Difference between revisions of "Lead-Lag Compensators"

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Generally the purpose of the Lead-Lag compensator is to create a controller which has has an overall magnitude of approximately 1. The lead-lag compensator is largely used for phase compensation rather than magnitude. A pole is an integrator above the frequency of the pole. A zero is a derivative above the frequency of the zero.

Adding a pole to the system changes the phase by -90 deg and adding a zero changes the phase by +90 deg. So if the system needs +90 deg added to the phase in a particular frequency band then you can add a zero at a low frequency and follow that zero with a pole at a higher frequency.

Lead and lag control are used to add or reduce phase between 2 frequencies. Typically these frequencies are centered around the open loop crossover frequency. A lead filter typically has unity gain (0 dB) are low frequencies while the lag provides a nonunity gain at low frequencies.[1]

 $LaTeX: C\left(s\right)=\frac{\tau_{1}s+1}{\tau_{2}s+1}$ Lead[2]

where

$LaTeX: \tau_{1}>\tau_{2}$ makes this a lead filter and
$LaTeX: \tau_{1}<\tau_{2}$ makes this a lag filter.

This is an extension of the Lead and Lag network described above. We can always stick a gain in front of something so reformulate $LaTeX: C\left(s\right)$ above to look like this

 $LaTeX: C\left(s\right)=k\frac{\tau_{a}s+1}{\tau_{b}s+1}$

Then a lead-lag controller would have the form

 $LaTeX: C\left(s\right)=k\left(\frac{\tau_{a}s+1}{\tau_{b}s+1}\right)\left(\frac{\tau_{c}s+1}{\tau_{d}s+1}\right)$ [3]

where

$LaTeX: k$ is a gain,
$LaTeX: \tau_{a}>\tau_{b}$ makes the first part a lead filter and
$LaTeX: \tau_{c}<\tau_{d}$ makes the last part a lag filter.

3 References

• Goodwin, G. C., Graebe, S. F., and Salgado, M. E. 2000 Control System Design. 1st. Prentice Hall PTR. ISBN 0139586539
• Franklin, G. F., Emami-Naeini, A., and Powell, J. D. 1993 Feedback Control of Dynamic Systems. 3rd. Addison-Wesley Longman Publishing Co., Inc. ISBN 0201527472

3.1 Notes

1. Franklin et all, pg. 412
2. Goodwin et all, pg. 170, Eqn. 6.6.1
3. Goodwin et all, pg. 170, Eqn. 6.6.1