Feed forward (control)
Feed forward control, also known as anticipatory control, is a control strategy that aims to predict and compensate for disturbances before they affect the system's output. Unlike feedback control, which reacts to deviations from the desired setpoint, feed forward control proactively adjusts the control signal based on measurements of known disturbances.
Mechanism
The basic principle of feed forward control involves:
- Measuring Disturbances: Identifying and measuring the key disturbances that influence the system's output.
- Modeling System Dynamics: Developing a model that describes how these disturbances affect the system's output. This model is often a transfer function or a similar mathematical representation.
- Calculating the Compensatory Control Action: Using the model and the measured disturbances to calculate the control action required to counteract the effect of the disturbances. This calculation aims to maintain the system's output at the desired setpoint.
- Applying the Control Action: Adding the calculated feed forward control signal to the existing control signal (which may include feedback control).
Advantages
- Improved Disturbance Rejection: Feed forward control can significantly reduce the impact of disturbances on the system's output, leading to more stable and accurate control.
- Faster Response: By anticipating disturbances, feed forward control can react more quickly than feedback control, which must first detect an error before taking action.
- Reduced Overshoot and Oscillations: The proactive nature of feed forward control can minimize overshoot and oscillations in the system's response.
Disadvantages
- Model Dependence: The effectiveness of feed forward control heavily relies on the accuracy of the system model. Inaccurate models can lead to poor performance or even instability.
- Disturbance Measurement Requirement: Feed forward control requires accurate and reliable measurements of the disturbances. The inability to measure all relevant disturbances can limit its effectiveness.
- Complexity: Designing and implementing feed forward control systems can be more complex than implementing feedback control systems, especially when dealing with multiple disturbances or nonlinear systems.
- Lack of Error Correction for Unmodeled Effects: Feed forward doesn't correct for unmodeled disturbances or inaccuracies in the system model itself. It is often used in conjunction with feedback control to address these shortcomings.
Applications
Feed forward control is used in a wide range of applications, including:
- Process Control: Controlling temperature, pressure, and flow in chemical plants and refineries.
- Robotics: Compensating for disturbances caused by gravity, friction, and external forces.
- Automotive Engineering: Controlling engine speed and fuel injection to optimize performance and reduce emissions.
- HVAC Systems: Maintaining constant temperature and humidity in buildings.
- Power Electronics: Compensating for voltage drops and current fluctuations in power supplies.