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LabVIEW System Identification Toolkit

By Ricardo Santa Olalla

Introduction

In the System Control Design the most important consideration is a well defined modelfor the plant that you want to control. The reason is that the entire design will be based

on this mathematical model. One way to obtain this model is by using a numerical

process known as system identification. This process involves acquiring data from a plantand then numerically analyzing stimulus and response data to estimate the parameters of

the plant.

System identification is a process that includes acquiring, formatting, processing, and

identifying mathematical models based on raw data from a real-world system. You then

validate that the resulting model fits the observed system behavior. If the results are

unsatisfactory, you revise the parameters and iterate through the process. Figure 1 showsa typical system identification flowchart.

Figure 1. System Identification Flowchart

The National Instruments LabVIEW System Identification Toolkit combines data

acquisition tools with system identification algorithms for accurate plant modeling. It

provides two tools, an assistant and a library of VIs, for identifying discrete single-inputsingle-output (SISO) and multiple-input and multiple-output (MIMO) linear systems.

Table of Contents

Model Estimation through the System Identification Assistant

Model Estimation through the LabVIEW VIsConclusion

References

Model Estimation through the System Identification Assistant


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Without prior knowledge of programming in LabVIEW, you can use the SystemIdentification Assistant to develop a model that reflects the behavior of a certain dynamic

system. You access the System Identification Assistant through the NI Express

Workbench. The Express Workbench is a new framework that can host multiple

interactive National Instruments tools and assistants.

To open the System Identification Assistant you go to Tools > System Identification

Toolkit > Launch System Identification Assistant.

Figure 2. Express Workbench

Under this environment you build your project through several steps. In the first step we

need to define how to acquire the signal coming from the plant. To do this, we add a step

and choose whether to acquire the data through DAQ or load it from a saved file. In thisexample we will use the data saved in a LabVIEW Measurement File (.lvm) from one of

the shipping examples.

To add the step you click on Add Step > Signal Input/Output > Load/Save Signals >Load from LVM. Click on the folder icon and navigate to the following folder:

C:\Program Files\National Instruments\ExpressWorkbench\System Identification

Assistant\Examples\dc_motorand choose the dc_motor_test1.lvm file.


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Figure 3. Load/Save Signals

Make sure that both Motor Input and Motor Output have a check next to them. This will

add both signals to the project.

We close that window and on the left side of the screen, you should now see the Load

from LVM step. To see the stimulus and response signals, drag the stimulus signal

(Motor input) over to the graph. To view the response signal select Add Display anddrag the Motor Output to the second graph.


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Figure 4. Model Input and Model Output signals

From this data, we will estimate the model of the motor. To do this we add a second stepby selecting Add Step>System Identification>Model Estimation>Parametric Estimation.

Make sure that the stimulus signal is Motor Input and the response signal is Motor

Output. Choose the autoregressivemoving average with exogenous terms (ARMAX)Model type. For more information on the types of models, check the LabVIEW System

Identification User Manual. On the other tabs you can choose the Model Order whichaffects the error in the system.


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Figure 5. System Identification Step


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Once we close the step setup, the Parametric Estimation will appear on the left side of the

window. To view the calculated values we add another display. Then we Drag Sys ID

model to the third display to see the coefficients that were calculated.

Figure 6. Signals with coefficients

Once we have the coefficients we can either save them to a file or use them to design our

control system. To do this, right click in the SysID model under Parametric Estimation.

Model Estimation through the LabVIEW VIs

The System Identification Toolkit also provides VIs that you can use to preprocess rawdata from a dynamic system and develop a model that reflects the behavior of that

system. These VIs enable you to customize a LabVIEW block diagram to achievespecific goals. You can also use other LabVIEW VIs and functions to enhance the

functionality of the application.

The first task is to provide the signals coming from the plant. For this example we willread the same LabVIEW Measurement file (dc_motor_test1.lvm) that we used before.

To read it we place the Read from Measurement File Express VI from the Express

Palette.


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Figure7. Read From Measurement File Express VI

This VI will open a window to configure the Express VI. Here you need to browse to

find the file to read. C:\Program Files\National Instruments\ExpressWorkbench\System

Identification Assistant\Examples\dc_motor.


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Figure 8. Read from Measurement File Express VI

Once you have the signals configured in the Express VI we will use another Express VI

called SI Model Estimation under the System Identification palette. This Express VI will

allow us to choose the model type, the dimensions, the order, and the type of data.


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Figure 9. SI Model Estimation Express VI

Just as in the previous section, select ARMAX as the type of model.


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Figure 10. Configure Model Estimation

Click Ok to accept the settings. The Express VI will generate the code in the background

for generating the coefficients and obtaining the system model.

Figure 11. Acquire Data and Model Estimation


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Now that we have the coefficients of our plant we can get the transfer function of thesystem using the system model output, so that it can be saved to a file or used by the

LabVIEW Control Design Toolkit.

Wire the system model out to SI Convert to Transfer Function of CDT VI. The output ofthis VI is the transfer function of the model which can be used in the NI LabVIEW

Control Design Toolkit to aid in the design the control system for the model plant.

In this example we will save the model transfer function into a file and we will display it

in the LabVIEW Front Panel. The first thing we need to do is convert the transfer

function into a Continuous-Time model by using the CD Convert Discrete to ContinuousVI. Then we save the model to a file with the CD Write Model to a File.

To display the transfer function equation we can use the CD Draw Transfer FunctionEquation VI.

Figure12. Draw and Save Transfer Function

To check if the model is a close representation of our plant, we need to simulate andcompare it to the original response from the motor. We simulate the model by using the

SI Model Simulation VIand then we just compare both signals.


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Figure 13. Model Validation

Figure 14. Transfer Function and Model Validation

Conclusion


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This document shows how to use the LabVIEW System Identification Toolkit to estimatea model of a plant, using stimulus and response signals. The toolkit provides two ways of

executing necessary algorithms. The first is through the System Identification Assistant

that uses ExpressWorkbench as the interface, and does not require any LabVIEW

programming knowledge. The second is by using the LabVIEW VIs to build customizedcode.

References

LabVIEW System Identification Toolkit User Manual


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