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Instrumentation

It seems a shame to send a craft out to near space, and not take as many measurements as our energy, weight and financial budgets can allow.  Visuals are a given: we'll have as many cameras as we can get away with.  But what else?

We'll leave GPS, communication and tracking to a section on their own: they're not just instrumentation, they're mission-critical elements, without which we won't be able to retrieve the payload.

As usual, the Cool Components and Sparkfun web sites are our sources of inspiration.  It turns out that there's a lot of options.  Here are the ones we're considering:

  • Accelerometer;
  • Gyro;
  • Thermometers (internal and external);
  • Digital compass/magnetometer;
  • Barometer - certainly static, and possibly dynamic (pitot);
  • Ionizing radiation;
  • Infra-red;
  • Ultra-violet;
  • X-ray;
  • Acoustic;
  • RF - including CMB detection.

So what are the options and gotchas?

Accelerometer - there's a good range of options, so we can afford to choose on the bases of price and power budget.  The two stand-out devices are the ADXL335 and ADXL345.  This turns out not to be a difficult choice.  The 335 draws 350mA at 3.0V, so just over 1mW power consumption.  The 345, at 2.5V, pulls 40uA for sample rates of 10Hz or less, up to 140uA for rates over 100Hz, giving a range of 100-350uW (or 0.1-0.35mW).  Strangely, of the two the breakout boards from Cool Components, the 345 is cheaper - bonus!

Gyro - there are two candidates that suggest themselves: the L3G4200D and the ITG-3200.  They are the same price, so our criteria are technical merit and availability.  At 3V, the '2000D pulls 6.1mA operating, 1.5mA in sleep mode, giving 18.3mW and 4.5mW respectively.  The '3200, at 2.5V, draws 6.5mA and 5uA - note the units!  That's 16.25mW and 12.5uW.  At full range, both devices can sample 2000ops; both use I2C at up to 400kHz; both have operating temperature ranges of -40 to 85oC.  Where the '2000D wins, however, is its 32-slot FIFO, with a programmable watermark interrupt (you can control how full it is before it interrupts the CPU to tell it to read data).  The datasheet for the '3200 shows a FIFO in its block diagram, but refers to it nowhere else.

But wait! HobbyTronics lists an IMU Fusion board that combines the low-power ADXL345 with an IMU3000 3-axis gyro (similar specs to the ones above), and adds a digital thermometer too - and cheaper than the individual devices, at £39.  Granted, the temperature sensor grounds out at -30oC, but insulation, plus heater, should keep it within range.  Off to a good start...

Thermometer - the internal sensor's taken care of with the IMU Fusion, so we only need external thermometers.  Best to have more than one, as one side will be sunwards, and the other in shadow, at any given time.  Fortunately, temperature sensors are cheap, with the TMP102 breakout boards coming in under £5 each - however, TMP102 only goes down to -20oC.  We need something better.  The DS18B20 is even cheaper than the TMP102, goes down to -55oC, and there's sample code for the mBed that can be adapted.

Digital compass/magnetometer - do we go for a cheap 2-axis digital compass, or the more expensive 3-axis version?  It's a tough one.  Will we realistically make use of full 3-axis data?  It might be interesting to map the Earth's magnetic fields as we travel.  No decision yet.

Barometer - well, so far we haven't found a small, cheap unit that goes much above 30,000 ft.  We'll leave that on hold for now.

Ionizing radiation - Geiger tube, right?  Wrong.  Geiger tubes eat electrons by the bucket-load.  Not only that, they need what are (from our point of view) spectacularly high voltages: 300-600V DC.  It's just not practicable.  If we're carrying any radiation sensing at all, it will probably be a film dosimeter of some sort, which means we won't get a real-time record, just an overall dose level.

Infra-red - it would be very nice to record through the IR spectrum, not just one overall measurement - a thermal imaging camera (still or video) would be ideal.  Unfortunately, there doesn't seem to be much around at a reasonable price point.  We may have to miss out this one.

Ultra-violet - still looking for a useful UV sensor or camera.

X-ray - "cheap" and "x-ray detector" don't seem to belong in the same sentence.  The film dosimeter may well have to do.

Acoustic - microphones abound, although ones that work at near-space temperatures less so - however, we'll not be hearing much in a 10mbar pressure environment!  Near-vacuum pressures may not do the moving elements of conventional microphones many favours anyway, so we'd probably need something like a piezo transducer with physical amplification (something equivalent to what the aural ossicles do, in the human ear).  The question remains, however, as to what value we would place on audio recordings, so this may well be sacrificial, or perhaps satisfied by the imaging subsystem(s) instead.

RF - it would be interesting to monitor a number of non-broadcast frequencies, relating the measurements to altitude.  We may in theory be able to see whether the cosmic microwave background becomes more evident once the terrestrial RF sources fade.  However, the RF given off by the electronics, not to mention that produced by the telemetry tramsmitter(s), is almost guaranteed to render experimental data useless - and it would be all but impossible to shield the electronics to the point where they generate no incidental RF.  We are still looking for an RF experiement around which to base payload.

 

However - an epiphany!  What might include a whole lot of our instrumentation, be highly programmable, have GPS, a self-contained power source, its own substantial memory, and even an RF downlink for very low altitudes?   Oh yes...an Android smartphone!  The bonus is that we now get redundancy in some of the mission systems too.  A quick trip to Carphone Warehouse, and we're now the proud owners of a Samsung GT-I5800.  It's only got Eclair (Android version 2.1r2), but that's absolutely all we need.  It's remarkably lightweight, and the cost of the phone (£50, plus a £15 topup) is far less than the collection of sensors it replaces, with a whole lot of extras alongside.  In deference to CAA regulations, we won't be using the transmitter unless it's very nearly at ground level, but a goodly stream of SMS messages to warn of imminent landing will be golden.

Last Updated (Thursday, 11 August 2011 23:54)