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Thread: Picaxe-based Satellite: PongSat-18X

  1. #1
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    Default Picaxe-based Satellite: PongSat-18X

    Hello,

    In this thread my son Joost and I want to introduce our near-space PongSat-18x project. We have several questions and hope that forum members are willing to join in design discussions; we'll keep you informed about our progress. The idea is that this miniature 18X-based satellite will fly within a month towards near-space.

    A pongsat (http://www.jpaerospace.com/pongsat/index.htm) is an experiment that fits inside a pingpong ball and will be flown to the near-space area (about 30 kms high if we are lucky) free of charge by company called JPAerospace. There have been some Basic Stamp and picaxe-based pongsats before. They have recorded the temperature at the beginning of the trip in the air; at some point the temperature gets so low that an ordinary battery freezes and thus the experiment finishes, long before the highest altitude has been reached.

    We have registered for the planned flight of 2 May, and we decided to aim a little bit higher. Our goals are:
    - To develop a picaxe-based pongsat that functions properly through the *whole* flight (this implies that very low temperatures (-70 Degr. C or less) may be encountered),
    - To do a large number of measurements (not just temperature) very regularly and thus learn about the atmosphere.
    - To test a number of sensors for their appliccability
    - To run the project like a real space project, i.e. build and test prototypes thoroughly and under "harsh conditions", thorough preparation etc.

    Like in a real space projects there is the risk of total failure in several stages, but that's what we happily take for given ;o) The road towards it is as exciting as the final flight and data retrieval.

    Here are some impressions of our designs:



    picture 1: Pongsat, print layout scetch and special battery that fits within the PongSat




    picture 2: Project elements: prototype testing model (based on Philips EE system, http://www.kranenborg.org/ee/ , with identical circuit)

    Central to the idea is a PICAXE-18X based flight computer that is designed for very harsh conditions: low voltage due to very low temperatures. The computer is powered by a special Lithium polymer battery that has a guaranteed functionality down to -65 degs C !



    picture 3: Circuit diagram

    The circuit reveals that the flight computer has similarities with Rev-Eds datalogger kit, however we use different components that can operate under very low voltages (1.8V); the battery can operate at very low temperatures but the voltage drop at low temperatures is considerable as well and may be below the "standard" 2.7V limit.

    The computer has 7 sensors (whereof 4 analog), all served by the 18X (how? thanks to hippy's pokings, see the following link: http://www.picaxeforum.co.uk/showthread.php?t=9086 ):

    - Internal temperature (digital, DS18B20)
    - External temperature (analog, thermistor outside)
    - Tilt sensor (internal, requires the pongsat to be in fixed position), to indirectly measure wind effect, obtained via Rev-Ed
    - Vibration sensor (another way of indirectly measuring wind speed, also via Rev-Ed)
    - Two light sensors (using LEDs with inverse charging/discharging) to measure darkening effect at high altitudes (inspired by wilf_nv, see http://www.picaxeforum.co.uk/showthread.php?t=5945 ). The LED will also be used in a normal way to indicate proper functioning at start-up
    - Battery voltage monitoring for measuring voltage drop at low temperatures (Using MAX6018 voltage reference device)

    All information will be stored in the special low-voltage 24AA512 EEPROM with an interval of approx. 20 secs.

    As you can see development is at full speed now (I'll do mostly hardware, Joost has begun programming, we do the prototype model together), we'll come back soon with a lot of issues that we would like to get help with. Meanwhile we are open for any discussions on the design!

    Jurjen & Joost Kranenborg
    Last edited by kranenborg; 04-04-2008 at 23:20.

  2. #2
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    Most inspiring & their space adverts are great => www.jpaerospace.com/away29-30/camera2.jpg .
    From where & how is this pong to be lofted & retrieved- balloon & parachute? Location/tracking beacon? Stan

  3. #3
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    Whoo, that sounds like an amazing adventure to be involved with and good luck. I'm sure it'll take pride of place on the mantlepiece once it's been into space.

    Probably late in the day but a few thoughts ...

    Do you need the RTC ? Use on-chip Timer 1 with the 32kHz as a Low Poer Osc and you can probably get an as equally accurate timebase / timer. That saves space and reduces current drain.

    Can you sandwich the PICXE between the R's ( on top and underneath ) ? That may give a little extra heating which keeps the PICAXE going for a little longer.

    What about a low-R which does work as a heater under the PICAXE ? You could drive that via PWM to adjust the temperature. Nothing to lose by draining the battery quicker if it eeks out a few extra seconds of PICAXE life.

    A compact design with components under/over each other, vertical R's may give a smaller core, small enough to get two batteries in there if lucky. SMT might help, an SMT PICAXE could perhaps be easier to keep warm ?

    Maybe a dead-bug wiring scheme would be even more compact but more fragile.

  4. #4
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    Stan: Somehere in California close to the Nevada desert these PongSats are flown together with other bigger and more advanced instruments into the air and back on earth again, using balloon-lifted platforms. A GPS system and transmitter is present (not a pongsat ;o) in order to locate the platform when they are back on earth again. Afterwards the pongsats are returned to their owners via mail

    Hippy: Your reactions are highly appreciated, here is the response to your remarks:

    Do you need the RTC ? Use on-chip Timer 1 with the 32kHz as a Low Poer Osc and you can probably get an as equally accurate timebase / timer. That saves space and reduces current drain.

    I have considered this option (as I currently need to place the MAX6018 voltage reference somewhere hanging in the air), but Timer 1 use requires pins 12 and 13 for the 32786 Hz crystal connection and these are exactly the extra ADC pins available when poking port B ... . I prioritized the option to use the two LEDS as optosensors, but I was close to choosing the timer use and thus saving circuit board space.

