Need help for a post-processing "trigger"

[dmontalvao]

Dear all,

Without getting into much detail, I have a system that is simultaneously introducing a sinusoidal Force (F) and measuring the sinusoidal response (X) of a structure under resonance. I need to acquire the whole time signal, including the free decay response in the end (please find attached a sample picture of the time signal and the corresponding data file), so I cannot use a trigger during the measurement. During the stationary response it may be seen that the signal has a slight negative slope: that is due to a phenomenon of dilatation of my structure that is dissipating heat at the same time. 

My problem is how to know exactly when my NI-USB 6216 DAQ has stopped the excitation force, which, in my sample waveform would be around 0.025s (I require an automatic process of identifying it).

Is there any VI, filter or trigger function that might provide me that information?

Thank you in advance for your help.

Kind regards,

Diogo Montalvão

[Joel_G]

Hi Diogo,

Perhaps it would help if you attached your VI.

Are you using an analog output for the excitation force?

If you control when your 6216 stops the excitation force, can you find out the exact timing by using the timing functions within LabVIEW?

Kind regards,

[Henrik_Volkers]

The brute force method:

Can't you use another analog input channel and capture the excitation voltage?

Or if you have a shaker with an amplifier that provide a current monitor output you can capture the excitation current and might find that due to the voltage control some amount of force is still introduced after setting the control voltage to zero 😉  and you might have to implement a system model that include the shaker and the output impedance of your amplifier :0

[johnsold]

Diogo,

The method Henrik suggests is my first thought also.

If you cannot do that for some reason, I would consider finding the envelope of the response signal and look for the change in slope. Prior to .025 s the slope is nearly horizontal.  Immediately after that point it is sharply downward.  The place where it changes is close to your "trigger" point.

Post a VI with that data saved as default or a text file with the data so we can work with numbers.

Lynn