13
Synchro Noise Filtering
Overview:
There are some applications where the 1250B may be installed in environments where
harmonic noise on the power line can cause an unstable reading of the synchro position.
Assuming that a stable, steady position reading is always better than a fluctuating reading,
INCON has devised a solution to these problems. Two types of position filtering have been
added to the 1250B’s firmware – a Turns Rate Threshold Filter, and an Averaging Filter. The
strength of each filter can be adjusted so that the best combination can be achieved for
individual applications. The factory default programming is for both of these filters to be
OFF.
Description:
The
Turns Rate Threshold
filter allows a position measurement of the synchro to be
accepted only if measurements of the synchro’s output voltages indicate that the
synchro is
rotating at a
rate of less than the programmed number of degrees per second
(either
clockwise or counter-clockwise).
If a rotation rate faster than the programmed number of
degrees per second is measured, the measurement is not acknowledged. It is assumed to be a
noise-induced aberration and the display will freeze at the last known good reading. The
nature of harmonic noise is that its high frequency will cause the synchro output signal to be
randomly measured higher or lower than the true signal voltage. This would be interpreted as
abrupt rotations of the
synchro
at a very high rotation rate. Therefore, this filter will be
effective in cancelling the destabilizing effect of harmonic noise on the power line. The
1250B will display the synchro position more faithfully, but more importantly, the accidental
accumulation of turns will be prevented.
This Turns Rate Threshold should be set at least TWICE the maximum expected synchro
rotation
rate
in Degrees Per Second. If harmonic noise is present on the power line, the
1250B may not follow the normal synchro rotation. The display will be frozen in one
position. In this case, increase the Turns Rate Threshold value significantly until a valid
reading of the synchro is attained. The value may be adjusted up or down as needed to reach
the desired filtering effect. Remember: a smaller Threshold value means a stronger filtering
effect; a larger Threshold value means a less effective filter.
The
Averaging Filter
causes the displayed value to change more slowly, as a number of
measured position values are averaged
. With this filter activated, the effects of noise-
induced aberrations are lessened, the position reading is very stable, but at the cost of a
somewhat sluggish display. The displayed position may lag behind the actual synchro
position by several seconds, depending upon the filter’s settings. This filter is especially
helpful in applications where a very stable, slow changing reading is required. A
programmable
Averaging Filter Threshold
is available to disable the averaging during
periods of faster rotation rates. When this Threshold is set to a number other than zero, the
filter will stop averaging while the synchro is being turned faster than the programmed
number of
display counts per second
. The filter will resume averaging when the rotation
rate drops below the programmed number of display counts per second.