OSHW Bluetooth telemetry board discussion & spec-out

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Reinhard

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Did some quick math, here are the expected accuracies, drifts, etc

For the IMU I chose, the ICM-20602, the noise is 1mg(RMS). Therefore the velocity drift is expected to be at 0.0098 m/s/s. This is with a max readable acceleration of 16g

The only IMU I could easily find with more than a 16g limitation, the LSM6DSO32TR(with double the max readable acceleration, at 32g), has a 5.2mg(RMS) noise floor. The velocity drift is 0.0529 m/s/s.

Here's a table with the expected tolerances, accelerometer alone

Seconds since armingICM-20602 Velocity ErrorApogee ErrorLSDM6 Velocity ErrorApogee Error
60 s0.588 m/s17.64 m3.174 m/s95.22 m
120 s1.176 m/s70.56 m6.348 m/s380.88 m
300 s2.94 m/s441 m15.87 m/s2380.5 m
600 s5.88 m/s1764 m31.74 m/s9522 m

I'm really tempted to go without a barometer if the ICM-20602, at least for the small flights I'm most likely to go for that's a perfectly fine velocity/apogee error for me.
Noise is not much of a problem, because you integrate your accelerometer data. The primary concern is nonlinearity and maybe things like offset drift.

Your table bases the error on "Seconds since arming". You can compensate your sensor offset right until liftoff, or at least keep the altitude and velocity values fixed to zero before liftoff is detected. This will reduce your errors significantly.

Reinhard
 

gtg738w

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For the FlightSketch, is there any chance that the software could be tweaked so that the acceleration data graph is visible on the PC rather than just on the phone?
Yes, The flight log page has been updated to match what is shown in the app. They now use the same chart package so everything looks familiar. The updates are tied to a couple of other features that are still in work so it may be a bit of a delay before it gets pushed to the main server. All of the existing flights have the data stored so they will be updated too.

From my perspective if you're going to implement remote arming you'd be better off using wifi instead of bluetooth. Same frequency, longer range, still very small form factor.
From what i've tested I don't think range is a factor. The trade is a MUCH lower power consumption for a slightly slower data transfer rate with Bluetooth. I'll have to let the hardware speak for itself but for what I fly I'd much rather have Bluetooth. Battery size is a much bigger factor for weight and volume than the altimeter itself.
 

Gip-Gip

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Finally got most everything assembled and noticed I somehow lost my AVR ISP programmer. :mad:

Still waiting on the IMU, that's on backorder due to chip shortage and all that, but I have everything else assembled and ready for latency testing, among other things...

IMG_20210504_152553260_HDR.jpgIMG_20210504_152611450_HDR.jpg

Even without programming I've figured out a few things I should change. First, As you can see, almost everything except the programming pins fit inside the gnome pictured, which I'm fairly impressed with. I could honestly trim them down and see how that goes, but I would also like to see if there is another way to connect this to a programming interface, perhaps like a card-edge style connector or something of that sort

Second, I wasn't sure how I would accomplish double sided pcb reflow soldering until I figured out you simply use a high temp solder paste on the first side and a low temp bismuth paste on the second side. With this knowledge I could move all the non-heat-sensitive passives to the back side of the PCB and all the ICs to the front.

Third, putting VIAs on the solder pads of the RN4871 is a bad idea, as all the solder flows to the ground/power planes. I should ignore KiCad's keepout zones for the part and stick to the manufacturer recommendations instead

Within the next week I should have the programmer and I'll be able to test the I2C latency of the RN4871. What I will most likely find is that the latency will probably be way too high for anything usable, and I will most likely have to either deal with a lower target sample rate, probably closer to 100hz, or worst case redesign the PCB to make the ATTINY84 the I2C controller instead of the RN4871.

Meantime, I'll either reorganize the repository and work on code standards, or just relax and wait for the programmer to come in
 

jderimig

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Kinda late to this thread but you may want to consider the ICM20601, it has 32g range and maybe footprint compatible, i haven't checked.
 

Gip-Gip

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Kinda late to this thread but you may want to consider the ICM20601, it has 32g range and maybe footprint compatible, i haven't checked.
Not late at all, still prototyping as far as things go. Also appears I don't have an option, as the ICM20601 is in stock and the ICM20602 has a 26 week lead-time

the noise is about 4x worse, meaning the drift will be pretty bad, but whether or not that truely matters will be down to testing

quick look at the data sheet shows the pins are identical
 

jderimig

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The noise difference will not be relevant for your application. It gets integrated out when you process the signal. We are using it a project quite a bit more exacting than yours, it will be fine. Gaussian noise is good, it gives you more resolution when you oversample.
 

Gip-Gip

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The noise difference will not be relevant for your application. It gets integrated out when you process the signal. We are using it a project quite a bit more exacting than yours, it will be fine. Gaussian noise is good, it gives you more resolution when you oversample.
Thanks for the suggestion!

Out of curiosity, what projects are you working on at the moment?
 
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