10.5 Digital Filtering - Rice Lake IQ plus 710 Installation Manual

10.5 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.
DIGFLx Parameters
Discontinued Product
The first two digital filtering parameters, DIGFLT1,
and DIGFLT2, 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.
The overall filtering effect can be expressed by adding
the values assigned to the two filter stages:
DIGFLT1 + DIGFLT2
For example, if the filters are configured as
DIGFLT1=16, DIGFLT2=8, the overall filtering effect
is 24 (16 + 8). With this configuration, each A/D
reading has a 1-in-24 effect on the displayed weight
value. Setting the filters to 1 effectively disables
digital filtering.
®
RATTLETRAP Filtering
R AT T L E T R A P d i g i t a l f i l t e r i n g ( R AT T R A P
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
s c a l e l e s s t h e v i b r a t i o n - i n d u c e d f l a w s . I t i s
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 and DIGFLT2) 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
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 paramete rs to eliminate th e
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 / DSPDIV
In the example in Step 2, with a threshold
weight value of 50 lb and a display division
value of 5D:
50 / 5D = 10.
set to 10DD 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.
DFTHRH should be
Appendix 69