This manuscript is going to focus on how the transmitter bender element oscillates after function generator stops.

In geotechnical laboratories, bender elements are mainly used as a couple; one is used as a transmitter and the other as a receiver. Function generator is an unavoidable element in these tests; it produces a signal that transmitter senses it and oscilloscope shows that. The goal of this text is to show what oscilloscope shows, is more than what is made by the function generator. The transmitter bender element starts to move as it senses the electricity (regarding a delay or phase), but it does not stop when the electricity goes off.

*Schematic sircuit*

Schematic sircuit

Persistence in the vibration of the transmitter is the result of its momentum and it applies more energy to the tested sample. In the other word when bender element is used as a transmitter in the lab, in spite of the function generator that can produce an exact unique cycle, bender element shows a decaying vibration after the strong initial cycle. It means that the straight tail in the below figure has some wave shape if we zoom enough.

Hear after the laboratory measurements on a bender element which vibrates in air is going to be presented. The goal is to show how amplitude of free vibration varies with frequency.

As the consequence of connecting the bender element, changes occurs in the free vibration part of the signal; these changes has small amplitudes compared to the generated signal. Their Order of magnitudes is 5~20 mV while the main signal is 20V! As they always exist, at least as a source of noise, they should not be ignored furthermore, we may find a way to use them. At the moment there is no way to use amplifier to scale them up; the only way to monitoring them is by scaling the vertical axis on the oscilloscope.

Does changing the scale has effect on the values or just on appearance?

At arbitrary frequency of 19 kHz two measurements are performed; in one of them vertical scale of 20mV is used and in the other 10mV. (data set:Transmitter-1-49kHz.csv). plotting the results on one graph shows just the scale is changed and amplification is not occurred (focus on free vibration part at the right hand side of the figure):

The pattern of the wave at the right hand side of the next figure differs from its left hand side; it clearly shows oscilloscope receives the signal from the transmitter after the main signal from function generator stops.

Two set of tests are performed, just using a transmitter, an oscilloscope and a function generator (as in the above scheme). Applied voltage is set to 20V and sine pulses with different frequencies are applied. In the first set of tests even frequencies are applied while in the second set odd frequencies are used. There is no difference between even and odd tests except experience in performing a better test. Range of the applied frequencies was 1~50 kHz. Max. amplitude of the signal in free vibration part is detected. In some cases, maybe when excitation frequency is close to the natural frequency of the bender element, non stability is detected in received signal; in these cases by zooming out on the vertical axis, the problem was solved. Frequencies of 16 and 32 kHz in the first data set and 17kHz in the second data set can be named.

These are the max. amplitude in free vibration part, versus applied frequency in two separate set of tests:

Discussion:

  • Grapphs are not matched exactly; it is possible that two different benderes have been tested otherwise, the approach is unreliable.
  • General simmilarity of the graphs shows even if two benders have been tested, they have been very similar ones.
  • Repeting the test with the same frequency have caused various results (vertically aligned dots); it means that measurements may not be reliable.
  • Magnitudes are very low (suppose 0.01 Volt). It is just 0.01/10=0.1(%) of the input signal! In the absence of the amplifier, it is not a high quality data.

Fourier analysis:

According to the above figures two natural frequencies of the transmitter can be concluded. Does it means that in Fourier analysis the majority would be these natural frequencies?

These are the results of the fft analysis on tail of the signals. Although low frequencies has majority but the resonant frequencies are not detectable.

Discusion:

  • Results do not approve the concluded natural frequency for benders.

functions

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  1. Code: Transmitter-01-02-2023.Rmd↩︎