Flowserve 400MD Logix - Manual (Page 11)

Figure 1.2 Functional Block Diagram of Logix 3400MD Digital Positioner Operating with other Instruments
Theory of Operation
Figure 1.3 Logix 3400MD Digital Positioner Block Diagram
NOTE: Variable names inFigure1.3 are reference names only and not accessible to the user. They are for reference use only.
The Logix 3400MD digital positioner receives power from the two-wire, fieldbus input signal. A digital signal, sent via fieldbus, is used as the command
source. A value of 0 percent is always defined as the valve closed position and a value of 100 percent is always defined as the valve open position.
Next, the command value is passed through a characterization/limits algorithm block. The positioner no longer uses cams or other mechanical means
to characterize the output of the positioner. This function is done in software, which allows for in-the-field customer adjustment. The positioner has
two basic modes: linear and custom characterization. In linear mode, the command signal is passed straight through to the control algorithm in a
1:1 transfer. If custom characterization is enabled, the command source is mapped to a new output curve via a 21-point, user-defined curve. In addi-
tion, two-user defined features, Soft Limits and MPC (Minimum Position Cutoff; in fieldbus terminology these are called FINAL_VALUE_CUTOFF_HI and
FINAL_VALUE_CUTOFF_LO), may affect the final command signal. The actual command being used to position the stem is called FINAL_VALUE. The
FINAL_VALUE is the actual positioning command after any characterization or user limits have been evaluated.
The Logix 3400MD digital positioner uses a two-stage, stem positioning algorithm. The two stages are comprised of an inner-loop, spool control and
an outer-loop, stem position control. Referring again to Figure 1.3, a stem position sensor provides a measurement of the stem movement. The control
command is compared against the stem position. If any deviation exists, the control algorithm sends a signal to the inner-loop control to move the
spool, up or down, depending upon the deviation. The inner-loop then quickly adjusts the spool position. The actuator pressures change and the stem
begins to move. The stem movement reduces the deviation between control command and stem position. This process continues until the deviation
goes to zero. The control algorithm is both proportional and integral. This algorithm will be further explained later in the document.
A more detailed example to explain the control function follows. Assume the following configuration:
• Unit will receive its command from the FBAP
• Custom characterization is disabled (therefore characterization is linear)
ST3000 FF
AI
Fieldbus
Logix 3400MD DP
PID AO XD
Valve
Stem
Position
Sensor
Tubed ATO
Sensor
Air Supply
Piezo Valve
Voltage
Inner Loop
Spool Control
Inner-Loop
Hall Sensor
Output
D/A Output
Percentage
Control
Algorithm
Deviation
Position
P
max
P
min
G
mult
Integration Summer
Inner Loop Offset
XD AO +
Control
Command
(CMD_USED)
(GAIN_UPPER)
(HALL_SENSOR)
(GAIN_LOWER)
(GAIN_MULTI)
(IL_OFFSET)
Linear Mode
Characterization
Soft Limits
MPC
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