Theodore Jenkins

Hi Peter, 

I have been reading your responses to other people for months now and i am always astounded by your knowledge and clarity of response. Thank you very much.

1 - the test-specimen is affixed to the moving part of the vibration exciter via a double ended stud bolt (See photo 1), and I beleive that this connection is not stiff enough ie the first mode is lower than the highest mode of interest. I beleive the setup is flawed, but i just need to be able to model it as best i can with FEA, i will address the concerns in the discussion of my thesis. 
The elastic support was my attempt at creating "weak springs" to restrict the rigid body motion of the part. i did not want to model the exciter as i do not know enough about it to model it with any certainty.

2 - I really should know that, thank you for bringing it up. i will find out ASAP. Currently, we generate a sine wave or a particular frequency and convert it to analogue signal, which is then passed into the exciter. 

3 - i dont know much about exciter tables. the vibration exciter sits on a heavy steel plank which rests on a table. photo 2 shows this.

4 - i have seen something similar in my vibrations class, however, we are trying to replicate the vibration of a pipe/pipeline. we are more just interested in the range of frequencies for which the accelerometer response is reliable before the mounting method reaches resonance. i.e., connecting a flat magnet on the curved surface would likely have some wobble (my theory) that would mean reduced accuracy. we are testing 0-10kHz.

5 - This makes so much sense, wow! thank you, i totally see what you mean. in the real experiment though, i assume there will be some of that motion, making the magnet wobble... this is just my assumption. experimental data shows a real spike (dB) at around 8kHz, therefore i believe this is what is happening. should I try to add some small excitation in another direction/point to try simulate this? 

Also, experimental data I am using is from this vibration exciter orientated in the vertical direction, so the centre of gravity is acting through the centre of the exciter i.e. no moment due to gravity on the test specimen/accelerometer/magnet etc. 

again, thank you for your reply and your time.

kind regards,