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A controller seeks to maintain the measured process variable (PV) at set
point (SP) in spite of unplanned and unmeasured disturbances. Since e(t) = SP
—
PV, this is equivalent to saying that a controller seeks to maintain controller error, e(t),
equal to zero.
A controller repeats a measurement-computation-action procedure at every
loop sample time, T. Starting at the far right of the control loop block
diagram above (click
for large view):
| ▪ |
A sensor measures a temperature, pressure, concentration or
other property of interest from our process. |
|
▪ |
The sensor signal is transmitted to the controller. The pathway
from sensor to controller might include: a
transducer, an
amplifier, a
scaling element,
quantization,
a
signal filter,
a
multiplexer, and other operations that can add delay and change the size, sign,
and/or units of the measurement. |
| ▪ |
After all electronic and digital operations, the result
terminates at our controller as the "wire in"
measured
process variable (PV) signal. |
| ▪ |
This "wire in" process variable is subtracted from set point in
the controller to compute error, e(t) = SP —
PV, which is then used in an
algorithm (examples
here and
here) to compute a controller output (CO) signal. |
| ▪ |
The computed CO signal is transmitted on the
"wire out"
from the controller on a path to the final control element (FCE). |
| ▪ |
Similar to the measurement path, the signal from the controller
to FCE might include filtering, scaling, linearization,
amplification, multiplexing, transducing and other operations that can
add delay and change the size, sign, and/or units of our original CO signal. |
| ▪ |
After any electronic and digital operations, the signal reaches
the valve, pump, compressor or other FCE, causing a change in the manipulated variable
(a liquid or gas stream flow rate, for example). |
| ▪ |
The change in the manipulated variable causes a change in our
temperature, pressure, concentration or other process property
of interest, all with the goal of making e(t) = 0. |
Design
Based on CO to PV Dynamics
The steps of the controller
design and tuning recipe include: bumping the CO signal to generate CO to PV dynamic process
data, approximating this test data with a first order plus dead time (FOPDT) model, and then using the model
parameters in rules and correlations to complete the controller design and tuning.
The recipe provides a
proven basis
for controller design and tuning that avoids wasteful and expensive
trial-and-error experiments. But for success, controller design and tuning
must be based on process data as the controller sees it.
The controller only knows about the state of the
process from the PV signal arriving on the "wire in" after all
operations in the signal path from the sensor. It can only impact the state of the process
with the CO
signal it sends on the "wire out" before any such
operations are made in the path to the final control element.
As indicated in the diagram at the top of
this article, the proper signals that describe our complete "process" from the
controller's view is the "wire out" CO and the "wire in" PV.
Complete the Circuit
Sometimes we find ourselves unable to proceed with an orderly controller
design and tuning. Perhaps our controller interface does not make it
convenient to directly record process data. Maybe we find a vendor's documentation to be
so poorly written as to be all but worthless. There are a host of complications
that can hinder progress.
Being resourceful, we may be tempted to
move the project forward by using portable instrumentation. It seems
reasonable to collect, say, temperature in a vessel during a bump test by
inserting a spare thermocouple
into the liquid. Or maybe we feel we can be more precise by standing right at the valve
and using a portable signal
generator to bump the process rather than doing so from a remote control
panel.
As shown below (click
for large view), such an approach cuts out or short circuits the complete
control loop pathway. External or portable instrumentation will not be recording the
actual CO or PV as the controller sees it, and the data will not be appropriate for controller design or tuning.
Every Item Counts
The illustration above is extreme in that it shows many items that are
not included in the control loop.
But please recognize that it can be problematic to leave out even a single step
in the complete signal pathway.
A simple scaling element that multiplies
the signal by a constant value, for example, may
seem reasonably unimportant to the overall loop dynamics. But this alone can change the size and even the
sign of Kp, thus having dramatic impact on best tuning and final controller
performance.
From a controller's view, the complete loop
goes from "wire out" to "wire in" as shown below.
Every item in the loop counts. Always use
the complete CO to PV data for process control analysis, design
and tuning.
Pay Attention to Units
As
detailed in this related article, signals can appear in a control loop
in electronic units (e.g., volts, mA), in engineering units (e.g. oC,
Lb/hr), as percent of scale (e.g., 0% to 100%), or as discrete or digital
counts (e.g. 0 to 4095 counts).
It is critical that we remain aware of the
units of a signal when working with a particular instrument or device. All
values entered and computations performed must be consistent with the form
of the data at that point in the loop.
Beyond the theory and methods discussed in
this e-book, such "accounting confusion" can be one of the biggest
challenges for the process control practitioner.
Return to the
Table of Contents to learn more.
Copyright © 2007 by Douglas J. Cooper. All Rights Reserved.
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