Solar Powered Mini Instrument Panel for Hang Glider

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We plan to create a basic instrument panel for a light sport aircraft, ultralight aircraft, or hang glider based on the Lattice Semiconductor MachXO2 programmable logic chip (and the Pico8 processor).

In this highly electronic ours, it’s hard to comprehend the idea that early aircraft often had no electrical systems to speak of. But aviation’s “golden age,” when passenger air service began to take hold, airmail delivery became more common, and the public’s view of pilots had progressed from “flying fools” to heroes such as Lindbergh began in about 1930 - lasting until just after WWII, when many returning GI’s (having become pilots in the service) snapped up surplus small aircraft the US government sold off. Throughout this period, the prevalent small private-piloted aircraft (such as the venerable Piper J-3 “Cub”) were being built with no battery, vacuum driven instruments, and an engine that required “hand propping” to start.

Many of these aircraft are still flying today, and their owners (who would like to update their instruments) are discovering that adding the wiring to do so can be an expensive process of adding the weight of a battery and copper to an airframe meant to weigh perhaps 700lbs empty. Thus, a small device that could replace the vacuum-driven instruments of the era with something lighter, more modern, and more reliable could find a home among those pilots who buy these older planes with the intent of flying them (not just keeping them for show).
Also, in the last five years, the FAA has opened up an entirely new class of aircraft and license under the general heading of “light sport aviation.” These aircraft are meant for the casual weekend flyer who doesn’t fly in poor weather conditions and doesn’t want to outfit his plane with a modern “glass cockpit.” LSA is one of the fastest growing segments of aviation now, and there’d be a market for a lightweight, simple, easily powered panel.

Finally, if the system were made light enough, it could also find a place on an ultralight or hang glider.

In 1937, the RAF specified what it considered to be the “basic six” instruments for flying under inclement weather conditions. Although it’s our intent to create a panel for “daylight visual flight rules” conditions (not “instrument” conditions for those who’d fly in London fog), the “basic six” list is a good starting point.

These instruments are an altimeter, an airspeed indicator, a directional gyro (to keep track of heading), an artificial horizon, a vertical speed indicator (to quantify climb and descent), and a turn and bank indicator. All of these instruments, although not strictly required for flying in daylight in clear weather, are advantageous for all pilots to have. They’re also, thanks to modern silicon micromachining, relatively easy things to make with a few chips and a good back-end processor such as a programmable logic device capable of also supporting a small “soft core” microcontroller such as the Mico8.

Our proposed design would make use of a MEMS accelerometer, MEMS gyro, pressure transducers, and magnetometer to collect the data for the basic six instruments.

The two pressure transducers (one facing into the direction of travel of the aircraft and one facing orthogonally) are adequate to derive airspeed and altitude. By itself, the orthogonal transducer provides a reading of barometric pressure, which plus or minus a known deviation caused by weather conditions reports data that with a lookup table or a little math can be converted to altitude. This transducer (the “pitot static” port on an aircraft) would have its pressure value subtracted from the pressure reported by the transducer at the end of a pitot tube that faces directly into the onrushing air in the direction of travel. The difference (with a little math) would give airspeed.
Most modern micromachined silicon pressure transducers report to a host microcontroller over either an I2C or SPI link, so we would implement the I2C or SPI “master” at the processor end using programmable logic and map it to a Mico8 processor that could format packets, collect data, and do the math to get altimeter and airspeed readings.

MEMS manufacturers these days have also managed to create multi-axis gyroscopes on silicon that would enable the reporting of heading in the manner of a directional gyroscope (around one axis), the reporting of data for turn and bank coordination (around a second axis), and the creating of an artificial horizon (around a third axis). Again, these gyros commonly report their data over either SPI or I2C.
Thus, with one 3-axis MEMS gyro and two pressure transducers, we’d already have five of our six basic instruments.

Finally, to get vertical speed indication, we’d use a MEMS accelerometer (also connected over I2C or SPI) in the same way.

Output of all these data would go to a small LCD screen (of character-only type or graphical nature), with the minutiae of driving the LCD possibly being handled by a hardware state machine in the FPGA to offload some of the work of the Mico8. We also anticipate needing perhaps a switch or two on the panel for setting initial headings or weather-related pressures. Switches into the processor could be debounced in a hardware state machine.

Powering all of this would be a battery charged by a solar cell. For charging we’d implement a small switched-capacitor supply that could stack capacitors in series and then rearrange them in parallel to buck or boost solar cell output while impedance matching the cell to the battery it’s charging. Typically solar cells have a spot on their I-V curves where this product is maximized (the “maximum power point”), and this can vary with ambient light conditions (see figure, reproduced here under GPL from Wikipedia).

To hit the maximum power point (shown here as a violet line), a pin somewhere on the MachXO2 would have its “threshold” value set where once the capacitor stack hit a certain voltage, a state machine would rearrange the configuration of capacitances to “switch” them from parallel to series (or vice versa) and dump charge into the coin cell battery. The objective would be to keep the capacitors somewhere on their charge curve where the impedance presented back to the solar array would be the Thevenin output impedance of the array itself. We suspect that with a properly chosen threshold on the detect pin (and perhaps a few timers to dither the point of switching a little) as well as a suitably chosen capacitance value, we could get pretty close to an optimally impedance-matched system.

Switched-capacitance is really the bucking/boosting technique of choice here for the reason that we are using a magnetometer to implement the “compass” function, and magnetic fields from a switching inductor could swamp the earth’s own field.

We’d simultaneously preserve the option of periodically charging a capacitor directly from the battery (perhaps through a high-impedance resistive divider to reduce loss) to test battery voltage. This would implement a poor-man’s A-to-D by comparing the charge time on a known capacitance to a timer value to find where on the battery charge/discharge curve you were in order to have a crude “gas gauge” for the battery.

Probably with some smart shopping, a design like this would have a BOM cost well under $50.