Rockwell Automation T3834 User Manual
ICS Regent
®
PD-6044
Industrial Control Services
1
Continuous Control Package for
W
INTERPRET
Continuous Control Function Block for PID
Control (T3834)
Issue 1,
March, 06
The Continuous Control package for WINTERPRET
is an
add-in software package which allows the user to create
co
ntinuous control function blocks for Regent application
programs. When installed on the PC, the continuous
control package is seamlessly integrated with the base
W
INTERPRET
software.
Continuous Control is used to maintain a measured
process signal at a pr
edetermined value and provides the
user with three different control modes: Proportional,
Integral and Derivative. These modes give Continuous
Control the term ‘PID’. Each function block allows the user
to define up to 16 control loops (also known as con
trollers).
Software Installation
The Continuous Control package is installed on the PC
running the WINTERPRET
application software. The
W
INTERPRET base package provides the necessary
installation software to install this add-in package. The
Continuous Control package should be installed at the
same time or after you have installed the WINTERPRET
base package.
Installation Procedure
IMPORTANT!
The files on the Continuous Control package diskette are
in compressed form. You cannot simply copy the files to
your hard drive — they must be decompressed before they
will run. You must have the WINTERPRET
base package
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INTERPRET
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Industrial Control Services
distribution disk in order to run the setup procedure to
install the Continuous Control package.
To install the Continuous Control package, use the
following sequence:
1. Insert the WINTERPRET base package distribution disk
into drive A: or B:
2. Start Windows (if it isn’t already running).
3. Choose Run from the Program Manager’s File menu.
4. Type a:\
setup.exe
in the text box. (if you inserted the
W
INTERPRET base package disk in drive B: type
b:\setup.exe
.) Choose OK or press ENTER.
5. In the WINTERPRET Setup dialog box enter the name of
the directory in which you have installed the
W
INTERPRET base package (This assumes that you have
already insta
lled W
INTERPRET). Choose Continue.
6. In the WINTERPRET Installation dialog box check the
Continuous Control package box.
7. Choose OK to have the setup program install the
Continuous Control package software.
When the installation is completed, you can r
un the
W
INTERPRET application and create continuous control
function blocks in your application programs.
Working With Continuous Control (PID) Function Blocks
Application programs consist of one or more logical units
called function blocks. Each function
block performs part
of a program. These function blocks allow you to create
modular programs that are easily updated and
manipulated. Each function block can be used in one or
many programs.
Different types of function blocks can be combined to create
a
program.
Continuous control function blocks let you coordinate and
interlock the loop control functions with the discrete and
sequential functions of ladder logic. The shared variables
in the Regent provide a convenient way to do this.
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INTERPRET
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Shared variables are data values internal to the Regent
that may be commonly referenced by all function blocks of
any application program. Use the Shared Variable Editor
to define the shared variables in the Regent.
Each controller may have up to 10 alarms associated with
it, each of which may be defined by assigning a name.
Continuous Control is used to maintain a measured
process signal at a predetermined value. A Continuous
Control process requires four fundamental items:
·
A process variable (PV) that can be measured
·
A s
etpoint (SP) at which the process variable should be
maintained
·
A control output that affects the process such that the
process variable is changed
·
A set of rules that govern how the controlled output
should be changed to make the process variable equal to
its setpoint
Most of the parameters in the Continuous Control loop
may be assigned a name instead of a constant value.
When a parameter is given a name, that value may be
dynamically modified by other application program
function blocks.
Examples of how
this dynamic modification might be used
include:
·
Assigning a name to the auto/manual parameter would
enable a ladder logic function block to place the
Continuous Control loop in automatic mode when
required by the process
·
Assigning a name to the setpoint p
arameter would
enable a ladder logic function block to modify the
setpoint automatically. Remote setpoint and ramp/soak
functions could be initiated automatically based upon
sequential process events.
