3-7
Level Controller Configuration
ADAPTIVE BIAS CONTROL
offering with the Model 3095 Level Controller. The Adaptive Bias
Control (ABC) algorithm is used when operating in Proportional (P) or
Proportional-Derivative (PD) modes. Adaptive Bias Control eliminates
limit cycling caused by the control element’s attempts to adjust the
level around a setpoint.
When to Use Adaptive
Bias Control
ABC is used when the application does not require accurate control
around the setpoint. In these applications, standard PID or PI control
can result in a large amount of limit cycling as the control element
attempts to maintain the level around the setpoint. Excessive limit
cycling results in unnecessary control element wear.
How ABC Works
The operator uses a 275 HART communicator or AMS to select a
maximum and minimum range around the setpoint for ABC. The ABC
algorithm restricts the Level Controller’s output to only the proportional
response to level changes within the selected ABC region, ignoring any
offset. If the level moves beyond the selected ABC region, the ABC
algorithm initiates the required control element response that will
return the level to the selected ABC region.
By providing a larger allowable level range, ABC reduces the number
of required level changes to be made by the control element. Limit cycle
elimination reduces control element operation, which may significantly
extend the operating life of the control element. In summary, ABC brings
the users the benefits of PID control while eliminating limit cycling.
FIGURE 3-5. ABC Operation.
ABC:
• is standard with all Model 3095 Multivariable Level Controllers.
• can be initiated in Proportional or Proportional/Derivative (PD)
control modes by using the Model 275 HART communicator or AMS.
• initiates only the P response to level changes within the upper
and lower limits as selected by the user, thus eliminating limit
cycling of the control element.
• allows the Level Controller to complete the P response and I
response when the level is outside the selected upper and lower
limits until the level is again within the specified range.
ABC Upper Limit
ABC Lower Limit
Setpoint
Proportional
Response Only
to Level Changes
3095\3095_12A
Rosemount’s Adaptive Bias Control algorithm is a standard
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