Real-time rectangular-wave transmitting metal detector platform with user selectable transmission and reception properties

• US 8,063,777 B2
• Filed: 12/14/2006
• Issued: 11/22/2011
• Est. Priority Date: 12/20/2005
• Status: Active Grant

• Current Assignee: Minelab Electronics Pty Limited (Codan Ltd.)
• Original Assignee: Minelab Electronics Pty Limited (Codan Ltd.)
• Inventors: Candy, Bruce Halcro
• Primary Examiner(s): HOFSASS, JEFFERY A
• Application Number:
  US12/158,441
• Publication Number:
  US 20090315707A1
• Time in Patent Office: 1,804 Days
• Field of Search: 340/540, 340/551, 340/657, 324/323, 324/326, 324/329
• US Class Current: 340/540
• CPC Class Codes: G01V 3/104 using several coupled or uncoupled coils G01V3/101 takes precedence

Abstract

A highly flexible real-time metal detector platform which has a detection capability for different targets and applications, where the operator is able to alter synchronous demodulation multiplication functions to select different types or mixtures of different types to be applied to different synchronous demodulators, and also different waveforms of the said synchronous demodulation multiplication functions; examples of the different types being time-domain, square-wave, sine-wave or receive signal weighted synchronous demodulation multiplication functions. The operator can alter the fundamental frequency of the repeating switched rectangular-wave voltage sequence, and an operator may alter the waveform of the repeating switched rectangular-wave voltage sequence and corresponding synchronous demodulation multiplication functions.

Claims

1. A real-time metal detector platform

having a transmitter connected to a transmit coil for the transmission of alternating magnetic fields when operational, a magnetic field receiving means connected to signal processing and assessment electronics,

the transmitter being adapted to apply a transmit signal to the said transmit coil when operational,

such that the transmit signal consists of a repeating switched rectangular-wave voltage sequence consisting of at least two substantially constant voltages and alternating rapid switching voltage transitions between the said at least two substantially constant voltages,

wherein the said repeating switched rectangular-wave voltage sequence contains at least two different time periods between successive alternating rapid switching voltage transitions,

a frequency spectrum of the said repeating switched rectangular-wave voltage sequence contains a plurality of different spectral magnitude peaks,

the highest magnitude of the said plurality of different spectral magnitude peaks being a highest magnitude spectral peak,

one of the said different spectral magnitude peaks being at a first harmonic fundamental frequency of the said repeating switched rectangular-wave voltage sequence,

wherein the said signal processing and assessment electronics contains synchronous demodulators,

wherein a time-domain synchronous demodulation multiplication function, a first time-domain synchronous demodulation multiplication function, is applied to at least one of the said synchronous demodulators,

such that the said time-domain synchronous demodulation multiplication function is zero or at least approximately zero value for at least one time period, a first time period,

and wherein the said first time period commences during a said alternating rapid switching voltage transition and ends when the said time-domain synchronous demodulation multiplication function changes from zero or at least approximately zero value to a substantially non-zero value,

and wherein a sum of products of the said time-domain synchronous demodulation multiplication function multiplied by receive signal transients resulting from the said alternating rapid switching voltage transitions applied to an input of the synchronous demodulator to which the said time-domain synchronous demodulation multiplication function is applied, is approximately zero,

the said first time-domain synchronous demodulation multiplication function is of a first fundamental frequency,

the first fundamental frequency being synchronous with a frequency of said different spectral magnitude peak of the said repeating switched rectangular-wave voltage sequence,

an output of the said signal processing and assessment electronics indicates the presence of selected environmental magnetic properties under the influence of the transmission alternating magnetic fields,

the said synchronous demodulators being implemented either in hardware or software or a combination of hardware and software.

2. A real-time metal detector platform as in claim 1 further characterised in that

a magnitude of the said spectral magnitude peak at a first harmonic fundamental frequency of the said repeating switched rectangular-wave voltage sequence and magnitude of the said highest magnitude spectral peak are within a factor of two in magnitude of each other.

3. A real-time metal detector platform as in claim 1 further characterised in that a square-wave synchronous demodulation multiplication function of a second fundamental frequency is applied to at least one of the said synchronous demodulators,

and a time-domain synchronous demodulation multiplication function of a third fundamental frequency is applied to at least another of the said synchronous demodulators,

the said second fundamental frequency and third fundamental frequency each being synchronous with a frequency of a said different spectral magnitude peaks of the said repeating switched rectangular-wave voltage sequence.

4. A real-time metal detector platform as in claim 1 further characterised in that at least the said first fundamental frequency or the said second fundamental frequency or the said third fundamental frequency equals the said first harmonic fundamental frequency.

5. A real-time metal detector platform as in claim 1 further characterised in that the a frequency of a said spectral magnitude peak of the said frequency spectrum of the said repeating switched rectangular-wave voltage sequence within a magnitude factor of two of the magnitude of the said highest magnitude spectral peak, is at a highest operational frequency,

wherein the said highest operational frequency equals half the reciprocal of a shortest period between successive said alternating rapid switching voltage transitions of the said repeating switched rectangular-wave voltage sequence,

and wherein at least one fundamental frequency of a synchronous demodulation multiplication function applied to a said synchronous demodulator is at the said highest operational frequency.

