Electronic – Ultrasound in air ranging system design

frequencyrange-detectorsignal processingultrasound

I'm designing an air ultrasound range finder that bounces ultrasound signals off a person's head to measure the distance of the device from the head. The goal is to get ~1mm accuracy as a device is actively being moved away from a person's head (range of distances is 0mm to ~250mm) so I need a pretty good update rate (>30 Hz). Of course, there are also some issues with the reflectance of the scalp and of the occlusion caused by a person's hair.

  1. How do I determine the best frequency transducer to use? I was thinking 40 kHz due to the low size components and the market availability.
  2. Should I go for narrow beam or wide beam? I think narrow beam would have better reflectance, but wouldnt they only work for specific ranges?
  3. When I mount these transducers onto this device, do they need to be at an angle relative to each other to optimize the reflection? Or can their transmit axis just be normal to the same plane?
  4. What kind of signal processing should I use? Obviously low bandwidth ultrasound transducers will have a fairly broad waveform, so I think correlation won't work too well. But will a threshold method be accurate enough? Maybe there are other methods I haven't considered?

Best Answer

The goal is to get ~1mm accuracy

Wavelength is determined by speed and frequency. Speed is approximately 340 m/s and therefore wavelength is 8.5mm.

So what you may ask. Any standing waves you might get will occur every 8.5mm and these could ruin you expected accuracy of 1mm.

You may then point out that you will use a pulse driven into the transducer. The 40kHz resonators I've come across are very "resonant" and generating a pulse may not be that easy.

I'm saying these things because I think you need to take them into account.

A narrower beam seems logical to me or else there could be several reflections from different objects coming back and obscuring your desired distance measurement. Also remember that narrow beam devices can still produce/be susceptible to side lobe interference.

As for your other questions I think you need to determine what you want to transmit before you think about signal processing.

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