Have a look at the most recent posts below, or browse the tag cloud on the right. An archive of all posts is also available.
TeleBT working with AltosDroid on an Android device provides everything needed to monitor a rocket in flight, record telemetry, and know how to walk right to the airframe after it's back on the ground.
The Bluetooth capability of TeleBT is also supported by AltosUI on Linux, and with a micro USB cable TeleBT works just like TeleDongle on Windows, Mac, and Linux systems running AltOS version 1.2.1 or later.
In my post about Batteries and Pyro Circuits, one of my suggestions was to remove the protection circuit board from LiPo cells used with Altus Metrum flight computers. To help out folks who want to do this themselves, I put together and posted a how-to with photos in the Documents section of our web site.
Keith and I have discovered a change in the behavior of the protection circuits integrated in the LiPo batteries we sell for use with Altus Metrum products that poses a risk for our customers. This post is meant to document what we now know, communicate changes we're planning as a result, and explain what we think flyers of our existing electronics and batteries may want to do to maximize their chances of successful flights.
Choosing batteries and designing pyro circuits for high power rocketry avionics involves a variety of trade-offs. Reliability is the highest concern, both because nobody wants to lose an airframe due to a failed pyro event, and also because airframes recovering anomalously have safety implications. But we also care about other factors including cost and weight, and usually want to minimize the complexity of both the electronics and the overall installation.
The objective of a pyro circuit is to dump enough energy rapidly into an electric match to cause it to catch fire. We need batteries both to power the electronics that decide when to fire the charge, and to provide the energy that actually ignites the match.
The two most common battery types seen in the rocketry hobby are alkaline cells, often the nominal 9V rectangular variety, and rechargeable batteries based on Lithium Polymer (LiPo) chemistry. LiPo cells are 3.7 volts per cell nominal voltage, are very light, and have a high energy density.
LiPo capacity is measured in units of current times time, so an 850mAh cell should be able to deliver 850 milliamps for an hour. The battery industry also uses something called a "C rate" to describe how fast the battery can be usefully discharged, wich is a multiplier relative to the battery capacity. So a battery with 850mAh capacity and a "2C" rating can deliver current at a sustained rate of 1700mA until discharged, while the same capacity at a "5C" rating can deliver 4250mA.
By comparison, most 9V alkaline batteries are actually composed of 6 individual 1.5V cells enclosed in a wrapper. It's hard to get hard numbers for capacity and discharge rate, since in an alkaline cell the two are not independent, and the discharge rate is related to the volume of each individual cell. The data sheet for an Energizer 522 shows just over 600mAh at a 25mA discharge rate, dropping to about 300mAh at a 500mA discharge rate.
Importantly for use in pyro circuits, LiPo cells have a very low source impedance, which means they can source immense amounts of current. It's not unusual for cells in the 1000mAh range to have ratings in excess of 30C! Because this rapid discharge ability can pose a risk of fire, it's common for LiPo cells to come with a "protection board" integrated into the battery assembly that is designed to limit the current to some rate such as 2C continuous duty.
In large airframes, or projects that involve staging, air-starts, or other complex pyro event sequences, the most reliable approach will always be to use separate batteries for the control electronics and the source of pyro firing power. In the limit, having separate pyro batteries for each pyro charge with the control electronics only providing the switching to connect the batteries to the charges could even make sense. But for most airframes, this is overkill, and the increases in mass, volume, and wiring complexity just don't make sense.
The challenge, then, is how to design electronics that will robustly initiate pyro events without negatively affecting the rest of the electronics when operating from a single shared battery.
Altus Metrum Pyro Circuits
The very first prototypes of TeleMetrum were designed to use a single-cell LiPo battery, and had an on-board 100mA charging circuit. Because we needed 5 volts to power the accelerometer, we had a small switching regulator that up-converted the LiPo voltage, and we used some of that regulator's output to charge a 1000uF capacitor. The pyro circuit used Fairchild FDN335N N-channel MOSFET switches in a low-side switching configuration to dump the energy stored in the capacitor through an attached ematch. Those FETs had an on resistance of under a tenth of an ohm in our operating conditions. The circuit worked very reliably, but the 1000uF electrolytic cap was huge and we struggled with the mechanics of such a large part hanging off the board...
