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
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!)