Choosing Identified Plant Structure
PID Tuner provides two types of model structures for representing the plant dynamics:
process models and state-space models.Use your knowledge of system characteristics and the level of accuracy required by your
application to pick a model structure. In absence of any prior information, you can gain
some insight into the order of dynamics and delays by analyzing the experimentally
obtained step response and frequency response of the system. For more information see
the following in the System Identification Toolbox documentation:- “Correlation Models” (System Identification Toolbox)
- “Frequency-Response Models” (System Identification Toolbox)
Each model structure you choose has associated dynamic elements, or model parameters.
You adjust the values of these parameters manually or automatically to find an identified
model that yields a satisfactory match to your measured or simulated response data. In
many cases, when you are unsure of the best structure to use, it helps to start with the
simplest model structure, transfer function with one pole. You can progressively try
identification with higher-order structures until a satisfactory match between the plant
response and measured output is achieved. The state-space model structure allows an
automatic search for optimal model order based on an analysis of the input-output data.When you begin the plant identification task, a transfer function model structure with one
real pole is selected by default. This default set up is not sensitive to the nature of the
data and may not be a good fit for your application. It is therefore recommended that you
choose a suitable model structure before performing parameter identification.Process Models
Process models are transfer functions with 3 or fewer poles, and can be augmented by
addition of zero, delay and integrator elements. Process models are parameterized by
model parameters representing time constants, gain, and time delay. In PID Tuner,
choose a process model in the Plant Identification tab using the Structure menu.7 PID Controller Tuning