Modal Testing System

[SamSSX]

Hi,

I have done a lot of structural dynamic analyses but have done little experimental modal analysis myself. Now I need to buy a modal testing system that I am going to use if needed in any project. I would like to hear your opinion and recommendation for such a system.

The structures of my interests weight from 50-200 lbs, and are the assemblies of machined and casted metal parts. Some of the structures may have rotating components. The modal frequencies of the interests are most likely to be less than 2000 Hz. Because we do not have a shaker table, I will do modal testing with impact hammer. The budget is around a few $10k range. I may also need to use to the system for vibration data acquisition, such as to measure the base accelerations to develop vibration environment spec.

What should I look for in a system like this, both in term of hardware and software?

On software side, it seems the LMS, Spectrum Dynamics and Vibrant(ME’Scope) have special modules for modal analysis. What are their ease of use and functionality?

What is the key specs in Dynamic Signal Analyzer, scan rate, dynamic range? Is 24-bit A/D enough or do I need 32-bit A/D? What is the minimum channels that I need?

Who else are players in modal testing system field beside the companies I already mention?

Thank you all in advance for any response.

Sam SSX

 

[GregLocock]

You need a minimum of 4 channels (a triax, which you'll move around, and the hammer). You will be buying a shaker fairly soon (grin), so it is nice to have an output channel.

24 bit is fine. You aren't pushing the envelope.

The last time I used EMA software was LMS , 6 years ago. It was powerful but perhaps a little obtuse.

I haven't used SD stuff for years but it tended towards the lower end of the market - fats simple, not many options.

Vibrant - never used it.

B&K, Prosig, Agilent(?) probably make a small modal system.

There used to be a very sweet system around called Star Modal.

There are three separate tasks - taking good data, and then analysing it, and the displaying the pretty pictures. Taking good data with a hammer is largely a a matter of experience rather than software.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[FGL]

I was in IMAC yesterday for conferences on Structural Dynamics. They had some exhibitors too. You should have come, it's nice to compare modal analysis softwares and instrumentation. ME'Scope is a good software very complete but it's not so esay to use. Since I was there I would recommend you OROS for 2 reasons:
1. They have their own Modal software, easy to use. It's only for an experimental use so there is no finite element (I think so). I've had a demo, it works very well. In your case, this software would be perfect.
2. It is based on the OROS technology, one of the best for vibration analysis. A colleague uses one and that's a pretty nice instrument. the software is very intuitive and powerful. And the price is even cheaper than LMS or B&K.
That's why I would go for it. Actually, I'm thinking about buying one for my department too and that's why I'm so excited about it. Finally, I saw a post a couple of weeks ago about how they are responsive for technical support. I think that's a good reason too cause when I need the analyzer, I don't want to wait a week to get an application engineer on the phone.

For the Hammer, I would choose Dytran.

Cheers

 

[SamSSX]

Thank you Greg and FGL for your inputs.

It seems I just missed the big gathering of modal testing venders.

I used HP/LMS (in unix box) modal test system over 10 years ago. Now to my knowledge, Agilent sold its mechanical data acquisition product line to VXI Technology and is out of modal testing market; m+p uses some of VXI’s hardware in its system; B&K (and maybe several others) uses ME'Scope as the software part of the system. OROS is an interesting prospect. Does its have tech support staff in US?

 

[FGL]

Hi SamSSX,
I know OROS has a facility in Virginia where you can calibrate your instrument or send it for maintenance. It also has reps around the country but I can't say exactly where. I recently talked to one of them from SC who was quiet technical (not a salesdroid!).

Cheers

 

[GregLocock]

Once you've narrowed your selection down get them all to come and give you a demo, on your test piece.

I can't get very religous about hammers, when push came to shove a 4 kg lump of cast iron with a force gauge screwed into it worked fine, when I needed a big one. The PCB ones are cheap and good enough, but the B&K ones are the ones I always preferred, simply because the B&K 2635 charge amps are probably my favourite front end. Since they don't exist any more then I guess that throws the field wide open.

A good way to check a hammer is to get a cylindrical test mass (a couple of kg will do). Screw an acc into one end, hold it cupped in your hand. Bonk the other end with your hammer.

You should get a flat accelerance line, and if your calibration chain is good it should be the inverse of the mass.







Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[garymondo]

Folks,
I work for SD (Spectral Dynamics) and we sell STAR and the data acqusistion system to go with it. We were at IMAC and showed our 4 input channel system (BobCat) and one output. With analysis software for gathering data it runs $8K. The STAR modal software runs anyhere from 5K to 14 K depending on what level of performance you want. Our modal expert resides in Detroit (248-269-6181 ask for Marc; he will also give you my number if you want it). We mfg the h/w and develop the s/w in the USA (San Jose)!
OROS - France, B&K - Denmark, LMS and m+p Germany, - getting the picture

Salesdroid,
gary