Flowserve 400MD Logix User Manual
Page 91
Figure 10.2 Gain Effect Diagram
This algorithm allows for quicker response to smaller steps yet stable control for large steps. (Refer to Figure 10.2.) 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 the factory defaults are maxi-
mum 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 which may prevent fully closing the valve.
Integration Summer: The integral summer within the Logix 3400MD digital positioner is clamped at +20.00 percent and –20.00 percent. If
the integration summer is fixed at +20% or –20%, 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 will clear the integral summer. The integral gain can then be returned to its original value.
Inner Loop Offset (IL_OFFSET): Referring to Figure 1.3 in section 1, three control numbers are summed to drive the inner loop spool position control: pro-
portional 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 replacing the Driver Module Assembly is necessary, or the software reset 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.
From the fieldbus configurator,
s
p
e
t
S
e
g
r
a
L
s
p
e
t
S
l
l
a
m
S
| deviation |
Minimum Gain
(GAIN_LOWER)
Maximum Gain
(GAIN_UPPER)
Pr
opo
rtional Gain
Gain Multiplier
(Determines Slope)
(GAIN_MULT)
Logix 3400MD Digital Positioner LGENIM3405-02 11/13
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