Friday, July 19, 2013

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



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:


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: 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.



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_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:

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.

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

Friday, July 12, 2013

Review: Hummer Brushless Gimbal for DJI Phantom and GoPro Hero 3

Figure 1: Hummer brushless gimbal, straight out of the box before any modifications

The Hummer Brushless Gimbal is a 2-axis direct-drive brushless motor camera gimbal designed specifically for use with the DJI Phantom quadcopter and the GoPro Hero 3. It comes with everything you need, fully assembled and configured, right out of the box. No configuration, assembly, soldering, or balancing is required, making it a true plug-and-play system. However, in my testing I found a couple modifications that can be done to make it better.


Material: 2mm carbon fiber plate
Weight: 146 grams (includes cables)
Firmware: Alexmos 1.3B1 (preinstalled)
Power: 3S LiPo (JST connector preinstalled)
Current: 0.3 to 0.4 Amps
Tilt Range: 180 degrees
Roll Range: 60 degrees

Installation and Connection

The top plate of the Hummer gimbal assembly has two holes that precisely match the positions of the mount holes under the DJI Phantom. Two screws are included to secure the top plate to these mount holes. The rest of the gimbal assembly is suspended from this top plate via O-rings. Each O-ring has a chunk of yellow foam inserted to provide vibration damping and stabilization. The foam itself is worthless because it's too soft and easily compressible, but I rectified this by replacing it with small squares of 1/2-inch thick DuBro foam. 

Figure 2: The Hummer gimbal comes with these useful connector cables pre-soldered to the main board
Fortunately the Hummer gimbal comes with connector cables pre-soldered, instead of providing header pins to connect them yourself. There is so little extra space under the DJI Phantom body that having header pins sticking up out of the control board would be inadvisable due to space constraints.

These cables include white signal wires to adjust pitch and roll. Configure your transmitter to apply greater or less than 1500 PWM to move roll and pitch, and exactly 1500 PWM to lock in the adjusted roll or pitch setting. I only connected the pitch control wires, since I have no need for roll to be anything other than level with the horizon.

Also very convenient is that the Gimbal includes a built-in video-out/power cable for your GoPro Hero 3. The end with the servo connector plugs into most video transmitters' video out, GND, and +5V pins (if your video tx doesn't supply a +5V out sufficient to power a GoPro, or you want to power your GoPro from its own battery, then you can leave this red wire disconnected). See Figure 2.

Figure 3: Nuts added to either side of bearing to prevent it from slipping out

Note that the bearing opposite the tilt motor is not well installed, and tends to pop out of the hole in the carbon fiber plate that seats it. I resolved this by simply adding a nut on either side of the bearing to keep it in place (fortunately the shaft came already threaded for this purpose). See Figure 3.

Testing the Hummer Brushless Gimbal

Figure 4 below shows the Hummer brushless gimbal and GoPro 3 installed on the DJI Phantom. The GoPro fits into the gimbal by friction alone, so it's very easy to remove when you want to--no need to muck with unreliable velcro straps.

It's also very easy to remove the mount when you don't want to use it. Just unhook the O-rings from the mount and leave the top plate and O-rings attached to the copter, or remove the two screws holding the top plate on as well, if you wish.

In my tests I found the Hummer to be an excellent gimbal, perfectly suited for the Phantom and GoPro 3. The only major issues I had were the low-quality foam that came supplied with it, and the problem of the bearing popping out, both of which were easy fixes.

Figure 4: Hummer brushless gimbal installed on DJI Phantom airframe
My first use of the Hummer Gimbal was on my ArduPhantom, testing my SplineNav code:

But for a more extreme test in high winds, I made this video, flying the ArduPhantom in 25 km/h winds with very bouncy turbulence down among the treetops. Watch how the trees flail around, especially near the end of the video as the oncoming typhoon gains strength. The aircraft was bouncing around severely, but the Hummer gimbal reduced the movement significantly, although not completely, considering it can only stabilize pitch and roll, not yaw or linear motions: