Pulse induction metal detectors

By PhD Iliya Kanchev

Revision 2.0 – june 2005

The Idea

 

 When a metal object is in an electro-magnetic field, it interferes with this field due to the self-induction of the metal object itself. If we transmit an electro-magnetic pulse with a certain duration and place a metal object near this transmitter, then the following effect will appear: After the initial effect of the pulse, a fading electric current pulse occurs in the metal object. This fading pulse is a result from the physics phenomenon self-induction. After the end of the transmitted electro-magnetic pulse, a secondary fading electro-magnetic pulse is transmitted from the object. If we can catch this secondary pulse, we'll have an information if a metal object is present near our transmitter. This is the basis on which the pulse metal detectors are developed and work. The main task for the pulse metal detectors' developers is catching this fading pulse and processing it.

 

Discrimination

 

The parameters of the secondary electro-magnetic pulse, as duration and amplitude, depend poorly on the kind of metal the object consists of. Therefore, the pulse-type metal detectors are not able to discriminate successfully the variety of metals. Nevertheless, there are patents offering a way for defining the very small difference in the secondary electro-magnetic pulse's types. Unfortunately, it can only be used when we have equally size metal objects, consisting of different metals, and which are placed on the same distance from the transmitting and receiving antenna of the metal detector. And that's why these patents usually don't meet a practical usage.

 

Electro-magnetic disturbances

 

 The pulse metal detectors, as any sensitive metering device, are affected by electro-magnetic disturbances. If during the receiving of the secondary electro-magnetic pulse from the metal object, placed on a distance from the receiving and transmitting antenna, another electro-magnetic signal from a random source is received, then it will add to the useful signal. Sources for radio disturbances may be: TV or radio, a working cell phone placed too close, radio stations, a nearby working TV receiver, a PC's monitor, the PC itself, a working electro-engine, a benzene engine that uses high voltage ignition coil, power lines. In the pulse metal detector there's a scheme for eliminating the electro-magnetic disturbances to the maximum possible degree.

 

Advantages of pulse metal detectors

 The main advantages of the pulse metal detectors are the high working stability, as well as the option for a huge searching coil. This grants them a huge depth penetration of the electro-magnetic pulse. Earth and water does not have any effect on them. That's why the pulse metal detectors are used primary for hunting in great depths, underwater, on the border between the earth and water, and at the beaches. The simple design of the searching coil, compared to the other types of detectors, and the minimum effect the temperature and other climate changes have on it, makes the detector a useful choice.

Disadvantages of pulse metal detectors

 This type of detectors are unable to discriminate metals and that's their main disadvantage. For this purpose, they're sometimes combined with magnetometers. These magnetometers react on magnetic metals only.

 

 

block diagram pulse induction

Click to enlarge pulse induction metal detector diagram

 

 

Consists of:  

 

  • -Transmitting and a receiving antennas Generator of electric current pulses
  • -Amplitude restrictor of the transmitted and received pulse
  • -Low-noise pre-amplifier
  • -Useful signal amplifier
  • -Scheme for compensating the magnetic field of the Earth
  • -Filter accepting the useful signal only and negating the external noises
  • -End-amplifier of the useful signal
  • -Scheme making the indication when a metal is present and detected
  • -Control block
  • -Power supply

 

Transmitting and receiving antennas

The transmitting antenna is used to emit an electro-magnetic pulse. The receiving antenna accepts the secondary electro-magnetic pulse, transmitted by the target. The antenna, as the theory for broadcasting of electro-magnetic waves is concerned, works in a reversible way. In other words, generally there is no difference between the transmitting and the receiving antenna. The transmitted and the received pulse have a different timing, and therefore the same antenna is generally used in the pulse metal detectors.

 

Generator of electric current pulses

 This generator serves for shaping the frequency and duration of the transmitted pulse. It can be quartz-stabilized or not. If so, this makes the whole metal detector much more stable and raises its ability to negate bad noises. In the microprocessor controlled detectors, the processor itself defines the duration and frequency of the transmitted pulse.

Amplitude restrictor for the transmitted and received pulse

The transmitted pulse may reach an amplitude of 400V - 500V. The forefront of the received secondary electro-magnetic pulse, if the target is close enough, may reach 20V - 50V too. For this reason, an amplitude restrictor has to be put in the lead-in circuit. It doesn't interfere with the sensitivity of the detector, because the useful signal in normal conditions is low enough. The amplitude restrictor decreases the signal in the entrance of the pre-amplifier to values of 0,2V to 0,7V.

 

Low-noise pre-amplifier

The already decreased by the amplitude restrictor signal reaches the lead-in of the pre-amplifier. The signal is amplified to the maximum value possible, which is when the noises in the amplifier, the antenna and the restrictor don't have any effect on the form and amplitude of the received useful signal.

 

Scheme for extraction of the useful signal

After it is amplified, the useful signal is extracted from the main input signal. This happens with a time selection. The precise selection is of great importance for the stable work of the detector. Usually, it is done by a microprocessor's control block.

 

Amplifier of the useful signal

The already extracted useful signal is amplified in an alternating current low-frequency amplifier. This amplifier has a quite narrow frequency characteristics, which helps separating noises from the useful signal. It is capped in a negative reverse connection. This reverse connection removes the "floating zero" of the operation amplifiers, on which the amplifier of the useful signal is based on.

 

Scheme for compensating the magnetic field of the Earth

The Earth's magnetic field creates an additional electromotive power in the receiving coil. This causes disturbances in the more sensitive metal detectors. Therefore, an additional scheme for compensating the magnetic field is installed in these type of detectors.

 

Filter accepting the useful signal only and negating the external noises

In order a metal detector to be sensitive enough and with as much as possible lower level of the unwanted noises, the low-frequency useful signal, after amplified, is processed by a narrow-band filter. Filters can be passive and active. The active ones are used in the more sensitive metal detectors and have a very narrow band for accepting in the low-frequency range. This contributes in a high grade for reducing the unwanted noises in the metal detector.

 

End-amplifier of the useful signal

Despite the high amplifying of the useful signal, it has a small amplitude. So that the amplitude of this signal is able to trigger the metal detector's indication, it should be additionally amplified by the end-amplifier.

 

Scheme making the indication when a metal is present and detected

A visual or sound indicator is used. The combination of the both indications is possible however. The visual indication system can be an arrow-like analog device, an LCD display or a LED indicator with a number of light diodes. The sound indication consists of a speaker or headphones. This is the indication system that offer the easiest use and more comfortable work with the detector, since it lets the operator focus on the work field.

 

Control block

The control block manages the whole metal detector. It may consist of discreet digital schemes, or be a microprocessor itself, which is the prefferent variant. In the professional metal detectors, the operator is granted control over the work of the microprocessor, which sets many adjustment parameters of the detector available to be easily modified. Examples are: width and frequency of the probing pulse, positioning in time for the selecting pulse from the scheme for extraction of the useful signal, etc.

 

Power Supply

 Due to its specifications, the pulse metal detector uses a significantly greater amount of energy power than the other types of detectors. Therefore, it needs a power source that is stable and with enough power output capacity.

 

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