define a Linear Time-Delay system in Mathematica and plot it's Bode and Nyquist diagrams

I have a characteristic equation of a time-delay system and i can't define it with StateSpaceModel or TransferFunctionModel command in Mathematica; Because these commands only work for linear systems without delay.

My purpose is to define a Time-Delay system in Mathematica 8.0.1 and plotting Bode Diagram and Nyquist Diagram for a system like this:

开发者_开发技巧g = ((s + 1) (1-E^(-2 s) + E^(-3 s)))/(s^2 + 2 s + 10)

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Judging from the documentation, it seems MMA expects transfer functions to be polynomial. The doc page for TransferFunctionModel says:

In TransferFunctionModel[{num,den},var] num must be a polynomial matrix and den can be specified as a polynomial matrix or just the common denominator polynomial.

All the examples I have seen have polynomials in the nominator and denominator.

Matlab's bodeplot also doesn't do pure time delays.

One sometimes hears the suggestion to use a Padé approximation of the exponential. This could be done with the mma function PadeApproximant.

However, this seems to be valid only for rather low frequencies (the first 180 degrees of phase change or so) as stated here.