    Can you sandwich the PICXE between the R's ( on top and underneath ) ? That may give a little extra heating which keeps the PICAXE going for a little longer.

    What about a low-R which does work as a heater under the PICAXE ? You could drive that via PWM to adjust the temperature. Nothing to lose by draining the battery quicker if it eeks out a few extra seconds of PICAXE life.

    A compact design with components under/over each other, vertical R's may give a smaller core, small enough to get two batteries in there if lucky. SMT might help, an SMT PICAXE could perhaps be easier to keep warm ?

    Maybe a dead-bug wiring scheme would be even more compact but more fragile.


    We have thought above the above options in a similar way, primarily opting for battery heating (max 50 mA can be drawn from the types used, but this may be less at lower temperatures, thus a control strategy is needed) using mosfet-switched resistors and using special IC sockets with pins sideways and connecting these together around the battery, thus maximizing thermal radiation heating (most of the air is gone, so radiation may remain as the most inportant thermal transport process). However, this approach makes it very difficult to add the various extra components (Still a lot of them, it is a pity that only port B supports programmable pullups, whilst I need them at the "official" inputs at port A) and I am very limited in the soldering tool sand experiences. The idea of use of SOIC parts is an interesting option however I did not take into account.

    For a next fligth it may be interesting to develop a special PongSat with "Climate Control" using hippys and my arguments above, and see whether this is technically feasible? Maybe developed purely via the Forum? This would require a thorough understanding of battery physics.

    In the meantime we have made progress; the prototype board is finished as shown below (circuit identical to pongsat) , and we are currently proceeding well with testing the sensors.



    Picture 4: Joost soldering long wire legs to the sensors for connecting them to the prototype board




    Picture 5: The finished (and apparently functioning) prototype board, with sensor locations indicated (only the voltage reference device is lacking).

    The sensors generally appear to work as expected, but the tilt sensor is att odds, we'll come back with a reaction on that one, as it behaves somewhat unexpectedly. Almost bedtime now.

    Some other issues that maybe someone can help us with:

    1) All semiconductor devices are rated down to -40 deg. C. What may we expect to happen when their temperature gets even lower? Does it help to lower the clock frequency of the 18X in that case? ( we alsready use a 100kHz i2c speed)

    2) During the project we have come to realize that maybe the biggest threat to the succesful conclusion of this project may be the mail transport toward JPAerospace and back; this one may look like a true bomb at customs. Is there anyone who could advice us on reducing this transport risk? Electronic cicruitry does attrack attention; we have had several posts with electronic greeting cards send from the Netherlands to Sweden (i.e. EU-internal) opened, in one case the card did not function.


    As you can see this project focuses much on technology design; we are very keen on thinking more on advanced sensing and control for future projects, in which the 18x will probably remain the key player.

    Next step: understanding the tilt sensor behaviour (this is the device: http://194.201.138.187/epages/Store....roducts/SEN010 ) and testign the two LEDs as optosensors. I will also start soldering the PongSat print, as it now seems that the circuit design is OK.

    Best regards,
    Jurjen
    Last edited by kranenborg; 06-04-2008 at 00:30.

  5. #5

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    Cool project

    The DS18B20 is only rated to measure down to -55, have you tested it in lower temperatures ? (dry ice maybe)

    What about isolation, dont know if it does any difference at those temperatures, with the low amount you can fit in there.

  6. #6
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    Why not use surface mount components (as far as available)? SOIC packages aren't too hard to solder even for a layman. Surface mount will need less board space and MUCH less volume (especially compared to socketed DIP), and I'd expect to to be more rugged, too. Probably too late right now, but if you do it again I'd volunteer to do the layout and PCB manufacturing (double-sided, which will save even more space) for free. Since surface mount will also result in a much lower height, you could even think of fitting two boards on top of each other, for even more space for additional circuitry.

    Wolfgang

  7. #7
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    Great project! Good luck on the flight!

    As a retired model rocketeer form the '70s, I'm amazed how far things have come along!

    If anyone hasn't checked out the PONGSAT site, do so.

    I do have a fcouple of questions:

    How long is the flight?

    How do you start and stop the data collection?

    I can't see the PONGSAT guys flipping switches on 500 ping pong balls.

    Myc

    Myc

  8. #8

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    Quote Originally Posted by Mycroft2152 View Post
    How long is the flight?
    I think I recall one was recorded as 2.5 hours...

    How do you start and stop the data collection?
    This could be a limiting factor on power, I noted the coin cell on some had a plastic tab to remove.

    regards,
    colin

  9. #9
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    Slurp,

    2.5 hours sounds about right.

    according to the website, they rmust receive the PONGSAT 6 days before launch.
    There may be some info in the PONGSAT project package about manually triggering projects.

    Note to Jurgen:

    You may want to check out the picaxe model rocket data collection projects for some ideas.

    Also, filling the PONGSAT with injectable foam insulation wil help maintain the temperature, though a real pita to remove. I used this trick during a model rocket egg lofting event to protet the egg from cracking. Managed to get 3rd place in the nationals.

    Myc

  10. #10

    Default Magnetic tilt sensor

    You mention that your having problems with the tilt sensor. Have you considered using the vertical component of the Worlds magnetic field to detect tilt? Honeywell and Philips produce a range of magnetic sensors that have the required sensitivity. Although they are only able to provide a raw bridge output of a few mV maximum, an instrumentation amplifier (single SOIC package) with a gain of about 500 will provide reasonable ADC resolution. This technique has been used for some years for detecting apogee on model rockets.

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