Web log in reverse chronological order, i.e. most recent entry first.
After getting gas quotes and talking to Doug Jones, I decided to abandon the idea of using Ethane/GOX for the igniter and decided to go with IPA/GOX, eventually moving to NOS like Andrew Case. The ethane was too expensive at $200 for 80 cubic feet. I decided to start with GOX as it is cheap and simplifies the plumbing a bit. I started putting the plumbing together recently and learned a lot about 37 degree flare fittings in the process which I will write up separately. I bought an inexpensive cart from Enco ($35) that I am going to mount everything on. I also bought a 1/2" aluminum plate from Metal Connections to bolt onto the cart top and use as a stiff mounting point, but the plate hasn't shown up yet. Metal Connections will cut it to size and includes shipping in its pricing. Not a bad deal. I am still acquiring plumbing, gas bottles, regulators, etc. but it is all coming along. I bought a 32" x 1.75" copper 145 remnant from Yarde Metals for only $74. I am trying to not use to much surplus stuff as that starts making it harder to reproduce, in whatever fashion, my results. But I'll take a deal if I can get it. The new Rev. B High Voltage Driver boards came in and they are perfect except that the bottom-side writing is mirror-imaged. Jeez, it's always something. I also found that Aircraft Spruce sells shielding braid for sparkplug wires that is a lot cheaper that the buck a foot stuff I had been using. Aircraft Spruce's is $0.35 a foot, P/N 11-02463.
My biggest accomplishment of the month was to create a design spreadsheet. I used the booklet How to Design, Build and Test Small Liquid-Fuel Rocket Engines . Something I have wanted to do for a while is to try and come up with closed form expressions for various design parameters like ISP and chamber temperature as a function of O/F ratio and chamber pressure. In the past, to get a handle on those sorts of issues, I would make a bunch of runs in PROPEP and then laboriously edit and graph the data in MS Excel. That took a lot of time and only gives you a two dimensional view of the data. My idea was to try and curve fit the PROPEP data using a commercial curve fitting program. I used a demo copy of Table Curve 3D and it really worked out well. TC3D is a fantastic program that is very easy to use. I think I had to look up one thing in the manual in a week of using it. Since the complete spreadsheet with all of the graphs and data is almost 7M bytes, I also created a slimmer version that just has the minimum necessary for the motor design section. When I compare the equations used in the booklet to Humble and Larson they seem simplified, so I'd be interested in hearing how useful and realistic everyone finds the booklet design procedure to be. I am sure there are errors here and there in the spreadsheet, so if something looks funny let me know and I'll look into it. By the time the spreadsheet gets to Rev C or so I hope it will be getting truly useful.
Small Motor Design Spreadsheet Lite Rev. A (89KB)|
At last, some real hardware. I had received the proto Rev A boards
the morning I was leaving on a two week business trip. Torture! I
finally got everything assembled and checked out upon my return. I made one electrical error (you can see
the jumper on top side) and had four minor mechanical problems. I will make a Rev B board that cleans up all of the errors
and that will be it for the HV Igniter Driver, pretty much completed. Since I am now dealing with artwork, schematics
, and list of materials I need to get the CVS system going for this stuff and then you can
be sure of which version is which. I will post the documentation at the end of this entry
and then copy it to the CVS directories later. BTW, rocketworkbench lost three whole directories,
including the blog, in February. It's all restored now and hopefully won't happen again. As
far as the igniter goes, it checked out fine. No interference with the controller (notice the
shielded wires) and the aluminum box helped as well. The aluminum block is a heat conductor between the IGBT and
the case. In a weight optimized version the case could be cut down and the block eliminated. The thermal
compound that goes on the block was removed so it wouldn't get all over while I was taking pictures.
I have started ordering parts for the gas system: a regulator and the Elderbrock tapered jet kit that John C.
recommended. I bought a three foot by one and one-quarter copper bar from my local recycler for $10. I don't
know what kind of copper it is, but hopefully it machinable enough to practice making chambers with. I am going to
continue using my kludged controller while I get the chamber built and plumbed, then go back and finish the controller.
