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Technical Discussions
Alex
,
Israel, Joined Sep 2017, 8

Alex

,
Israel,
Joined Sep 2017
8
08:35 Dec-11-2019
Influence of pulse frequency on measurement of material thickness change

Hi guys,

I'm a bit confused with the following question. Assuming the signal sampling frequency is sufficiently high, how the ultrasonic pulse frequency influences the measurement precision of material thickness change in a pulse-echo mode?

The example of the problem is this. Material with c=3.4 mm/microsec is grinded by a 0.1 mm steps from top to bottom, signal sampling frequency is very high (1.25 GHz) and the frontwall and backwall echoes are clearly visible. What is the basic difference if a transducer with 1MHz, 2.25 or 3.5 MHz or higher is used for that?

 
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Anmol Birring
Consultant,
Birring NDE Center, Inc., USA, Joined Aug 2011, 832

Anmol Birring

Consultant,
Birring NDE Center, Inc.,
USA,
Joined Aug 2011
832
11:02 Dec-11-2019
Re: Influence of pulse frequency on measurement of material thickness change
In Reply to Alex at 08:35 Dec-11-2019 (Opening).

No difference, but you got to use zero crossing method.

 
 Reply 
 
Wiesław Bicz
Engineering,
PBP Optel sp. z o.o., Poland, Joined Feb 2009, 276

Wiesław Bicz

Engineering,
PBP Optel sp. z o.o.,
Poland,
Joined Feb 2009
276
12:38 Dec-11-2019
Re: Influence of pulse frequency on measurement of material thickness change
In Reply to Alex at 08:35 Dec-11-2019 (Opening).

Time of flight measurement has not much to do with frequency, but with the method of signal analysis. This was already discussed on this forum many times. Frequency of signal is of much lower importance than for example the accuracy of amplitude measurement.
Zero crossing method mentioned by Anmol is one of the simplest possibilities.

 
 Reply 
 
Alex
,
Israel, Joined Sep 2017, 8

Alex

,
Israel,
Joined Sep 2017
8
14:16 Dec-11-2019
Re: Influence of pulse frequency on measurement of material thickness change
In Reply to Wiesław Bicz at 12:38 Dec-11-2019 .

Hi Anmol and Wieslaw, thank you for your response.
I use envelope extraction and cross-correlation to estimate the time of flight. And use this method consistently between all acquired signals on the course of material thickness change.

Can you though, describe why the pulse wavelength has nothing to do with the distance change measurement? Can I measure smaller and smaller increments of the thickness change only by carrying for sufficiently high sampling period?

 
 Reply 
 
Wiesław Bicz
Engineering,
PBP Optel sp. z o.o., Poland, Joined Feb 2009, 276

Wiesław Bicz

Engineering,
PBP Optel sp. z o.o.,
Poland,
Joined Feb 2009
276
18:08 Dec-12-2019
Re: Influence of pulse frequency on measurement of material thickness change
In Reply to Alex at 14:16 Dec-11-2019 .

There are two critical factors in TOF measurement:
The most important factor is the accuracy of amplitude measurement.
In the case if you want to measure the distance to some pulse measured from the excitation moment, the synchronization between the pulser excitation and sampling clock is also critical.
In the case of measurement between two pulses practically only the accuracy of amplitude measurement. Sampling rate must be sufficient from the point of view of Nyquist criterion, but is of significantly lower importance than the amplitude measurement accuracy.
And naturally the algorithm, that calculates this is very important and the final result depends on its quality.
If you are interested in checking this, I can send you a simple paper, where I have explained this.

 
 Reply 
 
Alex
,
Israel, Joined Sep 2017, 8

Alex

,
Israel,
Joined Sep 2017
8
08:31 Dec-15-2019
Re: Influence of pulse frequency on measurement of material thickness change
In Reply to Wiesław Bicz at 18:08 Dec-12-2019 .

Thank you Wiesław, I will be happy to get the paper.

 
 Reply 
 
Alex
,
Israel, Joined Sep 2017, 8

Alex

,
Israel,
Joined Sep 2017
8
08:58 Dec-15-2019
Re: Influence of pulse frequency on measurement of material thickness change
In Reply to Wiesław Bicz at 18:08 Dec-12-2019 .

Wiesław,
Even if I fulfill the minimum Nyquist criterion and have 10 bit vertical range, the sampling points won't fall each on the maximum of two received pulses. Why do you state that for the time of flight measurement the sampling frequency is of much lower importance?

 
 Reply 
 
Wiesław Bicz
Engineering,
PBP Optel sp. z o.o., Poland, Joined Feb 2009, 276

Wiesław Bicz

Engineering,
PBP Optel sp. z o.o.,
Poland,
Joined Feb 2009
276
12:23 Dec-16-2019
Re: Influence of pulse frequency on measurement of material thickness change
In Reply to Alex at 08:58 Dec-15-2019 .

If you are too close to Nyquist criterion it will be necessary to take in account not only one, but many points - correlation would be the best solution. If you have sampling rate that is more than 3 times higher in many cases only two points (with good SNR): one before zero transition and one above.

 
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