It turns out that 3.7 volts is way more than enough to fire a typical low-current e-match or equivalents like the Quest Q2G2 igniters. In fact, bench testing with a good digital oscilloscope showed that a typical e-match with resistance of 1.3-1.8 ohms will fire in approximately 13 microseconds when hit with the nominal 3.7 volts from a LiPo.
So, starting with our v0.2 boards, we switched to using the LiPo battery voltage directly to fire the pyro charges, eliminating the clunky electrolytic capacitor entirely. We also switched to the Fairchild FDS9926A dual N-channel MOSFET whose specs are better in all regards for our application. The on resistance is down around 40 milli-ohms in our circuit, such that the current ratings are much higher (FET current limits are primarily driven by how much heat is generated due to current flowing through the channel's on resistance).
Because using the LiPo voltage directly means we're effectively temporarily putting a very low resistance across the battery during the pyro events, the input voltage to the voltage regulator gets pulled down. To ensure the processor could "ride through" these events, we added a 100uF surface mount bulk capacitor on the 3.3 volt regulated voltage rail, which bench testing demonstrated was more than sufficient to maintain processor operation through pyro events. And that is essentially the same pyro circuit on all the boards, both TeleMetrum and TeleMini, that we have shipped to date.
The LiPo batteries we source and sell with our electronics come with a protection board and cable terminated in a 2-pin, 2mm "JST" connector. The specs on the protection board have always been "2C continuous", but we observed the ability to source much higher currents for short periods such as the 13 micro-seconds or so required to fire an e-match. Thus these protection circuits seemed just fine .. we could fire e-matches with a burst of current but were protected against short circuits in the wiring or our boards by the 2C continuous limit.
Unfortunately, the most recent batch of batteries we sourced seem to have a much "twitchier" protection circuit. We can draw more than 2C for short bursts, but not as much as with prior boards, and not for as long an interval. With a 1 ohm power resistor on the pyro terminals of one of our boards, we get about 9 milliseconds of power before the protection circuit cuts in and shuts the battery down. The power stays down until all load is removed, which at least means the board is turned off and back on again, and in some cases could even mean the battery is unplugged and re-plugged since we draw trace current to keep the GPS memory alive even when the power is turned off, and at least some of the new batteries see that as enough to keep the power turned off after an over-current event.
For many e-matches, this isn't an issue, since 9 milliseconds is way longer than the 13 microseconds needed to fire the charge. Unfortunately, we've discovered that many of the e-matches bought and used in the rocketry hobby are actually made for use in the fireworks industry, where it is desireable to retain continuity after firing so that series connections of e-matches all can fire even if some fire faster than others! This means that while the resistance goes up some after firing, sometimes the drain on the battery remains sufficient to cause the protection circuit to kick in even after the pyro charge has fired.
What We're Doing About It
If we remove the protection circuit from the LiPo (or jumper around it), all existing Altus Metrum products will operate successfully with pyro charges thave have an effective resistance of as low as 1 ohm. That's lower than any e-match or Q2G2 we've ever seen, so in effect what this means is that if you have an existing Altus Metrum flight computer, and you remove the LiPo protection circuit, you're good to go. This does not really make things any more "dangerous", since our battery chargers are all current limited and our discharge patterns will never cause heating of the battery. Frankly, in a rocket, the most likely way to cause a problem with a LiPo is by smashing or puncturing the actual battery during bench work or during a crash... and those cause the same problems with or without a protection board present.
In the future, we will ship batteries that have either a much higher C rating on the protection circuit, or have no protection circuit at all.
The number of problems reported by actual customers that we think should be attributed to LiPo protection circuit boards is very low, and we suspect most of our customers who are happily flying their boards can continue to do so. Ground testing where you fire pyro charges (or at least e-matches) using RF to issue the commands (not USB, since the LiPo charger is running any time USB is connected!) will confirm whether there's a problem. If the board resets (does startup beeps) after a pyro event, or shuts down completely (no LED activity), then you have a problem. If the matches light and the board keeps running, you're good to go.
