Flowserve 3400IQ Digital Positioner - Manual (Page 32)

Logix 3400IQ Digital Positioner FCD LGENIM3401-00 – 06/06
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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 parameters 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 3400IQ 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.