changing signal length

[leongchuan]

i get a error message Error -41201 for my program
"System Identification Toolset: The signal length is too small for computation, or the stimulus and response signals are not the same length."

the stimulus signal is generated from the program and the response signal from a daq card
is there a vi that allows me to change the length of a signal?

[Kristi_P]

It sounds like you may not be using a VI in the System Identification Toolkit properly. Which VI is giving you the error? Right-click on the VI that is in error and view the Help. Make sure that you are wiring inputs in the same way the VI is expecting them. You can also probe the inputs of the VI and/or use the Array Size VI to determine how long the arrays are that you are passing in.

If you continue to see problems, please provide some more details on what is drawing the error and what you are trying to do.

Regards,

Kristi H

[leongchuan]

thanks for the reply

I am using the SI Model Estimation VI from the system identification toolkit
i tested my signals from your tips and found out where the problem is
my stimulus signal is an uniform white noise at a sample size of 128 which i believe is not a problem
my response signal is coming out of a transfer function vi and the sample size of the signal is 1

is there any way to increase the sample size of my response signal to 128?
i have tried change the stimulus signal to 1 to match my response signal but i still get the above error

i extracted out some part of my program to illustrate my point

[Kristi_P]

It sounds like you may not be using the SI Estimate ARX Model.vi correctly. If you look at the help for this VI, it describes the inputs it is expecting. If it's the case that the single value you receive for your response is the same throughout the entire 128 sequence of uniform white noise stimuli, then you could create an array of 128 elements using the Initialize Array.VI. However, you should consider whether or not a single point response actually represents the complete response to the input signal. In other words, does a single value held over X amount of time represent the response to your input over X amount of time (128 elements), or do you need to sample your response longer?

I hope this helps. Be sure to take a look at some of the shipping examples for the System Identification Toolkit as well in Help->Find Examples.

Regards,

Kristi H

[leongchuan]

Thanks for the reply

let say i am using a simple motor as my plant, with the input as voltage and the output as the speed.
i use white noise as the stimulus and i got my motor speed from the encoder counter after passing thru a transfer function
i suppose a single value input held over a time interval will give me the same motor speed.Does that mean i can create an array of 128 elements and use it as my response signal?

secondly how do i sample my response longer(any vis?)?im actually not sure of how to do it. my response signal comes from the pid advance transfer function vi.

[Kristi_P]

If white noise (128 elements) is your stimulus signal, then your system will respond with 128 output values corresponding to the inputs applied with the stimulus signal. These are the signals that should be sent to the System Identification VI. I would recommend looking through some of the shipping examples that show how the ARX model is used in other applications.

I hope this helps.

Regards,

Kristi H