Thermo Scientific APEX 100, User Manual, Page: 276 / 442

Page 276
Thermo Scientific APEX User’s Guide
Theory of Operation
Your APEX operates on the balanced coil full-loop detection system. Traditionally, three
equally spaced coils surround the aperture or opening through which inspected material
passes. The central coil connects to an oscillator circuit to produce a magnetic field. The
coils on either side of the central coil receive this signal. These are the receiving or input
coils.
Since the input coils are equally spaced from the oscillator, they receive equal amounts
of signal. The coils are wound in such a way that their signals oppose each other so the
net signal across them is zero. When a piece of metal enters the magnetic field, it alters
the field strength around it. As this metal passes through the aperture, it changes the
balance of the receiving coils so that the net signal is no longer zero.
The APEX uses more than three coils in its design. Up to three pairs of oscillator coils
provide higher levels of sensitivity over traditional methods. Parallel and series
arrangements of coils are used. The APEX still uses two receive coils to produce the
metal signal.
A digital signal processor (DSP) processes this signal. The DSP performs the product
compensation, phasing, residual compensation filtering, and produces a reject signal.
Your APEX detector is a high performance, measuring instrument. The quality of the
installation has a direct effect upon performance and reliability. Please read the
installation instructions completely prior to installing the unit.
Definition of Various Product Effects
Metal detectors are capable of detecting metal by measuring two effects: resistive and
reactive effects.
Resistive Effect
Electrically conductive materials and many food products by nature are electrically
conductive. Salt and moisture content combine to produce resistive effects that must be
overcome in order to detect small metal contaminants.