However, any Altus Metrum customer with LiPo batteries sourced from us or our distributors who is worried about this problem (even if you haven't seen problems in ground testing or previous flights), and who doesn't want to try soldering on the battery circuit board yourself, is welcome to contact us about removing the protection circuit for you. We won't charge anything other than shipping.
To take advantage of this offer, just send email to firstname.lastname@example.org telling us how many of which capacity batteries you have that you'd like updated, and we'll respond with an RMA number and shipping details.
Going Even Further
As previously indicated, with the LiPo protection circuits removed, all of our current products will work reliably with at least 1 ohm across the pyro terminals. That should cover all real-world flying conditions just fine, but we're not satisfied yet.
We've designed a new pyro control circuit that transfers the maximum possible energy to the load regardless of battery voltage without ever allowing the voltage to the processor to droop at all. We're testing this new circuit in various prototypes now, and if it pans out it will probably show up first in MegaMetrum and then trickle down to TeleMetrum and TeleMini as those products are updated later this year. The new pyro circuit tolerates 0 ohms (dead shorts) on the pyro terminals for as long as the battery can provide current, which is as good as it gets. We think the circuit is clever enough that we'll probably write more about it once we're finished validating it.
Altus Metrum is pleased to announce our first-ever "Shiny Black Friday" event! We're combining a special, one-time discount on TeleMini products with a price reduction on TeleMetrum boards ... and a new product introduction!
For one day only, Friday, 23 November 2012, we are offering a special $50 discount on TeleMini Starter Kits (regular $225, now only $175!) and TeleMini boards (regular $125, now only $75!). This discount is available only for direct orders from our web site, see below. TeleMini is a baro-only recording dual deployment altimeter with radio telemetry and direction finding features. Learn more about TeleMini at
We are also pleased to announce that TeleMetrum is back in stock after a brief production delay. To celebrate, we are dropping the price of extra TeleMetrum boards from $350 to $325. Starter kits will continue to sell for $400. TeleMetrum is a dual-deploy altimeter with accel and baro sensors, GPS, and radio telemetry and direction finding features all on one board! Learn more about TeleMetrum at
Last, but certainly not least (unless we're talking size or mass!), we are proud to introduce a new product... MicroPeak! MicroPeak is a tiny board that provides highly accurate peak barometric altitude recording. About the size and weight of a US dime with battery ready to fly (18x14mm, 1.9 grams), this is the ideal altimeter for model rocket contests. MicroPeak is so easy to use that it's also the ideal altimeter for young people working on rocket related science fair projects, etc! MicroPeak sells for $50, learn more at
Find more information on all Altus Metrum products at http://altusmetrum.org. The special Black Friday discount on TeleMini products is available only on orders placed through Bdale's web store at http://auric.gag.com.
Thank you for making 2012 such a great year for Altus Metrum, LLC. We're having fun, and look forward to introducing several exciting new products in the weeks and months ahead!
When Keith and I released a new version of AltOS, the open source firmware and software system associated with our Altus Metrum hobby rocket avionics systems last week, we forgot to mention one small but really neat new feature Keith spontaneously added a few months ago. When a rocket is on the rail waiting to launch, we now beep out the igniter continuity status over the radio link instead of just sending periodic audio beeps.
This is particularly useful when flying TeleMini, which is just too small to include an on-board beeper. Previously, you had to check the ground station computer to see if the board powered up correctly and was ready to fly. Now all you need is an HT tuned to the right frequency, which is easy to have in your pocket or clipped on your belt while preparing a rocket for flight!
Son Robert and I made heavy use of this last weekend flying at the Tripoli Colorado Fall Frenzy 2012 launch, where 4 of our 5 flights were in TeleMini-equipped rockets set up for apogee-only electronic recovery deployment with motor backup. I also found it really handy when setting up YikStik3 on the flight line at Airfest 18. It was hard to hear the altimeter beepers on the two TeleMetrum boards buried deep in the airframe, but they were loud and clear through the HT in my back pocket!
