I was working with a new (old model) accelerometer without signal conditioning or amplification and I am picking up a ~50Hz sine wave acceleration (on FFT) when holding it in my hand. The signal remained despite covering it with insulators. Similarly noted when others are asked to hold the accelerometer. The sine wave disappears leaving noise when the accelerometer is left on the table.
The fan was off. No fluorescent lamp around. I don't think it is EMF induced as it should be apparent even when left lying around.
I don't see such observation when using newer accelerometer with in-built signal conditioners.
Could not get much on literature search as most are talking about induced WBV at 50Hz. But a book by Ian Sinclair, Sensors and Transducers (attached) in page 156 states 'A moving human generates a Doppler signal whose frequency is centred around 50Hz, and amplifiers for a Doppler system have their maximum sensitivity at this frequency.'
Is anyone aware of this phenomena and can help with some enlightenment.
https://books.google.com.my/books?id=s_WIb91uKK8C&pg=PA156&lpg=PA156&dq=is+the+human+body+shaking+at+50hz&source=bl&ots=cN3n6sSHqI&sig=3WeRHg7wANTW9mEsatM4L81Jbz8&hl=en&sa=X&ei=-GCLVYOOK-XdmAWEqICADA&redir_esc=y#v=onepage&q=is%20the%20human%20body%20shaking%20at%2050hz&f=false
I never experience such peak holding and accelerometers in hand. I suppose it is a sort of amplification of the alternate current magnetic field due to the human body.
I am looking at repeating the observation with the main power to the building switched off. I can't do that now as my ADC is dependent on AC power supply. I am working on replacing my current ADC with NI MyRIO which can run on battery. Perhaps if the setup is good, I might even perform the test in the wilderness far from the power grid and see if I am getting the same finding.
I think Pier Paolo is right and the idea of repeating the experiment in the USA looks interesting. I have worked with accelerometers a lot, but never experienced anything like this. Most probably the primary source is not related to the human body.
Having used acceleromterers of many types for many years, I concur with the above authors.
/C
Apparently in the past I have been using IEPE accelerometers connected to National Instruement, NI9234 DSA for data acquisition. I did not get such problem as the IEPE accelerometers behaves as a 'floating' signal source. The accelerometer that I am referring to in this question is Knowles BU27315 which is a charge type piezoelectric accelerometer. Unlike the IEPE accelerometers, BU27315 is covered by a conductive metal casing powered by an external battery 1.5V. It gets grounded when applied to the (bare skin) human body. The NI9234 is also ground-referenced. Hence I was getting ground loop noise with power line frequency (50Hz). The pseudodifferential connection mode only removes the DC offset but not the AC noise. I should have been using differential input configuration. Unfortunately it seems NI9234 is not allowing me to select a differential setting.
Perhaps equipment such as this may come handy.
http://adelab.com.au/wp-content/uploads/HUMBUG-handout.pdf
I suggest that you connect the grounds of all the instruments you are using... I promise you, this will disappear !
Hi
It seems that you have nailed the root cause.
Why not spray paint the BU27315 or use an insulating sticker when applying it to your test subject?
/Claes
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