Gas station without pumps

2016 February 12

Ultrasonic rangefinder without amplifier

Filed under: Data acquisition,freshman design seminar — gasstationwithoutpumps @ 21:20
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I’m still working on doing the ultrasonic rangefinder project  myself, though I have already determined that it is feasible for the freshmen in the design seminar. In More testing for ultrasonic rangefinder, I gave a digital filter and showed it working with a non-resonant electret microphone and amplifier and pings from a Maxbotix rangefinder.  In Ultrasonic rangefinders arrived, I looked at generating good 40kHz pings while listening to the direct sound with a microphone and amplifier.

Yesterday and today, I tried setting up a rangefinder without an amplifier, using the resonant piezoelectric receiver, instead of the non-resonant microphone and amplifier.

This rangefinder has no amplifier, just a high-pass RC filter to block 60Hz and audio interference.

This rangefinder has no amplifier, just a high-pass RC filter to block 60Hz and audio interference.

I used this circuit in lab today to demonstrate the use of the digital oscilloscopes. We looked at the signal to the transmitter and the signal back from the receiver. I showed them the use of an extra channel (and an extra digital output from the Teensy board—pin 13 to flash the on-board LED) to trigger the sweep when the burst signal started.

If I make the initial burst loud enough (which requires making it last longer, with more cycles to build up the resonance), I can see the return echo even with no amplification:

The signal-to-noise ratio is poor, but the echoes from 1, 2, and 3 round trips are clearly visible at around 4ms, 8ms, and 12 ms.

The signal-to-noise ratio is poor, but the echoes from 1, 2, and 3 round trips are clearly visible at around 4ms, 8ms, and 12 ms.

The delays are a little longer than I would have expected, but I don’t know whether this is because of sloppy measurement of the distance, failure to check the temperature and relative humidity and compute the speed of sound more accurately, or longer ringing than expected and not seeing the beginning of the burst. All probably contribute to the error.

We can zoom in on the raw recorded signal around the first echo:

The zoomed-in raw data shows background noise around ±4mV and the first echo around ±12mV.

The zoomed-in raw data shows background noise around ±4mV and the first echo around ±12mV.

It is not entirely clear where the echo starts—I’m guessing around 4020µs, when I would have expected it closer to 3880µs.  That would be about a 2.4cm error in measuring the distance to the table, which is possible, but a larger error than I would have expected (the bottom of the table is not smooth, but the fluctuations in distance are 1cm, not 2.4cm).  The 3.6% difference in time corresponds to a very low temperature—maybe –30°C, and my bedroom is not that cold! (It is probably around 17°C, for a speed of sound around 342 m/s, which would suggest a delay of 3860µs, even earlier than the 3880µs I estimated when using 340m/s as a crude estimate of the speed of sound.

The second bump around 4800–5200µs is probably the vibrations from the mode change that we saw in the transmitter recordings in Ultrasonic rangefinders arrived.

The digital filtering improved the signal-to-noise ratio, from about 3:1 to about 6:1 for the first echo. Amplifying the signal from the receiver before digitizing it should improve the signal-to-noise ratio, since much of the noise we’re seeing is the noise of the analog-to-digital converter—using more of its range should clean the signal up a lot.

3 Comments »

  1. […] project , More testing for ultrasonic rangefinder, Ultrasonic rangefinders arrived, and Ultrasonic rangefinder without amplifier, I tried today to look at creating pings with a non-resonant transmitter: a cheap, mid-range 8Ω […]

    Pingback by Ultrasonic rangefinder with loudspeaker | Gas station without pumps — 2016 February 17 @ 23:30 | Reply

  2. […] rangefinder project , More testing for ultrasonic rangefinder, Ultrasonic rangefinders arrived, Ultrasonic rangefinder without amplifier, and Ultrasonic rangefinder with loudspeaker I tried measuring the (magnitude of) impedance of the […]

    Pingback by Ultrasonic transmitter impedance | Gas station without pumps — 2016 March 13 @ 13:00 | Reply

  3. […] good 40kHz pings while listening to the direct sound with a microphone and amplifier. In Ultrasonic rangefinder without amplifier, I tried seeing if we could use the transducers without an amplifier. In Ultrasonic rangefinder […]

    Pingback by Ultrasonic rangefinder tests with tiny loudspeaker | Gas station without pumps — 2016 August 18 @ 23:31 | Reply


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