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Technical Discussions
talortang
talortang
05:58 Jun-01-2009
direction control of guided waves

Could anybody help me solve the problem with direction control of guided waves?

In my research, I need to control guided waves, torsional waves, to propagate in a single direction, that is unidirectional propagation, I know that it can be carried out when arranging two coils which have an interval of 1/4 wavelength of guided waves, but I can't get detailed information about it, especially electrocircuit design such as phase shift circuit.

I have searched for many ariticles, but rarely related information could be gotten, can anybody help me solve this problem? or advise some articles?

Thank you!

Sang Kim
Consultant, NDT Trainer
Guided Wave Analysis LLC, USA, Joined Feb 2008, 44

Sang Kim

Consultant, NDT Trainer
Guided Wave Analysis LLC,
USA,
Joined Feb 2008
44
23:59 Jun-02-2009
Re: direction control of guided waves
In Reply to talortang at 05:58 Jun-01-2009 (Opening).

You can do direction control of guided wave with two or multiple probes and time-delaying circuit. Follow this procedure for controlling the direction of guided wave:
1) Find the velocity of torsional wave mode in the applied material; The torsional mode velocity is about 3250 m/sec in steel. The fundamental torsional mode velocity is the same to shear wave velocity of the material.
2) Decide the operating frequency of your wave such as 128 kHz.
3) Calculate the wavelength with the velocity and frequency. In this example, it is about 1 inch.
4) Calculate ¼ of wavelength of the wave. It is 0.25 inch.
5) Install two probes in 0.25 inch of center-to-center separation
6) For transmitting the guided wave, make electric circuit so that the transmitters trigger the high-power electric currents with a delay of ¼ period to each other. In this example, 1/(4*128000) sec
7) For receiving the guided wave, make electric circuit that can add the two signals after delaying ¼ of period from each other.
8) For good direction control, you need two transmitters and two receivers.
9) If you use multiple probes, you will have better direction-controlled signal.

If you need simple test in the lab for a paper, try to use one probe that is bonded at ¼ wavelength separation (The center of probe should be ¼ wavelength separation from the edge of specimen) from the end of specimen such as pipe or plate. Then you will have perfect direction-controlled signal with only one channel equipment.

Sang

talortang
talortang
10:46 Jun-08-2009
Re: direction control of guided waves
In Reply to Sang Kim at 23:59 Jun-02-2009 .

Thanks! I don't need simple test in the lab for a paper, but long time and deeply research for controlling the direction of guided wave.
I have known the procedure for controlling the direction of guided wave, and I have painted the figures of vibration of two particles in one period ,which stand for two transmite coils bonded at 1/4 wavelength separation and delaying 1/4 of period from each other.
But unfortunately, I don't know how to upload the figures to forum, so, I have to e-mail to you with the figures.
I have two problems with the procedure.
1)when the vibration direction of two particles ,which stand for two probes installed at 1/4 wavelength of center-to-center separation, are the same or the converse, we get different results when two waves compound,as the figures show. which one should I choose? It seems that we can get pure direction-controlling when the vibration direction of two particles are the converse,but the amplitude of first half-period wave is half of the amplitude of second half-period wave, this wave may affect our test result. However, we can't get ture direction-controlling when vibration direction of two particles are the same. In one direction, we get half-period wave; in another direction, we get one period wave, which amplitude is two times as the half-period wave.
So, should I choose the converse one? Then how to avoid the affect as a result of different amplitude of the first half-period and the second half-period? or whether it will affect the test result?
2)for good direction contorl, I know two transitters are necessary, but why we need two receivers, too? when we use two transitters installed as you said in your message, we can control guided waves to propagate in a single direction, then receiving the single-dierction guided waves, one receiver is enough, why do we need two receivers? You said it is for good direction control, could you explain it in details?

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