×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Total Harmonic Distortion of vibration shaker system

Total Harmonic Distortion of vibration shaker system

Total Harmonic Distortion of vibration shaker system

(OP)
Hello There,

I have question regarding our electrodynamic vibration shaker system.

Recently we have noticed some abnormal sound (like turning fork) from the system when it is used with head expander. It occurs at the frequencies,300 Hz, 600Hz, 900Hz, 1200 Hz, 1500Hz, 1800 Hz and so on. This sound appears even without the head expander and the situation get worsen with head expander.

We conducted, total harmonic distortion test at vertical axis with bare head expander as mentioned below.


Control acceleration = 1g

Test Frequency(Hz)
5
10
20
30
500
900
1200
1500

CH1 THD +N %
N/A (5Hz)
N/A (10Hz)
8.49286 (20Hz)
7.44264 (30Hz)
8.98874 (500Hz)
53.0711 (900Hz)
26.9406 (1200Hz)
9.44025 (1500Hz)

(CH2 THD + N %)
N/A (5Hz)
N/A (10Hz)
7.02918 (20Hz)
6.28316 (30Hz)
7.08433 (500Hz)
45.2265 (900Hz)
20.5199 (1200Hz)
9.21418 (1500Hz)





Control acceleration = 5g

Test Frequency(Hz)
500
900
1200
1500

(CH1 THD+ N %)
5.1278 (500Hz)
17.4902 (900Hz)
6.4243 (1200Hz)
2.0185 (1500Hz)

(CH2 THD +N %)
6.01514 (500Hz)
12.2896 (900Hz)
6.2920 (1200Hz)
2.7729 (1500Hz)




We think that above calculated THD values are very high and a clear distortion is visible in the graph of the head expander (pls see attached graphs).

Shaker manufacturer advised that system is working fine after reviewing these results.

If you can review this data and provide any feedback would be highly appreciated.

Thank you

BR,
Prasanna

RE: Total Harmonic Distortion of vibration shaker system

Looks like there may be a shaker resonance near 900 - 1000 Hz. This is the most audible frequency for hearing, so it is most noticeable. Look for loose parts/fasteners outside and inside shaker. Try a different mounting location.

Walt

RE: Total Harmonic Distortion of vibration shaker system

(OP)
Hello Walt,

Thank you for the response and suggestions. It's true that sound gets most audible when it passes through ≈ 900Hz frequency .
I've tried with different control accelerometer locations for bare armature condition and still i can hear a clear sound when the frequency sweep through those frequencies. Sounds like the issue with the armature?

BR,
Prasanna

RE: Total Harmonic Distortion of vibration shaker system

There could be an issue with armature support (looseness or crack) or with housing mount (gimble?) or housing base on support/foundation. Some detective work is required!

Walt

RE: Total Harmonic Distortion of vibration shaker system

When I started out we used to have mechanics assigned at random to the modal lab, and I knew nothing. We often used to damage the voice coils in the small shakers due to inappropriate setups, basically lateral forces on the stinger. I'd expect that to cause 3rd order distortion, followed sooner or later by an expensive smell of burning insulation.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Total Harmonic Distortion of vibration shaker system

(OP)
To give you some more background, The shaker system capacity is 600kgf (random force).Typically we use the shaker for vibration ESS testing. Total moving mass is around 16kg including fixturing and all moving parts of the shaker. The Grms value for these tests does not exceed 8 Grms. Test profiles are created based on MIL-STD-810 and DO-160 standards.

In fact we are relatively new for vibration testing field as well as we intend to use this machine mostly for ESS tests and couple of DVTs.

The thing that concerns me is that shaker manufacturer's response on this inquiry. Even though we report this issue with all of this test data , they claim that the "system is working fine". They mentioned that as long as the response is between the 3dB tolerance limit we would get sufficient accuracy for our testing (regardless of the distortion).

I would investigate further and try to find some clue. Can anyone please suggest some steps for diagnosis?

Any suggestions would be highly appreciated.

BR,
Prasanna

RE: Total Harmonic Distortion of vibration shaker system

I would look at the power/current consumption by frequency to see if there is a resonance that the shaker is overcoming. Even if the control response is flat; if the shaker is drawing 50% more power at the frequency where you hear the noise, that would certainly limit your ability to drive the shaker at its rated performance at that frequency, which would make it noncompliant to its specification. Moreover, if the shaker is indeed drawing excess power to drive at that frequency, it would be generating mechanical harmonics, by virtue of it clipping its signal, that would cause the controller to have to compensate elsewhere in the frequency space, which will likewise draw additional power, and reduce the life of the driving circuits.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now
The Great Project Profitability Debate
A/E firms have a great opportunity to lead the world into the future, but the industry’s greatest asset—real-time data—is sitting wasted in clunky, archaic ERP platforms. Learn how real-time, fully interactive dashboards in a modern ERP allow you to unlock data that will shape the future of the world. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close