Teledyne 440, Системное руководство, Страница: 220 / 256

A – Operating Theory
A-2 © Teledyne TSS DPN 402196 Issue 4.1
Figure A-1: Pulse induction waveforms (not to scale)
In Figure A-1 (a) a voltage pulse of amplitude –15V and width 1500µs drives current into a single
search-coil. The polarity is not important for the purposes of this explanation.
The current flowing in the coil rises in a logarithmic manner, as shown in Figure A-1 (b). The SEP limits
this current to a maximum of 20A.
At the end of the 1500µs ‘drive’ period, the SEP removes the voltage drive to the coil and the current
begins to fall rapidly – though not instantaneously – towards zero as shown in
Figure A-1 (b).
The surrounding magnetic field collapses very quickly as the coil current falls towards zero. This rapid
change of magnetic field induces eddy currents in nearby conductive targets, and the magnetic fields
associated with these eddy currents induce a voltage in the coil. See
Figure A-1 (c).
Because the current changes at a much higher rate at the end of the drive pulse than at the start, the
eddy currents are much stronger while the current decays to zero.
The initial high voltage peak (250V) that occurs in the coil immediately after the end of the drive pulse
is due largely to the self inductance of the coil.
After the initial voltage peak, the shape of the decaying waveform will depend upon the nature and the
proximity of any nearby conductive material. The 440 System uses this feature to determine the range
between the coil and the target.
A.2 Waveform Measurement
By making measurements upon the decaying part of the voltage waveform, the 440 System measures
the range between each search-coil and any conductive target nearby.