Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 91)

Publication 1757-UM006A-EN-P - May 2002
Configurating the 1757-FIM 4-5
The application involves controlling the level of a 2000 gallon surge
tank with a steady-state 100 gallon per minute (gal/min) process feed
and recycle line. A fieldbus approved smart pressure transmitter is
being used to monitor the level in the surge tank. A fieldbus approved
valve positioner is being used to regulate the control valve in the
process feed line.
The following are some pertinent characteristics about this application
and the corresponding ProcessLogix control strategy for reference.
• The goal is to recycle the process fluid back to the process with
a minimum swing in the recycle feed rate.
• The tank level set point (SP) is 50 percent.
• The tank level low (LO) alarm is 25 percent and the low-low
(LOLO) alarm is 15 percent.
• The tank level high (HI) alarm is 70 percent and the high-high
(HIHI) alarm is 85 percent.
• The ProcessLogix control strategy includes a tank level Control
Module (CM) named CM101 and a pump control CM named
CM102.
• The CM101 includes a ProcessLogix Proportional, Integral,
Derivative (PID) function block configured to operate as a
Proportional and Integral (PI) two-mode controller.
• The tuning for the PI controller is “loose” to allow some swing
in the level of fluid in the tank.
• The CM101 includes a ProcessLogix Data Acquisition
(DATAACQ) function block to provide the alarm flags for the
LO, LOLO, HI, and HIHI tank level alarms.
• The CM101 includes a fieldbus Analog Input (AI) function block
to integrate the tank level indicating signal from the ST3000FF
smart pressure transmitter with the control strategy. It includes a
fieldbus Analog Output (AO) function block to integrate the
Logics 1400 Valve Positioner with the PI controller output from
the control strategy.
• The CM102 includes a ProcessLogix Device Control (DEVCTL)
function block to control the recycle pump through
corresponding Discrete Input and Discrete Output Input/Output
Module function blocks. It also includes links to the LO and
LOLO alarm flags for the DATAACQ block in CM101 for device
safety override and output override interlocks, respectively.
• The recycle pump is to be restarted at 35 percent.