Modern industrial automation has evolved significantly from basic single-loop control to advanced, multi-loop control systems. In practice, processes often require more sophisticated control methods due to dynamic interactions, non-linear behaviors, and varying disturbances. This article explores six widely used types of complex control loops, highlighting their principles, structures, benefits, and typical applications.
1. Cascade Control System
Basic Concept
Cascade control is one of the earliest and most widely applied forms of complex control. In this configuration, two controllers are arranged in series: the output of the primary (master) controller becomes the setpoint for the secondary (slave) controller. This strategy is particularly effective for processes with significant time delays or frequent disturbances.
System Structure
Primary Controller (Master): Regulates the main process variable based on the setpoint.
Secondary Controller (Slave): Receives the master output as its setpoint and controls a secondary variable that directly influences the primary variable.
Primary Loop (Outer Loop): From master controller to final process output.
Secondary Loop (Inner Loop): A faster loop that acts to stabilize rapid disturbances.
Advantages
Fast response to disturbances entering the secondary loop.
Reduces the impact of process dead time.
Compensates for actuator non-linearity.
Provides better overall control quality and robustness.
Tuning and Implementation
Common tuning methods include:
Step-by-step (sequential) tuning: Tune the secondary loop first, then the primary.
Two-step method: Fix secondary controller first with primary in manual.
Single-step method: Pre-configure secondary parameters based on process type.
2. Ratio Control System
Basic Concept
Ratio control maintains a defined ratio between two or more process streams. It is essential in mixing or blending operations where maintaining proportional flow is critical, such as in chemical reactions.
Controlled Variable: Flow rate of secondary (dependent) material.
Reference Variable: Flow rate of primary (independent) material.
System Structures
Single-loop ratio control.
Double-loop ratio control.
Cascade ratio control.
Implementation Methods
Multiplicative Scheme: Multiply the primary signal with a ratio constant to set the secondary loop.
Divisive Scheme: Divide the secondary signal by the primary to compute and control the ratio directly.
Advantages
Ensures consistent material proportions.
Minimizes waste and improves product quality.
Multiplicative methods are easier to stabilize; divisive methods offer direct ratio control.
3. Selective Control System (Override Control)
Basic Concept
Selective control is used to prevent process variables from exceeding safety or operating limits. The system uses signal selectors (high/low) to dynamically choose the most critical input.
Structure
High Selector: Outputs the highest signal among inputs.
Low Selector: Outputs the lowest signal.
Applied between multiple sensors/controllers and the actuator.
Applications
Steam pressure control.
Fuel switching in furnaces.
Safety override in pressure or temperature control.
Anti-Integral Windup Techniques
Due to one controller being inactive (open-loop), integral windup may occur:
Output Clamping.
Integral Cutout.
External Feedback.
4. Feedforward Control System
Basic Concept
Unlike feedback control, which reacts to process changes, feedforward control acts before the disturbance affects the process. By measuring disturbances directly, feedforward compensates for them in advance.
Common Structures
Pure Feedforward.
Feedforward + Feedback (Additive or Multiplicative).
Applications
Heat exchangers.
Distillation columns.
Boilers.
Design Approach
Static Feedforward: Based on steady-state process models.
Dynamic Feedforward: Includes dynamic response to disturbances.
5. Split-Range Control System
Basic Concept
Split-range control involves one controller managing multiple actuators (typically valves) across different operating ranges. It is used when a single actuator cannot meet the entire control demand.
System Characteristics
Two or more control valves operate over different ranges.
Can be configured for same-direction or opposite-direction actuation.
Key Considerations
Valve flow characteristics must be matched.
Avoid dead zones and overlaps.
Ensure leak-free valve operation.
Controller tuning must account for nonlinear transitions.
6. Three-Element Control System
Basic Concept
Used primarily in boiler drum level control, this method involves three measured variables: drum level, steam flow, and feedwater flow. This multi-input system stabilizes drum level despite load changes.
Structure
Primary Element: Drum water level.
Secondary Elements: Steam flow and feedwater flow.
Advantages
Compensates for both load disturbances and supply variations.
Maintains safe and stable boiler operation.
Summary Table: Comparison of Six Control Strategies
| Control Type | Main Feature | Typical Use Case | Key Advantage |
|---|---|---|---|
| Cascade | Two nested loops | Temperature control, flow | Fast disturbance rejection |
| Ratio | Maintains proportion between streams | Mixing, combustion | Accurate blending |
| Selective (Override) | Uses high/low selector logic | Pressure/temp limit control | Safety and soft constraint |
| Feedforward | Acts before disturbance affects output | Boilers, heat exchangers | Proactive compensation |
| Split-range | One controller, multiple actuators | Multi-stage flow/pressure | Extended control range |
| Three-element | Uses 3 measurements for better control | Boiler drum level | Precise level under load change |
These six complex control strategies, when properly understood and implemented, can significantly enhance process stability, product quality, and operational safety. As industrial systems become increasingly automated and data-driven, mastering these control techniques is essential for engineers and technicians alike.