Thanks, Keith! So much improvement in flight-line experience from so little code...
This release is, frankly, long overdue. Getting here has been harder and taken much longer than any of us intended. Fortunately, now that we've reached this milestone, I believe that making regular progress towards the vision we have for a "1.0" release will get easier.
FreedomBox is far from the only thing that I'll be working on after my upcoming early retirement from HP, but hopefully I will succeed in putting far more time towards the project soon than I have been able to in the past.
I am pleased to announce that HP accepted my application for participation in this year's Enhanced Early Retirement program. After 26.5 years, my last day at HP will be 31 August 2012.
This isn't because I have a problem with HP, quite the contrary! I feel immensely privileged to have worked on so many interesting things with so many smart people for so long. And there are many exciting things happening right now, including high profile projects like Odyssey and Moonshot expanding both ends of the server scaling continuum with Linux, and the immense commitment to and investment in Open Source represented by the use of Open Stack and Ubuntu LTS in building HP Cloud. The temptation to stay and continue to help guide HP's Open Source engagement was strong. I'm certainly under no pressure to leave... in fact, I think I managed to shock my management with my decision to retire!
But I'm very excited about "taking a break" from full-time employment for a while to focus more time on things that matter a lot to me. My son and I have a number of projects we'd like to work on together as he enters high school. The side business I started two years ago with Keith Packard to design, build, and sell avionics solutions for high power model rockets is growing and could use more attention. I'm in the midst of designing the processor board for another amateur radio satellite. And of course, I'm looking forward to putting more "quality time" towards Free Software projects like FreedomBox and Debian!
Since they do not depend on my employment status, I do not expect any immediate changes in the various roles I play outside of HP. I will continue to serve on the board of the Linux Foundation representing individual affiliates, on the board of the FreedomBox Foundation, as Chairman of the Debian Technical Committee, and as President of Software in the Public Interest. And while I hope to travel less than I have in recent years, I look forward to continuing to speak and otherwise participate in key Free Software conferences around the world from time to time.
After I retire, the primary point of contact with HP on Open Source matters will be the Director of HP's Open Source Program Office, Phil Robb. But if there are open issues regarding HP and Open Source that I can help with over the next month, feel free to contact me and I'll see what I can do to help.
We've had sporadic Altus Metrum customer reports about RF susceptibility issues with their TeleMetrum installations. In almost every case, these problems have been completely resolved by either making sure the system battery has sufficient charge before launch, or through the application of standard engineering techniques such as twisting wire pairs to reduce differential coupling. However, even when every technique we could think of had been applied, once in a while someone still had issues.
Around the time of LDRS this year, the incidence of such reports seemed to increase. One customer, in particular, had an installation in which he virtually always saw continuous resets of the board once his 54mm airframe was put on a launch rail, and several customers reported seeing board resets during ejection charge firing. Keith and I saw a board reset during main charge firing happen in person at NCR's Oktoberfest, and with a couple days available to work together after that launch, we decided it was time to figure out what was really going on.
Here's what we've learned.
In bench testing, it quickly became clear that the problem was the 3.3 volt power supply rail getting pulled down far enough to reset the CPU. This most frequently happened during ejection charge firing, when the input of the LDO regulator is pulled down by the near-short presented by the e-match when a pyro FET is turned on. To keep the 3.3 volt rail voltage up during firing, we include a 100uF bulk capacitor on the regulator's output. In all of our prior bench testing, we never saw the 3.3 volt rail droop significantly. Clearly something had changed... or maybe several things had changed?
The first thing I wondered about was whether the new Kalman filtering code, which requires more compute cycles from the processor, might be consuming enough more power to pull the rail down faster during charge firing. After poking around at it, though, we have no data to suggest the new code makes a measurable difference in power consumption.
The next thing we pondered was that at least some of the e-matches we and others are using in the hobby now come from the fireworks industry, where it is apparently considered a feature for the match to retain continuity after firing. This means the input of the LDO gets held down for longer than with the e-matches we used to use and Quest Q2G2 igniters that open when fired. But that still didn't make sense as the root cause, as we chose the FET firing time such that even with a dead short across the igniter terminals, the 3.3 volt rail wouldn't be pulled down far enough to cause trouble during firing.
