Gas station without pumps

2016 March 13

Ultrasonic transmitter impedance

Filed under: freshman design seminar — gasstationwithoutpumps @ 13:00
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Continuing the series  Ultrasonic rangefinder project , More testing for ultrasonic rangefinderUltrasonic rangefinders arrivedUltrasonic rangefinder without amplifier, and Ultrasonic rangefinder with loudspeaker I tried measuring the (magnitude of) impedance of the transmitter in the same way that I measured loudspeakers.  That is, I put the transmitter in series with a resistor, and drove the pair from a function generator, measuring the voltages across the transmitter and the resistor simultaneously with a pair of voltmeters.  I did this in the circuits lab, so that I could use the high-quality multimeters and function generators there—I no longer have a multimeter at home that is good at high frequencies.

I did not have my laptop handy while I was making the measurements, so I recorded them on paper and typed them up later.  It would have been good to have plotted the data as I collected it, so that I could get more points in  regions where there was rapid variation in impedance.  I’ll probably have to go back to the lab later this week and collect another data set, with much finer spacing between 30kHz and 70kHz.

The transmitter behaves like a capacitor of 1.76nF at low frequency, and 1.36nF at high frequency. Near the resonance, things get complicated. Unlike the inductive loudspeaker, where impedance goes up at the resonance, the capacitive transmitter has decreased impedance at the resonance. [Update:2016 March 15, fixed the capacitance from µF to nF.]

I used a 100Ω series resistor, arbitrarily chosen.  In retrospect it would have been better to use a 2.2kΩ resistor, to more closely match the impedance of the transmitter in the region of interest between 30kHz and 70kHz.

There isn’t a single resonance at 40kHz, like I expected, but three resonances (maybe more), around 39.2kHz, 42kHz, and 55KHz.  There seems to be an antiresonance around 43 kHz.  The multiple resonant frequencies are probably due to different vibration modes, and may account for some of the difficulty in getting a short burst produced by the transmitter—even if I can cancel one mode, there is another mode still being somewhat excited.

The first resonance is around 39.2kHz, which is probably in spec (not that these cheap transmitters have data sheets that I can access—more expensive ones are generally 40kHz±1kHz for the resonance).  At the resonant frequency, the impedance is about 610Ω, so a ±3.3V swing would require ±5.4mA.  (Note: since my measurements were not very closely spaced here, I may have missed the peak, which could be as low as 500Ω.)

The digital output pins of the Teensy 3.1 board I’m using for the rangefinder are only specified to 2mA (except a couple of high-drive pins, which I’m not using).  Using inverters with 5V power to drive the transmitter would need  8.2mA, but the inverters I have are only specified to 4mA, so I might want to use two or even three in parallel for each pin of the transmitter.

Of course, the measurements I made were for steady-state frequency response, not the response for starting the transmitter moving from being motionless—there may well be a different impedance that is relevant then (I suspect that more current may be needed when the transmitter isn’t already moving at the resonant frequency).

1 Comment »

  1. […] I had to spend a lot of time in the lab today, I decided to redo the work mentioned in the post  Ultrasonic transmitter impedance. That is, I tried measuring the (magnitude of) impedance of the transmitter at different frequency […]

    Pingback by Ultrasonic transmitter impedance | Gas station without pumps — 2016 March 15 @ 23:28 | Reply

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