Feedforward control, feedback control, and feedforward-feedback control comparison

. Feedforward control belongs to open-loop control, while feedback control belongs to closed-loop control with negative feedback.

Typically, a set-point control system adjusts based on the deviation obtained by comparing the measured value with the given value, which falls under closed-loop negative feedback regulation. Its characteristic is that regulation only occurs after a deviation in the controlled variable appears; if a disturbance has occurred but no deviation is produced, the controller will not take action. Therefore, the regulatory action of feedback control lags behind the disturbance action.

Feedforward regulation operates based on the disturbance action. Feedforward control measures the disturbance and directly introduces it into the control device, allowing it to counteract disturbances more promptly than feedback control.

Now, let’s take a heat exchanger control scheme as an example to intuitively illustrate feedforward control and feedback control:

1. In feedforward control systems, the disturbance variable is measured, whereas in feedback control systems, the controlled variable is measured.

In a pure feedforward control system, the controlled variable is not measured, and in a pure feedback control system, the disturbance variable is not measured.

1. Feedforward control requires a specialized controller, while feedback control typically uses a universal PID controller.

Feedback control follows the PID regulation principle and is commonly implemented using universal PID controllers, DCS, PLC control systems, etc. The controller used in feedforward control is a feedforward controller, whose regulation characteristics are determined based on the specific features of the controlled process.

1. Feedforward control can only counteract measured disturbances, whereas feedback control can address all disturbances.

In a feedforward control system, if a disturbance cannot be measured, it cannot be compensated. In contrast, in a feedback control system, any disturbance that affects the controlled variable can be counteracted to some extent.

1. Feedforward control can theoretically achieve zero error, while feedback control inevitably results in some error.

Feedback control brings the system to dynamic stability, keeping the controlled parameter fluctuating dynamically near the setpoint but not precisely fixed at the setpoint.

Feedforward control, in theory, can achieve error-free regulation.

1. Limitations of Feedforward Control:
   A. In practical applications, process characteristics often vary with load, and the dynamic behavior of objects is diverse and difficult to measure accurately, which can easily lead to overcompensation or undercompensation. To compensate for the inaccuracies in feedforward control, feedforward and feedback control systems are often combined to form a feedforward-feedback control system.
   B. Industrial processes are subject to multiple disturbances. Equipping all of them with feedforward controllers would require high equipment investment and extensive effort.
   C. Many feedforward compensation outcomes cannot be verified with current detection technologies.
   D. The effectiveness of feedforward control is limited by the accuracy of the feedforward control model.
   E. Feedforward control algorithms often involve approximations.

Principles for Selecting Feedforward Control:

1. When a system is subject to frequent, large-magnitude disturbances that are measurable but uncontrollable, feedforward control may be selected.
2. When the control process has a long time lag and feedback control alone cannot achieve satisfactory results, feedforward control may be selected.
3. The selection of feedforward control must adhere to economic feasibility.
4. After deciding on a feedforward control strategy, if static feedforward meets the process requirements, dynamic feedforward is not necessary.

Advantages of Feedforward-Feedback Control Systems:

1. From a feedforward control perspective, the addition of feedback control reduces the requirement for high precision in the feedforward control model and allows for compensation of unmeasured disturbances.
2. From a feedback control perspective, the feedforward control action provides timely coarse adjustments for major disturbances, significantly reducing the burden on feedback control.

Application Example of Feedforward-Feedback Control:
Now, let’s take two heat exchanger control schemes as examples to intuitively illustrate feedforward-feedback control:

1. Heat Exchanger Feedforward-Feedback Control Scheme

2、Feedforward-Feedback Control Scheme 2 for Heat Exchanger

Explanation: This control scheme can overcome the nonlinearity of the system or function as a variable-gain controller.