My little altimeter

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Adrian A

Adrian A

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A couple of months ago I decided to see how small I could get a altimeter/accelerometer/locator beeper. A picture is attached. It's small enough to fit into a BT-20 (17 mm) nosecone with just the USB-mini connector and the ON/OFF/mode switch exposed.

I'm still in the middle of programming the microcontroller, but I just got the board to spit out all the analog telemetry through the USB connector, so I think it's time for a birth announcment. :D


Here are the features:
  • Mass is 8.6 grams, including battery. I believe this makes it the lightest altimeter with battery, other than some un-advertised versions of the picoalt.
  • Barometric altimeter. Resolution: ~3 feet/count at low altitude. Range: 30,000+ ft. 100,000+ ft as an option
  • 2-axis accelerometer. Range: +/- 35 or 70 G in thrust axis, +/- 17 or 35 G in lateral axis. ~0.03 G/count resolution
  • Chip temperature. Resolution: ~0.1 F
  • Battery voltage
  • End-to-end temperature compensation for all analog measurements
  • Included Li-poly battery never needs replacing
  • USB interface for data interface and battery charging. No separate $30+ transfer board required.
  • Safe Li-poly charger built-in, includes current and voltage limit
  • 64k EEPROM for flight data. (updated:Current rev has 16 Mbits flash memory!)
  • 5 flights recorded separately. Recorded data rate variable by flight mode.
  • Beeper for locating, launch countdown, and mode feedback.
  • 3 mounting holes
  • 5 LEDs, including 3 that operate in flight for night flights
  • Pushbutton switch for on/off and changing modes
  • USB drivers and Excel spreadsheet for analysis
  • Bare-bones Apogee deployment (requires separate battery & arm switch)

I designed this with the idea in mind of using it for low-powered rocket altitude competitions, so it friction fits into a BT-20 nosecone, and one of the mounting holes can be used to attach the streamer/parachute cord.

[updated]
The 2 axes of acceleration measurement can be used to record the post-burnout drag for Cd calculations, and the dynamics of the launch rod interactions, deployment, and landing. The actual thrust curve of the motor can be reconstructed using the velocity-dependent Cd. In some circumstances, the 2nd axis can provide roll rate information.

When I get the first boards working and flight-tested I'll offer them for sale officially. I'm thinking of offering it around $100. If all goes well, I'll make some tube adapters for different tube sizes and I'll bring these to the Northern Colorado Rocketry event in the first week of June to do some free demos.
If there are any software features people are interested in, let me know. [strikethrough] I have a couple of spare I/O pins, but no way to get them in this rev of the board without soldering directly onto the TSSOP lead. [/strikethrough]

I have quite a few bare boards, and I can populate them at home according to demand. I've run across a number of board errors, but nothing I can't work around, so far. If there is enough interest, I'll re-spin the board to eliminate the haywires, and I may even get the surface mount assembly done professionally if I think I can sell enough of them.

[Update: Yes the Parrot altimeters are for sale. The latest specs, user's manual, comparison, and example flight data are available at www.featherweightaltimeters.com
 
VERY nice :D

Now I really wish I could make it to the June launch...


Any chance of bringing it in July as well?
 
Thanks. As for the July launch (July 7), the official household scheduler (not me) says "I suppose." Woo hoo!

At the last event, I arrived at 3:30 on Saturday, just when some of the last people were breaking down and pulling out.
 
Nice :D

I'll look for you at the July launch - the June launch perfectly coincides with the day of my high school graduation :rolleyes:
 
Originally posted by cjl
Nice :D

I'll look for you at the July launch - the June launch perfectly coincides with the day of my high school graduation :rolleyes:
Click to expand...

just think, It will be the last day school will interfere with a launch :) :D (congrats BTW)

Ben
 
Originally posted by ben
just think, It will be the last day school will interfere with a launch :) :D (congrats BTW)

Ben
Click to expand...

College isn't school?
 
Does the locator beeper start after the apogee has been reached, or does it continually sound even while it's still on the pad waiting to be launched?

