Linearize Your Process
for Optimal Performance at Any Production Rate
Reprinted with permission from CONTROL
Magazine, August 1997.
Process loop tuners create a dynamic simulation of your
process to calculate the tuning parameters required. The
controller bumps the process off its setpoint to examine its
response to the outputs. To get good data, the loop tuner needs a
period of linear response - during intervals when significant
disturbances are unlikely. So what happens if your process
oscillates toward one end of the range and is sluggish at the
other?
ExperTune Inc, Hartland, WI, claims to have the answer with
a loop characterizer—the latest module for its PID loop
analysis tools. According to John Gerry, president, "The
characterizer linearizes your process so you get uniform
performance across the entire range. You can run at optimum for
any production rate."
"Many loops have the wrong valve characteristic,"
adds Greg Shinskey, ExperTune's PID loop consultant, "so
optimum tuning can only be realized at one value of controller
output." Most digital controllers now can add a nonlinear
characterizer to the loop. "It's no longer necessary to
change the valve," claims Shinskey. "ExperTune can
automate the selection of the proper characteristics for the
valves in your loops."
The existing relationship between the controller output and
its effect on the process variable can be found by stepping the
output through its range manually and allowing the process to
settle out after each step. ExperTune plots the resulting data to
present the existing characteristic. It then lets the user fit
the characteristic with either a piecewise linear characterizer
or a smooth hyperbolic function, using a simple drag-and-drop
procedure.
Continues Gerry, "When you have selected a curve,
ExperTune then converts your curve into either a set of x-y
coordinates or into source code in FORTRAN, BASIC or C to apply
to your controller. When characterization is complete, tuning at
any operating point should apply equally to all."
The characterizer module can be used where servo control is
important, to linearize flow loops, to control jacket temperature
in split-range chemical reactors or slave loops in cascades, or
for any nonlinear loop where the setpoint will change.
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