Rev A Eagle format board file (Try a right-click and Save Target As if clicking the link doesn't work)
Rev A Eagle format schematic (Try a right-click and Save Target As if clicking the link doesn't work)
|This is a no update update of the "other" igniter project. After a phone conversation with Charles Poole he convinced me that maybe capacitive discharge did have some advantages. I am not willing at this point to go off on that tangent, but I decided to split the electronics between two boards: one with the controller and solenoid drivers (HV Controller)and one with the coil driver (HV Driver). That ended up solving several issues. First, I can physically separate the HV from the rest of the circuitry and further shield it, second it makes the driver a separate unit so it can be just a simple switch or something more exotic like a CD driver, third, I made the driver through-hole instead of surface mount so it makes that part of the design more accessible to electronics challenged rocketeers, and fourth, I added a 555 timer circuit so the HV Driver module can be used stand-alone for a low cost solution. Just add power and sparks will fly, no controller required. I ordered some nice little Hammond powder coated, flanged, aluminum enclosures for the driver is it should look very nice packaged. I was originally going to order both boards at the same time (a bit cheaper that way), but as time dragged on I decided to build the HV Driver board first. I was hoping to have that board competed this week but my boards are a little late coming in from CustomPCB (FedEx's fault) and they won't arrive until next week. I am leaving tomorrow on a two week business trip so it will be next month before the driver gets built. Which is too bad, making this is the "no update" update. Funny, I really thought that this project would take a month when I started and I have worked on many complex engineering projects so I should know better. I now am resigned that it will be a year plus to finish the igniter and then even more time to build a small motor. Fun project though!|
I had a chance to put in some work on the HV igniter over the holiday break. I solved the problem of EMI causing the microcontroller to freakout
by putting a braided shield around the ignition lead and then grounding the braid. I covered the ends of the braid with heatshrink tubing to
keep the ends from fraying. The controller now works OK as long as I keep the spark plug about 12" away. The one exception is at turn-on. There is
typically a small glitch probably caused by current in-rush into the coil as the magnetizing inductance charges up. I wrote a loop that sends a
rolling count out the serial port and when power is applied to the coil you see a small glitch in the count. Notice count number 86 in the screen shot below:
I think this performance is good enough at this point to go ahead and layout the board, keeping in mind that all the electronics will need to be in a metal box for shielding. I also decided to put the coil driver IGBT on its own board so that it can be located away from the main PCB, keeping the HV as far away as possible from the main board. Since the two boards (main and driver) can be fabbed as part of the same job, there is no additional cost other than I'll get less total boards from a minimum lot. No big deal since the first batch is a prototype.
I also built a thermocouple amplifier to try out the idea of using a thermocouple as an ignition sensor. The response time of the exposed junction K-type thermocouple I tried out was quite fast, way less than 100 milliseconds. Check out the plot made with my trusty Dataq DI-194RS:
I generated the thermal step function by holding the thermocouple against an ice cube and then plunging it into a hot cup of coffee. The y-axis thermal scale is reasonably accurate. Omega makes a thermocouple, the Pipe Plug Probe for $39 that has the exposed junction option needed for a fast response time. It has a stainless steel overbraid and is good to 2500 psi, although the max temperature rating is only 480°C. Hopefully the thermocouple won't melt above that temperature, but we'll see.
Thom McGaffey moved to Arizona which will be another set back as Thom had bought all the gas equipment. Thom may come off as a bit gruff on Arocket, but in person he is a great guy to work with and I'm going to miss him :(
In September I thought I was pretty close to getting a board done. Jamie Morken volunteered to write the
microcontroller code and after we went over everything Jamie pointed out I needed a couple of more I/O pins
on the AVR (a 32 pin ATmega8). After trying a few workarounds it was clear that I would have to switch to a
44 pin Atmega16.
