Feedback and feedforward control systems are two common types of control systems used in industrial automation and process control. They differ in their approach to handling disturbances and achieving desired control objectives.

1. Feedback Control System:

• Principle: In a feedback control system, the output of the system is continuously monitored and compared to a reference or setpoint value. Any deviation between the actual output and the desired setpoint is used to adjust the system inputs or control actions to minimize the error.

• Operation: The feedback loop typically consists of four main components: a sensor or measurement device to measure the system output, a controller to compare the measured output to the setpoint and generate a control signal, an actuator to adjust the system inputs based on the control signal, and the process or plant itself.

• Advantages:

  • Provides closed-loop control, which continuously adjusts the system to maintain desired performance in the presence of disturbances, uncertainties, or changes.
  • Offers robustness and stability, as it reacts to changes in the system and corrects errors in real-time.
  • Can handle nonlinearities and dynamic behavior in the system.

• Applications: Feedback control systems are widely used in various industrial processes and applications, such as temperature control, pressure regulation, speed control in motors, and level control in tanks.

2. Feedforward Control System:

• Principle: In a feedforward control system, the control action is based on a prediction or estimation of the disturbances or external influences affecting the system. Instead of reacting to the output error, the system anticipates the disturbances and adjusts the control inputs preemptively to minimize their effects on the output.

• Operation: The feedforward loop typically consists of a disturbance model or predictor that estimates the effect of disturbances on the system output, a controller that generates a compensating control signal based on the predicted disturbances, and the process or plant itself.

• Advantages:

  • Provides proactive control by anticipating and mitigating disturbances before they affect the system output, leading to improved performance and stability.
  • Can reduce the reliance on feedback control and improve system efficiency by preemptively compensating for known disturbances.
  • Particularly effective in applications where disturbances are predictable or have a significant impact on system performance.

• Applications: Feedforward control systems are commonly used in applications where disturbances are known or can be measured in advance, such as in chemical processes, flow control systems, and motion control applications.

Comparison:

• Feedback Control: Reacts to deviations between actual output and setpoint.

• Feedforward Control: Anticipates and compensates for known disturbances before they affect the output.

• Feedback Control: Relies on measured feedback to adjust the system.

• Feedforward Control: Relies on predictions or estimates of disturbances to adjust the system.

• Feedback Control: Provides closed-loop control and is more robust to uncertainties.

• Feedforward Control: Provides proactive control but may require accurate disturbance modeling.

In practice, a combination of feedback and feedforward control strategies, known as cascade control, is often used to achieve optimal system performance by leveraging the strengths of both approaches.