Rockwell Automation T3834 ICS Regent Continuous Control Package for Winterpret - Manual (Page 4)

Continuous Control Package for W INTERPRET (T3834)
4
Industrial Control Services
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
difference between the setpoint and the process variable.
This can be represented by:
M
n
= P = K
c
e
n
where,
M
n
= controller output
P = proportional action
K
c
= proportional constant, Gain
e
n
= 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:
M
n
= P + M
r
= K
c
e
n
+ M
r
where,
M
r
= 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 integral term can be expressed as:
I
n
= I
n -1
+ (T
S
/T
l
) * e
n
where,
n = current integral action
I
n-1
= previous integral action