Since I was faced with that setback I decided to build a small prototype of the AVR microcontroller circuit and check out the noise situation that I was so worried about last month. I also learned how to use interrupts and use the AVR onboard timers so it was a fun little project. I didn't bother to write all the code (or get Jamie to do it) and just set up the 16 bit timer1 to put out a 1kHz square wave. Once the timer is programmed it runs independently of any other code that is running so it is like mini-multitasking in the microcontroller. To imitate the other code (for solenoid control, ignition detection, etc.) that will be running in the main loop I programmed two output pins to drive two LED's that blinked in an on/off sequence. That way it would be easy to tell if the AVR died or freaked out when the spark plug was sparking. Well, the square wave came out and the spark plug sparked , but the LED's went wacky. When I removed the coil 12V (oh yeah, I used the old sparkplug/coil prototype Thom and I had used before except the coil and plug are rigidly mounted to a small aluminum plate. The coil is connected to a large 12V gel cell and the controller to a 9V battery), the microcontroller went right into the main loop like nothing had ever happened. If there was a lot of EMI I would expect the program counter and register contents to be scrambled and the whole thing would just hang. Somehow the controller was able to recover quite nicely! Obviously there is some kind of EMI issue that needs to be solved so I am first going to try adding a snubber on the IGBT to reduce the flyback dV/dt, second I'll shield the coil/plug and/or processor, then third add optical isolation to the driver and isolated power if absolutely necessary. To design the snubber I really need to know what the coil HV waveforms look like so I need a HV scope probe. I have been avoiding this issue since HV scope probes are kind of expensive ($300-$600). I found a few used ones on Ebay for about $100 but decided to try and build one first. There is a nice explanation and example at Basics of High Voltage Probe Design and Homemade High Voltage Probe (> 40kV) . I wasn't able to find any of the required high voltage resistors at the usual online sources so I called Caddock directly. I talked to a very friendly applications engineer who recommended the MX440 11kV 10Meg resistors as a place to start. When I asked for one sample he replied "I won't send you one but I will send you two. I never send one of anything". Nice guy! It will be interesting to measure the actual output voltage as I really have little idea of what it is.
On another note, as I mentioned in another Arocket post, I was fortunate enough to see Elon Musk of RocketX speak at Stanford University. In the course of his talk he mentioned that they were using ethernet for internal communications, which got me thinking. I had bought an AVR ethernet demo board from EDTP Electronics for another project ( Easy Ethernet AVR ). The EDTP board comes with C source code for an ImageCraft compiler. There is also source code for the free AVR Freaks gcc compiler (which Jamie plans on using for the igniter board) elsewhere on the 'net. I have a CodeVision compiler and didn't want to take the time to install the gcc compiler ( Winavr ). Fortunately it only took a few minutes to fix the compiler errors and produce working code (well, I was able to ping it) using my CodeVision compiler. A pretty complete solution (support for tcp, udp, icmp, arp) fits in only 5KBytes out of 16KBytes of flash memory. Pretty impressive! And that 5K includes some UART code for debug messages so I bet it could be reduced to only 3 or 4K for tcp support only. The ethernet controller (Realtek RTL8019AS) is also only $8. If CAN turns out to be a big hassle, ethernet looks like a pretty attractive alternative.
After a summer slow down, this igniter project is moving again. I am currently
laying out the PC board and hope to send it out this weekend. I 'm using Eagle sw
and have found that I have to make my own layout packages for library parts 90% of the time.
That really slows the process down. I am also a bit nervous about induced noise from the coils
primary voltage flyback causing the microcontroller to reset. I am going to set up a little
experiment to test that this weekend as well before I send the PCB out for manufacturing.
BTW, I have been using CustomPCB (http://www.custompcb.com) in Malaysia for board
manufacturing lately. $72 gets you a double sided, 4" x 5.5", solder masked and panelized
(they cut up the 4x5.5 panel into as many of your pieces as fit) fedex'd to your door 5
business days after ordering. Much better than ExpressPCB or Olimex. The owner, Gary Cho,
has been very helpful and responsive to all of my questions. A global economy success story.
One issue I had on the PCB was heatsinking the surface mount coil driver (the board is almost
all surface mount). There are a lot fewer heatsinking options in SMD than through-hole.
I talked to an applications engineer at Aavid/Thermalloy and he recommended a heatsink that
should do the job. I hope.
