Sunday, November 19, 2017

New sensor board in the making

I'm trying to be far more careful now about how I build up and qualify my boards. I got 6 boards made from Osh Park and fully stuffed one of them yesterday. Today, I'm doing a step by step qualification to make sure everything works well.

I attached a Fio V3 to the board with a temporary header and rubber bands (someone should make Cleco fasteners for 0.1 inch headers!!) and used a Sparkfun TMP102 breakout board I had lying around as a generic I2C device to test the pressure sensor slots (so I don't risk my actual, expensive pressure sensors).

First, I learned that unless I de-populated the pullup resistors and decoupling cap on the TMP102 breakout, the bus did not work properly. I suspect it was the pullups that were overwhelming the bus, given that my board already has pullups.

With that fixed, I was able to verify that each of the pressure sensor slots were interrogated correctly by my bus scanner and that the TMP102 showed up where I expected it to.

Here is a picture of my test setup, followed by a picture of the signals as digital and analog traces on the scope. You can see how each of my 4 I2C buses is lit up in sequence by my bus scanner program.

Turbulence at the top of my mast

I installed a little tuft to estimate the amount of turbulence at the top of my mast:

As you can see, it is in view of my video camera. The result of a test run on the freeway is here (and note also impact with a tree branch ... happily no harm done).

Here are some things I noticed:

1. The piece of copper wire holding the tuft in place is a little bit wiggly, which could be exaggerating the movement of the tuft.

2. That said, even when there were few road bumps and the wire was staying put, the tuft was wiggling around quite a bit.

3. It seems difficult to estimate the frequency of the oscillations but I'm going to suspect it's just broad spectrum noise.

4. If this is any indication of what things look like on a crowded runway, I'll have to do some serious digital filtering to basically reject everything except the bandwidth of interest, which is probably no faster than 0.5 Hz or so.

With that in mind, then, the following is my plan of attack:

1. Build up and qualify a reliable new probe.

2. Test it with the 60 mW XBee modules, pushing the sampling rate as high as I possibly can while still getting reliable comms.

3. Thus oversampled, now add a simple lowpass filter on the display.

4. Give myself a TODO item to learn about Kalman filters and implement a more proper filtering strategy later.

Sunday, November 12, 2017

Furl the staysails!

I made a mast to fit on my 2006 Toyota Prius to permit further testing. The mast is an 8 foot section of aluminum tubing; at that height, if there is any turbulence, then it is likely to also occur at takeoff and landing and should probably be digitally filtered. In any case, it's a cheap way to do tests.

The mast is made of an 8-foot length of 1" square 1/16" wall aluminum tubing. At the bottom, it is riveted to a large T-strap hinge which is attached to a piece of plywood which is in turn held onto the car's roof rack bar with a pair of U-bolts. I have some ropes with carbiner hooks attached to the four jacking points under the bumpers, and from these and the roof rack I have 4 stays made of paracord. The lines come off and the mast folds down for transport. I don't deploy it unless I know I'm on a stretch of road where I'll have clearance.

Here it is, stowed and deployed:

I drove the rig around today on a 45mph road and on the freeway, with a camera attached, to determine how it held up. Stability seemed good. I'm generally not worried about excessive vibrations.

Here is the 45mph run:

And, after gaining some confidence, this is the run on the freeway:

For a subsequent test, I'd like to make a dead-simple wind vane and stick it up in view of the camera to get a baseline for how turbulent the flow is up there and how that is affected by vehicles ahead of me on the freeway.

Sunday, October 29, 2017

My board eats chips

After a very frustrating time spent with the innards of Probey, I have come to a disturbing conclusion: My circuit board eats chips.

By which I mean, for some reason, the circuit I designed and built has been destroying the ($50 each, thank you very much ... did I mention nothing in aviation is cheap?) pressure sensor chips. Right under my very own eyes, one chip that was doing just fine and answering as it should at I2C address 0x29, after being plugged into the board to test the various sockets, just stopped working and did not respond to I2C messages at all.

I am ensuring the board is powered off prior to connecting and disconnecting things. BUT it may well be that the Fio V3 is not really powering off the buses when it's turned "off" so long as it has a battery connected, and perhaps the resulting discharge when parts are connected and disconnected causes them to fail. I am trusting the Fio to turn itself off while I swap chips with the battery connected. Maybe I should disconnect the battery before swapping chips.

Perhaps also the All Sensors chips are particularly sensitive to power transients and ESD? I don't know.

I put together an independent breadboard test rig to test the chips away from any other weird circuitry:

This rig is my determinant of whether a chip has been eaten or not. And in fact, again, a "good" chip showed up on 0x29 in the test rig and in the probe, then stopped responding in the probe, and back in the test rig was also not responding. So the probe board eats chips. Simple as that.

Probey McProbeFace taking shape

I finally got enough pressure sensors to fully populate the probe, and put it together. The one missing part is a more durable back cover that can be secured with a screw. The following photos show some of the tubing installed, and the Taoglas adhesive patch antenna with a Zip-tie for strain relief.

Sunday, October 22, 2017

Rough idea of new probe

This is a rough idea of what the new probe, with the new PCB and supports, will look like. Some dimensions and positions will change, but nothing major:

New board making progress

Today I got the new board put together with the 3D printed components. I have IC sockets for the pressure sensors so I do not need to solder them in. I have some more sensors on order to fill up the slots, and then hopefully we'll be ready for another flight test!