Difference between revisions of "FAQ: Why is it better to use a lead controller than a PD controller?"

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{{FAQ page
 
{{FAQ page
| chapter = Introduction
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| chapter = Frequency Domain Design
| contributor = Richard Murray
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| contributor = Elisa Franco
| date = 17 May 2008
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| date = 3 Dec 07
 
| answer =  
 
| answer =  
This is a template page for creating a FAQ. You should replace the information above with the appropriate information for your FAQ and replace this text with the answer to your FAQIf your FAQ page is complicated, you may want to substitute this page (use <nowiki>{{subst:FAQ page ...}}</nowiki> and then customize it.
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The lead controller helps us in two ways: it can increase the gain of the open loop transfer function, and also the phase margin in a certain frequency range. But why could we not use just a PD controller, which increases the phase over a wider range of frequencies?
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The use of the derivative action should always be limited: it helps improve the control performance when the error signal changes rapidly, but for the same reason it could amplify unwanted disturbances on the measured outputHence, a lead compensator rolls off the gain at high frequency. The same transfer function can be realized by coupling a PD controller with a low pass filter (whose break frequency is chosen to be above the frequency of the zero from the PD controller).
 
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Latest revision as of 16:55, 15 June 2008

(Contributed by Elisa Franco, 3 Dec 07)

The lead controller helps us in two ways: it can increase the gain of the open loop transfer function, and also the phase margin in a certain frequency range. But why could we not use just a PD controller, which increases the phase over a wider range of frequencies?

The use of the derivative action should always be limited: it helps improve the control performance when the error signal changes rapidly, but for the same reason it could amplify unwanted disturbances on the measured output. Hence, a lead compensator rolls off the gain at high frequency. The same transfer function can be realized by coupling a PD controller with a low pass filter (whose break frequency is chosen to be above the frequency of the zero from the PD controller).