Thom McGaffey and I had worked on machining the igniter body about a month ago but the original design was too hard to accurately machine. Thom redid the design and has machined and welded a beautiful prototype from 304 stainless. I didn't get a picture of it but I will next time.
Decided to use the Tecumseh small engine coil. It is not as high performance as the Ford coil and it runs hotter, but it is $18 versus about $60 and has a nicer form factor. The Tecumseh coil has a square hole in the center that fits a 3/8" carriage bolt. It also fits over the sparkplug insulator which makes for some interesting packaging possibilities.
I have been talking to the guys at Portland Aerospace (PSAS) about their use of the CAN bus as I am designing in both CAN and I2C (and manual bit switching) interfaces. I hope to meet up with them at BALLS and get an up close look at their avionics architecture & implementation. They will be launching their airframe with both an N and O motor at BALLS.
I found a part number for the Ford coil-on-plug and Jamie Morken found a few numbers from some units being sold on eBay.
Did some more testing of the coil/sparkplug yesterday. Put some thermal goop on the heatsink which kept the IGBT cooler. Just used the Ford "coil on plug" as it has the best performance of all the coils tested. It is interesting that all the other coils had turns ratios of about 50 whereas the Ford unit had a turns ratio of almost 70. That combined with a 38mH magnetizing inductance makes this one a winner. Now I just have to find the Ford part number and how much it costs.
Played around with the two best spark plugs from last time, the BM6F and the 10mm NGK racing plug. Somehow the racing plug didn't seem quite as impressive as it did the first time we tried it. Still produced a very hot spark, but it was more or less the same as the BM6F. The racing plug has a drawback of a 3/4" threaded barrel ("reach" in sparkplug nomenclature) but not having a protruding electrode should help avoid any potential fouling issues. As a result we decided to go
with the NGK plug. It's less than $3 so it is certainly cost effective. Tried various duty-cycles and found a range of 40 to 60% would work. 60% gave a bit hotter spark, 40% a bit weaker. At 10% it was extinguished or close to it. 1KHz was the best overall frequency so we decided to just go with 1KHz and 50% duty cycle.
Worked through the preliminary design of the controller board. It will have something for everybody (at least the prototype): AVR controller, CAN bus interface or parallel control lines, two channel temperature sensing (chamber and heatsink), two channel solenoid control, status read back via the AVR or parallel lines. Should have boards in two weeks. Question though: are there strong preferences for SMD or through-hole? Either will work and through-hole may have some advantage because it is easier to heatsink through-hole devices. But it will be smaller with SMD. Any opinions?
Thom has a design for the chamber and is starting on that. It'll be GOX and Hydrogen. The prototype will have outboard gas regulation so we can easily change pressure. The body will be a chunk of stainless steel with diffusion welded SS tubing to the gas ports. Holley solenoids for gas control. Looking at reusing large threaded CO2 cartridges as the flight gas storage. Will start testing the prototype chamber a week from Monday (July 7).
So far so good!
Thom and I got together yesterday to do some coil and sparkplug testing. The basic circuit was a heavy duty 12V battery connected to a coil that was switched to ground through an IR IRGS14C40L IGBT. The source (emitter) of the IGBT had a 0.15 ohm 1% resistor in series with it for current monitoring. The IGBT gate was driven with a function generator square wave. We focused on trying to measure the magnetizing inductance of the coils, but also checked out some more subjective things such as apparent Hottest Spark Switching Frequency and Hottest Spark Coil/Plug Combo. The coils were the same as before: Tecumseh small engine coil, Jamie's Ford Coil-on-Plug, motorcycle coil, and small-block Chevy coil. I ordered some plugs from Sparkplugs.com, so the sparkplug line-up is now: NGK R0045J-9
10mm surface discharge racing plug, a Champion RJ12C regular car plug, a NKG BM6A small engine sparkplug, a NKG BM6F compact small engine sparkplug, and last, a tiny 10mm NGK CM-6 plug. The CM-6 is used in a lot of model engines because of its small size. All of the plugs are less than $3.
