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Inputs/Outputs Menu
Analog output characteristics (continued)
“Tri-linear” analog
output
The "Tri-linear" output brings benefits over the “Linear output” discussed
before. It is illustrated below (4-20 mA output is shown, 0-20 mA or 0-5 V
settings are similar).
Compared to the “Linear” mode, the expected range of measure is Range 2.
A Range 1 and 3 are available to show the measures falling out of this
Range 2, but normally at a lower resolution. Expected measurements for the
measured process are supposed to be in Range 2 most of the time, and in
Range 1 or 3 occasionally (problems, calibration, line stop, etc.). The
benefits are:
• The PLC can compute the measurement over a large range (1, 2 and 3).
• The PLC can compute a higher resolution signal for the expected
measuring range (Range 2: MH > M > ML).
• Carefully selecting the set points allows for an individual resolution for
each range, so a different resolution can be applied to Range 1, 2 and 3,
allowing to tailor the analog output to the actual conditions.
As before, the downside is that any measure below or over the Range 1, 2
and 3 will have the same signal locked at 4 mA and 20 mA respectively, but
Range 1, 2 and 3 should cover a larger range than in the “Linear” mode.
Settings must made in balancing these aspects.
Settings
For each output, set MLL, ML, MH, and MHH in the current measuring unit
(e.g. °C for a temperature output). When a compound unit is selected, the
smallest unit will be used (e.g. ppb for a "ppm-ppb" compound unit). Also set
AOL (Analog Output Low) and AOH (High) in mA (or Volts).
These points should be set keeping in balance the following conditions (see
illustration above):
• The smaller is Range 2, the better is the analog signal resolution within
the expected range of measure.
• Size of Range 1 and 3 should be set to deliver an adequate level of
resolution for the measures falling out of the expected range of
measure.
• In Range 0 the analog outputs only shows that measurement is below
the MLL value. Similarly in Range 4 the analog output only shows that
measurement is over the MHH value.
The formula to compute the measurement knowing the current or the
voltage and the resolution R is given in the following table:
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