Rice Lake 720i Programmable Indicator/Controller - Installation Manual - Manual (Page 113)

Appendix 107
11.7 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
d i s p l a y e d we i g h t . T h e v a l u e a s s i g n e d t o e a ch
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
d i v i s i o n s . W h e n a s p e ci f i e d n u m b e r o f
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
t h e i n d i c a t o r d i s p l a y t o d e t e r m i n e t h e
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.