My ambition is to send APM aloft to the edge of space on the end of a balloon, and since it gets a little cold up there, I decided to take a look at what I would need to keep things running well.
NASA have kindly given the world a mathematical atmospheric model which shows that if I can meet the challenge at -56°C, I should be OK at any altitude. Here is the temperature of the atmosphere according to NASA:
So, having established a boundary condition, I fired up APM and measured the current draw at various voltages. My measurements showed me that APM will chew anywhere between 1.3W and 2.3W, from 5V through to the 7.2V that I will probably run it on using a 2S LiPo battery.
I then constructed a small EPS enclosure model in SolidWorks and ran a simple thermal study with a boundary temeprature of -56°C and an internal heat power source of varying levels (see above). The EPS enclosure I modelled is a 10mm thick box and just large enough to fit a fully assembled APM1 with no accessories - obviously I will have to have some cable penetrations and a few other compromises in the final design, however the model suggests I can keep it comfortably above freezing with only 3.5W of power, 2.3W of which APM will generate of its own accord
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To make this work, I will probably use a heater resistor powered by the relay that I can PWM to top-up the heat as necessary with a closed-loop controller using the on-board temperature sensor data for feedback. Obviously I will lose the OAT measurement, which will have to be subsituted by an externally mounted NTC thermistor.
So, now I have a first-pass heater power value for my mission power budgeting. I will refine and optimise this as I progress with the design of the electronics installation on HDwing.
Onward and upward (eventually!)
Comments
Alan, I think enough LiPo's have reached near-space to demonstrate that this battery technology isn't a problem.
Also, most data available is from parachute-borne payloads where their return trajectory is quite rapid and hence long durations at low temperatures only occur on the way up. The data I've looked at since posting this blog suggests that the residual heat from the launch is enough to keep things warm on the way up, but an extended return flight could be a problem.
My X-Plane simulations show that the return flight could take a couple of hours. Descent rates of between 500-1500'/min make 100k feet last quite a long time, especially through the tropopause where the TAS is begining to drop from the insane speeds of higher up.
Attempted to fly with my APM 2.0 some 2 weeks a go, in -20c here in ukraine, the APM 2.0 went crazy.......I think the APM is really sensitive to temperature......
The barometer has a built in thermometer and the AP_Baro library has a get_temperature call that might be useful for your PID controller attached to the heater. Perhaps you've already seen that but though I'd point it out in case you hadn't.
All good info - thanks guys!
I am mostly concerned with keeping the gyros and accelerometers at a relatively stable temperature to avoid the need to run an extensive calibration exercise. The gyro gains are apparently particularly sensitive to temperature extremes.
Thanks for the links to the HAB projects. I enjoy reading through other's experiences.
@Jonathan, the temperature at the tropopause doesn't change that much, so -56°C is a good minima to work with.
I generally agree with Paul; I've flown nearly 100 HAB payloads, many of them with very exposed electronics, and have never had a temperature-related hardware failure. There are temperature sensitive components (for example, we use TTL timing sources rather than crystal oscillators to send clocks into our microprocessors, and you want to stick with a lithium-based battery chemistry), but most components are pretty OK with getting cold. Is there a specific temperature sensitivity that you've identified on the APM?
If your APM will be in the same box as your transmitter, you'll have a heat source "built in." 20C temperature differentials aren't uncommon in our relatively uninsulated containers that contain the transmitter, though, admittedly, we're running the TX pretty hard.
Alternatively, you could use dry ice to get the APM that cold at standard atmospheric pressure... find out what stops working. Just be careful about how you cool it down if you go that route - it is possible to break stuff by transitioning the temperature too quickly.
On very early flights, we ran resistive heaters out of an abundance of caution, but we quickly found that the weight (particularly the battery) and complexity of those systems could be much better used elsewhere in our payloads. So... if it's necessary, absolutely implement it, but I'd suggest making sure that there's not an easier solution.
Well insulated box works very well, indeed, 15km without any prob
Have you ever thinked about peltier cell? keep warm when it needs and keep cold just reversing polarity, quite simple!
Interesting comment Monroe! Thanks for the tip.
I guess that means there are a whole bunch of conflicting requirements to make this thing work. I need to get it through the tropopause without icing up and dying on me, but also I need to keep it from cooking when it gets into sub-orbit as well as at sea level.
I will think some more! :)
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