Feedforward MFA Controller

Feedforward is a control scheme to take advantage of disturbance signals. If a process has a significant measurable disturbance, a feedforward controller can be used to reduce its effect before the feedback loop takes corrective action. A good feedforward controller can economically improve the control system performance.

Feedforward compensation can be as simple as a ratio between two signals, or involve complicated energy or material balance calculations. The Feedforward MFA is a general-purpose feedforward controller. It does not attempt a perfect cancellation of the disturbances, which is difficult to implement in industrial applications due to changing process dynamics and operating conditions. A feedback/feedforward MFA control system diagram is illustrated in the following graph, where Gp1 is the main process and Gp2 is the process with the disturbance as input and the process variable as output.

Feedback and feedforward MFA control system

Feedforward MFA Controller Configuration

Since the feedback MFA adapts, the feedforward MFA can be designed in a simple form with parameters of a feedforward gain and time constant. The feedforward gain can be estimated by using the following formula: Kfc=-Kp2/Kp1,

where Kp1 and Kp2 are the estimated static gain for processes Gp1 and Gp2, respectively. In order to assure that the feedforward action rejects the disturbance, the rules for selecting the sign are (1) if processes Gp1 and Gp2 have the same sign, the FF's gain should be negative; and if processes Gp1 and Gp2 have different signs, the FF's gain should be positive. The feedforward time constant can be an estimate of the time constant of Gp2. This is related to how fast the disturbance will affect the process PV.