system identification & Its transfer function

[RF328]

Hi there

I'm trying to get a transfer function of an AC servo moter system in torque mode with input to the driver of 0-10V  & output of the motor in pulses.

The stimulas is 0-1V MLS signal & the output of the motor is position in pulses.

With the attached input & output data (MLS+data.xls) inputing into the ARMAX model, I get the required transfer function.

However, when I use the same input data using the SI simulation Model to simulate the system output, I find out that I need to scale up the simulated output by a factor of 340 in order to match the actual logged system ouput.

My questions are :

1. Can someone let me why this happen ?

2. Now, we know that by the the Transfer function created by ARMAX model, in order to get the actual output, we need to scale up the Transfer function by a factor of 340. Can anyone tell me how to add this factor (in this case 340) into the Transfer function created by ARMAX model so that I can achieve the required output ?

see attachment for the LV vi & the data.

Thank you in advance.

[Jasper S]

Hello,

Are you certain the the 340 difference is in actually not an offset?  From your code, I can see that you filter out the mean of the signal, is it possible that this is 340?

The System Identification toolkit is not designed necessarily to modify the system parameters that it detects.  However, you could definately unbundle the cluster containing your coefficients and modify them by a given factor.

Do you have the Control Design toolkit?  This toolkit will allow you to add a series step after the System Identification code, which will allow you to add your factor on. 

Regards,

Jasper S

[RF328]

Hello Jasper

Thank you for your reply.

I did not filter out the mean of the signal. The vi is there as option only.

I do have control design toolkit. May I know which vi to use in order for me to add factor to the transfer function ?

Best Regards

[Jasper S]

You can use a Control Design VI called CD Series to append a transfer coefficient to your transfer function.  You should be able to set this to the factor that you are seeing, and shift your system back to where it needs to be.

Regards,

Jasper S