9-DOF Sensor Instead of Absolute Encoders?

The PANOPTES baseline design uses an iOptron mount, an iEQ30 or an iEQ45. These do not have absolute encoders, so they can't always determine the position of the two axes. Normally they'll remember where they were when last used, but if the user has loosened the clutch to adjust things, the scope has collided with the mount, there was a failure to save state during power down, or some such, then the mount won't have accurate info, and needs to be manually homed: aimed at the celestial pole. This runs contrary to the goal of a robotic telescope: it should require as little human assistance as we can reasonably achieve.

We've experimented with including a 3-axis accelerometer in the camera box, a sensor that allows us to determine which way is down when the camera is not moving. This is pretty useful, but it can't distinguish, for example, between pointing down on the east side of the mount and on the west side of the mount. Basically, for each position on one side of the mount, there is a symmetric location on the other side where the sensor output (i.e. which way is down) is the same.

I recently came across an interesting little breakout board for an MPU-9250, a device with three 3-axis sensors: an accelerometer, a gyroscope and a magnetometer (i.e. a compass).

While I can't think of a way to use a gyroscope in this application, the magnetometer may allow us to distinguish different orientations of the camera box w.r.t. the magnetic field lines of the earth. Of course, that assumes that the local conditions, including the electronics and all the nearby metal in the mount, cameras and lenses, doesn't interfere. But interference is pretty likely.

Fortunately, once the mount is aligned to the celestial pole, there are only two degrees of freedom: declination and right ascension. So not all combinations of down and north are valid: for any one direction for down, only two compass values make sense, and they are in opposite directions. So, even if the compass resolution is pretty poor, it may still be possible to distinguish between the two sides of the mount.

A complication with using the breakout board is that the sensor is a 3.3V device, while the Arduino Micro that we use runs on 5V. The exceedingly minimal documentation for the board says it has a voltage regulator for the power line, but says nothing about level shifting the signal lines. So, I may destroy the thing just hooking it up to the Micro. Sigh.

If anyone has pointers to previous work on this, please do share! Thanks.