Checkline TI-MINIMAX User Manual
Page 141
MINI-MAX Ultrasonic Bolt Tension Monitor
cycle is lost, the MINI-MAX will “peak Jump” to another cycle later in time with
greater amplitude. Often times the transducers have a very slow ramp up time in
their construction. In other 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 MINI-MAX 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
MINI-MAX 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 MINI-
MAX 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.