108 km/h SplineNav with Arducopter 3.2

SplineNav is now a feature of the latest Arducopter, version 3.2! Robert Lefebvre posted this flight of his traditional helicopter flying a SplineNav circuit at 108 km/h:

New SplineNav 0.3: Smoother Yaw, and Manual Yaw Override

After a few months of hard work, SplineNav 0.3 is now ready to fly! This SplineNav release started with the goal of simply reducing the yaw jitter issues of SplineNav 0.2, but in the end it turned into a major rewrite of the code.

New Features:

• Smoother Yaw: Updates x and y spline derivative calculations faster, in a separate loop, for a more precise yaw control. Uses a fast approximation of the atan2 function to compute the yaw angle from the spline derivative.
• Features yaw rate smoothing for much smoother video, avoiding the small yaw "jerk" at each waypoint.
• Allows manual override of yaw control, then takes back auto control of yaw if the pilot centers the stick when yaw is directed approximately in the direction of the spline curve.
• Now enters SplineNav mode smoothly, without an initial jerk.
• Uses it's own flight mode, so unlike SplineNav 0.2 it doesn't override your Circle mode. To use SplineNav 0.3, set one of your FLTMODE parameters to 14 in the Full Parameter List.

SplineNav Waypoints

We collected the waypoints for this video while flying FPV, and flipping the channel 8 switch at each point to record. Then we loaded the waypoints into Mission Planner, and made some small adjustments:

Waypoints recorded during FPV flight and adjusted in Mission Planner

The GPS track indicates it hit all the waypoints quite precisely, on each lap for a total of 3 laps (72 waypoints in total):

GPS track from dataflash log shows SplineNav hitting each waypoint precisely over 3 complete laps

The waypoints had a range of altitudes set, from 7 to 35 meters. Here is the flight profile from the autopilot logs, imported into Google Earth:

GPS flight profile from dataflash logs after 3 complete laps

Hardware Used:

Airframe: 3D Robotics Quad, with some modifications.
Autopilot: 3DR APM 2.6, with external compass/GPS module
Motors: T-Motor MT2216 KV800
Props: APC 11x4.7
Gimbal: WIND-1 two-axis brushless
Camera: GoPro Hero 3 Silver
Telemetry: 3DR 433 MHz
R/C: FlySky TH9X(ER9X FW) + 2.4GHz FrSky DJT module + V8R7-II rx
FPV: ImmersionRC 5.8GHz 600mA + FatShark Predator + SecurityCamera2000 CMQ1993X
Also: These mods for longer range FPV.

SplineNav 0.3 Firmware Installation:

1. First make sure your quad copter is flying well with ArduCopter version 3.0.1, since SplineNav 0.3 is based on this firmware version.
2. Go to https://github.com/mavbot/SplineNav, and click the "Download ZIP" link.
3. In the special Ardupilot version of Arduino, go to File -> Preferences and set your sketch directory to the path of the "SplineNav-SplineNav-0.3" directory from the extracted zip archive.
4. Restart Arduino, then choose File -> Sketchbook -> ArduCopter from the menu.
5. From the ArduPilot menu, make sure your HAL Board is set correctly.
6. Connect your copter's APM via USB, and from Arduino's Tools menu make sure the serial port is set correctly.
7. Click the Upload arrow button and wait for the code to compile and upload to your APM.
8. Set one of your FLTMODE parameters to 14 in Mission Planner's Full Parameter List, Set your waypoints (either with Mission Planner or with the channel 7 or 8 switch), then go fly!

Note: If you have any special requirements, such as a frame or orientation other than quad X, remember to make those adjustments in the code before compiling. Or if you'd rather not compile yourself, please contact us to get a hex file you can upload directly via Mission Planner.

Parameters:

Here are the speed and acceleration parameters we used for this video (set in Mission Planner):

WPNAV_SPEED: 1350 cm/s
This should make SplineNav go about 49 km/h on straight segments.

WPNAV_SPEED_UP: 1000 cm/s

WPNAV_SPEED_DN: 650 cm/s

WPNAV_LOIT_SPEED: 1500 cm/s
Should be set higher than WPNAV_SPEED.

WPNAV_ACCEL: 180 cm/s/s
Nice low value for a smooth steady start.

Also, the following parameters are #defines in the splinenav.h source code, and can be changed at compile time:

SPLINE_TENSION: 1.6
Higher tension splines curve more tightly at waypoints, but straighter in between waypoints. A tension value of 2 makes it a Catmull-Rom spline. We found that slightly lower tensions tend to give nice loose curves for smooth aerial video.

SPLINE_CURVE_ACCEL_MULTIPLE: 2.0
This allows for twice the max curve acceleration as set in the WPNAV_ACCEL parameter. Added this parameter to allow for smooth slow starts without making tight curves overly sluggish.

SPLINE_JERK: 200.0 cm/s/s/s
Jerk is the maximum rate that SplineNav increases or decreases acceleration as it flies the curve.

SPLINE_LOOP: true
This makes SplineNav loop the waypoints forever until you exit out into another mode.

