Sunday, February 16, 2020

Ideas for a next-generation probe

Losing the static probe

I have a friend who is an aeronautical engineer at nearby NASA Ames, at Moffett Field. He's been a great resource in helping me think about my probe. He has designed 5-hole and 9-hole alpha/beta probes for the NASA Orion spacecraft, so this sort of thing is not new to him.

Based on his feedback and my flight test data, it's clear I'll likely need to do some sort of calibration of the probe, based either on CFD simulations or wind-tunnel data, or a combination of both. Luckily, my Reynolds number range is not huge, so I should be able to get some pressure ratio calibration that will work for my entire range of dynamic pressures.

He has pointed out to me that, since I'm all in on calibration anyway, I don't technically need this bulky static pressure probe sticking out. I could measure any additional pressure between any of the holes on the ball nose. All I need is 3 pressures, to determine 3 unknowns (α, β, q). Once I determine q, I can subtract that from whatever the barometer reading is, from whichever probe hole I choose to connect it to, and get a "true" barometric reading.

Discovering the ESP32-WROOM-32U

While I was living under a rock, the world has gone all in on ESP8266 processors and Wi-Fi "Internet of Things" modules. The next step from there on is the ESP32-WROOM-32U which has lots of memory, can be programmed via the Arduino IDE, and comes with all the Wi-Fi I need. And it costs $4.50 per part. Yes, you read that right.

With all that memory, I can afford to embed the calibration tables in the probe itself, meaning that anyone can grab the probe, connect to Wi-Fi, and receive data sentences at 20 Hz in the form:



α = angle of attack
β = angle of yaw
q = dynamic pressure
p = barometric pressure
T = temperature

This means that people don't need to use "my" host Raspberry Pi software to get immediate, useable airdata.

A smaller form factor

Given all the above, it should be possible to fit the whole thing into a 1 inch diameter by about 8+ inch long housing, still with the transparent polycarbonate tube that is our trademark. :) If we add a USB interface, it should be possible for someone to get a probe, put it together, and then program it with the Arduino IDE themselves without having to take it back apart!

Recall we got ~ 12 hours of battery life with a 1 Ah LiPo battery. With a 3.2 Ah 18650 battery, regardless of how much power the ESP32 consumes to transmit on WiFi, it is very likely we could get similar if not better battery life.

The installation flexibility of such a small package would be awesome! You could simply zip-tie it to any sort of "curved" mounting ... we would not need to add any screw holes.

Wired installation

To install this in a wired fashion, we have an ongoing to-do item to prototype an RS-485 driver. With that and a standard R/C airplane "battery eliminator circuit" to step down the ship 12VDC to something near 5V, which we could then down-regulate to 3.3V, we'd have a wired system with the comms going over a single twisted pair. More on our experiments with that soon.

Of course you might ask: But the wired probe will not need Wi-Fi. Will you use a different microprocessor so that you don't waste all that Wi-Fi functionality? To which I answer: $4.50. I can afford to standardize on the part with the integrated Wi-Fi and just ignore the Wi-Fi, and it's still cheaper than most of my alternatives.

No comments :

Post a Comment