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## Re: sys variables for control theory

**From**: |
Andy Adler |

**Subject**: |
Re: sys variables for control theory |

**Date**: |
Thu, 6 Nov 2003 10:51:33 -0500 (EST) |

On Wed, 5 Nov 2003, Doug Stewart wrote:
>* These thoughts are for those that use octive for control theory and design.*
>* I was trying to model a PID controller and use it in a closed loop*
>* negative feed back loop. The PID numerator is a quadratic equation and*
>* the denominator is first order. Thus the numerator is of higher order*
>* than the denominator. Octave's tf2sys.m does not like this but Matlab's*
>* tf.m thinks it is OK. I understand that you can't do a step or bode etc.*
Is this allowed? If so, I'll need to appologize to my students.
I've been teaching that a control system with more poles than zeros
(ie numerator of higher order than denominator) is inherently unstable.
Are there control gurus out there that can clarify?
Thanks
Andy
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