3.11 Padé Approximation for Dead Time

As discussed in the previous section the transfer function for a pure time delay is e^–qs, where q is the time delay. Some control system design techniques require a rational transfer function; the Padé approximation for dead time is often used in this case.

A first-order Padé approximation is

A second-order Padé approximation is

Example 3.8: Application of the Padé Approximations for Dead Time

Consider the first-order + dead time transfer function, where the time-delay dominates the time constant

The first-order Padé approximation yields the transfer function

and the second-order Padé approximation yields

A comparison of the step responses of g(s), g₁(s), and g₂(s) is shown in Figure 3-16. Notice that the first-order approximation has an inverse response, while the second-order approximation has a "double inverse response." The reader should find that there is a single positive zero for g₁(s), and there are two positive, complex-conjugate zeros of the numerator transfer function of g₂(s).

Most ordinary differential equation numerical integrators require pure differential equations (with no time delays). If you have a system of differential equations that has time delays, the Padé approximation can be used to convert them to delay-free differential equations, which can then be numerically integrated.

One of the many advantages to using SIMULINK is that time delays are easily handled so that no approximation is required.