Rockwell Automation T3834 ICS Regent Continuous Control Package for Winterpret - Manual (Page 6)

Continuous Control Package for W INTERPRET (T3834)
6
Industrial Control Services
The amount of derivative action is defined by the
derivative time constant: rate time (T
D
). 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
both proportional and integral. When derivative control is
combined with proportional control and integral control,
the output equation is expressed as:
M
n
= K
c
[e
n
+ I
n -1
+ (T
S
/T
l
) * e
n
+ (T
D
/T
S
) * d
e
] + 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.