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Assume you have a system model that is controlled by a PID controller (feedback). You want to add feedforward to improve the characteristics of the controlled system. How can you design the transfer function of the feedforward? (Cf. this image)

On slide 18 of this presentation, it is suggested to use a gain and a lead-lag filter:

K*(T1*s+1)/(T2*s+1).

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    $\begingroup$ It depends on the approach for the controller. Can you specify the system and what do you want to control? $\endgroup$ – leCrazyEngineer Jul 17 '16 at 1:31
  • $\begingroup$ @leCrazyEngineer It was a general question. $\endgroup$ – Karlo Jul 18 '16 at 11:13
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The idea of using a feedforward component in a control system is to provide near instant action of the plant output to the input command. So a suitable choice of transfer function which connects the command to the plant input directly is an inverse model of the plant. The feedback control can then serve to 'clean up' any error in the feedforward component perhaps due to modeling error or drift in the parameters.

Usually the feedforward DC gain used is less than or equal to 1.0.

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