·
Continuous Control alarms can be assigned names to
ena
ble them to be referenced in ladder logic function
blocks
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INTERPRET
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Proportional Control
Making the controlled output a continuous variable that is
proportional to the process error provides a precise means
of controlling the process. The process error is the
diff
erence between the setpoint and the process variable.
This can be represented by:
Mn = P = Kcen
where,
Mn = controller output
P = proportional action
Kc = proportional constant, Gain
en = process error = setpoint (SP) - process variable (PV)
In practice this representation is not satisfactory because
the output is zero when the error is zero. Usually some
output is required to maintain the process variable at the
setpoint. This is called the output bias. The rules
governing the control output then become:
Mn = P + M
r
= Kcen + Mr
where,
Mr = output bias
With the addition of the output bias we have an output
value to keep the process variable at the setpoint. This
indicates that there is a finite controller output for a finite
process error. In many cases this finite value will not bring
the process variable back to the setpoint due to system
disturbances or setpoint changes. In effect, it becomes
necessary to continuously adjust or reset the output bias to
bring the process back in control.
Integral Control
The most common method used to automatically reset the
output bias is integral control. Integral control
automatically adjusts the output bias as long as there is a
process error. This is accomplished by integrating the error
with respect to time. The int
egral term can be expressed as:
In = I
n -1
+ (TS/Tl) * en
where,
n = current integral action
I
n-1
= previous integral action
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INTERPRET
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TS = loop sample time
Tl = reset time
The amount of reset action is defined by the integral time
constant: reset time (Tl). The re
set time represents the time
required for the integral sum to equal the proportional
action. This assumes that the process error remains
constant. Thus a smaller value for reset time creates more
reset action. A larger value for reset time creates less res
et
action.
The integral term can be added to the equation to provide
an automatic reset function. When integral and
proportional control are combined the control output is
expressed as:
Mn = P+l+Mr = Kc [en + I
n-1
+ (TS/TI) * en] + Mr
Derivative Control
A
third type of control action is derivative control.
Derivative control causes the control output to change
based upon the rate of change of the error. It acts as an
anticipatory control action used to compensate for sudden
process changes or upsets. The derivative action can be
represented as:
Dn = (TD/TS ) * d
where,
Dn = derivative action
TD = derivative time constant, rate time
d = differential:
for error differential, de:
de = (en - e
n-1
)
where,
en = current error
e
n-1
= previous error
for process differential, dp:
dp = (PVn - PV
n-1
)
where,
PVn = current process variable
PV
n -1
= previous process variable
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INTERPRET
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Industrial Control Services
The amount of derivative action is defined by the
derivative time constant: rate time (TD). The rate time is
the time period over which the error rate of change is
extrapolated and a control action is based. A larger rate
time causes more derivative action. A smaller rate time
causes less derivative action. Derivative action is never
used alone but only in conjunction with proportional or
bo
th proportional and integral. When derivative control is
combined with proportional control and integral control,
the output equation is expressed as:
Mn = Kc [en + In
-
1
+ (TS/Tl) * en + (TD/TS) * de] + M
r
Often all three control modes are used. This three-mode
control is referred to as PID control, which has been widely
accepted as a general set of rules suitable for most
continuous control systems. The PID algorithm can be
"tuned" to reflect the unique dynamics of a particular
process by modifying the proportional, integral and
derivative constants (gain, reset time and rate time) until
the desired system response is achieved.
Continuous Control Package for W
INTERPRET
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Using the Continuous Control (PID) Editor
Continuous Control function blocks are created using Add
Function Block or Insert Function Block from the Program
Editor. After you have created a new function block the
Continuous Control Editor window is opened as shown in
Figure 1.
Figure 1. The Continuous Control (PID) Editor Window.
The Continuous Control Editor lets you create an
d edit
continuous control function blocks. When you create a
continuous control function block you will enter your
parameters in the dialog boxes.
It should be noted that output and output bias variables
must be unique to a controller.
Using drop-down
menus you can select commands to
configure controllers in the function block, print the
function block and perform a host of other options.
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