Signal voltage resulting from the disturbance of an electromagnetic field within the volume of a sensitive area is compared with a reference ac voltage for polarity information, which identifies the material. System output amplitude and polarity indicate approximate size and type of metal, respectively

Lee, R. D.

English

NASA Technical Reports Server

November 1970	 Brief 70-10511 
NASA TECH BRIEF 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Metal Detector System 
Power Driver	 k-i Oscillator I Amplifier Amplifier	 I 1fo = 24.259 kHzI 
Balance 
Transformer 
ET Phase 
Shifter 
Transmitter EB1 EB2 Eref 
Coil
90 Phase Eox
10 Hz 
Low Pass
Out 
H 4 Metallic Detector Filter Object Nulling Amplifier Circuit 
Sensor - 
Coil X	 R 
E R	 . E1 Summing 
Transformer I	 Shifter 
E N2 Eref. R 
Bandpass E
1OH Z 
Low Pass
Out
 
Amplifier
.[_ s } o 
Detector Filter 
Amplifier
Preliminary Block Diagram for Metal Detector 
The problem: 
To design a detector which can perform an incon-
spicuous search of personnel moving into areas where 
metallic objects are prohibited. 
The solution: 
The detection and identification of ferrous and 
nonferrous metallic objects is accomplished with a 
phase-sensitive (polarity) detector. The signal volt-
age resulting from the disturbance of an electro-
magnetic field within the volume of a sensitive area 
is compared with a reference ac voltage for polarity 
information, which identifies the material. The system 
output amplitude and polarity indicate the approxi-
mate size and type of metal, respectively. 
How it's done: 
The system (see block diagram) operates as follows: 
the oscillator provides an ac voltage at 24.259 kHz 
for the driver and power amplifiers. The power ampli-
(continued overleaf) 
This document was prepared under the sponsorship of the National
	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately ownedrights. 
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fier feeds an ac voltage ET to the transmitter coil, 
which sets up a magnetic field. The sensor coil is lo-
cated 3 to 4 feet from the transmitter coil, with its 
plane parallel to that of the transmitter coil. The sensor 
coil therefore links the transmitter coil field, H, and 
develops a signal voltage, ER, which is summed 
with a voltage, E BL, from the balance transformer 
connected in series with the transmitter coil. The ac 
voltages ER and EBL are of opposite phase, and when 
they are summed, the result is a small residual voltage, 
EN!. The summing transformer adds E. 1 to EE12 
with the nulling circuit providing both correct phase 
and amplitude of EE12 to form a null at the output of 
the summing transformer. The summing transformer 
voltage EN2 is coupled to the bandpass amplifier 
which has a center frequency identical to that of the 
oscillator. The bandpass amplifier drives the 900 or 
"X" phase-sensitive detector and the 0°or "R" phase-
sensitive detector. The reference voltage for the phase 
detectors is provided by the power amplifier through 
a separate phase shifter for each detector circuit. The 
phase detector outputs are dc voltages (E ox for the 
90° detector and EOR for the 0° detector). The ampli-
fied voltages E ox and EOR are filtered and amplified 
by a pair of 10-Hz low-pass-filter amplifiers. The 
amplifier outputs are displayed on a two-channel re-
corder that indicates a dc voltage level near zero for an 
undisturbed field, H. A ferrous object passing between 
the two sensor coils will disturb the magnetic field, H, 
and cause an amplitude increase in the sensor coil 
signal voltage, ER. The increase in ER unbalances the 
summed voltages in the balance and summing trans-
formers. The change in EN2 having the same phase 
polarity as ER, is amplified to voltage EPF. For fer-
rous objects, E PF increases the dc voltage of the phase-
sensitive (polarity) detectors E OX and EOR from 
their ambient levels. These amplified and filtered 
positive dc voltages represent a ferromagnetic object 
disturbing the field, H. The magnitude of the dc 
change from the ambient level is proportional to the 
permeability, size, and geometry of the object.
The duration of the dc change depends on the length 
of time the metal object remains between the.sensor 
coils; the voltages will return to their ambient dc 
levels when the metal object passes through the region 
linking the two coils. 
A nonferromagnetic object between the detector 
coils will absorb energy from the field, H, and cause 
an amplitude decrease in ER, unbalancing the summed 
voltages in the balance and summing transformers. 
The change inEN2 is amplified to EPN. The phase 
polarity of EPN causes the phase-sensitive (polarity) 
detector dc voltages Eox and E OR to decrease from 
their ambient dc levels. The 10-Hz low-pass-filter 
amplifier will filter and amplify these negative dc 
voltages which represent a nonferromagnetic object 
disturbing the field, H. These negative voltages de-
flect the recorder pen as long as the nonferrometallic 
object remains between the coils. 
Notes: 
1. The detector is insensitive to metal objects below 
a certain size, typically coins, watches, and pens. 
2. The output amplitude is substantially independent 
of the speed at which the metal object moves 
through the detector system. System output . may 
be signalled by colored lights, an aural alarm, 
or a chart recorder. 
3. Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP70-1051 1 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546.
Source: R. D. Lee
Ames Research Center 
(ARC-10265)
. 
. 
Brief 70-10511	 Category 01


Metal detector system

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