Scratch-built brushless gimbal for the Black Widow

After flying the Black Widow for a while, I decided it was time to expand its capabilities by adding a camera gimbal. Since we're writing the year 2013, servo gimbals are outdated - brushless is the way to go!

The Black Widow doesn't use standard mounting points for off-the-shelf gimbals, and I wanted to integrate a non-stabilized FPV camera into the construction - so scratch built was pretty much the only option. Didn't stop me when I designed the frame itself, so ahead I went... and the following design came to be:

Huge thanks to Frosch2006 from the fpv-community.de forums for the awesome 3D model and render.

The overall design is inspired by the VectorDD GoPro gimbal, however my design is slightly more compact and based on glass-reinforced plastic as opposed to aluminum. Both axis have two points of attachment, spreading the load for better stability and less strain on the motors' ball bearings. The large mounting point on top is for a dedicated, non-stabilized FPV camera. As with the Black Widow itself, I did the parts design in 2D, and got the pieces CNC'd from GRP (2mm thickness).

For the gimbal electronics, I decided to go with the open source Martinez v3.1 controller, operating two RCTimer HP2212 70T gimbal motors. The controller software is being written by a team of people on the fpv-community.de forums which I frequent, that surely contributed to the decision. The alternative was the significantly more expensive and closed-source AlexMos controller - it has the same dimensions and connections as the Martinez board, so my gimbal can be run by either one.

Build

The parts were CNC'd by a forum colleague of mine and arrived here.

After filing the slots a little to make sure all parts fit together snugly, I glued everything with epoxid glue and set aside for the night:

While the glue was setting, I took to soldering the electronics. First I added the required pin headers for the power connector, two motors and the I2C sensor interface to the controller:

Soldered the sensor leads to the MPU6050 breakout and twisted the cable (for better flexibility and less signal interference):

And finally created the wiring harness for the camera - OSD - transmitter combination. This got a bit complicated:

• Video signal needs to go from the camera to OSD, and from the OSD to the TX
• The camera (Sony Super HAD 600TVL) requires 12V
• The transmitter (ImmersionRC 5.8G 25mW) outputs 5V
• The OSD is powered independently, video cables going to and from it must not have a + wire

To get all that fixed, I soldered this cable assembly:

Slighly less of a mess after attaching the camera and heatshrinking the OSD connectors and the voltage regulator together:

Next morning glue on the mechanical parts has set, and the build continued. Motors, controller and sensor all installed and wired:

Camera scwered into place:

And the whole thing attached to the tubes of the Black Widow frame:

The more eagle-eyes readers might notice that the rest of the frame is somewhat missing. That is indeed the case - a motor has died on the last Black Widow flight, and to check the wiring and install new motors, I had to disassemble the frame. Since I also need to access the power distribution inside to install the 12V BEC for the gimbal, the timing is actually pretty fitting.

Firmware flashing and configuration

The open-source Martinez controller runs on Brugi software, available here: https://code.google.com/p/brushless-gimbal/

I've gone with the latest version available at the time, v49 r69, complete with a TCL-based GUI by meister. Flashing the board is same as any other Arduino-compatible board (those of you who have ever flashed a MultiWii or an APM FC will be very familiar with the process). The important point is that the Martinez board uses the Arduino Uno bootloader to save space on the board, not the standard Pro Micro loader found on most 328p-based Arduino-compatible boards. Also important is to disconnect the motors and not connect 12V power before basic configuration is done - only the sensor and USB are connected.

After flashing, meister's GUI can directly connect to the board. The GUI looks like this:

Hitting Connect, the board restarts and the config values are loaded into the GUI. First thing to do is to set up the sensor. Hit the large Star button to view live pitch and roll values in the graph. Then check or uncheck axisReverseZ and axisSwapXY until you get correct values shown in the graph as you move the sensor around.

After that, disconnect from USB, connect the motors and 12V, and connect to USB again. At this point the camera must be installed, so the weight and balance of the gimbal are correct. Set direction and PWM percentage for both motors (roll and pitch) - it roughly corresponds to the power applied to the motor. If you get oscillations (overshoots), decrease it. Increase if the gimbal skips, not having enough momentum to continue rolling or pitching. After that's done, play around with the PID values to get fast enough and precise enough correction going on.

When the configuration is finished, don't forget to hit Save to flash, so the controller actually memorizes them and reloads the next time you power up. The whole procedure from flashing to final test took me ~30 minutes.

Test video

Since the copter isn't much flyable at the moment without motors, I went ahead and made a short handheld test video in my room.

For a grand total of ~30 minutes of firmware flashing, basic setup and tuning, I think it's pretty good. Definitely not perfect yet - at 35s some unsteadinness can be seen, and the pitch motor seems to oscillate very quickly at certain angles. Those I'll tackle when the copter is flying again though.