Dakota Ultrasonics
150
words, the amplitude of the first cycle is lower in amplitude than the second cycle.
For this reason, selecting the proper phase is very important.
The
Auto Set
feature built into the
MAX II
will do a lot of this work for you. However,
it’s always a good idea to understand how to interpret the waveform. The following
diagrams offer a brief explanation:
Correct Phase (Polarity) – Fig. 1
Incorrect Phase (Polarity) – Fig. 2
Refer to the illustrations above. The first thing to note is that the screen shots above
are split into positive and negative regions. The centerline is assumed zero, and the
two regions in the upper portion of the display are positive, while the two lower halves
are negative. Refer to
Fig.1
. Notice that the height or amplitude of
W1+
is greater
than the height of
W2+
from the zero crossing (centerline). These are the positive
cycles, as they are in the upper regions. Now consider that the height of
W1-
is lower
in amplitude than
W2-
. These are the negative cycles as they are in the lower
regions. Keeping that in mind, the
MAX II
uses a zero crossing (Flank) style of
detection. Therefore, the detection will always occur to the right side of the positive
or negative cycle being detected. With this said, the detection in
Fig.1
(B)
is
occurring on the zero crossing of the positive first cycle
W1+
. In this example, the
correct (positive) polarity has been selected.
Why? I’m getting to that! Refer to
Fig.2
. Notice that when the bolt is tightened, the
signal amplitude has decreased due to the applied stress, and a peak jump has
occurred from
W1-
to
W2-
at
(B)
. If we look at the height of
W1-
it has a much lower
amplitude than
W2-
. Therefore, when stress was applied to the bolt, the amplitude of
W1-
was decreased beyond the threshold
(A)
and the
MAX II
jumped to the next
available signal with sufficient amplitude. However, this could never happen in
Fig.1
.
This is because
W1+
has more amplitude than
W2+
, and a peak jump could never
occur without losing the signal entirely.
Good Rule of Thumb: Always look at the height of the positive and negative cycles
separately. Ask yourself if the first cycle (from left to right) is greater in height
(amplitude) than the second. If so, chances are you’ve selected the correct phase
(polarity).
If a situation does occur, as in
Fig.2
above, and you’re able to notice the peak jump
during the process of loading, increasing the gain or lowering the threshold will
generally correct the error. However, if it goes unnoticed, all your measurements will
be in error. Therefore, it’s critical that the user takes the necessary time to select the
proper phase in the initial setup process.
Summary of Contents for MAX II
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