Hi, I have to do frequency respons 1

[sasank]

Hi,
I have to do frequency response analysis for automotive frames and cradles. Before starting the analysis I have some questions. Can any body please answer my questions?
My questions.

1) What is frequency response analysis ?
2) what is Low frequency and High frequency responses.
3) When and why we have to do the frequency response analysis for the automotive structures ?

Please answer my questions.
Thanks
Kamalakar Rao

 

[hechengli]

I think,
frequency response analysis is, give one sinusoidal input, find the output, usually, for linear system, the frequency is same as the input, find the gain of magnitude and phase shift. I think there are many experiment equiment can do that.

the find the natural frequency of the automobile and reduce the vibration of the automobile by passive or active method, or maybe for control purpose.

 

[IRstuff]

The bodyframe transfers energy from the suspension system to the occupants and the interior. If there are resonances that are generally too high amplitude or in a bad frequency range, it can make for, at best, an uncomfortable if not impossible ride.

Additionally, resonances affect the ability of the fasteners within the vehicle to do their job of holding things together. Too much vibration and things shake apart and start buzzing and rattling.

TTFN

 

[sasank]

Thanks for the reply.
I wanted to know some more information about frequency reposnse ana;ysis.

To my knowledge, there are three methods in frequency response.They are
1) Displacement appraoch
2) Velocity approach
3) Acceleration approach

My question is which of these approaches, we need to follow ? To be more clear,

When do we do displacement, velocity or acceleartion responses required. Also, when some body is talking about the Low frequecny response ( say about 0-200 Hz) of the vehicle, why that low frequency responce required ?
When we have to study the high frequency response ( say 200-600 Hz)of the structures? Can you please answer my questions? I think, I am clear in asking the questions.

Thanks
Kamalakar

 

[GregLocock]

The choice of displacement velocity or acceleration is usually dpeendent on the frequency range you need. On vehicles most of the time you use acceleration, for modal analysis.

LF response (more typically 1-40 Hz) affects durability, shake, ride comfort, handling and so on. There are also some nois eproblems that can be caused by these low frequencies.

40 Hz+ gives noise problems, and on the body at least, local issues such as booms being amplified by specific panels.

Cheers

Greg Locock

 

[MikeyP]

To add to Greg's comments: Measuring displacement on a vehicle body with displacement transducers could be impractical (for a modal analysis at least) as the transducers have to be grounded with respect to the test structure. Accelerometers are the commonest transducer for these applications and hence acceleration is the most common measured parameter but laser vibrometers are becoming more popular now and these measure velocity.

Michael

 

[Rob45]

Please don't take this as a sarcastic or rude response, but it is apparent from your question that you need to do some basic reading on this subject. Books have been written addressing the questions you've asked, but perhaps a more basic text on vibrations would be the place for you to start.
A starting place would be "Theory of Vibration" by William T. Thomson.

 

[sasank]

Guys,
Thanks for response. I have another question? If we look for acceleartion response, and plot the acceleartion Vs Frequency graph, How can we use this information? To be more clear, suppose if I get the peak response (acceleration ) in the rane of say 150-200 Hz, How this information can be used?

Sasank

 

[IRstuff]

Generally, you are not looking for absolute acceleration vs frequency, but rather transmissibility of a stimulus at different frequencies to identify resonances. So, your peak transmissibility is then looked at relative to how much gain (Q) there is. It's not usual for a system to have a Q of 10, where the response is 10 times the magnitude of the input.

In such cases, you need to evaluate the likelihood of getting such a stimulus and whether there is a problem with having such a high response. In many cases, such responses are unacceptable because the vibration level or noise becomes too high, or ultimately, the system will shake itself apart.

TTFN

 

[GregLocock]

In our cars the prime reason for looking at 150 to 200 Hz would be noise, not vibration. If the engine mount bracket had a high peak at 150 Hz then it will tend to degrade the performance of the engine mount at that frequency, and you would get a series of peaks at 150 Hz on a waterfall plot of interior noise against engine speed. Similarly, if your suspension mounting points have resonances in the wrong place you can see them on the coarse road noise spectrum.

If you are at an early stage of the design then you won't be able to test for these effects, you'll just have to rely on the noise predictions from the computer models (good luck) and the prior experience of the design people, who will nominate 'do not exceed levels' for compliances. A typical requirement for an engine mount bracket is that it should have no local modes at less than (max firing frequency *1.15) and a dynamic stiffness of 10 times that of the engine mount over the same frequency range. There is some redundancy in that spec, and it assumes that you know what your engine mounts will look like. Later on in the program you can drop your point mobilities into a noise path analysis and see if they are important.

Your noise and vibration people should also be building a modal alignment chart showing you where you can have the modes for major subsystem, graphed against the typical causes of excitation.
Cheers

Greg Locock

 

[satyaram]

Hi Iam doing a project on static analysis of heavy vehicle suspension mounting bracket , what type of static loads shall i consider for the analysis and what is the factor of saftey taken while designing the bracket

 

Sir,
Here is my question
1. for what kind of applications FFT analysers are used?
2. Can I use it for vbrations in bearings. If so what is the mounting procedure for accelerometers?
3. What way can we use a FFT for preventive maintenance and condiotion monitoring.
4. What r the things that we can interpret from a signature of a frequency responce?
Thks