What's wrong with the vibration shaker?

[zhappybird]

Hi There,
I have a VTS(vibration test system)-100 shaker. To make sure it works correctly, I have done a acceleration frequency response test for the bare shaker. In which, I place an accelerometer on the top of the shaker platform, use a function generator and amplifier to drive the shaker at sweeping frequency from 2 Hz to 6000 Hz. I also use a polytech laser vibrometer to detect the displacement of the point on the shaker platform close to the location of the accelerometer. I use LABVIEW to control the oscilloscope and function generator. Attached are my result. From the acceleration frequency response graph, I see several peaks indicating the "resonance" frequency, according to the vibration theory, the acceleration should be a continuous function of the frequency, it should be a smooth line without any resonance peaks. What is wrong with my measurement? or the shaker is bad? I have made sure the accelerometer is good.
Thanks.
Mike

 

[dragoneyes001]

could you be getting harmonics? do these same results show on every try?

 

[Dr.D]

I suspect that the difficulty arises from trying to make an overly simple mathematical model fully describe a real physical system. All real systems have infinitely many modes of vibration, yet we never develop models of that level of complexity. You have just run headlong up against reality!

 

[Bystander]

Looking at a picture of the little beast, you could be getting mode plus harmonic for the bracket.