padecoef

Padé approximation of time delays

Syntax

[num,den] = padecoef(T,N)

Description

[num,den] = padecoef(T,N) returns the Nth-order Padé Approximation of the continuous-time delay exp(-T*s) in transfer function form. The row vectors num and den contain the numerator and denominator coefficients in descending powers of s. Both are Nth-order polynomials.

Examples

Second Order Pade Approximation

Use padecoef to estimate the value of $$e^{-2s}$$ to second order.

[a,b] = padecoef(2,2)

a = 1×3

     1    -3     3

b = 1×3

     1     3     3

The result indicates that the second order approximation is

$$f(s)\approx \frac{a}{b}=\frac{s^2-3s+3}{s^2+3s+3}.$$

Compare the approximation to the actual value at $$s=0.25$$.

f_approx = @(s) (s^2 - 3*s+3)/(s^2 + 3*s + 3);
f_actual = @(s) exp(-2*s);
abs(f_approx(0.25) - f_actual(0.25))

ans = 
2.6717e-05

Input Arguments

T — Time delay

scalar

Time delay, specified as a real numeric scalar.

Data Types: single | double

N — Order of approximation

scalar

Order of approximation, specified as a real numeric scalar.

Data Types: single | double

More About

Padé Approximation

The Laplace transform of a time delay of T seconds is exp(-Ts). The padecoef function approximates this exponential transfer function by a rational transfer function using Padé approximation formulas. [1]

References

[1] Golub, G. H. and C. F. Van Loan. Matrix Computations. 4th ed. Johns Hopkins University Press, Baltimore: 2013, pp. 530–532.

Version History

Introduced in R2008a