I have an industrial application using an ultrasonic sensor to detect whether transparent film is present in a plastic ring. Ring minimum diameter is > 30mm. Sensing distance is 90mm. Sensor transducer looks about 10 - 15mm diameter - it's covered by a membrane so I can't measure easily. I don't know the frequency but let's assume 20kHz giving a wavelength of about 18mm.
Film is nominally at right angles to ultrasonic beam but I'm finding that when it's distorted the sensor doesn't detect. It's as though the sound is reflected off-axis by the angled film. I can't increase the sensitivity because I'll start sensing the background.
+-------------+ | | | Sensor ) ) ) ) ) ) ) ) ) ) ) ) ) ) | | +-------------+ |Target |Background
One option I'm considering is making an ultrasonic waveguide as sketched below.
+-------------+ ---------------- | | | Sensor ) ) ) ) ) ) ) ) ) ) ) ) ) ) | | +-------------+ ---------------- |Target |Background waveguide tube
The idea is that I could guide the wave to within 30mm of the target and better capture the echo and guide it back to the sensor.
My question is what tube diameter would form an optimum waveguide given the 18mm diameter wavelenght? Or is there a reason this is a bad idea? I'm also assuming a steel or hard plastic tube with smooth internal bore would improve my chances.
[Edit 2015-02-27] The manufacturer has replied, "Adding a sonic tube or choke to the sensor may help with the problem. It should have an ID of 12-14mm and no longer than 20mm with the current software configuration in the sensor."
+-------------+ ------ | | | Sensor ) ) ) ) ) ) ) ) ) ) ) ) ) ) | | +-------------+ ------ |Target |Background 20mm waveguide tube
Can anyone suggest what physics principle would be involved with this solution? i.e., Why would this work in preference to me extending the tube as close as possible to my target? Why would he refer to it as a choke?