M4.4 Converting Discrete Models to Continuous

Discrete-time models are often developed based on the sampled inputs and outputs, using parameter estimation. Control system design, however, is usually performed based on continuous models. It is important, then, to be able to convert discrete models to continuous. Fortunately, the Control Toolbox makes this easy via the d2c command.

sys = d2c(sysd, method)  % discrete to continuous

where sysd is the LTI object for a discrete model, sysc is the continuous object to be created, and method is the type of hold placed on the input variable. We always assume that a zero-order hold ('zoh') is used for the discrete model; this is also the default if no entry is made for method.

Consider the discrete transfer function model for the van de Vusse reactor, vdv_tfd. It can be converted to the continuous-time model using the following MATLAB command:

» vdv_tfc = d2c(vdv_tfd,'zoh')

Transfer function:
  -1.117 s + 3.147
---------------------
s^2 + 4.643 s + 5.382