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Slowly Picking Away

Work’s been quite busy the past little while, so I haven’t been able to get as much machining done as I’ve wanted to — That’s been additionally hampered by needing a new shaft coupling for the Y axis on my mill, which is currently on its way.

A few weeks ago, I did get enough time to take the prototype bulkhead ring for a spin on the lathe, and played around with the feeds/speeds quite a bit. I think I’ve got things somewhat dialed in for the facing/boring steps, and as expected I need some fairly light cuts to be able to get to ideal cutting speeds. Still having some trouble getting good chip breaking, so not perfect yet. Face boring proved to be quite difficult — I’ve found a speed that doesn’t stall the machine, and doesn’t sing too badly, but definitely far from ideal.

Experimenting with the CNC Lathe
Experimenting with the CNC Lathe

I’ve been putting some thought into the sonar payload itself, primarily with how I’m going to manage record all the data. Luckily the sonar bandwidth is very small compared to that of a Synthetic Aperture Radar so the data rates needed are orders of magnitude lower, but some very preliminary estimates are around 85 Mbit/sec at a worst case for I+Q samples from a 2×12 element interferometric array on either side of the sonar. For an hour’s worth of imaging, that’s nearly 40 GB of data to store! I could optimize the data rate by something like BAQ compression, but I think that rate should be manageable without compression.

An SSD would clearly fit the bill for storage, but could potentially be a pain to get the data into directly from an FPGA. I did a bit of searching and found the Inforce 6540 single board computer, which offers a Snapdragon 805 processor, both USB 3.0 and SATA ports, and a Linux image. I could potentially route the data from the ADC array through an FGPA into a Cypress EZ-USB controller into the ‘6540, which could then write to an SSD with a proper file system — I could then connect the SSD to my computer at home to download and process the data. I’ll dig around a bit more to see if the processor can handle the sustained data rate over SATA, but this seems to be a good candidate for a mission computer to handle sonar and potentially even future camera payloads.


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