The tests were conducted by monitoring the voltage waveform on the series current monitoring resistor while varying the signal generator frequency. The IGBT heatsink temperature was monitored with a thermocouple temperature meter. Theoretically, if the secondary is open (ie, not in discharge), the coil will act like an inductor. If a fixed voltage is put across an inductor it will generate a linear current ramp. Once the inductor saturates (can no longer increase the energy storage in the magnetic field) the effective inductance will drop sharply and the current ramp should increase its slope sharply. Measuring the slope in the low slope portion gives us effective magnetizing inductance (L= V/delta I/delta t). The peak current that could be supplied in the linear region was also measured. Peak current plus magnetizing inductance gives use energy storage (E= 0.5*L*I^2). In reality, only one of the coils could be driven into true saturation. The rest would limit due to the circuit resistance plus internal coil resistance. One coil, the motorcycle coil, didn't produce a linear ramp but instead produced a single-time-constant-RC looking waveform. I'm not sure what that was from and will have to check into it some more later.
The Chevy and motorcycle coils ran very cool as did the IGBT when driving them. Low peak currents pay off in thermal performance. The Tecumseh ran hot and the IGBT ran very hot. We decided that on all tests we would shutdown when the heatsink temp reached 150ºF. The Tecumseh had to be shutdown after about 10 seconds max. The Ford coil could be run for about a minute before the temperature max was reached. All coils performed (subjectively) best at about a 1KHz switching frequency.
Plugs (subjectively): CM6 was the wimpiest of the bunch. An anemic spark. Champion RJ12C was OK The BM6 (not sure if it was the A or F) produced a very bright, hot spark The NGK R0045J-9 was an arc welder. A full spark curtain surrounding the surface discharge electrode when driven at the optimum frequency. Very cool. Too bad racing plugs only come in 3/4" reach.
Coils (subjectively): Interestingly, subjective performance went up as size went down. Chevy, OK Motorcycle, somewhat better Ford Coil-on-plug, hot intense spark Tecumseh, noticeably hotter more intense spark.
However, they all, plugs and coils, worked. So I think that really any of them can be successfully used. At this point it looks like the racing plug is the most interesting, especially since it doesn't have a protruding electrode to erode, and is design for hot race car conditions. The Tecumseh coil is great, but it is such a current hog that it is at a thermal disadvantage. That would leave the Ford coil as the best looking one right now (Jamie has good instincts!).
The IGBT worked great with no problems through several hours of testing abuse without any clamp/protection/snubber circuitry. I did replace it at one point thinking it was shorted but it turned out I had shorted the test leads. Thom successfully lit his nitrous/butane microtorch several times in the racing plug discharge. No problems there! We had to kluge some coil to sparkplug leads for the Chevy and motorcycle coils using regular wire. That produced a ton of RF so that the temperature meter wouldn't work. That will have to tested/fixed some more before using a controller.
Next steps: A bit more testing to try and understand the motorcycle coil Take scope photos for documentation Try pulse-width modulating the IGBT Build a prototype PCB and ignition chamber
Thom bought a bunch of stainless stock and a bottle of hydrogen to use for the chamber. Should be fun!
Updated Resistance/Inductance Measurement Spreadsheet|
I went over to a company owned by some friends of mine (Amicronix Test Systems) that has a small metrology lab. They have an HP 3458A 8 digit voltmeter that will resolve 10 micro ohms. I hadn't received the mosfets from Digikey so I couldn't take the inductance measurements. I settled for taking the equivalent series resistance (ESL) measurements of the coils primaries and secondaries and measuring the coil turns ratios.
I first measured the turns ratios by driving the primary with a small (40mV to 250mV) sine wave and measuring the secondary peak-to-peak voltage. There was a lot of "smear" on the low voltage waveform that made the measurement difficult.
I then tried a higher voltage of 1Vp-p on the primary and measured the secondary again. These "high drive" measurements came out much better. I recorded the voltages and the respective phases in the attached spreadsheet. The phase gives the "dotted" sense of the terminals. Transformers are notated with dots on the windings to show the relative phase of one end of each winding.
As I expected, the secondary (high voltage lead) was in phase with the "+" side of the primary on the two coils that had a marked polarity.
