We're pleased to report that following four days of intense graft, including some hair-pulling over our Vulture 2 spaceplane's cantankerous Raspberry Pi rig, we've finally got some custom Low Orbit Helium Assisted Navigator (LOHAN) mission parameters loaded into our rocket ship's Pixhawk autopilot.
Here's Linus chatting to Tridge earlier this week, as the pair got down to work:
Those of you of a nervous disposition regarding autopilot programming are advised to look away now, because here's what we're implementing in APM:Plane, according to Tridge's notes:
Startup in AUTO mode
We have added a new ArduPilot INITIAL_MODE parameter for what mode to boot into. For LOHAN it will be AUTO.
Ascend wait will be an AUTO mode command that is used to wait for LOHAN to ascend to the needed altitude, wiggling servos on the way. This command will complete when one of the following conditions is met: vehicle is descending at higher than a set speed, vehicle sees an X acceleration above a given threshold (rocket has fired).
While ascending the servos will move through their full range every wiggle_time seconds.
We have a new ACC_HOLD_THRESH parameter in m/s/s. If the X acceleration is ever above that level in AUTO mode then the vehicle will move servos to the trim position and hold them there for ACC_HOLD_TIME seconds (we will use 20 seconds). This parameter will operate whenever in AUTO mode.
The MAV_CMD_NAV_LOITER_DESCEND command will be used during the primary descent of the vehicle after the rocket has fired. It will take a target latitude/longitude a target altitude and a loiter radius. The vehicle will fly towards the target lat/lon then circle at that position until its altitude drops below the given altitude target. At that point the mission will continue.
Autotune in AUTO mode
We have a new MAV_CMD_DO_ENABLE_AUTOTUNE while will enable autotuning during AUTO flight. This mission command will be just before the LOITER_DESCEND stage of the flight. It will then disable when LOITER_DESCEND has finished. This will allow the aircraft to use automatic PID tuning while descending.
So, in summary: We've got an ascent mode where the servos wiggle every X seconds to prevent freezing. At rocket ignition, the Pixhawk detects the acceleration and locks the control surfaces in neutral position.
Then, after 20 seconds, the Pixhawk will go into MAV_CMD_NAV_LOITER_DESCEND mode.
Simple as that. However, there is more. Quite sensationally, we've acquired a high-powered radio rig (details to follow) which will allow Linus and Tridge to monitor the aircraft's status live, and - if necessary - tweak settings on the fly.
For those of you who've never had the pleasure of working with APM Mission Planner, here's a quick grab from yesterday as Tridge and Linus were hewing parameters from the living code:
And here are just two of our new parameters in Mission Planner - ACC_HOLD_THRESH and ACC_HOLD_TIME - showing how we can easily alter values as required for testing and the final flight:
We've still got work to do crunching a few predicted aircraft flight prediction numbers to tweak the final settings, but it's all in good hands. We're obliged to Paul Riseborough for answering Tridge's call for some assistance in interpreting flight simulation and expected performance data provided by the Vulture 2 design team.
Paul's an autopilot software developer with more than enough Right Stuff for the job, so we're sure readers will join us in raising the traditional pint or two to him, and indeed Tridge and Linus.
Later today, myself and Linus will strap to the Vulture 2 to the van roof...
...and take to the road to test the MAV_CMD_NAV_ASCEND_WAIT command in AUTO mode. We'll also be collecting data on GPS, magnetic compass, servo and battery performance, checking the airspeed sensor, as well as grabbing some vid on the Raspberry Picam.
So, that'll keep us on the streets for a while, and we'll report back next week as to how things went. ®
More from the lovely LOHAN:
- You can find full LOHAN coverage right here.
- If you're new to LOHAN, seek out our mission summary for enlightenment.
- There are photos our our magnificent Vulture 2 spaceplane here, and detailed structural plans here.
- For your further viewing pleasure, we have all our photographic material stored on Flickr.
- Our LOHAN and Paper Aircraft Released Into Space (PARIS) videos live on YouTube.
- We sometimes indulge in light consensual tweeting, as you can see here.