Rockwell Automation T8019 Trusted Process Control Algorithm Software Package - Manual (Page 17)

Trusted
TM
Process Control Algorithms T8019
Issue 8 Sep 07 PD-T8019 17
2.7. Incremental PID Function
A PID is a process regulator. Using the feedback concept, an output is regulated according to the
difference between its current value (PV) and its required value (SP).
The IPID function block is an improved version of the PID_II block with incremental calculation,
derivative term switch for PV or error, and a derivative term filter. The IPID block is recommended for
all new applications. It is also a relatively simple retrofit for the PID_II to provide a more stable output.
IPID PV
X
OUT
SP
Auto
XO
Kp
Ti
Td
Ts
X
MAX
X
MIN
LIMIT
HI
LIMIT
LO
INC
STOP
DEC
STOP
D
ERR
Tdf
D
QP
D
QI
D
QD
D
XOUT
Fwd
Auto is the auto/manual mode select (type BOOLEAN). When set to TRUE the IPID calculation is in
auto mode. XO is the manual set-point and is type REAL. Kp is the proportional constant (type
REAL). Ti is the integral time constant (type TIMER). If this is set to zero the function provides only the
P and D terms. Td is the derivative time constant (type TIMER). Kp, Ti and Td may be dynamically
changed in Auto mode without process bump.
Fwd switches between Forward and Reverse action (type BOOLEAN). When Fwd is FALSE and Kp is
positive, an increasing PV causes a decreasing Xout (reverse action). When Fwd is TRUE and Kp is
positive, an increasing PV causes an increasing Xout (forward action). If Kp is negative, the action is
also inverted.
Ts is the sample time (type TIMER). If the specified sample time is less than the application program
scan time, then the effective Ts will be the greater of the application scan time or PID function
execution interval. Changing Ts when in Auto mode may cause a process bump due to recalculation of
dynamic variables.
Tdf is the derivative action filter time constant (type TIMER). This may be used to reduce reaction to
process noise, or to spread derivative action over time. Tdf sets the exponential time constant of a first
order lag. If set to zero, the derivative action will act on the process fluctuations immediately.
D
ERR
switches the derivative action input (type BOOLEAN). When set to TRUE, the derivative action is
calculated based on the change in error (SP – PV). When set to false, the derivative action is
calculated based on the change in PV. This allows a choice on whether setpoint changes demand a
derivative kick or not. Note that when D
ERR
is false, the proportional action will still react to setpoint
changes, but with less step than P + D action.
The IPID calculation is incremental; at each scan, the required change in output is calculated. This
means that the P,I and D settings may be dynamically tuned without significant change to the output.
The total change at each scan is output as D
XOUT
(type REAL). The change due to the P,I and D terms
are output as D
QP
, D
QI
and D
QD
(all type REAL). These may be used for continuous incremental action
or for commissioning data.