Comparison of Control Loops

Control loops are the building blocks of control systems and come in various configurations, depending on the number of variables being controlled, the type of control, and other factors.

Below is a comparison table that outlines the key features of commonly used types of control loops such as open, closed, cascade, feedforward, ratio, and split range loops.

Parameter	Open Loop	Closed Loop	Cascade Loop	Feedforward Loop	Ratio Control Loop	Split Range Loop
Principle of Operation	No feedback is used. Output is neither measured nor adjusted to achieve a desired result.	Uses feedback to adjust the control action based on the output.	Two control loops where the output of one serves as the set point for another.	Anticipates disturbances and proactively adjusts the system.	Maintains the ratio between two variables.	Uses multiple control devices to handle different ranges of a process variable.
Theory	A predetermined input directly controls the action, irrespective of the output.	Measures output, compares to a set point, and adjusts accordingly.	Main loop controls a process variable, secondary loop controls another variable.	Identifies disturbances and proactively makes adjustments before they affect output.	Measures two variables, adjusts one to maintain a set ratio with the other.	The controller splits its output to different control elements based on setpoints.
Advantages	Simple design. Generally less expensive and less complex.	Higher accuracy due to feedback. Adaptable to system changes.	Better disturbance rejection. Enhanced control accuracy.	Preemptively deals with disturbances. Enhances output quality.	Ensures consistent ratios. Useful for processes like blending or mixing.	Efficiently manages processes with a wide operation range.
Disadvantages	No correction for disturbances. Potential inaccuracy.	More complex design. Potential for instability.	More complex setup. Needs tuning of two controllers.	Requires knowledge and model of the disturbance. Complexity increases.	Requires accurate measurement. If one variable falters, the ratio is affected.	Potential for overlap or dead zones. Needs careful tuning.
Response to Disturbances	Poor	Good	Good	Good	Good	Variable
Accuracy	Low	High	High	Moderate to High	High	Moderate to High
Stability	Variable	Generally stable	Generally stable	Can improve stability	Generally stable	Generally stable
Complexity	Simple	Moderate	Complex	Moderate to Complex	Complex	Moderate to Complex
Cost	Low	Moderate to High	High	Moderate to High	High	Moderate to High
Control Action	Pre-determined	Dynamic	Multiple, Dynamic	Dynamic	Dynamic	Dynamic
Tuning Required	No	Yes	Yes	Sometimes	Yes	Yes
Application Examples	Timers, Basic motors	HVAC, Industrial processes	Complex systems with interacting variables	Systems with measurable disturbances	Mixing, Blending processes	Heating and cooling systems
Feedback Required	No	Yes	Yes	Yes	Optional	Optional

Brief explanation:

Principle of Operation: The fundamental principle that the control loop operates on.
Response to Disturbances: How well the loop can handle changes in the system or environment.
Accuracy: How precisely the loop can control the process variable.
Stability: The loop’s ability to maintain control without oscillating.
Complexity: The difficulty level in understanding, implementing, and maintaining the loop.
Cost: The relative financial expense of implementing and maintaining the loop.
Control Action: Describes the method of control action undertaken by the loop.
Tuning Required: Whether the control parameters need to be tuned for optimal performance.
Application Examples: Examples of real-world applications where each type of control loop is used.
Feedback Required: Whether the loop needs feedback from the process variable.