One of the big changes between v1.0 and v1.1 on TeleMetrum was that the newer boards incorporate a better reset circuit. This helps ensure the GPS chip always comes up running at power on, which was a problem at temperature extremes with older boards. However, a side-effect of this change is that a v1.1 board will reset any time the 3.3 volt rail drops below 3.15 volts, whereas older boards didn't trip until a much lower voltage. So the recent increase in reports might just be related to more v1.1 boards being placed in service?
While experimenting on the bench, we observed that injecting RF energy into the input of the LDO regulator had the effect of pulling down the output voltage, presumably because the internal reference source accumulates charge and is fooled into thinking the output is too high. Since our designs all have the power switch contacts ahead of the LDO, the wires going out to the switch and back are effectively an antenna... as are, to a lesser extent, the wires going to the e-matches. There is some variability from part to part in just how badly the LDO reacts. But by attaching a tuned length of wire as an antenna to the LDO input and playing around, we were finally able to reproduce the problem reliably on a test board at my bench!
On further analysis, we realized that the output of the USB battery charger chip and the input of the LDO both expect a 1uF bypass cap to ground. At some point, those looked redundant and we eliminated one of the two. Unfortunately, we weren't internalizing the fact that the switch leads were between the two caps, and the one we left was on the output of the charger and not at the input of the LDO. Placing a suitable bypass cap right at the input of the LDO turns out to have a truly dramatic effect on RF immunity!
Once we realized that RF getting into the LDO input was the problem, Keith pointed out that we used to see "noise" in the accelerometer data on earlier boards that was caused by the 3.3 volt rail moving slightly during radio transmit, which we fixed with a hardware change on v1.1. We are now convinced that this was at least partly related to RF coupling to the LDO input, not just the change in power consumption on the LDO output. We didn't realize what was going on in earlier testing because we often didn't have ematches wired up, so RF coupling was minimal. But going back to flights logged with v1.0 boards that included deployment, and studying the magnitude of the "steps" in acceleration data observed when the transmitter was on, Keith was able to compute the amount the 3.3 volt rail must have sagged if the real acceleration wasn't changing... which in some cases was as much as 180mV! We think this proves that RFI could cause the LDO to drop its output voltage below the reset controller set point on v1.1 boards.
Based on these observations, I'm making two hardware changes for the next version of TeleMetrum (version 1.2), and Keith is also making a software change. We have tested all of these changes on real boards both on the bench and in test flights, and the net effect is a major improvement in the RF immunity of TeleMetrum.
The first hardware change is moving to a slightly lower trip voltage on the reset controller. Instead of 3.15, the new part trips at 3.00 volts nominal. This gives us more "headroom" to tolerate 3.3 volt rail droop during charge firing, and will allow the board to operate longer on a given battery charge. This change is not relevant for v1.0 and prior.
The second hardware change is adding an appropriate bypass capacitor right at the LDO input. This requires a PCB update, but it's possible for me to update existing production boards by adding an 0402 cap right across the appropriate pins on the regulator chip.
The software change prevents our altimeters from turning on the radio transmitter while an ejection charge is firing. Since the RF transmitter draws substantial additional power, this should help keep the 3.3 volt rail from drooping. This may not really matter, but it feels like the right thing to do. This change will be part of our next stable firmware release.
We think most TeleMetrum customers need not worry about these updates. But if you have seen odd resets on the rail or during ejection charge firing in flight with a TeleMetrum v1.1, feel free to contact me about updating your existing board to include these improvements.
This release includes a fix for the defect in version 1.0.1 that prevented rebooting an altimeter from idle to pad mode over the radio link. This affects both TeleMetrum and TeleMini boards, and re-flashing the altimeter firmware is required to pick up this fix.
We also restored the pre-1.0 mode selection behavior for TeleMetrum at power on. This is also an altimeter firmware change, but does not affect TeleMini.