Steerable three-dimensional magnetic field sensor system for detection and classification of metal targets
Most metal detectors (electromagnetic induction (EMI) sensors) or nuclear quadrupole resonate (NQR) use a loop antenna to create a magnetic field in the environment. The magnetic field interacts with the objects. For trasient EMI metal detectors, the magnetic field creates eddy currents in the metal object which is then detected by some suitable magnetic receiver or sensor. For NQR sensors that are designed for explosive detection, the magnetic field interacts with the explosive and causes a small magnetic signal to be transmitted that can be detected with a suitable magnetic field sensor. In both cases, an important component of the sensor is the ability to project a strong magnetic field at the site of the target. Loop type magnetic field transmitter coils are commonly used for this task. The problem with loop antennas is the magnetic field strenght is a strong function of distance from the antenna. Along the axis of the loop antenna, the field strength varies approximately as 1/(r^3), where r is the distance from the plane of the loop of the object.
The Johns Hopkins University has developed and patented a planar magnetic field transmitter antenna that creates a magnetic field that has a more uniform shape and a higher intensity as a function of distance from the antenna. The increased magnetic field strength as a function of distance from the antenna results in a sensor that is more sensitive for direction. In the ideal case of an infinite current sheet, the magnetic field is constant with distance from the antenna to object. For a real antenna, the infinite current sheet is approximated by a collection of parallel current carrying elements (e.g., wires or traces on a printed circuit board). The magnetic field strength then becomes a function of the wire spacing. Rough estimates of field strength as a function of distance from an antenna with realistic wire spacing have been made and the field falls off at about 1/r or better. This is a major increase magnetic field strength compared to a loop antenna.
Patent Status: U.S. patent(s) 7030759 issued.CONTACT: