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Zynq-based Red Pitaya Open Instrument Platform + Ultrasound Xcvrs = micron-resolution caliper

Xilinx Employee
Xilinx Employee
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In the last couple of weeks, the Red Pitaya team in Slovenia has been playing with ultrasound. In case you’re new to the multi-talented Red Pitaya, it’s an open-source, programmable instrument platform with two 125Msamples/sec analog input channels and two 125Msamples/sec analog output channels. The board is based on the Xilinx Zynq SoC, which runs a variety of instrumentation programs ("apps") that turn the Red Pitaya into multiple instruments including an oscilloscope, a spectrum analyzer, and a waveform generator. (See “Zynq-based Red Pitaya Open Instrumentation Platform blows past $50K Kickstarter funding goal by 5x” for more details.)

 

 

Red Pitaya.gif

 

The Red Pitaya Open Instrument Platform

 

 

One of the intriguing features of the Red Pitaya platform is that it can implement multiple instruments at the same time, which is what the recent experiments in ultrasound-based distance measurements use as a foundation. The Red Pitaya team has been connecting inexpensive ultrasonic transmitters and receivers to the Red Pitaya’s analog outputs and inputs and they’ve used these inexpensive devices to make distance measurements with 1-micron resolution. I find that pretty stunning and nothing I’d have cooked up on my own with a Red Pitaya. (Note: It looks like they’re using about $5 worth of transducers and $100 worth of coaxial cabling, connectors, and adapters.)

 

The distance-measurement scheme uses some elemental physics. Based on a handy online calculator on the Georgia State University Web site, sound travels through dry, room-temperature air at about 347m/sec or 347mm/msec. A 40KHz sine wave has a period of 0.025msec. Multiply those two figures together and you get 8.675mm/cycle. So you can determine the distance between the ultrasonic transmitter and receiver by counting cycles of delay between the sourced and received 40KHz signals. Here’s a short video to demonstrate:

 

 

 

 

You see in this first video that the Red Pitaya team comes up with 8.5mm per 40KHz cycle using graph paper, which is within about 2% of the calculated value, plenty close enough for the experimental level we’re working with here.

 

This next video, published a week later, shows a refined technique using phase lag to measure differential delay between waveforms, which yields micron resolution (though perhaps not accuracy). First you see a pair of the transmitted and received waveform zero crossings aligned in time. Pay particular attention to the way the operator zooms the image by drawing a box around the portion of the waveform getting attention. That’s just the way you’d want to zoom waveforms in the 21st century. No more time-base rotary knobs and waveform-positioning verniers. Just outline what you want to see.

 

 

 

 

Really, the test setup is only slightly more refined in this second video. You can see that the ultrasonic transducers are fastened to the jaws of a micrometer-vise with double-sided foam tape and paper clips—to measure microns. Amazing!

 

A couple of zoom-adjust-zoom iterations closely overlaps the crossings. Then, small adjustments of the vise jaws cause the zero crossings to separate. The easily-measured time differential between the transmit and receive waveform zero crossings gives a differential measure of the spatial separation between the ultrasonic transducers with amazing micron-level differential resolution, as calculated by the Red Pitaya’s control software running on the attached laptop.

 

For more info, see the Red Pitaya blog.

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