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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 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.
Once again, the Auto Set feature built into the MAX II will attempt to optimize and
select the polarity for you. Use of this feature is further explained in the sections that
follow.
12.8 Manually Locating the Echo
The echo can be manually located by simply adjusting the delay and width values of
the MAX II. While this is process is very tedious and will most often not be used
because of the Auto Set feature, it should be explained for general control of the
instrument. The following procedures outline how to locate the echo using delay and
width.
Adjusting the Delay to Locate the Echo
128
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