Thermo Scientific APEX 100, User Manual

Page: 130 / 442

Page 130
Thermo Scientific APEX User’s Guide
Activating Reject Suppression (Reject 1)
See page 131. (This function is normally used only in gravity-feed and pipeline
applications.)
Suppression Reject
Trigger
Understanding Reject Suppression (Reject 1)
This function is normally used only in gravity-feed and pipeline applications. Before you
can set parameters for the reject-suppression function, you must first activate the
detector’s suppression capabilities. To help you understand why suppression is useful
and how it works, here is some theory and a real-world example of the suppression
function in action.
Theory Behind the Amplitude-Suppression Function
In a typical gravity-feed or pipeline application that diverts contaminated product for
only one second and where vibration resulting from gate opening and closing lasts for
three seconds, there is a two-second window where product continues to flow, but
where product may be erroneously tagged by the detector as contaminated. This,
clearly, is undesirable for a number of reasons: first, the product now flowing is not (in
all likelihood) contaminated; and second, allowing the vibration signal to affect the
detector would produce a fatal feed-back loop. This problem is resolved by suppressing
the vibration signal just enough to prevent the detector from interpreting the vibration
signal as contamination.
For example, if you apply an amplitude-suppression value of 75% to the vibration-plus-
product signal from the search head, 75% of the signal goes to the detector’s circuits for
evaluation (meaning the signal is analyzed to see whether it looks like a contaminated
product is currently passing through the search head). In this example, by a process of
trial and error (by starting at 100% and working down in 5% increments), we have
discovered that 75% amplitude suppression prevents vibration being tagged as
contamination.
Example of How Gate Vibration Can Affect the Search Head
Let’s look at an example. (Looking at the figure on page 304 will help you understand
the explanation that follows). A milling company is monitoring the inflow of grain from
their suppliers. The grain falls down a duct under the influence of gravity and passes
through the search head, which surrounds the duct. Whenever contamination is
detected, a gate immediately downstream of the detector slams closed in
50 milliseconds, diverting the contaminated product to the reject bin. After one second,
the gate slams open in 50 milliseconds, allowing the product to again flow normally.
Unfortunately, the vibration in the duct caused by the gate slamming open and closed is
communicated to the search head, which interprets the vibration as the passage of
contaminated product. To prevent this occurring, any reject signal from the detector is
suppressed for a period of time—in our example, 3.0 seconds—after contamination is
first detected.