Mathcad need denom and numer funcitons

[PNachtwey]

I am often doing control theory work where I have a transfer function in the s or z domain. There is no easy way to automatically get the numerator or denominator of the transfer function. This must be done manually which is a pain and is sometimes forgotten which create errors. There should be numer and denom functions or is there some other method of extracting the numerator or denominator? I have Mathcad 13.

 

[pstuckey]

Numer and denom are two of the new functions added in MC14.

Peter

 

[PNachtwey]

That is good to know. Its about time these functions are part of Mathcad.

Peter Nachtwey

 

[excognito]

denom and numer have been part of Mathcad since at least v11 and features in one of the Quicksheets.

Try dn(x,y):=denom(y/x^2)-> and nm(x,y):=numer(y/x^2)->

(they are symbolic operators)

Stuart

 

[pstuckey]

Interesting, those do indeed work in MC11, 13 and 14. According to the MC14 release notes, Numer and Denom are "new features" in MC14 (See #2 under symbolics enhancements).

http://www.ptc.com/support/mathcad_relnotes.htm

Peter

 

[PNachtwey]

Wow! Mathcad sure has kept that a secret! I can't find numer or denom in the help at all but I tried it and it works! I can now generator coefficients for by difference equations automatically from the z domain transfer function. I use numer and denom and then coeffs,z.

I will try out the numer and denom functions with polyroots tomorrow to see if I can easily generate poles and zeros. Then my pole/zero plots will update automatically too.

I wonder what other secrets there are.

My next complaint is that often Mathcad produces transfer functions where the sign is wrong for both the numerator and the denominator. Example -x/-y. Is there and easy way to make this x/y?

After that I often see functions the look like this (-b+a)/3 instead of (a-b)/3. I know this is small stuff but I would like to use my Mathcad examples in a book someday and it would be nice if I didn't have to manually edit so much.

It never hurts to ask.

Peter Nachtwey

 

[IRstuff]

versions prior to M14 used Maple and contained many undocumented Maple features. Don't know if there's any compendium of those anywhere.

TTFN

FAQ731-376