Logix 3400MD Digital Positioner FCD LGENIM3404-00 – 9/09
20
Position Span Flag in CALIBRATE_FLAGS
Position span is a check during stroke calibration to verify that the valve
stem moved. The algorithm waits to see if no movement is detected
when the valve is automatically stroked open. Anything that could
prevent the valve from stroking will generate a Position Span error (no
supply pressure, malfunctioning spool valve).
7.13.2 Control and Tuning
Setting P + I Parameters
Using the Host configurator, you can set individual tuning parameters. A
few key points are mentioned below. (See Figure 11.)
GAIN_UPPER, GAIN_LOWER, and GAIN_MULT
: These three param-
eters are related by the following formula.
Proportional Gain =
Maximum Gain - | deviation | x Gain Multiplier
If Proportional Gain < Minimum Gain, then Proportional Gain =
Minimum Gain
This algorithm allows for quicker response to smaller steps yet stable
control for large steps. Setting the gain multiplier to zero and max gain
= min gain results in a typical fixed proportional gain.
The higher the gain multiplier, the larger the required deviation before
the gain increases. Default values upon initiating a RESET to factory
defaults (under LOAD_EE_DEFAULTS) are maximum gain = 2.0,
minimum gain= 1.0, and gain multiplier= 0.05. These values will allow
stable control on all Valtek control product actuator sizes.
Integral Gain (IGAIN)
: The integral gain is primarily for deviations due
to temperature drift within the inner loop spool control. The factory
default value is 10. Although higher numbers can speed the time it
takes to reach zero deviation, it can add overshoot if too large. It is
recommended that maximum and minimum gains be adjusted while
leaving integral gain fixed at 10. Integration is disabled below a stem
position of 3 percent and above a stem position of 97 percent. This is to
prevent integration windup from calibration shifts due to lower pressure
or a damaged seat that may prevent fully closing the valve.
Integration Summer
: The integral summer within the Logix 3400MD
digital positioner is clamped at +20 percent and -20 percent. If the
integration summer is fixed at +20 percent or -20 percent, it usually
indicates a control problem. Some reasons for a clamped integration
summer are listed below:
• Stroke calibration incorrect.
• Any failure which prevents stem position movement: stuck spool,
handwheel override, low pressure.
• Incorrect inner loop offset.
• Loss of air supply on a fail in place actuator.
Writing a zero to integral gain (IGAIN) will clear the integral summer.
The integral gain can then be returned to its original value.
Inner loop offset (IL_OFFSET)
: Three control numbers are summed to
drive the inner loop spool position control: proportional gain, integral
summer, and inner-loop offset.
Inner-loop offset is the parameter that holds the spool in the ‘null’
or ‘balance’ position with a control deviation of zero. This value is
written by the positioner during stroke calibration and is a function
of the mechanical and electrical spool sensing tolerances. However,
if it becomes necessary to replace the driver module assembly or the
software RESET calibration constants has been performed, it may be
necessary to adjust this value. The method below should be used to
adjust inner-loop offset.
Or simply perform a new stroke calibration.
Figure 11: Logix 3400 Block Diagram
Sensor
-
+
+
+
+
Summer
Integration
I
Offset
Loop
Inner
+
-
Gmult
Pmin
Pmax
Deviation
F
OUNDATION
Fieldbus
Signal
Command In
(AO Block)
F
OUNDATION
Fieldbus
Transducer
Block
CONTROL
COMMAND
Output
D/A
Percentage
Algorithm
Control
Supply
Air
Control
Spool
Loop
Inner
ATO
Tubed
Sensor
Position
Stem
Voltage
Valve
Piezo
MPC
Soft Limits
Characterization
Linear Mode
Output
Inner-Loop
