|Sunday, 20 August 2006|
A metal detector operates by measuring the change in an electromagnetic field when a target object enters the field. Metal detectors work by generating a low intensity magnetic field from a current flowing through a coil of wire and examining the various types of distortions produced in this field by a surrounding matrix of materials. Metals have one important property possessed by no other elements that enables such to be readily distinguished with suitably sensitive apparatus. Metal detectors typically identify the type of a metal object located in a background environment by analyzing the characteristics of an electrical signal attributable to the target. By generating an alternating magnetic field in the vicinity of a metal object, electric currents are induced in the object which in turn set up a magnetic field around the object which distorts the original field. By detecting this distortion, the metal object can be located. Some metal detectors identify target type by analyzing the phase relationship between a signal transmitted into the ground and a received signal. Due to their electrical and magnetic properties, metal objects provide phase changes in the received signals which are characteristic of the objects in question. By monitoring the received signals the presence of a metal object can be detected and classified according to the phase character of the received signals. Metal detectors in the food industry typically radiate an electomagnetic signal having a single operating frequency in the range of fifty kilohertz to two megahertz. These detectors use phase sensitive synchronous detection in order to detect the metal in the presence of background noise.
Metal detectors are typically operated using a transmit coil and a receive coil within a single coil assembly. Metal detectors include a signal generator which supplies a sinusoidally time-varying voltage to a transmit coil to produce a time-varying electromagnetic field. Usually a receive coil, is connected to a detector circuit which amplifies and evaluates the voltage induced within the receive coil by the transmit coil. The transmitter coil generally generates a pulsed magnetic field, which excites eddy currents in metal objects passing through the portal. The receiver coils receive the magnetic field created by the eddy currents in the metal objects passing through the portal, as well as the magnetic field created by the transmitter coil. Many metal detectors include various oscillatable controls which may be preset as desired according to the type of metal detecting operation to be carried out. There are two modes of operation which are frequently employed. In a ground cancellation mode, the detector is adjusted so as to ignore the response generated by layers of permeable ferrous material within the soil. In the second mode, the discrimination mode, the metal detector circuit is adjusted to discriminate between various types of target objects. Most metal detectors of this type consist of a generally rectangular box having a handle on the top and a rigid shaft connected to the bottom of the box frame and extending outwardly therefrom. A disc or ring shaped head member is attached to other end of the shaft.
Metal detectors are based on the measurement of changes to the electromagnetic field, which are brought about by metal objects located in a zone to be monitored. An electromagnetic field is created in a given region where a hidden object may be located. The change in the field is indicated in a receive signal derived from a coil located in the electromagnetic field. The metal detector has circuitry that transmits a periodic signal to the transmit coil as the search head is manually swept over a ground surface to detect buried metal objects. The detection circuits may be of the analog type with a minimum of digital logic or may be highly digital, using microprocessors and microcontrollers with a minimum of analog circuitry. The circuit in the metal detector is preset to a threshold level and any increase in the detection signal causes the generation of an audible and visual signal for alerting the operator. The sensing coils (transmit coil and receive coil) and sensing circuit are used to detect presence of a metal object in the zone by comparing perturbations in the magnetic field caused by the presence of the metal object to a threshold value. The presence of any conductive or magnetic material within the changing flux field of the transmit coil alters the basic relationship between the transmit coil and the receive coil, causing a detectable change in the voltage induced in the receive coil. Metal detectors can emit an audible signal when their search coil passes over detected metal objects.