Next, I measured the primary and secondary ESR's using a four-wire kelvin connection. I was able to decently resolve 1 milli ohm, but below that the measurements were a little noisy. The HP 4184 has the provision to increase the ADC integration time, but I couldn't figure out the menu system. I also couldn't find the manual (almost everyone had already left for the day). On a whim, I decided to measure the secondary with respect to both ends of the primary (+ and -). The secondary impedance was always higher with respect to the "minus" side of the primary than the "plus" side! I was surprised, because that means the + side is the common point with the secondary, just the opposite of what I would have thought (see attached schematic). To read the schematic you'll have to download the viewer from ExpressPCB. I seem to recall that automotive sparkplugs fire when the electrode is negative so electrons flow from the center electrode to the rim. Is that correct? If so, this picture is consistant with that scenario. But also if this is true, I should really redo the turns ratio measurements as I may have been driving the coil in such a way as to bias the secondary with the primaries drive level. It's a small error, but should be corrected.
I am not 100% sure of my results because of the noise problem I had with the voltmeter. Because of the noise I really only had 4 significant digits. I need at least one more to really be sure of these results. To reduce noise I either have to average some samples or increase the integration time (the voltmeter has a dual slope integration converter). I couldn't figure out how to do that by just fiddling around, I need to read the manual. Unfortunately Amicronix just sent the meter out for calibration so I can't use it again for a couple of weeks. Thom has a 6 and a half digit voltmeter so I am going to go over to his house on Wednesday and try again. If the IGBT's come in we can also do some inductance testing. Thom also reports he received the thermocouples for ignition detection.
I would also like to get some pictures posted this week!
Resistance Measurement Spreadsheet|
Since I want to use non-surplus parts for the igniter, I ordered three different size ignition coils, two from the web and one from my local cheap auto parts store (Kragens). I bought a cheap Chevy pre-'74 small block V8 coil at Kragens for $14, ordered a 12V universal motorcycle coil from JCWhitney for $18, and a Tecumseh small engine coil for $18. That kind of covers the range of big/medium/small. We'll see what works. Jamie Morken also sent me his coil which is a Ford "coil on plug" unit he bought on Ebay. It is pretty small and light so it seems worth looking into. I'll see if my local Ford dealer can identify a part number for it.
I decided to use the International Rectifier IRGB14C40L IGBT to drive the coil. It's $3.75 to $3.96 from Digi-Key in single units depending on the package. I'll order some TO-220's for testing and, depending on power dissipation with the chosen coil, go with a surface mount device for the PCB. The IRGB14C40L is a 400V clamped device that will switch 20A. It was designed to drive coils so it has a built-in clamp as well as a 5V drive spec. Paul Kelley recommended the MJ10012 darlington. That is an old Motorola 400V 10A device that is hard to get in the US now. It also would require a more elaborate drive and clamping scheme, but it probably is pretty cheap. I would guess it is still being made by some mainland Chinese company. Infineon has a pretty cool device that has an internal current measuring resistor (BTS2140/45/65). Unfortunately I couldn't find a source for it.
I bought two NOS Powershot solenoids to test as igniter gas switches. I talked to Holley technical support and they told me the Powershot is NOT rated for continuous duty. The maximum on time is 14 seconds followed by a minimum off time of 20 seconds. I want to try using a current modulated switch to coax effectively continuous duty from the Powershot. Has anybody tried current or pulse-width modulating them?
Thom McGaffey gave me a NGK BM6A sparkplug which is much smaller than a car sparkplug. Bill Bullock sent along a REALLY small NGK BM6F chainsaw motor sparkplug. The BM6F doesn't have a crush washer though. I'm not sure if that will be a problem. Thom is going to machine a flame injector for testing.
Thom is also working on a design for trying a thermocouple for ignition sensing. I'm going to try the ion detection method and the glow plug. Andrew Case is working on optical detection.
Next step: as soon as the rest of the coils come in I'll measure the magnetizing inductance, leakage inductance, ESR, saturation current, turns ratio, and weight of each coil. Then add the switch and see what kind of HV they produce.