My parasitic harmonics are moving

[KalleH]

When using a 3DOF servohydraulic shaker, I have lately encountered some odd harmonics in my 3DOF response signal. The frequency content of the non-stationary noise that I try to simulate is 5-35 Hz.

When measuring using a MTS flextest system, the parasitic harmonics end up at around 80Hz. All three response signals include three harmonics.

When measuring using an eDaq system and antother set of signal conditioners, the three harmonics move up to 491, 494 and 500Hz.

I use the same ICP accelerometers in both cases.

Does anyone have a reasonable explanation for this behaviour?

Thanks!

/Karl

 

[KalleH]

Never mind.

In the case of the flextest system, I acquired the signals through the auxiliary channels. Apparently, these are not low pass filtered to prevent aliasing.

Hence, my conclusion is that the frequencies around 80Hz are simply the ones around 500Hz aliased.

Sorry!

/Karl