If a general-linear polynomial, transfer function, zero-pole-gain, or state-space model cannot represent the model you want to estimate, you can define a model by revising a template VI.

You can find template VIs in the \vi.lib\addons\System Identification\User-Defined Model Templates.llb. You then can estimate the model you define by using the SI Estimate User-Defined Model VI. This VI enables you to estimate some other model representations in addition to the general-linear, transfer function, zero-pole-gain, and state-space models that the System Identification VIs directly support. For example, you can use this VI to estimate nonlinear models. With this VI, you also can estimate linear models that you define early.