Rice Lake 820i Programmable Indicator/Controller - Installation Manual - Manual (Page 101)

Appendix 97
10.7 Digital Filtering
Standard digital filtering uses mathematical averaging
to eliminate the variant digital readings that the A/D
converter sends periodically because of external
vibration. Digital filtering does not affect the indicator
measurement rate, but does affect the settling time.
The selections from 1 to 256 reflect the number of
readings averaged per update period. When a reading
is encountered that is outside a predetermined band,
the averaging is overridden, and the display jumps
directly to the new value.
DIGFLTx Parameters
The first three digital filtering parameters, DIGFLT1,
DIGFLT2, and DIGFLT3, are configurable filter
stages that control the effect of a single A/D reading
on the displayed weight. The value assigned to each
parameter sets the number of readings received from
the preceding filter stage before averaging.
A rolling average is passed to successive filters for an
overall filtering effect that is effectively a weighted
average of the product of the values assigned to the
filter stages (
DIGFLT1 x DIGFLT2 x DIGFLT3)
within a time
frame corresponding to the sum of the values (
DIGFLT1
+ DIGFLT2 + DIGFLT3)
.
Setting the filters to 1 effectively disables digital
filtering.
RATTLETRAP ® Filtering
RATTLETRAP digital filtering (RATTRAP
parameter set ON) uses a vibration-dampening
algorithm to provide a combination of the best
features of analog and digital filtering. The
RATTLETRAP algorithm evaluates the frequency of
a repeating vibration then derives a composite
displayed weight equal to the actual weight on the
scale less the vibration-induced flaws. It is
particularly effective for eliminating vibration effects
or mechanical interference from nearby machinery.
Using RATTLETRAP filtering can eliminate much
more mechanical vibration than standard digital
filtering, but will usually increase settling time over
standard digital filtering.
DFSENS and DFTHRH Parameters
The digital filter can be used by itself to eliminate
vibration effects, but heavy filtering also increases
settling time. The DFSENS (digital filter sensitivity)
and DFTHRH (digital filter threshold) parameters can
be used to temporarily override filter averaging and
improve settling time:
• DFSENS specifies the number of consecutive
scale readings that must fall outside the filter
threshold (DFTHRH) before digital filtering
is suspended.
• DFTHRH sets a threshold value, in display
divisions. When a specified number of
consecutive scale readings (DFSENS) fall
outside of this threshold, digital filtering is
suspended. Set DFTHRH to NONE to turn off
the filter override.
Setting the Digital Filter Parameters
Fine-tuning the digital filter parameters greatly
improves indicator performance in heavy-vibration
environments. Use the following procedure to
determine vibration effects on the scale and optimize
the digital filtering configuration.
1. In setup mode, set the digital filter parameters
(DIGFLT1–DIGFLT3) to 1. Set DFTHRH to
NONE. Return indicator to normal mode.
2. Remove all weight from the scale, then watch
the indicator display to determine the
magnitude of vibration effects on the scale.
Record the weight below which all but a few
readings fall. This value is used to calculate
the DFTHRH parameter value in Step 4.
For example, if a heavy-capacity scale (10000
x 5 lb) produces vibration-related readings of
up to 50 lb, with occasional spikes to 75 lb,
record 50 lb as the threshold weight value.
3. Place the indicator in setup mode and set the
DIGFLTx parameters to eliminate the
vibration effects on the scale. (Leave
DFTHRH set to NONE.) Find the lowest
effective value for the DIGFLTx parameters.
4. Calculate the DFTHRH parameter value by
converting the weight value recorded in Step
2 to display divisions:
threshold_weight_value / display_divisions
In the example in Step 2, with a threshold
weight value of 50 lb and a display divisions
value of 5 lb: 50 / 5 = 10. DFTHRH should be
set to 10D for this example.
5. Finally, set the DFSENS parameter high
enough to ignore transient peaks. Longer
transients (typically caused by lower vibration
frequencies) will cause more consecutive
out-of-band readings, so DFSENS should be
set higher to counter low frequency transients.
Reconfigure as necessary to find the lowest
effective value for the DFSENS parameter.