Photic

Photic is a collection of utilities for writing high power rocket flight computer software. It was created to bring power and elegance to what is an oft-overlooked aspect of high power rocket development.

Photic caters to low power microprocessors by minimizing its memory footprint and external library dependencies. This also makes Photic highly portable. It is particularly suitable for Arduino or other embedded environments which lack a complete C++ standard library.

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Patterns

The utilities in Photic can simplify programming patterns commonly seen in high power flight software. Several examples are shown below.

Liftoff Detection

The flight computer must wait for liftoff. Acting on a single accelerometer reading would be unwise, so the programmer typically takes a rolling average of acceleration readings over a period of time for more robust liftoff detection. For this and similar tasks, Photic provides a special capacitated data structure called a History:

Photic::History<100> accelReadings; // Capacity 100
while (!accelReadings.atCapacity () && accelReadings.getMean () < 30) // ~3 Gs
{
    imu.run ();
    accelReadings.add (imu.getVerticalAcceleration ());
}

Sensor Interfaces

Hardware-in-the-loop and hardware-out-of-the-loop simulation is a powerful tool for validating software in flight-like conditions on the ground. These simulations typically require a way of spoofing sensor readings for consumption by the flight software. Photic enables this with two abstract interfaces--IMUInterface and BarometerInterface--which standardize communication with these sensors.

The programmer can implement different sensor interfaces under this abstraction and easily swap between them based on whether the software is running in simulation or production, e.g.

Photic::IMUInterface* pImu =
#ifdef HARDWARE_IN_THE_LOOP
    new SimulationIMUInterface (); // IMU interfaced with simulation
#else
    new MyIMUInterface ();         // IMU interfaced with physical sensor
#endif

Navigation

The rocket's altitude, velocity, and acceleration at a point in time are useful for informing control and recovery decisions. However, the rocket's state can be difficult to track accurately. In an abominable oversimplification of a complex algorithm comparable only to scikit-learn, Photic provides RocketTracker.

RocketTracker is a self-calibrating 1-DOF Kalman filter that tracks a rocket's vertical state throughout flight. It does this with almost zero user input by accessing the rocket's sensors directly through the aforementioned sensor interfaces. Usage looks a bit like this:

using namespace Photic;
...
RocketTracker::Config_t config = RocketTracker::getDefaultConfig ();
config.pImu = new MyIMUInterface ();
config.pBarometer = new MyBarometerInterface ();
RocketTracker tracker (config);
...
// Returns <altitude, vertical velocity, vertical acceleration>
Vector3_t rocketState = tracker.track ();

The navigation filter itself is a standalone class KalmanFilter which offers greater configurability if RocketTracker feels like too much of a black box. Usage for each class is thoroughly documented in their respective headers.

The best navigation solution will be highly specialized to the rocket it flies on. However, most high-power rockets are similar enough that RocketTracker will perform admirably if a good 9-DOF IMU and barometer are available. The following graph compares a rocket's velocity as estimated by RocketTracker and a StratoLogger, a COTS flight computer (data taken from this flight):

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Other Features

• History data structure for efficient sensor reading analysis
• BarometerInterface and IMUInterface abstract sensor interfaces
• RocketTracker self-calibrating Kalman filter navigation utility
• KalmanFilter for greater navigation configurability for advanced users
• Matrix data structure and supporting MathUtils for common GNC math

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Arduino Installation

Download this repository as a zip and import it in Arduino IDE via Sketch > Include Library > Add .ZIP Library.

Then, include all of Photic with the following header:

#include <Photic.hpp>