Wednesday, April 27, 2011

Feed Forward Control

 We all are familiar with the feedback control used in a closed loop controller. This one is something new; came across today from the lecture at MESCE by Prof. C. Chandrsekhar - IITM. Combined feedforward plus feedback control can significantly improve performance over simple feedback control whenever there is a major disturbance that can be measured before it affects the process output. In the most ideal situation, feedforward control can entirely eliminate the effect of the measured disturbance on the process output. Even when there are modeling errors, feedforward control can often reduce the effect of the measured disturbance on the output better than that achievable by feedback control alone. However, the decision as to whether or not to use feedforward control depends on whether the degree of improvement in the response to the measured disturbance justifies the added costs of implementation and maintenance. The economic benefits of feedforward control can come from lower operating costs and/or increased salability of the product due to its more consistent quality. Feedforward control is always used along with feedback control because a feedback control system is required to track setpoint changes and to suppress unmeasured disturbances that are always present in any real process. The continuously stirred tank reactor is under feedback temperature control. Feedforward control is used to rapidly suppress feed flow rate disturbances.
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a) open-loop b) feed-forward c)closed loop.

This is a control technique that can be measured but not controlled. The disturbance is measured and fed forward to an earlier part of the control loop so that corrective action can be initiated in advance of the disturbance having an adverse effect on the system response.
Thus Feedback control is typically used to regulate a variable (or variables) in a control systems design which has time varying disturbances, and or operating parameters. It is also used when the accuracy afforded by feedforward controls is not adequate to meet the application performance specifications.

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