0
\$\begingroup\$

In ultrasonic distance measurement a train of pulses is sent using typically a 40Khz piezo transducer. After some time the same piezo transducer is used to receive the pulses and distance calculated by Time Of Flight.

https://www.jtaibwing.com/index.php?main_page=product_info&products_id=367861

The pulses are generally created by MAX232 to generate a high voltage of 10V needed to generate pulses of sufficient intensity

I was wondering if just 1 pulse could be used. Instead of 10V, we could use say 20V to generate a single pulse. On way back, just detect that 1 pulse.

\$\endgroup\$
4
  • \$\begingroup\$ Maybe it could. But how it would be simpler to detect a single pulse from noise, instead of detecting a tone burst of 40 kHz which provides redundancy to minimize errors? \$\endgroup\$ – Justme Mar 8 at 17:06
  • \$\begingroup\$ A train of pulses need pattern matching. A single pulse does not. A single pulse could be falsely triggered so use a higher intensity pulse to improve signal to noise ratio. The receive circuit could be used to just stop a h/w timer in an MCU so its certainly easier than pattern matching at high speed in MCU. \$\endgroup\$ – wonderfuliot Mar 8 at 17:08
  • \$\begingroup\$ IMO, the best way is to use a PRBS sequence modulated carrier of 40kHz. Then use cross correlation to detect target and eliminate the dispersed echo. \$\endgroup\$ – Marko Buršič Mar 8 at 18:12
  • \$\begingroup\$ There is no train, and there is no pattern. It is a burst of a single-frequency sine wave. Because the transmitted burst is at one specific frequency, a simple bandpass filter can separate it from background noise way better than an impulse, which the world is full of. \$\endgroup\$ – AnalogKid Mar 8 at 18:17
3
\$\begingroup\$

I was wondering if just 1 pulse could be used.

  • Trying to emit a single ultrasound pulse from a highly resonant piezo transducer will be impossible. Try it and see. To do so you need a wideband transducer and, as far as I'm aware, they are not common at 40 kHz.

  • Trying to get a highly resonant piezo transducer receiver to work with a single incoming ultra sound pulse will be problematic to say the least.

Piezo transducers are highly resonant. If you have a decent data sheet that has got a model of the equivalent circuit, this can be simulated very easily.

Here's some data from a Murata transmit/receive pair (redlines added by me): -

enter image description here

If you then modelled it with an RLC and fed it a step or pulse you would seen the transient response graph oscillating with a general exponential decay like this: -

enter image description here

Picture above taken from this interactive RLC bandpass calculator. Notice how in the lower graph that one step of voltage as an input produces a succession of decaying sinewaves.

And this is just from what is produced in the air from the transmitter. The combined effect of transmitter and receiver will have approximately double the Q factor and the received sinewave will take even longer to decay.

That's why many cycles are used to activate the piezo transmitter rather than a single pulse. But, if you can find a wideband transducer, the Q factor will be a lot lower and a single pulse is more attainable but, the downside is that the receiver sensitivity falls through the noise floor unless you use high power transmit pulses.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.