WW
 
Nice alt. you got there. That's amazingly small.:)
 
Does the locator beeper start after the apogee has been reached, or does it continually sound even while it's still on the pad waiting to be launched?
Click to expand...

The design at the moment has it beeping at 1 Hz (countdown metrononome) until liftoff is detected, so that you can tell it's ready for the G detection. If this is an issue I could probably have a user-controlled setting to disable that feature. The beeper is driven by the microcontroller, so I have control over the frequency. Away from the beeper's resonance frequency the tone is considerably quieter, and I'll save the resonant beeps for the locator function. Battery-wise, short beeps at 1 Hz shouldn't be an issue for 10s of hours.

could you do one for DD?
Click to expand...

I'm thinking about it. Once I get the software on this one just the way I want it I'll start looking into what I would have to do for DD. Certainly I would need a larger and/or separate battery or cap for the charge. There are safety inhibits and continuity checks I haven't worked out yet. There was a thread recently with a lot of good information on this topic that I'll check out when I get there. I'm currently intrigued by making my Blue Ninja a 2-stage rocket with an air-lit upper stage and full recovery systems on each stage. A friend of mine is also working on a CLV/Orion model and I've gotten him thinking about making the second stage an air-lit stage as well.
 
Nice alt. you got there. That's amazingly small.
Click to expand...

Thanks. Anyone have a suggestion for an easy way to get access to an accurate balance? After a reasonably exhaustive search, the only other altimeters I've seen that can fit in a BT-20 tube are from Adept and Apogee, and both are heavier, with battery, than this one. I'd like to get an accurate mass, because I think this may be the world's lightest recording altimeter.

Also, anyone have a suggestion for a name? I've made a few flyers with the title "Featherweight Altimeter" but I'm open to suggestions. 50% off for someone comes up with a good one!
 
I got a scale like this on eBay for measuring epoxy:
https://snipurl.com/1kdue

Just search for .1g digital scale and there are plenty in the $10-20 range that go up to 500g. Most of them are made in China, but they are good enough for most uses.

(try to find one with cheaper shipping though.)
 
the board layout is impressively compact. A few issues though:

There's not much point in having both acceleration and pressure measurement unless you are doing deployment or ignition. The pressure readings should give much more accurate altitude data than the accelerometer, so the accelerometer would only really be useful for detecting liftoff and burnout precisely.

You're going to want a better way to mount the battery. That doesn't look very sturdy.

Also, what's the sampling rate? 64K doesn't sound like a lot if you're sampling three (or four?) values with high resolution.
 
Originally posted by Adrian A
After a reasonably exhaustive search, the only other altimeters I've seen that can fit in a BT-20 tube are from Adept and Apogee, and both are heavier, with battery, than this one. I'd like to get an accurate mass, because I think this may be the world's lightest recording altimeter.
Click to expand...

Robert DeHate at PicoAlt makes a range of tiny recording/non-recording altimeters with or without deployment.
 
The PerfectFlite ALT15K is also a small (BT-20) recording altimeter. It weighs in at 15 grams with battery so not nearly as light as yours.
Click to expand...

Yes, this one appears to be the one I saw at the Apogee website. I think I got the impression that Apogee designed or produced it, but maybe they just sell it for them?

Robert DeHate at PicoAlt makes a range of tiny recording/non-recording altimeters with or without deployment.
Click to expand...

Yes, cool stuff there. I like the pico P1. Nice and simple and cheap.
 
There's not much point in having both acceleration and pressure measurement unless you are doing deployment or ignition. The pressure readings should give much more accurate altitude data than the accelerometer, so the accelerometer would only really be useful for detecting liftoff and burnout precisely.
Click to expand...

