein said integrator has an input for receiving sample signals supplied by said sampling unit, said integrator being provided for integrating a predetermined number of sample signals occurred during said predetermined time interval Δt preceding an actual time t.

3. A detection device as claimed in claim 2 , characterized in that said transmitter includes a transducer, said transducer being an ultrasonic transducer and having a typical carrier frequency of about 40 KHz.

4. A detection device as claimed in claim 3 , characterized in that said transmitter is provided for generating either an unmodulated or a modulated wave on said carrier.

5. A detection device as claimed in claim 2 , characterized in that said transmitter is provided for generating either an unmodulated or a modulated wave on said carrier.

6. A detection device as claimed in claim 2 , characterized in that said receiver comprises a selective bandpass filter provided for calibrating said receiver upon the carrier frequency.

7. A detection device as claimed in claim 2 , characterized in that said verification unit comprises a timer, said timer being provided to be activated upon receipt of said preliminary alarm signal and de-activated and reset after each disappearance of said preliminary alarm signal, said timer being provided for generating said effective alarm signal upon exceeding a predetermined timeslot.

8. A detection device as claimed in claim 1 , characterized in that said transmitter includes a transducer, said transducer being an ultrasonic transducer and having a typical carrier frequency of about 40 KHz.

9. A detection device as claimed in claim 8 , characterized in that said transmitter is provided for generating either an unmodulated or a modulated wave on said carrier.

10. A detection device as claimed in claim 8 , characterized in that said receiver comprises a selective bandpass filter provided for calibrating said receiver upon the carrier frequency.

11. A detection device as claimed in claim 8 , characterized in that said verification unit comprises a timer, said timer being provided to be activated upon receipt of said preliminary alarm signal and de-activated and reset after each disappearance of said preliminary alarm signal, said timer being provided for generating said effective alarm signal upon exceeding a predetermined timeslot.

12. A detection device as claimed in claim 1 , characterized in that said transmitter is provided for generating either an unmodulated or a modulated wave on said carrier.

13. A detection device as claimed in claim 12 , characterized in that said receiver comprises a selective bandpass filter provided for calibrating said receiver upon the carrier frequency.

14. A detection device as claimed in claim 12 , characterized in that said verification unit comprises a timer, said timer being provided to be activated upon receipt of said preliminary alarm signal and de-activated and reset after each disappearance of said preliminary alarm signal, said timer being provided for generating said effective alarm signal upon exceeding a predetermined timeslot.

15. A detection device as claimed in claim 1 , characterized in that said receiver comprises a selective bandpass filter provided for calibrating said receiver upon the carrier frequency.

16. A detection device as claimed in claim 15 , characterized in that said verification unit comprises a timer, said timer being provided to be activated upon receipt of said preliminary alarm signal and de-activated and reset after each disappearance of said preliminary alarm signal, said timer being provided for generating said effective alarm signal upon exceeding a predetermined timeslot.

17. A detection device as claimed in claim 1 , characterized in that said verification unit comprises a timer, said timer being provided to be activated upon receipt of said preliminary alarm signal and de-activated and reset after each disappearance of said preliminary alarm signal, said timer being provided for generating said effective alarm signal upon exceeding a predetermined timeslot.