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Phase sign switch in measurement

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Hello all,

I am performing a dynamic test using a piezo actuator PI 843.60 with a built-in displacement sensor in close loop and a force sensor from PCB 208C02. The modules I am using are NI 9269 to send a sinusoidal signal and NI 9215 to receive the data. I was wondering if anyone has ever have problem in which the phase between the input signal to the actuator and the displacement signal back from the sensor switches sign at a certain frequency. I analyzed the data with fft in Matlab and it doesn't seem like we have any resonance. I attach the graph I am getting.

Thank you for the help.

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Solution
Accepted by topic author DavidMu

Try unwrapping phase.

Unwrapped Phase.png

You can use this VI found in the LabVIEW palettes:

Unwrap Phase Context Help.png

Doug
NI Sound and Vibration
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Thank you!

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