True that the accelerometer doesn't really help with the altitude measurement, but it's fun, the data is interesting and useful in other ways. Probably the most useful function is that it provides a very direct measurement of the rocket's Cd after burnout. If I were tweaking my rocket for maximum altitude, this would let me see evaluate my progress in that. How much does filling in the spiral grooves really help? Or sanding the fins?
I'm also looking forward to measuring the acceleration of the nosecone during the ejection. Recently I snapped 4 20-lb strands of steel beading wire that attached a very light piston to the rocket on a C11 ejection. How close am I to breaking the nosecone attach cord on my other rockets?

You're going to want a better way to mount the battery. That doesn't look very sturdy.
Click to expand...
The battery is epoxied onto the board, and the battery terminals are soldered. :D It never needs to come off because it gets recharged whenever you plug it in with the USB connector. In the picture, the board with the battery attached was a failed 1st try at solder reflow, so I taped off the battery leads rather than connect them to the board.
 
Also, what's the sampling rate? 64K doesn't sound like a lot if you're sampling three (or four?) values with high resolution.
Click to expand...

This part of the software isn't done yet, but here are some example calculations:

Pre-launch through 2G detection: X and Y acceleration at 40 Hz, stored in RAM in a 1-second circular buffer. No EEPROM write cycles while it's waiting on the pad.
G detection through G+10 seconds: X and Y acceleration at 40 Hz. Pressure at 10 Hz. Temperature, battery voltage, and voltage reference at 1 Hz.
G + 10 seconds for TBD minutes: Pressure, temperature, battery voltage, and reference voltage at 1 Hz for TBD minutes.

With 2 bytes per measurement channel, that works out to 1600 bytes for acceleration and 280 bytes for the other measurements during the initial high-rate portion. Then if I have 4 minutes of the 1 Hz data, that's 1920 bytes. Altogether each flight would need about 3.8 kbytes, and the serial EEPROM I'm using has 8kbytes (64kbits), so there would be room for 2 flights. Taking out the reference voltage and spacing out the post-launch measurements more, maybe I could set it up for 3 flights. I was looking at EEPROM the other day, and there are other options for 128k and 256k in the same small package size, but the pin-outs are different.:mad: The other parts are only about $.50-$1.00 more expensive, so I'm kicking myself a bit I didn't design for them. Another board update into the rev-2 bucket.

If there's enough interest that I'm confident I'll make more than a handful, rev 2 will come sooner rather than later so that I don't have to do the haywires each time.
 
Originally posted by Adrian A
If there's enough interest that I'm confident I'll make more than a handful, rev 2 will come sooner rather than later so that I don't have to do the haywires each time.
Click to expand...

Hi Adrian:
Cool project and I'm very interested in Rev2, depending upon the cost, that is! I didn't see a ballpark price for the hardware. I might be able to help out with the software if you sell a developer's version.
Best wishes,
Will
 
I'm thinking about selling it for around $100.

You can get a Zilog development kit directly from Digikey for $40, and it includes a complete IDE and C compiler and libraries. I made a simple interface board on perfboard that provides the 3.3V power and 6-pin interface needed by the Zilog programmer. I can send you a plan for it if you solder, or sell one ($30?) if you don't. I'll send you the source code and the schematic if you agree to share the code you come up with. I'm a systems engineer first, an electronics designer second, and a C coder last, so any help would be appreciated! :)
 
Originally posted by Adrian A
I'm thinking about selling it for around $100.

You can get a Zilog development kit directly from Digikey for $40, and it includes a complete IDE and C compiler and libraries. I made a simple interface board on perfboard that provides the 3.3V power and 6-pin interface needed by the Zilog programmer. I can send you a plan for it if you solder, or sell one ($30?) if you don't. I'll send you the source code and the schematic if you agree to share the code you come up with. I'm a systems engineer first, an electronics designer second, and a C coder last, so any help would be appreciated! :)
Click to expand...

Hi Adrian:
Count me in for one of those at $100. I can solder, so plans would be lovely. I'm a C coder first, a systems engineer second, and an electrical designer third so maybe this will work out! :)

I haven't done a project on sourceforge.net yet, but maybe that is the watering hole for an open source project like this?
Best wishes,
Will
 
Hi Adrian,

What about to use those free pinouts to use on dual event (drogue and main) mode?

