Driving seat vibrations while driving a bus

[Mccoy]

Hi everyone,
I've recently measured accelerations on the driving seat of a bus for industrial hygiene purposes. The reference configuration is an orthonormal cartesian system with a 3-sensors arrangement, the z axis being the vertical (parallel to the driver's backbone). Accelerations have been measured while driving along the usual bus route. I noticed one behaviour which I'm not able to explain: The signal or temporal series of z-axis accelerations, applying an n=30 mobile average (n = 1 second), displays a clear periodical fluctuation with a ciclicity of about 40-50 seconds(longer cycles are present).
Does it sound sensible that the vibrational modes of the system may overlap producing such long-wave impulses? Bus is 33 feet (11 meters)long and has air suspension; seat has steel spring damping system.
A low-pass filter is applied; accelerations are RMS values, road was paved and in reasonable conditions
Thanks in advance for any help

 

[TwnB]

Hi McCoy,

Check out this web site. These guys are experts in this field. Hope it helps.

http://www.mrfluid.com

 

[GregLocock]

I doubt that a vibrational mode in a bus would have such a low frequency, and I'm not sure what you mean by 'overlap' unless you mean 'beating'. In that case, I suppose it is possible that you could have two modes being excited in a correlated fashion that are only 0.02 Hz apart - but it doesn't seem very likely.

If you are looking at the shape of the envelope then it doesn't, really, have much to do with the human perception of vibration, unless it is strongly dominated by one frequency band.

I suggest you read up on papers by Mike Griffin at ISVR at Southhampton University in the UK.



Cheers

Greg Locock

 

[Mccoy]

Thanks for the replies.

TwnB,
I browsed the site you suggested, though it didn't help much in my case, it was fascinating to know about the new suspensions with fluids whose rheological properties are adjusted based on the vibration field. Just amazing!

GregLocock,
unfortunately I never had a chance to consult Griffin's "Handbook of human vibration" and only know a few of his recent papers on the subject. In the vehicle I studied, according to ISO 2631-1, there is really no significant health risk related to seat vibrations (az = 0.37 ms-2 average). This applies to all measured intercity buses (European models)except the old models, with old-fashioned suspension system. Since I'm writing a tech paper on the subject, I was concentrating on some fine details. Your guess might make up a useful work hypothesis to propose,in lack of other tentative explanations.

best regards