Filter Selections; Analog Filtering; Digital Filtering - Rice Lake IQ plus 800 Installation & Operation Manual

9-4 IQ plus 800/810 Installation Manual

FILTER SELECTIONS

Finding the right filter settings takes both patience and experimentation. The
following subsections will help you understand how filtering works.

Analog filtering

Analog filtering uses electrical components (usually special capacitors
and other "voltage smoothing" elements) to make the DC voltage from
the load cell to the A/D converter as free of surges and fluctuations as
possible. These capacitors have a tendency toward smoothing out the
major, high-frequency voltage bumps, while easily tracking the smaller,
low-frequency changes. Analog filtering is generally more effective
than digital filtering in preventing voltage fluctuations caused by
electrical or radio frequency interference. Voltage fluctuation caused by
such electrical interference usually shows as a regular rolling up and
down of the display.
In Table 9-1, notice that settings for increased filtering effect that screen
out more electrical noise, also tend to increase settling time, therefore
slowing down the display in achieving Standstill. Generally, use the
lowest filtering effect which yields a quiet or stable display.
It is generally the best practice to attempt to clear interference first with
just the analog filter so the signal will be clear entering the A/D
converter. When testing the effect of the analog filter in this manner, set
the digital filter at its lowest setting, 1. If the highest rate of analog
filtering does not stabilize the display, leave the analog filter on at 8 Hz
or 2 Hz, and begin increasing the digital filter settings.
Table 9-1.
Analog filter selections
ALGFLTR
setting...
None
8 Hz
2 Hz
*Settling times are relative and dependent on interference level.

Digital filtering

Standard digital filtering is basically a software function. It uses
mathematical averaging to try to eliminate the variant digital readings
that the A/D converter sends periodically because of external vibration.
Digital filtering does not effect the indicator's measurement rate, but
does affect the settling time. The selections from 1 to 128 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.
Filtering
effect...
Low (>25 Hz)
Medium (>8 Hz)
High (>2 Hz)
Approximate
settling time*...
64 ms
200 ms
800 ms