MPPT in iNav software
Solar panels produce the most watts when operated at some ideal voltage called Vmppt.
Normally people use special hardware called an MPPT controller to draw
power from the solar at that voltage and then convert it to the right
voltage for a battery. We have worked out a program for iNav so that this extra
hardware and weight won't be needed. The idea is to adjust the throttle higher if the voltage is higher than ideal and to lower it if the voltage is lower than idea. Since we already have a computer onboard that knows the battery voltage and controls the motors, we can get things operating at the ideal voltage just using some software. This will help keep weight down on solar airplanes we make. We also save any conversion losses or power limitations of the MPPT hardware. You can see this in the video and the picture after that.
Here is a shot of the program:
A shot of the supercapacitor, flight controller, volt meter:
The solar out the window:
The power meter on propeller:
really works. As clouds go by the throttle adjusts to keep the voltage
constant. And it is getting good power out of this small 25 watt
panel. Note that the flight controller and DJI Air Unit are taking part
of the power from the solar. In the real plane we plan to have more
solar, like 92 watts, so the electronics will be a smaller fraction of
The supercapacitor is 16.7F and 16V. This is enough to run the motor at full throttle for around 6 seconds (really it starts about 150 watts and drops off fast). It should be enough time for normal takeoff and probably even enough for VTOL takeoff.
I changed the program so that we can use the throttle on the remote. The software still adjust the motors (OVERRIDE THROTTLE) to keep at some target voltage; however, now we can use the throttle stick on the remote to adjust the target voltage! :-) It works well. Also, now we can do a low-stick-throttle and the solar can charge the supercapacitor to a higher voltage. Then when we move the stick up (and make lower target voltage) we can use up the energy stored in the supercapacitor. We need this for takeoff and landing control. Now we should be able to. Here is the updated program (at least main part).
So a low throttle stick targets 16 volts and a high throttle stick targets 11.7 volts.
And a video showing how this works with solar and supercapacitor.
So we should be able to have speed control while the computer keeps the voltage something safe without any hardware MPPT. Takeoffs and landings should be possible.
After landing the current plan is to put something over the solar very quickly to really turn it off. However, we might put a switch on the plane to turn it off. Or we could just unplug connectors from the solar. Right now the lowest position of the throttle on the remote really means "targe 16 volts please" which is not the same as "off". It could keep the motor off for maybe a minute, but eventually it would turn on the props to keep from going higher if the solar is still working. So we have to be sure and have some way to really shut things down for good.