FPV Crash into Obstacle on RTL

This was the first time I've crashed while flying FPV. Crashes are usually caused by a series of mistakes, and this crash was no exception:

Mistake #1: Some of the hills around the flying area rose to at least 100 meters above the launch point, but I had the Return to Launch (RTL) minimum altitude set to only 50 meters above launch altitude. Note: I later realized that setting the RTL alt even higher wouldn't have helped anyway, because the current ArduCopter code foolishly limits RTL height to 80 meters).

Mistake #2: I flew behind the hill at such a low altitude that the forest cover on one of the hill's ridges came between me and the vehicle, thus scrambling my video signal (the 5.8GHz video is so high frequency that even vegetation can scramble the signal quite easily; whereas my RC control on 2.4GHz remained solid through the entire ordeal).

Mistake #3: Upon losing the video I did not back up or gain some altitude, but instead just kept going forward for a couple seconds, hoping video would come back. This just made the problem worse.

Mistake #4: When the video signal didn't come back, I switched into autopilot-controlled RTL mode, without gaining some altitude first. Thus the autopilot, already well above the set RTL minimum altitude, simply initiated a high speed beeline back towards the launch point...

RTL Code Change:

To help prevent this kind of accident happening again, I made a change to my local ArduCopter code to make it so that RTL always climbs first before heading back to launch, no matter what altitude that RTL is invoked at (although my code does provide for an absolute altitude limit if the Fence feature is turned on).

I've been using the code for a few weeks, and found that it makes FPV flying much safer, without having to set an inconveniently high RTL altitude; especially in high mountainous terrain or around tall trees. I submitted the code as the RTL Always Climb pull request to ArduCopter.

Hardware:

Airframe: ArduPhantom (DJI Phantom case, stock ESC, props, and battery)
Motors: T-Motor MN2214 upgrade for DJI Phantom
Autopilot: 3DR APM 2.5
Gimbal: Hummer 2-axis brushless gimbal for DJI Phantom and GoPro 3
Camera: GoPro Hero 3 Silver
GPS: 3DR ublox LEA-6H
Telemetry: 3DR 433 MHz
R/C: FlySky TH9X(ER9X FW) + 2.4GHz FrSky DJT module + 2W WiFi booster, V8R7-II rx
FPV: ImmersionRC 5.8GHz 600mA tx + CL antenna, FatShark Predator goggles + 11db patch antenna
Ground station: Mavbot Finder (first time actually used to find lost aircraft!)

New SplineNav Version 0.2 on a 63 km/h Tour of Chinese Lotus Pond

I've now finished coding up SplineNav version 0.2. It's almost a complete rewrite from version 0.1. Besides flying smoother and using processor resources more efficiently, it now restricts maximum speed to prevent lag. For example, if you're flying fast into a strong headwind, then altitude may get low due to insufficient downward thrust, and position may also start lagging target, resulting in corner cutting. SplineNav now tracks this, and continuously and smoothly adjusts target speed to allow the copter to keep up without major altitude loss. So it's now completely safe to crank up the speed settings and fly SplineNav really fast.

 

SplineNav Waypoints

I collected the waypoints for this video while flying FPV, and flipping the channel 8 switch at each point I wanted to record. Then I loaded the waypoints into Mission Planner, and made some small adjustments (Figure 1). I also checked them by flying SplineNav at low speed first, and watching via FPV how close it got to the trees.

Figure 1: Waypoints recorded during FPV flight and adjusted in Mission Planner

After the low speed check I felt comfortable to fly it at max speed, although it was still very nerve racking, because it was zipping by a few meters from those weeping willow trees at about 60 km/h. I'm not sure I could have reacted in time to prevent my copter going to the bottom of the pond had a GPS error caused it to brush the willow branches and careen out of control!

 

Flight Log Track

After the flight, I loaded the log data into Google Earth and exported a KML file to overlay on the map, using Mission Planner's handy KML Overlay feature (Figure 2).

Figure 2: Purple track is GPS recorded track during SplineNav flight

As you can see, the GPS track indicates it hit all the waypoints very precisely, expect it slightly missed waypoint 8, which I attribute to the copter having just flown right next to a large building which could have caused GPS signal reflections.

The waypoints had a range of altitudes set, for a more interesting flight. Here is the flight profile from the data logs, imported into Google Earth:

Altitude profile generated in Google Earth from flight log data

Hardware Used

Airframe: ArduPhantom (DJI Phantom case, stock motors, ESC, and battery)
Autopilot: 3DR APM 2.5
Gimbal: Hummer 2-axis brushless gimbal for DJI Phantom and GoPro 3
Camera: GoPro Hero 3 Silver
GPS: 3DR ublox LEA-6H
Telemetry: 3DR 433 MHz
R/C: FlySky TH9X(ER9X FW) + 2.4GHz FrSky DJT module + V8R7-II rx
FPV: ImmersionRC 5.8GHz 600mA + FatShark Predator goggles

 

Software

This test was done using the excellent new ArduCopter 3.0.1 release code, that I then modified to include and call the SplineNav code.

