Manual Chapter 5

6-3
When using the sweep mode calculations are completed in the following
manner. If readings were taken from each laser target at 1° intervals (with
the position as the Y- axis and the rotational position as the X-axis), a sine
wave would be formed. Even when only part of the sine wave is completed,
UltraSpec Alignment software can complete the remainder of the curve.
Called curve fitting, all values used to determine the machine moves can be
obtained from the completed sine waves.
CSI recommends you sweep at least 90°, however accuracy may be
reduced even at 90°. At the very least, this could result in more machine
moves being required to achieve satisfactory alignment. Each head will
store up to 180 readings (the higher the number, the greater the accuracy),
however, all 180 readings are not required to determine the sine wave. You
must have at least three readings and they should be spaced at large inter-
vals in the shaft rotation.
After the sweep data has been transferred from the heads to the analyzer,
the data will be automatically curve fit. The quality of the data is shown by
the curve fit percentage. If more points are located off the sine wave (the
greater distance from the curve), the lower the percentage will be. A high
number of points on the curve means that most of the data were acquired
from points that lay on the sine curve.
Bearing faults, rubs, and looseness are all problems which can cause points
not to fall on the curve. These mechanical problems can cause all data to
have some low levels of variability. This variability may appear as random-
ness, or “noise”. This noise is not generally of much concern when the level
of misalignment is high, however, as the amplitudes measured from mis-
alignment decrease,
the ratio of noise to the signal increases and the percentages of
fit may worsen
. Although the program will automatically condition the data
for an improved fit in such a circumstance, there may be times when the
user chooses to change methods or to manually condition the data using the
View Data function (see Sine Fit under “Additional Data Detail” on page
5-34).