Saturday, December 17, 2022

A new probe, probe-ably

Among the things I'm working on these days is a new probe. The current probe, if you recall, looks like this:

It works pretty well, but it has a number of important disadvantages:

  1. It is large and heavy.
  2. The brass static pressure probe is unwieldy.
  3. The sheer amount of labor required to put it together is immense!
I didn't really anticipate this last factor, but it's important! So -- I am now working on a new one. It will look something like this:

Note how I have preserved the "thru-bolts" assembly, which has worked very well for me in the past, but I have tried to integrate the spacers and other doo-dads as part of the 3D printed pieces, to save time and hassle. Some important attributes of this probe are:

More compact. I am cramming everything into a 2-sided, 4-layer PCB that fits inside the 2" diameter tube.

Less is more. I am eliminating the battery charging circuitry; it will be externally powered either from ship's power or from a flashlight battery holder. So far I don't even have battery voltage monitoring, but I might want to add that. We'll see.

Built-in adjustment. Since it is very light, it can have a series of simple built-in pivots allowing it to be adjusted as needed.

The above is not the entire design -- I have some thoughts about the mountings as well. But this is what I have drawn up so far. I will post more soon.
You might notice that the pressure hole scheme is different: There are 2 holes on either side of the "ball nose" instead of the bulky static pressure probe. I have performed a wind tunnel calibration of this configuration and I believe that, with the data I have, I can get good results. I will be sharing separately the story of my recent wind tunnel work. Stay tuned for that.

I have also been designing the PCB for this. In the process, I am moving from AutoDesk Eagle to KiCad, which is really exciting. The PCB work in progress is at:

One of the things I'm hoping to try is to add a 5x2 1.27mm pitch header to the edge of the board (look for J1 in the renderings), and use an Adafruit pogo pin clip for programming. The pogo pin clip would connect to a "programmer" board which would contain all the USB/serial logic and transistor network necessary for programming this thing with the standard Arduino utilities. Hopefully, this makes it easy to program, but also saves precious board real estate.

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