Shaker positions for full body vehicle testing

[MikeyP]

Some rather interesting (to me anyway!) issues have come up in some recent threads prompted by comments from Greg Locock ("Harmonics and Modes" and "physical significance of frequency response function &quot

I have a question for the NVH people out there re shaker positioning for vehicle testing.

The current trend is for independent front and rear sub-frame chassis design in relatively small vehichles.

1) How does this influence your choice of shaker positions?

2) How does this influence the quality of your results ?

I don't have direct experience, but from what I have heared through my contacts, many NVH fully trimed body modal testers tend to stick with the same excitation positions no matter what the design of the car (usually 1 shaker at the front rear corner exciting vertically and one at the front exciting at a compound angle). I would imagine that a front and rear sub-frame design would require at least 4 shakers (2 on each frame) for adequate testing.

I'm sure there is an element of "We do it the way we have always done it.". I also realise that some NVH departments will be more progressive than others and that there will be limited time available to experiment with new techniques out there in the real world where you may only have a vehicle in the lab for 1 or 2 days as well as limited money for new equipment.

It just seems strange to me that the civil and military aerospace industry is always desperate to try out new methods which will improve their testing while the NVH world seems relatively sluggish.

Any comments?

PS No one could ever describe me as a "fully trimed body modal tester"!

M

 

[GregLocock]

Subframes, by design, make it very hard to get energy into the suspension, if the shaker is on the body. And vice versa of course. Unfortunately subframe modes are crucially important (both rigid and flexural) - the rbms are critical to secondary ride (that's why I hate subframes) and the flex modes usually give you a noise problem, either roadnoise or driveline out of balance or both.

Our current recommendation is probably to take the subframe and engine off the trimmed vehicle if the prime purpose is for correlation with a trimmed FE model. To me as an experimental (not analytical) engineer this sucks, since the results are borderline meaningless in their own right.

However, if we are looking for trouble, and don't have to worry about the FE boys, then the best way to get a shaker position is to feed your mechanic a lot of beer and test his patience by trying a whole bunch (say 5) of different exciter locations and directions. Then compare the FRFs of each one to a subset of important accelerometer locations and make a call based on evenness of peaks, and non-grunginess of the data. Then do a full survey and second survey on 1/3 of the points using a different shaker location and direction. Then frighten yourself silly doing a thorough check of reciprocity.

By the way (1) I loathe compound excitation. A virtual beer to the person who can guess why!

By the way (2) best practice in vehicle modal testing does not seem significantly different to me now to how it was in 1982

By the way (3) and I'm not wildly keen on Multishaker if it comes down to it.
Cheers

Greg Locock

 

[Rob45]

I have one observation for MikeyP who wrote:
"It just seems strange to me that the civil and military aerospace industry is always desperate to try out new methods which will improve their testing while the NVH world seems relatively sluggish."

One big reason for this is cost, and the "civil and military aerospace 'industry'" doesn't have to produce a product to be sold at a profit. The rest of us do.