Feedback and Feedforward Systems Notes

Definition: Feedback is an electric signal sent from the output to the controller to calculate the difference between the output and the required value.
Function: Allows the controller to understand the past state of output.
Error Calculation: Controller calculates the error and adjusts the system input to achieve the desired output.
Closed-Loop System: Known as a feedback system, it feeds back output into input to clear errors and maintain output quality.
Automation: The feedback control system automates sensing, calculating, and manipulating tasks, requiring communication between elements.

Good Accuracy: Ensures the output is close to the desired value.
Oscillation Tendency: Slight tendency towards instability due to oscillations.
Reduced Non-linear Effects: Minimizes the non-linearity in system response.
Disturbance Rejection: Can reduce the impact of external disturbances or noise.
Quality Improvement: Enhances product quality.
High Input Variation Response: Quick to respond to changes in input.

Components: Consists of controllers, compensators, feedback structures, and elements:
- Sensors: Measure actual values of controlled variables.
- Controllers: Compare measured values to desired values, calculate errors, and adjust the process.
- Actuating Devices: Adjust inputs based on controller signals.
Error Detection: The difference between measured and desired values is used to correct the process.
Disturbance Handling: Sensors detect disturbances; controllers respond accordingly to maintain desired outputs.

Positive Feedback System:
- Input and output values are added together.
- Increases the output activation until a limiting constraint is reached.
Negative Feedback System:
- Output affects operation to reduce changes; output value is subtracted from input value.
- Advantages: High stability, accuracy, reduced sensitivity to disturbances, and high bandwidth.

Disturbance Rejection: Effectively rejects disturbances impacting the system.
Reduced Sensitivity: Minimizes system sensitivity to uncertainties.
Steady-State Error Reduction: Decreases errors in output states.
Improved Stability: Enhances overall system performance and stability.
High Bandwidth: Supports quick response to changes.

Potential Instability: Incorrect design can lead to instability.
Complexity: Feedback control can complicate system design.
Delayed Response: Only acts after change is detected, potentially leading to lag.
Online Measurement Limitation: Some controlled variables cannot be measured continuously.

Definition: Anticipates disturbances and compensates before they affect the process.
Limitations: Cannot fully compensate due to variations in models and control actions.
Model Requirement: Requires knowledge of plant behavior to act proactively.

Proactive Actions: Acts before disturbances influence the controlled variable.
Quick Response: Adjusts according to process models quickly.

Measurement Challenges: Online measurement of disturbances may not be feasible in many applications.
Accuracy Dependency: Performance heavily relies on the model's accuracy.

Reactive vs. Proactive: Feedback reduces error after they occur; feed-forward anticipates and mitigates errors before they occur.
System Knowledge: Feedback requires minimal process knowledge; feed-forward needs detailed plant model.
Structure: Feedback systems often lack a feed-forward structure, while feed-forward includes feedback elements.
Use Cases: Feedback can handle non-linear or variable systems; feed-forward cannot.
Measurement Focus: Feedback looks at output to determine error; feed-forward adjusts inputs based on process inputs.

Feedback Control: A system focusing on output adjustments to minimize errors.
Feed-Forward Control: A system anticipating disturbances and adjusting proactively.
Positive Feedback: Enhances output; can lead to instability.
Negative Feedback: Reduces output deviation, promoting stability.