Jun 6, 2017

Project: Pixhawk 2.1 Cube upgrade
Objective: Test Pixhawk 2.1 Cube and Reach+ GPS as well as UGCS compatibility
Flight Date: 6/6/2017
Flight Location:
 Airframe: Tarot 810
Sensors: NA
Crew: Burnett and Benz

Swap from Pixhawk to the Cube Pixhawk 2.1 with Reach+ GPS was less than 20 minutes.  Initial thoughts are that hardware appears higher quality than the original 3DR equipment.

Hardware Observations:

Threaded hard points on autopilot base for mounting
Connection cables have cable protectors
Cable ends use a quick release that will be less susceptible to breakage
Reach+ GPS integrates the safety switch and a second barometer
Included power module supports up to 8S.  No more integrating an attopilot or some other questionable alternative

Intangibles

Reach+ GPS can produce raw .obs files for post-processing if raw logging is enabled (results are untested but we can confirm it does produce a .obs file when processing the dataflash logs)

Compass calibrates and accurately accounts for proper declination (with Pixhawk we always saw a 15 degree heading error that we were unable to correct; we had to rely on GPS heading corrections when moving to resolve this in automode)

KDE incompatibility seems to be resolved. ESCs and motors do not lose calibration randomly during power up.  This was an issue we could never reliably correct with the original Pixhawk

Issues

Reach+ and Reach+ RTK are designed to work with Arducopter 3.5 which is still in beta.  As of June 6, the beta version will not upload to the Pixhawk 2.1 Cube we have. The major issue with AC 3.4.6 is the aircraft fails pre-arm check for GPS 1 despite having a low PDOP and > 15 satellites in view.  This can be bypassed by arming in stabilize / altitude hold, then switching to loiter or auto mode.

UGCS

The autopilot connected to UGCS without any issues. UGCS was able to upload mission and arm/disarm as required. Telemetry data appeared accurate. However, ALL configuration and calibration has to be performed in Mission Planner before hand.  We will be conducting more flight testing with UGCS in the future. We are concerned that RTK functionality will require Mission Planner, which is disappointing because we are impressed with the DEM following functionality of UGCS (tested with P4). Also, after over coming the initial learning curve, mission planning with UGCS has become much more efficient.

Reach+ RTK

The base station appears to work. Due to the issues acquiring Arducopter 3.5 we were never able to confirm that corrections were being sent to the onboard GPS.  When we go live with RTK we are concerned that RTK may disable SBAS.  This means if RTCM corrections stop coming in (e.g. telemetry drop out), positioning goes back to uncorrected code-based positioning which can be highly imprecise.  The mitigation to this is to have two GPS units with one in conventional non-RTK mode, however, we'll have to investigate how the aircraft handles the blending of the two positons.

Flight

The test pilot said the PIDs out of the box were not optimized and of the 12+ builds he's done, this was the most dangerous.  This may be because the Cube Pixhawk 2.1 represents a major shift in hardware and the defaults don't adequately account for the new sensors. After autotune, the pilot felt this was the most solid hardware we've flown to date (Phantom 3/4 excepted).

Conclusion

These results give us enough confidence to switch our other Tarot 810 to the Cube Pixhawk 2.1. We think the hardware improvements mentioned earlier are going to save us a lot of troubleshooting time in the field.  In addition, the added redundancy that is inherent in the Pixhawk 2.1 design gives us more confidence that a catastrophic autopilot failure is less likely (yes it's happened....).