Measurement Computing eZ-Record rev.2.1 - Manual (Page 13)

13
January 2001 eZ-Record Manual
Analyzer Tab - Trigger Panel
Triggering is the method used by the analyzer to start capturing and
processing data. To capture continuous data, select Free Run. To capture
transient data, select Input Channel and specify the channel where the
analyzer should expect to “see” the signal.
For example, if you are using an impact hammer for modal analysis, the
hammer is attached to the specified input channel. For this type of test, you
may also want to average a set number of acquisitions before saving the data
(See Number of Averages). You may also want the option to automatically or
manually reject measurements caused by double hammers and overloads.
(See Block Rejection.) (Also see: Hints for Trigger Setup and Double
Hammer Rejection.)
Trigger conditions are used by the analyzer to determine the trigger signal’s
attributes required to start data acquisition. Triggered acquisition is typically
used to capture a specific recurring event. Because the analyzer is always
capturing data, and storing it in the memory buffer, you can specify a pre-
trigger that tells the analyzer how much data you want captured before that
event occurs. This also insures that you get the full event and not just the
data that follows it. On the other hand, you may know that something is
occurring x amount of time after an event. In that case, you may want to set
up a post-trigger. A post-trigger tells the analyzer to look for an event, then
wait a specific amount of time and then start capturing data.
A trigger is specified by the signal level (as a percent of the Full Scale), the
signal’s location on a slope (ascending/positive; or descending/negative).
Other factors that determine what data is captured when a trigger condition
occurs are the amount of time (as a percentage of the frame/block of data)
associated with a pre- or post-trigger.
To understand the concept of Pre and Post Triggering you must remember
that when the green light on the front of the Medallion is glowing, it is
“seeing” data and sending that data to the DSP. If you have not clicked the
acquire button, this data is not processed. However, the DSP has 4 MB of
memory (memory buffer). It therefore has the capability to “remember” this
unprocessed data.
That said, lets look at trigger modes and what they mean.