29
en
a very slow ramp up time and the amplitude of the first cycle is lower than the second cycle. For this reason,
selecting the proper polarity is very important.
The AUTO SET function in your gauge will do a lot of this work for you. However, it is always a good idea
to understand how to interpret the waveform. The following diagrams offer a brief explanation:
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.
Referring to Fig.1. Notice that the height or amplitude of W1+ is greater than the height of W2+ from the
zero crossing (centreline). These are the positive cycles (they are in the upper region). Now consider that
the height of W1- is lower in amplitude than W2-. These are the negative cycles (they are in the lower
region). With this in mind, the gauge 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? Referring 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 gauge 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.
Note: 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, it is probable that you have selected
the correct phase (polarity).
If a situation does occur, as in Fig.2 above, and you are 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 is critical that you take the necessary time to
select the proper polarity in the initial setup process.
The AUTO SET function of your gauge will attempt to optimise and select the polarity for you. Use of this
feature is further explained in “Auto Set” on page 29.
If you want to adjust the polarity manually, see “Polarity” on page 31.
15 MEASUREMENT - OPTIONS
15.1 AUTO SET
Your gauge includes a function for locating the detection point of a bolt end echo that is not currently in the
viewable region of the display. This AUTO SET function uses a variety of algorithms to evaluate the positive
and negative cycles and then automatically selects and sets the values of the following functions:
•
POLARITY
•
GAIN
•
DELAY
•
WIDTH
•
GATE
Figure 1
Correct polarity (phase)
Figure 2
Incorrect polarity (phase)