6. A real-time metal detector platform as in claim 1 further characterised in that the said signal processing and assessment electronics contains a broadband preamplifier with low-pass filtering action with a −

3 dB frequency cut-off of at least greater than the said highest operational frequency,

wherein the magnetic field receiving means is connected to an input of the broadband preamplifier and an output of the broadband preamplifier is connected to the said synchronous demodulators, or further filters.

7. A real-time metal detector platform as in claim 1 further characterised in that a said time-domain synchronous demodulation multiplication function is substantially non-zero for periods in the latter half of a period between two successive alternating rapid switching voltage transitions,

and an output of a synchronous demodulator to which the said synchronous demodulation multiplication function of substantially non-zero value for periods in the latter half of a period between two successive alternating rapid switching voltage transitions is applied, is processed by the said processing and assessment electronics to assess and indicate a ferrous nature of a metal target under the influence of the transmission alternating magnetic fields.

8. A real-time metal detector platform as in claim 1 further characterised in that an operator may alter at least one of the said square-wave synchronous demodulation multiplication functions to select a different said square-wave synchronous demodulation multiplication function.

9. A real-time metal detector platform as in claim 1 further characterised in that an operator may alter at least one of the said time-domain synchronous demodulation multiplication functions to select a different said time-domain synchronous demodulation multiplication function.

10. A real-time metal detector platform as in claim 1 further characterised in that an operator may alter at least one of the said time-domain synchronous demodulation multiplication functions and square-wave synchronous demodulation multiplication functions to select a different said time-domain synchronous demodulation multiplication function or a different square-wave synchronous demodulation multiplication function, or a different said time-domain synchronous demodulation multiplication function and a different square-wave synchronous demodulation multiplication function.

11. A real-time metal detector platform as in claim 1 further characterised wherein an output of the said broadband preamplifier is connected to at least one said filter,

the filter is connected to an input of at least one said synchronous demodulator,

wherein a forward transfer function of the filter attenuates frequencies at the harmonics of the fundamental frequency of the synchronous demodulation multiplication function applied to the said synchronous demodulator to which the said filter is connected,

wherein the filter may contain any combination of low-pass or notch action or both, and may be implemented in hardware or software,

and wherein an operator may alter the said fundamental frequency of the said synchronous demodulation multiplication function applied to the said synchronous demodulator to which the said filter is connected,

and the said forward transfer function of the filter may be altered correspondingly.

12. A real-time metal detector platform as in claim 1 further characterised in that an operator may alter at least one of the said synchronous demodulation multiplication functions to select sine-wave synchronous demodulation multiplication functions.

13. A real-time metal detector platform as in claim 1 further characterised in that an operator may alter at least one of the said synchronous demodulation multiplication functions to select a receive signal weighted synchronous demodulation multiplication function,

wherein the said receive signal weighted synchronous demodulation multiplication function has a greater absolute magnitude multiplication factor for a least some periods than other substantially non-zero signal weighted synchronous demodulation multiplication function periods,

when a magnitude of a time derivative of a receive signal at an output of the said broadband preamplifier or said filter resulting from a metal target under the influence of the said transmission of alternating magnetic fields from the said transmit coil and received by the said magnetic field receiving means,

is greater than during other periods,

the said receive signal weighted synchronous demodulation multiplication function may include time-domain synchronous demodulation multiplication function or frequency-domain synchronous demodulation multiplication function or both.

14. A real-time metal detector platform as in claim 13 further characterised in that the said receive signal weighted synchronous demodulation multiplication function has a magnitude frequency spectrum relatively attenuated in high frequencies, all harmonics of the said magnitude frequency spectrum above the fifth being less than one tenth of the fundamental frequency of the said receive signal weighted synchronous demodulation multiplication function.

15. A real-time metal detector platform as in claim 1 further characterised in that an operator may alter a synchronous demodulation multiplication function applied to a synchronous demodulator.

16. A real-time metal detector platform as in claim 1 further characterised in that an operator may alter at least one type of synchronous demodulation multiplication functions such as time-domain synchronous demodulation multiplication functions, square-wave synchronous demodulation multiplication functions, sine-wave synchronous demodulation multiplication functions or receive signal weighted synchronous demodulation multiplication function applied to a synchronous demodulator to a different type of synchronous demodulation multiplication function applied to the same said synchronous demodulator.

17. A real-time metal detector platform as in claim 1 further characterised in that an operator may alter the said fundamental frequency of the said repeating switched rectangular-wave voltage sequence.

18. A real-time metal detector platform as in claim 1 further characterised in that an operator may alter the waveform of the said repeating switched rectangular-wave voltage sequence.

19. A real-time metal detector platform as in claim 1 further characterised in that an operator may alter the said repeating switched rectangular-wave voltage sequence and synchronous demodulation multiplication functions.

20. A real-time metal detector platform as in claim 1 further characterised in that the said broadband preamplifier has a slew rate limit exceeding the maximum expected slew rate.