Re: sys variables for control theory

[Andy Adler]

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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