A Stratux AHRS image for beta testers

I am excited to finally offer a Stratux image for anyone who would like to help beta test my initial AHRS solution!

Requirements

To use this image, in addition to the normal Stratux requriements, you’ll need to connect an MPU-9250 sensor to your Stratux. The OpenFlightSolutions AHRS board is a great choice. You can also use a RY836AI (tested), a SparkFun MPU9250 breakout board (not yet tested), XINY MPU9250 BMP280 (not yet tested) or your own home brew.

The code also has a driver for the BMP280, which also exists on the OpenFlightSolutions AHRS board, the RY836AI and the XINY. If you have one of these boards or a separate dev board, the pressure altitude and rate of climb will be used in the algorithm.

Download

You can download the image from the Releases page of the GitHub project page. Just burn it like any other Stratux image.

Sensor Pinout

Pinouts for the MPU-9250 are as follows:

MPU-9250 Pin	Raspberry Pi Pin
VDD	Pin 01
GND	Pin 06
FSYNC	Pin 09
SDA1	Pin 03
SCL1	Pin 05

The Raspberry Pi also has ground on pins 14, 20, 25, 30, and 39 that you can connect GND or FSYNC to.

If you use the RY836AI, you can also connect its GPS:

GPS Pin	Raspberry Pi Pin
TXD	Pin 10 (RXD0)
RXD	Pin 08 (TXD0)

With the RY836AI, you should also connect its VDD to Pin 02 or 04 (5V) instead of 01 (3.3V).

Be sure to affix the sensor firmly to the Stratux box. The sensor must not be able to move around in flight or the AHRS will give false indications.

Changes to Stratux

Once you’ve connected the board and burned the image to your SDCard, you’re ready to go.

There are just a few changes to cyoung/stratux. First is a new Attitude Indicator in place of the old “paper airplane.” Tilting the sensor should result in some motion of the AI, though possibly not in a way that makes sense – because we haven’t calibrated it yet.

This is done via a “Calibrate AHRS Sensors” button on the “Settings” page.

I also added a “CPU Load” area to the “Status” page.

Finally, there are “Attitude Sensor” and “Altitude Sensor” switches on the “Settings” page. If your Attitude Indicator doesn’t respond when you move the sensor, be sure that these switches are on!

Stratux Startup

When this fork of Stratux is first started, it tries to connect to the sensors and, once successful, performs a separate “calibration.” The purpose of this calibration is to determine a preliminary zero value for the gyro and accelerometer.

It is very important when doing this calibration that the sensor remain fairly still. It is usually OK for it to be sitting on the glareshield with the engine running, though at these early (beta release) stages it might be better to start the Stratux up and ensure it is operating before starting the engine. It will not calibrate if the sensor is waved around or wiggling too much.

You will know that the sensor is calibrated when the heading on the attitude indicator swerves to show 90 degrees. Then you can pick up the sensor, wave it around and see how the attitude indicator responds.

Sensor Orientation

As noted earlier, it is crucial that the sensor remain in one position in your aircraft–if it slides around or wiggles, it will lose orientation. OpenFlightSolutions has a really nice base with a grippy bottom surface for their OpenFlightBox that works great for me. The OpenFlightBox AHRS board also affixes very firmly to the Raspberry Pi, so there is no chance for wiggling. With another sensor like a RY836AI, a XINY, etc. it is important to firmly affix the sensor to the case, Raspberry Pi, or something else so that it does not wiggle.

In this initial release, the Stratux will need to be oriented roughly at “right angles” to the pitch, yaw and roll axes of the airplane, so give some thought to how it will be situated in your aircraft. It can sit in any orientation as long as one of the sensor’s X, Y, or Z axes (positive or negative) points forward, one toward the left wing, and one up. I put mine on my glareshield with the antennae at the rear–see the picture.

It will need to be oriented to within say 5° of the airplane’s axes to provide good data to the algorithm. Eventually I may write a more sophisticated self-calibration routine that will orient itself automatically from any which angle, but for now we need to tell it precisely which direction is which.

Sensor Calibration

Now we will perform a “calibration.” This is not a full sensor calibration, it is just a means to tell the AHRS algorithm the orientation of the Stratux box in the airplane, i.e. which sensor axis corresponds to which aircraft axis. It needs to know which end of the sensor will point forward (along the roll axis), which end will point up (along the yaw axis), and which toward the left wing (along the pitch axis). This only has to be done once, as long as you don’t change this gross orientation of the sensor to your airplane. The setting will be stored with your other settings on your SDCard. It goes without saying that this is best done on the ground, with the engine off…

The calibration routine uses gravity to tell the algorithm which way is which. On the Settings page, if you click on “Set AHRS Sensor Orientation”, it will guide you through this process.

Once you click this button, you will first orient the sensor so that the end that will be pointing forward in flight is instead pointing straight up.

“Straight” just means that it is pointing up more than the other two axes, say a pitch angle of 70° or more. Then the axis which feels the most gravity will be considered “forward.”
Once you are holding the sensor in this attitude, “nose-up,” then click the “Set Forward Direction” button.

Now you just place the sensor in the orientation it will ultimately rest in the airplane.

Then click the “Set Up Direction” button and gravity will tell the algorithm which direction will be Up.

From this, we can calculate which direction will be toward the left wing, so there is no need to tell the algorithm this.

You’re now ready to fly!

GPS-Only Attitude

There is already a “GPS-only” attitude system recently integrated into Stratux. This was developed by @AvSquirrel. It is considered a “pseudo-attitude” system because it does not use independent sensors to determine attitude–it bases its attitude calculations only on GPS data.

It is excellent code and works quite well. However, because it doesn’t have sensors, it has several deficiencies. Most notably, its attitude lags the aircraft’s actual attitude by a second or so; and it cannot correctly sense uncoordinated maneuvers.

I retained this excellent code for users who do not have attitude sensors, which presumably will be the majority. My fork will use the GPS-only attitude if there is no sensor present, or if the “Sensors” switch in the Hardware section of the Settings page is “off.” Otherwise, it uses my AHRS calculations.

Disclaimer