Vehicle dynamics: rigid modes PSDs question

[pietro82]

Hi all,

I performed some measurments of pitch, roll and bounce accelerations of a vehicle. I installed three accelerometers placed as a triangle and through few trivial formulas (ie roll_acc=(a_lh-a_rh)/(2*l), where l is the distance between the accelerometers and a_lh and a_rh are the accelation at two sides of the vehicle) I derived the vehicle pitch, roll and bounce accelerations. I drove the vehicle on a straight asphalt road.
These are the PSDs of the mentioned signals.

http://img690.imageshack.us/img690/6311/40024043.jpg

What seems strange to me is all the PSDs have a peak around the same frequency (2.25) . May you be so kind to explain to me the reason of that?
thank you in advance.

Best regards

 

[GregLocock]

I got a units problem when I looked at the plots, the vibration appeared to be in (m/s)^2

Anyway, the passenger is primarily sensitive to what s happening to him, and that is the vertical input from the seat, and longitudinal on the back of the seat. Pitch and roll are secondary.

Cheers

Greg Locock


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[IRstuff]

normally, it would have been (m/s^2)/sqrt(Hz)

TTFN
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[pietro82]

Hi all,

sorry for the unit problem. I calculated the psd from a linear acceleration in m/s^2 and from an angular acceleration in °/s^2.

IRstuff, how do you guess the rms value from a PSD?

Greg, Look at this reasearch report

http://www.hse.gov.uk/research/rrhtm/rr321.htm

page 66 fig 5.8. The vertical acceleration is always lower than the horizontal ones, due to ISO standard post-processing. The vibration level at the seatbase is always lower than the one at the seat. In my opinion this is due to the tractor angular accelerations and I want to evaluate somehow their effect on the seat accelerations. Since the seat is rigid on the horizontal directions, the seat linear acceleration due to the pitch is: pitch acceleration times the vertical distance between the pitch center and the seat position. The same with roll.
What do you think? Any other suggestions are appreciated.

Thanks again.

 

[IRstuff]

You're supposed to do the numerical integration across the entire bandwidth, but seeing as it's only picture, my "guess" was a brute-force eyeball integration of curve.

"The vibration level at the seatbase is always lower than the one at the seat. "


That's troubling; a properly designed seat, particularly for a vehicle with no suspension, is supposed to attenuate the vibrations so that the passenger receives an attenuated vibration. It's also possible to design the seat to minimize angular motions, and it should certainly not create more motion than the vehicle generates.

TTFN
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[GregLocock]

Well that would be in the ideal world. In practice the resonance of the human body on the seat does cause some pretty neat resonances, at least on passenger car seats.

Cheers

Greg Locock


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[patprimmer]

Greg

Maybe that depends on if the car is parked or moving and what is going on in the seat.

Regards
Pat
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[pietro82]

Thanks for your replies.

IRstuff: I don't want to design a seat suspension to minimize the angular motions, I think it's better to decouple systems so to design a cab suspension to minimize the angular movements. But the first question is: which rigid modes it's better to minimize?
Tractor cab have this kind of suspension system:

http://www.machineshift.com/blog/blog/wp-content/uploads/130.jpg...

that it is designed to minimize pitching.

Greg: you're on right.