With this feature, it's become a flight computer more than a simple altimeter! ;)

Regards,

Marcelo Hacker
 
I haven't done a project on sourceforge.net yet, but maybe that is the watering hole for an open source project like this?
Click to expand...

This seems fine, based on some of their documentation. I haven't gotten around to setting up a project there for this, but I will.

What about to use those free pinouts to use on dual event (drogue and main) mode?
Click to expand...

I'm getting pretty tempted to have a go at this. I've been working some on what changes I want to make to the layout, and I'm getting motivated to plunk down another $150 for my next set of prototypes, even without attempting DD. And if I do that, for about the same money I could add some board area for a 2-channel dual deployment with safing and continuity/resistance checking, and connectors. With the extra parts, DD version would not be able to fit inside an 18mm tube unless I made it really skinny (and probably not even then, considering the connector height) but I could probably still get it into a 24 mm or 29 mm tube, depending on whether I mount the battery on-board. It could be smaller than any of the DD altimeters I've seen so far. If I go this route I'll also update the no-deployment, 18 mm board in the same run.

In any case, this time I'll at least have a via or an SMD resistor attached to each of my signals so I don't have to probe around on TSSOP leads any more!

In the meantime, I'm still shooting to have a functional set of software for the original rev1 board in time for the NCR June launch. Too many fun things to distract me from work!
 
I designed a Dual event altimeter pretty much just like that that measures about 1.1" x 3.0" (doesn't use mini USB). I haven't built it yet, though, since I'm working on other projects. As you said, it doesn't seem feasible to fit it into an 18mm tube, no matter how long you make it.

And for probing, throwing in a little via or plated hole is always enough for me.
 
Originally posted by Adrian A
And if I do that, for about the same money I could add some board area for a 2-channel dual deployment with safing and continuity/resistance checking, and connectors. With the extra parts, DD version would not be able to fit inside an 18mm tube unless I made it really skinny (and probably not even then, considering the connector height) but I could probably still get it into a 24 mm or 29 mm tube, depending on whether I mount the battery on-board. It could be smaller than any of the DD altimeters I've seen so far.
Click to expand...

What about leaving the altimeter control board the same size and having a connector to run a cable to a separate "loads handling" board? That board would have the stuff to handle pyrotechnics. It could have the connections for a separate load driving battery.
 
What about leaving the altimeter control board the same size and having a connector to run a cable to a separate "loads handling" board? That board would have the stuff to handle pyrotechnics. It could have the connections for a separate load driving battery.
Click to expand...

That's an option, though one of the trickiest parts about this is getting the signals from the spare microcontroller pins out to the same location, given all the other signals I have packed in there. Also, connectors will be the biggest thing on the board, other than the battery. That's why the connections for the programming interface are just some plated holes.

I was thinking about laying out the DD part of the board next to the original, with enough space between to cut the DD part off if not needed. Then I realized that with the way I'm doing my own PCB panelization (40 little boards per panel, at with the current design), I can put different boards on the same panel and all it would cost is panel area. If I adjust the layout of the original functions for the DD version, I think I can get a more optimal design overall.

Last night I read through the basilj threads (thanks you guys for all the good information), and I'm still tempted to use 1 analong input to measure the igniter resistance and look for shorts on the output FETs. I'm thinking of having 3 inhibits to the pyro circuits: A high-side mechanical switch, that when closed, enables the health monitor circuit, followed by a single high-side P-channel enable FET controlled by the MCU, and then a low side FET for each output channel. I think I can do a good health check for both channels using one 8 x 1k resistor array, one intstrumentation op amp, and a 1mA current source. The schematic is hand-drawn at the moment. Maybe I'll scan it?

The idea is to test the circuit with dummy loads, perhaps while connected to a computer away from the pad, then open the switch, install the real loads, go out to the pad, and then turn the switch on and listen for the right sort of beeps to know you're good to go.
 

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