US 5,691,640 AGranted

Forced balance metal detector

AssigneeRamsey Technology, Inc.

InventorKevin James King

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Abstract

Metal detector (1) including an oscillator (10) generates an RF field in the vicinity of the head (2). Two reference signals (13, 14) are generated, one signal (14) being in phase with the oscillator (10) and the other signal (13) being shifted from the oscillator signal by 90.degree.. The signal (8,9) from the receive coils (6,7) is amplified and quadrature demodulated into two signal channels, each channel giving the orthogonal component of the original signal (8,9). The signals of each channel are digitized and read by a microprocessor (37) that controls the amplitude of the balance signals (43, 49). One signal (43) is in phase with the oscillator signal (11) and the second signal (49) is shifted 90.degree. from the oscillator signal (11). Signals (43,49) return to detection circuitry (6,7), maintaining balance on each channel. By monitoring the magnitude of the phase shifted and unshifted signals (49,43), the microprocessor (37) determines the characteristics of the product. In a second version, metal detector (62) includes a digital phase shifting circuit (77) but only one phase sensitive detector (84). The circuit (77) is manipulated by microprocessor (89) to maximize RF signal (87). The balance circuits (92, 100) remove signal (87). By monitoring the control signals (79) used for balancing, the microprocessor (89) determines characteristics of a product passing in the vicinity of the head assembly (63).

Claims19

Dependent claims

A metal detector, comprising:

(a) an electromagnetic field generator, the generator generating a radio frequency signal having a first phase characteristic;
(b) an electromagnetic field sensor, the electromagnetic field sensor producing an output signal, the electromagnetic field sensor detecting:
(i) the first phase characteristic of the radio frequency signal produced by the electromagnetic field generator;
(ii) an amplitude of the radio frequency signal;
(iii) a frequency of the radio frequency signal;
and
(iv) variations in the first phase characteristic, amplitude and frequency caused by foreign objects entering an electromagnetic field adjacent to the electromagnetic field sensor;
(c) a feedback circuit, the feedback circuit being interconnected to and receiving the output signal of the electomagnetic field sensor, the feedback circuit comprising:
(i) an electrical interconnection to a first signal, the first signal being substantially identical to the radio frequency signal produced by the electromagnetic field generator;
(ii) an electrical interconnection to a second signal, the second signal being a composite of the first signal and a phase shifted component of the first signal;
and
(iii) an electrical interconnection of the first signal and the phase shifted component of the first signal to the electromagnetic field sensor;
(d) a microprocessor, the microprocessor comprising:
(i) at least one input data port, the input data port being interconnected to the first signal and the phase shifted component of the first signal emanating from the feedback circuit;
and
(ii) at least one data output port;
and
(e) a first multiplier, the first multiplier being interconnected to:
(i) the data output port;
(ii) the first signal;
and
(iii) the feedback circuit, the first multiplier modulating the first signal with a signal appearing at the data output port to produce a modulated signal which is thereby interconnected to the feedback circuit;
and
(f) a second multiplier, the second multiplier being interconnected to:
(i) the data output port;
(ii) the phase shifted component of the first signal;
and
(iii) the feedback circuit, the second multiplier modulating the phase shifted component of the first signal appearing at the data port to produce a modulated signal which is thereby interconnected to the feedback cirsuit, the microprocessor alternatively selecting the first signal and the phase shifted component of the first signal to appear at the data output port so as to maintain an electrically balanced condition in the electromagnetic field sensor, the microprocessor monitoring the manipulations so as to determine:
(A) signal manipulations attributable to maintaining field sensor balance in the absence of a product passing through a region near the field sensor;
and
(B) signal manipulations attributable to maintaining field sensor balance in the presence of a product passing through a region near the electromagnetic field sensor.

28 more elements

A foreign object detector for determining the characteristics of a product in the vicinity of a sensing head, comprising:

(a) a radio frequency oscillator, the radio frequency oscillator having a frequency and a phase;
(b) an exciter coil electrically interconnected to the oscillator;
(c) an electromagnetic field detector, the electromagnetic field detector sensing a signal emanating from the exciter coil;
(d) a feedback circuit, the feedback circuit comprising:
(i) a phase sensitive detector having two input signals, the two input signals being:
A. a signal that is derived from a phase difference between the radio frequency oscillator signal;
and
B. a phase shifted component of the signal emanating from the radio frequency oscillator;
and
(ii) an electrical interconnection to the electromagnetic field detector by a controller, the controller receiving an input signal derived from the phase sensitive detector, the input signal being proportional to a phase difference between the two input signals to the phase sensitive detector, the controller:
A. selectively enabling the feedback circuit to send field detector balancing signals derived from the radio frequency oscillator and the electromagnetic field detector to the electromagnetic field detector so as to maintain electrical balance of the electromagnetic field detector;
B. storing information regarding balancing signals sent to the electromagnetic field detector;
and
C. categorizing balancing signal manipulations sent to the electromagnetic field detector so as to determine a characteristic of a foreign object.

11 more elements

Dependencies are inferred from the claim text.