The latest SplineNav code, already integrated into my own branch of ArduCopter 3.0.1, is available here: https://github.com/mavbot/SplineNav

 

SplineNav 0.2 Firmware Installation

Warning: Only install SplineNav if your copter is already working well with ArduCopter Version 3.0.1, and if you're experienced enough to test fly it safely.

1. Download the code with this link: https://github.com/mavbot/SplineNav/archive/SplineNav-0.2.zip and extract the zip file.

2. In the special Ardupilot version of Arduino, go to File -> Preferences and set your sketch directory to the path of the "SplineNav-SplineNav-0.2" directory from the extracted zip archive.

3. Restart Arduino, and choose File -> Sketchbook -> ArduCopter from the menu.

4. From the ArduPilot menu, make sure your HAL Board is set correctly.

5. Connect your copter's APM via USB, and from the Tools menu make sure the serial port is set correctly.

6. Click the Upload arrow button and wait for the code to compile and upload to your APM.

7. Set your waypoints (either with Mission planner or with the channel 7 or 8 switch), then go fly!

Note: Since there is not yet any SPLINENAV mode in Mission Planner, SplineNav for now just commandeers CIRCLE mode. So switch to CIRCLE mode on your transmitter when you're ready to fly your waypoints with SplineNav.

 

Parameters

Here are the speed and acceleration parameters I used for this video (set in Mission Planner):

WPNAV_SPEED: 2000 cm/s
My copter can't fly 2000 cm/s, but SplineNav correctly kept the speed adjusted to what my copter can actually handle, and according to the GPS data it reached a maximum velocity of 1760 cm/s (63 km/hour).

WPNAV_SPEED_UP: 350 cm/s
WPNAV_SPEED_DN: 450 cm/s
WPNAV_LOIT_SPEED: 2500 cm/s
WPNAV_ACCEL: 500 cm/s/s

Also, the following parameters are #defines in the splinenav.h source code, but hopefully they will eventually become configurable parameters:

SPLINE_TENSION: 1.4
Higher tension splines curve more tightly at waypoints, but straighter in between waypoints. A tension value of 2 makes it a Catmull-Rom spline. I found that slightly lower tensions tend to give nice loose curves for smooth aerial video.

SPLINE_JERK: 500.0 cm/s/s/s
Jerk is the maximum rate that SplineNav increases or decreases acceleration as it flies the curve.

SPLINE_LOOP: true
This makes SplineNav loop the waypoints forever until you exit out into another mode.

Coding: Flying 3D Spline Curves between Waypoints with ArduCopter 3.0

Figure 1: Mission Planner waypoints and telemetry track after 3 spline circuits

I wanted my ArduCopter to fly smooth continuous curves between waypoints for some nicer aerial video, so I coded up this SplineNav class. Each spline segment is a cubic polynomial curve defined in 3D space, with 1st and 2nd derivatives continuous wherever two segments meet at a waypoint. This gives a nice smooth transition between curve segments during flight (Figure 1). It also saves time and battery, since abrupt speed and direction changes are avoided.

Figure 2: Autopilot logged track after 3 spline circuits

The SplineNav class, like CIRCLE mode, calls the loiter controller, which is what allows it to precisely control position along the curve using GPS and inertial navigation (thanks to the brand new ArduCopter 3.0 firmware). Unlike CIRCLE mode, in SplineNav the autopilot takes full control of setting altitude as well, so it can fly complex 3D curves. However, it also references your WPNAV_SPEED, WPNAV_SPEED_DN, and WPNAV_SPEED_UP parameter settings to avoid flying too fast, or climbing or descending too quickly.

For this video I wanted a slightly more wild ride, so I increased my WPNAV_SPEED_DN to 250 cm/s, and WPNAV_SPEED_UP to 350 cm/s. I left WPNAV_SPEED at 500 cm/s for now, but will try increasing it later. Figure 2 shows the 3D track in Google Earth. If you reduce these UP/DN speed parameters you will get the same 3D track for these waypoints, but at the steep up and down locations SplineNav will reduce its travel speed along the 3D curve.

Video

I shot this SplineNav demonstration video with my ArduPhantom. The GoPro is mounted on a Hummer 2-axis brushless gimbal designed for DJI Phantom.

Source Code

Here's the source code for developers and brave testers to test and suggest fixes and improvements. There's a .PDE file and an .H file, both of which go in your ArduCopter 3.0 sketch folder:

SplineNav.h
SplineNav.pde

Follow the directions in the SplineNav Readme to make the ArduCopter 3.0 code call SplineNav. Then compile with the special ArduPilot version of the Arduino IDE, and upload to your copter. Set your waypoints with Mission Planner, or with the channel 7 switch, and go test out SplineNav.

Warnings

Make sure your loiter is rock solid before you test, because SplineNav relies on the loiter controller. Also, the first time you test a new set of waypoints, you should probably keep WPNAV_SPEED low, perhaps just 200 cm/s or so. That way you'll have enough time to take over control in case the spline curve intersects any solid objects!