®
15
Detect Modes
E-Z Tec IV and V metal detectors can operate in either
of two detect modes. The most appropriate mode for
a given application depends largely upon
environmental conditions and upon the likely mix of
tramp metal in the product stream. The discussion
below provides guidelines on selection of the best
detect mode for your operation.
MODE 1
The metal detector control processes an analog signal
from the receiver coils of the metal detector. A typical
example of a metal signal is shown in Figure 5-A. If
the magnitude of the signal rises above a
predetermined detection threshold, the metal detector
will output a detect signal. Note that the analog signal
has a positive and a negative portion. In detect mode
1 the metal detector will detect on both portions of the
signal. The detect signal typically begins with the first
portion of the signal crossing the detection threshold
(at point “A”) and ends after the second portion of the
signal returns through the opposite detection threshold
(at point “B”). Mode 1 is usually the most sensitive
and reliable method of detection and is, therefore,
used in most applications.
MODE 2
Occasionally a metal detector is used in an
environment where significant interference is
experienced. Typical sources of interference are
lightning, static electricity, and nearby equipment with
inductive loads. These types of interference usually
cause analog signals which have only one polarity.
Figure 5-B shows typical unipolar noise signals at “D”,
“E” and “F”. Each of these signals would cause a metal
detector to false detect when operating in Mode 1
because they exceed the detection threshold level.
The metal detector can be adjusted to ignore many
of these signal types by using Mode 2 detection. Refer
again to the analog signal of Figure 5-A. Notice that
the metal signal has both a positive and negative
polarity. Mode 2 detection does not detect a unipolar
signal. It holds off detection until the second polarity
of the signal occurs. For the signal in Figure 5-A the
detection would occur at point “C”. It would last a
predetermined length of time and reset. The noise
signals in Figure 5-B would be ignored because they
are unipolar.
In general, Mode 2 detection may be somewhat
less sensitive than Mode 1 detection. Also Mode 2
detection may allow metal to pass undetected in
certain instances. Figure 5-C shows an analog
signal representing a small piece of metal followed
by a large piece of metal in the product stream.
The beginning of the signal is similar to the
beginning of the signal shown in Figure 5-A; but as
it moves toward the negative detection threshold,
the signal of the small piece of metal is absorbed
by the signal of the larger piece of metal following
it. The larger piece of metal is detected at “G”. Note
that the small piece of metal is not detected.
Extremely large pieces of metal will cause larger
sections of the product stream to be ignored
because the electronics will require more time to
recover. Therefore, the probability of missed metal
increases with the size of the tramp metal. Normally
this area is small because the tramp metal is small.
The probability is low that metal will be missed, but
the possibility exists and must be considered before
one decides to use Mode 2 detection.
If the metal detector works reliably using Mode 1
detection, Eriez recommends use of this mode.
Some applications are electrically noisy and the
number of false trips is high enough that the metal
detector cannot be used in Mode 1. In this case
one must decide whether to use Mode 2 detection
with an increased probability of missing a small
percentage of metal. The unit will ignore most noise
and still detect the overwhelming majority of the
metal. Mode 2 is much better than no protection.
However, Eriez recommends its use only when
absolutely necessary.
Every unit leaves the factory with an Individual Unit
Specifications sheet. This sheet will indicate
whether the unit is set for Mode 1 or Mode 2
detection. Most units will be set for Mode 1
detection. If the detection mode of the unit must be
changed, use switch SW2-1 on the Output Control
panel of an E-Z Tec IV or V analog metal detector
(in the Power Supply housing) or use the Detect
Mode menu option on an E-Z Tec IV or V MPC
metal detector. Detailed instructions for these
procedures are given in later sections.
Summary of Contents for E-Z TEC IV
Page 58: ...58 Figures cont Figure 5 Detect Modes...
Page 62: ...62 Figures cont Figure 12 Metal Detector Mounting Feet Figure 13 Conveyor Belt Splices...
Page 63: ...63 Figures cont Figure 14 Electrical Connections...
Page 64: ...64 Figures cont Figure 15 Analog Version Control Panel...
Page 67: ...67 Figures cont Figure 19 Flowchart of MPC Menu Structure...
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