Teledyne 440, System Manual, Page: 219 / 256

A – Operating Theory
DPN 402196 Issue 4.1 © Teledyne TSS A- 1
A Operating Theory
The 440 System locates a target by:
1. Inducing a pulse of current in the conductive material of the target.
2. Using three independent coils to detect the magnetic fields associated with the currents
induced in the target.
3. Calculating the position of the target from the relative strengths of the signals on each channel.
The Pulse Induction method of target detection, used by the Teledyne TSS 440 System, provides
considerable advantages over alternative magnetometer-based systems:
❐
Pulse induction can detect almost any type of conductive material – not just ferrous metals.
❐
Terrestrial magnetism has no effect upon the measurements.
❐
The System can compensate electronically for the proximity of the ROV body, regardless of its
magnetic heading.
❐
The System uses a simplified scaling procedure that allows a high degree of measurement
accuracy and stability.
This appendix describes the technique of Pulse Induction, and the method used by the 440 System to
derive the signal strength and the target co-ordinates.
A.1 Pulse Induction
NOTE
Because of the special characteristics of 316 stainless steel, it is very difficult to
establish strong eddy currents in this material. For this reason, you would experi-
ence considerable difficulties in detecting targets made exclusively from 316 stain-
less steel.
A current flowing through any coil will create a surrounding magnetic field. The strength of that field at
any instant in time will be proportional to the instantaneous magnitude of current. If the current in the
coil changes, the strength of the magnetic field will vary in proportion to the changes in magnitude of
coil current.
These variations in magnetic field strength will induce voltages in conductive targets that lie near the
coil. The magnitude of eddy currents that flow in the target because of these induced voltages will
depend upon two factors:
❐
The electrical characteristics of the target material.
❐
The rate at which the current in the coil changes.
Any eddy currents flowing in the target material will produce magnetic fields of their own and these
‘secondary’ fields will induce a measurable voltage in the coil as they change.
The 440 System uses this principle to detect the presence of conductive material near the search-coils.
See
Figure A-1 , which shows a single cycle of measurement on one channel: