Rockwell Automation T3834 ICS Regent Continuous Control Package for Winterpret User Manual
Page 5
Continuous Control Package for W
INTERPRET
(T3834)
P D - 6 0 4 4 M a r c h , 0 6
5
T
S
= loop sample time
T
l
= reset time
The amount of reset action is defined by the integral time
constant: reset time (T
l
). The reset 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 reset
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:
M
n
= P+l+M
r
= K
c
[e
n
+ I
n-1
+ (T
S
/T
I
) * e
n
] + M
r
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:
D
n
= (T
D
/T
S
) * d
where,
D
n
= derivative action
T
D
= derivative time constant, rate time
d = differential:
for error differential, d
e
:
d
e
= (e
n
- e
n-1
)
where,
e
n
= current error
e
n-1
= previous error
for process differential, dp:
d
p
= (PV
n
- PV
n-1
)
where,
PV
n
= current process variable
PV
n -1
= previous process variable