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MODULAR MULTIVARIABLE CONTROL BLOCK (CC)
An Advanced Control Block in the MANTRA Process Control System
The Modular Multivariable Control Block (MMC) controls two interacting
process variables (PV) by manipulating as many as three different controller outputs (CO).
For example, when controlling both temperature and pressure in a vessel, the
temperature and pressure will interact with each other. As temperature increases, the pressure will also increase. If two separate PID controllers
are used, the temperature control loop will cause disturbances in the pressure loop.
The MMC controller creates a model of how temperature affects pressure, and as the temperature
increases, it will adjust the pressure actuator, thus eliminating the disturbance.
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How:
The Modular Multivariable Controller takes into account interactions
between multiple process variables and controller outputs.
The MMC decouples the interacting responses. Still, the controller is modular and allows for a controller output to be put into manual control without disturbing other process variables.
The PV1 and PV2 active lists determine in what order the controller
outputs are used to compensate for each PV deviation from the setpoint.
If a controller output saturates or the actuator is put in manual mode, then the second most active CO is used.
If there are more controller outputs than process variables, a
controller output can be selected to follow a target value. This allows for optimizing the process performance by operating it under the most
cost-effective operating conditions.
Control objectives are ranked in the order of importance.
To maintain PV1 at the setpoint is the most important goal. Next, PV2 will be maintained at the setpoint. Lastly, a selected CO will be driven to its target value.
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Use Examples:
Distillation
Process Variable 1 Top composition
Process Variable 2 Bottom composition
Controller Output 1 Reflux ratio
Controller Output 2 Steam flow
Controller Output 3 Overhead pressure
Chemical Reactor
Process Variable 1 Temperature (Zone 1)
Process Variable 2 Temperature (Zone 2)
Controller Output 1 Steam flow setpoint
Controller Output 2 Fuel pressure setpoint
Crude oil catalyst cracking unit optimization
Controller Output 1 Feed
Controller Output 2 Feed composition
Constraint Variable 1 Wet gas compressor RPM
Constraint Variable 2 Air blower RPM
Constraint Variable 3 Air vane position
Constraint Variable 4 Lower regenerator temperature
Constraint Variable 5 CO/CO2 ratio
Constraint Variable 6 Steam generation
Constraint Variable 7 Heavy oil %
Features:
- Self-Tuning
- Bumpless transfer
(rate limiting of the controller output)
- Full Alarming
- Anti-windup implementation
- Online changing of the Target List
- Online changing of the Target Values
- Control of both integrating and
nonintegrating processes
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Key Parameters:
Process Control Parameters
Setpoint (SP)
Process Variables PV1 and PV2
Controller Output for CO1, CO2, & CO3
Manual CO Value for CO1, CO2, & CO3
Auto-Manual for CO1, CO2, & CO3
PV1 active list for CO1, CO2, & CO3
PV2 active list for CO1, CO2, & CO3
Target Value for CO1, CO2, & CO3
Target CO
PV/CO Model Parameters
Model Gains
Model Time Constants
Model Deadtimes
Filter Time Constants
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Limits and Alarms
Positive Dev HiHi Alarm
Positive Dev Hi Alarm
Negative Dev Hi Alarm
Negative Dev HiHi Alarm
Minimum CO Value for CO1, CO2, & CO3
Maximum CO Value for CO1, CO2, & CO3
Rate Limit Pos for CO1, CO2, & CO3
Rate Limit Neg for CO1, CO2, & CO3
Other Parameters
CO Direction for CO1, CO2, & CO3
Controller Speed
Noise Level
PV Tuning Limit
Tuning Step Size for CO1, CO2, & CO3
Start Tuning for CO1, CO2, & CO3
Tuning Status for CO1, CO2, & CO3
Warm Initialization
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